SINISTRAL FAULTING ALONG THE NORTHERN CORDILLERAN MARGIN: IMPLICATIONS FOR INSULAR-INTERMONTANE CONNECTIONS AND THE POLARITY OF JURA-CRETACEOUS SUBDUCTION

Jura-Cretaceous sinistral motion has played an essential but under-appreciated role in the accretionary history of the northern Cordillera. Large-scale sinistral motion has been recognized since early attempts to reconstruct geologic relations (e.g., Coney, 1981; Plafker et al., 1989) and plate motions (Engebretson et al., 1985), and is inherent in most models based on paleomagnetic data (e.g., Enkin, 2006). But where are the sinistral faults?

Field and geochronologic studies in and adjacent to the central and southern Coast Mountains Batholith (CMB) document the existence of two sinistral fault systems that were active during Early Cretaceous (and possibly Late Jurassic) time. The outboard fault system is developed within 160-105 Ma plutons in the outboard (western) portion of the CMB, and records ~800 km of offset of the Banks Island Assemblage and southern Wrangellia from an original position adjacent to the northern portion of the Alexander terrane and Wrangellia. The inboard fault system is interpreted to have offset the Insular terrane and outboard portion of the CMB a total of ~800 km southward relative to the Intermontane terrane and inboard (eastern) portion of the CMB. The southern part of the outboard CMB and the northern part of the inboard CMB are distinctive in that they both record stationary and continuous magmatism from ~160 Ma to ~100 Ma, whereas all other the Cordilleran arc segments record eastward-stepping magmatism from 160-140 and 120-100 Ma and a magmatic lull from 140-120 Ma. The occurrence of this unique magmatic history in both outboard and inboard segments of the CMB (1) ties the Insular terrane to the Intermontane terrane during Late Jurassic-Early Cretaceous time, (2) precludes the existence of a major suture zone within the Methow-Tyaughton-Gravina-Dezadeash-Kahiltna basin, and (3) requires all segments of the CMB to have faced westward. Collapse of these basins and final accretion of the Insular terrane at ~100 Ma signals the transition from sinistral motion during Late Jurassic-Early Cretaceous time to dextral motion during Late Cretaceous-Paleocene time. The timing and amount of sinistral motion on these two fault systems is consistent with moderate-translation models of Cordilleran paleogeography (e.g., Butler et al. 2001; Umhoefer, 2003; Wyld et al., 2006; Rusmore et al., 2013).