Palaeomagnetism of Cretaceous to Tertiary sediments from southern Tibet: evidence for the extent of the northern margin of India prior to the collision with Eurasia

Abstract

Palaeomagnetic studies have been carried out on mid-Cretaceous to latest Palaeocene sediments from the Tethyan Himalaya on the northern margin of the Indian Plate near the localities of Gamba (28.3°N, 88.5°E) and Duela (28.0°N, 89.2°E) in southern Tibet. A total of 61 sites (more than 700 cores) have been sampled mainly from limestones (Zongshan Formation, Jidula Formation Member II, Zongpu Formation), sandstones (Jidula Formation Member III) and graywackes (Gamba Group Member II), with ages well defined from detailed lithological and biostratigraphic analyses. Carriers of characteristic remanences are magnetite in limestones and graywackes, and hematite in sandstones. Reliable palaeodirections were obtained from 35 sites (356 specimens). Positive fold tests, antiparallel normal and reverse polarities, a northward trend of the palaeolatitudes with decreasing age, and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences. The results have been compared with expected palaeodirections according to available data for the Indian subcontinent based on its present-day geometry. The late Maastrichtian sites (fourteen sites, 156 specimens) and the middle-Late Palaeocene sites (fourteen sites, 113 specimens) yield palaeolatitudes of 5.7°S and 4.0°N for the sampling localities in southern Tibet, respectively. These palaeolatitudes are much further north than expected and imply that crustal shortening between the Tethyan Himalaya and the Indian Shield since the time of India-Eurasia collision has occurred on the order of 1500 km at the longitude of Gamba/Duela. Comparison of our observed palaeolatitudes with palaeolatitudes for the Lhasa block during Cretaceous and Palaeocene times indicate that initial contact of the northern Indian margin with the Lhasa block was already established at 65-60 Ma with suturing completed at about 55-50 Ma.