Position of the Lhasa terrane prior to India-Asia collision derived from palaeomagnetic inclinations of 53 Ma old dykes of the Linzhou Basin: constraints on the age of collision and post-collisional shortening within the Tibetan Plateau

P>Seventeen sites were drilled from ca. 53 Ma old mafic dykes intruded in the Linzizong Formation of the Linzhou Basin for palaeomagnetic studies. From 10 sites a higher coercivity component demagnetized between 20 and 100 mT could be isolated. Detailed rockmagnetic analyses reveal Ti-rich titanomagnetite as the remanence carrier, which indicates that the rock is not much altered and the remanent magnetization is likely of primary origin. This is supported by a positive fold test. Tilt correction could be performed by the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns post-dating the dyke emplacement. Bedding corrected directions give an overall mean direction of D/I = 12.3 degrees/27.2 degrees (alpha(95) = 10.6 degrees, k = 21.7, N = 10) corresponding to a palaeolatitude of 14.4 degrees +/- 5.8 degrees N. Comparison with previous Cretaceous data mainly from the Takena Formation yields a stable position of the Lhasa terrane during Cretaceous and Early Eocene. The difference between expected palaeolatitudes determined from the APWP of Eurasia and observed ones reveals relative northward movement of the Lhasa terrane of ca. 1847 +/- 763 km since early Eocene. This is attributed to indentation of India into Asia and implies a considerable amount of north-south crustal shortening. Together with the extent of 'Greater India', we can derive an age for the India-Asia collision between ca. 53-49 Ma with a 95 per cent confidence limit of +/- 6 Ma.