Episodicity and the dance of late mesozoic magmatism and deformation along the northern circum-pacific margin: nerussia to the cordillera

ABSTRACT New U-Pb geochronology and a large and growing geochemical data base has clarified the age and composition of the extensive magmatic belts of NE Russia, allowing a better understanding of their relation to the plate tectonic evolution of the circum- Pacific margins of Russia, Alaska and the North American Cordillera, as well as to plate tectonic events that formed the Arctic Ocean. Prior to the magmatism described here, the paleo-Pacific margin of Siberia (now the Verkhoyansk-Kolyma orogen), Arctic Chukotka and the Brooks Range, Alaska, shared passive margin sedimentation from the Triassic to mid-Jurassic. Deformation along this once contiguous margin began with closure of deep-water basins and culminated with emplacement of oceanic and arc rocks onto the margin with continuing shortening of the margin into the Jura-Cretaceous. The Main Kolyma batholith belt (calc-alkaline granites to granodiorites) (158-144 Ma, peak 150±3 Ma) and the paired Uyandina-Yasachnaya volcanic arc are thought to have resulted from west-dipping subduction. Northern belt plutons (calc-alkaline) are 134-123 Ma and mostly post-date the E-W trending oroclinal bend of the northernVerkhoyansk fold belt. Calc-alkaline granites to granodiorites spanning 125-100 Ma intruded across Chukotka (Artic Russia) and Alaska during extension, right lateral shear and subduction zone retreat towards the Pacific perhaps coeval with the onset of rifting in the Amerasia Basin of the Arctic. The Uda-Murgal arc evolved independently along the Taigonos and Okhotsk part of the Russian margin, between 150-130 Ma and 107-97 Ma, and was followed by the establishment of the margin-parallel Okhotsk-Chukotka volcanic belt (OCVB), erupted in a neutral to slightly extensional tectonic regime, beginning 106 Ma, but, mostly between 98-80 Ma and ending ~ 76 Ma. Age equivalent magmatism in the Bering Strait is associated with continued extension, melting and remobilization of the deep crust. Between 80-60 Ma, subduction-related magmatism stepped south to the outer Bering Shelf and then to the Aleutians before 46 Ma. A similar process occurred in Russia with the pre-55 Ma onset of opening of the Sea of Okhotsk and step-wise migration of the subduction zone to eastern Kamchatka and Kuril Islands. These events broadly coincide with the opening of the Eurasia Basin of the Arctic. Profound differences occur in the tectonic setting of magmatism through time along strike of the subducting margin from Russia to the North American Cordillera. From 125 to 60 Ma, the NE Russian and Alaskan sectors of the margin underwent mostly neutral to extensional tectonism while the Cordillera underwent crustal shortening. Although vast amounts of paleo-Pacific oceanic lithosphere were subducted beneath both sectors of the margin, it appears that in a mantle reference frame, the Cordillera moved mostly towards its subduction zone while the overriding continental plates in NE Russia and Alaska moved parallel to or away from their subduction zones, allowing magmatism to take place in tectonically neutral or extensional settings. The coupled timing of trench-towards versus trench-away events related to motion of the overriding plates led to the observed “dance” of magmatism and deformation along the northern circum-Pacific margin.