Lithospheric aging, instability and subduction

Tectonophysics, 32 (1976) 331-351 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

LITHOSPHERIC

AGING, INSTABILITY

AND SUBDUCTION

N.J. VLAAR and M.J.R. WORTEL Vening

Meinesz

(Submitted

Laboratory,

State

University

of Utrecht,

Utrecht

(The

Netherlands)

October 3, 1975; revised version accepted February 27, 1976)

ABSTRACT Vlaar, N.J. and Wortel, M.J.R., tonophysics, 32: 331-351.

1976. Lithospheric

aging, instability and subduction.

Tec-

The subduction behaviour of oceanic lithosphere in relation to its age is studied in detail. It is shown that the penetration depth of subducted lithosphere increases with increasing lithospheric age. In all cases where sufficient data are available, the relation proves to be unique. The controlling property appears to be the amount of gravitational instability of the part of the lithosphere concerned with respect to the surrounding upper mantle. The instability depends, through the density and temperature, on the time elapsed between creation and subduction. It is concluded that gravitational instability of the oceanic lithosphere-upper mantle system is a major cause of plate tectonics. The structure of individual subduction zones is interpreted accordingly.

INTRODUCTION

It is generally accepted that a convective process is responsible for plate tectonics. There is no general agreement, however, concerning the specific nature of the mechanism. Common in all convection theories is the requirement of gravitational instability, due to temperature differences, maintaining the flow of mass. Apart from cubic expansion or contraction under the influence of temperature differences, phase transitions must also have a pronounced effect on gravitational instability. The gravitational instability as such, has been held responsible by several investigators as the driving force of plate tectonics. Horizontal instability resulting in gravitational sliding of the lithosphere off the crests of oceanic ridges has been proposed a.o. by Jacoby (1970). Press (1973) deduces from the inversion of seismic data that a density inversion exists in the upper mantle and demonstrates that ample gravitational

332

energy is available in the upper mantle-lithosphere system for driving the motion of the plates. In a recent Paper (Vlaar, 1975) a semi-quantitative model for the dri.ving mechanism has been proposed. It was corroborated that the plate motion is maintained by horizontal instability resulting in gravitational sliding off the ridges, thus creating a vertical instability under the spreading ridge axes where hot mantle material rises to create new oceanic lithosphere and part of the upper mantle. Vertical instability is also manifest in some subduction zones where cooled lithosphere is consumed by the upper mantle again. A crucial role in the convective mechanism is played by the cooling of the lithospheric plate after its creation. The lithosphere cools by heat flow through the ocean bottom. The loss of heat is reflected by the ocean-bottom topography which has been shown to be controlled by thermal contraction (Sclater and Francheteau, 1970; Mater et al., 1971). The possible occurrence of phase transitions upon cooling of the lithosphere has been proposed by Forsyth and Press (1971). Yoshii (1975) used surface waves to arrive at a zoned structure of the Pacific Ocean lithosphere, the zoning depending on age. From these studies we may derive an estimate of 70 m.y. for the age at which the oceanic lithosphere reaches a steady-state structure. We propose that another effect of the cooling must be that, on aging, the vertical instability changes sign. The lithosphere near the ridges is stably stratified with respect to the upper mantle, whereas by subsequent cooling it must become increasingly unstable. As the time elapsed since the creation of the lithosphere is the main controlling parameter for the cooling process, the state of vertical instability must also depend on this time, and hence must be a function of only the age of the part of lithosphere involved. Through the spreading rate, this age depends on the distance from the spreading axis. The increasing instability with time may result in subduction. In the older parts of the oceanic lithosphere this may be manifest by downbuckling and subsequent under- and overthrusting of oceanic plates. It is more usual that subduction takes place at the contact of an oceanic and a continental plate. The vertical motion of the downgoing slab and the structure of the subduction zone is controlled mainly by buoyancy forces resulting from the density contrast of the slab material and the surrounding mantle material, which, in turn, is directly related to the time elapsed since the creation of the relevant part of the lithosphere concerned. In the present paper we will demonstrate that a unique relationship exists between the sinking history of a subducted oceanic lithosphere and its age. In this context the relevant age of a subducted part of a lithospheric plate is the age it had at the beginning of its descent, which equals the length of the period during which it has been cooling. Hereafter, this age is also referred to as the age of the corresponding subduction zone. Age determinations are inferred from published studies of magnetic lineations and from the results of the Deep Sea Drilling Project. Our data on the structure of the various subduction zones are entirely based on the results of numerous studies of seismicity and focal mechanisms.

333 The deepest earthquakes in a subduction zone are taken to indicate the maximum depth reached by portions of lithosphere that have retained their brittle identity at least to such a degree that earthquakes can be generated. Though it is not possible to follow the flow of material which before constituted the lithospheric plate, it appears to be reasonable to assume that resorption results in assimilation. As, in our approach, gravitational instability and associated buoyancy forces determine the vertical subduction behaviour, we feel confident to state that the deepest earthquakes determine the level at which the plate assumes gravitational stability. For convenience in the following discussion we will briefly summarize some current hypotheses which bear upon the structure of subduction zones. Isacks et al, (1968) found an approximately linear relationship between the down-dip length of seismic zones and convergence rates of lithospheric plates (normal to the plate contact) as computed by Le Pichon (1968). They put forward two hypotheses in order to explain this relationship. In the first hypothesis it is assumed that the seismic zones were created during the current cycle of sea-floor spreading and underthrusting, for which a duration of 10 m.y. was derived. Thus, in this case the deepest part of a seismic zone is thought to be the leading edge of a subducted plate. In the second hypothesis, this period of 10 m.y. is regarded as the approximate time constant for assimilation of the subducted oceanic lithosphere into the upper mantle. In both hypotheses the down-dip length of a seismic zone equals the amount of unde~h~sting that has taken place during the last 10 m.y. Deffeyes (1972) found a linear relationship between the age of the sea floor and the resorp tion time for a number of subduction zones. In order to account for the dip of a descending plate, Luyendyk (1970) proposed the relation: dip = sin-l

(u,/+)

where u, and ug are the velocities of the downward and converging motions respectively, averaged over the appropriate period of plate consumption. Luyendyk suggests that differences in the dip of the plates, descending in the earth’s subduction zones, are mainly caused by differences in the convergence rates Ue. Along the same lines he attributes differences within a zone to lateral variations in the component Ue, due to deferential distances from the pole of rotation involved. Many numerical investigations have been made on the temperature distribution and the stress field in a subducted plate and the surrounding upper mantle (e.g. McKenzie, 1969; Toksiiz et al., 1971; Smith and Tokdz, 1972; Toksijz et al., 1973). They usually start from an assumed geometry and are directed towards getting insight into physical phenomena related to subduction, rather than into the primary causes of the subduction process itself. In summary it can be said that k~emati~al aspects have been emphasized and that with some exceptions (Deffeyes, 1972; Truchan and Larson, 1973; Uyeda and Miyashiro, 1974; Forsyth, 1975) the possible age-dependent nonuniformities that may have been present in the portions of oceanic litho-

Fig. 1. Subduction zones are indicated by the thick barbed lines, The underlined zones are included in the classification (see Table I). References from which data on the tectonic setting and the structure of the subduction zones have been taken are: Tonga-Kermadec, Sykes (1966), Isacks et al. (1969); Indonesia-Philippines, Fitch (1970), Fitch and Molnar (1970); Kuriles, Sykes (1966), Bulletin of the International Seismological Centre (event: Sea of Okhotsk, Aug. 30, 1970); Honshu, Isacks and Molnar (1971); MarianasIzu Bonin-Ryukyus, Katsumata and Sykes (1969); Aleutians-Alaska, Tobin and Sykes (1966), Stauder (1968); Western North America, Tobin and Sykes (1968), Silver (1971); Central America-Mexico-Lesser Antilles, Molnar and Sykes (1969); Peru-Chile, Ocala (1966), Santa (1969), Stauder (1973), Swift and Carr (1975); South Sandwich, Forsyth (1975); New Zealand, Hamilton and Gale (1968), Smith (1971), Scholz et al. (1973); New Britain-Solomon Islands-New Hebrides, Denham (1969), Johnson and Molnar (1972). Moreover, Rothd (1969) has been consulted as a general reference.

sphere at the time they began to be underthrust, have been neglected in the study of subduction zones. In this study it proved to be helpful to divide the earth’s subduction zones into age classes. The following classes were selected: class 1: older than 70 m.y., class 2: 40-70 m.y., class 3: O-40 m.y., with a subdivision into 3A (15-40 m.y.) and 3B (O-15 m.y.). For all seismic zones in Fig. 1 it has been shown in studies of seismicity, focal mechanisms and sea-floor spreading that they mark sites where ocean floor is being reabsorbed into the mantle. Table I shows the classification of the subduction zones and the references used in the age determinations. Within a few zones a subdivision is made into

335 TABLE

I

Classification of subduction

zones

Age class

Subduction

1. (>70 m.y.)

Kuriles

zone

Honshu Izu Bonin Marianas Java Tonga

2. (40-70

m.y.)

Kermadec Aleutians + Alaska Central America (85-95OW) Peru-Northern Chile (north of 36%) Sumatra

3A. (15-40

3B. (O-15

m.y.)

m.y.)

South Sandwich (south of 58’S) Central-North Chile (36-42’S) Central-South Chile (42-46’S) Western North America (40--5O’N) Mexico ** (95-lo5°w)

References

*

Larson and Chase (1972); Larson et al. (1973) Larson and Chase (1972); Larson et al. (1973) Heezen et al. (1972) Winterer et al. (1969); Heezen et al. (1972) Veevers et al. (1973) Burns et al. (197 2); Larson and Chase (1972) Larson and Chase (1972) Peter et al. (1970); Taylor and O’Neill (1974) Herron (1972) Herron (1972);

Hart et al. (1974)

Sclater and Fisher (1974); et al. (1972) Forsyth (1975)

Von der Borch

Herron and Hayes (1969) Klitgord et al. (1973) Atwater (1970) Sclater et al. (1971);

Herron (1972)

* References from which the age data used in the classification have been taken. The geomagnetic reversal-time scale of Heirtzler et al. (1968) and its modification and extensions by Talwani et al. (1971), Larson and Pitman (1972) and Larson and Hilde (1975) are used throughout this paper. ** The Mexican zone comprises both class 3A and 3B; however, the age data do not allow a subdivision.

two or three parts because significant age differences can be distinguished. Only subduction zones of which the tectonic setting and the age are adequately known, are considered in the classification. This implies the exclusion of several seismic zones directly associated with complicated inland or marginal seas: e.g. the Ryukyu and Mindanao zones along the western boundary of the Philippine Sea, The Tyrrhenian and Aegean zones in the Mediterranean and the New Guinea, New Britain, Solomon islands and New Hebrides zones at the boundaries of the Solomon Sea and the Coral Sea.

336 ANALYSIS

AND RESULTS

Class 1

This class (see Table I) comprises nearly all well-known deep earthquake zones. Their average dips and maximum focal depths are summarized in Fig. 2. The maximum focal depths are in the range 580-685 km. Of the two hypotheses (see introduction) advanced by Isacks et al. (1968), the former, in which the 10 m.y. period is considered to be the duration of the present episode of sea-floor spreading and subduction, requires a worldwide interruption of sea-floor spreading just prior to 10 m.y. B.P. Extensive mapping of magnetic anomalies and data from deep sea drilling (e.g., Maxwell et al., 1970; Pitman and Talwani, 1972) do not provide evidence for such a worldwide phenomenon. The latter hypothesis, in which the 10 m.y. period is taken to be an approximation to the time required for resorption of the cold underthrust slab, is the more acceptable (Oliver et al., 1973). If we take into account the refinement made by Deffeyes (1972) concerning the agedependency of the resorption time, a relation between down-dip length of a seismic zone and the convergence rate is indeed apparent. Such a relation, however, does not give insight in the vertical motion of subducted oceanic lithosphere. Luyendyk (1970) investigated the correlation between the convergence rate and the dip of the Tonga, Kermadec, Java and Kurile zones. The data could be explained by his dip-rate relationship, if an average sinking rate V, between 4 and 6 cm/year was assumed. Thus, after approximately the same amount of cooling the slabs in the class-l zones appear to sink to depths in the small range 580-685 km at

Fig. 2. Schematic represenation of the subduction zones of age class 1. The lines show the average dips and the maximum focal depths of the zones. References from which data are taken are listed in the caption of Fig. 1.

337

about the same rate. This is considered as strong evidence for the dominating effect of the gravitational instability of the slabs on their vertical motion. To the Kermadec and Tonga zones we could only assign minimum ages of 90 m.y. and 100 m.y., respectively. As the other zones in this class are older than 90 m.y., it appears that we lack data in the interval 70- 90 m.y. Class 2 In the classification of these four zones (see Table I), several complications in the spreading history of the ocean floor involved were encountered. In the east-central Pacific a reorganization of spreading centres has taken place during the Cenozoic era (Herron, 1972; Mammerickx et al., 1975). The age of the ocean floor near the Aleutian and Sumatran trenches increases in the direction towards the adjacent ocean basins (Peter et al., 1970; Sclater

ANTARCTIC

*oL-160

100

PLATE

90

80

70

Fig. 3. The pattern of lithospheric plates in the east-central Pacific. Double lines indicate the location of active spreading centres (after Stover, 1973). The hatched double bars show the approximate location of crest segments of the extinct ridge (after Herron, 1972). Numbers indicate approximate ages (in m.y.) of the oceanic lithosphere (Herron, 1972). The dotted lines show the boundary between lithosphere created at the present East Pacific Rise and that generated at the extinct ridge crest. A-A’ and B-B’ indicate the locations of the sections shown in Figs. 4 and 6, respectively. a: Tehuantepec Ridge; b: Cocos Ridge ; c : Nazca Ridge.

338

.

I

Fig. 4. Projection of hypocentres of events that occurred in the Middle America seismic region during the g-years period 1965-1973, onto a vertical plane approximately parallel to the trench (line A-A’ in Fig. 3). The hypocentres were taken from the PDE (Preliminary Determination of Epicenters) data file of the National Earthquake Information Center of the U.S. Geological Survey (formerly NOAA and USCGS). Only hypocentres deeper than 60 km were plotted. The Tehuantepec Ridge intersects the trench between 95” W and 96” W.

and Fisher, 1974). This has been taken into account in “dating” the corresponding subduction zones. We will briefly describe some features of the individual zones. The structure of the Aleutian-Alaskan subduction zone varies strongly along the strike of the zone (Tobin and Sykes, 1966; Rothe, 1969; Cormier, 1975). In the western part of the Aleutian arc the variations may well be attributed to the rapid change in direction of motion of the Pacific Ocean floor relative to the arc (McKenzie and Parker, 1967). Herron’s (1972) data indicate that the central and northeastern part of the Cocos plate is older than the western and southern edges. This is clearly reflected in the topography of the Guatemala Basin and in the heat-flow values (Anderson, 1974). The Tehuantepec Ridge which intersects the trench near 95” W (see Fig. 3) delineates the boundary between the Guatemala Basin and the younger northwestern part of the Cocos plate. The part of the Middle American zone west of the Tehuantepec Ridge which has a maximum age of

Fig. 5. Vertical section oriented parallel to the Sumatra and Java seismic zones. Data were taken from the PDE-file (see caption of Fig. 4) for the, period 1965-1973 and from Rothe (1969) for the years 1953-1964. Only hypocentres deeper than 60 km were plotted.

339

approximately 20 m.y. near 95”W, falls under class 3 and will be called the Mexican zone. The remaining part east of 95”W is a member of class 2 and will be referred to as the Central American zone. Figure 4 demonstrates that the deepest foci (maximum 275 km) are found east of the Tehuantepec Ridge, where the oldest lithosphere of the Cocos plate is descending. A correlation between lithospheric age and focal depths in this seismic region has previously been suggested by Truchan and Larson (1973). Figure 5 shows a striking contrast in maximum focal depths between the Sumatra zone (class 2) and the Java zone (class 1). A change in the rate or direction of plate convergence cannot readily account for this very abrupt change (Fitch and Molnar, 1970; Fitch, 1970). The most characteristic feature of the Peru-Northern Chile zone is a pronounced gap in seismic activity at depths between 300 km and 525 km. Deep seismic activity occurs in some parts of the zone at depths between 525 km and 660 km (&ala, 1966; Santa, 1969; Stauder, 1973; Swift and Carr, 1975). With the exception of the Pea-Noshes Chile zone all deep seismic activity, considered in our age classes, occurs in subduction zones where old Mesozoic lithosphere is consumed (class 1). At present the oceanic lithosphere at the Peru-Chile trench (north of 36”s) is only 45-53,m.y. old (see Fig. 3). Analysis of directions and rates of spreading and plate convergence, taken from Herron (1972) and Minster et al. (1974), revealed that during the last 10 m.y. this zone has evolved from a class-l zone to a class-2 zone. The subducted lithosphere above the gap began to descend at an age between 45 m.y. and approximately 70 m.y. The deeper parts were over 70 m.y. of age at the initiation of their descent (class 1). , With the incorporation of the transitional situation in the Peru-Northern Chile zone it follows from the references cited in Fig. 1 that the subduction zones of class 2 have maximum focal depths in the range 175-300 km. A short duration of the present period of undert.hrusting relative to that for the deep earthquake zones is not a satisfactory explanation for the limited penetration depth of the subducted class-2 lithosphere, because it is not consistent with evidence from volcanic activity on land and from sea-floor spreading (see Herron (1972) and Malfait and Dinkelman (1972, p. 258) for the Central American zone and Katili (1973) for Sumatra). If, after Deffeyes (1972), the resorption time is taken to be the down-dip length of a seismic zone divided by the subduction rate, then, with the rates of Minster et al, (1974), we find resorption times shorter than 9 m,y. for the class-2 lithosphere. As these are shorter than those for older subducted lithosphere (Deffeyes, 1972), this is evidence for age-dependent subduction behaviour. However, the Peruvian zone provides evidence for the possibility that the maximum penetration depth is not determined by resorption, Although the upper part of the under-thrust slab (i.e. above the gap) has a length of about 800 km, it apparently does not sink to depths greater than 200-250 km.

This is explained by the limited initial density contrast, inherent to its age.

340

Therefore, characteristic

we consider the established depth range of 175-300 for subducted lithosphere of class 2.

km to be

Class 3 Except for the South Sandwich arc, the class-3 zones appear to be located in regions when the North and South American plates approach active spreading centres. The age of the oceanic lithosphere being underthrust in the Central Chile zone is indicated in Fig. 3. A subdivision is made into two latitude ranges (see Table I). A projection of hypocentres of earthquakes that occurred in this region during the years 1965-1973, onto a vertical north--south trending plane is shown in Fig. 6. The limited penetration depth and the small down-dip length of the subducted lithosphere (see cross-sections in Stauder (1973) and Swift and Carr (1975)) cannot be explained by assuming that Central Chile is in a youthful stage of plate convergence. Vergara and Munizaga (1974) showed that Central Chile has been a plate-contact zone probably since Mesozoic but certainly since Miocene times. Since the pole of the relative motion between the Nazca plate and the South American plate, as determined by Minster et al. (1974) at 52” N 91” W is approximately 90” away from Central Chile, the structural change shown in Fig. 6 near 42”s cannot be attributed to a variation in relative plate motions. Figure 6 demonstrates that the offset in the Chile Ridge (see Fig, 3) resulting in a lithospheric age difference at the plate contact, is clearly reflected in the seismicity of the area. The situation in Western North America (40” N-50” N) where the Juan de Fuca plate is consumed beneath the continent (Atwater, 1970; Silver, 1971), is very similar to that in Central-South Chile. Forsyth (1975) described the tectonic situation along the South Sandwich arc where also an offset in a spreading ridge crest is involved, resulting in lithospheric age differences at the trench. In the southern part (south of

Fig. 6. Projection of hypocentres of events that occurred in the Chilean seismic zone (between 33”s and 50”s) during the period 1965-1973, onto a north-south trending vertical plane (line B-B’ in Fig. 3). Hypocentres were taken from the PDE-file (see caption of Fig. 4). The eastward prolongation of the offset in the Chile Ridge intersects the Chilean coast near 42” S.

341 58” S) the maximum focal depth is 150 km and the intermediate earthquakes indicate down-dip compression, whereas in the northern part the maximum focal depth is 180 km and the intermediate earthquakes indicate down-dip extension. We agree with Forsyth who explained this difference in terms of gravitational body forces dependent on lithospheric age. With the exclusion of the Mexican zone, we can subdivide class 3 into class 3A covering the subduction zones where lithosphere between 15 m.y. and 40 m.y. of age is being underthrust, and class 3B covering the range O15 m.y. The former class, then, includes the Central-North Chile zone (36” S-42’S) and the South Sandwich zone (south of 58” S). Their maximum focal depths span the range 150-185 km. Also the northwestern end of Sumatra may probably be added to this small class. If this would be correct, it would set the lower limit at 130 km. The latter class includes the Central-South Chile zone (42”S-46”s) and the contact between the Juan de Fuca plate and the North American plate. Their maxima are 120 km and 103 km, respectively. The Mexican zone, in which the age data do not allow a subdivision, seems to comprise both the 3A and 3B classes (see Fig. 4). Relationship focal depth

between

the age of the subducted

The main results obtained

so far are compiled

lithosphere

and maximum

in Fig. 7. A distinct correla-

2

t

Fig. 7. Relation between the age of oceanic lithosphere at the time it began to be underthrust in a subduction zone and the maximum focal depth of the earthquakes occurring within this subducted lithosphere. The numbers indicate the age classes. The horizontal bars correspond with the widths of the age classes (see Table I). The vertical bars indicate the depth ranges covered by the maximum focal depths in the subduction zones of the corresponding classes. No data appear to be available in the age interval 70-90 m.y. However, the age determinations of the class-l zones are not accurate enough to define a separate age class.

342

tion between the age of oceanic lithosphere at the time it began to be underthrust in a subduction zone and the maximum focal depth of the earthquakes occurring within this subducted lithosphere is demonstrated. Although PDE (Preliminary Determination of Epicenters) data have been used in the construction of the sections shown in Figs. 4, 5 and 6, the maximum focal depths on which the relationship in Fig. 7 is based were taken from the cited regional studies (see Fig. 1) and from Rothe’s (1969) catalogue. DISCUSSION

Phase changes and resorption On aging of the oceanic lithosphere, two of its physical parameters which are pertinent to the process of subduction, are subject to changes: (1) temperature decreases; and (2) density increases. These parameters are related through cubic expansion and phase transitions. Christensen and Salisbury (1975) established some relations between geophysical features of ocean basins and the age of the ocean floor. They arrived at some significant ages which coincide with the limits of our age classes: anomalous upper-mantle velocities (between 7.2 km/see and 7.8 km/see) are common to 15 m.y.; oceanic layer 3 continues to increase in thickness to 40 m.y. and a strong Bouguer anomaly associated with the presence of an anomalous upper mantle persists to 70 m.y. Forsyth and Press (1971) suggested that phase changes take place upon cooling of the oceanic lithosphere. This would imply that gravitational instability is enhanced. When cooling has resulted in gravitational instability, the density determines the negative buoyancy forces and, upon reheating after subduction, the depth at which stability will be re-established. The rheological state in which this depth is reached depends on the temperature of the slab and of the surrounding mantle. This also plays a primary part in the process of resorption, which therefore is an obscuring factor in identifying the slab’s penetration depth. Hence, discrepancies may exist between the maximum focal depth of earthquakes occurring in the slab and the depth of penetration. We propose that penetration depth and the depth of assimilation, both depending on the temperature, are approximately equal. As the oceanic lithosphere has been derived from the upper mantle, and as its history depends criticalfy on its temperature, this proposition appears to be sound. Moreover, it has been shown by numerical calculations (Minear and Toksoz, 1970) that for a deeply penetrating slab (our class 1) the deepest e~hquakes are at a depth where the temperature of the slab is close to the temperature of the surrounding mantle. There is no reason to assume that the same does not apply to younger slabs. In this light we may state the following: (1) When oceanic lithosphere younger than 15 m.y. (class 3B) is being underthrust, or more likely, is being overthrust by continental lithosphere, it

343

is reheated rapidly to the temperature of the upper mantle. As a consequence, gravitational stability and resorption are attained within a short time and at a shallow level, thus resulting in (apparent} subhorizontal underthrusting. As a consequence, earthquakes generated between 60 km and the indicated maximum depth range are very rare. (2) The depth ranges of classes 3A and 2 are representative of the maximum focal depths all along the strikes of the zones involved. The .large gap between the depth ranges of class 1 and class 2 (see Fig. 7) suggests that the interval 70-90 m,y, in which no data are available, is of a transitional nature. This may be attributed to the part the olivine-spinel phase change plays in the subduction process. Because of the lower temperature this phase change, normally being situated at a depth of approximately 400 km, may take place at a shallower level in a descending slab. On the basis of the results of Schubert and Turcotte (1971) and Turcotte and Schubert (19?1), the following implications may be envisaged. A lithospheric slab younger than 70 m.y. does not penetrate to depths greater than 300 km and its temperature is too high to elevate the phase boundary. Thus, the phase change does not affect the subduction in classes 2 and 3. In a very cold slab (older than 90 m.y.), the phase boundary is displaced upward to a depth of 260 km, which enhances the negative buoyancy force by a factor of about two. The slab then is forced to sink to the depth of approximately 650 km, Going through the transitional interval from 70 m.y. to 90 m.y., the gravitational instability is drastically increased. A hypothetical penetration depth-age curve should have a steep slope in this interval. This indeed is apparent in Fig. 7. Plate cmwergence

In the oldest zones (class 1) ~avitatiun~ instability may have resulted in spontaneous downbuckling and subsequent over- and und~~h~sting of oceanic lithosphere, which is typical for island-arc structures. Within the global system of relative plate motions patterns may arise which imply a decrease in the age of the oceanic lithosphere near a consuming plate boundary. In this way a subduction zone class 2 or 3 may evolve. In the age classes 2 and 3A vertical forces appear to be present. At plate boundaries of the Juan de Fuca-North America type (class 3B), where vertical instab~ity has not yet been developed, ove~h~sting of the continental plate appears to be the primary cause of the continuing plate consumption. The dip angle of a subducted plate depends, among others, on the convergence rate normal to the plate contact and the vertical velocity (Luyendyk, 1970). We propose that the vertical velocity in turn is determined by the amount of instability and hence the age of the downgoing slab, which should effect the dip angle accordingly. However, the dip angle must also be influenced by the motion of the plate contact with respect to the mantle. This unknown factor obscures the situation considerably. In numerical modelling of subduction processes, above considerations

of

344

should be taken into account, in particular the age-dependence temperature and density at the convergence zone.

of the initial

Detachment is supposed to take place (McKenzie, 1969) when two continental plates collide at a subduction zone. The oceanic lithosphere in front of one of them, becomes detached and sinks on its own (Garpathians). In principle this mechanism may also be applicable when the evolution of a subduction zone is such that, within a short time interval, increasingly younger lithosphere is subducted. This is probably the case in the PeruNorthern Chile zone. An age discontinuity in a subducted slab may exist as a result of the development of a new spreading ridge crest in an old basin. The presence of the relatively young and hot oceanic lithosphere does not necessarily stop the subduction process, but it may cause detachment at a certain depth in the subduction zone. The upper part, consisting of the younger lithosphere, may not be able to pass the olivin~p~el phase transition whereas the older part had sufficiently cooled to do so. As a consequence the older part becomes detached and sinks to greater depths. remarks

on some te~t~~i~~l~y complex seismic regions

(1) The evolution of the complicated F~i~~ppineSea is only partially understood. The development of the Parece Vela Basin between the northtrending Palau-Kyushu Ridge and the West Mariana Ridge, and the Mariana Basin between the latter ridge and the Mariana arc is explained by Karig (1971) as a result of interarc extension which started in the Miocene and Late Pliocene, respectively. No agreement exists concerning the evolution of the western Philippine Basin which has a well-developed trench system along its western boundary. If the model of Uyeda and Ben-Avraham (1972) in which the western basin is taken to be a part of an old ocean floor (older than 100 m.y.), including an extinct segment of the Kula-Pacific Ridge, is correct, we would expect the Mindanao and the Ryukyu subduction zones to be of the “deep” type (class 1). However, only in the southern half of the Mindanao zone (between 2”N and 10”N) are deep earthquakes known (Fitch and Molnar, 1970). On the other hand, deep sea drilling results (Ingle et al., 1973) do not indicate ages older than Paleocene in the western Philippine Sea floor. Maximum focal depths in the Ryukyu zone and the northern half of the Mindanao zone are in correspondence with class-2 depths (Fitch and Molnar, 1970; Katsumata and Sykes, 1969). No sites have been drilled off the southern Mindanao zone where deep earthquakes occur. Therefore, we can only say that on the basis of our results we expect these deep earthquakes to be associated with old portions of lithosphere (class 1). At the southwestern tip of the Mariana arc the deep and in~~ediate seismicity ends rather abruptly. The seismic activity along the Yap and Palau

345

trenches is extremely low and of the shallow type. This abrupt transition has to be linked to the age difference between the Caroline Basin which is only 14-18 m.y. old east of the Palau trench (Bracey, 1975) and the old Mesozoic Pacific ocean floor east of the Mariana arc. (2) New Zealand is located on the contact of the Indian and Pacific plates. From NE to SW the structure of this contact shows important changes as a result of the fact that the plate boundary cuts through the continental plateau which, apart from New Zealand’s North Island and South Island, comprises the Lord Howe Rise, the Chatham Rise and the Campbell Plateau. Although differences in seismicity are apparent, the North Island seismic zone may be considered as a continuation of the Tonga and Kermadec zones. The Fiordland seismic region with maximum focal depths of about 160 km (Smith, 1971; Scholz et al., 1973) is related to subduction of the Tasman Sea floor beneath the South Island. The Central Tasman Sea floor has been created by spreading during the period from 80 m.y. B.P. till 60 or possibly 50 m.y. B.P. (Hayes and Ringis, 1973; McDougall and Van der Lingen, 1974). However, the Southern Tasman Sea, which is involved in the underthrusting beneath Fiordland, originates from the southeastern part of the mid-Indian Ocean Ridge which started to separate Australia from Antarctica about 50-55 m.y. B.P. (Bowin, 1974). Taking into account the convergence rate of 3.7 cm/year (Minster et al., 1974), it turns out that the deepest parts of the lithosphere in the Fiordland zone ,were probably not older than 50 m.y. at the initiation of the underthrusting. The maximum depth of 160 km in the Fiordland region does not fall in the range 175-300 km we found for class 2. Possibly the proximity of the continental-type Lord Howe Rise influences the development of a subduction zone. This may also be the case in the North Island seismic zone where a lateral transition takes place from the oceanic lithosphere at the Hikurangi trench to the continental lithosphere of the Chatham Rise. (3) Mediterranean Sea. In the Tyrrhenian earthquake zone the maximum focal depth appears to be 485 km. Hypocentres of intermediate and deep events delineate a narrow WNW-dipping seismic zone, thus suggesting that the Ionian Basin lithosphere is descending beneath the Calabrian arc. Focal mechanisms of intermediate and deep events are in support of this interpretation (Ritsema, 1971,1972). However, the moderate shallow seismicity indicates that at present this region is not a site of active under-thrusting. If, indeed, the Ionian Basin lithosphere has descended beneath the Calabrian arc, then, on the basis of our results and the depths that the possibly detached pieces of lithosphere appear to reach, it seems justified to assign a Mesozoic age to the western part of the Eastern Mediterranean Basin floor. Ritsema (1971,1972) proposed an active overriding of both the Calabrian am and the Hellenic arc over the Ionian Basin floor. The great differences in dip and maximum focal depth between the Tyrrhenian and Aegean seismic zones have as yet not been explained. (4) Caribbean Sea. According to Molnar and Sykes (1969) underthrusting

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of the Atlantic Ocean floor takes place beneath the Lesser Antilles arc. A maximum focal depth of 232 km is reported by Sykes and Ewing (1965). A tentative age of 90 m.y. for the equatorial Atlantic directly east of the arc may be inferred from the results of Pitman and Talwani (1972). It is doubtful whether this subduction zone is representative of the afore-mentioned transitional interval. Possibly, the presence of the thick sedimentary cover on the South American continental margin which extends in northern direction all along the arc (Ewing et al., 1973), has resulted in less effective cooling than in open oceanic basins with thin sedimentary covers and has kept the maximum focal depth in the class-2 range. However, the evolution and tectonic setting of the Caribbean are of such a complexity that we must await further understanding of the region’s specific features before we can include the data in a worldwide analysis of subduction zones. (5) Recent hypotheses concerning the tectonics of the New Guinea-New Britain-Solomon islands region involve a number of small plates needed to explain the regional seismicity (Johnson and Molnar, 1972). In general it is assumed that the contact of the Indian (or Australian) plate and the Pacific plate changed significantly at the time New Guinea reached the trench of a north-dipping subduction zone. At present, it does not seem to be of much use to speculate on the implications of our results for the evolution of this extremely variable area. CONCLUDING

REMARKS

In all cases where adequate data are available for dating the history of subduction zones, a unique relation between the history and the depth of subduction of oceanic lithospheric material and the time elapsed between its creation at spreading centers and its subduction, has been well established. This remarkable feature enables us to make a number of speculative statements: (1) Plate tectonics is part of a convective process involving the oceanic lithosphere and probably the entire upper mantle above a depth of 750 km. The process is maintained by gravitational instability in a lithosphere-upper mantle system which is not capable of loosing its heat by simple conduction. The gravitational instability is manifest at ocean ridge crests where hot material rises and in subduction zones where a cooled lithospheric slab is being subducted. The depth of penetration can be related to the amount of cooling the lithosphere has been subject to since its creation, which in turn determines the gravitational instability of the layering of the lithosphere relative to the deeper layers. (2) While subduction usually takes place at continental margins where an oceanic and a continental plate collide, it appears to be that spontaneous subduction is possible in oceanic regions provided the oceanic lithosphere has reached the amount of instability inherent to our class 1. Such may be the case in the Tonga-Kermadec region. An initial stage of downbuckling may

347

be present at the Canton trough (Rosendahl et al., 1975), which indeed concerns old oceanic lithosphere. It is plausible to expect that downbuckling and subduction eventually will take place in the oldest parts of the Atlantic Ocean near the continental margins. As these epicontinental basins are filled with a thick sediment cover this may give rise to tectonic phenomena such as mountain building. (3) As is evidenced that cooling of the oceanic lithosphere is instrumental to the occurrence and character of subduction it is to be expected that the tectonic history of complicated areas can be understood in the light of our hypothesis if more relevant data will become available. We are confident that deep earthquakes in turn are to be associated with remnants of an old oceanic lithosphere, even if no other evidence is present. In this light, the intermediate and deep Hindu Kush, Carpathian, and Spanish earthquakes can be explained. Obviously, our results are of interest in the reconstruction of paleo-subduction zones. (4) Subduction of oceanic lithosphere which has been created recently (class 3B) results in apparent subhorizontal underthrusting and is accompanied by lower and predominantly shallow seismicity. The upper basaltic layer of the underthrust slab, which has not been cooled sufficiently, is heated to near its melting point again. This appears to be the case in the western part of North America, where recent large-scale uplift took place and which region is characterized by volcanism and the inital stages of rifting (Anderson, 1971; Decker and Smithson, 1975). (5) Though the subduction process of oceanic lithosphere and its relation to its cooling history appears now to be well established as being part of a convective cycle, it is not clear why and where rifting of the lithosphere, which must be the onset of the convective process, will take place. If initial rifting takes place in a continental lithosphere it can be made plausible that blanketing by sedimentary layers plays a major role. However, in oceanic regions, where active ridges are initiated and also disappear, there appears to be no evidence for preference for specific areas relative to others for the initial stages of ocean spreading. All that can be stated is that the oceanic lithosphere is such a poor conductor for heat that convective processes are necessary for the earth to transport its internal heat to the surface.

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