Cenozoic tectonic processes along the southern Alaska convergent margin

Abstract

The southern margin of Alaska is arguably the most tectonically complex part of the plate boundary that defi nes western North America. The tectonic confi guration of this plate boundary is defi ned by an active subduction zone, two active volcanic arcs (the Aleutian and Wrangell arcs), some of the largest strike-slip fault systems on Earth (the Denali and Tintina faults), and an allochthonous crustal block, the Yakutat microplate, that is currently colliding in southeastern Alaska (Fig. 1; Plafker and Berg, 1994). In addition, large-magnitude earthquakes and extremely high topography mark this area as a dynamic, actively deforming continental margin. The most accepted model for the tectonic development of southern Alaska is based on the “terrane tectonics” paradigm. The generalized framework of this model is that distinct crustal blocks (i.e., oceanic plateaus, volcanic arcs, rifted continental margins) are passively transported with subducting oceanic crust. As the terrane follows the oceanic crust into the subduction zone, the more buoyant terrane jams the subduction zone and forces the subduction zone to step outboard of the “jammed” segment, and thus the terrane becomes accreted or welded to the continental margin (i.e., the upper plate of the subduction zone). The fi nal products of this process are crustal-scale blocks (i.e., terranes) that are fault bounded and that have no “geologic” affi nity to adjacent crustal blocks along the continental margin. In the case of the northern Cordillera, the now “accreted” terranes are tectonically transported northward as rigid blocks by margin-parallel strike-slip faults. An important point for this model is that the major strikeslip faults along the continental margin are the driving force for northward translation of terranes. The terrane conceptual framework has been applied to tectonic settings around the globe, but southern Alaska was the center