Paleomagnetic and Geochronologic Results of Latest Cretaceous Lava Flows From the Lhasa Terrane and Their Tectonic Implications

Abstract

To position the Asian southern margin before the India‐Asia collision, paleomagnetic and geochronologic studies were performed on the Dianzhong Formation lava flows from the Shiquanhe area of the westernmost Lhasa terrane (LT). Zircon U‐Pb analyses dated the lava flows to ~69.5 ± 2.5 Ma. The characteristic remanent magnetization directions contain antipodal polarities and pass fold tests, implying that they are primary magnetizations; this interpretation is supported by rock‐magnetic analyses and petrographic observations. Forty‐four site‐mean directions were divided into 17 statistically independent direction groups. The group‐mean direction after tilt correction is Ds = 43.3°, Is = 30.3°, k = 28.0, α95 = 6.9°. The corresponding paleopole at 47.8°N, 181.4°E (A95 = 6.4°) yields a paleolatitude of 16.6° ± 6.4°N for the Shiquanhe area of westernmost Tibet (32.34°N, 80.12°E). Consistent paleolatitudes for the southern margin of the LT calculated from the western and central part of the LT indicate that the leading edge of the LT was aligned relatively W‐E. When compared with the reference pole at 70 Ma for Eurasia, this new paleopole suggests that crustal shortening between the Shiquanhe area and stable Asia was 1,500 ± 800 km. This is supported by the crustal shortening (600–1,000 km) absorbed by Cenozoic thrust and fold belts within this area, indicating that the magnitude of crustal shortening within Asia north of the India‐Asia suture zone was similar in the central and western part of the plateau.

A Stable Southern Margin of Asia During the Cretaceous: Paleomagnetic Constraints on the Lhasa‐Qiangtang Collision and the Maximum Width of the Neo‐Tethys

“…However, both sections A and C recorded geomagnetic polarity reversals. Both sections are dominated by normal polarity, which is consistent with the geomagnetic polarity time scale (Gradstein & Ogg, ) at the time interval between ~113 and ~72 Ma, as bounded by the underlying and overlying volcanic rocks (Bian et al, ; Ma, Yang, et al, ). Two sites in section A (JG4 and JG7) include both reversed and normal polarities, revealing four geomagnetic polarity reversals within section A (Figure d).…”

“…Moreover, the red beds from the Linzhou basin in the eastern part of the Lhasa terrane may have suffered from remagnetization (Liang et al, ). Only a few late Cretaceous paleomagnetic results are available from the western part of the Lhasa terrane (Chen et al, ; Ma, Yang, et al, ; Tang et al, ; Yi et al, ). Considering that the volcanic results from the Lhasa terrane include less than 20 directional groups, determining whether the secular variation of the paleomagnetic field has been averaged may be a critical issue for the volcanic results (Ma, Yang¸ et al, ; Tang et al, ; Yi et al, ).…”

“…Only a few late Cretaceous paleomagnetic results are available from the western part of the Lhasa terrane (Chen et al, ; Ma, Yang, et al, ; Tang et al, ; Yi et al, ). Considering that the volcanic results from the Lhasa terrane include less than 20 directional groups, determining whether the secular variation of the paleomagnetic field has been averaged may be a critical issue for the volcanic results (Ma, Yang¸ et al, ; Tang et al, ; Yi et al, ). Additionally, the limestone data set reported by Chen et al (), which only comprises three sites (22 specimens), is too small and does not permit a test for potential inclination shallowing.…”

“…Notably, the dual polarities of the Jiega Fm limestones (see detailed paleomagnetic results in section ) imply that their age is not confined to the Cretaceous long normal superchron, as Chen et al () suggested was the case for the limestones in the northeastern Shiquanhe area. Zircon U‐Pb ages determined by laser ablation‐multicollector‐inductively coupled plasma‐mass spectrometry dating of the underlying Duoai Fm of the Zenong Group and the overlying Dianzhong volcanics are ~113 and ~72 Ma, respectively (Bian et al, ; Ma, Yang, et al, ). We conservatively conclude that the Jiega Fm limestones were deposited during the time interval of ~113–72 Ma, which is roughly the same age as the Shexing Fm redbeds from the eastern part of the Lhasa terrane.…”

Comment on “Comparing Paleomagnetic Study Means With Apparent Wander Paths: A Case Study and Paleomagnetic Test of the Greater India Versus Greater Indian Basin Hypotheses” by David B. Rowley

“…Global Apparent Polar Wander Path of Torsvik et al () in the coordinates of (a) Eurasia (blue curve), India (black curve), the Lhasa terrane according to the reconstruction of van Hinsbergen et al (; green curve), and the Tibetan Himalaya according to the reconstruction of van Hinsbergen et al () following the GIB hypothesis (orange curve), or assuming no Greater Indian extension (purple curve); (b) Eurasia (blue curve), India (black curve), the Lhasa terrane according to the reconstruction of Ingalls et al (; orange curve), and the Tibetan Himalaya according to the reconstruction of Ingalls et al (; green curve). Paleomagnetic data compilation as in van Hinsbergen et al (), with additional recent poles from Qin et al () and Bian et al () for the Tibetan Himalaya and Meng et al (), Ma et al (), and Tong et al () for the Lhasa Block.…”