Elbow tectonics: Smashing, translating, and rotating outboard terranes of the Syringa embayment of the Laurentian accretionary margin

Abstract

The Mesozoic tectonic margin in west-central Idaho, USA, continues to be a world-class geologic venue, instigating debates and inspiring hypotheses that shape our understanding of tectonics processes and related strain regimes worldwide. This field guide1 is a snapshot of up-to-date interpretations along with new data that fuels ongoing discussion of the processes that have shaped this corner of the tectonic margin—a field-based road-log tour of the enigmatic structural “elbow,” defined by an ~90° bend (a transition from N-S to E-W orientation) in the Mesozoic arc-continent boundary at the latitude ~46°N. This trip is composed of three transects that showcase the rocks and structural markers documented within this complicated region. These transects are organized as follows: (1) along the N-S segment of the terrane boundary near Riggins, Idaho; (2) a W-E orientation along the South Fork of the Clearwater River to Elk City, Idaho, roughly perpendicular to tectonic grain; and (3) along and across the complex “elbow” bend, where the boundary’s orientation shifts abruptly from N-S to E-W. Bedrock exposures and outcrops along these transects yield many opportunities to see field-based evidence for the complex, protracted tectonic and structural evolution of the arc-continent boundary, while also considering the critical preexisting tectonic framework that exerted influence on the overprinting accretionary event, as well as the late-stage, extensional system that has since reactivated and dismembered the region.