Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India‐Asia collision?

Abstract

Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1°(35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9°(41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence-21°N (19.8°N, 23.1°N)-is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya-Asia collision was situated at~20°N and took place by~50 Ma.

Lower Cretaceous Xigaze ophiolites formed in the Gangdese forearc: Evidence from paleomagnetism, sediment provenance, and stratigraphy

“…In the light of Abrajevitch et al (2005), the Xigaze ophiolite was previously taken as geological evidence (Jackson et al, 1991;Kodama, 2009) for the existence of a northward dipping equatorial intra-oceanic subduction zone and island arc system within Neotethys (e.g., Abrajevitch et al, 2005;Hébert et al, 2012;van Hinsbergen et al, 2012). Our results, however, demonstrate that the Xigaze ophiolite can no longer be viewed as a geological archive of near-equatorial subduction within the Neotethys.…”

“…The plaeomagnetic inclination shallowing effect has been frequently documented in clastic sedimentary rocks (e.g., Jackson et al, 1991). The two most common methods used to perform this correction are based on the elongation/inclination (E/I) correction of the ChRM distribution (Tauxe and Kent, 2004), and the anisotropy of susceptibility or remanence ( Comparative studies show that these two methods provide correction values that are comparable within error (e.g., Huang et al, 2013).…”

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

“…Especially, Lippert et al () reevaluated available paleomagnetic data using a statistically rigorous filtering criteria and concluded that the LT was located at high tropical/low subtropical latitudes (e.g., ~20°N) since early Cretaceous. Later paleomagnetic investigations further supported this conclusion (e.g., Ma et al, ; Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Guo, et al, ; Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Waldrip, et al, ; Yang, Ma, Zhang, et al, ). The precollisional paleolatitude of the LT dated the TH‐LT collision to ~55–50 Ma and suggests that less than 1/3 of Cenozoic convergence is partitioned into Asian lithosphere (Lippert et al, ; van Hinsbergen et al, ).…”

“…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.…”

“…However, Sun et al () published a data set of 17 red bed sites in the Shexing Fm (Maxiang area) that had a steeper inclination (35°). These discrepant inclinations observed in different sections of the Shexing Fm red beds imply that inclination shallowing may exist to varying degrees (Cao et al, ; Lippert et al, ; Sun et al, ; Tan et al, ; van Hinsbergen et al, ) or that the problem of insufficient averaging of paleomagnetic secular variation may exist not only for the volcanics (e.g., Cao et al, ; Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Guo, et al, ; Lippert et al, ; Ma et al, ; Sun et al, ; Tan et al, ; Yang, Ma, Zhang, et al, ) but also for the red beds (Yang, Ma, Zhang, et al, ).…”