Slow and ultra-slow spreading at mid ocean ridges provide a window into mantle deformation processes. At first order, core-complexes exhume mantle to the ocean floor along large-offset normal faults (Cann et al., 1997; Tucholke et al., 1998). These detachment structures form through weakening by serpentinization and melt infiltration of abyssal peridotites that facilitate the large amount of extension needed to bring mantle lithosphere from ~10-15 km depth to the surface (Escartín et al., 2001). More recent work in the Southwest Indian Ridge (SWIR) and in Western Alpine ophiolites shows that other weakening mechanisms such as dynamic grain recrystallization and melt infiltration may also control the formation of detachment faults in the mantle (e.g., Kaczmarek & Müntener, 2008; Linckens et al., 2011; Warren & Hirth, 2006). Similarly, at continental rifts, magma-poor margins experience a phase of mantle exhumation preceding seafloor spreading initiation (Franke, 2013; Manatschal, 2004; Peron-Pinvidic et al., 2013; Tugend et al., 2018; Boillot et al.,1989).
Seismic reflection and refraction experiments at the Newfoundland-Iberian conjugate margins show large domains (in excess of 50 km) of exhumed mantle (McIntosh et al., 2013; Pedrera et al., 2017; van Avendonk et al., 2006, 2009). This is also observed at magma-poor margins in the Angola-Gabon margin, East India margin, and Australia-Antarctica conjugate margins (Gillard et al., 2013; Harkin et al., 2019; Peron-Pinvidic et al., 2013, McCarthy et al. 2020). Drilling and dredging in Iberia and Newfoundland have confirmed the presence of peridotite and serpentinite (Boillot et al., 1987; Manatschal et al., 2007). Overall, the character of magma-poor margins is that of exhumed or denuded mantle in along oceanic core-complex-like structures and a limited amount of volcanic activity prior to seafloor spreading initiation (Franke, 2013; Peron-Pinvidic et al., 2013; Tugend et al., 2018) (Fig. 1). Understanding the dynamics and kinematics of the mantle lithosphere, which makes up the bulk of lithospheric strength (Buck, 2006), is therefore fundamental to understanding extensional tectonics. While there is a fair amount of evidence for mantle exhumation, the transition from mantle exhumation to seafloor spreading at magma-poor margins is still poorly constrained and leads to the broad use of “ocean-continent transition” (OCT) or “continent-ocean transition” (COT) to characterize this key domain for addressing breakup processes and the birth of a Mid Oceanic Ridge. For example, at the Newfoundland- Iberia conjugate margins, the transition is highly controversial due to differing interpretations of a strong magnetic anomaly, which may or may not represent a sharp or diffuse boundary between lithospheric necking and seafloor spreading (Bronner et al., 2011; Nirrengarten et al., 2017, 2018). Similar debates surround the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA) of the Eastern North American rifted margin (Biari et al., 2021; Shuck et al., 2019). Whether or not these magnetic anomalies represent paleomagnetic isochrons or whether they are magma bodies generated by another, earlier, ultimate rifting process has important implications for characterizing the rift-to-drift transition.