New Contributions to the Lhasa-Qiangtang Collision and Inclination Discrepancy from Early Cretaceous Paleomagnetic and Geochronological Results in the Western Lhasa Terrane

To better understand the inclination discrepancy of Cretaceous paleomagnetic data obtained from the Lhasa Terrane and further constrain the Lhasa-Qiangtang collisional process, a combined paleomagnetic and geochronological study has been carried out on the Early Cretaceous Risong Formation redbeds and volcanic rocks, dated at ~107-120 Ma, located at two different limbs of folds in the Wuma area of the western Lhasa Terrane. Characteristic remanent magnetization directions of 56 sites provide a tilt-corrected site-mean direction of Ds = 19.2°, Is = 44.1° with α95 = 6.2°, corresponding to a paleopole at 72.0°N, 189.8°E with dp/dm = 4.9°/7.8° and a paleolatitude of 25.9° ± 4.9°N for the reference point (32.41°N, 83.39°E). Though the site-mean direction for all the 56 sites passes positive fold tests, the site-mean inclination for 32 sites (33.1 ± 2.3°) distributed at the northern limb of folds is ~15.4° shallower than that for 24 sites (48.5 ± 2.0°) distributed at the southern limb of folds. Tests for inclination shallowing show that the studied Risong Formation redbeds did not suffer significant inclination shallowing. The Cretaceous foreland basin widely developed in the Wuma area indicates that the inclination discrepancy observed from the Early Cretaceous Risong Formation redbeds may result from the syntectonic sedimentation. The site-mean inclination for 56 sites (Ic= 41.7 ± 1.6°) after the syntectonic-sedimentation-correction is basically consistent with the observed site-mean inclination, supporting that paleomagnetic data obtained from two different limbs of folds are still reliable for paleogeographic reconstructions though the syntectonic sedimentation may cause the deviation of paleomagnetic inclinations from monoclinal strata. Our new results, combined with the reliable Early Cretaceous paleomagnetic datasets observed from the western Lhasa and Qiangtang terranes, reveal that the Lhasa-Qiangtang collision in the western part occurred before ~120 Ma.