Evidence for Magma-Rich and Magma-Poor Characteristics Across the Continent-Ocean Transition Offshore Central Nova Scotia as Deduced From Vp/Vs Ratios Using 4-Component Seismic Data

Abstract

To improve identification of crustal rock types within the continent-ocean transition (COT) offshore Nova Scotia based on the standard approach of analyzing P-wave velocities (V_p), we incorporate S-wave velocity (V_s) modeling and determine V_p/V_s ratios. In this work, we construct detailed layered V_p and V_s models using four component wide-angle ocean bottom seismometer data from profiles SMART-2 and -3 across the central and southwestern Scotian margin, respectively. Along profile SMART-3, the lower continental crust displays low V_p/V_s ratios ($\sim$1.7), akin to felsic granulite. V_p/V_s ratios (1.7–1.9) within the COT suggest that syn-rift volcanism and magmatism resulted in a basalt-dominated upper crust overlying a thick layer (4–10 km) of laterally heterogeneous gabbroic underplate that jointly cover an estimated profile area of $\sim$1,230 ${\text{km}}^2$. On Profile SMART-2, we observe upper continental crust (V_p/V_s 1.7) with highly felsic composition within the continental and the COT domains. Serpentinized mantle (V_p/V_s $>$ 1.9) is interpreted within the seaward-most segment of the transitional lower crust, alongside gabbroic underplate, based on its V_p/V_s ratio of 1.7–1.8 and high V_p of 6.9–7.4 km/s, notably thickening beneath the East Coast Magnetic Anomaly near the landward COT limit. Syn-rift volcanism and magmatism SMART-2 profile area is estimated at $\sim$ 513 ${\text{km}}^2$. Our results portray the southwestern Scotian margin as magma-rich, the northeastern as magma-poor, and the central margin as a hybrid zone amalgamating characteristics from both. Regardless of margin type, the thinnest transitional crust is consistently observed toward its seaward termination leading into the full-scale seafloor spreading.