The Nova Scotia margin is located in a unique position bounding volcanic margins to its south and non-volcanic margins to its north. Previous studies of the structure during the MARIPROBE Program (1999-2004) and OETR Play Fair Analysis (2009-2011) resulted in margin-normal wide-angle refraction OBS transects that crossed the Scotian offshore in four locations (Figure 1; 3 SMART profiles and 1 OETR profile together with other relevant regional MCS surveys). These profiles show significant variations in rifting style along the margin: (i) a volcanic-type margin in the southwest; (ii) a non-volcanic margin in the centre, with a sharp boundary between highly faulted continental crustal blocks and oceanic crust; and (iii) an extremely amagmatic margin in the northeast, with exposed mantle within the continent-ocean crustal transition and thin oceanic crust seaward. However, without any along-margin continuous coverage it has not been possible to determine the exact location and nature (e.g. gradual/abrupt/complex) of the identified lateral variations in the continent-ocean transition zone.
In the Fall of 2010, a margin-parallel OBS profile named OCTOPUS (Ocean-Continent Transition Offshore Profiling Using Seismics) was acquired in water ∼4-km deep using 20 Dal/GSC OBS (Figure 2). The OCTOPUS profile examines, in a continuous manner, the transition from non-volcanic to extreme amagmatic rifting associated with a major change in crustal reflectivity recorded on the GXT Nova Span MCS Line 5100. This profile is about 200 km long, consists of 20 4-C OBS, is coincident with NovaSPAN Profile 5100, and crosses OETR and SMART-1 OBS profiles (Figure 3) . An array consisting of eight air guns (total volume = 68.2 L) towed at 7-m depth was fired at 60-s intervals to produce an average shot spacing of 125 m. The wide-angle data were recorded at a sampling rate of 4 ms . OBS #5 data cannot be analyzed due to a timing error.
Thus far the vertical geophone and hydrophone data were analyzed using the rayinvr layered velocity modeling/inversion approach (Lau et al., 2019). The results show that the continent-ocean transition at the northeastern Nova Scotia Margin displays highly variable short-wavelength tectonic structures along strike. Widespread serpentinized mantle is found beneath the transitional crust with exhumed serpentinized mantle present only locally.