MESOZOIC EVOLUTION OF THE NORTHERN CORDILLERAN MARGIN OF LAURENTIA: WHAT WE KNOW, WHAT WE DON’T KNOW, AND IMPLICATIONS FOR LITHOSPHERE DEVELOPMENT (Invited Presentation)

During Jurassic-Cretaceous time the northwestern and northern margin of Laurentia evolved from a series of plates and microplates bounded by subduction and transform systems into a diffusely deforming plate boundary zone. This presentation highlights the geophysical and geological constraints on this history.

Although Alaska is almost exclusively composed of non-Laurentian fragments, much of interior, western, and northern Alaska was continental crust or margin sediment that was close or adjacent to the Laurentian margin by Jurassic time (Arctic, Farewell, and Yukon-Tanana terranes and para-autochthonous Laurentia). The Arctic terrane in Alaska has a unique history compared to the other blocks as it evolved from an outward (away from continent) subduction system to a collision with an oceanic island arc during the Jurassic and then moved away from Laurentia during opening of the Canada basin in the Cretaceous. In contrast, in southern Alaska the Wrangellia composite terrane’s (WCT) juvenile crust evolved during a latest Triassic–Cretaceous arc-forearc-subduction complex built during east to north-directed subduction (in modern coordinates). Closure of the ocean basin between WCT and North America (NA) varied from south to north, with evidence for southern regions experiencing Jurassic closure and partial re-opening, forming the Gravina-Kahiltna basin, before final mid-Cretaceous closure along the entire WCT-NA via an east-dipping interface. The connection between the southern and northern AK subduction zones and arcs is unclear; during this time the Farewell terrane of west-central AK remained a stable continental fragment with marine sedimentation.

By Late Cretaceous time all subduction zones were closed within what is now Alaska – Yukon-BC; the only active subduction boundary was from the Pacific basin. However large-scale strike-slip displacement along lithosphere-penetrating faults and major crustal-scale shortening continued to alter the crustal thickness and composition of the western and northwest Cordilleran margin through the early Cenozoic. Major unresolved problems include: 1) the extent of continental crust under arcs and basins of western - southwest Alaska and the polarity of subduction for these arcs; and 2) the location and extent of pre-Cenozoic strike-slip faulting (dextral and sinistral).