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Petrologic model of the northern Mississippi Embayment based on satellite magnetic and ground-based geophysical data
Earth and Planetary Science Letters, 70 (1984) 115-120
115
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
[4]
Petrologic model of the northern Mississippi Embayment based on satellite magnetic and ground-based geophysical data Herman H. Thomas Geophysics Branch, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 (U.S.A.)
Received July 25, 1983 Revised version received April 24, 1984
A petrologic model of the northern Mississippi Embayment, derived from gravity, seismic and rift data, is evaluated by converting the model to a magnetization model which is compared with satellite magnetic anomaly models. A magnetization contrast of approximately - 0 . 5 4 A / m , determined from the petrologic model of the embayment compares favorably to values of - 0.62 A / m and - 0.45 A / m from a M A G S A T United States Apparent Magnetization Contrast Map and a published P O G O magnetization contrast model, respectively. The petrologic model suggests that the magnetic anomaly low associated with the Mississippi E m b a y m e n t m a y be largely due to the intrusion under non-oxidizing conditions of low Curie temperature gabbroic material at the base of the crust of the embayment. Near-surface mafic plutons, bordering the Mississippi Valley Graben, appear from aeromagnetic data to have higher magnetizations than the deeper gabbroic material; however, it is impossible to ascertain if this is due to compositional differences or similar material at shallower (lower temperature) depths. These results indicate that variations in the Curie temperatures of intrusions accompanying rifting m a y account for a large part of the wide range of magnetic anomalies associated with presently inactive rifts with normal heat flow.
1. Introduction
Both P O G O [1] and M A G S A T [2] anomaly maps show the magnetic anomaly low as seen by aeromagnetic surveys [3] over the Mississippi Embayment. The feature is thought to be a reactivated Precambrian rift [4] or the failed arm of a late Paleozoic triple junction associated with the opening of the Gulf of Mexico [5]. In a recent paper, von Frese et al. [6] completed preliminary modeling of the Mississippi Embayment P O G O magnetic anomaly using the Austin and Keller [7] modification of a regional geophysical analysis by Ervin and McGinnis [4]. A proposed mantle-derived intrusion at the base of the crust was modeled so as to explain the satellite magnetic low as well as a regional gravity high. In this study, the same Austin and Keller [7] profile, used by von Frese et al. [6], becomes the starting point for a somewhat different approach, 0012-821X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
the purpose of which is to obtain a petrologic interpretation directed toward an understanding of the extreme variation in rift-produced magnetic anomalies. Density-seismic velocity data from the crustal profile is used to determine the rock types that make up the layers in the profile. Magnetic susceptibilities, typical of values for each rock layer, are used to form a magnetization profile which is then compared to an Apparent Magnetization Contrast Map [8] derived from satellite anomaly data. The Apparent Magnetization Contrast Map is a model of the anomaly field which is constructed by determining magnetic moments for dipoles in an equal area grid at the earth's surface such that the resulting field results in a least squares best fit to the anomaly field. The map, despite being an additional step removed from the actual measurements, offers closer geographic coincidence between the anomalies and their source regions than do satellite anomaly maps [8].
116 2. C r u s t a l p r o f i l e
3. P e t r o l o g i c
The Ervin and McGinnis [4] gravity profile (Fig. 1) extends from Yellville, Arkansas southeastward to Scottsboro, Alabama. It crosses the Mississippi E m b a y m e n t and the Mississippi Valley G r a b e n which lies within the northern part of the embayment. The Ervin and McGinnis model was modified by Austin and Keller [7] as the result of an analysis of group and phase velocity dispersion of Rayleigh waves recorded near St. Louis, Missouri and Oxford," Mississippi. The Austin and Keller cross-section (Fig. 2) is used in this study because of the additional seismic constraint and because it has one less crustal layer. In addition, its use allows a comparison to the magnetization model of von Frese et al. [6].
The first step in constructing a magnetization model from the crustal profile (Fig. 2) is to convert the density-seismic velocity layers to rock layers. Neither densities nor seismic velocities uniquely define rock compositions; they can however, along with other data, provide constraints on rock types. The crustal profile plus geophysical data from Ervin and McGinnis along with rift petrology generalizations, particularly those of Logatchev [9], form the basis for the conversion in this discussion. Both seismic velocities and density (Fig. 2) are consistent with an interpretation that layer I is the upper mantle which is thought to be made up largely of peridotites with local eclogites [10,11]. Based on a seismic refraction profile of nearby normal craton [12] compared with a profile of the Mississippi E m b a y m e n t [13], Ervin and McGinnis postulated that layer II was "anomalous" low-density mantle material. This description is consistent with that of a basaltic partial melt from the mantle [14,15]; layer II would then be gabbro, the plutonic equivalent of basalt. Layer III is assumed to be the pre-existing lower crust, within the rift context that is employed here, which along with the upper crust was
95 °
90 °
85° 40°
insights
O
g
~
Vp~ 4.2-4.3
v,=2, ~o.o
]5° ~ 0 10
~
Vp = 6.1 6.3
20
U
vs = 3.6
30
D
vp= 8.s-o.8
~ V p
= 7.3-- 7.4 Vs = 4.0
E ~. a
Vp= 4.9-5,0 Vs=Z9 p = 2.60 k=O.O
50
p= Z= k = 0.038 Ys = 3JB p = 2.97 k = 0.013
p = 317 k=O0
Go
Vp = 8.0--8,1 VS = 4.6 k.O~
9(1°
Fig. 1. "G" gravity profile of Ervin and McGinnis [4] with the approximate limits of the Mississippi Valley Graben.
Fig. 2. Gravity-seismic velocity cross-section along the "G" profile modified from Austin and Keller [7]. Seismic velocities in km/s, densities in g/cm3 and magnetic susceptibilities in SI units.
117 domed upward during the early stages of rifting. There is no unanimity of opinion on the composition of the lower crust, and indeed it appears to be laterally and vertically inhomogeneous [16,17]. Xenolithic studies [18,19] plus upturned crustal sections, such as the Ivrea Zone [20,21], indicate that the lower crust is comprised of igneous intrusive rocks as well as amphibolite to granulite grade m e t a m o r p h i c rocks of variable composition. Smithson [17] proposes an average dioritic composition for the lower crust but emphasizes that diorite itself may not be important as an individual rock type. On the other hand, some proponents of an early global crust [22] for the earth speculate that early large-scale melting would produce specifically andesitic liquids due to high partial pressures of water in the melts [23,24]. The modeled P-wave velocity, 6.5-6.6 k m / s (Fig. 2), is indicative of an intermediate composition relative to a general range of 6.4-7.4 k m / s for the lower crust [16], and the P-wave and S-wave velocities are in agreement with velocities of 6.65 and 3.81 k m / s for a quartz diorite sample measured at 6 kbars [25] pressure. There is general agreement on the composition of the upper crust, layer IV; however in this area, mafic plutons, possibly intruded during the later Mesozoic [4] reactivation of the rift, border the Mississippi Valley Graben. One of these intrusions, the Covington pluton, is responsible for a local 30 mgal. Bouguer gravity high in the observed data along the profile [4]; this short-wavelength anomaly disappears, however, when integrated into the regional gravity model. Consideration of this local anomaly, though not justified in a regional two-dimensional model, has merit in the petrologic model, especially as these plutons appear to have expression in both aeromagnetic maps as well as in the Apparent Magnetization Contrast Map. Layers V and VI consist of sediments probably deposited during isostatic subsidence of the rifted region.
4. Magnetization profile The second step consists of assigning susceptibilities to the crustal layers as constrained by their
assumed rock types. Despite ranges of several orders of magnitude within a rock type [26,27], attempts have been made at estimating susceptibilities [28]. Values used in this study are best estimates or averages used in other modeling efforts with no attempt at modification to obtain a better fit between the modeled and the observed anomaly. Layer I (Fig. 2), the upper mantle, is assigned a susceptibility of 0.0 based on the study of Wasilewski et al. [29] in which peridotitic and eclogitic xenoliths representative to the upper mantle were found to have negligible magnetization. Layer II, the mantle-derived gabbroic intrusion, might nominally have a value of about 0.038 in SI units [27]; however, it has been shown [30,31] that varying degrees of oxidation of titanomagnetite solid solutions in plutonic rocks can produce a range of Curie temperatures from - 1 5 3 ° C , that of ulv6spinel, to 580 °C, for magnetite. Thus, the postulated layer II gabbro with a Curie temperature of less than 300 °C could have a susceptibility of 0.0 if emplaced at 30 km depth under any geothermal gradient larger than 1 0 ° C / k m . It should be noted that this suggestion differs from the von Frese et al. [6,32] alternative of a shallow Curie isotherm. Evidence above is suggestive of an intermediate, between granite and gabbro, rock type for layer III. A susceptibility of 0.013 in SI units, a value typical of rocks of andesitic composition [28,33], is used for layer III due primarily to a lack of magnetic data for intermediate plutonic rocks, especially diorites. A surface magnetics study [34] of the southeastern margin of the Mississippi Valley Graben gives good evidence for the susceptibility of layer IV, the upper crustal rocks. Within the graben, basement rocks have modeled susceptibilities typical of either granite, 0.0088 in SI units, or the near surface mafic plutons bordering the graben, 0.038 in SI units. A 0.038 susceptibility is used throughout layer IV as Bouguer gravity indicates that the Covington pluton is almost continuous across the Mississippi Valley Graben along the profile. This susceptibility is almost certainly high within the graben, but there is no way of knowing how much granite is present. Fig. 2 shows, however, that the thickness of layer IV, within the graben, is so small
118
Fig. 3. A section of an Apparent Magnetization Contrast Map [8] based on MAGSAT satellite magnetic anomaly data. Contour interval is 0.1 A/m.
ceptibility assigned to the proposed gabbroic body intruded into the lower crust in the area of the Mississippi Valley Graben. The data of Haggerty [30,31] demonstrate that unoxidized mafic plutonic rocks initially contain ulv6spinel-dominated titanomagnetite solid solutions. Unless subsequent oxidation exsolution or true exsolution occurs within the solid solutions, then the rocks can have very low Curie temperatures. Reported examples include Curie temperatures of approximately 2 5 0 ° C and 1 5 0 ° C [35] for an Oki Dogo, Japan, xenolithic gabbro and a xenolithic pyroxene garnet granulite from the Kilbourne Hole in the southern Rio Grande Rift, respectively; both samples are thought to have equilibrated under reducing conditions (lower oxygen fugacity than the fayalitemagnetite-quartz buffer). Plotted along the profile (Fig. 4) are magnetiza• tions from the M A G S A T Apparent Magnetization Contrast Map (Fig. 3) together with the modeled magnetizations. Two major discrepancies between the curves are readily apparent; a difference in the magnetization levels and a lack of correspondence of the magnetization minima in an east-west direction. The magnetization level problem is not im-
1.4
that this uncertainty will have little effect on the total magnetization. Layers V and VI consist of the upper two sedimentary layers of the Austin and Keller profile. Sediments routinely have low susceptibility values [26,28], and lacking information to the contrary, we assign a value of 0.0 to layers V and VI. The magnetization profile, normalized to the 40 km thickness of the M A G S A T Apparent Magnetization Contrast Map (Fig. 3), is then calculated as the sum of the products of the layer thicknesses and induced magnetizations (in a 40 A / m field) divided by 40 km.
1.2
1.0
The petrologically determined magnetization cross section depends heavily on a very low sus-
/
o.s !
# / I~I
A/m
0.6
iI
\
/
0.4
0.2
0.0
N.W. YELLVILLE
5. Results and discussion
jr. -~/ /
= 100
, 200
i 300
~ 400
= 500
i 600
S.E. SCOTTSBORO
Km
Fig. 4. MAGSATApparent Magnetization Contrast (solid line, values taken from contours Fig. 3) and model values (dashed line) along the profile. Model values calculated every 50 km.
119 mediately solvable; there exists a zero level uncertainty in all measured magnetic anomaly data [2], but magnetization contrasts of approximately - 0 . 5 4 , - 0 . 4 5 and - 0 . 6 2 A / m for the petrologic model, the von Frese et al. model and the Apparent Magnetization Contrast Map, respectively, are in reasonable agreement. The spatial disagreement between the magnetization minima can be attributed to resolution problems in the satellite data, or more importantly to the coincidence of the observed magnetization low with the Mississippi Valley Graben (Fig. 3) which has little expression in the regional gravity maps from which the crustal profile was constructed.
6. Conclusions Rifts, globally, appear to be associated with magnetic anomaly lows or highs. Magnetic highs, as exemplified by the Bangui [36] and Kentucky [37] paleorifts, are thought to be fairly well understood as are magnetic anomaly lows caused by shallow Curie isotherms as is the case with the Rio Grande Rift [38]. Less well understood are rifts such as that of the Mississippi E m b a y m e n t which have magnetic lows and possibly normal heat flow [39]. In order to understand better the Mississippi E m b a y m e n t low, a petrologic model was constructed and its magnetization evaluated. The best fit between the model and satellite magnetic anomaly daCa occurred with magnetized near-surface mafic plutons, normally magnetized lower crust and a non-magnetic mafic intrusion at the base of the crust. This type of intrusion in some other rifts has high modeled susceptibility and appears to be largely responsible for anomaly highs. It is suggested that the low magnetization for the Mississippi E m b a y m e n t intrusion could be a consequence of emplacement under non-oxidizing conditions such that the survival of titanomagnetite solid solutions results in a low Curie temperature. If the product of the local geothermal gradient times the depth of emplacement exceeds the reduced Curie temperature, then zero magnetization results. The somewhat higher magnetization
of the near-surface mafic plutons, such as the Covington pluton, may be due to lower temperatures at shallower depths. These results m a y have application to the magnetic anomaly lows of other paleorifts such as the South American Amazon and Takatu features [40] as well as other structures with high gravity and subdued magnetics such as the Scranton Gravity High [41].
Acknowledgements The author thanks Gene C. Ulmer and three anonymous reviewers for comments, particularly for pointing out my misinterpretation of a point in a referenced article. Discussions with H.V. Frey and M.A. Mayhew are also appreciated. The manuscript was typed by Barbara Conboy.
References 1 R.D. Regan, J.C. Cain and W.M. Davis, A global magnetic anomaly map, J. Geophys. Res. 80, 794-802, 1975. 2 R.A. Langel, J.D. Phillips and R.J. Hoerner, Initial scalar magnetic anomaly map from MAGSAT, Geophys. Res. Lett. 9, 269-272, 1982. 3 Tennessee Valley Authority, Southern Appalachian tectonic study, W.M. Seay, Task Force chairman, TVA, Knoxville, Tenn., 1979. 4 C.P. Ervin and L.D. McGinnis, Reelfoot rift: reactivated precursor to the Mississippi embayment, Geol. Soc. Am. Bull. 86, 1287-1295, 1975. 5 K. Burke and J.F. Dewey, Plume-generated triple junctions: key indicators in applying plate tectonics to old rocks, J. Geol. 81, 406-433, 1973. 6 R.R.B. von Frese, W.J. Hinz¢, L.W. Braile and A.J. Luca, Spherical-earth gravity and magnetic anomaly modeling by Gauss-Legendre Quadrature .Integration, J. Geophys. 49, 234-242, 1981. 7 C.B. Austin and G.R. Keller, personal communication, 1979. 8 M.A. Mayhew and S.C. Galliher, An equivalent layer magnetization model for the United States derived from MAGSAT data, Geophys. Res. Lett. 9, 311-313, 1982. 9 N.A. Logatchev, Main features of evolution and magmatism of continental rift zones in the Cenozoic, in: Tectonics and Geophysics of Continental Rifts, I.B. Ramberg and E.R. Neumann, eds., pp. 351-366, D. Reidel, Dordrecht, 1978. 10 P.G. Harris, A. Reay and I.G. White, Chemical composition of the upper mantle, J. Geophys. Res. 72, 6359-6369, 1967.
120 11 D.A. Carswell, Possible primary upper mantle peridotite in Norwegian basal gneiss, Lithos 1,322-355, 1968. 12 S.W. Stewart, Crustal structure in Missouri by seismic-refraction methods, Seismol. Soc. Am. Bull. 58, 291-323, 1968. 13 K. M c C a m y and R.P. Meyer, Crustal results of fixed multiple shots in the Mississippi embayment, in: The Earth beneath the Continents, J.S. Steinhart and T.J. Smith, eds., Am. Geophys. Union, Geophys. Monogr. 10, 370-381, 1966. 14 D.H. Green and A.E. Ringwood, The genesis of basaltic magmas, Contrib. Mineral. Petrol. 15, 103-190, 1967. 15 H.S. Yoder, Jr,, Generation of Basaltic Magma, National Academy of Sciences Printing and Publishing Office, Washington, D.C., 1976. 16 S.P. Smithson and S.K. Brown, A model for Lower Continental Crust, Earth Planet. Sci. Lett. 35, 134-144, 1977. 17 S. Smithson, Modeling continental crust: structural and chemical constraints, Geophys. Res. Lett. 5, 749-752, 1978. 18 R.R. McGetchin and L.T. Silver, A crustal-upper mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike, J. Geophys. Res. 77, 7022-7037, 1972. 19 E. Padovani and J. Carter, Aspects of the deep crustal evolution beneath south central New Mexico, in: The Earth's Crust, Its Nature and Physical Properties, J. Heacock, ed., Am. Geophys. Union, Geophys. Monogr. 20, 19-55, 1977. 20 K.R. Mehnert, The lvrea zone, a model of the deep crust, Neues Jahrb. MineraL~Abh. 125, 156-199, 1975. 21 D.M. Fountain, The'I~vrea-Verbano and Strona-Cenarizones northern Italy: a cross-section of continental c r u s t - - n e w evidence from seismic velocities of rock samples, Tectonophysics 33, 145-165, 1976. 22 P.D. Lowman, Jr., Crustal evolution in silicate planets: Implications for the origin of continents, J. Geol. 84, 1-26, 1976. 23 I. Kushiro, Melting of hydrous upper mantle and possible generation of andesitic magma: An approach from synthetic systems, Earth Planet. Sci. Lett. 22, 294-299, 1974. 24 B.O. Mysen and A.L. Boettcher, Melting of a hydrous mantle, II. Geochemistry of crystals and liquids formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen, and carbon dioxide, J. Petrol. 16, 549-593, 1975. 25 F. Press, Seismic velocities, in: Handbook of Physical Constants, S.P. Clark, Jr., ed., Geol. Soc. Am. Mem. 97, 195-218, 1966, rev. ed. 26 D.H. Lindsley, G.E. Andreasen and J.R. Balsey, Magnetic properties of rocks and minerals, in: Handbook of Physical
27
28
29
30 31 32
33 34
35
36 37
38
39
40
41
Constants, S.P. Clark, Jr., ed., Geol. Soc. Am. Mem. 97, 543-552, 1966, rev. ed. H.M. Mooney and R. Bleifuss, Magnetic susceptibility measurements in Minnesota, Part. II. Analysis of field results, Geophysics 8, 383-393, 1953. T. Nagata, Reduction of geomagnetic data and interpretation of anomalies, in: The Earth's Crust and Upper Mantle, P.J. Hart, ed., Am. Geophys. Union, Geophys. Monogr. 13, 391-398, 1969. P.J. Wasilewski, H.H. Thomas and M.A. Mayhew, The Moho as a magnetic boundary, Geophys. Res. Lett. 6, 541-544, 1979. S.E. Haggerty, The redox state of planetary basalts, Geophys. Res. Lett. 5, 443-446, 1978. S.E. Haggerty, The aeromagnetic mineralogy of igneous rocks, Can. J, Earth Sci. 16, 1281-1291, 1979. R.R.B. von Frese, Regional anomalies of the Mississippi River aulacogen (abstract), Presented at the Fifty-second Annual Meeting of the Society for Exploration Geophysicists, Dallas, TX Texas, 1982. B.D. Marsh, personal communication, 1980. T.G. Hildenbrand, Model of the southeastern margin of the Mississippi Valley Graben near Memphis, Tennessee, from interpretation of truck-magnetometer data, Geology 10, 476-480, 1982. P. Wasilewski and M.A. Mayhew, Crustal xenolith magnetic properties and long wavelength anomaly source requirements, Geophys. Res. Lett. 9, 329-332, 1982. R.D. Regan and B.D. Marsh, The Bangui magnetic anomaly: its geologic origin, J. Geophys. Res. 87, 1107-1120, 1982. M.A. Mayhew, H.H. T h o m a s and P.J. Wasilewski, Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee, Earth Planet. Sci. Left. 58, 395-405, 1982. M.A. Mayhew, Application of satellite magnetic anomaly data to Curie isotherm mapping, J. Geophys. Res. 87, 4846-4854, 1982. C.A. Swanberg, B.J. Mitchell, R.L. Lohse and D.D. Blackwell, Heat flow in the upper Mississippi Embayment, in: Investigations of the New Madrid, Missouri Earthquake Region, F.A. McKeown and L.C. Pakiser, eds., U.S. Geol. Surv. Prof. Pap. 1236, 185-189, 1982. M.B. Longacre, W.J. Hinze and R.R.B. yon Frese, A satellite magnetic model of northeastern South American aulacogens, Geophys. Res. Lett. 9, 318-321, 1982. W.H. Diment, T.C. U r b a n and F.A. Revetta, Some geophysical anomalies in the eastern United States, in: The Nature of the Solid Earth, E.C. Robertson, ed., pp. 544-572, McGraw-Hill, New York, N.Y., 1972.
115
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
[4]
Petrologic model of the northern Mississippi Embayment based on satellite magnetic and ground-based geophysical data Herman H. Thomas Geophysics Branch, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 (U.S.A.)
Received July 25, 1983 Revised version received April 24, 1984
A petrologic model of the northern Mississippi Embayment, derived from gravity, seismic and rift data, is evaluated by converting the model to a magnetization model which is compared with satellite magnetic anomaly models. A magnetization contrast of approximately - 0 . 5 4 A / m , determined from the petrologic model of the embayment compares favorably to values of - 0.62 A / m and - 0.45 A / m from a M A G S A T United States Apparent Magnetization Contrast Map and a published P O G O magnetization contrast model, respectively. The petrologic model suggests that the magnetic anomaly low associated with the Mississippi E m b a y m e n t m a y be largely due to the intrusion under non-oxidizing conditions of low Curie temperature gabbroic material at the base of the crust of the embayment. Near-surface mafic plutons, bordering the Mississippi Valley Graben, appear from aeromagnetic data to have higher magnetizations than the deeper gabbroic material; however, it is impossible to ascertain if this is due to compositional differences or similar material at shallower (lower temperature) depths. These results indicate that variations in the Curie temperatures of intrusions accompanying rifting m a y account for a large part of the wide range of magnetic anomalies associated with presently inactive rifts with normal heat flow.
1. Introduction
Both P O G O [1] and M A G S A T [2] anomaly maps show the magnetic anomaly low as seen by aeromagnetic surveys [3] over the Mississippi Embayment. The feature is thought to be a reactivated Precambrian rift [4] or the failed arm of a late Paleozoic triple junction associated with the opening of the Gulf of Mexico [5]. In a recent paper, von Frese et al. [6] completed preliminary modeling of the Mississippi Embayment P O G O magnetic anomaly using the Austin and Keller [7] modification of a regional geophysical analysis by Ervin and McGinnis [4]. A proposed mantle-derived intrusion at the base of the crust was modeled so as to explain the satellite magnetic low as well as a regional gravity high. In this study, the same Austin and Keller [7] profile, used by von Frese et al. [6], becomes the starting point for a somewhat different approach, 0012-821X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
the purpose of which is to obtain a petrologic interpretation directed toward an understanding of the extreme variation in rift-produced magnetic anomalies. Density-seismic velocity data from the crustal profile is used to determine the rock types that make up the layers in the profile. Magnetic susceptibilities, typical of values for each rock layer, are used to form a magnetization profile which is then compared to an Apparent Magnetization Contrast Map [8] derived from satellite anomaly data. The Apparent Magnetization Contrast Map is a model of the anomaly field which is constructed by determining magnetic moments for dipoles in an equal area grid at the earth's surface such that the resulting field results in a least squares best fit to the anomaly field. The map, despite being an additional step removed from the actual measurements, offers closer geographic coincidence between the anomalies and their source regions than do satellite anomaly maps [8].
116 2. C r u s t a l p r o f i l e
3. P e t r o l o g i c
The Ervin and McGinnis [4] gravity profile (Fig. 1) extends from Yellville, Arkansas southeastward to Scottsboro, Alabama. It crosses the Mississippi E m b a y m e n t and the Mississippi Valley G r a b e n which lies within the northern part of the embayment. The Ervin and McGinnis model was modified by Austin and Keller [7] as the result of an analysis of group and phase velocity dispersion of Rayleigh waves recorded near St. Louis, Missouri and Oxford," Mississippi. The Austin and Keller cross-section (Fig. 2) is used in this study because of the additional seismic constraint and because it has one less crustal layer. In addition, its use allows a comparison to the magnetization model of von Frese et al. [6].
The first step in constructing a magnetization model from the crustal profile (Fig. 2) is to convert the density-seismic velocity layers to rock layers. Neither densities nor seismic velocities uniquely define rock compositions; they can however, along with other data, provide constraints on rock types. The crustal profile plus geophysical data from Ervin and McGinnis along with rift petrology generalizations, particularly those of Logatchev [9], form the basis for the conversion in this discussion. Both seismic velocities and density (Fig. 2) are consistent with an interpretation that layer I is the upper mantle which is thought to be made up largely of peridotites with local eclogites [10,11]. Based on a seismic refraction profile of nearby normal craton [12] compared with a profile of the Mississippi E m b a y m e n t [13], Ervin and McGinnis postulated that layer II was "anomalous" low-density mantle material. This description is consistent with that of a basaltic partial melt from the mantle [14,15]; layer II would then be gabbro, the plutonic equivalent of basalt. Layer III is assumed to be the pre-existing lower crust, within the rift context that is employed here, which along with the upper crust was
95 °
90 °
85° 40°
insights
O
g
~
Vp~ 4.2-4.3
v,=2, ~o.o
]5° ~ 0 10
~
Vp = 6.1 6.3
20
U
vs = 3.6
30
D
vp= 8.s-o.8
~ V p
= 7.3-- 7.4 Vs = 4.0
E ~. a
Vp= 4.9-5,0 Vs=Z9 p = 2.60 k=O.O
50
p= Z= k = 0.038 Ys = 3JB p = 2.97 k = 0.013
p = 317 k=O0
Go
Vp = 8.0--8,1 VS = 4.6 k.O~
9(1°
Fig. 1. "G" gravity profile of Ervin and McGinnis [4] with the approximate limits of the Mississippi Valley Graben.
Fig. 2. Gravity-seismic velocity cross-section along the "G" profile modified from Austin and Keller [7]. Seismic velocities in km/s, densities in g/cm3 and magnetic susceptibilities in SI units.
117 domed upward during the early stages of rifting. There is no unanimity of opinion on the composition of the lower crust, and indeed it appears to be laterally and vertically inhomogeneous [16,17]. Xenolithic studies [18,19] plus upturned crustal sections, such as the Ivrea Zone [20,21], indicate that the lower crust is comprised of igneous intrusive rocks as well as amphibolite to granulite grade m e t a m o r p h i c rocks of variable composition. Smithson [17] proposes an average dioritic composition for the lower crust but emphasizes that diorite itself may not be important as an individual rock type. On the other hand, some proponents of an early global crust [22] for the earth speculate that early large-scale melting would produce specifically andesitic liquids due to high partial pressures of water in the melts [23,24]. The modeled P-wave velocity, 6.5-6.6 k m / s (Fig. 2), is indicative of an intermediate composition relative to a general range of 6.4-7.4 k m / s for the lower crust [16], and the P-wave and S-wave velocities are in agreement with velocities of 6.65 and 3.81 k m / s for a quartz diorite sample measured at 6 kbars [25] pressure. There is general agreement on the composition of the upper crust, layer IV; however in this area, mafic plutons, possibly intruded during the later Mesozoic [4] reactivation of the rift, border the Mississippi Valley Graben. One of these intrusions, the Covington pluton, is responsible for a local 30 mgal. Bouguer gravity high in the observed data along the profile [4]; this short-wavelength anomaly disappears, however, when integrated into the regional gravity model. Consideration of this local anomaly, though not justified in a regional two-dimensional model, has merit in the petrologic model, especially as these plutons appear to have expression in both aeromagnetic maps as well as in the Apparent Magnetization Contrast Map. Layers V and VI consist of sediments probably deposited during isostatic subsidence of the rifted region.
4. Magnetization profile The second step consists of assigning susceptibilities to the crustal layers as constrained by their
assumed rock types. Despite ranges of several orders of magnitude within a rock type [26,27], attempts have been made at estimating susceptibilities [28]. Values used in this study are best estimates or averages used in other modeling efforts with no attempt at modification to obtain a better fit between the modeled and the observed anomaly. Layer I (Fig. 2), the upper mantle, is assigned a susceptibility of 0.0 based on the study of Wasilewski et al. [29] in which peridotitic and eclogitic xenoliths representative to the upper mantle were found to have negligible magnetization. Layer II, the mantle-derived gabbroic intrusion, might nominally have a value of about 0.038 in SI units [27]; however, it has been shown [30,31] that varying degrees of oxidation of titanomagnetite solid solutions in plutonic rocks can produce a range of Curie temperatures from - 1 5 3 ° C , that of ulv6spinel, to 580 °C, for magnetite. Thus, the postulated layer II gabbro with a Curie temperature of less than 300 °C could have a susceptibility of 0.0 if emplaced at 30 km depth under any geothermal gradient larger than 1 0 ° C / k m . It should be noted that this suggestion differs from the von Frese et al. [6,32] alternative of a shallow Curie isotherm. Evidence above is suggestive of an intermediate, between granite and gabbro, rock type for layer III. A susceptibility of 0.013 in SI units, a value typical of rocks of andesitic composition [28,33], is used for layer III due primarily to a lack of magnetic data for intermediate plutonic rocks, especially diorites. A surface magnetics study [34] of the southeastern margin of the Mississippi Valley Graben gives good evidence for the susceptibility of layer IV, the upper crustal rocks. Within the graben, basement rocks have modeled susceptibilities typical of either granite, 0.0088 in SI units, or the near surface mafic plutons bordering the graben, 0.038 in SI units. A 0.038 susceptibility is used throughout layer IV as Bouguer gravity indicates that the Covington pluton is almost continuous across the Mississippi Valley Graben along the profile. This susceptibility is almost certainly high within the graben, but there is no way of knowing how much granite is present. Fig. 2 shows, however, that the thickness of layer IV, within the graben, is so small
118
Fig. 3. A section of an Apparent Magnetization Contrast Map [8] based on MAGSAT satellite magnetic anomaly data. Contour interval is 0.1 A/m.
ceptibility assigned to the proposed gabbroic body intruded into the lower crust in the area of the Mississippi Valley Graben. The data of Haggerty [30,31] demonstrate that unoxidized mafic plutonic rocks initially contain ulv6spinel-dominated titanomagnetite solid solutions. Unless subsequent oxidation exsolution or true exsolution occurs within the solid solutions, then the rocks can have very low Curie temperatures. Reported examples include Curie temperatures of approximately 2 5 0 ° C and 1 5 0 ° C [35] for an Oki Dogo, Japan, xenolithic gabbro and a xenolithic pyroxene garnet granulite from the Kilbourne Hole in the southern Rio Grande Rift, respectively; both samples are thought to have equilibrated under reducing conditions (lower oxygen fugacity than the fayalitemagnetite-quartz buffer). Plotted along the profile (Fig. 4) are magnetiza• tions from the M A G S A T Apparent Magnetization Contrast Map (Fig. 3) together with the modeled magnetizations. Two major discrepancies between the curves are readily apparent; a difference in the magnetization levels and a lack of correspondence of the magnetization minima in an east-west direction. The magnetization level problem is not im-
1.4
that this uncertainty will have little effect on the total magnetization. Layers V and VI consist of the upper two sedimentary layers of the Austin and Keller profile. Sediments routinely have low susceptibility values [26,28], and lacking information to the contrary, we assign a value of 0.0 to layers V and VI. The magnetization profile, normalized to the 40 km thickness of the M A G S A T Apparent Magnetization Contrast Map (Fig. 3), is then calculated as the sum of the products of the layer thicknesses and induced magnetizations (in a 40 A / m field) divided by 40 km.
1.2
1.0
The petrologically determined magnetization cross section depends heavily on a very low sus-
/
o.s !
# / I~I
A/m
0.6
iI
\
/
0.4
0.2
0.0
N.W. YELLVILLE
5. Results and discussion
jr. -~/ /
= 100
, 200
i 300
~ 400
= 500
i 600
S.E. SCOTTSBORO
Km
Fig. 4. MAGSATApparent Magnetization Contrast (solid line, values taken from contours Fig. 3) and model values (dashed line) along the profile. Model values calculated every 50 km.
119 mediately solvable; there exists a zero level uncertainty in all measured magnetic anomaly data [2], but magnetization contrasts of approximately - 0 . 5 4 , - 0 . 4 5 and - 0 . 6 2 A / m for the petrologic model, the von Frese et al. model and the Apparent Magnetization Contrast Map, respectively, are in reasonable agreement. The spatial disagreement between the magnetization minima can be attributed to resolution problems in the satellite data, or more importantly to the coincidence of the observed magnetization low with the Mississippi Valley Graben (Fig. 3) which has little expression in the regional gravity maps from which the crustal profile was constructed.
6. Conclusions Rifts, globally, appear to be associated with magnetic anomaly lows or highs. Magnetic highs, as exemplified by the Bangui [36] and Kentucky [37] paleorifts, are thought to be fairly well understood as are magnetic anomaly lows caused by shallow Curie isotherms as is the case with the Rio Grande Rift [38]. Less well understood are rifts such as that of the Mississippi E m b a y m e n t which have magnetic lows and possibly normal heat flow [39]. In order to understand better the Mississippi E m b a y m e n t low, a petrologic model was constructed and its magnetization evaluated. The best fit between the model and satellite magnetic anomaly daCa occurred with magnetized near-surface mafic plutons, normally magnetized lower crust and a non-magnetic mafic intrusion at the base of the crust. This type of intrusion in some other rifts has high modeled susceptibility and appears to be largely responsible for anomaly highs. It is suggested that the low magnetization for the Mississippi E m b a y m e n t intrusion could be a consequence of emplacement under non-oxidizing conditions such that the survival of titanomagnetite solid solutions results in a low Curie temperature. If the product of the local geothermal gradient times the depth of emplacement exceeds the reduced Curie temperature, then zero magnetization results. The somewhat higher magnetization
of the near-surface mafic plutons, such as the Covington pluton, may be due to lower temperatures at shallower depths. These results m a y have application to the magnetic anomaly lows of other paleorifts such as the South American Amazon and Takatu features [40] as well as other structures with high gravity and subdued magnetics such as the Scranton Gravity High [41].
Acknowledgements The author thanks Gene C. Ulmer and three anonymous reviewers for comments, particularly for pointing out my misinterpretation of a point in a referenced article. Discussions with H.V. Frey and M.A. Mayhew are also appreciated. The manuscript was typed by Barbara Conboy.
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