[Show abstract][Hide abstract] ABSTRACT:
The Nova Scotia margin off Eastern Canada is located at a transition from volcanic margins in the south to non-volcanic margins in the north. The southern Nova Scotia margin is largely volcanic, evidenced by seaward dipping reflections (SDR) coinciding with a strong East Coast Magnetic Anomaly (ECMA). The northern part of the margin shows no evidence of major volcanism where the ECMA weakens and eventually disappears. In the central part of the margin, no SDRs have been observed but the ECMA is still prominent. This raises doubt about where the transition from a volcanic to a non-volcanic style margin exactly occurs. In order to study this transition, three wide-angle refraction lines were acquired in 2001 across the northern, central and southern parts of the margin, respectively. The middle line extends from the Lahave Platform into the Sohm Abysmal Plain. The velocity model derived from the refraction data shows no evidence for a magmatic underplated layer beneath the continental crust, suggesting non-volcanic rifting. In addition, a 120-km wide region of partially serpentinized mantle is observed within the ocean-continent transition zone, similar to other non-volcanic margins in the North Atlantic. Together, these observations are difficult to reconcile with significant volcanism, and the ECMA with reduced amplitude in this region is not likely to be related to volcanic activity as it is farther to the south. The velocity model also shows that the continental crust is thinned during rifting initially within the middle and lower crust beneath the hinge zone, and subsequently within the upper crust. The thinned upper continental crust extends over a distance of 180 km up to a highly faulted basement structure; the continent-ocean boundary is then located some 60 km farther seaward than an earlier interpretation by Keen et al. (1991) based only on reflection seismic data. Keen C. E., MacLean B. C. & Kay W. A., 1991. A deep seismic reflection profile across Nova Scotia continental margin, offshore eastern Canada. Can. J. Earth Sci., 28, 1112-1120.
[Show abstract][Hide abstract] ABSTRACT:
The Nova Scotia margin is located at a transition from volcanic margins in the south to non-volcanic margins in the north along the Eastern Atlantic continental margin system. South of the Nova Scotia margin, seaward dipping reflections (SDR) and a high-velocity lower crustal layer are observed across the ocean-continent transition (OCT), indicative of volcanic margins. North of the Nova Scotia margin, no SDR is observed and thin crust overlies serpentinized mantle across the OCT. Along the Nova Scotia margin, an SDR sequence is observed but only on the southern-most part neighboring Georges Bank. The East Coast Magnetic Anomaly, possibly related to an igneous wedge, extends further to the north but disappears south of the Scotian Basin. In order to understand the crustal structure of this transition from volcanic to non-volcanic margin, three wide-angle seismic reflection/refraction (WAR/R) profiles were acquired across the full width of the Nova Scotia margin in 2001, to delineate the crustal features from continental crust to oceanic crust. The northern profile (Line 1) crosses the Scotian Basin and coincides with existing multi-channel seismic reflection (MCS) profile 89-1; the middle profile (Line 2) crosses the Lahave Platform and coincides with MCS profiles 88-1 and 88-1a; and the southern profile (Line 3) crosses the southwestern part of the margin. Preliminary results for the middle WAR/R profile indicate that serpentinized mantle is observed below the OCT, possibly overlain by thin oceanic crust. Oceanic crust is about 6 km thick, 1-2 km thicker than that of the northern profile. This indicates increasing volcanism and magma generation from the northern to the southern parts of the margin. WAR/R profiles were collected across the conjugate NW-Moroccan margin in 2001, one of which is 60 km to the north of Line 1 in the plate reconstruction at the time of rifting. Similar amounts of crustal extension and widths of transitional crust are shown along these two conjugate WAR/R profiles. No WAR/R profiles are available in conjugate positions to the Nova Scotia margin lines 2 and 3. However, by analogy to Line 1, we suggest that the Moroccan margin will also be characterized by increasing volcanism toward the south.