Paleomagnetism of Paleocene‐Maastrichtian (60–70 Ma) Lava Flows From Tian Shan (Central Asia): Directional Analysis and Paleointensities

Abstract

The Tuoyun volcanics from the Tian Shan range (Central Asia) give an opportunity to investigate the variability of the Earth's magnetic field during the Cretaceous and Early Paleogene. In the paper we focus on Maastrichtian-Paleocene basalts and report new paleomagnetic results from 70 lava flows (respectively 45 directional groups) sampled near Tuoyun village (75.33°E; 40.18°N) from three distinct sections. Combined with previous results, our new data set yields a virtual geomagnetic pole at φ S ¼ 180.2°E, ϑ S ¼ 49.5°N with an angular standard deviation S ¼ 20:8 ∘ j 23:1 ∘ 18:8 ∘ (for N ¼ 93 directional groups). The mean inclination I s ¼ 36.2°is indicative of an emplacement of the Tuoyun volcanics at a paleolatitude around 20°N, consistent with the high dispersion k ¼ 14.6 of the directions. Such an inclination value is 10-15°l ower than the predictions from the apparent polar wander paths for stable Europe and East Asia. This observation can be partly explained by crustal shortening within the East Asian plate during the India-Asia convergence, but also needs to invoke a local field anomaly in Central Asia as previously proposed. Our absolute paleointensity experiments conducted on two lava flows and two baked sedimentary layers yield a mean virtual dipole moment of 58.9 ± 6.9 ZA•m 2 , consistent with the values at 60-70 Ma in the global paleointensity database. Compared to relative paleointensity results during the Cretaceous Normal Superchron and during the Plio-Pleistocene, the relative variability in intensity-supposed to be a proxy for the geodynamo's activity-estimated here at 31 ± 5% (N ¼ 15) from pseudo-Thellier experiments possibly corroborates a correlation between geomagnetic reversal frequency and paleosecular variation rate during the past 120 Myr. An open question is whether the changes in the frequency of reversals correlate with the gradual temporal changes of the magnetic field during stable periods, referred to as paleosecular variation (PSV). An increase of PSV rate with reversal frequency has often been proposed (e.g., Biggin et al., 2008). Identifying such a correlation however depends on our ability to accurately quantify PSV through geological time. This is

An Initial Collision of India and Asia in the Equatorial Humid Belt

“…For instance, the APWP for Asia (Cogné et al, 2013) suggests a more southerly position for Asia at ∼50 Ma which theoretically requires a southward shift of Asia during the Cretaceous and Early Paleogene. Moreover, Meng et al (2020) reported a paleomagnetic pole from the latest Cretaceous and Paleocene (∼70-60 Ma) volcanics from the Tuoyun Basin at the northern margin of the Tibetan Plateau that yielded a paleolatitude of 21.3° ± 3.8° N (Site location at 40.2°N, 75.3°E). The observed paleolatitude is much lower than that calculated from the APWP of Eurasia (38.8° ± 3.0° N, Torsvik et al, 2012).…”

“…Our updated paleomagnetic pole fully supports the initial collision model proposed by Yi et al (2011) and confirms an initial contact between India and Asia by ∼61 Ma at ∼7°N (Figure 4). Using our updated Dianzhong pole (∼64-60 Ma) and the Tuoyun pole (∼70-60 Ma) from Meng et al (2020) yields a latitudinal crustal shortening of 5.3° ± 4.3° (∼600 ± 480 km) that is consistent with estimates based on structural geology (Li et al, 2015;van Hinsbergen et al, 2011). It is difficult to decipher the overall shortening budget in the Asian interior as the motion of stable Asia needs refinement given the complexities suggested in this and other studies (e.g., Cogné et al, 2013).…”

“…The observed paleolatitude is much lower than that calculated from the APWP of Eurasia (38.8° ± 3.0° N, Torsvik et al., 2012). This large discrepancy cannot simply be attributed to Cenozoic crustal shortening in Asia (Meng et al., 2020). Although the lower inclination was interpreted by the authors as a local anomaly in the Earth’s magnetic field (Meng et al., 2020), it can alternatively (and more simply) be interpreted as a more southerly positioned Asia continent that is consistent with our results.…”

High-resolution petrographic evidence confirming detrital and biogenic magnetites as remanence carriers for Zongpu carbonates in the Gamba area, South Tibet

“…On the paleolatitudinal comparison based on reliable This is a provisional file, not the final typeset article paleomagnetic poles, paleomagnetism provides a direct constrain on timing and locus for the initial collision between India and Asia (e.g., Dupont-Nivet et al, 2010;Najman et al, 2010;Yi et al, 2011Yi et al, , 2021 The Indian plate was subjected to rapid northward motion toward Asia during the Cretaceous and Paleocene (Patriat and Achache, 1984;Yin and Harrison, 2000;van Hinsbergen et al, 2011). The kinematics of the northern margin of India can be constrained by the Cretaceous and Paleogene paleomagnetic data obtained from the Tethyan Himalaya (Besse et al, 1984;Patzelt et al, 1996;Tong et al, 2008;Yi et al, 2011;Yang et al, 2015Yang et al, , 2019Ma et al, 2016;Meng et al, 2019Meng et al, , 2020Y. Zhang et al, 2019;Yuan et al, 2020).…”

High-Resolution Petrographic Evidence Confirming Detrital and Biogenic Magnetites as Remanence Carriers for Zongpu Carbonates in the Gamba Area, South Tibet

“…The Indian plate was subjected to rapid northward motion toward Asia during the Cretaceous and Paleocene (Patriat and Achache, 1984;Yin and Harrison, 2000;van Hinsbergen et al, 2011). The kinematics of the northern margin of India can be constrained by the Cretaceous and Paleogene paleomagnetic data obtained from the Tethyan Himalaya (Besse et al, 1984;Patzelt et al, 1996;Tong et al, 2008;Yi et al, 2011;Yang et al, 2015;Ma et al, 2016;Yang et al, 2019;Meng et al, 2019;Meng et al, 2020;Yuan et al, 2020). For the lack of contemporary volcanic rocks, the Late Cretaceous to Paleocene sedimentary rocks from the Tethyan Himalaya are especially crucial for reconstructing the overall process of the India-Asia collision.…”