Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

Abstract

We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide‐angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35‐km‐thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ∼6 km thickness. In the distal margin we find a 60‐km‐wide zone with seismic velocities between 5.0 and 6.5 km s−1 that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s−1 and higher beneath this thin crustal layer in the continent‐ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25‐km‐wide zone where seismic velocities increase gradually from 6.3 km s−1 just below the top of acoustic basement to 7.7 km s−1 at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s−1 in basement ridges to 7.0 km s−1 at its base, overlying mantle velocities between 7.8 and 8.2 km s−1. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma‐starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent‐ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.

Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey)

“…At the eastern Grand Banks of Newfoundland (SCREECH 2, Fig. 16A zone 2), the ocean-continent transition is marked by 25 km wide zone, where seismic velocities increase gradually from 6.3 to 7.7 km/s at 5 km below basement (Van Avendonk et al, 2006). The authors interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle.…”

“…These Vz graphs can be compared to compilations for typical continental crust (Christensen and Mooney, 1995), Atlantic-type oceanic crust (White et al, 1992), and serpentinized upper mantle from previous work (Dean et al, 2000;Van Avendonk et al, 2006). On the base of this analysis of the final models and the Vz graphs, three distinct domains can be distinguished in the final models (A) unthinned continental domain (B) a domain of crustal thinning (necking domain), which can be divided in two subdomains, one where the lowermost crustal layer thins extremely or might pinch out and a second in which crustal thinning is located mainly in the upper crustal layers, and lastly (C) a domain westward of the S1 magnetic anomaly which also can be divided in two subdomains based on lower crustal velocities.…”

The final rifting evolution at deep magma-poor passive margins from Iberia-Newfoundland: a new point of view

“…This domain shows geophysical and geological characteristics, different from those of classical continental or oceanic crusts (Chian et al 1999;Funck et al 2004;Van Avendonk et al 2006;Lau et al 2006). For instance, the seismic velocities of the upper part of the basement (V P 4.5-7 km/s) are lower than those of the adjacent continental crust whereas those of the lower part (V P [ 7.6 km/s) are too high to represent either oceanic layer 3 or underplated mafic continental crust .…”

“…However, as expose in this paper we favour the hypothesis of mantle exhumation and are confident that our model correctly explains the geophysical characteristics (see below). A typical oceanic velocity structure is observed westward of magnetic anomaly M0 in the Iberia margin (Whitmarsh et al 1990) and eastward of magnetic anomaly M1 on the Newfoundland side (Van Avendonk et al 2006;Lau et al 2006).…”

“…2 and discussed in the following. a Iberia -Newfoundland composite transect based on geophysical data (van Avendonk et al 2006;Shillington et al 2004;Krawczyk et al 1996;Pickup et al 1996;Pérez-Gussinyé et al 2001) and drill hole data (ODP Legs 103,149,173 and 210 Scientific Results). The transect coincides with the Screech 2 reflexion /refraction profiles on the Newfoundland margin and integrates the IAM9; LG12, ISE1 and ISE17 lines on the Iberia margin (for location of the transects see map in the lower right corner).…”