TRANSITION FROM OCEANIC TO CONTINENTAL CRUSTAL STRUCTURE - SEISMIC AND GRAVITY MODELS AT THE QUEEN-CHARLOTTE TRANSFORM MARGIN

Seismic refraction data have been interpreted along a line crossing th e Queen Charlotte transform, just north of the triple junction where t he Explorer Ridge intersects the continental margin. These data, obser ved at three onshore sites, help to define the structure of the contin ental crust beneath the Queen Charlotte sedimentary basin. Sediment th icknesses of up to 4 km were determined from a coincident multichannel reflection line. Beneath the sediments, velocities increase from abou t 5.5 to 6.3 km . s(-1) at 8 km depth, then increase from 6.5 to 6.7 k m . s(-1) at 18 km depth. Below this depth, the lower crust is partly constrained by Moho wide-angle reflections at the three receiving site s, which indicate a lower crust velocity of 6.8-6.9 km . s(-1) and a M oho depth of 26-28 km. The crustal velocity structure is generally sim ilar to that in southern Queen Charlotte Sound. It is in contrast to t he velocity structure across Hecate Strait to the north, where a promi nent mid-crust interface at similar to 15 km depth was observed. Seism ic velocity models of the continental crust provide constraints that c an be used in modelling gravity data to extend structures across the o cean-continent boundary. Along the profile just north of the Queen Cha rlotte triple junction, the gravity ''edge effect'' is very subdued, w ith maximum anomalies of <25 mGal (1 meal = 10(-3) cm . s(-2)). To sat isfy the gravity data along this profile, the modelled crustal thickne ss must decrease to oceanic values (5-6 km) over a horizontal distance of 75 (+/-10) km, which gives a Moho dip of about 14 degrees. Farther north, refraction models across Hecate Strait provide similar constra ints for gravity modelling; the gravity data indicate horizontal trans ition distances from thick to thin crust of 45 (+/-10) km, comparable with, but slightly smaller than, those nearer the triple junction, and Moho dips at an angle of 18-22 degrees. The greater thinning near the triple junction is consistent with mass flux models in which ductile flow in the lithosphere is induced by the relative motion between ocea nic and continental plates.