Abstract The San Juan–southern Gulf Islands Archipelago of Washington State, USA and western Canada is located on the upper plate of the Cascadia subduction zone, in the forearc between the trench and volcanic arc. Onland and island investigations show many faults within the region that primarily represent old, inactive faults associated with transport, subduction and accretion of tectonostratigraphic terranes. However, until recently little geologic investigation and mapping have been done in the offshore. From these narrow straits, channels and sounds we have collected and interpreted high-resolution multibeam echosounder bathymetric data, 3.5 kHz sub-bottom and Huntec seismic-reflection profiles, and piston-cores to identify and date recently active faults. Previous studies by us focused on the earlier recognized active Devils Mountain fault zone that bounds the southern part of the Archipelago and the recently reported newly mapped active Skipjack Island fault zone that bounds the northern part. These transcurrent fault zones appear to be deforming and rotating the Archipelago. We concentrate on the unique deformation occurring within the seaways to determine the relationship and styles of faulting associated with these active bounding fault zones and relate the fault geometry and kinematics to one other subduction complex, the New Hebrides island arc of Vanuatu.

Abstract Sucia Island, part of the San Juan archipelago of western Washington, is underlain by sedimentary rocks of the Upper Cretaceous Nanaimo Group and Eocene Chucka-nut Formation (molasse). The Chuckanut overlies the Nanaimo along an important thrust fault. In addition to the geology, this trip is designed to provide excellent views of many species of sea and land birds that inhabit the offshore waters.

Abstract The mid-Cretaceous San Juan Islands–northwest Cascades thrust system is made up of six or more nappes that are a few kilometers or less thick, up to one hundred kilometers in breadth, and that were derived from previously accreted Paleozoic and Mesozoic terranes. This field trip addresses many questions regarding the tectonic evolution of this structural complex, including the homeland of the terranes and the process of post-accretionary dispersal that brought them together, how thrusting in the San Juan Islands might have been related to coeval orogenic activity in the neighboring Coast Plutonic Complex, and the origin of blueschist metamorphism in the thrust system relative to subduction and nappe emplacement. The geology of this trip has many counterparts in other outboard regions of the Cordillera, but some aspects of the tectonic processes, as we understand them to date, seem to be unique.