Two-phase evolution of accretionary margins: examples from the North American Cordillera

Abstract

The Coast Mountains orogen, southeastern Alaska, and the Salmon River suture zone, western Idaho, record the accretion of oceanic crustal fragments to the North American Cordilleran margin. Structural elements associated with these boundaries include a western accretion-related thrust belt and steeply dipping post-accretionary shear zone. The shear zones, the Coast shear zone, southeastern Alaska, and western Idaho shear zone, Idaho, are steeply dipping crustal-scale structures characterized by steep isotopic and geophysical gradients. The presence of syndeformational tabular calc-alkaline tonalitic plutons concordant with shear zone fabrics, locally preserved dextral strike-slip indicators, and pervasive down-dip mineral and elongation lineations are consistent with an intra-arc transpressional origin for both structures. In addition to strike-slip translation, the structures accommodated exhumation of high pressure–moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure–high temperature batholithic rocks to the east. Similarities between the structures in Alaska and Idaho suggest that the observed two-stage history of underthrusting of accreting crustal fragments followed by modification by intra-arc transpressional shear zones is a common tectonic process. Our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post-accretion deformation and provides insight into lower-crustal processes presently operative under modern strike-slip–accretionary boundaries. In addition, the intra-arc transpressional shear zones (e.g. the Coast and western Idaho shear zones) are structures that could have accommodated paleomagnetically determined large-magnitude transcurrent displacements.