LATE CRETACEOUS PALAEOMAGNETIC RESULTS FROM SIKHOTE ALIN, FAR EASTERN RUSSIA: TECTONIC IMPLICATIONS FOR THE EASTERN MARGIN OF THE MONGOLIA BLOCK

Abstract

We present palaeomagnetic results from Late Cretaceous welded tuffs in the Kisin Group (Monastirskaya Suite, Primorskaya Series) collected at 27 sites from the Sikhote Alin mountain range. A high unblocking temperature magnetization component (>590°C) was isolated after stepwise thermal demagnetization from 25 sites. Combined with previously reported data, reliable characteristic remanence directions from 39 sites are distributed at eight areas ranging from 46.7°N, 138.1°E to 43.4 °N, 134.8°E in the Sikhote Alin Block. The bedding-tilt test is positive for two area mean directions and inconclusive for the remaining six areas. The data set for all 39 sites reveals a positive bedding-tilt test at the 99 per cent confidence level, and their tilt-corrected mean direction is D= 335.6°, I= 54.4° (α95= 8.5°), corresponding to a palaeopole at 71.5°N, 38.9°E with A95= 9.9°. This westerly direction is ascertained through the tilt-corrected mean direction (D= 331.1°, I= 53.5°, α95= 8.5°) based on the 25 data selected from four areas (Kema river, Terney, Plastun and Moryak-Rybolov) where each data set passes the positive bedding-tilt test or reveals an increase in the precision parameter after tilt correction. Palaeomagnetic declination indicates that the Sikhote Alin Block has rotated counterclockwise by 41°± 16° with respect to the Eurasian continent between Late Cretaceous times and 53–50 Ma. Compared with palaeomagnetic data from the surrounding regions, we find that the rotation recorded in Sikhote Alin extends westward into the interior of the Mongolia Block. The eastern margin of the Asian continent experienced both counterclockwise rotation of the eastern part of the Mongolia Block and clockwise rotation of the eastern part of the North China Block over the Cretaceous. We interpret these data in terms of a strong net horizontal force towards the ocean side, acting on the lithosphere at the eastern margin of the Asian continent between the Late Cretaceous and 53–50 Ma. Intermittently occurring upwellings of mantle and associated horizontal flows may have played an important role in producing the net horizontal force acting on the continental block during Late Cretaceous times.