Along‐Strike Variations in Structure of the Continent‐Ocean Transition at the Northeastern Nova Scotia Margin From Wide‐Angle Seismic Observations

Abstract

Our 2‐D‐layered velocity model along strike northeastern Nova Scotian margin, constrained by wide‐angle seismic Profile OCTOPUS and coincident 9‐km streamer Profile GXT‐5100, displays highly variable basement structures interpreted to define four distinct zones within the continent‐ocean transition: Zone I, with thin (1–2 km) and laterally uniform upper crust (5.2–5.5 km/s) and high‐velocity lower crust (6.5–7.7 km/s); Zone II, with velocities of 5.5–7.5 km/s and a high vertical velocity gradient (∼1.1/s) characteristic of exhumed serpentinized (up to 90%) mantle (ESM) above ∼3.5‐km‐thick slightly (<30%) serpentinized mantle layer of reduced mantle velocities (7.5–8.0 km/s); Zone III, with 1‐ to 2‐km‐thick upper crust and velocity of ∼5.2 km/s above a ∼6‐km‐thick, moderately (<60%) serpentinized mantle layer (velocity ∼6.4–7.9 km/s); and Zone IV, with lower crust (∼0.7–3.0 km thick and with velocity of 6.1–6.6 km/s) between upper crust (velocity ∼5.0–5.4 km/s) and a slightly serpentinized mantle layer (<40%; velocity ∼7.3–8.0 km/s). These continent‐ocean transition structures represent embryonic oceanic or rifted continental crust, and ESM. By combining our results with those from two dip‐oriented crossing profiles, we map a regional track of non‐ESM landward of Profile OCTOPUS with mostly uniform along‐dip width, but a limited extent and variable width section of ESM seaward. Our results indicate that there can be a remarkable amount of short‐wavelength (e.g., 50 km) lateral structural variability within individual rifted margin segments but that mapping it requires a grid of modern wide‐angle profiles.

New Insights Into the Rift to Drift Transition Across the Northeastern Nova Scotian Margin From Wide‐Angle Seismic Waveform Inversion and Reflection Imaging

“…(2018) identify a similar COT zone except with no mantle exhumation (Figure 2a). This along‐margin structural complexity of the COT at the northeastern Nova Scotian margin is further supported by OCTOPUS, a margin‐parallel OBS profile that runs along the COT, crosses both the SMART‐1 and OETR‐2009 profiles and displays highly variable short‐wavelength (∼50 km) crystalline basement structures (Lau et al., 2019). In both the SMART‐1 and OETR‐2009 models, thinner than normal oceanic crust is imaged and interpreted as juxtaposed with the COT crust, but the more detailed information about the onset of seafloor spreading and the role of magmatism in the breakup has not yet been captured due to data and/or imaging limitations.…”

“…Lau et al. (2019) analyzed a margin‐parallel seismic profile that crosses our seismic profile at 267 km distance. Around the intersection point, the margin‐parallel profile reveals a northeast‐dipping reflector from the basement top down to 14 km depth.…”

“…Beneath the valley, reflector sequences are observed at large depths, among which some isolated reflectors are extremely bright. Lau et al (2019) analyzed a margin-parallel seismic profile that crosses our seismic profile at 267 km distance. Around the intersection point, the margin-parallel profile reveals a northeast-dipping reflector from the basement top down to 14 km depth.…”

“… Study area and seismic profiles. (a) Bathymetry map of the northeastern Nova Scotian continental margin with the location of the OETR‐2009 (Lau et al., 2018), OCTOPUS (Lau et al., 2019), and SMART 1 and 2 (Funck et al., 2004; Wu et al., 2006) wide‐angle seismic profiles. The numbers alongside the OETR‐2009 profile indicate along‐profile distance from the landward end, as used in the data analysis.…”

“…The dependence on a priori information is minor because we make no assumptions regarding the Moho or other intrabasement boundaries, and the resolution is high as the models are discretized using fine grids (Van Avendonk et al, 2006;Zelt et al, 2003). (Lau et al, 2018), OCTOPUS (Lau et al, 2019), and SMART 1 and 2 (Funck et al, 2004;Wu et al, 2006) wide-angle seismic profiles. The numbers alongside the OETR-2009 profile indicate along-profile distance from the landward end, as used in the data analysis.…”