Geophysical Investigation of Salt Tectonics and Deeper Structure in the Eastern Magdalen Basin, Atlantic Canada.

Magnetic anomalies and related salt structures in the E. Magdalen basin give a window in to the effects of regional tectonics on basin formation. The Magdalen basin contains up to 18 km of Paleozoic sedimentary rocks resting on the crystalline basement of the Acadian Orogeny. Carboniferous rocks are intensely deformed as a result of regional faulting and salt tectonism, which was initiated following deposition of the Viséan Windsor Group. Clusters of short wavelength magnetic lineations, associated with gravity anomaly lows, define NNE and ENE trending linear belts, which enclose rhomboidal zones of very low amplitude magnetic anomalies. Seismic profiles show the lineations to be related to zones of deformation and diapir collapse near to the margins of the salt structures. Euler deconvolution models indicate both shallow (<300 m) and deeper (<1 km) fault or contact type magnetic sources. Shallow sources are interpreted to result from mineralisation associated with alteration in salt impregnated iron rich sedimentary rocks, brecciated during salt activity. Deeper sources are related to faulting and are linked to the deeper structure. The base event, the deepest regionally mappable seismic horizon at between 2-5 s. TWTT ( ~ 5-11 km), is associated with an unconformity at the base of the Windsor Group, sampled by the Cap Rouge well. Salt structures are related in both trend and location to faults and associated topographic highs in the base event. The orientations of structures in the eastern Magdalen basin are generally consistent with regional deformation associated with middle Carboniferous dextral strike-slip motion on the E trending Cobequid-Chedabucto fault system. As suggested by the strain ellipse model, this produced NW compression and ENE dextral motion in the E. Magdalen basin from Viséan to at least Westphalian D. This resulted in thrusting of the base event to the SE of the Magdalen Is. and NE of Cape Breton Is. and dextral motion on faults which bound the southern edges of rhomboidal structures.