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A seismic refraction study of the Queen Charlotte fault zone

University of British Columbia

The margin between the continental North American and oceanic Pacific plates west of the Queen Charlotte Islands is uniquely marked by an active transform fault zone. The region is the locus of oblique convergence between the two lithospheric plates. West of the fault zone the absent continental shelf is replaced by a 25 km wide scarp-bounded terrace at 2 km depth which separates the oceanic and continental crust. An onshore-offshore seismic refraction survey was carried out in 1983 across the Queen Charlotte Islands region. Thirty-two explosive charges and several airgun lines were recorded on eleven land-based and six ocean-bottom instruments. A subset of the resulting data set was chosen to study the structure of the Queen Charlotte Fault zone and adjacent terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structural model of the fault region. Underlying the deformed terrace sediments is an upper 3 km thick faulted unit with velocity 3.8 km/s and a high gradient. The lower crustal region is on average 10 km thick and has velocity 5.3 km/s and a slightly lower gradient. Beneath this unit the Moho increases in dip from 5° to 19° eastward. The terrace velocities are anomalously low compared to the adjacent oceanic and continental crustal structures. Velocities of the oceanic crust are consistent with those observed in ophiolite sequences. The velocity structure of the continental crust is not well-defined; however, vertical offset of 1.1 km is seismically observed on the Rennell-Louscoone Fault on Moresby Island. Two tectonic mechanisms are proposed to explain the anomalous terrace structure. Subduction of the oceanic lithosphere beneath the terrace would accrete sediments to the seaward edge of the terrace and subduct material beneath it. Upthrusting of terrace material along near-vertical fault planes would result from compression at the transform fault above an inactive subduction zone. A combination or alternation of the two mechanisms would explain the observed fault zone structure.

Text

2010-06-20

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http://hdl.handle.net/2429/25867

Geophysics

University of British Columbia

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