PALEOMAGNETISM OF CRETACEOUS ROCKS FROM EASTERN QIANGTANG TERRANE OF TIBET

Thermal demagnetization has successfully isolated high unblocking temperature characteristic directions from paleomagnetic samples of Lower and Middle Cretaceous redbed formations from eastern Qiangtang Terrane, Tibet. The tilt-corrected mean direction for the Lower Cretaceous is D/I = 57.7-degrees/50.2-degrees with alpha(95) = 10.9-degrees and N = 12 sites, and that for the Middle Cretaceous is D/I = 39.7-degrees/51.6-degrees with alpha(95) = 8.5-degrees and N = 11 sites. Both formations pass the fold test, and the latter possesses two polarities. The magnetization predates the Eocene and was probably acquired during Cretaceous time. Comparison of the mean directions with expected directions from the stable interior of Eurasia indicates significant clockwise rotation but little paleolatitudinal change for the sampling area. Deformation and rotation of these rocks are presumably related to the collision of India with Asia and its subsequent northward indentation into Asia. Existing paleomagnetic data from Tibet and its environs seems to be compatible with a crustal shortening/thickening model (Dewey et al., 1988) in that the continental crust directly north of the Indian indenter was substantially shortened north-south, whereas in the vicinity of the Western and Eastern Syntaxes, latitudinal displacement was minimal but tectonic rotation was very important. More data are needed to further differentiate between this model and the lateral eastward crustal extrusion model (Molnar and Tapponier, 1975).