PALEOMAGNETIC STUDY OF MESOZOIC CONTINENTAL SEDIMENTS ALONG THE NORTHERN TIEN-SHAN (CHINA) AND HETEROGENEOUS STRAIN IN CENTRAL-ASIA

A paleomagnetic study of rocks from the northern foot of the Tien Shan and the southern border of the Dzungar Basin, east of Urumqi (44.2-degrees-N, 86.0-degrees-E), spanning ages from middle Jurassic to early Tertiary was carried out to constrain the tectonic evolution in central Asia since Mesozoic time. Five middle Jurassic sites reveal a remagnetized direction close to the present Earth field in geographic coordinates: D = 6.6-degrees, I = 72.6-degrees (alpha-95 = 7.4-degrees). Thirteen out of 17 upper Jurassic and lower Cretaceous sites yield a characteristic direction (stratigraphic coordinates) of D = 12.7-degrees, I = 48.6-degrees (alpha-95 = 5.5-degrees). Nine of 16 upper Cretaceous and lower Tertiary sites provide a characteristic direction of D = 12.5-degrees, I = 51.3-degrees (alpha-95 = 6.9-degrees). The latter two directions pass fold and reversal tests. The pole positions are close to each other and to the Besse and Courtillot [1989, 1990] Eurasian apparent polar wander path, for ages ranging from 130 to 70 Ma. However, the difference in paleolatitudes amounts to about 5.9-degrees +/- 3.7-degrees, which could indicate significant continental shortening in the Altai Mountains and perhaps further north, subsequent to India-Asia collision. The pole positions from the Dzungar Basin are close to those found for the Tarim [Li et al., 1988a], leading to an insignificant paleolatitude difference (3.0-degrees +/- 6.9-degrees), but showing a larger difference in declination (8.6-degrees +/- 8.7-degrees). These paleomagnetic results are compatible with a model of heterogeneous deformation in the western part of the collision zone between India and Siberia. A significant shortening in the Altai, a slight counterclockwise rotation of the Dzungar block, the westward-increasing shortening in the Tien Shan with attendant clockwise rotation of the Tarim block are all consistent with this model, in which Tibet, the Tien Shan and the Altai undergo differential strain along strike in a relay fashion, with the total India-Siberia convergence remaining approximately constant.