Deformation of the Bellingham Basin in the Northern Cascadia Forearc as Inferred from Potential Field Data

The Bellingham basin, spanning onshore and offshore regions of northwestern Washington state and southwestern British Columbia, is deforming under north-south shortening in the north Cascadia forearc. Accommodating the regional strain are Holocene-active faults within the basin that have been traced both offshore and onshore on the basis of gravity, aeromagnetic, and limited seismic data. In this study, we add 160 new gravity measurements to an existing database to better define the geometry of the Bellingham basin and its relation to recently discovered NW-trending faults. The new gravity data, spaced at ~ 1 km in the study area, were collected to address gaps in the irregular spatial distribution of existing data and extrapolate deformation recorded in coastal areas eastward into the basin. Regional-residual separation methods and derivative maps suggest that the Bellingham basin is segmented into three smaller basins. The southeast-trending Birch Bay fault extends 30 km into the basin, in agreement with previous work. The Sandy Point fault to the south of Birch Bay and the Drayton Harbor fault to the north appear as pronounced NW-SE trending lineations in magnetic data but are not as apparent as the Birch Bay fault in the new gravity data. The new data indicate that the northern margin of the Bellingham basin follows an arcuate path, southeastward from Birch Bay, then curving northeastward to connect with the Boulder Creek Fault. Two cross-sectional 2.5D models crossing the Bellingham basin show that the Birch Bay fault is steeply dipping and closely associated with a NW-SE trending anticlinal structure involving the underlying Chuckanut Formation and older rocks. An industry seismic line located ~2 km north of the Birch Bay fault shows an anticline involving Quaternary strata, consistent with the cross-sectional models. Results from the study suggest that the Bellingham basin contains evidence of Holocene-active faulting that, like other forearc basins to the south, is accommodating regional strain.