Palaeomagnetic results from Palaeocene basalts from Mongolia reveal no inclination shallowing at 60 Ma in Central Asia

Abstract

We present the results of a palaeomagnetic study of 277 cores drilled at 35 sites, in 32 basaltic flows from three Early Palaeocene volcanic regions in the Gobi Desert, Mongolia, at Sumber Uul (62.2 Ma; 42.6°N/104.0°E), Tulga (62.0 Ma; 43.2°N/104.1°E) and Khuts Uul (57.1 Ma; 43.2°N/104.6°E) localities. Samples from Sumber Uul (62.2 ± 0.9 Ma) and Khuts Uul (57.1 ± 0.8 Ma) localities were dated using the K-Ar Cassignol–Gillot technique. Stepwise thermal and alternating field demagnetizations isolated a stable A component of magnetization carried by single domain (SD) to nearly SD magnetite. We interpret this A component to be the primary magnetization of these basaltic lava flows.The Sumber Uul and Tulga data were combined and recomputed at the Sumber Uul locality because of their similar ages. The palaeopoles computed from the A components lie at λ= 85.2°N, φ= 92.5°E, dp/dm= 3.9/4.9 (n= 14 flows) for Sumber Uul-Tulga (average age: 62.1 ± 5.9 Ma) and λ= 69.6°N, φ= 148.0°E, dp/dm= 6.3/7.3 (n= 14 flows) for Khuts Uul (average age: 57.1 ± 0.8 Ma). The palaeomagnetic inclinations are steeper than expected at the sites and consequently our palaeopoles occupy a near-sided position with respect to the 60 Ma reference apparent polar wander path (APWP) pole for Europe (Besse & Courtillot 2002). However, they appear to fully conform to the new high-resolution APWP poles for the 65–42 Ma period of Moreau et al. (2007). Following these authors, we interpret this anomalous near-sided position of our poles as arising from a rapid true polar wander (TPW) event in the Palaeocene, highlighted by a cusp at anomalies 26–25 (61–56 Ma), inexistent in the European APWP of Besse & Courtillot (2002). We conclude that our new data do not reveal any anomalous shallow inclinations in the Central Asia Palaeocene effusive rocks which is consistent with the Late Jurassic/Early Cretaceous age of Mongol-Okhotsk ocean closure and amalgamation of Amuria and Siberia, forming a rigid entity since then.