A quasi-linear structure of the southern margin of Eurasia prior to the India-Asia collision: First paleomagnetic constraints from Upper Cretaceous volcanic rocks near the western syntaxis of Tibet

Abstract

We report the first combined geochronologic and paleomagnetic study of volcanic rocks from the Shiquanhe and Yare Basins at the westernmost Lhasa Terrane, which aims to provide an accurate constraint on the shape and paleoposition of the southern margin of Asia prior to the India-Asia collision. Three new 40 Ar/ 39 Ar ages of 92.5 ± 2.9 Ma, 92.4 ± 0.9 Ma, and 79.6 ± 0.7 Ma determined by fresh matrix or feldspar from lava flows suggest a Late Cretaceous age for the investigated units. Characteristic remanent magnetizations have been successfully isolated from 38 sites which pass positive fold and/or reversal, conglomerate tests and are hence interpreted as primary in origin. The two paleopoles obtained from Yare and Shiquanhe yield consistent paleolatitudes of 13.6°N ± 9.6°N and 14.2°N ± 2.7°N, respectively (for a reference site of 31.5°N, 80°E), indicating that the southern margin of Asia near the western syntaxis was located far south during the Late Cretaceous time. A reconstruction of the Lhasa Terrane in the frame of Eurasia with paleomagnetic data obtained from its western and eastern parts indicates that the southern margin of Eurasia probably had a quasi-linear orientation prior to the collision formerly trending approximately 315°E. This is compatible with the shape of the Neo-Tethys slab observed from seismic tomographic studies. Our findings provide a solid basis for evaluating Cenozoic crustal shortening in the Asian interior and the size of Greater India near the western syntaxis.

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

“…ID, paleopoles abbreviation; Fm, Formation; Gp, Group; volc, volcanics; Slat/Slon, latitude/longitude of the sampling area; Plat/Plon, latitude/longitude of the paleopole; A 95 , radius of the circle of 95% confidence; dp/dm, semiaxes of elliptical error of the pole at 95% confidence; Paleolat, the paleolatitudes of the eastern sector of the LT were calculated at an eastern reference point (29.3°N, 91.1°E); other paleolatitudes were calculated at a western reference point (31.7°N, 80.1°E); n / N , number of samples/sites used to calculate the Fisherian mean; F/R means a positive fold/reversal test; D means coexistence of dual polarity. References: 1, Tan et al (); 2, Sun et al (); 3, Cao et al (); 4, Huang et al (); 5, Chen et al (); 6, Chen et al (); 7, available data analyzed by van Hinsbergen et al (); 8, available data reanalyzed by Lippert et al (); 9, Liebke et al (); 10, Dupont‐Nivet et al (); 11, Chen et al (); 12, Yi et al (); 13, Ma et al (); 14, Chen et al (); 15, Yang, Ma, Zhang, et al ().…”

“…Therefore, the precollisional southern margin of the LT at the early Cretaceous should have been located at a mean paleolatitude of ~18.8°N at ~117–132 Ma (Chen et al, ; Ma et al, ; Yang, Ma, Zhang, et al, ). For the far western part of the LT, only paleomagnetic results from Chen et al (), Yi et al (), and this study are available. Chen et al () reported Cretaceous paleomagnetic results of three limestone sites (22 specimens) from the LT.…”

“…For example, the ages of the Dianzhong Fm in the Cuoqin area show a wide variation from ~93 to 99 Ma by the 40 Ar/ 39 Ar dating of plagioclase (Tang et al, ) to ~117–121 Ma by the U‐Pb zircon dating of zircon (Yang, Ma, Zhang, et al, ). In the Shiquanhe area, the Dianzhong Fm volcanic rocks have been dated to ~60.1 Ma and ~92 Ma by K‐Ar and 40 Ar/ 39 Ar dating, respectively (I44C004002, 2005) (Yi et al, ). Therefore, precise dating of the Dianzhong Fm volcanic rocks is critical for reconstructing the paleolatitude evolution of the LT.…”

“…Previous studies proposed that the timing of the India‐Asia collision ranged from ~70 Ma to ~20 Ma (e.g., Aitchison et al, ; Hu et al, ; van Hinsbergen et al, ; Yin & Harrison, ). Therefore, many paleomagnetic investigations of Cretaceous to Eocene age rocks from the LT have been performed to determine the position of the precollisional leading edge of Asia, as well as to better understand the collision process (when and where) (e.g., Hu et al, ; Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Guo, et al, ; Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Waldrip, et al, ; Li et al, ; Liebke et al, ; Lippert et al, ; Ma et al, ; Pozzi et al, ; Tan et al, ; Yang, Ma, Zhang, et al, ; Yi et al, ). Most of these studies focused on the upper Cretaceous Shexing Formation (Fm) and the Linzizong Group, which contain sedimentary and volcanic units.…”

“…In addition, different units of the Linzizong Group sedimentary and volcanic sequences are characterized by a diversity of radiometric dates, varying from 69 to 40 Ma mainly using U‐Pb and 40 Ar/ 39 Ar methods (e.g., He et al, ; Huang et al, ; Tan et al, ). Some researchers, therefore, suggested that there may be a latitudinal movement of the LT after it collided with TH at this time interval (e.g., Chen et al, ; Yi et al, ). For instance, a systematic study on the Dianzhong, Nianbo, and Pana Fms from the Linzhou and Nanmulin basins showed that the paleolatitude of the LT progressively increased with decreasing age from 6.6° ± 6.3°N at ~64–60 Ma, to 9.7° ± 6.3°N at ~60–50 Ma, to 12.7° ± 5.6°N at ~50–44 Ma (reference point at 30°N, 91°E) (Chen et al, , ).…”