Formation and evolution of the Nova Scotian rifted margin: Evidence from deep seismic reflection data

Abstract

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Seotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and subsidence data. They show significant thinning of the continental crust over a zone about 200 km wide. The mode of extensional deformation is probably a combination of pure and simple shear; there is evidence for simple shear in the crust. The continent‐ocean boundary lies near the seaward edge of synrift salt below the continental rise. A 100‐km‐wide zone of very thin (approximately 9 km or less) continental or transitional crust extends seaward from the outer shelf to this boundary. Reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction, perhaps modified by extensional faulting. This margin is nonvolcanic, 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 margin segment than it is on the volcanic margin to the south. It seems likely that the prerift fabric of the continental lithosphere controls this width. A narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism. However, significantly narrower zones of crustal extension occur on other nonvolcanic margins, so factors in addition to rift width, such as the rate of rifting, may also be important.

An investigation of salt tectonic structural styles in the Scotian Basin, offshore Atlantic Canada: 1. Comparison of observations with geometrically simple numerical models

“…To first order, the large-scale synrift geometry of a rifted margin will depend on the distribution of lithospheric extension across the margin. In designing the models we assume that (1) lithospheric extension is uniform with depth, (2) the extension varies across the 200 km margin in an approximately linear manner that is consistent with crustal models developed by Keen and Potter [1995], Funck et al [2004], and Wu et al [2006], and (3) the stretched margin is in isostatic equilibrium in a configuration determined by the thickness, density and thermal state of the extended lithosphere. Synrift salt and clastic sediments load the margin and the system is flexurally isostatically balanced in a submarine configuration including loading by the water.…”

“…We chose h cc = 35 km, h oc = 6.5 km, and densities r cc = r oc = 2860 kg m −3 , and r cm = r om = 3225 kg m −3 , which allows the depth of the top of the oceanic crust to be calculated. Corresponding depths across the margin, assumed to be 200 km wide between positions X 1 and X 2 (a geometry consistent with recent seismic refraction surveys [Keen and Potter, 1995;Funck et al, 2004;Wu et al, 2006]), are calculated by linear interpolation because the lithosphere is assumed to thin linearly across the margin.…”

“…The Scotian Basin represents one of the deepest MesozoicCenozoic depocenters fringing the North American and African rifted margins systems [e.g., Wade and MacLean, 1990;Keen and Potter, 1995], and it contains multiple saltrelated structures that are classified by us into three principal structural styles (denoted SS) (Figures 1,3,4,and 5) : SSA, an open-ended roho system (listric, basinward dipping normal faults soling out onto allochthonous salt) with a synkinematic sediment wedge [Shimeld, 2004;Ings and Shimeld, 2006] (according to Schuster [1995], "the term roho was coined about 1970 at Shell Oil Co. from "C. C. Roripaugh's (a Shell geophysicist) Moho" to describe a high-velocity zone characterized by a thin band of high-amplitude discontinuous reflectors that form the base of most of the faults and allochthonous salt features in an area"); SSB, a linked salt tectonic system (landward extension and seaward contraction/salt sheet formation are dynamically linked) with listric normal faults, allochthonous canopies and roho systems [Jackson et al, 2003]; and SSC, salt diapirs and intervening minibasins. Despite recent improvements in seismic data and interpretations, the reasons for such contrasting structural styles, and the synrift basin-scale crustal configuration, are poorly understood.…”

Variations in rifting style and structure of the Scotian margin, Atlantic Canada, from 3D gravity inversion

“…5d). Keen & Potter (1995b) compared the crustal structure observed on multichannel seismic reflection lines 89-1 and 88-1 that are approximately coincident with SMART lines 1 and 2, respectively. In particular, they noted that on line 88-1, in the centre of the margin, many normal faults appear to dip to the NW and terminate in the middle crust, suggesting a mid-crustal weak zone beneath this part of the margin.…”

“…Previous studies, especially those involving multichannel seismic reflection data, have provided images of the crust and overlying sedimentary basin (e.g. Keen et al 1991;Keen & Potter 1995b). Velocity structural models, derived from wide-angle reflection/refraction data collected in 2001 as part of the Scotian Margin Regional Transects (SMART) experiment, provided information on crustal geometry and velocities across the SW, central and NE parts of the margin Funck et al 2004;Wu et al 2006).…”