MAGMATISM AND DEFORMATION ALONG THE NORTHERN CIRCUM-PACIFIC MARGIN FROM NE RUSSIA TO THE CORDILLERA

An evolving geochronology database for NE Russia can be compared to that available from Alaska and the northern Cordillera to give insight into the space-time patterns of subduction linked magmatism along the northern circum-Pacific margin. Coupled with constraints on the nature and age of regional deformation, magmatism can be classified as syn-shortening, syn-extensional or neutral tectonic settings. The combined data reveal similar episodicity of magmatic events but contrasts in tectonic setting along strike of the margin. These different histories are only beginning to be appreciated and are intriguing but poorly understood in terms of their plate tectonic and geodynamic context.

Contrasts in deformational history date back to the Permo-Triassic where eruption of Siberian Traps and rifting in NE Russia contrast to shortening in the Canadian Cordillera (closure of Slide Mountain Basin). The main Mesozoic shortening event in NE Russia and Alaska occurred ~ 150- 130 Ma, during a “lull” in both magmatism and deformation in the Cordillera. Magmatism in NE Russia and Alaska was voluminous in the timespan 120- 65 Ma but took place in neutral to extensional tectonic settings compared to the history of near continued convergence in the Cordillera. Geologically, these changes in the tectonic setting of magmatism (occurring near the Canadian- Alaskan border) resulted in a relatively narrow zone of magmatism and parallel zones of shortening and strike-slip in the Cordillera compared to broad and complex patterns of magmatism as subduction migrated Pacific-ward, culminating in subduction zone step-outs to Kamchatka (~65 Ma) and the Aleutians ~45 Ma.

The near coincidence in timing of events (albeit of differing nature) suggests that the absolute motions of the continental plates involved are coupled in a global plate framework and that their motion towards or away from subduction zones are important in dictating the origin of their associated magmatic belts and the tectonic settings for magmatism.