Evidence of large Cainozoic crustal shortening of Asia

Abstract

PALAEOMAGNETIC studies of late Cretaceous–early Tertiary red beds in southern Tibet indicate that they were magnetised at about 8°N and, therefore, that they subsequently moved north about 22° (ref. 1). Xiangyuan et al.¹ collected 12 samples across the thickness of red beds which overlies an early late-Cretaceous stratum (Tsu mulong group) in Linzhou. Fossils of late Cretaceous marine fauna were found within the red bed itself, and Foraminifera (Lockhatia sp.) of early Tertiary age were found near an outcrop of the volcanic assembly that unconformably overlies the red beds. Three of their samples were tested for stability using alternating field demagnetisation with an applied field of 50–300 oersted. No obvious secondary magnetisation was found. These red beds are north of the Indus–Tsangpo suture². Xiangyuan et al.¹ compared the virtual geomagnetic pole from Tibet with those from rocks of the same age in India to show that this portion of Tibet was indeed not part of India in the late Cretaceous–early Tertiary. We show here that these new palaeomagnetic data also require about 2,500–3,500 km of north–south shortening in Asia since its collision with India.