EPISODICITY AND THE DANCE OF MAGMATISM AND DEFORMATION ALONG THE NORTHERN CIRCUM-PACIFIC MARGIN: NE RUSSIA TO THE CORDILLERA (Invited Presentation)

An evolving geochronology and geochemical database for magmatic rocks in NE Russia delivers information on space-time patterns of subduction-linked magmatic activity that can be compared to Alaska and the Cordillera, providing a broad view of the nature and episodicity of magmatism and deformation along the northern circum-Pacific margin.

Our compilation classifies convergent margin magmatism as syn-shortening vs. syn-extensional/neutral in terms of tectonic setting. In the Mesozoic, magmatic and deformational events appear temporally linked but opposite in nature along strike of the margin. In the Permo-Triassic (when the northern circum-Pacific margin first developed) eruption of Siberian Traps and rifting in NE Russia is paired with shortening in the Canadian/US Cordillera (closure of Slide Mountain and Havallah back-arc basins). Magmatism and crustal shortening in the mid-Jurassic was significant in the Cordillera but minor and mostly offshore in Russia and Alaska. The main Jura-Cretaceous shortening event in NE Russia and Alaska occurred ~ 155- 125 Ma, during an important “lull” in magmatism and deformation in the Cordillera. Magmatism in NE Russia and Alaska was voluminous in the timespan 120- 65 Ma (as in the Cordillera), but occurred in a neutral to extensional tectonic setting compared to substantial shortening in the Cordillera. Differences take place near the Canadian-Alaskan border and through time resulted in relatively narrow zones of magmatism and parallel belts of shortening and strike-slip in the Cordillera versus broader and more complex patterns of magmatism in Alaska and Russia where subduction migrated Pacific-ward with time, culminating in step-outs to Kamchatka (~65 Ma) and the Aleutians ~45 Ma.

The remarkable synchroneity but decided contrasts in magmatic flareups, lulls and deformational events suggests that global factors such as the absolute motions of continental plates or mantle convection control the nature of this “coupling”. It has long been known that North America moved westward with respect to the locus of subduction along its western margin in the Mesozoic. Eurasia, coupled to North America across the Arctic, mostly moved away from its subduction zones in a plate tectonic dance that helped produce the pattern of linked episodicity seen along the northern circum-Pacific margin.