Paleomagnetic constraints on the paleolatitude of the Lhasa block during the Early Cretaceous: Implications for the onset of India–Asia collision and latitudinal shortening estimates across Tibet and stable Asia

Abstract

Interbedded volcano-sedimentary sequences are well exposed in the northern part of the Lhasa block in southern Tibet. Zircon U–Pb dating results from two samples indicate that the emplacement age of the Duoni Formation volcanic flows is 120.2±0.5Ma. Paleomagnetic results from 235 progressively demagnetized volcanic rock samples (25 sites) and 41 sandstone samples (5 sites) indicate that the dominant remanence carriers are Ti-poor titanomagnetite and Ti-poor titanohematite in the volcanic samples and Ti-rich titanomagnetite in the sandstone samples. Rock magnetic investigations, systematic demagnetization behavior, positive fold test results, and direct petrographic identifications all indicate that the paleodirections recorded by the chemically stable magnetic particles are primary thermal remanent magnetization in the volcanic flows and primary detrital remanent magnetization in the sandstones. The tilt-corrected ChRM mean direction is D/I=356.4°/16.4° with α95=6.3° (N=19), corresponding to a paleopole position of λp=66.9°N, φp=281.2°E with A95=6.1°. Combined with previously published results, the geochronological dating and paleomagnetic analysis indicate a paleolatitude of 13.1±2.7°N for the southern margin of the Lhasa block during the Early Cretaceous. Therefore, the southernmost margin of the Eurasian continent likely remained at the low-middle paleolatitude of 13.1±2.7°N between the Early Cretaceous and the Paleocene. Based on comparisons to results from the Tethyan–Himalayan block and the reference poles from stable India and Eurasia, the low-middle paleoposition of the Lhasa block during the Early Cretaceous through Early Paleocene suggests that the initial contact between India and Asia occurred at ca. 59.3Ma. Under the assumption of a rigid Eurasian plate, this timing implies that a total collision-related latitudinal convergence of 1450±400km (13.1±3.7°N) has been accommodated by folding, thrust faulting, normal faulting, crustal thickening, intracontinental subduction in Tibet and central Asia and southeastward continental extrusion of the Indo-China block from the eastern syntaxis between the Lhasa block and stable Asia.