Crustal structures and tectonism in southeastern Alaska and western British Columbia from seismic refraction, seismic reflection, gravity, magnetic, and microearthquake measurements

Graduation date: 1972Seismic refraction measurements along two unreversed lines\ud indicate that the earth's crust is 26 km thick in southeastern Alaska\ud and 30 km thick along the Inside Passage of British Columbia. The\ud crust in southeastern Alaska, north of Dixon Entrance, consists of\ud a layer 9 km thick with a seismic velocity of 5.90 km/sec, a layer\ud 7 km thick with a seismic velocity of 6.30 km/sec. and a layer 10 km\ud thick with a seismic velocity of 6.96 km/sec. The crust along the\ud Inside Passage of British Columbia, south of Dixon Entrance, consists\ud of a layer 13 km thick with a seismic velocity of 6.03 km/sec, a layer\ud 5 km thick with a seismic velocity of 6.41 km/sec, and a layer 12 km\ud thick with a seismic velocity of 6.70 km/sec. The velocity of the\ud mantle below the M discontinuity is 7.86 km/sec in southeastern\ud Alaska and 8.11 km/sec in British Columbia.\ud A compilation of Bouguer gravity data along the Inside Passage\ud from northern Vancouver Island to northern southeastern Alaska\ud indicates near-zero anomalies between steep gradients offshore and\ud near the western margin of the Coast Mountains. A two-dimensional\ud gravity model, constrained by seismic refraction measurements,\ud suggests that the thickness of the crust is constant beneath the region\ud of near-zero gravity anomalies and indicates a step-like transition\ud between oceanic and continental structure.\ud Seismic reflection, gravity, and magnetic measurements,\ud obtained during a 1970 cruise of the R/V Yaquina, help to determine\ud upper crustal structures in Dixon Entrance. Gravity models, constructed to agree with these data and the measurements of previous\ud investigators, indicate sediment thicknesses of nearly 3 km east\ud of Learmonth Bank and west of Celestial Reef. Magnetic models\ud suggest large lateral changes in basement susceptibility. Either\ud highly metamorphosed rock or basaltic intrusions can account for\ud these changes in susceptibility. Folded sediments suggest post depositional\ud distortion due either to regional compression or to\ud major local intrusions. Several linear gravity features, observed\ud in northern Dixon Entrance, disappear north of Graham Island.\ud Either the structures responsible for the gravity features end or\ud thick layers of basalt, extending northward from Graham Island,\ud obscure the effect of the structures.\ud A single-station survey detected microearthquakes at nine\ud locations in western British Columbia and southeastern Alaska. The\ud majority of the observed distant microearthquakes probably originated\ud in the Queen Charlotte Islands fault zone. However, observed\ud nearby microearthquakes indicate a microearthquake seismicity of\ud several events per day along the mainland coast of British Columbia.\ud Temporary seismic arrays located at a site along the central\ud portion of Chatham Strait near the Chatham Strait fault and at a site\ud in Glacier Bay recorded few nearby microearthquakes. Arrivals at\ud the arrays permitted the location of distant microearthquakes, however,\ud with epicenters in the vicinity of northern Lynn Canal and along\ud the Fairweather fault