INTERPRETATION OF 3-DIMENSIONAL SEISMIC-REFRACTION DATA FROM WESTERN HECATE-STRAIT, BRITISH-COLUMBIA - STRUCTURE OF THE CRUST

As part of a multidisciplinary investigation of the structure and tect onics of the Queen Charlotte Basin and underlying crust. deep multicha nnel seismic reflection and coincident crustal refraction data were co llected in 1988. Energy from the reflection air-gun array source was r ecorded at land sites at offsets appropriate to record crustal refract ion and wide-angle reflection data. Refraction data recorded in a broa dside geometry provide good three-dimensional coverage of western Heca te Strait. These data are modelled using tomographic inversion techniq ues to determine the three-dimensional velocity structure of the crust in this region. The one-dimensional average velocity increases rapidl y with depth to 6.5 km/s at 7 km depth. Velocities from 7 to at least 12 km depth remain approximately constant and are associated with rock s of the Wrangellia terrane. Significant lateral velocity variations, including large differences in near-surface velocities attributable to surface features, relatively low velocities representing interbedded Tertiary sediments and volcanics. and a deep high-velocity anomaly tha t may represent the root of an igneous intrusion, are mapped. Wide-ang le reflections from the Moho are used to determine the thickness of th e crust. The Moho is at 29 km depth beneath the east coast of the Quee n Charlotte Islands. This is deeper than the Moho observed below Queen Charlotte Sound and as deep as, or deeper than, that below Hecate Str ait. Crustal thinning during Tertiary extension was thus greatest bene ath the surface expression of the Queen Charlotte Basin, leaving the c rust under the islands considerably thicker than under the basin. In a n alternate or additional explanation, compression at the continental margin during the last 4 Ma may have been taken up by thickening or un derplating of the continental crust beneath the islands. If the Pacifi c plate is subducting beneath the islands. the Moho observations const rain the slab to dip greater than 20-26-degrees.