A model for recent plate interactions off Canada's west coast

Abstract

Detailed re-examination of existing magnetic anomaly data reveals the fine structure of variations in spreading rates and directions at the Juan de Fuca and Explorer Ridges during the last 10 million years. A geometrical model using these variations delineates the theoretical history of the interactions between the lithospheric plates involved. These interactions demonstrate the independent movement of the Juan de Fuca and Explorer plates and the development of the Sovanco Fracture Zone. The latter was apparently initiated E–W at 7 Ma, rotated clockwise to 120° and may have been the site of up to 50 km of crustal shortening. The model demonstrates that subduction rates at the Canadian continental margin declined from 5 cm/yr to a present 1.5 cm/yr and that recent relative movements are compatible with the N–S compression observed from earthquakes. It also suggests that the existence of both E–W and NE trending faults in the downgoing lithosphere beneath Vancouver Island, shows that a triple junction remained static near the northern end of Vancouver Island from 10–4 Ma, and predicts a buried northern edge of subducted material striking NE in this area.

Preliminary geologic interpretation of a seismic reflection profile across the Queen Charlotte Island fault system, Dixon Entrance, Canada-United States

“…The present plate boundary is defined by the active dextral Queen Charlotte fault whose movement is estimated at a rate of about 5-6 cm/yr (Minster and others, 1974;Chase and others, 1975;Riddihough, 1977). The interpretation of profile A-A 1 ( fig.…”

The geomagnetic response across the continental margin off Vancouver Island: Comparison of results from numerical modelling and field data.

“…This tectonic regime is a subduction boundary between the Juan de Fuca and the America plates (RIDDIHOUGH, 1977;1978;1982;KEEN and HYNDMAN, 1979). The convergence vectors for the Juan de Fuca -America plates are oriented almost normal to the bathymetric trend in the vicinity of our profile (Riddihough, personal communication).…”

Program for earthquake hazards assessment in the Pacific Northwest

“…Offshore, on the continental shelf, deformed Quaternary sediments are found where converging plates meet (Barnard, 1978). A convergence rate of 4-4.: cm/yr between these two plates and a convergence direction of approximately N. 50° E. have been estimated by Riddihough ( 1977Riddihough ( , 1984. The regional free-air and Bouguer gravity anomalies show a pattern common in subduction zones: an elongated gravity low along the toe of the continental slope and a gravity high inland over the coastal mountains.…”