TILTING AND TRANSLATION IN WESTERN BRITISH COLOMBIA: 40AR/39AR GEOCHRONOLOGY AND PALEOMAGNETISM OF THE SMITH ISLAND BUTEDALE AND ECSTALL PLUTONS AND IMPLICATION FOR THE BAJA-BC HYPOTHESIS

The emplacement location and translation history of deep-seated plutons in western British Columbia have been key ingredients to the Baja-BC hypothesis, which proposes that the westernmost terranes of British Columbia have experienced thousands of kilometers of late Cretaceous northward translation. We investigate three Cretaceous plutons, the Smith Island, Butedale, and Ecstall, using paleomagnetism and ⁴⁰Ar/³⁹Ar geochronology to produce an internally consistent dataset from which robust tectonic interpretations can be formed. Spatial trends in hornblende and biotite ⁴⁰Ar/³⁹Ar cooling ages from the Smith Island and Butedale plutons show evidence of reheating by the Ecstall and Quottoon plutonic complex, respectively, but there is no suggestion of tilting in either pluton’s cooling age trends as there was in the Ecstall. The mean paleomagnetic directions of the Smith Island and Butedale plutons are most consistent with tilt-corrected directions from parts of the Ecstall that are not affected by reheating. The mean paleomagnetic directions from reheated parts of the Ecstall pluton become concordant with the mean direction of the adjacent Quottoon plutonic complex when adjusted for 15-20˚ of tilt, implying tilting occurred before and after reheating. The virtual geomagnetic poles given by the three plutons provide estimates of northward translation that are consistent with the Baja-BC hypothesis, and could be further constrained given additional paleomagnetic sampling.