Search

Two wide-angle seismic reflection/refraction profiles were acquired to determine the velocity characteristics of the Meguma terrane and adjacent Avalonia northeast of the Cobequid-Chedabucto Fault Zone (CCFZ) in eastern Canada. Line 99–1 is located along the Scotian margin whereas line 99–2 crosses the Scotian margin and onshore Nova Scotia and extends into the Gulf of St. Lawrence. Velocity models were derived by forward modelling of travel times. P-wave velocities of 5.5–6.0 km/s and 6.0–6.4 km/s are interpreted to be metasedimentary rocks of the Goldenville and Halifax groups and granitic rocks, respectively. This inference is based on comparison with laboratory velocities of a suite of rock samples, offshore sampling, links to onshore geology with multibeam bathymetry, and gravity modelling. On both lines a low velocity zone (LVZ) >350 km wide and up to 15 km thick is present at mid-crustal levels. The LVZ is not observed beneath onshore Nova Scotia or northeast of Orpheus Graben, the offshore extension of the CCFZ. The crustal thickness of the Meguma terrane varies between 31 and 38 km; in contrast, the Moho is at a depth of 41 km beneath Avalonia. The velocity of the lower crust on line 99–2 is 6.8–7.1 km/s beneath the LVZ and Avalonia, and 6.6 km/s beneath onshore Meguma terrane. Poisson's ratios calculated from P- and S-wave velocities are 0.19–0.23 in the Meguma terrane crust; in contrast, Poisson's ratios of 0.24–0.25 are indicated for Avalonian crust. • Seismic reflection/refraction, gravity, and rock density/velocity data were obtained. • Two velocity/depth models were generated onshore and offshore of Nova Scotia, Canada. • Both models show an extensive low-velocity zone in the offshore Meguma terrane. • Meguma terrane has crustal structure different from that of adjacent Avalonia. • Avalonia extends only 30 km under Meguma terrane in faulted contact. [ABSTRACT FROM AUTHOR]

The central Nova Scotia margin off Eastern Canada is located at a transition from a volcanic margin in the south to a non-volcanic margin in the north. In order to study this transition, a wide-angle refraction seismic line with dense airgun shots was acquired across the central Nova Scotia margin. The 500-km-long transect is coincident with previous deep reflection profiles across the Lahave Platform and extending into the Sohm Abyssal Plain. A P-wave velocity model was developed from forward and inverse modelling of the wide-angle data from 21 ocean bottom seismometers and coincident normal-incidence reflection profiles. The velocity model shows that the continental crust is divided into three layers with velocities of 5.5–6.9 km s−1. The maximum thickness is 36 km. A minor amount (∼5 km) of thinning occurs beneath the outer shelf, while the major thinning to a thickness of 8 km occurs over the slope region. The seaward limit of the continental crust consists of 5-km-thick highly faulted basement. There is no evidence for magmatic underplating beneath the continental crust. On the contrary, a 4-km-thick layer of partially serpentinized mantle (7.6–7.95 km s−1) begins beneath the highly faulted continental crust, and extends ∼200 km seawards, forming the lower part of the ocean-continent transition zone. The upper part of the transition zone consists of the highly faulted continental crust and 4- to 5-km-thick initial oceanic crust. The continent–ocean boundary is moved ∼50 km farther seawards compared to an earlier interpretation based only on reflection seismic data. The oceanic crust in the transition zone consists of layer 2 and a high-velocity lower crustal layer. Layer 2 is 1–3 km thick with velocities of 5.6–6.0 km s−1. The high-velocity lower crustal layer is 1–2 km thick with velocities of 7.25–7.4 km s−1, suggesting a composite layer of serpentinized peridotite and gabbroic layer 3. Oceanic crust with normal thickness of 5–7 km and more typical layer 3 with velocities of 6.95–7.3 km s−1 is observed at the seaward end of the profile. [ABSTRACT FROM AUTHOR]