Cretaceous and Tertiary Structures in the Northern Cordillera and Their Bearing on Arctic Evolution

Maurice Colpron¹, Donald C. Murphy¹, and JoAnne L. Nelson^2
1Yukon Geological Survey, Whitehorse, YT, Canada.
²BC Geological Survey, Victoria, BC, Canada.

The northern North American Cordillera is dissected by major faults of Cretaceous and Tertiary ages that shuffle the complex distribution of previously amalgamated terranes and obliquely truncate terranes of different paleogeographic affinities. First-order Tertiary faults include: 1) NW- and WNW-striking dextral strike-slip faults (e.g. Tintina, Denali, Kobuk), 2) NE-striking dextral strike-slip faults (e.g. Kaltag, Iditarod-Nixon Fork) and 3) the NE Brooks Range-North Yukon thrust belt which arcs clockwise from E- to S-striking. Their coeval timing, geometry and relationships imply that the strike-slip faults are mutually cross-cutting and the NE-striking set is the southern wrench boundary to the NE Brooks Range-North Yukon thrust belt. When these considerations are applied in palinspastic restoration, the result is a Late Cretaceous paleogeography in which elements of the Eurasian Arctic Alaska terrane are re-aligned with near-identical parts of Farewell terrane about 1000 km southwest of Arctic Alaska’s current position. The NE-striking faults along which this realignment occurs are proposed to be a primary kinematic link between the evolutions of the Pacific-Cordilleran, Arctic and Atlantic realms.

Northwest-striking Cretaceous faults broadly delineate terranes of peri-Laurentian affinity in Yukon and British Columbia. In southern Yukon and northern British Columbia, these include Kechika, Cassiar and Teslin-Thibert-Kutcho faults which collectively account for about 250 km of dextral displacement of the Intermontane region with respect to parauthochtonous North America. Many of these faults are coeval with Early to mid-Cretaceous magmatism, constraining displacement to 115-95 Ma. In Yukon, the NW-striking dextral strike-slip fault system is inferred to be the lateral wrench fault for NW-directed thrusting along the Tombstone thrust system of western Yukon and counterparts in east-central Alaska.

By contrast, mid-Cretaceous transcurrent faults bounding the Intermontane region to the SW (e.g. Grenville Channel, Pootlash, Tchaikazan and Cadwallader-Ferguson) are mostly sinistral fault systems coeval with Cretaceous magmatism at 123-85 Ma. The apparent similarity in magnitude with the dextral system to the NE, implies that the Intermontane block was escaping to the NW in mid-Cretaceous, feeding the NW-directed Tombstone thrust system at its leading edge.

Restoration of the Tertiary faults aligns the Cretaceous Colville basin of the northern Brooks Range in a foreland position with respect to the NW-directed Tombstone thrust system as well as the N-directed Jura-Cretaceous Brooks Range thrust belt. This configuration provides a direct source for enigmatic Permian igneous clasts in Albian strata of Colville basin which are only matched in Permian arc rocks of the Yukon-Tanana terrane, occurring in the hanging wall of the Tombstone thrust system.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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