DISTRIBUTED DEFORMATION IN SOUTHERN AND WESTERN TIBET DURING AND AFTER THE INDIA-ASIA COLLISION

Field and radiometric data are used to describe and date strain and st ress states in southern (longitude 88 degrees to 91 degrees E, latitud e 28 degrees to 30 degrees N) and western Tibet (longitude 79 degrees to 82 degrees E, latitude 30 degrees to 34 degrees N). We factorize de formation into syncollisional and postcollisional, and we present stre tching lineation and displacement orientation maps, two sections acros s the Indian shelf sequence, and stress orientations calculated from m esoscale fault slip data. In southern Tibet, syncollisional stretching and displacement directions trend 9 degrees+/-46 degrees and displace ment is top to south. Synkinematic, low-grade metamorphism is dated at 50 Ma at one locality in the Indian shelf sequence underlying the mai n mantle thrust of the Indus-Yarlung suture. This implies Paleocene on set of continental collision for the investigated section. Postcollisi onal structures comprise a ''backthrust'' group, which includes forela nd- and hinterland-directed thrusts, reverse and strike-slip faults, a nd folds. It dominates postcollisional deformation, is concentrated al ong the Indus-Yarlung suture, and portrays N-S compression (sigma(1) t rend of 8 degrees+/-17 degrees, sigma(2) of 97 degrees+/-17 degrees). A ''strike-slip'' group consists of conjugate strike-slip faults, is c oncentrated in east trending, narrow, highly deformed zones, and indic ates that N-S compression is locally compensated by E-W extension (sig ma(1) of 15 degrees+/-29 degrees, sigma(3) of 103 degrees+/-30 degrees ). Synkinematic muscovite dates postcollisional deformation as late ea rly Miocene (17.5 Ma) at one locality at the suture. Strike-slip and o blique normal (sigma(3) of 60 degrees+/-23 degrees, sigma(1) of 144 de grees+/-21 degrees) and normal (sigma(3):114 degrees+/-16 degrees) fau lting, dated between late Miocene and Recent and including active defo rmation, represents (dominant) E-W and minor N-S extension due to E-W stretching of southern Tibet and oroclinal bending along the Himalayan arc. Restoring syncollisional and postcollisional deformation yields a minimum of 67% (258 km) shortening across the Indian shelf sequence. Incorporating recently published contraction estimates across the eas tern Himalaya yields minimum shortening between undeformed India and t he Indus-Yarlung suture of 66% (536 km). The Himalaya-Tibet orogenic s ystem south of the Indus-Yarlung suture had an initial width of greate r than or equal to 811 km in the southern Tibetan section. In western Tibet, imbrication of an ophiolite sequence of the Bangong-Nujiang sut ure is top to south (stretching lineation trend of 15 degrees+/-18 deg rees), and sigma(3) of active deformation trends ESE. Faulting along t he Shiquanhe fault zone, which transfers displacement from the norther n part of the Karakorum fault to a system of rifts in western central Tibet, indicates dextral strike-slip alternating with sinistral-obliqu e normal faulting and block rotations around vertical axes during a pr olonged shearing history. The Indian Shelf sequence south of Mount Kai las shows top to south imbrication (stretching lineation trend of 52 d egrees+/-60 degrees). Both Indian shelf rocks and (?Oligocene-Miocene) Kailas conglomerates record backthrusting and backfolding (sigma(1) o f 33 degrees) and Recent E-W extension (sigma(3) of 85 degrees+/-28 de grees).