TECTONIC EVOLUTION OF THE MESOZOIC SOUTH ANYUI SUTURE ZONE, EASTERN RUSSIA: A CRITICAL COMPONENT OF PALEOGEOGRAPHIC RECONSTRUCTIONS OF THE ARCTIC REGION

The South Anyui suture zone consists of Late Paleozoic-Jurassic ultramafic rocks and Jurassic-Cretaceous pre-, syn-, and post-collisional sedimentary rocks. It represents the closure of a Mesozoic ocean basin that separated two microcontinents in northeastern Russia, the Kolyma-Omolon block and the Chukotka block. Previously, it has been correlated to similar age rocks in the Angayucham suture zone in Alaska. In order to understand the geologic history and improve Mesozoic paleogeography of the Arctic region, we obtained U-Pb ages on pre- and post-collisional igneous rocks and detrital zircons from sandstones in the suture zone. We identified four groups of sedimentary rocks. Triassic sandstones were deposited on the southern margin of Chukotka from 240–212 Ma. Mid-Jurassic volcanogenic sandstones were derived from a continental margin arc along the Kolyma-Omolon block south of the Anyui Ocean. A sample from this unit had no pre-Jurassic zircons and a main peak at 166 Ma. Suture zone sandstones yield Late Jurassic maximum depositional ages and likely predate the collision. A mid-Cretaceous syn-collisional sandstone has a maximum depositional age of 125 Ma. These rocks were intruded by post-kinematic plutons and dikes with ages of 109 Ma and 101 Ma that post-date the collision. We interpret a seismic reflection line through the South Anyui suture zone to indicate south-vergence of thrusting of the Chukotka block over the Kolyma-Omolon block, opposite of most existing models and opposite of the vergence in the Angayucham suture zone, the postulated along-strike equivalent in Alaska. This suggests Chukotka and Arctic Alaska may have different pre-Cretaceous histories, which could solve space problems with existing reconstructions of the Arctic region.