Paleomagnetic Results From Triassic Rock Formations of Junggar Basin, NW China

Abstract:

Our knowledge of the kinematics and duration of collisional events between the Chinese blocks and Siberia is still inadequate. Likewise, the accretionary history of the Central Asian fold belt itself is contentious. The primary goal for this study was to address key questions about the geographic positions of the Junggar Basin in Paleozoic and Mesozoic times and its tectonic relationship with other major Asian blocks by generating high-quality paleomagnetically inferred paleolatitudes and basin histories. Another objective is to see what it can tell us about intraplate motions within central Asia and to recognize vertical-axis tectonic rotations in the region. We collected 348 samples from 42 sites in Paleozoic and Mesozoic stratigraphic sections of east Junggar Basin. Useful results were obtained by progressive demagnetization for sandstone samples of Xiaoquan Formation dated paleontologically as mid to late Triassic. The ChRM direction for the Xiaoquan Formation is directed easterly with intermediate to steep downward inclination in geographic coordinates. After tectonic correction, the mean direction of the ChRM becomes northeasterly with intermediate to shallow inclination, which corresponds to a mean paleolatitude of 20.7° within a 95% confidence band of ± 12 degrees. It is obvious from the data that Junggar and other major parts of eastern Asia did not occupy the same relative positions in terms of paleolatitudes in Late Triassic. Paleomagnetic declinations indicate clockwise vertical-axis rotations of R = 98.3° +/- 16° for the Late Triassic rock unit relative to North China Block (NCB), and 12.4° +/- 16.2° relative to South China Block (SCB). The paleomagnetically inferred latitudinal displacements between Junggar and SCB are general statistically insignificant, as well as the rotational data. These data suggest that Junggar Basin may have been very close to SCB during the Triassic time. A plausible cause of the rotation is the westward rotational closure of the South and North China Blocks, which is inferred to have occurred during the Late Triassic according to several geologic and tectonic analyses.