CRUSTAL STRUCTURE OF THE MEGUMA TERRANE PARALLEL TO THE COAST OF NOVA SCOTIA: CONSTRAINTS ON ITS ORIGIN

A marine wide angle reflection/refraction (WAR) experiment was run along the coast of Nova Scotia. The purpose of the experiment was to determine the lateral and vertical extent of the velocities typical of the Meguma Terrane and the velocity relationships to adjacent terranes. During the experiment 14 digital ocean bottom seismometers (DOBS) were launched and recovered. The airgun array fired 3983 shots over a range of 503 km. The source was an array of 6 airguns with a total capacity of 98.4 L (6000 in3). The modeling of the seismic waves indicate that rocks with velocities of 5.5-5.7 km/s typical of the siliciclastic rocks of the Meguma Supergroup are less than 3 km thick parallel to the coast. The average velocity of the crust is 6.2-6.4 km/s with a maximum velocity of 6.6 km/s consistent with the laboratory velocity measurements on the Musquodoboit granite. The total crustal thickness is 36-37 km and the Moho is flat. The base of the lower crust is highly reflective. This reflectivity is observed on both the vertical incident and wide angle reflection profiles.

These measurement are consistent with the results of the first offshore refraction experiment in the region and are within the range associated with Paleozoic terranes. The low velocity at the base of the crust and thickness is not typical of the adjacent Avalon Terrane. These lower velocities are also inconsistent with a substantial thickness of mafic underplating. Four models for the evolution of the Meguma Terrane are compared with the constraints from seismic profiles: tectonic emplacement of the Meguma over the Avalonian basement, a plume modified orogeny, Tibetian style collapse, and wide spread regional extension producing igneous activity after the amalgamation of the Meguma and Avalon Terranes.