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Geophys. J. Int. (1998) 134, 573^595 An enhanced image of the Pamir^Hindu Kush seismic zone from relocated earthquake hypocentres G. Pegler* and S. Das Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK. E-mail: [email protected] Accepted 1998 March 16. Received 1998 March 12; in original form 1997 May 28 S U M M A RY We determine the shape of the seismic zone in the Pamir^Hindu Kush region de¢ned by [30^420N, 68^780E] by obtaining improved hypocentral locations with 90 per cent con¢dence limits of less than 30 km (the depth error bar for most of the earthquakes) of about 6000 shallow and intermediate-depth earthquakes. Available S and depth-phase arrival times are also used together with the P-wave arrival times in the joint hypocentre determination technique. To obtain the best possible hypocentral locations, the study region is divided into three depth ranges, 0^60, 60^160 and >160 km. The 0^60 km depth zone is then subdivided laterally into 19 blocks, with the deeper regions divided into two blocks each. The improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate-depth seismicity in the Pamir^ Hindu Kush region is most simply explained by a single S-shaped seismic zone, 700 km long and no more than 30 km wide and with most activity concentrated at 100^300 km depth. The main features observed are: (1) the eastward steepening of the north-dipping Hindu Kush seismic zone through to its overturning at its eastern end beneath the Pamirs, where it dips to the southeast; (2) the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the Hindu Kush seismic zone; (3) the very abrupt cut-o¡ in intermediate-depth seismicity at 90^110 km depth with no extension to shallower depths under the Pamirs, and with a persistent gap between the intermediate and shallow seismicity in the northern Pamirs; and (4) the unusual horizontal T-axes for intermediate-depth earthquakes of the Pamir seismic zone, which align with its curvature. This study shows that the seismic zone under the Hindu Kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity, whereas the Pamir region has horizontal T-axes that follow the trend of the contorted seismic zone. This suggests that the Pamir seismic zone is a slab deformed due to £ow in the upper mantle. Key words: earthquakes, Pamir^Hindu Kush, P- and T-axes, subduction. I NT ROD UC T I O N The study of intermediate and deep earthquakes and associated stress axes within subducting slabs was a major piece of evidence that led to an understanding of global plate dynamics (Isacks & Molnar 1971). Interest in intermediate and deep intraslab earthquakes has grown recently with increasing awareness of its importance in understanding slab metamorphism (Kirby, Engdahl & Denlinger 1996a; Kirby et al. 1996b) and its ability to provide constraints on models of * Now at: StatSci Europe, Osney House, Mill Street, Oxford OX2 0JX, UK. ß 1998 RAS mantle dynamics. Intermediate-depth (&70^300 km) earthquakes usually occur in regions where oceanic lithosphere has been subducted for 10 Ma or longer (Chen & Molnar 1983; Molnar 1988); intermediate-depth earthquakes not associated with subducting oceanic lithosphere are rare. The Pamir^ Hindu Kush region of central Asia is one of the most active regions of intermediate-depth seismicity and by far the most active region of intermediate-depth seismicity not obviously associated with the ongoing subduction of oceanic lithosphere. The seismicity of this region has been the subject of many studies (e.g. Nowroozi 1971; Vinnik, Lukk & Nersesov 1977; Billington, Isacks & Barazangi 1977; Vinnik, Lukk & Mirzokurbonov 1978; Chatelain et al. 1980; Roecker et al. 1980; Roecker 1982; Katok 1988; Ni & Fan 1989; Fan, Ni & 573 574 G. Pegler and S. Das Wallace 1994; Mellors et al. 1995) and a variety of models have been proposed to explain the intermediate-depth seismicity. The models fall broadly into two categories. (1) Models that imply that the intermediate-depth seismicity beneath the Pamirs has a di¡erent origin from that beneath the Hindu Kush. Such models generally propose northward subduction of the Indian plate beneath the Eurasian plate for the Hindu Kush and southward subduction of the Eurasian plate beneath the Pamirs (e.g. Chatelain et al. 1980; Fan & Ni 1989; Burtman & Molnar 1993; Fan et al. 1994). (2) Models that consider the intermediate-depth seismicity beneath the Pamirs and Hindu Kush to be a single but highly contorted seismic zone (e.g. Billington et al. 1977; Vinnik et al. 1977). In this study we attempt to ascertain the con¢guration of the intermediate-depth seismicity beneath the Pamir^Hindu Kush region and to gain insight into the orientation of the principal stress directions within the seismic zones by relocating the seismicity and by examination of the Harvard centroid moment tensor (CMT) solutions (Dziewonski et al. 1983^1994). T E C TO N IC SET T I NG A N D PR EV I O US S E I S M IC ST U D I E S I N T H E R E GI O N The Pamir^Hindu Kush region is located at the western syntaxis of the Himalayas, in the broad deformation belt created by the collision of the Indian and Eurasian plates (Fig. 1). The tectonic evolution of the region can broadly be divided into three stages (Windley 1988). The ¢rst stage saw the northward migration of several plates from Gondwanaland and the formation of magmatic arcs, with the closure of Tethys during the Mesozoic^Lower Tertiary. During the second stage, the plates accreted to the southern margin of Eurasia, with the ¢nal collision, that of the Indian plate along the Indus suture zone, occurring around 50^40 Ma. The third stage of evolution of the region is the post-collisional indentation of the Indian plate into Eurasia, which has resulted in as much as 2000 km of crustal shortening across the collision zone. The intermediatedepth seismicity under study here is essentially restricted to the Pamir^Hindu Kush region, although there are rare intermediate-depth earthquakes beneath the Karakoram and the Tibetan plateau. The Pamirs have an average elevation of between 4 and 5 km and consist of a collage of sutured terrains which accreted to Eurasia during the Triassic^Early Cretaceous. The Karakoram lies to the south of the Pamirs and is geographically and geologically continuous westwards into the eastern Hindu Kush (Rex et al. 1988; Searle 1991, 1996a). The southern boundary of the Hindu Kush and Karakoram is marked by the Shyok suture zone, which has recently been reactivated and is often referred to as the Main Karakoram Thrust (MKT). The Kohistan and Ladakh blocks are island arc terrains which lie to the south of the MKT and form part of the 2500 km long Trans Himalayan batholith that formed during the closure of Tethys. The present-day relative plate motion of India with respect to Asia, within the Pamir^ Hindu Kush region, is approximately due north at 45 mm a{1 (DeMets et al. 1990). This convergence is accommodated from the Himalayan thrust front in the south, right through to the Tien Shan in the north (Burtman & Molnar 1993). There is much debate concerning the present partitioning of the total convergence across the deformation belt, in particular the relative importance of crustal shortening by thrust faulting or the extrusion of Eurasia along strike-slip faults around the indenting Indian plate (Molnar & Tapponnier 1975; Tapponnier & Molnar 1976, 1977, 1979; Burtman & Molnar 1993; Searle 1996a). The existence of intermediate-depth earthquakes beneath the Pamir^Hindu Kush region has long been recognized. The ¢rst published study of an earthquake in this region was the 1911 February 18 Pamir earthquake (Galitzin 1915), later also studied by Je¡reys 1923). In a historical aside, it is interesting to note that Turner showed that deep-focus earthquakes existed in 1922 and Je¡reys used the 1911 Pamir earthquake, which was a shallow shock, to argue against this possibility in 1923! Gutenberg & Richter (1954) located several earthquakes at depths greater than 200 km beneath the Pamir^ Hindu Kush for the period 1907 to 1950 and Richter (1958) found more than 70 earthquakes here, with nine being >7.0 magnitude at intermediate depths between 1904 and 1955. One of the ¢rst detailed seismic sections through the Pamir^ Hindu Kush region was published by Nowroozi (1971), which shows two thin zones of intermediate-depth seismicity. The ¢rst zone underlies the Pamirs, has a NE^SW trend, and is de¢ned by seismicity between depths of 70 and 175 km. The second zone beneath the Hindu Kush has an E^W trend and is de¢ned by seismicity between 175 and 250 km depth. Vinnik et al. (1977) suggested that the intermediate-depth seismicity represented a single zone of activity and was caused by a rigid region of the `tectosphere' subject to tectonic stresses from the surrounding material. This interpretation was based on the coincidence of a high-velocity zone at depths of up to 300 km, the intermediate-depth seismicity beneath the Hindu Kush and the outcrops of Precambrian rocks in the region. The ¢rst study to suggest subduction of oceanic lithosphere as the source of the intermediate-depth seismicity beneath the Hindu Kush and the Pamirs was that of Billington et al. (1977). They studied selected earthquakes from the International Seismological Summary (ISS) and the International Seismological Center (ISC) catalogues, as well as the best-located events of Nowroozi (1971) and identi¢ed a contorted Benio¡ zone running E^W beneath the Hindu Kush and NE^SW beneath the Pamirs. They interpreted the seismicity as representing a single seismic zone, but the possibility of there originally having been two subduction zones of opposing polarity was also considered. Chatelain et al. (1980) and Roecker et al. (1980) using data from microseismicity studies in 1966, 1967, 1976 and 1977 concluded that two subducting slabs were present and that they might represent the subduction of two small intracontinental basins beneath the Pamirs. Much of the interpretation of Roecker et al. (1980) and Chatelain et al. (1980) was based on the identi¢cation of gaps in the seismicity, which were seen in all of their data sets. More recent geophysical studies of the Pamir region have also favoured subduction in opposing directions beneath the Pamir and Hindu Kush, rather than one contorted zone (e.g. Hamburger et al. 1992; Burtman & Molnar 1993; Fan et al. 1994). These studies conclude that continental crust and lithosphere are subducting to depths of around 200 km beneath the Pamirs. However, all the above-mentioned studies that propose continental subduction beneath the Pamirs are essentially studies concentrating on the Pamir seismic zone alone, with little discussion of its interaction with, or connection to, the Hindu Kush seismic zone. The local ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 575 Figure 3. Relocated seismicity for the depth range 75^100 km for the period 01/01/1964^12/31/1992, together with all available CMT mechanisms. Solid dots represent epicentres with 90 per cent con¢dence limits ¦30 km. The size of each dot is related to the earthquake magnitude, the size key being shown in Fig. 5. Hollow dots represent the remaining relocated events which have greater hypocentral uncertainties and are plotted at half the diameter of the solid dots for events with equivalent mb . (Note that the smallest dots may appear solid but are actually hollow circles.) Figure 4. Same as Fig. 3 but for the depth range 100^125 km. ß 1998 RAS, GJI 134, 573^595 576 G. Pegler and S. Das Figure 5. Same as Fig. 3 but for the depth range 125^150 km. The size key relating the dot size to the earthquake magnitude is shown. The same key is used for all seismicity plots in this paper. studies of Roecker et al. (1980) and Chatelain et al. (1980) generally did not consider data much beyond the limits of the intermediate-depth seismicity. The goal of this study is to examine a much larger seismicity data set than previously studied. Our study area covers the entire Pamir and Hindu Kush regions and attempts to ascertain whether the Pamir^ Hindu Kush seismic zones represent a single contorted zone of seismicity or two unrelated seismic zones. Figure 6. Same as Fig. 3 but for the depth range 150^175 km. ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 577 Figure 7. Same as Fig. 3 but for the depth range 175^200 km. R E L O CAT E D S EI SM IC I T Y To study the seismicity of the Pamir^Hindu Kush region in detail, we have relocated about 6000 earthquakes within the region of interest for the period 1964^1992 using phase arrival times reported by the ISC and the joint hypocentre determination technique (JHD) developed by Dewey (1971, 1983). The ISC reports mainly P-, some pP- and a few S-wave arrival times. Richter (1958, Ch. 19, p. 311) wrote: `Depths are often seriously overestimated by taking sP for pP. Sometimes a majority of stations recording a given earthquake report a large sP and overlook or fail to ¢nd pP. This is common with intermediate shocks under the Hindu Kush'. The ISC has informed us that they rarely use depth phases. We have used available S and depth phases, in addition to the P-wave arrival times, in this study. In order to produce well-constrained relocated hypocentres, the region is subdivided into a number of blocks, both horizontally and vertically. Seismicity within the Pamir^Hindu Kush region extends to depths of around 300 km, although at least one apparently well-constrained event has a depth of around 380 km (Katok 1988). The ISC hypocentres appear to be broadly divisible into three depth zones (0^60, 80^150 and 180^300 km), each zone separated by a depth band in which there are relatively few hypocentres. Guided by this, we divide the entire data set into three depth zones, 0^60, 60^160 and >160 km. The 0^60 km depth zone was subdivided laterally into 19 blocks. Due to the fact that the earthquakes in the deeper regions were spatially clustered, these two depth ranges needed to be divided only into two blocks each. The horizontal areas of all the blocks used are similar in size (Pegler 1995). A group of representative ß 1998 RAS, GJI 134, 573^595 earthquakes from each block was then selected to be processed as a joint hypocentre group. All earthquakes within each block were then relocated using the SE89 (single-event) algorithm (Dewey 1971, 1983) and station-phase adjustments determined from the JHD relocation for that block. The relocated hypocentres were then combined into a single catalogue of events. If a block did not have su¤cient well-recorded events for a JHD, then a single-event relocation was carried out for events in that block. The details of the relocation are fully reported in Pegler (1995). The ISC lists 9127 events within the region bounded by 30^420N and 68^780E for the period 1964^1992. All events for which ISC reported fewer than six P-wave arrival times were discarded, leaving 7151 events of which 5904 were successfully relocated. 3260 events were relocated with 90 per cent con¢dence limits of less than 30 km. The best-constrained earthquakes are those at intermediate depths. Of the 929 earthquakes relocated with 90 per cent con¢dence limits of less than 10 km, 828 have depths greater than 60 km. The relocated seismicity is shown in a series of depth ranges and sections. To highlight the most reliably relocated events, the events which could be relocated with 90 per cent con¢dence limits of less than 30 km (in latitude, longitude and depth, but usually the errors in latitude and longitude are less than 30 km so for most events it is the error in depth) are shown as solid dots in all seismicity plots in this paper. When shown, the locations of events which were either not relocated or relocated with greater uncertainty are indicated by hollow circles and plotted at half the scale of the more reliably relocated events. Events for which the ISC was unable to determine depths were generally also not successfully relocated by us. 578 G. Pegler and S. Das Figure 8. Same as Fig. 3 but for the depth range 200^225 km. R E L O CAT E D E A RT H QUA K E HYPOCENTRES IN DIFFERENT D E P T H R A NG E S A N D IN V E RT ICA L C RO S S - S E CT I O N S In this paper, we shall plot mainly relocated hypocentres, the comparison with ISC hypocentres for all cases being shown in Pegler (1995). In general, we ¢nd that after relocation the hypocentres lie in narrower zones than those de¢ned by the ISC estimates. The relocated seismicity, in di¡erent depth ranges, is shown in Figs 2^9. It is also shown in a series of twelve 800 km long sections, the locations of these sections being shown in Fig. 10. The six sections N^N', O^O', P^P', Q^Q', R^R' and S^S' shown in Figs 11^16 are 50 km wide N^S sections roughly perpendicular to the Hindu Kush seismic zone. The four sections T^T', U^U', V^V' and W^W' shown in Figs 17^20 are 50 km wide sections taken perpendicular to the dip of the Pamir seismic zone. A 120 km wide section, X^X', across the eastern end of the Pamir seismic zone is shown in Fig. 21, and a 120 km wide section, Y^Y', parallel to and encompassing almost all of the intermediate-depth seismicity of the Pamir seismic zone is shown in Fig. 22. In order to examine the region where the Hindu Kush and Pamir seismic zones `meet', a set of thinner, more arc-perpendicular sections, Z1 to Z8, are presented. Fig. 23 shows the location of this set of sections, the sections being shown in Figs 24 and 25. Fig. 26 shows the seismicity of the Karakoram region in map view and in a section denoted KA^KA'. A complete description of the relocated seismicity in all depth ranges and in all vertical sections is given in Pegler (1995). We discuss only the most important features in the Appendix and summarize the results in the next section. SU M M A RY OF T H E FE AT U R E S S E E N I N T HE R E L O CAT E D SE I SM IC I T Y The intermediate-depth seismicity beneath the Pamir^Hindu Kush region occurs in a 700 km long S-shaped zone, no wider than 30 km throughout most of its length and with most activity concentrated between depths of 100 and 300 km. This zone is not continuous throughout its length, nor at all depths within any given section, but includes highly active clusters of events and several seismic gaps. The seismic zone dips towards the north beneath the Hindu Kush and to the southeast beneath the Pamirs. Seismicity along the Hindu Kush seismic zone suggests strongly that the zone extends to depths as shallow as 60 km and may extrapolate to the surface in the region of the MMT or MKT. Beneath the Hindu Kush, the northward dip of the seismic zone, at depths greater ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 579 Figure 9. Same as Fig. 3 but for the depth range 225^250 km. Error ellipses represent projections of the 90 per cent con¢dence ellipsoid. than 100 km, steepens from around 500 at 690E, becomes vertical and overturns at its eastern end, at around 71.50E. As the Hindu Kush seismic zone becomes vertical, seismicity indicating N^S shortening occurs just to the north of and slightly shallower than the Hindu Kush seismic zone. Almost all CMT mechanisms within the Hindu Kush seismic zone between 100 and 200 km depth have subvertical T-axes, and B-axes which closely approximate the arcuate strike of the seismic zone. There is an inverted V-shaped fork in the Hindu Kush seismic zone at its eastern end, at depths greater than 200 km. Intermediate-depth seismicity of the Pamir seismic zone displays less activity than the Hindu Kush seismic zone and appears to end abruptly at between 90 and 110 km depth and nowhere does it clearly extend to shallower depths. Seismicity along the northern margin of the Pamirs extends to depths of 70^80 km in some sections, but the minimum horizontal distance between this seismicity and that of the intermediatedepth seismic zone beneath the Pamirs is 70 km. CMT mechanisms along the Pamir seismic zone are more varied than along the Hindu Kush seismic zone. Unusual horizontal T-axes paralleling the strike of the seismic zone suggest that bending along the seismic zone is the dominant feature east of 720E. Thus, the stresses in the slab here do not have the classical pattern for downgoing slabs controlled by gravity but appear to depend instead on the contortion of the seismic zone. ß 1998 RAS, GJI 134, 573^595 Finally, seismicity beneath the Karakoram appears to de¢ne a triangular wedge down to 100 km, which is bounded at the surface by the MKT in the southwest and by the southern margin of the Tarim basin in the northeast. D I S C USS I O N A N D C O NC LUS I O N S Whereas most studies conclude that the intermediate-depth seismicity beneath the Pamir^Hindu Kush region represents some form of subduction of lithosphere, there has been much debate in the literature as to the nature of the material that the seismicity represents and whether it is caused by one or two opposing subduction zones (e.g. Billington et al. 1977; Chatelain et al. 1980; Hamburger et al. 1992; Burtman & Molnar 1993; Fan et al. 1994). We discuss the main factors surrounding this debate with emphasis on the implications of the evidence from this and other studies of the seismicity of the region. We shall argue that the Pamir^Hindu Kush seismic zone displays several features which are more simply and elegantly explained by a single contorted zone of seismicity. What does the intermediate-depth seismicity represent? The results of this and other studies of the Pamir^Hindu Kush seismic zone indicate that the intermediate-depth seismicity occurs along thin (<30 km wide) slab-like seismic zones 580 G. Pegler and S. Das Figure 11. Relocated seismicity section N^N'. Hypocentres plotted are for the period 01/01/1964^12/31/1992. Solid dots represent hypocentres with 90 per cent con¢dence limits ¦30 km. Hollow dots represent the remaining relocated events which have greater hypocentral uncertainties and are plotted at half the diameter of the solid dots for events with equivalent mb . (Note that the smallest dots may appear solid but are actually hollow circles.) CMT mechanisms are plotted as back-hemisphere projections with compressional quadrants shaded. The numbers at the centre of the top edge of the ¢gure give the longitude (E) and latitude (N) of the centre of the section, and the abscissa the distance in kilometres from this point. The topographic pro¢le (height in metres) along the line of the section is plotted above the seismicity section. (e.g. Billington et al. 1997; Roecker et al. 1980; Chatelain et al. 1980). These zones are similar to those seen along present-day subduction zones where oceanic lithosphere is being subducted. The near-vertical T-axes seen along the Hindu Kush seismic zone at intermediate depths are commonly seen within subducting oceanic plates which descend to intermediate depths (Isacks & Molnar 1971; Billington et al. 1977; Roecker et al. 1980; Chatelain et al. 1980). Therefore, the intermediate-depth seismicity beneath the Pamir^Hindu Kush might be considered to represent subducted oceanic crust (e.g. Billington et al. 1977; Chatelain et al. 1980). However, the youngest oceanic rocks within the western syntaxis of the Himalayas represent the closure of Tethys some 50^40 Ma, and furthermore there is no evidence for any subduction-related volcanism within the Figure 12. Same as Fig. 11 but for the seismicity section O^O'. ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 581 Figure 13. Seismicity section P^P'. Tertiary. Assuming the Hindu Kush seismic zone reaches the surface in the vicinity of the MMT at 690E, the total subducted along-slab length of the Hindu Kush seismic zone, down to 300 km depth, is around 500 km. The present-day convergence rate between India and Eurasia is about 45 mm a{1 (DeMets et al. 1990). At this rate it would take about 10 Ma to subduct a 500 km long piece of lithosphere. Even at only half this rate it would still only take around 20 Ma to subduct 500 km of lithosphere. Therefore, it is very unlikely that any of the seismicity at depth beneath the Hindu Kush can represent any Figure 14. Seismicity section Q^Q'. ß 1998 RAS, GJI 134, 573^595 part of the former Tethys oceanic lithosphere; it must represent more recent subduction. Chatelain et al. (1980) argued that the lack of recent volcanism within the region, combined with the presence of intermediate-depth seismicity, indicates a very brief period of subduction in the late Tertiary, involving the subduction of two small oceanic basins beneath the Hindu Kush and Pamirs. However, as mentioned above, there is absolutely no geological evidence for the existence of any such basins. Other authors have suggested that the intermediatedepth seismicity represents subduction of continental crust 582 G. Pegler and S. Das Figure 15. Seismicity section R^R'. The inset shows seismicity for the same orientation and scale as the main ¢gure, but for a section with its centre displaced by half a degree in latitude and extending from 200 to 400 km. The main purpose of this inset is to show the seismicity below 300 km, especially one earthquake near 400 km depth. (e.g. Roecker 1982; Hamburger et al. 1992; Burtman & Molnar 1993; Fan et al. 1994). Roecker (1982) found evidence for low velocities associated with the Hindu Kush seismic zone, shallower than 200 km, from a local tomographic inversion for P and S velocities, and suggested that this was indicative of the subduction of continental crust, rather than oceanic crust. Vinnik et al. (1977), however, found evidence for abnormally high velocities at greater than 200 km depth within the Hindu Kush seismic zone. A more recent study by Mellors et al. (1995) also implies the presence of a high-velocity zone at greater than 200 km beneath the Hindu Kush seismic zone. Results presented in Pegler (1995) con¢rm the presence of a high-velocity zone beneath the Hindu Kush, but do not resolve at what depth it occurs. Several authors point out that the existence of a low-velocity zone shallower than 200 km and a high-velocity zone deeper than 200 km depth beneath the Hindu Kush need not be incompatible and might represent the transition between the subduction of continental and Figure 16. Seismicity section S^S'. ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 583 Figure 17. Seismicity section T^T'. oceanic crust (e.g. Roecker 1982; Burtman & Molnar 1993; Mellors et al. 1995). Beneath the Pamirs, there is little clear evidence for the existence of either unusually high or unusually low velocities. It has been suggested that the Pamir seismic zone represents the subduction of Asian continental crust down to depths of around 200 km (Hamburger et al. 1992; Burtman & Molnar 1993; Fan et al. 1994). Whether or not continental crust can be subducted to depths of 200 km is unclear (Burtman & Molnar 1993) and no examples of continental subduction to such depths within other regions have been convincingly demonstrated. Figure 18. Seismicity section U^U'. ß 1998 RAS, GJI 134, 573^595 Continuity of the Pamir^Hindu Kush seismic zone To ascertain whether the Pamir and Hindu Kush seismic zones represent one or two subducting plates, it is important to examine the region where the two zones approach each other most closely. Billington et al. (1977) ¢rst described the Pamir^ Hindu Kush seismic zone as a single contorted zone, in which part of the zone beneath the Pamirs may have become overturned. However, Roecker et al. (1980) and Chatelain et al. (1980) using data recorded over relatively short periods from local stations showed the existence of a 50 km gap between the 584 G. Pegler and S. Das Figure 19. Seismicity section V^V'. Pamir and Hindu Kush seismic zones. Roecker et al. (1980) and Chatelain et al. (1980) cited this gap as the most convincing evidence that the two zones represented two subducting slabs, rather than a single subducting slab that had overturned beneath the Pamirs. The data from this study also shows a gap of around 50 km, at all depths, between the Pamir and Hindu Kush seismic zones, but we do not consider this as strongly suggesting that the two seismic zones are therefore representative of two di¡erent directions of subduction. First, this gap is not the only gap within the 700 km long intermediate-depth seismic zone. A smaller, 30 km gap exists around 750E within the Pamir seismic zone, and within any 25 km depth range there are numerous 20^50 km gaps along the seismic zone. The gap between the Pamir and Hindu Kush seismic zones would appear to be the largest but its existence need not favour interpretation of the Pamir^Hindu Kush zone as two separate subduction zones, rather than one contorted subduction zone. In the one-slab model, the gap in seismicity could be interpreted as evidence for a tear in the plate where it has overturned and detached from its shallower continuation. Assuming the intermediate-depth seismicity occurs along a particular interface (or narrow zone), it is then possible that, owing to the tear, there is a length over which the interface no longer exists, resulting in a gap in seismicity. In the two-slab model, the gap occurs between two plates with opposing subduction directions. We are not aware of any other Figure 20. Seismicity section W^W'. ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 585 Figure 21. Seismicity section X^X'. region where two subducting slabs, subducting in opposing directions, are geographically as close to each other as the twoslab model for the Pamir^Hindu Kush region would require. Perhaps the best example of two opposing subduction zones in close proximity is between the Manila and Philippine trenches beneath the Philippine Islands. However, the gap between earthquakes associated with the two subducting slabs at depths >150 km beneath the Philippines is over 300 km. For depths below 100 km, the extrapolation of the trends of the Pamir and Hindu Kush seismic zones across the seismic gap between them shows a remarkable alignment of the zones. The Hindu Kush seismic zone bends signi¢cantly from an E^W trend in the west to a more NE^SW trend in the east, resulting in an apparent alignment with the strike of the Pamir seismic zone. If the two zones are of separate origin, one should expect to see a di¡erence in their trends, or an o¡set between the depth contours either side of the gap that separates the two zones, as shown by Leith & Simpson (1986), but this is certainly not the case. Figure 22. Seismicity section Y^Y'. ß 1998 RAS, GJI 134, 573^595 The inferred bend in the Hindu Kush seismic zone at its eastern end leads to a problem of spatial accommodation of the bent slab at depth. This problem would be ameliorated if the fault identi¢ed by Billington et al. (1977) within the northdipping Hindu Kush seismic zone extends further to the east than suggested by Billington et al. (1977), or, alternatively, if a separate fault exists within the eastern part of the seismic zone at depths greater than 200 km. This inferred fault not only extends across the near-vertical Hindu Kush seismic zone but also extends beyond the zone and forms the inverted V displayed most clearly in section Z3 (Fig. 24). This forked feature is also seen in the data of Roecker et al. (1980), but not in the data of Chatelain et al. (1980). This indicates that although their local data may provide accurate hypocentres, the brief periods of operation of the networks do not give a complete picture of the geometry of the Pamir^Hindu Kush seismic zone. Indeed, the gap in seismicity around 70.60E at 200^250 km depth seen within all of the local studies of 586 G. Pegler and S. Das Figure 23. Location map of Z seismicity sections. Epicentres plotted are for the period 01/01/1964^12/31/1992 for the depth range 75^300 km. Only epicentres with 90 per cent con¢dence limits ¦30 km are shown. Roecker et al. (1980) and Chatelain et al. (1980) is ¢lled by both the ISC and the relocated events presented in this study. Surface expressions of the intermediate-depth seismicity In considering whether the Pamir^Hindu Kush seismicity represents one or two directions of subduction, it is important to ascertain if the intermediate-depth seismicity continues to shallower depths, or if it can be associated with any sutures or fault systems at the surface. As clearly shown in the seismic sections within this study, the Hindu Kush seismic zone extends as shallow as 60 km and possibly reaches the surface in the region of the MMT or MKT. Indeed, it is widely accepted that the Hindu Kush seismic zone represents northward subduction (Billington et al. 1977; Roecker et al. 1980; Chatelain et al. 1980; Hamburger et al. 1992; Burtman & Molnar 1993). However, as shown above, it is less clear whether the Pamir seismic zone represents the subduction of Asian lithosphere to depths of 200 km beneath the Pamirs. Studies which adhere to subduction of the Asian lithosphere to the southeast beneath the Pamirs either point to the Tien Shan (Hamburger et al. 1992) or the northern margin of the Pamirs (Chatelain et al. 1980; Burtman & Molnar 1993; Fan et al. 1994) as the surface locus of the subduction (Fig. 27a). The CMT mechanisms of Fig. 2 clearly demonstrate signi¢cant N^S shortening beneath the Tien Shan and northern Pamirs, although strike-slip faulting also contributes signi¢cantly to deformation along the northern margin of the Pamirs. Hamburger et al. (1992) studied the shallow seismicity beneath the Tien Shan and northern Pamirs, and presented a single NW^SE seismicity section which most closely corresponds to section W^W' (Fig. 20). Hamburger et al. (1992) interpreted their seismicity section as thrusting beneath the Peter I range within the 0^12 km depth range, underlain by thrusting in the 17^35 km depth range, which extrapolates to the surface in the Tien Shan. Hamburger et al. (1992) further postulated that this south-dipping zone might be an up-dip continuation of the intermediate-depth seismicity of the Pamir seismic zone. The shallow seismicity beneath the northern Pamirs and Tien Shan in section W^W' shows similar trends to the section of Hamburger et al. (1992), with a zone of shallow seismicity extending from the surface at the southern margin of the South Tien Shan and dipping southwards to about 40^50 km depth. However, if this zone does link up with the Pamir seismic zone at depth, then it is aseismic over a length of 140 km, between about 40 and 100 km depth. Invariably, published sections ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 587 Figure 24. Seismicity sections Z1^Z4. Hypocentres plotted are for the period 01/01/1964^12/31/1992. Only hypocentres with 90 per cent con¢dence limits ¦30 km are shown. Open boxes represent the projection of CMT T-axes onto the plane of each section. Each of the sections is 20 km wide and only contains hypocentres with 90 per cent con¢dence limits of less than 30 km, as well as the projections of all available T-axes obtained by the CMT solution. across the Pamir seismic zone show a clear gap between the intermediate-depth seismicity and the shallower seismicity along the northern margin of the Pamirs. The gap between the intermediate-depth seismicity of the Pamir seismic zone and the shallow seismicity beneath the northern Pamirs and Tien Shan is shortest in seismic sections T^T' and U^U' (Figs 18 and 19), where it reaches a minimum of 50^60 km. Burtman & Molnar (1993) identify an aseismic portion of the subducting Asian lithospheric plate, between depths of about 40 and 100 km. However, sections T^T' and U^U' imply that the gap in seismicity between the two zones of activity corresponds to a much smaller depth range, and is more of a horizontal gap, rather than an aseismic depth window. This con¢guration is more consistent with two seismic zones of separate origin than with subduction of Asian lithosphere down to 200 km depth. Nowhere does the intermediate-depth Pamir seismic zone extend shallower than 90 km, and its top appears to be marked by an abrupt cut-o¡ in activity. Furthermore, T-axes within a substantial part of the Pamir seismic zone indicate ß 1998 RAS, GJI 134, 573^595 that along-arc bending is the most signi¢cant force acting within the seismic zone. This stress pattern would not be expected within an actively subducting slab, continuous from the surface to a depth of around 200 km. All of the above observations suggest that the intermediate-depth Pamir seismic zone is not due to subduction of Asian lithosphere down to 200 km depth. N^S shortening is clearly occurring within the northern Pamirs and Tien Shan, but there is no conclusive evidence to suggest that this extends to depths greater than 50 km and certainly not to 200 km. The topography of the region highlights another di¡erence between the Pamir and Hindu Kush seismic zones. Sections N^N', O^O' and P^P' show a marked topographic high (the Hindu Kush) above the region where the Hindu Kush seismic zone bends sharply at around 100 km depth. The topographic high is greatest above the centre of the Hindu Kush seismic zone (sections O^O' and P^P') and decreases to the east and west. In section R^R', where the seismic zone in the one-plate model is interpreted to tear as it overturns, the topography shows a 588 G. Pegler and S. Das Figure 25. Same as Fig. 24 but for the seismicity sections Z5^Z8. more gentle incline northwards into the Pamirs. Beneath the Pamir seismic zone, the topography is essentially £at, above what would be the bend in the subducting Asian lithosphere, in the two-subducting-plates model. We do not suggest that there is a causal link between the topographic relief and the geometry of the seismic zones at depth, but merely point out the di¡erence in topography above the two seismic zones. Steepening and along-strike termination of the Hindu Kush seismic zone Another feature of the Pamir^Hindu Kush seismic zone which is more easily explained by a single seismic zone, rather than two opposing seismic zones, is the steepening of the Hindu Kush seismic zone from west to east, becoming vertical and then overturning at its eastern end. This steepening was noted by Chatelain et al. (1980), who suggested that it might be caused by the indenting Indian plate. However, Chatelain et al. (1980) preferred not to extrapolate this model through to the ultimate tearing and overturning of the eastern part of the seismic zone. It seems unnecessary to interpret the Pamir seismic zone, which dips steeply to the southeast and displays seismicity within the same depth range as the Hindu Kush seismic zone, only 50 km away, as a separate feature, especially as the Hindu Kush seismic zone itself overturns to dip to the southeast at its eastern end. To the west, the Hindu Kush seismic zone is clearly bounded by the sinistral strike-slip Darvaz and Chaman fault systems. The Chaman fault system extends for around 1000 km to the south of the Hindu Kush and marks the western boundary of the indenting Indian plate. The eastern Hindu Kush appears to continue geologically eastwards into the Karakoram (Searle 1996a) with no major suture or fault system separating the two mountain ranges. One question requiring an explanation is why the high level of seismicity beneath the Hindu Kush is not continued east of 71.60E, beneath the Karakoram. It is suggested here that the seismicity now underlying the Pamirs was formerly an eastward continuation of the seismicity beneath the Hindu Kush, and represents what was once a 700 km long, north-dipping subduction zone that underlay the Hindu Kush and Karakoram. The eastern limit of the intermediate-depth seismicity of the Pamir seismic zone is approximately along the northern extension of the Karakoram fault up into the northeastern ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone 589 Seismicity under the Karakoram (a) (b) (c) Figure 27. (a) Schematic section through the Pamir region (Fan et al. 1994, copyright by the American Geophysical Union). GKF~Gissal^ Kokshal fault; PT~Pamir Thrust; MBT~Main Boundary Thrust; MMT~Main Mantle Thrust; IS~ Indus^Tsangpo Suture. (b) Our interpretation of the Pamir^Hindu Kush seismic zone. HK~Hindu Kush seismic zone; P~Pamir seismic zone; KaF~Karakoram Fault, N~north. The Pamir seismic zone is concluded to have torn away from the Hindu Kush seismic zone and overturned at depths greater than 90 km. The stippling and the two dotted lines with arrows are used to illustrate this over-turning. (c) Schematic section for the Karakoram from Fan et al. (1994, copyright by the American Geophysical Union). MCT~Main Central Thrust; MBT~Main Boundary Thrust; MFT~Main Frontal Thrust. margin of the Pamirs. It is further suggested that the Karakoram fault marks the eastern limit of the Pamir^ Hindu Kush seismic zone, and that dextral movement along the Karakoram fault has accommodated the overturning of the single, north-dipping, subducting slab to the east of 71.60E. Fig. 27(b) shows a possible con¢guration of the Pamir^Hindu Kush seismic zone at depth, based on the model of a single, northward-subducting system, which has torn and overturned beneath the Pamirs. ß 1998 RAS, GJI 134, 573^595 Fan et al. (1994) suggested that the subduction of the Asian lithosphere beneath the Pamirs extends further to the east beneath the Karakoram. Fan et al. (1994) interpreted their section D^D' (Fig. 16a of Fan et al. 1994) as representing steep subduction of the Asian plate, which is being bent down by the more gently dipping Indian plate (Fig. 27c). However, section D^D' of Fan et al. (1994) is over 200 km wide and the seismicity at greater than 100 km depth is seismicity projected from the Pamir seismic zone. We have relocated the seismicity in the Karakoram region, shown in Fig. 26 in map view and section. Section KA^KA' reveals a much clearer picture of the Karakoram seismicity than section D^D' of Fan et al. (1994). Note that the CMT mechanism plotted in Fig. 26, outside section KA^KA', is event 10 of Fan et al. (1994), which was included within section D^D' of Fan et al. (1994), and indicates how intermediate-depth seismicity from the Pamir seismic zone was included within their seismicity section for the Karakoram. Section KA^KA' de¢nes a triangular wedge, bounded by the MKT to the southwest and the Tarim basin to the northeast, producing a pop-up structure for the Karakoram. This interpretation ¢ts in well with a geological section taken across the Karakoram (Searle & Tirrul 1991) in which the MKT is interpreted as a deep-seated, breakback thrust, extending to around 100 km beneath the Karakoram. Section KA^KA' also implies thrusting of the Kunlun over the Tarim, by as much as 200 km and down to 100 km depth. A gravity pro¢le across the Karakoram, Kunlun and Tarim basin would appear to support the above ¢ndings. Fig. 28 shows the isostatic gravity anomaly pro¢le across the region (Molnar 1988). Three negative anomalies are seen which correspond to the foreland basin of the Himalayas, the Karakoram and the Tarim basin (Molnar 1988). Molnar (1988) suggested that in order to account for the gravity anomaly across the southern margin of the Tarim basin by a simple plate model, in which the Tarim basin underthrusts the Kunlun, the thrusting would probably have to extend over 100 km southwest beneath the Kunlun. Such a length of underthrusting is consistent with the seismicity of section KA^KA'. Molnar (1988) went on to argue that the negative gravity anomaly beneath the Karakoram was evidence for abnormally cold mantle, which is consistent with what might be expected beneath a zone of intense crustal shortening. Furthermore, Molnar (1988) suggested that the downwelling of mantle material beneath the Karakoram may be suppressing the Moho in this region, allowing the unusually deep seismicity (Fig. 28). In the two-slab model for the Pamir^Hindu Kush seismic zone, the Karakoram region lies well to the south of the area where downwelling of the mantle might be expected. However, within the single-slab model the Karakoram marks the region where most downwelling of mantle material is expected. Implications for mantle dynamics The seismic zone under the Hindu Kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity. The Pamir region, however, has horizontal T-axes that follow the trend of the contorted seismic zone. This implies that the Pamir seismic zone is under horizontal shear and appears to be a slab caught up in £ow within the upper 590 G. Pegler and S. Das acceleration (mGal) ISOSTATIC GRAVITY ANOMALIES 100 0 –100 elevation (m) KARAKORAM HIMALAYA KUNLUN 5000 TARIM BASIN INDUS PLAIN 0 Crust Mantle Lithosphere Asthenosphere 500 km Figure 28. Sketches of isostatic gravity anomalies, topography and an interpreted section across the Himalaya, Karakoram and Kunlun (after Molnar 1988). mantle. The observed seismicity under the Pamirs is thus a manifestation of the deformation of the slab induced by this £ow. AC K NOW L E D GM E N T S One of the authors (GP) was supported by the NERC studentship GT4/91/GS/109. We would like to thank Jim Dewey for use of his earthquake relocation programs. Gary Pavlis provided profound thought-provoking comments and we are very grateful to him for this. R E F E R E NCE S Avouac, J.-P. & Tapponnier, P., 1993. Kinematic model of active deformation in central Asia, Geophys. Res. Lett., 20, 895^898. Billington, S., Isacks, L.B. & Barazangi, M., 1977. Spatial distribution and focal mechanisms of mantle earthquakes in the Hindu Kushö Pamir region; a contorted Benio¡ zone, Geology, 5, 699^704. Burtman, V.S. & Molnar, P., 1993. Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir, Tech. rept. Spec. Pap., 281, Geological Society of America. Chatelain, J.L., Roecker, S.W., Hatzfeld, D. & Molnar, P., 1980. Microearthquake seismicity and fault plane solutions in the Hindu Kush region and their tectonic implications, J. geophys. Res., 85, 1365^1387. Chen, W.P. & Molnar, P., 1983. Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere, J. geophys. Res., 88, 4183^4214. DeMets, C., Gordan, R.G., Argus, D.F. & Stein, S., 1990. Current plate motions, Geophys. J. Int., 101, 425^478. Dewey, J.W., 1971. Seismic studies with the method of joint hypocenter determination, PhD thesis, University of California, Berkeley, CA. Dewey, J.W., 1983. Relocation of instrumentally recorded pre-1974 earthquakes in the South Carolina region, in Studies Related to the Charleston, South Carolina, Earthquake of 1886öTectonics and Seismicity, pp. Q1^Q9, ed. Gohn, G.S., US Geol. Surv. Prof. Paper 1313. Dziewonski et al. 1983^1994. Centroid-moment tensor solutions 1977^1993, Phys. Earth planet. Inter., 33^83. England, P.C. & Houseman, G.A., 1989. Extension during continental convergence with applications to the Tibetan Plateau, J. geophys. Res., 94, 17 561^17 579. Fan, G. & Ni, J.F., 1989. Source parameters of the 13 February 1980, Karakoram earthquake, Bull. seism. Soc. Am., 79, 945^954. Fan, G., Ni, J.F. & Wallace, T.C., 1994. Active tectonics of the Pamirs and Karakoram, J. geophys. Res., 99, 7131^7160. Galitzin, B., 1915. Sur le tremblement de terre du 18 fevrier 1911, Comptes Rendues, 160, 810^813. Gutenberg, B. & Richter, C.F., 1954. Seismicity of the Earth and Associated Phenomena, Princeton University Press, Princeton, NJ. Hamburger, M.W., Sarewitz, D.R., Pavlis, T.L. & Popandopulo, G.A., 1992. Structural and seismic evidence for intracontinental subduction in the Peter the First range, central Asia, Geol. Soc. Am. Bull., 104, 397^408. Isacks, B. & Molnar, P., 1971. Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes, Rev. Geophys. Space Phys., 9, 103^174. Je¡reys, H., 1923. The Pamir earthquake of 1911 February 18, in relation to the depths of earthquake foci, Month. Not. R. astr. Soc., Geophys. Suppl., 1, 22^31. Katok, A.P., 1988. On the deepest earthquake in the PamiröHindu Kush zone, Izv. Acad. Sci. USSR Phys. Solid Earth, Transl., 24, 649^653. Kirby, S., Engdahl, E.R. & Denlinger., 1996a. Intermediate-depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs, in Subduction: Top to Bottom, American Geophysical Union, Geophys. Monogr., Vol. 96, Washington, DC. Kirby, S.H., Stein, S., Okal, E.A. & Rubie, D.C., 1996b. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere, Rev. Geophys., 34, 261^306. Leith, W. & Simpson, D.W., 1986. Seismic domains within the Gissar-Kokshal seismic zone, Soviet central Asia, J. geophys. Res., 91, 689^697. Mellors, R.J., Pavlis, G.L., Hamburger, M.W., Al-Shukri, H.J. & Lukk, A.A., 1995. Evidence for a high-velocity slab associated with the Hindu Kush seismic zone, J. geophys. Res., 100, 4067^4078. Molnar, P., 1988. A review of geophysical constraints on the deep structure of the Tibetan Plateau, the Himalaya and the Karakoram and their tectonic implications, Phil. Trans. R. Soc. Lond., A326, 33^88. Molnar, P. & Tapponnier, P., 1975. Cenozoic tectonics of Asia, e¡ects of a continental collision, Science, 189, 419^426. Ni, J. & Fan, G.W., 1989. Fault plane solutions of earthquakes and active tectonics of the Pamir-Karakoram region, EOS, Trans. Am. geophys. Un., 70, 1226. Nowroozi, A.A., 1971. Seismotectonics of the Pakistan plateau, eastern Turkey, Caucasus, and Hindu Kush regions, Bull. seism. Soc. Am., 61, 317^341. Pavlis, G.L. & Hamburger, M.W., 1991. Aftershock sequences of intermediate-deep earthquakes in the Pamir-Hindu Kush seismic zone, J. geophys. Res., 96, 18 107^18 117. Pegler, G., 1995. Studies in seismotectonics, PhD thesis, Department of Earth Sciences, University of Oxford. Rex, A.J., Searle, M.P., Tirrul, R., Crawford, M.B., Prior, D.J., Rex, D.C. & Barnicoat, A., 1988. The geochemical and tectonic ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone evolution of the central Karakoram, North Pakistan, Phil. Trans. R. Soc. Lond., A326, 229^255. Richter, C.F., 1958. Elementary Seismology, W.H. Freeman & Co., San Francisco, CA. Roecker, S.W., 1982. Velocity structure of the Pamir-Hindu Kush region: possible evidence of subducted crust, J. geophys. Res., 87, 945^959. Roecker, S.W., Soboleva, V., Nersesov, I.L., Lukk, A.A., Hatzfeld, D., Chatelain, J.L. & Molnar, P., 1980. Seismicity and fault plane solutions of intermediate depth earthquakes in the Pamir-Hindu Kush region, J. geophys. Res., 85, 1358^1364. Searle, M.P., 1991. Geology and Tectonics of the Karakoram Mountains, John Wiley, New York, NY. Searle, M.P., 1996a. Geological evidence against large-scale preHolocene o¡sets along the Karakoram fault: implications for the limited extrusion of the Tibetan plateau, Tectonics, 15, 171^186. Searle, M.P., 1996b. Geological Map of North Pakistan (and adjacent areas of northern Ladakh and western Tibet) 1 : 650,000 scale, Dept Earth Sciences, Oxford University. Searle, M.P. & Tirrul, R., 1991. Structure and thermal evolution of the Karakoram crust, J. geol. Soc. Lond., 148, 65^82. Tapponnier, P. & Molnar, P., 1976. Slip-line ¢eld theory and large scale continental tectonics, Nature, 264, 319^324. Tapponnier, P. & Molnar, P., 1977. Active faulting and tectonics in China, J. geophys. Res., 82, 2905^2930. Tapponnier, P. & Molnar, P., 1979. Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia, and Baykal regions, J. geophys. Res., 84, 3425^3459. Turner, H.H., 1922. On the arrival of earthquake waves at the antipodes, and the measurement of the focal depth of an earthquake, Mon. Not. Ry. Astr. Soc. Geophys. Suppl., 1, 1^13. Vinnik, L.P., Lukk, A.A. & Nersesov, I.L., 1977. Nature of the intermediate seismic zone in the mantle of the Pamir-Hindu Kush, Tectonophysics, 38, T9^T14. Vinnik, L.P., Lukk, A.A. & Mirzokurbonov, M., 1978. Quantitative analysis of velocity inhomogeneities of the PamirsöHindu Kush upper mantle, Izv. Acad. Sci. USSR Phys. Solid Earth, Transl., 14, 319^328. Windley, B.F., 1988. Tectonic framework of the Himalaya, Karakoram and Tibet, and problems of their evolution, Phil. Trans. R. Soc. Lond., A326, 3^16. A PPE N D I X A : D ETA I LE D D I S C US S I O N OF R E L O CAT E D S EI SM IC I T Y Relocated seismicity in di¡erent depth ranges 1. 0^75 km depth range (Fig. 2) Most of the shallow activity is concentrated along the narrow band extending from 690E, 38.50N to 780E, 400N, along the northern margin of the Pamirs. This is the Gissar^Kokshal seismic zone (Leith & Simpson 1986), de¢ned by the Darvaz^ Karakul and Vakhsh fault systems. The Darvaz fault marks the boundary between the northwestern margin of the Pamirs and the Tadjik basin, and trends roughly NE^SW, merging with the E-W-trending Vakhsh thrust around 710E, 39.50N. Earthquake focal mechanisms (e.g. 11/01/78 and 10/26/84) along the Darvaz fault are consistent with geological evidence that the Darvaz fault is a highly active, sinistral strike-slip fault with a slip rate of around 10^15 mm a{1 (Burtman & Molnar 1993). The Vakhsh thrust, to the north of the Peter I range, and the Trans-Alai thrust, to the northeast of the Pamirs, display active N^S thrust faulting (mechanisms 12/18/77 and 08/12/88), although there is also evidence for right-lateral strike-slip faulting within the region, as suggested by mechß 1998 RAS, GJI 134, 573^595 591 anisms 08/25/83 and 04/17/90. North of the Pamirs there is active NNW^SSE thrusting at the northern margin of the South Tien Shan (e.g. 10/13/85 and 05/06/82). There is also evidence for NW^SE dextral strike-slip faulting along the Talaso^Ferghana fault (mechanism 02/13/82). The NW^SE trend of seismicity running from 770E, 310N to 730E, 350N represents shallow northeastward subduction of the Indian plate beneath the Himalayas. West of 730E, to the west of the Hazara syntaxis, the earthquake mechanisms suggest northwestward subduction of the Indian plate. Nearly due north of the Hazara syntaxis, between 36.80N and 37.50N, there is a N^S-trending cluster of events which indicate E^W extension (e.g. mechanism 03/05/90). This cluster of events relocates within the 40^60 km depth range and not at the surface. However, further north, at 38.50N, there is another mechanism displaying E^W extension. Geological evidence for E^W extension within this region of the central Pamirs is seen in N^S-trending extensional faults around a lake near Ozera Kara Kul (73.50E, 390N) (Searle 1996a). Burtman & Molnar (1993) suggested that this E^W extension might be indicative of a cessation of crustal thickening within the Pamirs, as did England & Houseman (1989) for Tibet. A notable feature of the seismicity in the 0^75 km depth range is the lack of signi¢cant seismicity along the NW^SE-trending dextral Karakoram fault. This fault is clearly seen on satellite images of the region, is generally regarded as one of the most signi¢cant tectonic features in the region, and is believed to have a high present-day slip rate of as much as 32 mm a{1 (Avouac & Tapponnier 1993). 2. 75^100 km depth range (Fig. 3) The seismicity in this depth range is restricted to a much smaller area than the shallower seismicity, and there seems to be little correlation between the 0^75 and the 75^100 km depth events. The relocated seismicity in the 75^100 km depth range can be considered as three clusters, marked a, b and c. Cluster a de¢nes an E^W trend (not seen in the ISC seismicity). The three CMT mechanisms within cluster a (04/27/85, 04/17/79 and 05/17/90) have horizontal T-axes, and for the ¢rst two events parallel the trend of this group of events. (This behaviour of the T-axes also will be seen in the earthquakes in the 100^125 km depth range in this region.) Cluster b shows a slight elongation in the NNE^SSW direction relative to the width of the relocated seismicity zone, and CMT mechanisms display subvertical T-axes and roughly horizontal N^S-trending P-axes. Cluster c has an arcuate trend, varying from E^W at its western end through to NE^SW at its northeastern end. The seismicity south of 360N is generally shallower than that of cluster c. Cluster c contains CMT mechanisms with predominantly subvertical T-axes, and B-axes which closely parallel the trend of the cluster. 3. 100^125 km depth range (Fig. 4) The earthquakes form a narrow S-shaped band of seismicity approximately 700 km in length and no wider than 40 km. Gaps in seismicity exist along the seismic zone which may suggest that this S-shaped seismicity does not represent a single feature. Two of the largest gaps in seismicity along this S feature occur where the trend of the S changes most 592 G. Pegler and S. Das sharply, and are marked d and e. Gap d was also identi¢ed by Chatelain et al. (1980) and Roecker et al. (1980). It is approximately 40 km long and marks the boundary between what is often referred to in the literature as the Hindu Kush (to the southwest) and the Pamir (to the northeast) seismic zones. Clearly, the shallower clusters a and c described for the 75^100 km depth range are located along the up-dip continuation of the S-shaped seismic zone. Cluster b occurs at the southern end of the Pamir seismic zone and straddles a portion of the seismic gap d. As discussed in the paper, we do not consider cluster b to be an up-dip continuation of the S-shaped seismic zone. The CMT mechanisms for earthquakes north of 370N show a variation of types and were also discussed by Fan et al. (1994). Two events (03/07/82 and 03/26/88) have horizontal T-axes which parallel the strike of the S-shaped seismic zone. This is similar to the behaviour of the T-axes in this region at the 75^100 km depth range. Some mechanisms between 370N and 380N have B-axes which parallel the NE^SW strike of the S-shaped zone and T-axes which plunge to the southeast, roughly parallel to the southeast dip of the S-shaped seismic zone in this region. However, all CMT mechanisms within the Pamir seismic zone between 100 and 125 km depth and west of 740E have P-axes which are perpendicular to the strike of the S-shaped zone and plunge to the northeast. South of 370N, within the Hindu Kush seismic zone, all CMT mechanisms have approximately vertical T-axes, and B-axes which generally parallel the strike of the seismic zone, that is the classical pattern for subduction. 4. 125^150 km depth range (Fig. 5) This depth range, though less active than the 100^125 km range, shows a continuation of the S-shaped seismic zone that was clearly seen in Fig. 4. The width of this S-shaped seismic zone is about 20 km for the well-relocated events at this depth. Again, gaps of up to 60 km in length occur along the seismic zone but the apparent continuity of the trend of the seismic zone between the gaps is remarkable. The seismic gap, marked d in Fig. 4, is wider at the 125^150 km depth range than at the 100^125 km depth range, but an extrapolation of the trend of the Pamir seismic zone around 370N, across seismic gap d, would link it up with the ENE^WSW-trending, slightly arcuate Hindu Kush seismic zone. Only three events from the 125^150 km depth range have CMT solutions, all are within the Hindu Kush seismic zone and two have near-vertical T-axes. 5. 150^175 km depth range (Fig. 6) The seismicity is sparse and the earthquakes are small. A large number of small events, assigned mb ~0.0, are reported by the ISC for the 150^175 km depth range, most of which had too few reported phase arrivals to be relocated. The well-constrained relocated events occur in four clusters, two beneath the Pamirs and two beneath the Hindu Kush. Each cluster is elongated along the S-shaped arc, suggesting continuity of this feature, although the combined lengths of the gaps in this depth range are almost of the same size as the arc lengths with seismicity. The curvature of the S-shaped feature has decreased from that seen for the 100^125 km range. The single CMT in the 150^175 km depth range is within the Hindu Kush seismic zone and again it has a vertical T-axis. 6. 175^200 km depth range (Fig. 7) The earthquakes are larger than in the 150^175 km depth range and have two dense clusters, one in the central and one in the eastern Hindu Kush seismic zone, as well as a narrow forked seismic zone in the western Hindu Kush and a narrow NE^SW-trending Pamir seismic zone. All six available CMT mechanisms in the central Hindu Kush cluster have steeply southeast-plunging T-axes. 7. 200^225 km depth range (Fig. 8) This depth range is characterized by intense seismic activity along the arcuate Hindu Kush seismic zone with many large events, with no activity (in the study period) along the Pamir seismic zone. Almost all available CMT mechanisms within the Hindu Kush seismic zone display near-vertical T-axes, although most of the T-axes plunge towards the northeast, rather than the southeast as for the 175^200 km depth range. The CMT mechanisms within the western and central Hindu Kush seismic zone have B-axes which parallel the zone; however, within the eastern part of the seismic zone a clear pattern is not obvious. A cluster of events occurs to the south of the main arcuate Hindu Kush seismic zone at its eastern end. This cluster of events, located near (36.40N, 71.20E), is also seen in the next depth range. 8. 225^250 km depth section (Fig. 9) The cluster at (36.40N, 71.20E) is elongated NE^SW in this depth range into a second zone of seismicity, paralleling the main arcuate Hindu Kush seismic zone, some 30 km to the southeast. The horizontal projections of the 90 per cent con¢dence ellipses for the better-constrained events show that the cluster of events to the southeast of the main Hindu Kush seismic zone are truly separated from the main seismic zone. Four of the six CMT mechanisms within the 225^250 km depth range in the eastern Hindu Kush seismic zone have E^W-oriented P-axes, and one event has a normal mechanism. 9. 250^300 km depth range (not shown; see Pegler 1995) At this depth, seismicity exists along the Hindu Kush seismic zone which extends eastwards of that within the 225^250 km depth range. The earthquakes were too small to have CMT solutions. Other deeper sections are also not shown, although one well-constrained event is located at around 380 km depth beneath the eastern end of the Hindu Kush seismic zone (Katok 1988) and is seen in the section R^R' shown later. Seismicity cross-sections The locations of the vertical cross-sections are shown in Fig. 10. A. Section N^N' (Fig. 11) The relocated earthquakes de¢ne a north-dipping seismic zone at 100^180 km depth beneath the Hindu Kush, which may extend to the surface approximately 300 km to the south of the centre of the section in the vicinity of the westward continuation of the Main Mantle and Main Karakoram ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone thrusts (MMT & MKT, Fig. 1). The north-dipping zone appears to be about 30^40 km wide and the best-constrained relocated events suggest a steepening of the seismic zone in the 100^180 km depth range relative to its dip between the surface and 100 km depth. The events at the northern margin of the South Tien Shan represent thrusting of the Tien Shan over the Ferghana basin. A south-dipping feature in the 10^40 km depth range at the northern margin of the Hindu Kush and focal mechanisms (07/03/84 and 04/14/80) suggest that the Hindu Kush is being thrust over the southern margin of the Tadjik basin. The shallow, di¡use seismicity between the Hindu Kush and Tien Shan (including the 07/05/90 event) indicates thrusting within the Tadjik basin. B. Section O^O' (Fig. 12) This section shows features similar to section N^N'. The north-dipping seismic zone beneath the Hindu Kush displays more seismicity in here than in N^N' and appears to have a convex-up curve, dipping at around 500 in the 70^140 km depth range and 80^900 in the 170^300 km depth range. The seismic zone may extrapolate to a cluster of events at 30 km depth, 270 km south of the centre of the section. We see a similar pattern in the vicinity of the westward continuation of the MMT and MKT, but if so there is a gap of 50^60 km with no well-constrained events. Thrusting of the Tien Shan over the southern margin of the Ferghana basin can be inferred from event 10/13/85. Another feature of section O^O' is di¡use shallow seismicity under the Tadjik basin. C. Section P^P' (Fig. 13) This section also shows the north-dipping Hindu Kush seismic zone in the 90^280 km depth range, with the well-constrained relocated hypocentres de¢ning a zone no more than 40 km thick. However, seismicity is not continuous along the zone but is concentrated in two areas, between 80 and 150 km and 180 and 240 km depth, which dip at 500 north and vertically, respectively. The seismicity in the 180^240 km depth range has been referred to as the Hindu Kush nest of activity (Pavlis & Hamburger 1991). At approximately 80 km depth, 100 km south of the centre of the section, the width of the seismic zone would appear to increase, although, as will become more evident in section Q^Q', this may represent an imbrication of the north-dipping seismic zone. Sparse seismicity in the 30^80 km depth range beneath the south-central Hindu Kush may indicate the up-dip continuation of the Hindu Kush seismic zone. Section P^P' also incorporates the northwestern margin of the Pamirs beneath which there is shallow seismicity. D. Section Q^Q' (Fig. 14) The north-dipping Hindu Kush seismic zone is still seen. The part of it in the 60^100 km depth range is more clearly imaged in this than any other section and de¢nes a narrow seismic zone, less than 30 km wide, which dips at approximately 200 to the north. This zone may extrapolate to a cluster of earthquakes at 40 km depth, 250 km to the south of the centre of the section. Beneath the highest part of the Hindu Kush range (100 km to the south of the centre of the section) there appears to be an o¡set (north-up, south-down) in the Hindu Kush seismic zone as it bends sharply at around 100 km ß 1998 RAS, GJI 134, 573^595 593 depth. As with section P^P', there are few earthquakes within the 150^180 km depth range and a region of high activity between 180 and 270 km depth. The cluster of events in the 180^270 km depth range shows an inverted V-shaped feature. The northern prong of the inverted V displays the higher level of seismicity and dips at approximately 800 to the north. As was seen in the 225^250 km depth range (Fig. 9), the northern prong of the inverted V is continuous with the seismicity along strike westwards around the Hindu Kush seismic zone; the other prong of the inverted V is caused by the cluster of events to the southeast of the main Hindu Kush seismic zone. Another notable feature of this section is in the 70^100 km depth range, from 80 km to the west of the centre of the section to 20 km east of the centre of the section, where a 100 km long, 30 km wide seismic zone, dipping at 100^150 north, links up with the north-dipping Hindu Kush seismic zone. This seismic zone represents cluster b of Fig. 3, and lies above and to the north of the Hindu Kush seismic zone, with focal mechanisms which show N^S shortening (09/15/86 and 02/05/90). E. Section R^R' (Fig. 15) This is the easternmost N^S section in which the Hindu Kush seismic zone is seen. (To the east of 71.50E, only the Pamir seismic zone exists, as we shall see later.) The most notable features of section R^R' are the two clusters of events in the 75^110 km depth range. The clusters appear quite broad (50 km) because section R^R' is slightly oblique to both the Hindu Kush and the Pamir seismic zones. The cluster of events at 230 km depth represents seismicity along the southern prong of the inverted V within the Hindu Kush seismic zone. The well-constrained events deeper than 150 km depth show the Hindu Kush seismic zone to be at least vertical, and the deepest events, a cluster at 280^290 km depth and a single well-constrained event at 380 km depth, suggest that the Hindu Kush seismic zone is overturned beneath about 240 km depth. F. Section S^S' (Fig. 16) Only one event is seen at around 80 km depth, along strike from where the Hindu Kush seismic zone exists further west. A cluster of events at the southern end of this section occurs at 20^50 km depth beneath the MCT and MMT. The dominant feature of this section is the di¡use seismicity in the 100^240 km depth range, which represents seismicity of the Pamir seismic zone. The intermediate-depth seismicity appears di¡use due to the obliqueness of this section to the Pamir seismic zone. G. Section T^T' (Fig. 17) The Pamir seismic zone is clearly seen in this section as a very narrow (20 km) seismic zone dipping to the southeast at approximately 600 between depths of 100 and 160 km. ISC epicentres (Pegler 1995) and the poorly constrained relocated events suggest that the Pamir seismic zone may extend to greater than 200 km depth. The cluster of events at 0^80 km depth, about 160 km north of the centre of the section, represents activity beneath the northern margin of the Pamirs. It is di¤cult to see any clear trends within this shallow 594 G. Pegler and S. Das seismicity, although a near-vertical feature seems to be fairly well imaged. Further to the north, the shallow activity represents thrusting at the northern margin of the Tien Shan. The cluster of events at around 30^50 km depth, 180^250 km to the south of the centre of this section, may represent thrusting along a gently northward-dipping plane, which may extrapolate to the surface around the Nanga Parbat syntaxis. H. Section U^U' (Fig. 18) The well-constrained relocated earthquakes in this section also show the Pamir seismic zone between 100 and 140 km depths and the less well-constrained events down to 250 km. The zone has a southeasterly dip of approximately 400 in the 100^140 km depth range. Shallow seismicity beneath the northern Pamirs is associated with the Darvaz^Karakul and Vakhsh fault systems. As with section T^T', it is di¤cult to see any obvious trends within this shallow seismic zone, although a near-vertical trend associated with the large 11/01/78 strike-slip earthquake is discernible. I. Section V^V' (Fig. 19) The intermediate-depth seismicity of the Pamir seismic zone in this section shows two well-constrained clusters of events. The shallower cluster at 100^120 km depth dips southeast at approximately 300 and contains two events with CMT mechanisms (12/04/92 and 08/20/91) which have T-axes paralleling the dip of the cluster. The deeper cluster of events at 170^220 km depth suggests that the dip steepens to approximately 700 between 120 and 220 km. The `two-slab' model would require linking up these earthquakes between 100 and 220 km depth to the seismicity of the shallow cluster (which is situated more than 130 km horizontal distance to the northwest, and includes the events 01/09/88, 10/26/87 and 11/03/90), even though there is no seismicity between 70 and 100 km depth to indicate such a link. The Darvaz^Karakul and Vakhsh fault systems again show activity in the 0^30 km depth range and the 01/31/77 event indicates thrusting along the northern margin of the South Tien Shan. The cluster of events between 40 and 70 km depth 80 km to the southeast of the centre of this section are the E^W extensional events near 730E, 370N (Fig. 2). The north-dipping trend of seismicity in the 20^60 km depth range, 200 km to the south of the centre of the section, was partly imaged in section T^T'. However, this section shows more clearly the northward dip of this feature, which may extrapolate to the surface in the vicinity of the MMT. J. Section W^W' (Fig. 20) The geometry of the Pamir seismic zone in this section is similar to that in section V^V'; however, the level of seismicity is higher and there is no gap in seismicity within the 120^170 km depth range. As with all the other sections taken across the Pamir seismic zone, the seismicity at the top of the southeast-dipping, intermediate-depth seismic zone terminates abruptly at around 90 km depth; it is seen most clearly in this section. Shallow seismicity is again seen at the northern margin of the Pamirs, although in this section it appears more like a south-dipping wedge than in sections T^T' and U^U'. Again, the two-slab model would require the slab to continue upwards and northwestwards to connect with the shallow cluster to the right of the centre of the section, though practically no seismicity exists in the region in between. K. Section X^X' (Fig. 21) This section is taken through the eastern end of the Pamir seismic zone. This is the least seismically active part of the intermediate-depth Pamir seismic zone. Indeed, for sections taken east of 740E it is necessary to consider data from a section at least 100 km wide in order to see any obvious geometry within the seismic zone. The ISC data for this section (not shown; see Pegler 1995) show a thin seismic zone dipping at approximately 700S between 100 and 150 km depth. The relocated seismicity de¢nes a more vertical trend and activity over a slightly smaller depth range. The shallow seismicity occurs predominantly beneath the Tarim basin but there is no clear suggestion of any trends within the seismicity. L. Section Y^Y' (Fig. 22) This section is approximately parallel to the trend of much of the Pamir seismic zone and gives another view on the seismicity. The top of the intermediate-depth seismicity of the Pamir seismic zone appears to have an undulating upper surface, which can be divided into three segments. The ¢rst runs from the Hindu Kush seismic zone (120 km from Y), where the top of the seismic zone is at approximately 75 km depth, along to the centre of section Y^Y', where its top is at about 100 km. The second segment continues for 100 km further northeast, with a convex upper surface, and the third segment continues until 220 km east of the centre of Y^Y', where the top of the Pamir seismic zone is at a depth of about 90 km. Sections Z1^Z8 A set of thinner, more arc-perpendicular sections is presented to examine the region where the Hindu Kush and Pamir seismic zones `meet'. Fig. 23 shows the location of this set of sections. Sections Z1 to Z8 are shown in Figs 24 and 25. Sections Z1 to Z5 show the north-dipping Hindu Kush seismic zone. However, this zone is only imaged shallower than 100 km in sections Z1 and Z2. The eastward disappearance of the up-dip continuation of the Hindu Kush seismic zone coincides with the ¢rst appearance of the cluster of events to the north of the Hindu Kush seismic zone (cluster b of Fig. 3). The eastward steepening of the Hindu Kush seismic zone in the 100^300 km depth range is also seen between sections Z1 and Z5, with the overturning of the seismic zone in Z4 and Z5. Sections Z1 to Z4 also allow a closer analysis of the inverted V-shaped structure seen in section Q^Q'. Within section Z1, between 180 and 220 km depth, there is a clear 850 dip to the south in a zone of seismicity which cuts across the northdipping seismic zone. This feature was noted by Billington et al. (1977), who identi¢ed the feature as a fault plane within the western part of the Hindu Kush seismic zone (Fig. 6. of Billington et al. 1977). However, sections Z2, Z3 and Z4 ß 1998 RAS, GJI 134, 573^595 Pamir^Hindu Kush seismic zone suggest that this feature has developed further eastwards within the Hindu Kush seismic zone than inferred by Billington et al. (1977). Seismicity beneath the Karakoram The intermediate-depth seismicity beneath the Pamirs terminates eastwards at around 75.50E. However, there are some intermediate-depth events further to the southeast, beneath the Karakoram. These events have been interpreted as evidence for the deep subduction of the Asian lithosphere beneath the Karakoram (Fan et al. 1994). Fig. 26 shows ß 1998 RAS, GJI 134, 573^595 595 the seismicity of the Karakoram region. Section KA^KA' is a section taken through this region oriented roughly perpendicular to the strike of the Karakoram fault, the MKT and the Himalayan thrust front to the south. Clearly, the seismicity de¢nes a triangular wedge which extends to a depth of about 100 km. The north-dipping southwestern boundary of the wedge intersects the surface along the MKT, and the northern boundary of the wedge intersects the surface at the southern margin of the Tarim basin. The CMT mechanism shown in section KA^KA' is that of the 02/13/80 earthquake and is interpreted as thrusting along the MKT at 77 km depth. Figure 1. Map showing the Pamir^Hindu Kush region. Topography is contoured at 1000 m intervals (0^1000 m: green; 1000^2000 m: yellow; etc.) Faults are marked in black. VT~Vakhsh Thrust; DKF~Darvaz^Karakul Fault; KaF~Karakoram Fault; AF~ Andarab Fault; HF~Herat Fault; PF~Panjer Fault; CF~Chaman Fault; ISZ~Indus Suture Zone; MBT~Main Boundary Thrust; MKT~Main Karakoram Thrust; MMT~Main Mantle Thrust; NP~Nanga Parbat; H~Haramosh; HS~Hazara Syntaxis; ATF~ Altyn Tagh Fault; AKF~Atushi^Keping Fault; TFF~Talas Ferghana Fault. Coloured contours represent the intermediate-depth seismicity of the Pamir^Hindu Kush seismic zone (red: 100 km; orange: 125 km; yellow: 150 km; green: 175 km; turquoise: 200 km; blue: 225 km; purple~: 250 km). The black arrow shows the direction of the plate motion of India relative to Eurasia (DeMets et al. 1990). ß 1998 RAS, GJI 134, 573^595 ß 1998 RAS, GJI 134, 573^595 Figure 10. Map showing location of seismicity sections. Contours represent the intermediate-depth seismicity of the Pamir^Hindu Kush seismic zone (red: 100 km; orange: 125 km; yellow: 150 km; green: 175 km; turquoise: 200 km; blue: 225 km; purple: 250 km). Faults are marked in black (Searle 1996b). Figure 2. Relocated seismicity depth range 0^75 km for the period 01/01/1964^12/31/1992, together with selected CMT mechanisms. Solid dots represent epicentres with 90 per cent con¢dence limits ¦30 km. Hollow dots represent the remaining relocated events which have greater hypocentral uncertainties and are plotted at half the diameter of the solid dots for events with equivalent mb . The size of each dot is related to the earthquake magnitude, the size key being shown in Fig. 5. (Note that the smallest dots may appear solid but are actually hollow circles.) Few ISC events with indeterminate depths are successfully relocated. Faults are marked in black. Contours representing the intermediate-depth seismicity of the Pamir^ Hindu Kush seismic zone are the same colours as in Fig. 1. CMT mechanisms are plotted as lower-hemisphere projections with compressional quadrants in red. The month, day and year for the CMT solutions are shown next to them. To maintain clarity in the ¢gure, only su¤cient CMT solutions to indicate the type of faulting in the region are shown. For example, in regions of high activity with many CMT mechanisms for earthquake clusters, only a few are shown. ß 1998 RAS, GJI 134, 573^595 Figure 26. Seismicity across the Karakoram region.The map view (upper) shows all relocated seismicity covering the period 01/01/1964^12/31/1992. The location of section KA^KA' (lower) is also shown. Contours represent the intermediate-depth seismicity of the Pamir seismic zone. CMT mechanisms for the 07/29/77 and 02/13/80 events discussed in the text are shown as lower- and back-hemisphere projections in map view and section, respectively. Fault are shown in black. MKT~Main Karakoram Thrust. Units etc. are the same as in earlier map-view and cross-section ¢gures. ß 1998 RAS, GJI 134, 573^595