GPS and seismicity constraints on the current tectonics of the Northern Canadian Cordillera




Leonard, Lucinda J.

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This thesis presents new precision Global Positioning System (GPS) data, new analyses of earthquake deformation rates, and geological analyses, which constrain current deformation in Yukon and adjacent areas of the northern Cordillera. The integration of these data with additional thermal constraints facilitates the large-scale characterization of intense deformation associated with the collision of the composite oceanic-continental Yakutat block with North America in the corner of the Gulf of Alaska. Two main approaches are used to constrain the first-order current tectonic deformation pattern: (1) high-precision continuous and campaign GPS measurements; (2) seismic deformation rates calculated using earthquake statistics from the seismicity catalogue. The oblique Yakutat-North America collision is mainly accommodated by thrust and strike-slip faulting in the Saint Elias region, but associated deformation extends a great distance (> 800 km) into the North America crust of eastern Alaska, Yukon and western Northwest Territories. The data indicate westward motion and counter-clockwise rotation of the southeastern Alaska forearc along the arc of the right-lateral Denali fault. Right-lateral transpression north of the Denali fault in Alaska is accommodated by clockwise rotation of a number of NE-trending crustal blocks bounded by sinistral strike-slip faults. To the northeast of the Yakutat collision, GPS data indicate a northeastward crustal motion across the Cordillera towards the eastern foldbelt. Earthquake mechanisms indicate strain accommodation mainly by thrusting in the Mackenzie Mountains and N-S dextral strike-slip faulting in the Richardson Mountains. It is likely that a small northward component of motion is transferred further north to the offshore Mackenzie Delta region, where infrequent large earthquakes may result, despite little historical seismicity. An additional study is presented on the use of quantitative estimates of coastal coseismic subsidence in great earthquakes at the Cascadia subduction zone. Seismic hazard assessments for Cascadia megathrust earthquakes are largely based on the rupture area predictions of dislocation models constrained by geodetic and thermal data. The models for the 1700 great Cascadia earthquake are tested against compiled coastal coseismic subsidence for past great earthquakes, as estimated from paleoelevation studies. Models for strain accumulation of 550-800 years (in the range of event frequency from marsh dating and offshore turbidites), predict coastal subsidence in good agreement with marsh paleoseismic data, except that discrepancies occur at the ends of the subduction zone. Estimated slip from comparisons of dislocation models with marsh coseismic data for the 1700 event is consistent with the magnitude 9 earthquake indicated by Japanese tsunami records.



earthquakes, Canada, Alaska