The Problem of the Mantle-Crust Mix: Lateral Inhomogeneity in the Uppermost Part of the Earth's Mantle*†

THE PROBLEM OF THE MANTLE-CRUST MIX: LATERAL INHOMOGENEITY IN THE UPPERMOST PART OF THE EARTH'S MANTLE"? Kenneth 1. Cook Department of Geophysics, University of Utah, Salt Lake City, Utah

Page 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 2. Definitions.. . . . . . . . . . . . . . . . . . . . . . . .................................. 297 3. Velocity Considerations, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 3.1. Variation of Velocity with Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 3.2. Variation of Velocity with Crystalline Rock Type . . . . . . . . . . . . . . . . . . . . 300 303 3.3. Variation of Velocity with D e p t h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Statement of the Problem.. . . . . . ........................ 303 5 . Summary of D a t a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Mid-oceanic Ridge System.. . . . . . . . . . . . . . 6.1. Mid-Atlantic Ridge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 6.2. Arctic Mid-oceanic Ridge. . . . . . . . . . . . . . . . . . . . . . . . . . . ....................... 313 7. Island Arcs.. . . . .

.........................................

315

7.2. Southern Antilles Arc. . . . . . . . . . . . . . . . 7.3. Tonga-New Zealand Arc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 7.4. Japanese Arc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Western P ar t of Mediterranean Sea... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 7.6. Other Island Arcs.. . . . . . . . . . . 8. Continents.. . . . . . . . . . . . . . . . . . . . . . . . ................. . . . . . . . . . . . 320 8.1. Continental Rift Areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 8.1.1. Gulf of Ad en . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2. Red Sea. . . . . . . .............................................. 320 8.1.3. East African R 8.2. Western Part of North America.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 8.2.1. Rift System.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 8.2.2. Basin and Range Province.. . . . . . . . . . . . . . . . . . . . . . 8.2.3. Colorado Plateau.. . . . . . . . ....................... 328 8.2.4. Montana.. . . . . . . . . . . . . . . . ....................... 330 8.2.5. Central Plateau of Mexico.. . . . . . . . . . . . . . . . . . . . . . . 8.3. Summary of Continents.. . . . . 9. A Suggested Model for the Active Tectonic Belts. . . . . . . . . . . . . . . . . . . . . . . . . 332 10. Evidence for Convection Currents . . . . . . . . . . . . . . . . . . . . . . . . . 334

* Contribution No. 37, Department of Geophysics, University of Utah. 't A condensation of this paper was presented a t the annual meetings of the Utah Academy of Science, Arts, and Letters, Provo, Utah, on April 14,1961 and the American Geophysical Union, Washington, D. C., April 18-21, 1961. 295

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KENNETH L. COOX

11. Example of M ................... ........ ...... 12. Other Implica Model. . . . . . . . . . . . ................... 12.1. Trends of Basin and Range Faults.. ................................. 12.2. Heat F l o w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3. Difficulties with the Model.. ............... . . . . . . . . . . . . . . . . . . . . 12.4. Possible Fracture Zone. . . . . . . . . . . . . . . . . 12.5. Explanation of Gutenberg Low-Velocity Layer. ...................... 13. Problems Concerning the MohoroviEid Discon 14. Abrupt or Gradational Boundary.. . . . . . . . . . . 15. Depth of Isostatic Compensation.. ....................................... 16. Area of Mantle-Crust M i x . . ....................... 17. Summary............................ Acknowledgments. ............. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

335 337 338

341 347

350

1. INTRODUCTION

The possibility of continued interchange of matter between the mantle and crust of the earth is one of the major problems about the crust-mantle system (Ewing, 1958, p. 186). During the past decade, refraction seismic surveys and gravity surveys have indicated that some form of a “mantlecrust mix” exists along certain restricted belts such as mid-oceanic ridges and island arcs. More recently, seismic and gravity studies have yielded surprising results which support the possibility that a mantle-crust mix exists in tectonically active belts within the continents. Thus the upper part of the mantle is probably inhomogeneous laterally in some areas; that is, it has lateral variations of density. The inhomogeneity may be widespread, in varying degrees, over the entire earth. The present paper summarizes some of the evidence to date for the possible lateral inhomogeneity of the earth’s uppermost mantle, and gives possible explanations of this inhomogeneity. The topic is obviously speculative, and much more data are needed before firm conclusions can be reached. It is hoped, however, that this review may stimulate more thought and discussion-and, above all, the gathering of more experimental data-on the subject. The idea of heterogeneity in the uppermost part of the mantle is not new. Tatel et al. (1953) stated that “the idea that the mantle rocks are uniform may be a result of ignorance and very limited measurements.” Byerly (1956, p. 127) stated the possibility that “the heterogeneity. . . for shallow depth continues to depths of the order of that of M (MohoroviEiE discontinuity) or more.” The investigators a t the Carnegie Institution of Washington emphasized that “the idea of a single, world-wide value of wave velocity for the outermost portion of the mantle is probably an erroneous simplification,” and that “there appear to be non-uniformities and regional geographic differences in the mantle of the earth, just as there are in the

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297

crustal rocks” (Anonymous, 1958, p. 582). Oliver, in discussing the controversial aspects of the earth model and the discrepancies obtained in seismic and gravity investigation of crustal structure, as for example, in plateau regions, concluded that if the present results “are proven to be typical of the region rather than anomalous, then i t may be necessary to assume a non-homogeneous mantle, that is, one with lateraI variations of density, to account for the discrepancy” (Oliver, 1958, pp. 193-194). 2. DEFINITIONS

The “normal” outermost part of the mantle is considered in this paper as that layer lying immediately below the MohoroviEiE discontinuity and having normal compressional wave velocities between 7.8 and 8.3 km/sec (Byerly, 1956, pp. 119-147; Drake et al., 1955, p. 128). The group of layers above the MohoroviEiE discontinuity, so defined, is customarily defined as the crust,l and has seismic velocities that are usually somewhat less than 7.8 km/sec. The “basaltic,” intermediate, or “third” layer, which is oftenthough not always-found at the base of the crust, has compressional wave velocities between 6.4 and 7.3 km/sec (Woollard, 1959, p. 1521). Above the basaltic layer, and separated from it by the Conrad discontinuity, is the “granitic” or “second” layer, with velocities of 5.5 to 6.2 km/sec (Woollard, 1959, p. 1523). The “first” layer, comprising the low-velocity sedimentary rocks, lies above this latter layer, and has velocities below 5.5 km/sec. I n layered areas where normal mantle velocities are found beneath normal velocities of the basaltic or granitic layer, or both, the interpretation of the seismic results is straightforward and the layers are designated simply as mantle, basaltic layer, or granitic layer, respectively, or by similar terms. The Conrad discontinuity is not found in all areas. The granitic layer appears to be absent beneath most typical oceanic areas (Woollard, 1960a). Tatel and Tuve (1955) believed that the MohoroviEiE discontinuity may be sharp, or may extend over a transition zone of several kilometers. Using the conventional definition of “crust,” the earth’s crust averages about 35 km in thickness beneath the continents and 5 km beneath the oceans. It is generally believed that the continental crust is thicker beneath major mountain systems and thinner in coastal regions and other lowlying areas. The oceanic crust is thickest under mid-oceanic ridges and island arcs and thinnest under the deep trenches associated with the island arcs (Woollard, 1960a, p. 108). Following Bullen (1954, p. 838), the term “homogeneous” will be used to refer to a region in which there are no changes in chemical composition or polymorphic transitions; there will, however, be density changes in such a ‘Benioff (1954), however, defines the bottom of the crust at a depth of about 700 km, which is the greatest depth of foci of earthquakes.

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KENNETH L. COOK

region arising from variations of pressure and temperature. Vertical inhomogeneity implies that, with change in depth, the material is not homogeneous in the above sense, and that a change in chemical composition or polymorphic transition does occur with changing depth. Such a change could result in layers of material whose density and seismic velocity could have symmetry with respect to both the axis of rotation and equatorial plane of the earth; yet the material would still be designated as vertically inhomogeneous. Lateral inhomogeneity implies a change in chemical composition or polymorphic transition a t the same depth. Ordinarily such a change would result in a lateral change in density and compressional wave velocity of the material. A change of this kind could be effected a t a given depth where the temperature changed laterally, as for example, near the periphery of a rising column of a convection current. The outer portion of the mantle is considered to extend from the MohoroviFi6 discontinuity to a depth of about 900 km, which, according to Birch (1952), probably marks the bottom of the inhomogeneous part of the mantle where transformation to high pressure phases of the minerals (as they are interpreted beneath the MohoroviEiE discontinuity) has become complete. The top part of this transitional zone is postulated by him to be a t a depth of about 200 km. It should be emphasized that the term ‘Linhomogeneity” as used by Birch for this region of the mantle, lying between the depth of about 200 and 900 km, applies to changes in phase, composition, or both, in a vertical direction only and not in a lateral (or horizontal) direction. Birch (1952, pp. 272-273) recognizes the possibility of a layer of eclogite between the base of the crust (MohoroviEiE discontinuity) and a depth of “several hundred kilometers.” The uppermost or outermost part of the mantle, as used in this paper, is tentatively defined for the following reasons as extending from the h4ohoroviEiE discontinuity to a depth of about 400 km: (1) this depth is of the same order of magnitude as that (413 km) given by Bullen (1940, 1954, 1955) as the boundary between his layers B and C (although Gutenberg, 1959a, p. 75, took this boundary tentatively a t 200 km), (2) this depth is in the range given by Birch as the bottom of possible eclogitic material, and (3) recent studies with Rayleigh waves indicate that the difference between the continental and the oceanic mantle extends to a depth of about 400 km (Dorman et al., 1960, p. 89). As additional data are obtained, it may be found necessary to revise this depth estimate for the bottom of the uppermost part of the mantle. In this paper, the Gutenberg low-velocity layer, as inferred by Gutenberg (l953,1955a, 1955c, 1955d, 1959a, p. 84) and confirmed by various investigators (as discussed below), is assumed to lie within the uppermost part of the mantle between the approximate depths of 100 to 250 km.

THE PROBLEM OF THE MANTLE-CRUST MIX

299

Neither of the boundaries a t depths of about 200 and 900 km seems to be sharp, and the transition from one zone to the other is probably gradual (Gutenberg, 1959a, p. 75). Although it is doubtful that they are major discontinuities, added support for their existence a s a t least minor discontinuities was given recently by Hoffman et al. (1961). Events on seismograms obtained from nine large quarry blasts (up to 2,138,000 lb of explosives) a t Promontory and Lakeside, Utah were interpreted by them as possible seismic reflections from layers within the mantle a t depths of approximately 190, 520, and 910 km.

3. VELOCITY CONSIDERATIONS 3.1. Variation of Velocity with Density

Figure 1 shows empirical plots of compressional wave velocity versus density of rocks (Woollard, 1959; Talwani et al., 1959a). Because the

1

2 Density

3 gm/cc

4

FIQ.1. Relation between compressional wave velocity and density of rocks. Data after Woollard (1959), from various authors.

300

KENNETH L. COOK

velocity depends on various moduli of elasticity in addition to density, there is a considerable amount of scatter in the data of the several investigators, and therefore different interpretations of general trend of the curves. The plots demonstrate that it is impossible to assign a unique rock type to any velocity value. However, for crystalline rocks in the range of velocities between about 7.0 and 8.3 km/sec, an essentially linear relationship obtains, and an increase in density of 0.1 gm/cm3 causes an increase in velocity of about 0.3 km/sec. Such a generalization should be used cautiously, however, as i t may not apply in individual rock specimens. 3.2. Variation of Velocity with Crystalline Rock Types

Table I gives laboratory data from Birch (1960) for the compressional velocity of several rock types a t pressures of 6000 and 10,000 bars (equivalent to depths of about 21 and 35 km, respectively, except for temperature effects) and room temperature. If temperature effects at these depths were considered, the values of the velocities would probably be about 0.1 to 0.2 km/sec less than those shown (Birch, 1958, Table 3, p. 165). The lower part of Fig. 2 (in part after Woollard, 1959) shows plots of laboratory data from Birch (1958; 1960) for the compressional velocity for average granite (based on many samples) and average gabbro (based on many samples) as a function of pressure and depth. The solid curves for granite and gabbro give velocity versus pressure a t 20°C, and the broken curves TABLE I. Compressional wave velocity in rocks at pressures of 6000 to 10,000 bars (depths of about 21 and 35 km, respectively, except for temperature effects) and room temperature (after Birch, 1960, p. 1093-1098 except as noted otherwise).

Rock

Basalt" 1 rock, 1 sample Jadeite 2 rocks, 6 samples Pyroxenite 3 rocks, 9 samples Dunite 6 rocks, 18 samples Peridotite Harzburgite 1 rock, 3 samples Eclogite 4 rocks, 10 samples Garnet Grossularite 1 rock, 3 samples Almandite-pyropeb 1 rock, 1 sample

Mean density,

Mean velocity, krn/sec

gm/cms

6000 bars

10,000 bars

2.59 3.25 3.27 3.35

5.82 8.49 7.84 7.94

8.53 7.95 8.02

3.37 3.36

7.90 7.71

7.95 7.80

3.56 3.95

8.83 8.01

8.99 8.07

After Hughes and Maurette (1957). Shattered and somewhat altered along the fractures.

i

"

,PNT"E

I M B i r c h , 195E t-7 Gutenberg. 1959

L~

Various authors: see T a b l e 2 in l e x t

1 I

0

0

Pressure

5000

in

45

50

55

yollord,

60

1959 I

65

70

75

10,000

~g/cm'

FIG.2. Compressional wave velocity for average granite and average gabbro (bottom curves) as a function of pressure and depth, from laboratory data (Birch, 1958) and for material immediately below the MohoroviCiE discontinuity in continental areas (top curves) from field data of various authors (in part after Gutenberg, 1959b). For granite and gabbro, the solid curves give velocity versus pressure a t 20°C and the broken curves give velocity versus depth after correction for the assumed temperatures shown in the bottom curve. Mean crustal depths after Woollard (1959).

302

KENNETH L. COOK

give velocity versus depth after correction for the assumed temperatures shown by the bottom curve. Immediately above the MohoroviEiE discontinuity in many areas, the basaltic layer (with conventional velocity of 6.4 to 7.3 km/sec) has a compressional velocity which permits an interpretation of gabbro for the base of the crust (Birch, 1958, p. 168). Within the range of 6 to 7 km/sec, however, the interpretation of the seismic data is not unique; a multiplicity of chemical and mineralogical compositions different from gabbro could give these velocities (Birch, 1958, p. 169). The upper mantle, immediately below the MohoroviEiE discontinuity, has until recently been generally believed to be composed of normal duniteperidotite (Hess, 1954, 1955, 1959; Gutenberg, 1955b, p. 28; Ewing and Press, 1 9 5 6 ~ )Dunite . is composed almost entirely of olivine, and peridotite contains a mixture of olivine and pyroxene. Table I gives the compressional velocity for average dunite and 3 samples of peridotite. Ringwood (1958, p. 21 1) postulates that material of the composition of dunite-peridotite extends beneath the Mohorovic'iE discontinuity to a depth of about 400 km (following the depth given by Bullen and Jeffreys), where a phase transition to garnet-peridotite occurs; olivine inverts to spinel. The garnetperidotite is postulated by him to persist to a depth of about 900 km (following the depth given by Bullen, Jeffreys, Gutenberg, and Birch), where a phase transition to a homogeneous spinel phase, which is chemically equivalent to the garnet-peridotite, occurs. The spinel phase is presumed to continue to the core of the earth. Recently the hypothesis that the MohoroviEiE discontinuity constitutes a phase change from basalt (or gabbro) to eclogite rather than a change in chemical composition has been revived (Kennedy, 1959, p. 500; Lovering, 1958; Holser and Schneer, 1957; Robertson e t nl., 1955, 1957; Fermor, 1914). Eclogite is composed of a jadeitic pyroxene and a pyrope-rich garnet. Basalts (or gabbros) and eclogites have essentially identical chemical compositions, but sharply contrasting mineralogy. At a temperature of 5OO0C, basalt glass crystallizes to gabbro a t pressures below 10,000 bars and to a rock made up dominantly of jadeitic pyroxene a t pressures above 10,000 bars (Kennedy, 1959, p. 500). Because these temperatures and pressures are estimated to exist a t the depth of the MohoroviFiE discontinuity beneath continents, the possibility of such a phase transition appears reasonable (Lovering, 1958). The density and seismic velocity of eclogite (Table I ) , which depend upon the proportion of its constituent minerals, are variable. According to Kennedy (1959, p. 499), the mean density of eclogite is 3.3 gm/cm3; whereas that of gabbro is 2.95 gm/cm3. The density contrast is therefore about 10%. The average seismic velocities for the eclogites in Table I, if

THE PROBLEM OF THE MANTLE-CRUST MIX

303

the temperature effect is considered, arc somewhat smaller than normal mantle velocities. It is obvious, however, that an increase in the proportion of high-velocity garnet (see Table I) would increase the seismic velocity of the mixture to that within the range of normal mantle rock. 33. Variation of Velocity with Depth

Within the depth range shown in Fig. 2, it should be noted that after applying temperature corrections, the velocity increases with the depth in a uniform layer of average granite and decreases slightly, for depths greater than about 10 km, in a uniform layer of average gabbro. The two uppermost plots in Fig. 2 show values of the velocities of compressional waves V (km/sec) , immediately below the MolioroviEib discontinuity in continental areas, as observed by various investigators, versus the values of the depth h (km), a t which the discontinuity was observed. For each of the plots, it was assumed that the velocity changes linearly with the depth over the range of the depth data, and each of the resulting straight lines was obtained by the method of least squares. The shorter straight line was given by Gutenberg (1959b) for the depth interval between 26 and 50 km only. The longer straight line was obtained by combining the data used by Gutenberg with more recent data, which are given in Table II.2 For this recent compilation, the rate of the decrease of velocity with depth is 0.0052 km/sec per km (which is about half of Gutenberg’s value of 0.011 km/sec per km) and therefore still appreciably exceeds the critical rate of about 0.0013 km/sec per km, which, according to Gutenberg (1959b, p. 348) , is required for a low-velocity channel. The equation of the longer line is:

V = 8.32 - 0.0052h km/sec

It should be emphasized that this formula in the present analysis applies for values of h lying between 26 and 72 km only.

4. STATEMENT OF THE PROBLEM

Many areas are now known, both in oceanic and continental regions, in which the simple crustal picture above does not apply. In these regions, the aThere is obviously some selection of data in such a compilation. For example, both Gutenberg and the author omitted data from Japan. Since Fig. 2 was compiled, a statistical analysis by Steinhart and Woollard (1961, p. 3631, using some of this data and additional seismic data on continents, led them to conclude that crustal thickness is generally not related to the upper mantle velocity. If no such relationship exists, then the least-square lines in Fig. 2 would be horizontal instead of sloping. There is some evidence, however, that if the sampling of the data is done along certain tectonic trends, as for example, along the Pacific margin, a correlation of crustal thickness and upper mantle velocity may be found.

304

KENNETH L. COOK

TABLE 11. Compressional wave velocities V and depth h of the MohoroviEib discontinuity reported for various continental regions from artificial explosions.

Region

Author

Arkansas Plateau of Mexico

Woollard (1960b) Woollard (1960b)

Eastern Montana Western part of Colorado plateau (E. of Bingham Canyon, Utah) California-Nevada Nevada-Arizona Chuquicamata, Chile Eastern part of Basin and Range province

Meyer et al. (1960) Woollard's re-evaluation of Tatel and Tuve (1955) data (this paper) Press (1960) Diment et al. (1961) Woollard (1960b) Berg et al. (1960)

h, km

V , km/sec

43 44 (43)* 48 48

8.15 8.2 (8.38)n 8.11 8.2

50 530) 70 72

8.11 8.11" 8.0 7.97

Since this value was assumed from data of Press (1960) in the California-Nevada region but not actually observed by Diment et al. (1961) in the Nevada-Arizona region, i t is included in Fig. 2 but was not included in computing the least-squares line in Fig. 2. * A reinterpreted value (Steinhart and Woollard, 1961, Table 10.1, p. 347), which was called to the author's attention after Fig. 2 (in which the earlier values are used) was prepared.

velocity of the rocks a t depth is too low to be considered as normal mantletype rock, yet not sufficiently low to be regarded conventionally as of the basaltic type found in the basaltic layer. A lateral inhomogeneity (that is, lateral variation in density) in the upermost mantle is proposed for these areas. The proposed lateral inhomogeneity in the uppermost mantle varies in degree from place to place and is of such a relatively small amount and a t such great depth that refined seismic instrumentation and techniques are needed to detect it. If refraction seismic data are used, the accuracy of the times of arrivals must be within 0.1 sec or better, and the same accuracy is necessary for the times of origin of the explosion. Moving-coil seismometers with a natural frequency of 2 to 8 cps are necessary. Special low-pass amplifiers, with large amplifications, have to be used. Camera or recorder speeds of the order of 5 cm/sec are desirable. To date, these required accuracies of times of arrivals have not generally been obtained in the seismograms of the permanent seismograph stations in operation throughout the world; and, of course, the times of origin of most earthquakes are far from this required accuracy. For these reasons, the lateral inhomogeneity in the uppermost mantle has not been generally recognized in seismograms of earthquakes. The effective depth of measurement of seismic velocities in refraction

THE PROBLEM OF THE MANTLE-CRUST MIX

305

crustal studies with portable equipment depends on the shot-detector distance, the energy of the blast at the shot point, and the sensitivity of the recording equipment. I n oceanic areas, for practical and economic reasons, the shot-detector distance is usually up to about 100 km in length (Woollard, 1960a, p. 108) ; and, accordingly, measurements of seismic velocities to effective depths of 20 to 25 km can be made. I n continental areas, the length of the spread with sensitive seismic equipment may be up to several thousand kilometers, provided the blasts are sufficiently large, and the effective depths of measurement are correspondingly greater. The compelling reasons for postulating lateral inhomogeneity in the uppermost mantle are as follows. 1. Recent seismic data indicate the inhomogeneity in some areas, especially along many active tectonic belts. The belts include mid-ocean ridges, island arcs, and rift systems in upland or plateau areas on continents. These results will be summarized presently. The early data obtained from explosion or earthquake studies of continental structure in several plateau areas indicated a surprisingly small thickness of the crust, which did not reflect the high elevation of the topography. These areas included, for example, the southern part of the Colorado plateau in Arizona and New Mexico (Tatel and Tuve, 1955, p. 45,47) and the plateau in the Transvaal (Gane e t al., 1956). A recent re-evaluation of the seismic data from explosions for some of the plateau areas, however, has given alternative interpretations which are consistent with the normal thickness of the crust in these areas as indicated from studies of surface waves generated by earthquakes (Hales, 1960; Hales and Sacks, 1959; Hales and Gough, 1959). Nonetheless, the discrepancies in the plateau areas have not been completely resolved (Woollard, 1959). The most convincing results have been given by Dorman e t al. (1960) and by Aki and Press (1961), who have found differences between the uppermost mantle under oceans and under continents. Details of these discoveries will be discussed later. 2. Recent gravity data indicate the inhomogeneity in some areas. Gravity studies indicate that isostasy obtains generally throughout the world. In particular, the gravity data show that the continents are in approximate isostatic equilibrium and that the crust should thicken in accordance with the increased elevation of the surveyed region. It now seems probable, however, that variation in crustal thickness alone does not account entirely for the isostatic compensation, but that variations of density within the uppermost mantle may also contribute to the compensation (Tatel and Tuve, 1955, p. 50; Griggs, 1960, p. 167). Moreover, gravity data suggest the possibility that the density of the upper mantle in areas of great crustal thickness is less than the mean mantle density (Woollard, 1959, p. 1541). 3. Crustal density variations, especially those within the basaltic layer,

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KENNETH L. COOK

indicate the inhomogeneity in some areas. Although the possible genetic relationship between the basaltic layer and the underlying mantle rock has not been proved, the lateral inhomogeneity of the basaltic layer, as indicated by variations of seismic velocity from 6.4 to 7.3 km/sec and its varying thickness, suggest that i t may be a zone of phase transformation within the underlying mantle rocks (Woollard, 1959, p. 1521). Recent seismic studies by the Russians in Central Asia indicate that the basaltic layer is usually thicker beneath areas of uplift and suggest that the mean crustal density possibly increases as the crust thickens beneath these uplifts. Thus the problem resolves itself into choosing a model in which the depth of compensation may be much deeper than the MohoroviEii: discontinuity. The maximum depth a t which isostatic compensation may manifest itself is still not known. Tuve and Tatel (1955, p. 50) state-without proof-that a continent may have a depth of compensation down to hundreds of kilometers. In the model suggested in this paper, density can increase somewhat with depth, and the level of compensation may be as deep as, or greater than, that proposed by Bowie (1917, p. 111-12), who found that the most probable depth of isostatic compensation, based on the PrattHayford hypothesis, was 124 kin beneath the Rocky Mountains and 57 km beneath the continental lowlands. The model assumed in this paper departs from either the Pratt-Hayford or the Airy-Heiskanen hypothesis of isostasy and involves some of the complexities of crustal structure, as suggested by Tuve and Tatel (1955, p. 36). The density contrast between the uppermost part of the mantle and the crust may be small, especially along active tectonic belts. The Gutenberg low-velocity layer is assumed.

5. SUMMARY OF DATA Table I11 summarizes the abnormally low compressional wave velocities available for this study that give evidence of lateral inhomogeneity in the uppermost mantle. Tlic rocks with these abnormally low velocities of generally 7.4 to 7.7 km/sec are interpreted as associated with the mantle but are probably not typical of mantle-type rock. Consequently they arc postulated in this paper as a mixture of mantle- and crustal-type rocks and designated as “mantle-crust mix.” I n many of the areas included in the table, the position of the MohoroviFi6 discontinuity, as defined above, is difficu!t or impossible to select; and its existence as a sharp, discrete layer becomes questionable in some of these areas. It will be demonstrated that these areas of abnormally low velocities lie principally, if not entirely, along the mid-oceanic ridges, the island arcs, or the landward extensions of the mid-oceanic ridges.

307

THE PROBLEM OF THE MANTLE-CRUST MIX

TABLE 111. Compressional wave velocities and other pertinent data obtained in areas of possible mantle-crust-mix type of rock.

-

_ _ _ _ _ ~

Av velocity of top layer, km/sec

Depth of ayer,O km

Locality

Sarth quakc E or Blast 1

Investigator

Reference

7.4

30

Mid-Atlantic Ridge

B

Ewing and Ewing

Ewing and Ewing.

7.38 7.5

18 7?9

Western Iceland Norwegian Sea

B R

BIlth Ewing and Ewing

BIlth, 1960 Ewing and Ewing,

7.66

-

East Pacific Rise (Albatross PI ateau)

R

Raitt

Raitt, 1956

7.5*

-

East Pacific Rise

7.5 7.6 7.4 7.5 7.4

8

East Pacific Rise

13

Caribbean (Vene suela Basin) Caribbean (Aves Swell, Grenada Trough Western Caribbean

(f0.04) 7.36

(10.'22)

7.6

16

25 (minimum -

1959

1959

Scripps Inst. o Woollard, 1960a Ocean. Scripps Inst. o Menard, 1960b Ocean. Officer et al., Officer et al. 1957

Ewing et al.

Ewing el al.,

R

Antoine

Antoine, 1959

B

Lamont Geol. Obs. Raitt; Raitt et al.

Woollard, 1960a

N. Modriniak

Eiby, 1958

Matuzawa

Byerly, 1956, p.

1957

7.5

50

South of South Georgia Tonga Islands (Tofua Trough) Auckland, New Zealand Japan

7.75

-

Japan

Hodgson

Byerly, 1956, p.

7.7

11-12

D. Fahlquist

Anonymous,

7.6 t o 7.gd

9-11

Western Mediter ranean (Tyrrenian Sea) Gulf of Aden

B

Woollard,

7.6

35

The Netherland

E

Lamont and Woods Hole Gees

7.6" 7.71 f 0.02 7.6

-

7.0-7.5

11-21

B

Raitt, 1956; Raitt el al., 1955

124

147

1960a, 196oc

1960a, p. 112 Byerly, 1956, p. 121-122

308

KENNETH L. COOK

TABLE III.4ontinued Av velocity of top layer, km/sec 7.8( ?)*

3arthquake E or ilast €

Depth of ayerp km

Locality

18

Gulf of Californil (north end) Sierra Nevada, Calif.

E

Shor

Shor, 1961

E

Byerly

Investigator

Reference

7.2 t o 7.4

23-31

7.6 f (?)

36f

Great Valley, Calif.

E

Guten berg

25

California-Nevada region Eastern p ar t of Basin and Range province Nevada test site t o Kingman, Ariz. Eastern Montani

R

Press

Byerly, 1939; p. 1956, 122-123 Gutenberg 1943, Fig. 2, p. 492 Press, 1960

B

Berg et al.

Berg et al., 1960

B

Diment et al.

Western Montani Central Plateau of Mexico

Bl

Diment el al., 1961

B B

Meyer et al.

Meyer et al., 1960 Woollard, 1960h

7.66’ 7.44 t o 7.59

25

7.81

28

7.58

34-4

7.44 7.6

22-30 33

Woollard

Depth below sea level. “At 4 locations. No velocity greater than 7.5 km/sec was observed despite u n usually long profiles” (Woollard, 1960a, p. 110). Interpreted by Woollard (1960a, p. 112) as the lower layer of crust. * Reversed profiles. High velocities observed at two stations only. Layer iriterpreted by Woollard (1960a) as within the crust. Only 200-lb shots were used. Seismic arrivals were not definite or consistent (oral communication from Shor, 1961). Interpreted as “gahbroic-ultramafic rock” by Press (1960, p. 1039).

6. MID-OCEANIC RIDGESYSTEM The mid-oceanic ridge system (Fig. 3) is part of a continuous worldencircling system about 75,000km in length (Ewing, 1960, p. 173; Heezen, 1960; Menard, 1960a; 1960b; Heezen and Ewing, 1961). The ridge system comprises the Mid-Atlantic Ridge in the northern and southern parts of

FIO.3. World mid-oceanic rift system (modified after Heezen, 1960 and Heezen and Ewing, 1961. Reprinted, with modifications, from “Geology of the Arctic,” with permission of University of Toronto Press).

THE PROBLEM OF THE MANTLE-CRUST MIX

309

3 10

KENNETH L. COOK

the Atlantic Ocean, the Mid-Indian Ridge in the Indian Ocean, and the East Pacific Rise in the Pacific Ocean. In many parts of the earth, the continuity of the ridges has been confirmed by soundings; in other parts of the earth the continuity is indicated by the belts of seismicity, which generally coincide with the central rift valley which is characteristic of all the ridges sounded to date, except for the East Pacific Rise. The landward extension of these ridges also apparently coincides with the belts of seismicity where graben development, major transcurrent faulting, or island arc development are still active today. Examples of such landward extensions include the East African rift valleys, the Red Sea graben, the Palestine rift, the Central Icelandic graben, the Great Alpine fault of New Zealand, the San Andreas fault of California, and the long rift-like Lynn Canal of the Alaska panhandle (Heezen, 1960, p. 103-104). 6.1. Mid-Atlantic Ridge

The Mid-Atlantic Ridge, which is a zone about 700 miles wide (1000 to 1200 miles if the flanks are included, according to Heezen, 1960), extends the full length of the North and South Atlantic Oceans, and through the Norwegian and Greenland Seas into the Arctic Ocean in one direction, and into the Indian and Pacific Oceans in the other direction (Ewing and Ewing, 1959, p. 305,315) .3 The ridge itself has a central mountainous system formed by block faulting. In the North Atlantic, the central rift valley averages more than 6000 ft in relief and ranges from 8 to 30 miles in width for hundreds of miles. As almost all the Mid-Atlantic earthquakes occur in it, the’rift valley is undoubtedly an active fracture in the crust of the earth; and crustal movement along this fracture generates the earthquakes (Heezen, 1960, p. 100). I n the South Atlantic, the ridge has roughly the same configuration, although several “median” rifts exist (Menard, 1960a, p. 274). English and German oceanographers have found gaps in the rift in some places in the North Atlantic, and in other places detailed surveys show i t to be discontinuous (Menard, 1960b, p. 1741). Thus, instead of a single continuous rift valley, the fracture is probably part of a rift system in which individual valley grabens form an en e‘chelon pattern along the belt. The oceanographers of Lamont Geological Observatory have recently confirmed that the rift in the Mid-Atlantic Ridge joins the rift in the MidIndian Ridge just south of the tip of Africa (Anonymous, 1960b). I n the equatorial region between 2” S and 15” N latitudes, the crest of the MidAtlantic Ridge is offset along a series of east-west “fracture zones” (Heezen et al., 1961). The zones extend as much as 600 miles west of the crest of the ridge, and scarp heights exceed 3000 ft for more than 100 miles. The evi*The rugged crest of the ridge rises about 12,000 feet above the eastern and western Atlantic basins.

THE PROBLEM OF THE MANTLE-CRUST MIX

311

dence suggests that all are left-lateral strike-slip faults with displacements of 50 miles to more than 600 miles. Seismic profiles were taken by Ewing and Ewing (1959) a t various locations over the Mid-Atlantic Ridge between the latitudes of about 30” N, which is south of the Azores, and 73” N, which is about 150 miles north of Jan Mayen Island in the Greenland Sea. Over this great distance along the Mid-Atlantic Ridge, they failed to detect normal velocities associated with the normal mantle down to a depth of 12.0 km below sea level, which was the maximum depth obtainable wit,h the maximum shot-detector distance of 70 km used. The abnormally low average velocity of 7.4 km/sec (with a maximum velocity of 7.5 km/sec), which was obtained in this oceanic ridge area, was interpreted by them as “a mixture of basalt and mantle material” (Ewing and Ewing, 1959, p. 315). I n the present paper, this material is designated as mantle-crust mix. On the basis of their seismic data and on the assumptions that isostatic equilibrium obtains and that the ridge is largely compensated (based on the small free-air anomaly along the profile across the ridge), the depth of the bottom of the mantle-crust mix along a profile across the ridge south of the Azores was computed to be about 30 km below sea level (Ewing and Ewing, 1959, Fig. 3). It is noteworthy that this depth is nearly as great as the value of the average depth of the continental crust, near the coast, which is estimated by Woollard (1959, p. 1531) to be approximately 32 km. A layer of basaltic volcanic rocks, 3 to 4 km thick, overlies the mantle-crust mix along the ridge. I n the rift valley province of the Mid-Atlantic Ridge in the North Atlantic, a heat-flow measurement by Bullard (1954) indicated a value of about 7 X cal/cm2 sec, which is about 6 times the average value of 1.2 x 10-o cal/cm2 sec observed in the Lower Step and abyssal floor of the eastern Atlantic (Heezen et al., 1959, p. 103; Bullard and Day, 1961). Iceland lies along the intersection of the Mid-Atlantic Ridge and the much broader submarine rise which extends from the British Isles to Greenland. The central Icelandic graben, which contains many active gaping fissures of great dimensions (known locally as gjars) aligned parallel to the main faults of the graben, is the landward extension in Iceland of the ceneral rift of the Mid-Atlantic Ridge (Heezen, 1960, p. 98, 104). Studies by Bernauer indicate that the post-glacial rate of extension in the graben is 3.5 meters/km/1000 years and that the geology and structure has striking similarities with the East African rift valleys. Along two profiles in the western part of Iceland, B&th (1960) found a layer with a velocity of 7.38 km/sec, a top depth of about 18 km, a thickness of 10 km, and a bottom depth of 28 km. The layer was overlain by material with a velocity of 6.71 km/sec and thickness of 15.7 km, and underlain by material whose velocity could not be determined from first arrivals. The bottom boundary of the 7.38-km/sec layer was indicated by arrivals of reflected

312

KENNETH L. COOK

energy, and interpreted by B%th as the MohoroviEii: discontinuity; its depth is nearly as great as that found in coastal areas on continents. There is no layer corresponding to granite, and the 6.71- and 7.38-km/sec layers were interpreted by B%thas both probably basaltic, but of different origin and composition. The 6.7l-km/sec velocity agrees well with that of the basaltic layer in the ocean bottom found in many investigations. The 7.38km/sec velocity corresponds with that found beneath the Mid-Atlantic Ridge, and is probably caused by mantle-crust mix. Thus the crustal structure beneath Iceland is neither typically continental nor typically oceanic, nor even intermediate between the two. I n the Norwegian Sea, along the continuation of the Mid-Atlantic Ridge structure, the mantle-crust mix material showed a velocity of 7.5 km/sec, and is known to extend to a depth of a t least 15 km because of the shot-detector distance used (Ewing and Ewing, 1959, p. 311 and Fig. 7 ) . These results, together with the existence of the central rift valley and the earthquake activity along the Mid-Atlantic Ridge, indicate that the ridge has been built by the upwelling of great amounts of basaltic magma along a tensional fracture zone. The extensional forces and the supply of basalt magma are presumed to come from convection currents deep in the mantle (Ewing and Ewing, 1959, p. 291). Thus the hypothesis was offered that the ridge and rift system can be attributed to convection currents in the mantle, and occurs in the zone of upwelling separating two convection cells (Ewing, 1960, p. 173). This hypothesis, which is described later in more detail and with some modification, will constitute one of our guiding principles. 6.2. Arctic Mid-oceanic Ridge

On the basis of the belt of earthquake epicenters and sparse soundings, the median ridge system, with a central rift, is believed to extend through the vicinity of Jan Mayen Island, through the Norwegian Sea in an inverted S shape, and through the north-south clefts in Nansen’s Sill lying west and north of Spitsbergen (Heezen and Ewing, 1961, p. 629, 638). I n the Arctic Ocean north of Spitsbergen, the Arctic Mid-oceanic Ridge, which was postulated here by Heeeen and Ewing (1961) as an extension of the MidAtlantic Ridge, has apparently been confirmed by the Skate and Nautilus echograms along three profiles. This region, tentatively designated the “Region of Seamounts” by Dietz and Shumway (1961, p. 1327), is characterized by a continuous string of peaks (or ridges) of various sizes and having a maximum relief of about 1000 meters. The topographic roughness suggests volcanism, but the asymmetry of many of the peaks (or ridges) suggests a fault-block origin. The Arctic Mid-oceanic Ridge, with a suggested (on the basis of sparse soundings only) rift, is presumed to turn east a t 85”N latitude, 0” longitude,

THE PROBLEM O F THE MANTLE-CRUST MIX

313

and extend approximately parallel to the Lomonosov Ridge along the epicenter belt about midway between Severnaya Zemlya and the Lomonosov Ridge (Heezen and Ewing, 1961, p. 638, Fig. l o ) , and continue to the Siberian continental shelf in the Skado Trough area. The rift system, believed to be associated with the mid-oceanic ridge, has apparently cut into the Siberian continental shelf to form the Skado Trough and-as evidenced by the continuation of the belt of seismicity into the Russian c o n t i n e n t extends across the Laptev Shelf, and continues southward along the Verkhoyansk trough (which is bounded by normal faults for 1000 km) into the interior of Siberia, where it possibly joins the Baikal rift valley seismic belt (V. V. Beloussov, April, 1961, oral communication; Heezen and Ewing, 1961, p. 638). 6.3. Mid-Indian Ridge

The Mid-Indian Ridge has features comparable with those of the MidAtlantic Ridge. Six profiles taken over the ridge during 1960 confirm that the ridge and the median rift exist and that the rift follows the belt of seismicity in the region (Heezen, 1960, p. 108). The gravity anomalies over the ridge show large variations typical of gravity anomalies over midocean ridges (Talwani and Worzel, 1960).The results of the seismic studies over the ridge are not yet available. 6.4. East Pacific Rise

The East Pacific Rise, a vast low bulge of the sea floor, is a continuation in the Pacific Ocean of the mid-oceanic ridge system, which extends from the South Pacific, under the western part of North America, into the northeasternmost Pacific (Heezen, 1960, p. 100; Menard, 1960b, p. 1742; 1961). From Mexico to New Zealand, the rise is about 13,000 km long, 2000 to 4000 km wide, and has an average relief of 2 to 3 km. Shallow earthquakes are common along the crest of the rise, but no median rift has yet been found, despite repeated crossings of the crest, except south of New Zealand4 (Menard, 1960a, p. 274). Instead, the East Pacific Rise is cut by major fault zones intersecting the rise a t approximately right angles to the general trend. The crest of the rise is offset or changes trend in several places where it is intersected by the fault zones, and along some fault zones the whole width of the rise has been displaced vertically by several hundred meters. By observing offsets of prominent northward-striking magnetic anomalies, large strike-slip displacements have been discovered on the Murray (rightlateral displacement of about 150 km) , Pioneer (left-lateral displacement 'Along the crest of the rise, the topography is characterized in some places by troughs and ridges trending parallel to the rise (Menard, 1960b, p. 1745). Because of an expected analogy between this area and the Basin and Range province of the United States, the author predicts that some of these features may eventually be shown to be grabens and horsts.

314

KENNETH L. COOK

of about 250 km) , and Mendocino (left-lateral displacement of about 1200 km) fault zones (Mason, 1958; Vacquier, 1959; Menard, 1960b; Vacquier et al., 1961; Mason and Raff, 1961; Raff and Mason, 1961; Raff, 1961). The Murray fault zone is about 500 km south of the Pioneer fault zone, which, in turn, is about 150 km south of the Mendocino fault zone. Minor lineations indicate that the crustal blocks between the zones have been distorted uniformly. I n the Ridge and Trough province off Oregon and Washington, the crests of median elevations have been faulted into grabens and horsts trending roughly parallel to the rise (Menard, 1960b, p. 1741). Across a belt about 800 km wide which extends along the crest of the East Pacific Rise, no compressional wave velocities greater than 7.6 km/sec have apparently been observed a t depth to date (1961) a t the few stations taken, This is in good agreement with the Mid-Atlantic Ridge seismic results. During the earlier surveys over the East Pacific Rise (Albatross Plateau area) , Raitt (1956) observed velocities of 7.66 and 7.36 km/sec beneath the crestal part of the rise; he considered these velocities as too low to be characteristic of the mantle. I n the more recent work over the rise a t four locations during the International Geophysical Year (presumably very near the crest of the rise), no velocity greater than 7.5 km/sec was observed, despite unusually long profiles (Revelle, 1958; Woollard, 1960a, p. 110). Menard (1960b, Fig. 4) shows a crustal section across the East Pacific Rise in which the largest measured velocities below the third layer (ocean crust proper) under the crestal part of the rise are 7.5 and 7.6 km/sec. The intensity of heat flow through the crust of the East Pacific Rise correlates closely with topography (Bullard e t al., 1956; von Herzen, 1959). Along the crest of the rise, the heat-flow values, as obtained from about a and 8 X cal/cm2 sec in a band dozen stations, range between 2 x a few hundred kilometers wide and 10,000 km long (Menard, 1960b, p. 1742). These constitute the highest heat-flow measurements on the ocean bottom to date (1961). On the west flank of the rise, however, an area as much as 3000 km wide and 6000 km long has abnormally low heat flow, t o 0.97 X cal/cm2 sec. Another belt of with values of 0.14 X low heat flow apparently exists on the east flank of the rise, but the data currently available are too insufficient to be certain. It should be emphasized that the 800 km-wide belt along the crest of the East Pacific Rise is characteristically anomalous in terms of topography (presence of ridges and troughs parallel to the trend of the rise), seismicity, high heat flow, and abnormally low seismic velocities a t a depth where normal mantle velocities are usually found in ocean basins. I n the areas where the abnormally low velocities are obtained, Menard (1960b, p. 1741) raises the question about the correctness in calling the third layer the

THE PROBLEM OF THE MANTLE-CRUST MIX

315

“crust.” Using the analogy with the Mid-Atlantic Ridge, the author suggests that the abnormally low velocity material beneath the crestal part of the East Pacific Rise is also a mantle-crust mix. Moreover, in the belt of abnormally low velocity, the existence of the MohoroviZid discontinuity as an abrupt boundary between rocks of contrasting density and velocity is questioned ; and the depth of isostatic compensation probably extends deeper into the uppermost mantle than is generally recognized. These features will be compared later with similar features in the Basin and Range province of the United States. Beyond the central anomalous belt and to about the midlines of the two flanks of the rise-at least on the better-known western flank-the heat flow is abnormally low, the oceanic crust remains thin, and the velocities in the mantle are normal (Menard, 1960b, p. 1745). Although little is known of the topography of this region, there apparently exist here volcanoes, low domes, and troughs with adjacent tilted fault blocks trending a t various angles to the crest of the rise. On the outer half of the western flank, the abnormally low heat flow persists, but the crustal thickness and mantle velocities are normal for ocean basins. According to Menard (1960b, p. 1745; 1961), a hypothesis of a youthful convection current in the mantle, suggested by Bullard et aE. (1956) to explain high oceanic heat flow, offers a simple qualitative explanation of the above features observed over the East Pacific Rise. The details are discussed later in treating of the earth model. 7. ISLAND ARCS 7.1 Carribbean Arc Over the island arc area of the eastern Carribbean, Officer et al. (1957) obtained seismic velocities of approximately 7.4 km/sec, which they recognized as low values of velocity for the upper mantle. They listed four possibilities to explain the low velocities in this area: 1. The upper mantle is composed of a material whose chemical composition is intermediate between an olivine basalt and a peridotite. 2. It is a physical intermixture of 6.4 and 8.4 gm/sec material having an average velocity of approximately 7.4 km/sec. 3. It is a peridotite material in a different phase or crystalline state, which could result in a lower seismic velocity. 4. It is a partially serpentinized peridotitic material, which could also result in a lowered velocity. I n a somewhat later paper treating also of the eastern Caribbean, Ewing et al. (1957) postulate a process by which an oceanic crust could conceiv-

316

KENNETH L. COOK

ably be converted to a continental crust. The process would contaminate the material in the upper mantle and the crust by widespread intrusion of a low-velocity primary differentiate, which migrated upward from deep in the mantle. The resulting material, with the abnormally low velocity of 7.4 km/sec, was designated by them as “subcrust,” inasmuch as the velocity was lower than that of normal mantle. The later seismic work in the eastern Caribbean by Officer et al. (1959, p. 106) and in the western Caribbean by Ewing et al. (1960) failed to obtain seismic velocities greater than 7.6 km/sec over either the island arc of the Greater Antilles and Lesser Antilles or the ridges and elevated regions adjacent to the island arc (Antoine, 1959, p. 73). I n the elevated regions, such as the Aves Swell, Beata Ridge, Cayman Ridge, and Nicaraguan Rise, the depth to mantle by extrapolation is of the order of 20 to 25 km (Officer et al., 1959, p. 106; Ewing, 1959, p. 1719; Ewing et al., 1960, Fig. 2 ) . Beneath the Nicaraguan Rise, the rocks with a seismic velocity of 7.6 km/sec were shown definitely to extend to a minimum depth of 20 km below sea level, and a minimum depth of 25 km or more is possible (Antoine, 1959, p. 74-75). Moreover, the crustal section beneath the oceanic ridge area is more complex than that in the oceanic basin areas, both structurally and in terms of the number of layers present. Along the island arc the depth to which the abnormally low velocities of 7.4 and 7.5 km/sec extend is probably near that found under the continental coastal plains (Officer et al., 1959, p. 106-107). Normal mantle velocities were obtained beneath the troughs and basins. After studying their additional seismic data taken in the eastern Caribbean area, Officer et a1 (1959, p. 107-108) believed that the measured velocity slightly lower than normal mantle velocity suggests that contamination, differentiation, or some other process has altered the normal mantle. In particular, they gave the following alternative explanations for the unusually low velocities: 1. The MohoroviFi6 discontinuity is assumed to exist a t a level about which the pressure and temperature effects produce a change in state of the material whose chemical composition is about the same above and below this level. Under these circumstances, in an active region like the Caribbean, a relatively thick section of this intermediate composition material might have been created. 2. Another possible process is the partial serpentinixation of peridotite, as suggested by Hess (1954). 3. However, neither of these processes by itself would explain the observed features. 4. Rather, the entire Caribbean area was probably extensively intruded by a differentiate of lighter material migrating up from deep in the mantle.

THE PROBLEM OF THE MANTLE-CRUST MIX

317

Officer et al. (1959) do not explain why a mantle-crust mixture by either processes 1 or 2 above fails to explain the observed features. It is noteworthy t o our discussion here, however, that their final hypothesis-which is the one most favored by them-admits to the possibility of inhomogeneity in the mantle, inasmuch as the existence of a lighter material differentiate would constitute such an inhomogeneity. To summarize, the Caribbean as a whole is intermediate between continental and oceanic structure, although some parts are very nearly either continental or oceanic (Ewing, 1959, p. 1719). The low seismic velocities of 7.4 and 7.5 km/sec in this area beneath the Greater and Lesser Antilles island arc, as well as beneath the Aves Swell and the Nicaraguan Rise, are conceivably caused by a mantle-crust mix. The region is still seismically and tectonically active, and transformation of material between crust-like rock and mantle-type rock is apparently now in progress.

7.2 Southern Antilles Arc The Southern Antilles island arc, also designated as the Scotian arc system, which connects South America with Antarctica, includes the South Shetland, South Orkney, and South Sandwich Islands, and South Georgia. The arc is an active structure of Pacific type analogous to that of the Caribbean loop, and the seismicity of this region is somcwliat higher than that of the Caribbean (Gutenberg and Richter, 1954, p. 42; Ewing and Heezen, 1956). The arc system enters Antarctica through the Palmer peninsula and extends inland as a narrow ridge separating the Ross Sea and Wedell Sea embayments to the vicinity of the Horlick Mountains (Woollard, 1960c, p. 476). I n Antarctica, the ridge is not continuous and is broken by several narrow valleys. Seismic measurements made by the Lamont Geological Observatory in the South Georgia area incident to the International Geophysical Year program included a traverse extending from about latitudes 48" S to 57" S and crossing the island arc. The limited measurements apparently indicate a crustal structural pattern comparable with that of the Caribbean. In particular, in the area south of South Georgia, the crust was found to have a layered structure. The deepest measured layer, which had a seismic velocity of 7.6 km/sec, is interpreted by Woollard (1960a, p. 112) as a basal crustal layer that is similar to that observed in the Caribbean. Though data in the South Georgia area are still sparse, the author suggests that this 7.6 km/sec velocity material is conceivably a mantle-crust mix similar to that found in the Caribbean, and its origin may be of a similar nature. It should be noted that the western end of the Southern Antilles island arc apparently forms a continuous belt with the oceanic earthquake belt and ridge which, as shown by Heeeen (1960, p. 102), extends as a branch

318

KENNETH L. COOK

off the East Pacific Rise from the vicinity of Easter Island to southern Chile. 7.3. Tonga-New Zealand Arc

Along the north-northeastward-trending Tonga-New Zealand island arc within the Tofua Trough, a seismic velocity of 7.6 km/sec was observed a t an estimated depth of 11.8 km a t the only seismic observing point in the trough (Raitt et al., 1955, p. 249-250; Raitt, 1956). The layer probably extends to a depth of a t least 18 km, which was the maximum depth obtainable with the shot-detector distance used. The Tofua Trough is a basin with a general depth of 1300 to 1700 meters and width of 25 to 35 km which separates the active volcanic chain of islands on the west from the main Tonga Ridge-which includes the Tonga Islands-on the east. The Tonga Trench, which is a slightly arcuate furrow convex to the east, lies immediately east of, and parallel to, the Tonga Ridge. This abnormally low seismic velocity indicates the possibility of a mantle-crust mix in this area beneath the Tonga Ridge, as well as beneath the Tofua Trough. I n the area north of Auckland, on North Island, New Zealand, three layers with velocities of 7.0,7.2, and 7.5 km/sec were found a t depths of about 11, 17, and 21 km, respectively, along a reflection seismic profile taken by Modriniak; the depth of the MohoroviEi6 discontinuity was about 23 km (Eiby, 1958, p. 657). I n the Wellington area, also on North Island about 350 miles south of Auckland, however, a refraction seismic profile by Eiby (1958, p. 657) revealed no velocity exceeding 6.2 km/sec for any layer overlying the MorohoviEi6 discontinuity, which lies a t a depth of about 18 km. As these profiles are situated on opposite sides of the subcrustal rift, a difference in crustal structure on opposite sides of the rift is indicated. The Alpine right-lateral strike-slip fault, on South Island, has essentially the same trend as the subcrustal rift on North Island, and is apparently continuous with it. On the basis of the distribution of the depth of foci of earthquakes, Eiby interprets the rift as a great wedge-shaped feature in which the following discrete layers are recognized: a crust to a depth of about 30 km (which corresponds to the thickness of continental crust near coastal areas), a “transition zone” between depths of about 30 to 100 km, and a subcrust between depths of about 100 to 370 km (the limiting depth taken normally for seismic activity in New Zealand). It should be emphasized that the New Zealand region is bounded on the east and west by oceanic crusts.

7.4. Japanese Arc Along the Japanese island arc, the Japanese seismologists, using seismograms from earthquakes, have for many years obtained seismic velocities

THE PROBLEM OF THE MANTLE-CRUST MIX

319

as low as 7.5 to 7.75 km/sec at depths as great as 50 km (Byerly, 1956, pp. 124, 138, 147). Explosion-seismic observations during the early 1950's resulted in the recording of a layer with a velocity of 7.4 km/sec a t a top depth of about 27 km and a thickness of about 5 km (Byerly, 1956, p. 138). It is suggested that these abnormally low velocities are possibly caused by a mantle-crust mix beneath Japan. [Added in proof. Recent heat flow measurements through the sea bottom cal/cm2 sec) show that the average heat flow is low (less than 1.0 x on the Pacific Ocean side of the Japanese islands and high (more than 2.0 X cal/cm2 sec) on the Japan Sea side, except where the MarianaBonin arc meets the main Japanese arc (Uyeda et al., 1962). This tendency is more pronounced in the northern part of Honshu. Measurements a t three localities on a profile taken along the 38"N parallel of latitude across the cal/cm2 sec in the bottom Japan trench resulted in a value of 1.14 X of the trench. Surprisingly the lowest heat flow value, 0.273 X l o t 6 cal/ om2 sec, did not occur a t the bottom of the trench but 100 miles west of the bottom of the trench approximately midway between the trench and Honshu, that is, east of the Japanese arc]. 7.6. Western Part of Mediterranean Sea

The western part of the Alpide belt is considered to extend along the active belt of seismicity from the Azores to northern Africa, to continue along an active arc across Sicily and along the Apennines, and-by way of the southern Alps-to pass through the Dinaric Mountains east of the Adriatic Sea and to continue to the Balkans and on eastward into Asia (Gutenberg and Richter, 1954, p. 70). Italy has more earthquakes than any other country except Japan. I n the Tyrrenian Sea, Fahlquist obtained velocities of 6.6 km/sec a t a depth of 8 km and 7.7 km/sec a t a depth of 11 to 12 km (Anonymous, 1960a; 1 9 6 0 ~ ) The . latter velocity suggests the possible existence of a mantle-crust mix. 7.6. Other Island Arcs

Little or no refraction seismic work with portable seismic equipment has apparently been done in the other island arc regions of the earth to ascertain whether compressional velocities of 7.4 to 7.7 km/sec, indicative of a mantle-crust mix, exist. On the basis of an analogy of the Caribbean area to that of the other island arc areas, where active belts of seismicity and volcanism exist, i t may be supposed that such velocities a t depth may be found in some of these areas eventually. Because not all island arcs necessarily have the same mode of formation, however, they may not all be underlain by a mantle-crust mix.

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KENNETH L. COOK

8. CONTINENTS 8.1. Continental Rift Areas 8.1.1. Gulf of Aden. The median rift of the Carlsberg Ridge, which is the designation for the northern part of the Mid-Indian Ridge, extends through the Gulf of Aden, whose coastal escarpments are formed by rift faults, and thcn bifurcates. One branch continues southwestward on land through the Ethiopian rift valleys to join the East African rift system, and the other branch continues northward along the Red Sca to join the Palestine rift (Jacobs et at., 1959, p. 287; Heezen, 1960, p. 102,104; Heezen and Ewing, 1961, p. 627). Seismic measurements in the Gulf of Aden area incident to the International Geophysical Year program have resulted in seismic velocities of 7.6 to 7.8 km/sec a t only two stations. The depth to this highvelocity material is computed to be greater than 9 to 11 km from the seismic work and cannot be reconciled with the crustal thickness to be expected from both the shallowness of the water in this area and with thc results of gravity surveys, which suggest that thc basc of the crust hcrc should be about 17 km below sea level (Woollard, 1960a, p. 113).Woollard (1960a, p. 113) interprets this high-velocity material as material within the crustal layer; yet he points out that such velocities have been identified with the mantle rock beneath thc crust. Again, using the analogy with the apparently similar structural condition found in the Mid-Atlantic Ridge, it is suggested that these velocities may be caused by a mantle-crust mix. 8.1.2. Red Sea. The Red Sea depression is interpreted as a gigantic graben which extends northward to join the Palestine rift, in which the Dead Sea and the Jordan Valley lie (Gregory, 1921; Lill and Revelle, 1958, p. 1013; Heezen, 1960, p. 104). The Red Sea graben extends southward to joint the Ethiopian rift system. The shot-detector distance of seismic measurements throughout the length of the Red Sea incident to the International Geophysical Year program were apparently insufficient to give the total thickness of the crust in this region. The maximum vclocities measured, which were 7.1 km/sec, are interpreted to be caused by a mass of basic rock that found egress along a crustal fracture systcm in this portion of the Red Sea graben (Woollard, 1960a, p. 113).It would be of interest to take longer shot-detector distances in the Red Sea graben to ascertain whether velocities of 7.5 to 7.6 km/sec occur. It would seem reasonable to expect such velocities a t greater depth because of the possibility of a mantle-crust mix in this rcgion. The possibility that the Red Sea depression is a paar, which is caused by the moving apart of two crustal blocks, and thus is essentially a tensional feature (Swartz and Arden, 1960), makes this area a key test area for such crustal studies. 8.1 .S.East African Rift System. The main rift extending southwestward

THE PROBLEM OF THE MANTLE-CRUST MIX

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through the Ethiopian rift valleys from the Gulf of Aden, continues southwestward into the extensive plateau region of East Africa, where it bifurcates to form, respectively, the east and west rift valley systems. Each of the two main valley systems comprises a series of great elongate grabens with an en e‘chelon pattern, rather than a single grand valley. Both the east and west rift valley systems are active seismically, and active volcanoes occur along or adjacent to the rifts (Willis, 1936; Sutton and Berg, 1958; De Bramaecker, 1959) . Tertiary volcanics are widespread in the rift areas, and the age of the faulting along the rifts is “not later than Oligocene and continuing until quite recent times” (Gregory, 1921, p. 31). Great Bouguer gravity minima of several scores of milligals occur across the rift valleys (Bullard, 1936). Recent investigations indicate that the East African rifts were probably caused by extension, as postulated originally by Gregory (1921), and supported by Vening Meinesz (in Heiskanen and Vening Meinesz, 1958) and, more recently, by Bullard, who has abandoned a compressional hypothesis favored by him in 1936 (Heezen and Ewing, 1961, p. 627). Theories of compression (Wayland, 1930; Willis, 1936) and strike-slip fault movement (McConnell, 1951) have been given to explain the rift valleys. No seismic velocity data with portable seismic equipment are apparently available yet (1961) for the East African rift area. The similarity of the structural setting and age of these rift valleys and the rift valleys of the eastern part of the Basin and Range province of the United States, however, indicates that velocities characteristic of a mantle-crust mix may eventually be found beneath the East and West African rifts and the plateau region between them. Recent evidence for an intermediate layer with a velocity of 7.19 kin/ sec has been found by Hales and Sacks (1959) from crustal structure studies in the plateau area of the Eastern Transvaal, Union of South Africa, a t an average elevation of about 1500 meters. The layer has a top depth of 28.2 km, a thickness of 8.4 km, a bottom depth of 36.6 km, with a velocity of 7.96 km/sec beneath the MohoroviEi6 discontinuity. The 7.19-km/sec velocity, though somewhat lower than that regarded as typical mantlecrust mix, suggests the possibility that velocities characteristic of the mix will be found in going northeast of the Transvaal toward the East African rift system, which lies about 800 miles north-northeast of the Transvaal a t the south end of Lake Nyasa. 8.9. Western Part of North America

In this review, following Heezen (1960, p. 105) and Menard (1960b, p. 1742) in part, the northward landward extension of the East Paciflc Rise is interpreted to comprise a zone about 700 to 800 miles in width (from the

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west coast of California on eastward) which includes Baja California; the central plateau of Mexico, the Basin and Range province; the Colorado plateau; the great lava and volcanic plateaus of Washington, Oregon, Idaho, northwestern Wyoming, and northern California; and the area of trenches or rifts in British Columbia and southeastern Alaska. 8.6.1.Rift System. The rift system along the crestal part of the landward extension of the East Pacific Rise, though not yet clearly established, probably consists of several “median” rifts, or subsidiary rift systems. This multiplicity of rifts is comparable to (1) the situation in the Mid-Atlantic Ridge in the South Atlantic, where two or more separate rifts may exist; (2) the branching of the main rift of the Mid-Indian rift in the Gulf of Aden area; and (3) the bifurcation of the branch of the rift that extends into East Africa. The principal median rift (or rift system) of the landward extension of the East Pacific Rise probably extends along the Gulf of California, the Sulton trough, the Great Valley of California, the Ridge and Trough province off Oregon and Washington, and (following Heezen, 1960) the rift-like Lynn Canal of the Alaska panhandle. The rift is discontinuous in places, and is complicated by the crossing of it by the great San Andreas and Garloek strike-slip faults, and perhaps another major fracture zone in the Oregon-Washington region. A major active belt of seismicity extends throughout the length of the rift, and large grabens and areas of high heat flow occur along the rift. The Gulf of California is probably a partially submerged rift valley bounded by normal faults (see D e Cserna, 1961) ; and a belt of seismicity and a band of high heat flow extend along i t (Menard, 1960b, Fig. 5 , 1961). The northern end of the Gulf is a graben (Beal, 1948, p. 1)which probably extends northward continuously onto land to include the Salton trough as part of the same graben. This graben is probably terminated on the north by the southeastward extension of the San Andreas fault zone. Hamilton (1961) has emphasized the change in tectonic style a t the latitude corresponding with the north ends of the Gulf of California-Salton basin trough and the Baja California-Peninsular Ranges highlands, and with the westward deflection of the San Andreas fault system. He postulates that the Gulf was formed by oblique rifting across the San Andreas system, and that Baja California may initially have lain against the mainland 300 miles to the southeast. Beneath the north end of the Gulf of California, Shor (1961) obtained a velocity of 6.7 km/sec for the typical basal tic^' oceanic-type layer at a depth of 10 km below sea level. This layer is overlain by a 5.7-krn/sec layer and underlain, a t a depth of about 18 km below sea level, by a layer with a velocity of 7.8(?) km/sec. The velocity of this bottom layer is un-

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certain because the seismic arrivals obtained with 200 lb charges were not definite or consistent. If future work corroborates this velocity, the material is possibly a mantle-crust mix which may extend continuously between the Basin and Range province and the East Pacific Rise. The structural relations in Baj a California and the adjacent continental borderland are complicated by the Aqua-Blanca fault, an active right-lateral strikeslip fault trending east-southeastward and located south of Ensenada, Baja California, Mexico (Allen et al., 1960; Krause, 1961) ; and additional data are desirable to ascertain whether this major transverse structure disrupts the continuity of the mantle-crust mix postulated along the crestal part of the landward extension of the East Pacific Rise. The Great Valley of California, which is about 450 miles long and 60 miles in maximum width, is apparently a special type of graben or rift valley. According to Lawson (1936, p. 1698), “the Great Valley has the structural features of a geosynclinal trough bounded on either side by a fault zone, the trace of which is, for the most part, obscured by later contributions to the sediments of the valley fill.” The valley has undergone continuous sinking since mid-Tertiary time. The mean thickness of the valley fill, of Cenozoic age, has been estimated from wells and seismic and gravity data to be 4 km in a section through Mt. Whitney transverse to the trend of the Sierra Nevada (Lawson, 1936, p. 1699). Locally within the valley, as for example in the area 21 miles south of Merced, the bottom of the rocks of Cenozoic age lie about 8200 ft below sea level. The evidence suggests the hypothesis that the Great Valley is a graben, comparable in size to the East African rift valleys, and that it is probably caused by tension. It should be noted, however, that Lawson (1939) explains this structural valley, not as a graben, but as a “subsidence” that has resulted from “a low angle thrust cutting through the sial into the dunite of the sima,” so that the region is uplifted by thickening of the crust; the subsidence followed the uplift to provide final isostatic balance. This structural problem is difficult, especially because i t is not known to what extent the lateral movement along the San Andreas fault zone, which lies along or near parts of the western margin of the Great Valley, may have complicated the relationships. The Sierra Nevada, which extend along the east margin of the Great Valley, are recognized by geologists as a great tilted fault block whose eastern front is marked by great multiple fault scarps (Lawson, 1936), but whose western margin is apparently covered with the valley fill in the Great Valley (Hoots et al., 1954). Axelrod (1957) postulates that the Sierra Nevada were uplifted principally during early Pleistocene time ; in one area the uplift was 3500 ft along faults, and an additional uplift of 1800 to 3000 ft was produced by warping. It seems reasonable therefore to suppose that the Great Valley, being presumably a structural block ad-

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KENNETH L. COOK

jacent to the Sierra Nevada, was caused by large-scale faulting as well as downwarping; but no faults of late Pliocene or early Pleistocene age with great vertical displacements have apparently been found along the east and west margins of the valley. The great “Foothills fault system,” which is possibly a strike-slip fault system with 10 to 100 miles or more of horizontal movement, extends for about 200 miles (with a width of up to 30 niiles) along the west foothills of the Sierra Nevada; but this faulting is apparently older than the date of the Sierra Nevada uplift (Clark, 1960, p. 483,493). Thus, additional data are needed to resolve the problem of origin of the Great Valley. The area of high heat flow in the Ridge and Trough province (in which the ridges and troughs are northward-trending) off Oregon and Washington (Menard, 1960b), as well as the inferred high heat flow along the line of Tertiary and Quaternary volcanoes along the Cascade Range in northern California, western Oregon, and west,ern Washington, indicate that the crestal part of the landward extension of the East Pacific Rise passes through this general region. The continuation of the belt of seismicity through Cape Mendocino into the Pacific and west of Vancouver (Gutenberg and Richter, 1954, p. 35) indicates that the San Andreas fault zonc, and hence the rift system, probably extends along this belt. The WillamettePuget Sound depression, which also lies in a belt of seismicity (Woollard, 1958, Fig. 5 ) , is possibly one of the grabens that lie within the rift system in the crestal part of the feature. A second rift (or rift system), which can be considered a minor branch of the main rift in California, apparently extends along the belt of grabens lying immediately east of the Sierra Nevada, which constitute the west boundary of the Basin and Range province. Some of these grabens have been delineated by gravity and seismic surveys (Pakiser and Kane, 1956; Pakiser et al., 1960; Pakiser, 1960). The rift follows an active belt of seismicity (Woollard, 1958, Fig. 5 ) . A third rift (or rift system), which is considered a major branch of the main rift through the Gulf of California, apparently begins in the vicinity of the northeastern tip of the Gulf of California and extends along the active belt of seismicity (Woollard, 1958, Fig. 5) through northern Mexico, western Arizona, central Utah, southeastern Idaho, western Wyoming. western Montana, and British Columbia (Fig. 3 ) . The rift is not continnous, but comprises a zone of rift valleys, which are en e‘chelon. The system of great trenches between Arizona and British Columbia (Eardley, 1951, p. 294) are included in this rift system. On the basis of regional gravity surveys, some of these trenches have already been proved to be rift valleys, as for example the newly discovered rift valleys in north central Utah (Cook and Berg, 1956; 1958; 1961), which are comparable structurally to

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the rift valleys of Africa and the median rifts of the Atlantic and Indian Oceans. The gravity data indicate that the Wasatch structural trough, in north central Utah, is a major rift extending about 160 miles in length, with a width up to about 16 miles in places, along the western side of the Wasatch Range (Cook and Berg, 1961; Cook et al., manuscript in preparation). Many of the other trenches, such as the Rocky Mountain trench (1000 miles long) and Purcell trench (200 miles long) in British Columbia, are probably rifts; the structure of these two trenches is compared by Daly (1912, p. 25-27; 600) to that of the middle Rhine and Dead Sea grabens.6 Recent gravity surveys over the Rocky Mountain trench for a distance of about 60 miles along its trend in southeastern British Columbia revealed three separate gravity lows which were interpreted as “downfaulted blocks along longitudinal and transverse faults within the trench,” which were evidently caused by Cenozoic block faulting (Garland et al., 1961, p. 2504). To summarize, the rift systems just described are interpreted to lie within the crestal part of the landward projection of the East Pacific Rise. Between the two outer rift systems lie, on the south, the Basin and Rangc province and, on the north, the great lava and volcanic plateaus of Washington, Oregon, Idaho, northwestern Wyoming, and northern California. Most of the Columbia plateau lavas are middle Miocene in age and in several places are in excess of 4000 ft thick and may be more than 5000 ft thick (Eardley, 1951, p. 451). The seismic data, though still sparse, demonstrate that seismic velocities characteristic of a mantle-crust mix exist throughout the crestal part of the landward extension of the East Pacific Rise. To date (1961) the available data are confined principally to the southern part of this region, namely, in the Basin and Range province, western and southern margins of the Colorado plateau, Montana, and the central plateau of Mexico. The seismic results in these areas will now be given. 8.2.2.Basin and Range Province. The Basin and Range province consists of grabens, horsts, and tilted fault blocks which trend generally northward in a direction parallel with the trend of the rift systems along the east and west borders of the province. The structural features of the Basin and Range province resemble in many respects those of the submerged blockfaulted rift mountain province of the Mid-Atlantic Ridge, as described by Heezen et al. (1959, Fig. 43). Throughout the Basin and Range province, (Added in proof. H. E. Landsberg called the author’s attention to his results in the active seismic and graben zone of the Rhine Valley, where lie found a very prominent wave train with a velocity of 7.56 km/sw (Landsberg, 1931, p. 251). He found also that beneath the mountain system west of the Rhine, this wave train was more prominent than the P. .)

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block-faulting, with concomitant uplift of the mountains and distension of the earth’s crust, has probably been in progress from lower Oligocene time (about 36 million years ago) to the present. Topographically expressed faults, however, probably date back only to late Pliocene (less than about 13 million years ago) or early Pleistocene time, though there may have been still earlier movements along such faults (Nolan, 1943, p. 183). On the basis of estimates of deformation of Miocene-Pliocene and younger rocks during the last 15 million years, Thompson (1959) postulated that the rate of distension in the Basin and Range province is about one foot per century. Recent studies of leveling data across new fault scarps and of the shorelines of the ancient Lake Bonneville indicate that some of the mountain blocks are continuing to be uplifted today. Perhaps the most surprising results of crustal studies on continents in recent years is the fact that a thick layer of material with a seismic velocity of 7.4 to 7.7 km/sec (7.8 km/sec in the Kingman, Arizona profile only) lies a t a depth of about 25 km throughout the southwestern part of the United States. The layer is indicated by refraction seismic surveys of various investigators. The material in the layer is interpreted in this review as a mantle-crust mix. Figure 4c shows a generalized, interpretative cross section across the western part of the United States; on the west the section crosses the south central part of the Great Valley of California. In the eastern part (Utah and eastern Nevada) of the Basin and Range province, the layer has a velocity of 7.44 to 7.59 km/sec (average of about 7.5 km/sec) , a thickness of 47 km, a top depth of 25 km, and a bottom depth of 72 kni (Berg e t al., 1960). In the southwestern part (California and Nevada) of the Basin and Range province, the layer has a velocity of 7.66 km/sec, a thickness of 25 km, a top depth of 25 km, and a bottom depth of 50 km (Press, 1960; Thompson and Talwani, 1959). An alternate interpretation given by Press (1960, p. 1045), but considered by him as less probable, is that the layer in California-Nevada has a velocity of 7.77 km/sec, a thickness of greater than 65 km, and a bottom depth of greater than 90 km, the top depth remaining the same. In the southern part of the Basin and Range province along a profile between the Nevada test site and Kingman, Arizona the layer has a velocity of 7.81 km/sec (Diment et al., 1961) a t a depth of about 28 km. These investigators did not obtain data to indicate the depth of the bottom layer in this region; but using their data in conjunction with that of Press (1960) for California and Nevada, they computed a layer thickness of 28 km, with a bottom depth of 53 km. Recent experimental seismic studies by the University of Utah in the eastern part of the Basin and Range province (Nevada-Utah) have demonstrated that the top boundary of the 7.5-km/sec layer is capable of not only reflecting seismic energy (Narans et al., 1961; Berg et al., 1961) but also probably convert-

PLAN OF ARCHED BUT UNDISPLACED BLOCK M AN

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FIG.4. (a) and ( b ): Diagrammatic representation of the convection-current hypothesis for the origin of various features associated with the western part of the United States (largely after Menard, 1960b, as applied to East Pacific Rise). (c) Layering of the earth along generalized, interpretive cross section across western part of United States; data along western part of section are projected long distances into the section. Data from I, Talwani et aZ. (1959c), projected south to profile; 11, Shor and Raitt (19581, projected north; 111, Gutenberg (1943; 1951a; 1951b; 1952; Press, 1956a); IV, Press (1960), projected north; V, Berg et al. (1960); and VI, Woollard (this review, reevaluation of Tatel and Tuve data, 1955). Diagram (a) and convection-cell part of ( b ) reprinted, with modifications, from Science by permission. 327

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KENNETH L. COOK

ing energy from compressional P waves to PS converted waves (Schwind et al., 1960). These studies have also confirmed that the top of the layer lies a t a depth of approximately 25 km. The depth of the MohoroviFi6 discontinuity of 72 km obtained by Berg et al. (1960) with refraction techniques in the eastern part of the Basin and Range province greatly exceeds the depth of 48 km obtained by Ewing and Press (1959) from Rayleigh phase velocity studies. Quite apart from this discrepancy, the present data indicate that the layer of possible mantle-crust mix has a great thickness, not less than about 25 km and possibly as much as 47 km, over a large area of the crestal part of the landward extension of the East Pacific Rise. These results must be considered in any hypothesis of the origin of the Basin and Range province. Although no heat-flow measurements have apparently been made yet (1961) in the Basin and Range province, the evidence of widespread volcanism in the region since lower Oligocene time up to Recent time indicates that the present rate of heat flow in this region is probably much cal/cm2 sec measured over greater than the average value of 1.2 x continents and a t the bottom of the oceans. The high heat flow can also bc inferred because of the analogy of this rcgion with the fault-block system of the Mid-Atlantic Ridge, where abnormally high heat flow has been measured and because this region lies on the landward extension of the crestal part of the East Pacific Rise. I n an earlier paper, the seismic velocities of about 7.5 km/sec were interpreted as intimately associated with the mantle, and it was suggested that the material with this velocity is different from gabbro-eclogite as postulated by Kennedy (1959) or that an intermediate phase exists between the gabbro and eclogite phases (Berg et al., 1960, p. 532). It was suggested further that this part of the Basin and Range province has undergone expansion by heat generated by an excessive accumulation of radioactive materials in the upper mantle and crust, or from heat of a different source; that the heat could cause a phase change in the rocks of the upper mantle; and that the volume increase in the rocks that accompanied the phase change could explain the distension of one foot per century in the Basin and Range province, as postulated by Thompson (1959). I n the presence of this inferred high heat flow, mantle rock has probably changed progressively into crustal-type rock in the mantle-crust-mix zone, with both vertical and lateral expansion (see Thompson, 1960) ; the process is probably still continuing today. 8.63.Colorado Plateau. According to Mackin (1959) , regional uplift in the Colorado plateau region, which now amounts to 5000 to 7000 ft, began early in the Cenozoic era and may continue today. He concludes that this amount of uplift, because i t cannot be accounted for by thermal expansion alone, is probably caused in part by a change in state a t depth.

THE PROBLEM O F T H E MANTLE-CRUST MIX

329

The earlier work of Tuve and Tatel (1954) and Tatel and Tuve (1955) indicated that the approximately 5.5- to 6-km/sec layer reaches depths of 29 km along their unreversed profile between Bingham Canyon, Utah and the area “in and beyond the Uinta and Wasatch Mountains,”e and 28 to 34 km in the Arizona-New Mexico part of the Colorado plateau along their “south” and “north” profiles, respectively, a t elevations of 4500 to 7000 ft, respectively. Although they interpreted material with normal mantle vclocities directly to underlie this layer, a reevaluation of their travel-time plot by Woollard during 1959 resulted in the following alternative interpretation of the crustal structure in Utah below the surface layer, whose velocity could not be evaluated from the data obtained (G. P. Woollard, written communication, Sept. 28, 1960) : Velocity, km/sec V1 = 5 . 2 Vz = 5 . 8 V3 = 6 . 3 = 7.3-7.4 Vs = 8 . 2 Total

v,

Thickness of layer, km 5.9 11.7 21.3 8.8

-

47.7

The general agreement of the velocities given in this re-evaluation by Woollard and those of Berg et al. (1960) for the eastern part of the Basin and Range province is excellent. The differences in depths and thicknesses of the layers are significant and far exceed any small differences related to the thin (less than 1 km, except beneath grabens) surface layer. A difference in crustal structure is therefore indicated in the region underlying the western part of the Colorado plateau as compared with that beneath the eastern part of the Basin and Range province. I n Fig. 4c, the re-evaluated data of Woollard and the data obtained by Berg et al. (1960) have been provisionally combined to show (1) an eastward thinning of the mantle-crust-mix layer beneath the western part of the Colorado plateau, in comparison with its thickness beneath the eastern part of the Basin and Range province; (2) a shallower depth of the MohoroviFid discontinuity beneath the western part of the Colorado plateau than beneath the eastern part of the Basin and Range province; and (3) a gentle eastward dip of the top of the mantle-crust-mix in the region between Salt Lake City and the central part of the Colorado plateau, but not as great a dip as that indicated by the re-evaluated data of Woollard for this region. The first ‘Although lying immediately north of the Colorado plateau proper, this upland area is provisionally included in the Colorado plateau for convenience in this review because the results of Tatel and Tuve (1955, p. 48) here were comparable with those obtained in the Colorado plateau proper in Arizona-New Mexico.

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KENNETH L. COOK

seismic arrivals at Fruitland and Neola, Utah, indicated that any downwarp of the top of the 7.5-km/sec layer beneath the Wasatch and Uinta Mountains does not exceed a few kilometers (Berg et al., 1960, p. 528). Additional data are needed to resolve the discrepancies of crustal structure in this region. The reinterpretation by Woollard of the Tatel and Tuve (1955) data over the southern part of the Colorado plateau in Arizona-New Mexico, a t an elevation of about 2000 meters, indicated a layer with a velocity of 7.36 km/sec, a top depth of 26 km, a thickness of 22 km, a bottom depth of 48 km, and a velocity of 8.15 km/sec beneath the MohoroviEiE discontinuity (Steinhart and Woollard, 1961, Table 10.1, p. 347). It should be noted that the 7.36-km/sec velocity is consistent with that obtained for the possible mantle-crust mix beneath the Basin and Range province. 8.2.4. Montana. Recent explosion studies of continental structure in Montana in a joint effort by investigators from the University of Wisconsin, the Carnegie Institution of Washington, and Princeton University (Steinhart and Meyer, 1961) have established a layer with seismic velocities characteristic of a mantle-crust mix. These data were obtained over a portion of the crestal part of the landward extension of the East Pacific Rise. Their results are considered preliminary, as the compilation of all the data is not yet (1961) complete. Although several other profiles were taken, the two principal reversed profiles were approximately northward-trending profiles. One, in eastern Montana, extended between Acme Pond and Fort Beck, and the other, in western Montana, extended between Sailor Lake and Cliff Lake (Meyer et al., 1961). The western profile lies partly along the southern end of the Rocky Mountain trench as mapped by Daly (1912, pl. 3, Fig. 24). I n eastern Montana, with a n elevation of 900 to 950 meters along the profile, the 7.58-km/sec layer has a top depth of 34 to 40 km, a thickness of 10 to 23 km, and a bottom depth of 50 to 57 km; a velocity of 8.07 km/sec was found beneath the MohoroviEiE discontinuity. I n western Montana, with an elevation of 1500 to 1850 meters, the 7.44-km/sec layer has a top depth of 22 to 30 km, a thickness of 5 to 24 km (increasing rapidly in thickness from north to south), and a bottom depth of 35 to 46 km; a velocity of 7.94 km/sec was found beneath the MohoroviFiE discontinuity. I n eastern Montana only, a second intermediate layer with a velocity of 6.97 km/sec, and a rather uniform thickness of about 17 km, was found above the 7.58-km/sec layer. The fact that the MohoroviEi6 discontinuity is shallower a t the higher elevations than a t the lower elevations is contrary to expectations. It should be noted, however, that the velocities of corresponding layers beneath the higher elevations tend to be somewhat lower in the mountains than in eastern Montana; moreover, the velocity of the ma-

THE PROBLEM OF THE MANTLE-CRUST MIX

33 1

terial immediately beneath the MohoroviFid discontinuity is about 0.13 km/sec less in the mountains than in eastern Montana, and indicates that some of the isostatic compensation probably takes place within the mantle. It is considered significant that, in southwestern Montana, (1) the velocity of the inferred mantle-crust-mix layer is in excellent agreement with that obtained by Berg et al. (1960) in the eastern part of the Basin and Range province and (2) the thickness of this layer is increasing rapidly in a southward direction toward the Basin and Range province. 8.2.5. Central Plateau of Mexico. Over the central plateau of Mexico, refraction seismic studies incident to the International Geophysical Year indicated a layer with a seismic velocity of 7.6 km/sec, a thickness of about 11 km, a top depth of about 33 km, and a bottom depth of about 44 km, below which a mantle velocity of 8.2 km/sec7 was found (Woollard, 1960b, p. 354). The data were taken a t an average elevation of about 2200 meters along a northwestward-trending unreversed profile parallel to, and just east of, the Sierra Madre Occidental. The Sierras are believed to be faultblock mountains which were uplifted a t the same time (late Tertiary or early Pleistocene) as the Gulf of California was downfaulted and which are of the same fault-block system (Eardley, 1951, p. 473). The material of 7.6 km/sec velocity, which is possibly a mantle-crust mix, is possibly continuous with the material of corresponding velocity found beneath the Basin and Range province and western Montana. 8.3. Summary of Continents

Beneath continents, seismic velocities of 7.4 to 7.7 km/sec occur along active belts of seismicity and volcanism which constitute the landward extensions of the mid-oceanic ridge system. The structural characteristics of these tectonically active continental belts are similar to those of the midoceanic ridge system, and include rift systems, fault-block mountain systems, and uplifted plateaus. High heat flow along these belts can be inferred from the recent volcanic activity and extensive outpourings of basaltic lava. Specific examples of areas where such seismic velocities have been found, are the Gulf of Aden, the central plateau of Mexico, the Basin and Range province of the United States, the western and southern parts of the Colorado plateau, and the Rocky Mountain trench region of western Montana. These velocities are believed to be caused by a mixture of rocks in a transitional phase between normal mantle and normal basal crust types. In turn, these phase changes are believed related to the crustal uplift and ex'A later reinterpretation of the same data gives a bottom depth of about 43 km and a mantle velocity of 8.38 km/sec; but because the profile was unreversed, this velocity may not be a true velocity (Meyer et al., 1961, p. 224).

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KENNETH L. COOK

tension occurring in these regions as a result of an upwelling of convection currents in the mantle beneath these regions. It should be emphasized that these seismic velocities (7.4to 7.7km/sec) are apparently restricted to the tectonically active belts and have not been found to date (1961) in continental areas regarded as tectonically stable (see, for example, Steinhart and Woollard, 1961, Table 10.1, p. 347). 9. A SUGGESTED MODELFOR

THE

ACTIVETECTONIC BELTS

A suggested model, which involves much speculation, will now be given in an attempt to account for the 7.4 to 7.7 km/sec seismic velocities in tectonically active regions such as oceanic ridges, island arcs, and certain continental belts. The following assumptions are made. 1. The hypothesis of systems of convection currents in the crystalline mantle of the earth, which exert strong drag forces on the crust, is assumed (Griggs, 1939; Heiskanen and Vening Meinesz, 1958; Vening Meinesz, 1960). The deformations must therefore be of a plastic kind. According to Vening Meinesz (1960, p. 26), a rising convection current in the surface layer of the mantle cannot flow out to all sides, as a hydrodynamic current in a viscous Newtonian fluid usually does, but it can flow t o one side only; moreover, the current can make a half turn only. This half-turn current lasts some 50-100 million years, corresponding to the period of orogeny in the region, and the current velocities are a few inches per year. Because of the extremely small value of the temperature conduction by the mantle rocks, the temperature and also the corresponding density deviations are carried along by the current. The drag forces exerted by these currents on the rigid crust, floating on the mantle, which may result in either compressional or tensional stresses, bring about both strong deformations of the crust and also a tendency to move the crust around the earth. Thus convection currents constitute the main driving force of the two great geologic processes of volcanism and diastrophism. 2. More specifically, the convection current is the main driving force causing any large crustal shift occurring on the mantle. Moreover, the convection current is the driving mechanism of uplift and distension (causing plateaus and rift systems) and downwarps (trenches) of the overlying earth’s crust, which result from the stresses formed incident to the plastic flow of the convection current in the mantle. Although the convection current causes the larger amount of heat flow in a given area, the heat alone from the rising column of convection current and the concomitant expansion due to phase change are probably insufficient by themselves to cause the great uplifts and distensions of the overlying crust. 3. The great belts of anomalous beat flow, both those having a much greater or less rate than the average value of 1.2 x cal/cm2 sec, are

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believed to be caused by these great convection currents which carry hot material upward toward the base of the crust in regions of uplift, giving an anomalously high heat flow, and cooled materials downward toward the core of the earth in regions of downwarp, giving an anomalously low heat flow (Bullard e t al., 1956, p. 177; Lill and Revelle, 1958, p. 1013). 4. The hypothesis of Lovering (1958) and Kennedy (1959) is acceptable insofar as the change of phase of eclogite to basalt (or gabbro) a t the MohoroviEi6 discontinuity is concerned, but not sufficient to be the sole driving force for the uplift of mountains (Hadsell, 1960). Their hypothesis apparently does not require a driving force from deep within the mantle, although Lovering’s hypothesis apparently requires heat a t the base of the crust that would follow the final stage of convective overturn of the type envisaged by Griggs (1939; Lovering, 1958). 5. The sizes of the convection cells, still unknown, probably are variable, and depend on both local conditions and also the size and strength of the adjacent convection cells within the mantle. 6. The rising columns of convection currents can occur beneath continents (Hill, 1957; Heiskanen and Vening Meinesz, 1958, p. 400; Vening Meinesz, 1960, p. 29), as well as beneath oceans. The locality of the upwelling of the convection currents can change from place to place as the crust shifts. 7. The periods of tectonic activity during the earth’s history are probably episodic instead of periodic (Vening Meinesz, 1960, p. 28). 8. The earth need not be expanding as a whole. The convection current hypothesis could result in some compressional areas and some tensional areas. On the basis of the above assumptions, the earth model in an area above a youthful convection current cell could be envisaged as follows. I n the general area above the column of upwelling convection currents, which may be several hundreds of kilometers in width and thousands of kilometers in horizontal length, the mantle-type rock (eclogite) is postulated to change in phase to become crustal-type rock (basalt or gabbro). The zone occupied by the mantle-crust mix, where the phase transformation is occurring, is probably of considerable thickness; and in this region the abnormally low compressional wave velocities of 7.4 to 7.7 km/sec are found. As this material is different in its physical properties, such as density and velocity, from that on either side horizontally, it causes a lateral inhomogeneity in the uppermost mantle. Within the central part of the zone of mantle-crust mix, the density and seismic velocities are probably more or less uniform a t any specific level a t which pressure and temperature conditions are about the same. At the peripheral parts of the zone, however, the physical properties of the mantle-crust mix probably grade laterally into

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those of normal mantle- or crustal-type rock, depending on the level under consideration, Similarly, in a vertical direction within the zone, the mantlecrust-mix material probably changes gradually with increasing depth to normal mantle material. The depth required for this phase transition between mantle- and crustal-type rock to take place in tectonically active continental areas is probably at least several scores of kilometers (apparently a t least 72 km in the eastern part of the Basin and Range province) and may prove to be as much as 100 km to 200 km or more. The depth of isostatic compensation must go to this depth, and may go beyond it, if still other effects-not now known-ccur a t still greater depths. I n a vertical direction within the zone of mantle-crust mix, the top of the zone is apparently more abrupt, perhaps because of a somewhat greater geothermal gradient here due to physical-chemical reactions or other causes. As shown in Fig. 4b (in part after Menard, 1960b), the tectonic forces manifest themselves as (1) uplift and distension over the rising column of convection currents, (2) translation along the middle part of the flanks, and (3) compression along the region where the currents are downflowing toward the core. More specific characteristics of these tectonic features are discussed below in reference to the example of this model. 10. EVIDENCE FOR CONVECTION CURRENTS Many arguments in favor of the convection current hypothesis have been given in previous papers by Vening Meinesz (see, for example, references in Heiskanen and Vening Meinesz, 1958) and will not be repeated here. These arguments make the hypothesis “highly probable, if not certain” (Vening Meinesz, 1960, p. 26). The hypothesis has been greatly strengthened in recent years by the discovery of the great world-encircling mid-oceanic ridge system with its accompanying uplift and rift system, and its belt of high heat flow, seismicity, and outpourings of basaltic lava, and especially by the discovery of the great horizontal displacements along directions perpendicular to the trend of these mid-oceanic ridges in both the Pacific Ocean and the equatorial region of the Atlantic Ocean (Vacquier, 1961). All of these features are discussed in detail elsewhere in this review.s As a result of studies of the motions of artificial satellites, additional evidence in partial support of a system of convection currents in the mantle has been given recently by the discovery of O’Keefe e t al. (1959a; 1959b) that the earth’s gravitational potential contains a third-order zonal harmonic component. According to Licht (1960), this component could not be ‘Added in proof. Recent discussions of continent and ocean basin evolution by spreading of the sea floor, in which the mechanism driving their spreading may be the Vening Meinesz thermal convection cell, are given by Heezen (1960), Dietz (1961s and b), Bernal (19611, Wilson (1960, 1961), and Weertman (1962).

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supported by an earth in hydrostatic equilibrium. He concludes that the Vening Meinesz theory of convect,ion currents is capable of qualitatively explaining the stresses associated with the third harmonic ; however, if the Vening Meincsz value for the viscosity of the mantle is accepted, the required thermal efficiency of about 70% becomes too high for a convective process, which has an estimated maximum of 4%. As Licht considers it improbable that convective efficiencies could be so large, he concludes that it remains to be seen whether a more realistic theory of convection with a lower efficiency could account for the observed coefficient of the zonal harmonic. It should be emphasized that on the basis of the third-order anomaly, Licht assumed that only three principal convection cells, which were symmetrical with respect to the axis of the earth's rotation, exist in the entire mantle. I n these, the current rises under the North Pole, falls a t approximately latitude 25" N, rises again a t latitude 25" S, and finally descends under the South Pole; thus the currents would move along parallels of longitude only. Preliminary results of more recent studies of satellite motions indicate that the earth's gravitational potential contains also a fifth-order zonal harmonic component (Newton, 1961) and, in addition, possibly a secondorder sectorial harmonic coefficient (Izsak and Kozai, 1961). If the application of Licht's reasoning to these additional components is tenable, the existence of smaller convection cells and also of currents with an eastwest component as well as a north-south component, is implied. This new approach to the evidence for convection currents requires additional satellite data to evaluate its contribution to the problem. It should be noted that a second interpretation of the odd higher order zonal harmonics is that the earth's interior has a high order of ordinary mechanical strength (O'Keefe, 1959). A third interpretation is that significant variations exist in the density of the mantle, and that the observed geoid is in isostatic equilibrium within the limits previously suggested by Heiskanen and Vening Meinesz (Carey, 1960, p. 310) ; this will be discussed further in relation to the Gutenberg low-velocity layer. 11. EXAMPLE OF MODEL Figures 4a and 4b, in part modified after Menard (1960b, Fig. 7)' show a schematic, generalized, and highly idealized representation of the above model as it might apply to a cross section across the western part of the United States and eastern part of the Pacific Ocean. Many details are omitted a t the scale chosen, and not all features shown are necessarily to scale because of the many uncertainties involved and because the data in some areas were projected long distances into the profile. The Basin and Range province, including its typical graben, horst, and tilted fault-block

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structures and underlain by a mantle-crust mix, is interpreted as lying over the region of tension and uplift (Fig. 4b). The main rising column of the youthful convection current is provisionally placed beneath the Wasatch rift valley system, which lies immediately west of the Wasatch Range (a horst). This, in turn, lies along the western margin of the Colorado plateau. This interpretation is believed comparable with one which would place a similarly rising column of convection current beneath the median rift in the Gulf of California that lies immediately west of the block-faulted Sierra Madre Occidental, which, in turn, lies along the western margin of the central plateau of Mexico. Effects of heating and uplift probably exist beneath the Mexico and Colorado plateaus, but there are presumably less effects of tension on this side of the convection column inasmuch as the convection current is postulated to flow principally to the west only, in accordance with the hypothesis of Vening Meinesz (1960) that it can flow to one side only. Over the Basin and Range province, the rising hot material within the mantle causes uplift and extension of the crust partly because of thermal expansion and physical-chemical changes taking place in the mantle-crustmix phase transformation zone, but principally because of the driving forces of the upwelling convection current itself. The general seismicity of the Basin and Range province as a whole and the concentrated belts of seismicity along the rift systems on its eastern and western margins in particular, indicate that the process of uplift and extension is continuing today, as it has since lower Oligocene time, which, according to Kulp (1961), was about 36 million years ago. The process of transformation of eclogite to basalt (or gabbro) is apparently still in progress and constitutes isostatic adjustment in action. Because gravity measurements show that the Basin and Range province is now approximately in isostatic equilibrium (Woollard, 1943), the transformation is apparently keeping pace with the environmental changes of pressure and temperature caused by the convection current. The isostatic adjustment is apparently being effected partly by the phase changes within the transition zone at depths between 25 and 72 km in the eastern part of the Basin and Range province (which is inferred to lie directly over the uprising column of the convection cell) and depths between 25 and 50 km in the southwestern part of the Basin and Range province, and partly by changes that extend to even greater depths. As would be expected over the uprising column of the convection current, the depth to the MohoroviEii: discontinuity is greatest and the compressional velocity (and hence probably the density also) of the mantle rock beneath the MohoroviEiE discontinuity is less than that in adjacent areas at the same depth. The fact that the top of the mantle-crust mix rises so

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near the earth’s surface as 25 km is noteworthy, inasmuch as the top of the mantle-crust mix is much higher than the level a t which normal mantle rock would be expected. As the process of uplift and expansion continues with geologic time, it is conceivable that a t least the upper part of the transition zone will eventually convert to a material with seismic velocities closer to those of the typical “basalt” ordinarily found at the base of the crust. The mantle-crust mixture exists apparently because the region is still undergoing tectonic activity and the rocks have not yet reached a state of equilibrium. I n the crust over the main area of the upwelling convection current, a system of tension cracks, parallel to the trend of the landward extension of the East Pacific Rise, is formed. The direction of elongation of most of the basins and ranges in the Basin and Range province is north-south, and hence along this trend. Over the crest of the East Pacific Rise, Menard (1960b, p. 1745) believes that arching of the mantle stretches and thins the crust, but the observed thinning is so great that translation of the crust toward the flanks of the rise is also required; and he therefore reasons that the horizontal limb of the convection cell moves the crust outward and thins i t a t the crest of the rise by normal faulting along the tension cracks. This situation is also probably true for the Basin and Range provincc because of the small thickness of normal crust overlying the mantle-crust mix. I n the oceanic part of the section in Figs. 4a and4b, the pattern is taken from the diagrammatic section of Menard (1960b, Fig. 7) and is not intended to conform with the known geology. Here, in the region of translation along the west flanks of the East Pacific Rise, the horizontal limb of the convection cell displaces the individual crustal blocks different distances by wrench faulting on fault zones because of variations in intensity of convection.s The great horizontal movements along the Murray, Pioneer, and Mendocino fault zones are believed to have occurred over this translation part of the convection cell. On the outermost flanks of the rise, where low heat flow occurs, the sinking convection current marks the outer limit of wrench faulting, and the crust here is apparently thickened by thrust faulting. OF THE MODEL 12. OTHERIMPLICATIONS

There are other implications of the model, some of which may be checked by field data. ‘In Figure 4b, a zone of “medium heat flow” is shown beneath the region of translation (rather than “low heat flow,” as shown by Menard in the region of translation over the East Pacific Rise) because the available data indicate that the heat flow off the coast of California is average (or medium) for a great dist.ance along the profile (see Menard, 1960a, Fig. 5 and Bullard et al., 1956, Fig. 6).

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12.1. Trends of Basin and Range Faults

Within the Basin and Range province, in going from the Wasatch Range toward the Sierra Nevada, the over-all trend of the basins and ranges is northerly, but there is apparently an increasing number of ranges, or segments of ranges, which trend northeasterly or northwesterly. This feature suggests that, in going westward within this region, an increasing amount of the translation component of the convection current increases, and thus causes shear cracks to form in the crust a t angles (in plan view) to the direction of translation. A statistical study of the directions of the trends of the main Basin and Range faults throughout the Basin and Range province would be of interest to test the validity of this observation. 12.2. Heat Flow

To date (1961),no heat-flow measurements have apparently been made in the Basin and Range province. Heat-flow measurements are desirable a t various places across the western part of the United States between the Pacific coast and the Rocky Mountain front, to ascertain whether a aonc of high heat flow occurs over the crestal part of the landward extension of the East Pacific Rise. 12.3. Difficulties with the Model

The model along the section in Figs. 4a and 4b does not explain the rightlateral movement along the San Andreas fault, and therefore apparently needs modification. Other factors, such as perhaps local eddying effects or variations in the intensity of convection in some areas, would result in cross stresses in a three-dimensional pattern (as compared with the twodimensional pattern of Fig. 4b) and possibly result in such a cross structure to explain the San Andreas fault feature. Also the element of time has not been considered. As explained by Menard (1960b, p. 1744), wholly unrelated stresses may have produced movement in different directions a t different times along the same zone of weakness. One of the keys to the problem is the dating of the major horizontal movements along the faults transverse to the trend of the East Pacific Rise (Mendocino, Pioneer, and Murray faults) in relation to the time of the beginning of major movement along the San Andreas fault zone. If the transverse faults are no longer active, then the movements of the crust lying above the western part of the convection cell in Fig. 4b no longer exist as shown. It is conceivable that the Basin and Range orogeny, after about 36 million years, has run its course and that the system of convection cells beneath the western part of the United States are now in a transition period of changing directions.1° This situation might account for the oblique tensional rifting loAdded in proof. The convection cell along the profile shown in Fig. 4b is in accord with the direction of left-lateral movement along the eastward-striking Garlock fault

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postulated by Hamilton (1961, p. 1316) to be now occurring along the San Andreas rift system. I n this connection i t should be noted that according to Vening Meinesz (1960, p. 28), “. . .the presence of a system of convection currents in the mantle must have two consequences. The drag forces exerted by these currents on the rigid crust, floating on the mantle, must in the first place bring about strong deformations of the crust.. ..But in the second place we may expect that all these forces together exert a moment on the rigid crust as a whole, which tends to move the crust round the earth.” It is therefore suggested that the present right-lateral movement along the San Andreas fault zone constitutes part of the second consequence as postulated by Vening Meinesz, and that this movement may be related to the possible counterclockwise rotation of the Pacific Ocean basin, as suggested by Benioff (1959). The right-lateral displacement along the Alpine fault of New Zealand, which probably trends into the Tonga trench, would be consistent with this direction of rotation (Hamilton, 1961, p. 1316). The same reasoning of Vening Meinesz can perhaps be used to explain the oblique tensional rifting in the Dead Sea-Red Sea region, the Cayman Deep in the West Indies, and the Snake River downwarp of southern Idaho (see Hamilton, 1961, p. 1316 for a possible Comparison of these areas to the San Andreas rift zone). lW.4. Possible Fracture Zone

I n Fig. 4a the great east-west fracture zone is extended across the western part of the United States. At present, however, no such great fracture zone is apparently known that extends as the eastward continuation of any of those in the Pacific Ocean. The possible eastward continuation onto the continent of the east-west Mendocino fracture zone, which offsets the continental shelf about 100 km with right-lateral displacementll in the Cape Mendocino area (Menard, 1960b, p. 1744), is worthy of further investigation. The eastward projection of the strike of this fracture zone cuts through the complexly deformed rocks of the Franciscan type and is in approximate alignment with the of southern California. A recent study of the offset of a dike swarm on either side of this fault indicates a total horizontal displacement of about 40 miles along the fault since late Mesozoic or early Tertiary time (Smith, 1962, pp. 87-88). Although the Garlock fault has probably been active up until late Cenozoic time, the geomorphic characteristics of the fault scarps do not indicate displacements as young as those along the San Andreas fault, which has moved intermittently throughout Recent time. Although several suggestions have been made, the problem of the abrupt ending of the Garlock fault where i t meets the right-lateral San Andreas fault zone on the west and the rightUThis displacement is in the opposite direction from the displacement of the Mendocino fault in the deep-sea floor farther to the west.

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north end of the Great Valley of California, the south margin of the Modoc lava plateau, the south tip of the chain of Tertiary and Quaternary extinct volcanoes and volcanic cones, and the southern termination of the Cascade Range. Vacquier et al. (1961, p. 1253) report that James Affleck of the Gulf Research and Development Company has found evidence from magnetics that the Mendocino feature may extend eastward from the coast for a t least 100 miles (160 km) ;there is no direct evidence of horizontal displacements, but strong lineaments and changes of magnetic character are present. As emphasized by Vacquier et al. (1961, p. 1257) , if the horizontal displacements along oceanic faults propagate into the western American continent a t the time they happen, there is no need to be disturbed about the discrepancy between the displacements of the faults a t sea and those on land a t about the same latitude unless one insists that the faults are younger than the San Andreas fault. This is contrary to present-day evidence, which indicates that the San Andreas fault is seismically active, whereas the oceanic faults west of 130" W longitude are seismically dead. Some great geofracture zones or lineaments across the western part of the United States, some of which have long been known, are summariecd by Osterwald (1961). Although the relationships are not yet entirely clear, some of these are apparently of the transcurrent-fault type, and the possibility that the horizontal displacement along them may be comparable to a t least that of the smaller fracture zones in the Pacific Ocean warrants investigation. I n particular, the geofracture zone extending northwestward through Colorado and Wyoming (Osterwald, 1961, Fig. 1 ) may connect with the northwestward-trending zone of crustal weakness along the Snake River downwarp12 and volcanic plateaus of Oregon-Washington. The Snake River downwarp is conceivably a great graben filled with basalt along a major crustal break which follows a line of earthquakc epicenters (Malde, 1959; Woollard, 1959, Fig. 5 ; Neumann, 1959, Fig. 1 ) that extend diagonally from Puget Sound, across the Columbia River plateau, along the northern boundary of the western Snake River Plain, and thence across the plain to northern Utah. Though no fault has apparently yet been demonstrated on the south margin of the Snake River downwarp, a northwestward-trending fault zone of great vertical throw (at least 9000 ft) has been established by gravity, seismic, and geologic studies along the northern part of the Snake River downwarp where it is coincident with the northern boundary of the Snake River Plain, Idaho (Malde, 1959). The movement along this fault has occurred since early Pliocene time. As mentioned earlier, the direction of the convection currents may have changed from those shown in Fig. 4b. Billings (1960, p. 363) suggests that in the Fairview Peak-Dixie Valley, Nevada earthquake, there is evidence la Since this interpretation was made, Hamilton (1961, p. 1316) haa independently suggested that the Snake River depression is perhaps a tensional rift in an early stage.

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that strike-slip movements of the San Andreas type are invading an area characterized by Basin and Range normal-fault structure. 16.6 ExpZanation of Gutenberg Low-Velocity Layer

Recent investigations using different approaches indicate that the Gutenberg low-velocity layer in the upper mantle is a universal feature beneath oceans and continents. The investigations include studies of earthquakegenerated G waves, the dispersion of Rayleigh waves, and the free oscillations of the earth. The G wave, first discussed by Gutenberg and named after him by Byerly, is a horizontally polarized shear wave with predominant periods of 50 to 180 sec which travels below both continents and oceans. The G wave, which is transient in character, is followed by a train of dispersed Love waves when the propagation path is continental; for oceanic paths, where the crust is thin, the Love wave train has brief duration or may be absent. The velocity of the G wave depends primarily on the properties of the outer mantle averaged horizontally over continental dimensions and is less susceptible to local heterogeneity (Press, 1959, p. 565). Press and Ewing (1956) and Landisman and Sat6 (1958) reasoned that G-wave velocity data require the existence of a low-velocity zone in the upper mantle a t a depth of about 100 to 200 km below the earth’s surface. Also using G waves, Press (1959) demonstrated that the low-velocity zone in the mantle exists under continents and oceans. He suggested that (1) the low-velocity zone in the mantle is a world-wide phenomenon, (2) the zone may be ascribed to a state near the melting point, (3) the zone may be the source of the primary basaltic magma, (4) the zone accounts for the longperiod nature of S waves, and (5) the zone may be the place where the mantle is effectively decoupled from the crust for tectonic processes and differential movements between crust and mantle. I n a study of the dispersion of mantle Rayleigh waves (vertically polarized waves with periods as large as several hundreds of seconds), Takeuchi e t al. (1959) assumed the Gutenberg earth model with a low-velocity laycr a t a depth of about 150 km and obtained results concordant with the observations, whereas the modified Jeffreys-Bullen model-with no low-velocity layer assumed-disagreed significantly with the observations. They concluded that their results gave additional evidence for the worldwide existence of the low-velocity zone. Also using the dispersion of Rayleigh waves, Dorman et aZ (1960, p. 114) discovered that large differences between oceanic and continental mantle structure extend to a depth of a t least 100 km, and smaller differences extend to a depth of about 400 km. Their interpretation is that the Gutenberg low-velocity channel begins a t about 60 km depth under the oceans and a t about twice this depth under the continents. Aki and Press (1961) give the alternative interpretation that the

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Gutenberg low-velocity zone occurs a t the same depth under oceans and continents but that the seismic velocity under the oceans is lower; with this interpretation, the minimum velocity in the low-velocity layer occurs a t a depth of about 140 km (Press, 1961, Fig, 10, p. 1460). Also, with this interpretation, Aki and Press (1961) give evidence of differences between the mantle under the Pacific Ocean and under the Atlantic and Indian Oceans-differences which are interpreted as a smaller shear velocity a t the top of the mantle under the Atlantic and Indian Oceans. Additional support for the low-velocity zone in the upper mantle has been given from recent studies of the free oscillations of the earth (generated by the great Chilean earthquake of May 22, 1960) by Alsop e t a2. (1961), Benioff et al. (19611, MacDonald and Ness (1961), and Pekeris e t al. (1961). I n these studies the Gutenberg model, in which a low-velocity layer was assumed, gave better agreement between experimental and theoretical predictions than the other models considered. Gutenberg (1954a, p. 346) believed that the low-velocity channel, with characteristic compressional velocities of about 7.8 km/sec (Gutenberg, 1959a, Fig. 4.4), is probably due to a greater effect of increase in temperature with depth than that of the increase in pressure a t the depths where the melting point of the material is approached, while above and below this channel the effect of the increase in pressure with depth prevails. An increase in temperature tends to decrease the velocity of elastic waves. Verhoogen (1960, p. 153-154) estimates a temperature gradient of a t least 10 to 15"C/km a t the top of the mantle, a rapid increase in temperature from the MohoroviEi6 discontinuity downward to reach possibly 11001200°C at a depth of about 100 km and about 1500°C a t a depth of 200 km. He estimates that between the depths of 100 and 200 km, the temperatures come sufficiently close for partial melting of the mantle material and the formation of basaltic magmas. As explained by Birch (1952, p. 259-260), the Gutenberg low-velocity zone would not require an approach to melting, but may be accounted for solely by a critical gradient of temperature of about 6"C/km in a homogeneous layer of ultrabasic rock a t the depth of this low-velocity layer. Birch considered this temperature gradient as not unreasonably high. The results of a recent theoretical treatment of the lowvelocity layer by MacDonald and Ness (1961, p. 1904) support these conclusions of Birch. I n the model in Fig. 4b, the top of the approximately horizontal limb (medium heat flow) of the convection cell in the region of translation is tentatively shown to extend in the mantle along a zone confined between the depths of 100 and 250 km below the earth's surface in order to make the zone conform roughly with the tentative limits of the low-velocity layer as

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given by Gutenberg (1959a, p. 81,84) .13 If the lateral heat flow in the restricted zone a t this depth (caused by the convection current alone), together with concomitant expansion and phase transformation of eclogite to basalt (or gabbro) in accordance with the pressure-temperature conditions, were sufficient by themselves to give rise to velocities characteristic of the low-velocity channel, the low-velocity layer would not exist everywhere on earth, but only in restricted belts (Cook, 1961). The low-velocity layer would probably be best defined within the horizontal limb of a convection cell, next best defined within the zone of turning of the uprising column of the convection cell, and poorly defined or entirely absent within the lowheat-flow region. Because the Gutenberg low-velocity layer is a continuous, world-wide phenomenon, however, it now appears that the convection currents should therefore produce only modifications of the low-velocity layer. These modifications could be in both the depth a t which the layer exists, as well as the velocities that it shows. Seismic studies should be made to ascertain whether the Gutenberg low-velocity layer shows different characteristics beneath large regions of high, medium, and low heat flow. Because many earthquakes apparently originate in the low-velocity channel, Gutenberg reasoned that the material here has relatively small elastic constants and therefore relatively small breaking strength ; that is, a “softening” of the rock exists. He postulated that some of the earthquake energy may be caused by phase changes now in progress within the layer. On the basis of the convection current model shown in Fig. 4b, the Gutenberg low-velocity layer is the level along which movements of the crust could take place relative to the mantle.

[Added in proof. I n a paper published while this review was in press, Ringwood (1962) postulates an inhomogeneous upper mantle in which the Gutenberg low-velocity zone is absent beneath Precambrian shields. On the assumption that the mantle immediately below the MohoroviEiE discontinuity is a multicomponent system composed dominantly of dunite and peridotite, Ringwood ( 1962, pp. 859-860) reasons that the temperature gradients in a homogeneous mantle are unlikely to be sufficiently high to =Added in proof. There is not general agreement among the proponents of the convection current hypothesis as to the depth of the top of the convection cells. Dietz (1961, p. 855), who considers the sea floor as essentially the outcropping mantle (because the sea floor is covered by only a thin veneer of sediments with some mixed-in effusives), considers that the sea floor marks the tops of the convection cells and slowly spreads from zones of divergence to those of convergence; and he considers the cells to have dimensions of several thousands of kilometers. Bernal (1961, Fig. 11, however, considers the top of a convection cell beneath a mid-oceanic rift t o be at an approximate depth of loo0 km, which he gives as the boundary between the “plastic” lower mantle and the “rigid” upper mantle.

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cause the formation of a low-velocity layer in the position implied by seismic data; and he therefore postulates a nonhomogeneous upper-mantle model. In the model, the continents are regarded as having segregatcd vertically as a result of differentiation by fractional melting of a hypothetical primitive upper-mantle material (chemically equivalent to 1 part of basalt plus 4 parts of dunite) which he designates “pyrolite.” Consequently the upper mantle in the model is chemically zoned. The dunite-peridotite zone below the MohoroviEi6 discontinuity extends downward, perhaps at a depth of 150 km beneath Precambrian shields, into the primitive pyrolite. Under oceans, the pyrolite may extend upward to the MohoroviFii discontinuity or, as an alternative, a thin zone of dunite-peridotite, perhaps 25 km thick, may occur between the crust and the pyrolite zone. I n active or recently active tectonic zones, including island arcs, the thickness of the dunite-peridotite zone may be intermediate between that beneath thc oceans and that beneath the Precambrian shield areas; and a transitional zone (designated by Ringwood as the “pyrolite transition zone”) , perhaps 100 km in thickness, is postulated. Within this transitional zone, the lowvelocity layer is explained on the basis of the mineral assemblages (principally because of the presence of plagioclase) in the primary pyrolite in terms of the postulated pressure and temperature; and the decrease in velocity of seismic waves is postulated to be greater under oceanic than under continental areas. With this model, phase changes occur within the low-velocity zone; a transition from garnet-bearing pyrolite to plagioclascbearing pyrolite, or vice versa, is postulated depending upon whether the region is undergoing uplift or subsidence. The prediction that no lowvelocity zone occurs beneath Precambrian shields is based largely on the low heat-flow values obtained to date over shield areas in Australia and Canada. I n a recent study by Brune and Dorman (1962) and Dorman and Brune (1962) of extensive seismic-wave data for the Canadian shield, the phasevelocity data of surface waves indicate an upper mantle with shear velocity increasing from 4.77 to 4.95 km/sec a t a depth of about 95 km and decreasing to a minimum of 4.43 km/sec a t a depth of about 170 km with a Gutenberg structure below. For the Canadian shield, therefore, the Gutenberg low-velocity layer is indicated, and the postulate of Ringwood was not corroborated here. However, the additional results of Brune and Dorman (1962) suggest the possibility of lateral inhomogeneity in the upper mantle. A comparison of their data with that from other regions “indicates large regional variations in shear velocity in the upper mantle and suggests the existence of high-velocity roots in the mantle under stable continental areas of low relief such as the Canadian shield and the central United States.”]

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PROBLEMS CONCERNING THE

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MOHOROVIEI~ DISCONTINUITY

The existence in many tectonic belts of the mantle-crust-mix velocities of 7.4 to 7.7 km/sec leads to an ambiguity in the conventional definition of the MohoroviEii: discontinuity, which was given earlier in this paper. In tectonically active continental areas, these abnormally low velocities are found a t relatively shallow depths (as, for example, a t a depth of about 25 km in the California-Nevada-Utah region), and they may extend to great depth before normal mantle velocities are found. I n continental areas, normal mantle velocities have been found beneath the mantle-crust mix. In areas where the mantle-crust-mix velocities are less than 7.8 km/sec, there is little difficulty. I n some areas, however, as for example along the Kingman, Arizona profile (Diment et aE., 1961), the velocity of the mantle-crust mix is apparently 7.81 km/sec, and y e t a t a somewhat greater depth-the more common mantle velocities are found, or interpreted to exist. Aspects of this problem of the definition of the MohoroviEii: discontinuity on the continents have been mentioned briefly by Berg e t al. (1960, p. 532), Press (1960, p. 1051 ; 1961, p. 1452), Diment et al. (1961, p. 208), and perhaps others. I n tectonically active oceanic areas, the mantle-crust-mix velocities are also found a t relatively shallow depths, and they may extend to relatively great depth before normal mantle velocities are found. To date (1961), no measurements of normal mantle velocities in the crestal part of the mid-ocean ridge areas have apparently been found beneath the mantle-crust mix because of the limited shot-detector distance that has been used. The depth to the layer with normal mantle velocity has often been estimated, however, from the existing velocity and gravity data. The problem in nomenclature arises along an oceanic profile across the midocean ridge over and near the crest of the ridge, where normal mantle velocities along the flanks of the ridge give way to mantle-crust-mix velocities over and near the crest. Aspects of this problem in nomenclature have been mentioned briefly by Menard (1960b, p. 1741). The mantle-crust-mix velocities of 7.4 to 7.7 km/sec are believed more closely associated with the mantle than with the crust in active tectonic belts because immediately above the material showing these velocities are usually found-with sharp velocity and density contrasts-much lower velocities which are definitely characteristic of the crust. Below the mantle-crust-mix layer, however, the contrast with the normal mantle is often not as marked. I n some tectonically active areas in the future, it may be found that the mantle-crust mix grades into the mantle without a sharp contrast in velocity or density. A specific name for the discontinuity a t the top of the mantle-crust mix is now desirable. It is not considered appropriate to extend the usage of the term “Conrad discontinuity’’ to apply to this discontinuity because in some

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areas both the Conrad discontinuity and this discontinuity apparently exist. If, beneath the bottom of the mantle-crust-mix layer, normal mantle velocities are found, the usual designation of MohoroviEi6 discontinuity will apply.14 14. ABRUPTOR GRADATIONAL BOUNDARY

I n tectonically stable areas, both continental and oceanic, the MohoroviEi6 discontinuity apparently constitutes an abrupt boundary between rocks of contrasting density and velocity. This sharp contrast is considered not as a chemical change from basaltic (or gabbroic) material in the lower crust to ultrabasic material in the upper mantle, but as a contact between different phases of the same chemical composition: basalt (or gabbro) as the low-pressure low-temperature phase and eclogite as the high-pressure high-temperature phase (Sumner, 1954; Lovering, 1958, p. 953). The contact is abrupt presumably because the two phases have reached equilibrium with their pressure-temperature environments. I n tectonically active areas, however, both continental and oceanic, the process of phase transformation is apparently taking place and the situation is complex. The pressure and temperature, which are the controlling factors, are predicated upon the age, size, and past history of the convection current, as well as of the regional and local geology. The mantle-crust-mix layer is probably the zone in which the major transformation of eclogite to basalt (or gabbro), or vice versa is taking place during the period of orogeny in the region. The layer is probably thick. I n such active belts, the existence of the MohoroviEii! discontinuity-which lies at the bottom of this layer-as an abrupt boundary between rocks of contrasting density and velocity is questioned (Drake, e t al., 1959). Rather, the bottom of the mantle-crust mix, containing dominantly eclogite and a small amount of basalt (or gabbro), may grade into eclogite with depth. I n this case the MohoroviEi6 discontinuity, instead of being a sharp boundary, may extend over a zone of considerable thickness. Under the continents, according to Frank Press, the MohoroviEi6 discontinuity could extend over a depth in“Added in proof. On the basis of a crustal model only slightly at variance with that commonly accepted and a novel concept of the evolution of continents and ocean basins by spreading of the sea floor, Diets (1962, p. 857) suggests that the term “crust” be used only to any layer which overlies and caps the convective circulation of the mantle. He reasons that because the sialic continental blocks do this, they form the true crust; whereas because the ocean floor apparently does not (because he considers the ocean floor as essentially the outcropping mantle), the ocean basin is “crustless.” He considers that (1) the MohoroviEii: discontinuity is a phasc change rather than a chemical boundary, (2) the overlying basalt (or gabbro) layer is chemically the same as the mantle rock but petrographically different, and (3) that the basaltic (or grabbroic) layer (as a change of phase) is thus also part of the mantle-a sort of “exomantle.”

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terval of as much as 10 km without being resolved into a single layer by seismic measurements (Kennedy, 1959, p. 503). 15. DEPTHOF ISOSTATIC COMPENSATION

I n tectonically active areas, the depth of isostatic compensation probably extends below the MohoroviEid discontinuity and deeper into the uppermost mantle than is generally recognized (Griggs, 1960). Eclogite changes to basalt with an increase in volume of about 15% (Lovering, 1958, p. 953). I n the region above the rising column of a convection current, a temperature rise therefore causes eclogite to transform to basalt and thus for the MohoroviEi6 discontinuity to become unstable and sink to greater depth. If the heat flow is sufficiently great, some transformation of eclogite to basalt (or gabhro) and a concomitant increase in volume, and decrease in density and compressional velocity, could probably occur a t depths far below the MohoroviEid discontinuity. The process of the effective downward migration of the MohoroviEid discontinuity to effect equilibrium once more probably involves much time. I n the active areas, while the downward migration of the MohoroviEid discontinuity is taking place, the density and compressional velocity of the material lying much deeper than the maximum depth to which the MohoroviEi6 discontinuity will eventually migrate, are probably less than the density and compressional velocity of normal mantle rock. Within this part of the mantle, short-period shear waves traversing such an area would probably show preferential attenuation in a manner comparable to that observed by Ewing and Press (1956a). They attributed the phenomenon to “pockets of magma.” A lateral inhomogeneity of the type just described would constitute such a “pocket” and explain the method by which isostatic compensation can be accomplished below the MohoroviEi6 discontinuity. 16. AREAOF MANTLE-CRUST MIX

The area of active tectonic belts beneath which mantle-crust-mix velocities are found is estimated tentatively to be a t least 10% of the earth’s surface. The data are sparse, and the value could conceivably be twice this estimate. To obtain this figure, it is assumed that the entire length of the mid-oceanic ridge system of 75,000 km (Vetter, 1960) to 80,000 km (Menard, 1960b, p. 1737) is underlain by a mantle-crust mix with an average width of 700 km. The estimate of width is believed conservative. The average width of the anomalous physical properties over the East Pacific Rise-and hence the width of the anomalously low velocities-is estimated by Menard (1960b, p. 1745) as 800 km. The width of the mantle-crust mix along a profile across the Mid-Atlantic Ridge south of the Azores is shown

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by Ewing and Ewing (1959, Fig. 3) as about 1500 km. The known width of the mantle-crust mix beneath the western part of the United States, between California and the central part of the Colorado plateau, is about 1000 km. I n the Arctic Ocean, the northward extension of the Mid-Atlantic Ridge, as shown by the earthquake epicenter belt and soundings from the nuclear submarines, extends parallel to-but about 200 miles away from-the Lomonosov Ridge in the direction of the Eurasian continent, and continues southeastward to enter the Asiatic mainland a t the northern tip of the Verkhoyansk trough in East Siberia (Heezen, 1960, p. 101; Hope, 1959; Heezen and Ewing, 1961). Though no seismic work has apparently been done yet in this part of the ridge, typical mantle-crust-mix velocities can probably be expected here also, as well as perhaps beneath the Verkhoyansk trough and the Baikal rift valley seismic belt. I n the over-all estimate, no area is added to include the mantle-crust mix beneath the continents, yet this area may be large because the mantlecrust mix probably exists beneath continents in tectonic belts which lie along the landward extensions of oceanic ridges. This has been shown for the landward extension of the East Pacific Rise into the western United States. Because the Mid-Indian rift extends into the region of the East African rifts and plateau, as well as along the Red Sea and the Palestine rift (Heezen, 1960, p. 101), it seems possible that mantle-crust-mix velocities will eventually be found here as additional crustal studies are made. The possibility of the landward extension into India of the northeastward branch of the Carlsberg Ridge, which passes through the island of Socotra and off the south coast of Oman, is worthy of investigation. It is noteworthy that the landward continuation of the aseismic Laccadive-Chagos Ridge, which in the Indian Ocean underlies islands of those names and also the Maldine Islands which lie between them, coincides with the location of the Deccan plateau basalts of tholeiitic compooiiion and Eocene age (Jacobs et al., 1959, p. 287). It would therefore be of special interest to ascertain whether the mantle-crust mix exists beneath this ridge. 17. SUMMARY

The material with compressional wave velocities of 7.4 to 7.7 km/sec, existing beneath many active tectonic belts, is postulated to be a mixture of mantle- and crustal-type rocks. The belts with such velocities include mid-ocean ridges, island arcs, and rift valleys (both oceanic and continental). The total area of these belts, though still incompletely charted, is estimated as a t least 10% of the earth’s surface, and may be twice this figure. The uplift and lateral extension, as well as the high heat flow and

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volcanism generally characteristic of the active belts, indicate expansion and an upwelling of convection currents in the mantle, thus giving lateral inhomogeneities in the uppermost mantle. The mantle-crust mix is postulated to comprise a mixture of eclogite and basalt (or gabbro) in the phase transformation zone, which may be of considerable thickness. I n such active belts, the existence of the MorohoviEii!discontinuity as an abrupt boundary between rocks of contrasting density and velocity is questioned, and the depth of isostatic compensation probably extends deeper into the uppermost mantle than is generally recognized. The northward landward extension of the East Pacific Rise into the western part of North America (in part after Heezen, 1960, and Menard, 1960b) is given as an example of one of these active tectonic belts and is postulated to include three “median” rifts and the following uplifted regions: Mexico and Colorado plateaus, the Basin and Range province, and the great basalt, lava, and volcanic plateaus of the western United States. The median rifts, which follow belts of seismicity and probable high heat flow, include (1) the rift system passing through the Gulf of California, Sulton trough, Great Valley of California, and out to sea along the San Andreas fault zone, to re-enter the continent in Alaska; (2) the belt of grabens immediately east of the Sierra Nevada; and (3) the major rift system which extends, as a branch of the first rift system above, from the Gulf of California through western Arizona, central Utah (including the newly discovered rift valleys reported by Cook and Berg, 1956; 1958; 1961), southeastern Idaho, western Wyoming, western Montana, and British Columbia. These rifts are comparable structurally to the rift valleys of Africa and the median rifts of the Mid-Atlantic and Mid-Indian Ridges. The fault-block mountains of the Basin and Range province are strikingly similar to the “rift-mountain” systems of the Mid-Atlantic Ridge. A model, based on the hypothesis of a convection current cell in the mantle as developed by Vening Meinesz and applied to the East Pacific Rise by Menard (1960b), is applied to a generalized crustal section across the western part of the United States. The model can be used to explain some features but gives difficulty in explaining others, and therefore needs modification. Several implications of the model are reviewed; these include trends of the Basin and Range faults, heat flow, explanation of the Gutenberg low-velocity layer, and a possible fracture zone across the western part of the continental United States. To avoid confusion in the nomenclature in crustal structure in active tectonic belts, it is suggested that a name be specifically assigned to the discontinuity at the top of the mantlecrust mix, as this discontinuity is distinctly different from either the Conrad or MohoroviEid discontinuities as conventionally defined,

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ACKNOWLEDGMENTS A helpful discussion with H. H. Hess during November 1960 on the possibility of a phase transformation of eclogite to basalt (or gabbro) beneath the Basin and Range province and Colorado plateau is acknowledged; but the author alone is responsible for the speculations expressed. Though the specific topic of this research was not formally supported, some of the University of Utah geophysical data referred to in this review, both that already published and that in preparation for publication, were obtained with former grants from the National Science Foundation, University Research Fund of the University of Utah, and the Utah Engineering Experiment Station of the University of Utah. REFERENCES Aki, K., and Press, F. (1961).Upper mantle structure under occans and continents from Rayleigh waves. Geophys. J. 5, 29'2-305. Allen, C. R., Silver, L. T., and Stehli, F. G. (1960). Agua Blanca fault-a major transAm. 71, 457verse structure of northern Baja California-Mexico. Bull. Geol. SOC. 482. Alsop, L. E., Sutton, G. H., and Ewing, M. (1961). Free oscillations of the earth observed on strain and pendulum seismographs. J . Geophys. Res., 66,631-641. Anonymous (1958). Seismic studies in the Andes. Natl. Acad. Sci. IGY Bull. No. 3. Transact. Am. Geophys. Un. 39,580-582. Anonymous (1960a). XI1 General Assembly of the International Union of Geodesy and Geophysics, Helsinki. Geophys. J. 3, 462-476. Anonymous (196Ob).Research vessel Vema returns from year-long voyage. Transact. Am. Geophys. Un. 41,672. Anonymous (1960~).Seismology and Physics of the earth's interior (including tectonophysics). Transact. Am. Geophys. Un. 41, 575. Antoine, J. W. (1959). Seismic studies in the Western Caribbean. Transact. Am. Geophys. Un. 41, 73-77. Axelrod, D.I. (1957). Late Tertiary floras and the Sierra Neveda uplift. Bull. Geol. SOC. Am. 68,lW. BBth, M. (1957). Shadow zones, travel times, and energies of longitudinal seismic waves in the presence of an asthenosphere low-velocity layer. Tmnsact. Am. Geophys. Un. 38,526-538. BBth, M. (1958).Channel waves. J. Geophys. Res. 63,583-587. BIlth, M . (1960).Crustal structure of Iceland. J. Geophys. Res. 65,1793-1807. Beal, C. H.(1948). Reconnaissance of the geology and oil possibilities of Baja California, Mexico. Geol. SOC.Am. Mem. 31,138. Beloussov, V. V. (1960).Development of the earth and tectogenesis. J. Geophys. Res. 65,4127-4146. Benioff, H. (1954). Orogenesis and deep crustal structureadditional evidence from seismology. Bull. Geol. SOC.Am. 65,385-400. Benioff, H.(1959). Circum-Pacific orogeny. Ottawa Dominion Obs. Pub2s. 20, 395-402. Benioff, H., Press, F., and Smith, S. (1961). Excitation of the free oscillations of the earth by earthquakes. J . Geophys. Res., 66, 605-619. Berg, J . W , Jr., Cook, K. L., Narans, H. D., Jr., and Dolan, W. M. (1960). Seismic investigation of crustal structure in the eastern part of the Basin and Range province. Bull. Seismol. SOC.Am. 50,511-535. Berg, J. W., Jr., Cook, K. L., Narans, H. D., Jr., and Learner, R. L.(1961).Seismic pro-

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Eardley, A. J. (1951). “Structural geology of North America,” 624 pp. Harper, New York. Eiby, G. A. (1958). The structure of New Zealand from seismic evidence. Geol. Rundschau 4 7 , 6 4 7 4 2 . Ewing J. I. (1959). Crustal structure of the Caribbean from seismic measurements (abstract). Bull. Geol. SOC.Am. 70, 1719. Ewing, J., and Ewing, M. (1959). Seismic-refraction measurements in the Atlantic Ocean basins, in the Mediterranean Sea, on the Mid-Atlantic Ridge, and in the Norwegian Sea. Bull. Geol. SOC.Am. 70,291-318. Ewing, J., Officer, C. B., Johnson, H. R , and Edwards, R. S. (1957). Geophysical investigations in the eastern Caribbean. Trinidad Shelf, Tobago Trough, Barbados Ridge, Atlantic Ocean. Bull. Geol. SOC.Am. 68,897-912. Ewing, J., Antoine, J., and Ewing, M. (1960). Geophysical measurements in the Western Caribbean Sea and in the Gulf of Mexico. J . Geophys. Res. 65, 4087-4126. Ewing, M. (1958). The crust and mantle of the earth. In “Geophysics and the IGY.” Am. Geophys. Un. Geophys. Monograph 2,186-189. Ewing, M . (1960). Earth’s crust below the oceans and in continents. Transact. Am. Geophys. Un. 41,172-173. Ewing, M., and Heezen, B. C. (1955). Puerto Rico trench topographic and geophysical data. Geol. SOC.Am. Spec. Paper 62,255-268. Ewing, M., and Heezen, B. C. (1956). Some problems of Antarctic submarine geology. Am. Geophys. Un. Monograph 1,7541. Ewing, M., and Landisman, M. (1961). Shape and structure of ocean basins. In “Oceanography” (M. Sears, ed.), pp. 3-38. Am. Aasoc. Adv. Sci. Publ. 67, Washington, D. C. Ewing, M., and Press, F. (1955). Geophysical contrasts between continents and ocean basins. Geol. SOC.Am. Spec. Paper 62,1-6. Ewing, M., and Press, F. (1956a). Long-period nature of S waves (abstract). Transact. Am. Geophys. Un. 37,343. Ewing, M., and Press, F. (1956b). Surface waves and guided waves. In “Encyclopedia of Physics” (S. Flugge, ed.), Vol. 47, pp. 119-139, Springer, Berlin. Ewing M., and Press, F. (1956~).Structure of the earth’s crust. In “Encyclopedia of Physics” (S. Flugge, ed.), Vol. 47, pp. 246-257. Springer, Berlin. Ewing, M., and Press, F. (1959). Determination of crustal structure from phase velocity of Rayleigh waves. Part 3. Bull. Geot. SOC.Am. 70,229-244. Fermor, L. L. (1914). The relationship of isostasy, earthquake, and volcanicity to the earth’s infra-plutonic shell. Geol. Mag. 51,6547. Gamburtzev, G. A., Koridalin, E. A., Balavadze, B. K., and Tvaltvadze, G. K. (1957). Structure of the earth’s crust in Georgia from geophysical evidence. Intern. Geol. Rev. 1,5748,1959. Gane, P. G., Atkins, A. R., Sellschop, J. P. F., and Seligman, P. (1956). Crustal structure in the Transvaal. Bull. Seismol. SOC.Am. 46,29%316. Garland, G . D., Kanasewich, E. R., and Thompson, T. L. (1961). Gravity measuwments over the southern Rocky Mountain trench area of British Columbia. 1. Geophys. Res. 66,249!5-2505. Gast, P. W. (19eO). Limitations on the composition of the upper mantle. J . Geophys. Res. 65,1287-1297. Gregory, J. W. (1921). “The Rift Valleys and Geology of East Africa.” 479 pp. Seeley, Service & Co., Ltd., London. Griggs, D. T. (1939). A theory of mountain building. Am. J . Sci. 237, 611-650.

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