Net movement on the Sandspit fault, which marks the western edge of the Queen Charlotte basin, is a combination of dextral strike slip movement with significant downdrop of the east block. Evidence of Neogene-Holocene strike slip on Queen Charlotte Islands includes slickensides and offset drainage patterns, topographic features, and geochemical anomalies. The northwest-trending fault parallels the better documented Rennel-Louscoone wrench fault system and the Queen Charlotte transform fault.

Continuous reflection seismic and magnetic profiling in western Hecate Strait was conducted to investigate the offshore extension of the fault zone. A broad magnetic trough in Hecate subbasin, colinear with the Sandspit trend, suggests a crustal dislocation developed in "basement" Cretaceous sediments and Upper Jurassic volcanic rocks. En echelon, gentle

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open folds in shallow Neogene bed rock, likely to be crustally pervasive, are draped over the indicated fault zone. Lengths of major fold axes range from 25 to 45 km, with dips on fold limbs ranging from 2 to 11°. Wavelength of folds averages 3 km with indicated shortening at the pre-Pleistocene unconformity of 1.5 to 2.5%. The curvilinear trend of fold axes and associated minor faults is oblique to the Sandspit trend. Lack of a through-going fault in Neogene sediment cover demonstrates that the zone is in an early or incipient stage of wrench-related structural development. Deep coupling movements along the buried fault zone are interpreted as the drive which has produced the observed shallow structural pattern.

Temporal and spatial relations of the major northwest shears suggest part of the North American-Pacific plate motion has been taken up by the Sandspit and Rennel-Louscoone faults. Earlier lateral movements along major faults in Queen Charlotte Sound, that may form part of a further Sandspit extension, might explain geophysical anomalies and tectonic events recorded in Insular Belt rocks.

Exploratory drilling in the late 1960s tested a number of wrench anticlines along a broad zone parallel to the offshore Sandspit fault trend.

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