Crustal structure of the central Nova Scotia margin and the transition from volcanic to non-volcanic rifting off eastern Canada.

摘要

New seismic results are presented to elucidate the crustal structure of the transitional region of the margin. A velocity model derived from wide-angle seismic data shows three major crustal zones. The continental crust consists of three layers with velocities of 5.5-6.9 km/s with a maximum thickness of ∼36 km. The rifted continental crust extends to a region of the faulted basement blocks. The basement immediately seaward of continental breakup consists of oceanic layer 2 and a high-velocity lower crustal layer which is interpreted as a composite layer of serpentinized peridotite and gabbroic layer 3. The ocean-continent transition zone contains a ∼170-km-wide partially serpentinized mantle layer with velocities of 7.6-7.95 km/s, overlain by highly faulted continental crust in the northwest and thin oceanic crust in the southeast. No evidence of underplating or excessive extrusion is observed, suggesting that the central Scotian margin is non-volcanic. NovaSpan Project MCS data show that excessive rift volcanism disappears ∼100 km north of the SW Scotian margin. Therefore, the major part of Scotian margin is non-volcanic except for the southwest-most part, and the East Coast Magnetic Anomaly with reduced amplitude along the major part of Scotian margin is not caused by an igneous wedge. The volcanic to non-volcanic margin transition takes place across a ∼100-km-wide zone, which is characterized by rugged basement topography and relatively deeper depths. Reconstructions of the Scotian/Moroccan margin pair to continental breakup and pre-rift positions are proposed based on the seismic results, magnetic anomaly and salt distribution from the opposing sides. Two types of asymmetrical crustal structures are presented along this margin pair. Across the northern segment, rifting and breakup are symmetrical; but post-rift compression of the High Atlas Mountains caused uplift and erosion on the Morocco side, resulting in a shallow Essaouira Basin with thin sediment offshore Morocco, in contrast to a deeper Sable Subbasin with thicker sediment off Nova Scotia. Across the central segment, uplift and erosion occurred off Nova Scotia at the early stage of rifting. The absence of syn-rift uplift and erosion off Morocco produced asymmetrical initial extension. However, the Jurassic breakup and subsequent seafloor spreading became more symmetric.