FORMATION AND EVOLUTION OF THE NOVA-SCOTIAN RIFTED MARGIN - EVIDENCE FROM DEEP SEISMIC-REFLECTION DATA

A deep marine seismic reflection profile was obtained across the Mesoz oic rifted continental margin off Nova Scotia, eastern Canada. This pr ofile crosses the Scotian Basin, one of the deepest basins on the marg in of eastern North America, and it complements other deep crustal sei smic data on this margin. The seismic data have been interpreted in co njunction with gravity anomaly and subsidence data. They show signific ant thinning of the continental crust over a zone about 200 km wide, T he mode of extensional deformation is probably a combination of pure a nd simple shear; there is evidence for simple shear in the crust. The continent-ocean boundary lies near the seaward edge of synrift salt be low the continental rise. A 100-km-wide zone of very thin (approximate ly 9 km or less) continental or transitional crust extends seaward fro m the outer shelf to this boundary. Reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construct ion, perhaps modified by extensional faulting. This margin is nonvolca nic, and the transition to the volcanic margin off the eastern United States occurs about 500 km southwest of the seismic line. The width of the zone of crustal extension is much greater on this nonvolcanic mar gin segment than it is on the volcanic margin to the south. It seems l ikely that the prerift fabric of the continental lithosphere controls this width. A narrow rift may be prone to vigorous asthenospheric conv ection and therefore to more voluminous volcanism. However, significan tly narrower zones of crustal extension occur on other nonvolcanic mar gins, so factors in addition to rift width, such as the rate of riftin g, may also be important.