A PALEOMAGNETIC STUDY FROM THE MONGOL-OKHOTSK REGION - ROTATED EARLY CRETACEOUS VOLCANICS AND REMAGNETIZED MESOZOIC SEDIMENTS

We collected 47 sites of Upper Triassic to Middle Jurassic siltstones and sandstones and two sites of Lower Cretaceous andesites in a large basin located south of the Mongol-Okhotsk suture. The suture separates the Siberian craton to the north from the Mongolian and Chinese block s to the south. Laboratory treatment and analyses identify the same po st-folding direction in all rocks. The mean direction (with a correspo nding palaeopole at 76.8 degrees N, 152.2 degrees E, A(95) = 4.2 degre es) of the overprint component (N = 49 sites) is significantly differe nt at the 95% confidence level from the expected time-averaged Brunhes or present-day field directions. The Lower Cretaceous andesites posse ss a stable remanent direction at high temperatures that is distinct a t 95% confidence limits from the overprint direction. The correspondin g pole (58.3 degrees N, 51.0 degrees E, dp/dm = 3.8 degrees/4.6 degree s), based on only fourteen samples, is significantly rotated 78.4 degr ees +/- 5.3 degrees counterclockwise and insignificantly displaced 2.7 degrees +/- 3.4 degrees north with respect to the Early Cretaceous re ference pole for Siberia. We argue that the rotation is likely tectoni c in nature and not due to a chance reading of the palaeosecular varia tion of the Earth's magnetic field. Both the palaeomagnetic data and t he folding patterns we observed in the field suggest that deformation associated with the suture continued after the Early Cretaceous and in volved sinistral shear. (C) 1998 Elsevier Science B.V. All rights rese rved.