Paleomagnetism and geochronology of Early Cretaceous volcanic rocks in the eastern segment of the Lhasa terrane, Tibetan Plateau, and their tectonic implications

Abstract

Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous provide important constraints on the evolution of the Neo-Tethys Ocean and crustal shortening within Asia. However, the Cretaceous paleogeography of the Lhasa terrane remains contentious. A direct way to study this issue is to conduct paleomagnetic investigations of the Cretaceous rocks of the Lhasa terrane; however, most previous Cretaceous paleomagnetic investigations of the Lhasa terrane were conducted in the middle and western segments of this terrane. Different vertical-axis rotations affected different parts of the Lhasa terrane following the India-Asia collision, and therefore paleomagnetic data from the western and middle segments of the Lhasa terrane cannot necessarily be used to directly constrain the paleolatitudes of its eastern segment. This study presents paleomagnetic data from the Early Cretaceous volcanic rocks with well-constrained ages from the Luolong area in the eastern segment of the Lhasa terrane. Isotope geochronology reveals that these rocks formed at 127−124 Ma. The mean high-temperature direction obtained from 17 paleomagnetic sites is declination/inclination (Ds/Is) = 21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of the precision parameter; α95—radius of the 95% probability ellipse around the mean direction; s—stratigraphic coordinates). Petrographic investigations, a positive fold test, a reversal test, and a paleosecular variation test indicate the primary origin of this characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern segment of the Lhasa terrane. Combined with reliable results from previous paleomagnetic studies, we draw the following conclusions. (1) During the Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and the eastern-middle segments may have been oriented nearly E-W. (2) Asia has accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys Ocean at ca. 125 Ma was 4185 ± 300 km.