GEOLOGIC RELATIONS ALONG THE INSULAR-INTERMONTANE BOUNDARY REQUIRE MODIFIED “BAJA BC”, “MEZCALERA SUTURE”, AND “HIT AND RUN” HYPOTHESES

Recent tectonic syntheses have invoked large-scale tectonic mobility along the boundary separating the Insular (IN) and Intermontane (IM) terranes during Cretaceous-Paleocene time. Some models (e.g., "Baja BC") propose ~2000 km of dextral transport on the Coast Shear Zone (CSZ), some propose closure of a ~2000-km-wide ocean basin ("Mezcalera Suture"), and some propose both suturing and large-scale dextral motion ("Hit and Run"). These interpretations are inconsistent with geologic relations in SE Alaska and coastal BC which preclude significant strike-slip displacement or closure of an ocean basin between the Yukon-Tanana/Taku terranes on the east (part of IM) and the Alexander/Wrangellia terranes on the west (part of IN). Significant strike-slip displacement on the CSZ is precluded by the occurrence of Neoproterozoic-early Paleozoic rocks of the Tracy Arm Assemblage (Yukon-Tanana terrane) and Permo-Triassic rocks of the Stikine terrane on both sides of the Coast Mountains Orogen. Geologic relations are consistent with ~150 km of dextral displacement on the CSZ, as proposed by Lanphere (1978). Closure of a large ocean basin during Cretaceous time is precluded by accumulation of Upper Jurassic-Lower Cretaceous strata of the Gravina belt on both the Yukon-Tanana/Taku terrane and Alexander/Wrangellia terrane, with no evidence of a suture zone within the Gravina basin or along its margins. Geologic relations are consistent with Gravina forming as a back-arc basin, as proposed by Berg et al. (1972). Suggestions that the critical geologic relations have been obscured by younger deformation/metamorphism/magmatism are not viable, as Early Cretaceous sinistral faults, mid-Cretaceous thrust faults, and Late Cretaceous-Paleogene dextral faults (with limited displacement) are all well preserved within and adjacent to the orogen. The documented relations record a fascinating history of large-scale (~800 km) sinistral motion during Early Cretaceous time, collapse of the Gravina basin and juxtaposition of IN and IM along mid-Cretaceous thrust faults, and dextral offset along the Chatham Strait fault (~150 km), Clarence Strait fault (~15 km), and CSZ (~150 km). The Mezcalera Ocean basin could have closed along the IN-IM boundary prior to Late Jurassic time, and faults with large-scale dextral offset may be located inboard of the IN-IM boundary.