What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

Abstract

The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40 Ar/ 39Ar and U-Pb zircon dates of~52 Ma for the upper Linzizong, and 40 Ar/ 39 Ar dates (~51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of~5°N, compared to the upper part (~20°N) and to underlying Cretaceous strata (~20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by~52 Ma at~20°N.

Reduced convergence within the Tibetan Plateau by 26 Ma?

“…Various studies found significantly different paleolatitude values: e.g., 10.0°± 5.3°N [Chen et al, 2010], 15.2°± 5.8°N [Liebke et al, 2010], 21.7°± 5.0°N [Dupont-Nivet et al, 2010], and 20°± 4°N [Huang et al, 2015]. Various studies found significantly different paleolatitude values: e.g., 10.0°± 5.3°N [Chen et al, 2010], 15.2°± 5.8°N [Liebke et al, 2010], 21.7°± 5.0°N [Dupont-Nivet et al, 2010], and 20°± 4°N [Huang et al, 2015].…”

Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

“…We assume that 25 ± 10 °C reflects a reasonable temperature uncertainty for lacustrine carbonate precipitation that could account for the overall warmer Eocene conditions and low latitude46, even at moderate to high paleoelevations. We then used the mean δ 18 O c (VPDB) of lacustrine micrites of −5.1 ± 1.1‰ (1σ) and the temperature-dependent fraction equation between water and calcite47 to calculate the δ 18 O psw (psw - paleo-surface water) relative to SMOW - Standard Mean Ocean Water of −2.8 ± 1.7‰.…”

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, ).…”

“…Notably, these Linzizong volcanic rocks were restudied by Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Guo, et al () and Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Waldrip, et al () with further rock‐magnetic analyses and petrographic observations, revealing that the lower units have been remagnetized. In addition, Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Guo, et al () and Huang, Dupont‐Nivet, Lippert, Hinsbergen, Dekkers, Waldrip, et al () argue that a quick latitudinal movement of the LT at this time interval is neither observed by paleopoles from the LT since Cretaceous (e.g., Lippert et al, ; Ma et al, ; Sun et al, ; van Hinsbergen et al, ; Yang, Ma, Zhang, et al, ) nor predicted by the apparent polar wander path of Eurasia or East Asia since Cretaceous (Cogné et al, ; Torsvik & Cocks, ; Torsvik et al, , ). Also, it is not supported by geological observations and reconstructions (e.g., Yin & Harrison, ; Johnson, ; van Hinsbergen, Kapp, 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.…”