PALEOMAGNETISM AND K-AR DATING OF PLEISTOCENE VOLCANIC-ROCKS ALONG THE ALTYN-TAGH FAULT, NORTHERN BORDER OF TIBET

Two basaltic andesite lava flows were sampled in the Puru area (36 deg rees 15'N; 81 degrees 30'E) north of the Altyn Tagh fault. The samplin g sites were distributed between 13 km and 18 km from the fault. The t wo lavas are dated by K-Ar methods to be 1.05+/-0.11 and 1.44+/-0.06 M a. Their palaeomagnetic directions show reversed polarity of the Matuy ama chron. Declination values for nine sampling sites on the upper lav a flow more than 14 km from the fault are concordant, with little vari ance (D = -166.3 degrees, I = -50.8 degrees, alpha(95) = 2.2 degrees), but declination at a site nearer the fault (13 km from it) (D = -173. 7 degrees) displays a counter-clockwise deflection of 7.4 degrees +/- 5.2 degrees from that of the other sites. This palaeomagnetic rotation is ascribed to tectonic rotation due to left-lateral slip on the Alty n Tagh fault during the last 1.0 myr. The area beyond 13 km from the f ault, however, does not provide any evidence for the large rotation (s imilar to 60 degrees) expected from the known strike-slip movement of 30 mm yr(-1). This implies that little deformation penetrates deep int o the Tarim craton from the fault. The lack of deformation of features along the Altyn Tagh fault is explained by the non-rigid behaviour of the narrow zone of the continental lithosphere near the fault and/or by low stress acting on the fault.