GEOLOGICAL EVIDENCE AGAINST LARGE-SCALE PRE-HOLOCENE OFFSETS ALONG THE KARAKORAM FAULT - IMPLICATIONS FOR THE LIMITED EXTRUSION OF THE TIBETAN PLATEAU

Two end-member models proposed to accommodate the convergence between India and Asia north of the Himalaya are (1) homogeneous crustal thick ening of the Tibetan plateau and (2) continental escape, or extrusion, of Tibet and southeast Asia, away from the indenting Indian plate. Fo remost among the arguments supporting the latter would be large-scale (similar to 1000-km) offsets and high present-day slip rates along the major strike-slip faults bounding the postulated extruding crust, not ably the Altyn Tagh Fault along the northern margin of Tibet and the K arakoram Fault along the SW margin. Satellite photographic interpretat ion and field mapping in the Karakoram mountains in Pakistan, the Nubr a-Siachen area of north Ladakh, and the Pamirs in Xinjiang show that a lthough the Karakoram Fault is extremely active today, geological offs ets along the right-lateral fault are probably less than 120 km. The 2 1 +/- 0.5 Ma Baltoro monzogranite-leucogranite batholith has been rota ted clockwise about a vertical axis 35 degrees-40 degrees into NW-SE a lignment, parallel with the Karakoram Fault, east of the Siachen glaci er, with a maximum offset of 90 km across the fault. The Bangong-Shyok suture zone similarly has a dextral offset of 85 km. The course of th e Indus River, which was antecedent to the rise of the Ladakh, Karakor am, and Himalayan ranges, has been offset dextrally by 120 km south of Pangong Lake. If present-day slip rates (approximately 32 mm/yr) (Avo uac and Tapponnier, 1993) are correct, only 4 Ma are required to obtai n a 120-km offset. There is no geological evidence for any larger-scal e pre-Holocene offsets, and it is suggested that the Karakoram Fault c annot have accommodated major eastward lateral motion of Tibetan crust . The fault has also exerted little or no influence on surface topogra phic uplift, cutting obliquely across the highest peaks of the Karakor am. Dextral motion along the central part of the Karakoram Fault has b een transferred in the north to the Rangkul, Murghab, and Karasu trans pressional faults in the central Pamir. North of Tashkurgan, the Karak oram Fault shows mainly normal motion around the Kongur and Mustagh At a gneiss domes (metamorphic core complexes) and the extensional Muji g raben. Minor dextral displacement has occurred along the Shiquanhe Fau lt, but this motion cannot be linked to the Jiale Fault of east Tibet. The southern end of the Karakoram Fault merges with the Indus (Yarlun g) suture zone near Mount Kailas and does not cut across the Himalaya. The lack of large-scale geological offsets along the Karakoram Fault, together with its very recent initiation (?5 or 4 Ma), suggests that it is related to the Pliocene-Quaternary northward indentation of the Pamir, and not to any long-term extrusion of Tibetan crust following t he Indian collision.