PLATE TECTONICS AND THE MAGMATIC BELTS OF NE RUSSIA AND ADJACENT ALASKA: A DECADE OF FIELD AND U-PB (SHRIMP-RG) GEOCHRONOLOGIC STUDIES

The extensive Mesozoic magmatic belts of NE Russia still remain poorly dated. Ten years of field-based efforts together with U-Pb dating in collaboration with J. Wooden utilizing the Stanford-USGS SHRIMP-RG have significantly contributed to clarifying age relations, permitting a more in-depth understanding of the plate tectonic evolution of the Arctic and Pacific margins of Russia and adjacent Alaska.

The paleo-Pacific-facing margin of Siberia (now the Verkhoyansk fold belt) and Arctic Chukotka share an orogenic history with the Brooks Range, AK, with deformation beginning with emplacement of oceanic allochthons at 160-170 Ma and culminating with shortening of the margin in the Jura-Cretaceous. The Main batholithic belt of the Verkhoyansk is 160-140 with most intrusions in the 155-150 Ma range. Inheritance of slightly older zircons in younger plutons, occasional inheritance of 1.8-1.9 Ga basement zircons and coeval arc volcanic sequences (155-150 Ma) revise earlier views of colliding arcs to the current view of an Andean style arc with a back-arc fold belt. Magmatism in the northern E-W trending portion of the Verkhoyansk is younger at 137-120 Ma, perhaps documenting the beginning of subduction zone move-out towards the Pacific and rift opening of the Arctic which reconfigured and dispersed older orogenic trends. Magmatism spanning 120-105 Ma occurred across Arctic Russia as well as Alaska in an extensional setting above a subduction zone retreating towards the Pacific. Along the Taigonos and Okhotsk part of margin, periods of shortening and magmatism developed above continent-ward dipping subduction whose position shifted to a lesser degree. The extensive Okhotsk-Chukotka volcanic belt (OCVB) post-dates the reconfiguration and rift history of the Russian Arctic and was erupted in a neutral to slightly extensional tectonic regime, ~ 98-80 Ma. Where the OCVB crosses the Bering Strait, it is associated with continued extension and crust remobilization as established by dating of zircons from crustal xenoliths and seismic reflection profiling. The subsequent jumps of the northern circum-Pacific subduction zones to the Aleutians in Alaska and to Kamchatka in Russia (with opening of the Sea of Okhotsk) were the final steps that led to the present configuration of this long-lived subducting plate margin.