JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B9, PAGES 15,779-15,795, SEPTEMBER 10, 1993 Three-Dimensional Elastic Wave Velocity Structure of the Western and Central Tien Shan S. W. ROECKER, 1 T. M. SABITOVA, 2 L. P. VINNIK, 3 Y. A. BURMAKOV, 3 M. I. GOLVANOV, 4 R. MAMATKANOVA, 2 AND L. MUNIROVA 2 Arrival times of compressional (P) andshear (S) waves generated by earthquakes at local andteleseismic distances andrecorded by seismographs located in the westem andcentral Tien Shanare used to determine one- and three-dimensional elasticwave velocity structures of the crustand upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in thisarea is 50 km. The three-dimensional structure of theupper crust reveals thicksediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity,suggesting the presence of an activedecollemont. These low velocities extend continuously to the southeast towardIssyk-Kul, suggesting a structural relationship between the two. However, rather thanbeingconsumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. Thelow-velocity sediments in the Fergana basin reach depths of 10 km andarebounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shanthanbeneath the western Tien Shan.This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than300 km depth. Thereis no evidence for a lithospheric root beneath thispartof the range; rather, thelow velocities implythepresence of a positive buoyancy force uplifting themountains. Evidence thatthislow-velocity region existed before the collision suggests that theTienShan maynotoweitsrejuvenation simply to its location at thenorthern edge of a strong Tarimbasin but rather to an anomalous upper mantle thatwaseasier to deform thanthe surrounding lithosphere. INTRODUCTION The Tien Shan is the most prominent mountain belt of central Asia. It extendsroughly east-west over a distance of about2500 km from the Kyzyl Kum desert to the Gobi desert, and reaches a maximum width of 400 km at its western end. The belt is composed of a system of severalparallel ranges with elevations greaterthan 4000 m (the highest peaksexceed 7000 m) separating depressions of 2000-3000 m elevation. This actively deforming area is surrounded by regions of stability, beingbordered to the northand westby the Kazakhshield, the Turanianplatform, and the Dzungarian basin,and to the south and east by the Tarim massif and the Trans-Tien Shan depression. On the basis of distinct depositionand accretionhistories, the Tien Shan can be divided into three fault bounded units [Kravchenko, 1979]. All three units consist largely of sedimentary rocks that formed during the late Proterozoic (in the north) to Cambrian (in the south). These units were accreted onto Eurasiabeginning in the early Paleozoic for the 1Department of Earth and Environmental Sciences, Rensselaer Pol_Etechnic Institute, Troy, New York. 2Institute of Seismology, Kirghiz Academy of Sciences, Bishkek, Kirgihizstan. øInstitute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia. 4Institute of Seismology, Uzbek Academy of Sciences, Tashkent, Uzbekistan. Copyright 1993by the American Geophysical Union. Paper number 93JB01560. 0148-02227/93/93JB-01560505.00 northern unit to the late Carboniferous for the southern unit [Burtman, 1975, 1987; Krestnikov, 1962]. The whole area appears to have been stablethroughout the Mesozoic, with very little relief in the late Cretaceous. Tectonicactivity resumed in the Oligocene, presumably as a consequence of the collision of India with Eurasia and has continuedto the present day. The present highlevel of activity is evidenced in north- south shortening accommodated along both east-west trending thrust faults and northwest-southeast trendingstrike-slip faults such as the Talasso-Fergana fault (Figure1). The region is seismically very active; several earthquakes with M s > 8.0 have occurred in this areasince1900. Analyses of large earthquake data [e.g., Ni, 1978; Vilkas, 1982;Nelson et al., 1987] have shown that thrusting occurs on moderately dipping (35' - 55') fault planes. Some events,for example, those alongthe southern border of the Tien $han andwithin the Fergana basin, have fairly shallow depths(10-20 km), while those on the faults to the north of the Tien $han can approach depths of 40 km or more. The deformation suggested by these earthquakes is in the form of upliftedblocks alongthrusts that bound these blocks to the north and south. The Tien Shan is located about 2000 km from the collisional front, and the dynamics of its rejuvenationare enigmatic. It hasbeen proposed [e.g., England and Houseman, 1985] that the Tien Shan is active because it is located at the northern edge of an abnormally strongpiece of lithosphere beneaththe Tarim Basin. The basin is believed to be strong because it displays little internal deformation, instead transferring stress from Tibet to the more normallithosphere at its northern edge. However, residuals of multibounce S phases [Grand and Helmberger, 1985] that traverse the lithosphere beneath the Tarim Basin indicate slow shear wave velocities which suggests that this lithosphere is relatively weak. 15,779