New paleomagnetic results for Ordovician and Silurian rocks of the Tarim Block, Northwest China and their paleogeographic implications

Abstract

The kinematics of the Tarim Block is poorly understood prior to the late Paleozoic and this study aims to clarify motions during Ordovician and Silurian times. We report the paleomagnetic record from 12 sites in Ordovician marine sediments and 21 sites in Silurian fine-grained sandstones and mudstones from the Aksu and Kalpin areas of northwestern Tarim. Together with our previously reported preliminary results, the Ordovician sediments isolate a sporadically-preserved pre-Cenozoic characteristic remanence with dual polarity. Although a positive fold test can only define the remanence as pre-Cenozoic, the corresponding paleomagnetic pole (33.7°S, 185.0°E, dp/dm = 2.7°/4.0°) is removed from post-Silurian paleopoles from Tarim and combined with evidence from magnetic fabrics indicating a quasi-primary origin. A characteristic remanence isolated at 17 sites in Silurian sediments from the Aksu and Kalpin areas includes 7 sites with suspected contamination from regional Permian igneous activity. The 10 remaining sites, all from Kalpin, yield a positive fold test and a paleopole at 19.1°N, 172.9°E (A95 = 5.5°) significantly removed from younger paleopoles from Tarim indicating a Late Llandovery to Wenlock remanence age. The new Mid-Late Ordovician paleopole places the Tarim Block at ~32°S, probably in the vicinity of the Arabian margin of Gondwana. Rapid APW between ~460 and ~433 Ma records large differential dextral motions of Tarim together with other continental blocks along the Paleo-Asian Ocean margin which brought the China continental blocks into proximity with Australia by mid-Silurian times in low (~9°N) northerly paleolatitudes.