PALEOMAGNETISM OF PALEOZOIC AND MESOZOIC SEDIMENTS FROM THE SOUTHERN MARGIN OF MONGOL-OKHOTSK OCEAN, FAR EASTERN RUSSIA

Abstract

This paper presents new data from paleomagnetic investigations of Middle Paleozoic (Early-Middle-Late Devonian, Early Carboniferous) and Middle Jurassic geological units from the Amuria block south of the Mongol-Okhotsk suture zone. With fold tests for all localities, and two polarities for the Devonian, our new results constrain the evolution of the Mongol-Okhotsk ocean from the Devonian to the Middle Jurassic. The corresponding paleopoles lie at 21.6°N, 6.3°E (dp/dm = 5.3°/10.6°) for Early Devonian, 26.3°N, 345.3°E (dp/dm = 6.4°/12.8°) for Early-Middle Devonian, 24.6°N, 12.9°E (dp/dm = 8.7°/16.9°) for Middle-Late Devonian, 40.5°N, 352.4°E (dp/dm = 9°/16.7°) for Late Devonian, 39.8°N, 31.6°E (dp/dm = 10°/15.5°) for Early Carboniferous, and 46°N, 37.9°E (dp/dm = 9.4°/13.4°) for Middle Jurassic. The poles confirm that the large Paleozoic Mongol-Okhotsk ocean closed during the Jurassic, ending up in the late Jurassic or early Cretaceous at the eastern end of the suture zone, as originally proposed on geological grounds. The new paleomagnetic results exhibit large tectonic rotations about local vertical axes, which we interpret as probably arising from both the collision process and left-lateral shear movement along the suture zone, related to incipient extrusion of Amuria due to indentation of India into Asia.