PALEOMAGNETISM OF LATE PALEOZOIC ROCKS IN THE TIANSHAN, NORTHWESTERN CHINA

A paleomagnetic study of late Paleozoic rocks in two areas of northwestern China helps to constrain the timing of collision between Tarim and the Junggar part of Eurasia along Tianshan. A primary Late Permian direction (D/I = 198/-59, K=24, A95=12.6) is derived from seven sites (53 samples) from the Heavenly Lake (Tianzhi) section in northern Tianshan that has both normal and reversed polarities and passes a fold test at the 99% confidence level. This result gives a latitude of 39-degrees-N in the Late Permian for southern Junggar. Twenty-two sites (144 samples) from the Devonian and Carboniferous rocks across Tianshan reveal postfolding remagnetizations some of which are interpreted as being acquired in the Permian due to the Tarim-Eurasia collision. The Late Permian paleomagnetic pole from the Heavenly Lake section is statistically indistinguishable from three published Late Permian poles from Tarim indicating closure of the Tianshan ocean and suturing of Tarim and Junggar before that time. These paleomagnetic poles from northwestern China are, however, significantly different from those of stable Eurasia, suggesting substantial later relative motion that can be partly accounted for by the progressive closure of the Mongolo-Okhotsk ocean and associated tectonic rotations. The absence of a discernable Permian ocean record west of the Khangay highland in central Mongolia requires that the rotations be accommodated within a diffusive plate boundary in the Eurasian continent. A Late Permian continental plate reconstruction is presented that takes into consideration these new paleomagnetic results and other geological constraints.