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 The Messinian Salinity Crisis (5.85–5.35 Ma) represents a nearly unprecedented unloading and loading event. During the Messinian Salinity Crisis, two important things happened in terms of surface load changes—the accumulation of thick evaporites represent a load addition, while the desiccation of the Mediterranean represents a load subtraction. The desiccation and evaporite deposition were followed by the rapid addition of water, refilling the Mediterranean during the Zanclean. The calculated flexural response to load changes imposed significant changes on the horizontal stresses in the upper crust of the Northern Apennines, an active orogenic system characterized by simultaneous zones of extension and compression that migrated eastward over time. We show that these flexural stresses (approaching ± 50 MPa), added to the preexisting stresses across the Northern Apennines, were large enough to have caused some areas in compressional stress regimes to flip to extensional regimes, and vice versa. Among other things, our model predicts that the Marches Apennines region, which was beginning to undergo compression at the leading edge of the orogen, should have experienced a brief interval of extensional deformation. Previous structural studies of this region have shown that there was, in fact, a brief period of extensional faulting near the onset of the Messinian Salinity Crisis, which was then followed by a return to compressional deformation, just as our model predicts. The hypothesis presented here provides a novel explanation for this extension occurring in an area that should have been undergoing compression, one in which the unique events of the Messinian Salinity Crisis generated significant flexural stresses that changed the deformational regime during the relatively brief time of the Messinian Salinity Crisis. We further suggest that this hypothesis may provide insight into similar unexpected tectonic episodes during this time period in other parts of the Apennines.