New paleomagnetic data from the Mongol–Okhotsk collision zone, Chita region, south-central Russia: implications for Paleozoic paleogeography of the Mongol–Okhotsk ocean

Abstract

We present a reconnaissance paleomagnetic study of rocks from several formations of the Chita region of south-central Russia (representative location λ=51°N, φ=116°E), within the Mongol–Okhotsk collision zone. Sections of siltstone and fine-grained sandstone, dated using fossils as Bashkirian to Moscovian epochs (USSR Carboniferous system, Middle Carboniferous), Early Permian, Late Permian and Triassic in age were sampled. The resulting collection from five stratigraphic sections is a total of 319 oriented samples from 51 sites. Generally, 5 to 10 samples per site were collected. Sample orientation was determined using both magnetic and sun-shadow azimuths. All paleomagnetic measurements were completed in the Paleomagnetic Laboratory at the University of Pittsburgh using a 3-component 2G Superconducting Rock Magnetometer (SRM) in a magnetically shielded room. Thermal demagnetization was completed using between 12 to 20 heating steps up to temperatures of 685°C. Principal component analysis of the demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may record the present-day Earth's (PDF) magnetic field [PDF I=69.4°, D=351.8°; component A, Ig=65.1°, Dg=356.5°, α95=6.2°, N (sites)=50]. The higher unblocking temperature magnetic component (B), was observed in the Triassic (BT, N=3), Late Permian (BLP, N=14), Early Permian (BEP, N=5) and Bashkirian to Moscovian epochs of the Late Carboniferous (BMC, N=8) sections. The B component differs significantly from component A, and is recorded by sites of both downward and upward directed magnetic inclinations in the Late Permian and Bashkirian to Moscovian epoch sections. Component BMC and BLP may represent primary remanent magnetizations. The Bashkirian to Moscovian epochs of the Late Carboniferous mean paleolatitude is λMC=19.9°±14.8° and the Late Permian mean paleolatitude is λLP=19.6°±14.5°. Both are similar to that expected from reference paleomagnetic poles from the North China block, but significantly different from paleolatitudes calculated using reference poles from the Siberian or European plates. We interpret the results of this study to suggest that the sampled sections were located near, or associated with, the North China Block during their deposition.