Unusually fracture-free seafloor in the southern North Atlantic between Newfoundland and Iberia explained through compounding tectonic inheritance since the Paleoproterozoic Open Access

Inspection of interpreted fracture zones (Srivastava and Roest, 1999; Matthews et al., 2011) throughout the Atlantic Ocean (Fig. 1A) reveals an anomalous region of seafloor devoid of fracture zones between Newfoundland (eastern Canada) and Iberia (southwestern Europe), with that region bounded by two major fracture zones, the Charlie-Gibbs fracture zone in the north and the Newfoundland-Azores fracture zone in the south. This localized lack of fracture zones is independently supported by predictions of minimal seafloor roughness relative to the rest of the Atlantic Ocean (Fig. 1B), computed from gravity observations (Whittaker et al., 2008). Considering that one of the founding principles of plate tectonics was that inherited lithospheric weaknesses control the localization of future oceanic fracture zones (Wilson, 1965; Fig. 2), a lack of such fracture zones between Newfoundland and Iberia may reflect some form of inherited lithospheric mantle strength or homogeneity (Martinez and Hey, 2022), with only the largest transforms aligning with onshore inherited structures (Taylor et al., 2009).

The role of inheritance in subsequent tectonism, particularly for the North Atlantic, is well recognized based on the evidence for two consecutive Wilson cycles (Wilson, 1966; Thomas, 2006, 2019; Chenin et al., 2015; Schiffer et al., 2020). During the first Wilson cycle, involving the final breakup of Rodinia and the creation of the Iapetus Ocean from 620 to 550 Ma (Whitmeyer and Karlstrom, 2007), the resulting jagged geometry of the rifted southeastern margin of Laurentia (North America), as interpreted by Thomas (2006, 2019) and Allen et al. (2009) and shown by the purple lines in Figures 1 and 3, comprised major promontories, such as the St. Lawrence promontory in Atlantic Canada. This massive promontory would subsequently impact Appalachian orogenesis (Thomas, 1977, 2006; Stockmal et al., 1987; Waldron et al., 2019; White and Waldron, 2022) and possibly predefine the geometry for subsequent Atlantic rifting and breakup (Williams, 1984; Thomas, 2006, 2019; Allen et al., 2009; pink line in Figs. 1 and 3). The relatively smooth along-strike geometry of the southeastern limit of the Paleozoic St. Lawrence promontory (Allen et al., 2009) as later mimicked by the breakup geometry of the southeastern limit of the Mesozoic Grand Banks (Thomas, 2006, 2019) aligns with the fracture-free zone of outboard seafloor (Fig. 1B). If related, the anomalous zone of seafloor between Newfoundland and Iberia could be attributed to tectonic inheritance from as early as the Neoproterozoic. Such a statement does, however, ignore the important question of why the St. Lawrence promontory developed in the first place and suggests that even older tectonic inheritance may be involved in its formation.

In this contribution, a new onshore-offshore tectonic map is constructed, and a cascading sequence of tectonic events, described chronologically beginning in the Paleoproterozoic during the formation of Laurentia, is presented as a possible explanation for the paucity of fracture zones in the seafloor between Newfoundland and Iberia, with the locations of the key inferred tectonic episodes numbered sequentially in Figure 3 and their evolution depicted schematically in Figure 4. If correct, this evolutionary scenario, first hinted at by Williams (1984), implies that lithospheric tectonic inheritance can persist for almost two billion years and can survive through multiple orogenic and rifting cycles.

The Laurentian core of North America comprises several Archean cratonic blocks (e.g., Superior craton) that were amalgamated during distinct episodes of Proterozoic orogenesis, culminating with their inclusion into the supercontinent Rodinia (Li et al., 2008). The largest Paleoproterozoic orogen of Laurentia was the Trans-Hudson orogen (St-Onge et al., 2009), whose easternmost branch corresponds with the New Quebec orogen (Fig. 3). The New Quebec orogen (episode 1 in Figs. 3 and 4) trends NNW–SSE and marks the terminal 1.82–1.795 Ga dextral transpressive collisional suture zone between the Archean Superior craton to the southwest and the Churchill province (Rae province) to the northeast (Wardle et al., 2002; St-Onge et al., 2009). The collision involved southwestward thrusting of the New Quebec orogen over the Superior craton, with the southernmost extent of the New Quebec orogenic belt being deflected southwestward around the stronger Superior craton (Fig. 3; White and Waldron, 2022). The southernmost New Quebec orogenic rocks were later reworked during the late Mesoproterozoic to early Neoproterozoic Grenville orogeny, which sutured Laurentia and Baltica to Amazonia during the final stages of the building of the supercontinent Rodinia (Li et al., 2008; Rivers, 2009).

Prior to the Mesoproterozoic Grenville orogenesis, the Labrador orogen (highlighted in Fig. 3 and corresponding to episode 2 in Figs. 3 and 4) was accreted through southward subduction to this portion of the Laurentian margin toward the end of the Paleoproterozoic from 1.71 to 1.6 Ga (Gower et al., 1992; Gower, 1996; Rivers, 1997; Whitmeyer and Karlstrom, 2007). The Labrador orogeny involved accretion of multiple arc-related terranes with accompanying pluton and batholith emplacement, resulting in significant crustal thickening (Gower, 1996; Funck et al., 2001). For Atlantic Canada, the Labrador orogen represents the oldest (White and Waldron, 2022) and possibly thickest crust (Funck et al., 2001) to have been reworked 500 million years later during the subsequent Grenvillian orogenesis (Fig. 3).

Breakup of the Rodinian supercontinent and the resulting opening of the Iapetus Ocean (episode 3 in Figs. 3 and 4) along the southeastern margin of Laurentia occurred during the Neoproterozoic from 620 to 550 Ma (Whitmeyer and Karlstrom, 2007; Thomas, 2014), leaving an irregular rifted margin consisting of distinct promontories and embayments of variable size and lateral extent (Thomas, 1977, 2006, 2019; Stockmal et al., 1987; Allen et al., 2009). The jaggedness of the margin later dictated the overall shape and curvature of the Appalachian orogen during the amalgamation of the Pangean supercontinent as the relative rheological strength of the Iapetan margin indented and deflected the weaker intra-oceanic and peri-Gondwanan accreted terranes during ocean closure (Stockmal et al., 1987; Thomas, 2006; Waldron et al., 2019; White and Waldron, 2022).

For the southeastern Iapetan Laurentian margin, the massive size of the St. Lawrence promontory (Fig. 3) is rivaled only by the Alabama promontory that impacted the southern Appalachians (Fig. 1). While the Alabama promontory formed through the rifting away of the Argentine Precordillera along the southern boundary of Laurentia at 535 Ma (Whitmeyer and Karlstrom, 2007), no formation mechanism has yet been proposed for the St. Lawrence promontory. Given the alignment between the southwestern limit of the New Quebec orogen, the southwestern limit of the Labrador orogen, and the southwestern limit of the promontory (known as the Sept Îles transform; Fig. 3), inheritance from a lithospheric-scale feature dating back to the Paleoproterozoic is suggested here.

Early Jurassic breakup of Pangea began in the Central Atlantic, south of the Newfoundland-Azores fracture zone, following a long period of rifting from the Late Permian to the Early Triassic (Frizon de Lamotte et al., 2015). Rifting later migrated northward between Newfoundland and Iberia from the Late Jurassic to the Early Cretaceous before breakup was finally achieved in the late Aptian (Frizon de Lamotte et al., 2015). Atlantic rifting (episode 4 in Figs. 3 and 4) was localized outboard of the Iapetan rifted margin, within the zone of Appalachian orogenesis, leaving terranes of Gondwanan affinity behind in Laurentia (Williams, 1984; Thomas, 2006). The resulting angular shape of the Grand Banks continental shelf offshore Newfoundland mimics that of its predecessor, the St. Lawrence promontory of the Iapetan rifted margin (episode 3 in Figs. 3 and 4), and its southwestern limit corresponds to a transform that follows the same trend as the Iapetan Sept Îles transform.

Inspired by Wilson (1965; Fig. 2), the evolution from Paleoproterozoic orogenesis to Mesozoic seafloor spreading is illustrated in Figure 4. Episodes 1 and 2 occur during the Paleoproterozoic and result in the emplacement of perpendicularly oriented orogenic sutures. During subsequent Iapetan rifting from the latest Neoproterozoic through to the Paleozoic, rifts localize along the boundaries of the thickened Labrador orogen but are offset by a transform that follows along strike from the main NNW-SSE orogenic suture between the New Quebec orogen and the Superior craton. Rifting and eventual seafloor spreading during episode 3 create the St. Lawrence promontory where the ridge segments are offset by roughly the width of the Labrador orogen. These geometries remain in place and are reinforced through offset terrane accretion during Appalachian orogenesis. Finally, Atlantic rifting in the Mesozoic (episode 4) localizes the rift axis within the zone of accreted terranes, forming the Grand Banks along the same long-lived transform boundary that originally formed the St. Lawrence promontory.

The Newfoundland-Azores fracture zone, which marks the southern boundary of the zone of fracture-free seafloor between Newfoundland and Iberia, clearly aligns with the seaward extrapolation of the southwestern edge of the Neoproterozoic St. Lawrence promontory (Sept Îles transform), which itself aligns with the western limits of the Paleoproterozoic New Quebec orogen and the Labrador orogen (Fig. 3). As postulated by Thomas (2014), such a long-lived structural feature likely reflects that the underlying lithospheric mantle fabric is aligned with the overlying brittle transform fault in the crust and partitions the ductile flow in the lithospheric mantle on either side of the transform. This major fracture zone also lies in close proximity to the along-strike transition from magma-rich rifting offshore southern Nova Scotia to magma-poor rifting offshore Newfoundland, as revealed from continent-ocean transition zone variations (Welford, 2024), perhaps suggesting a long-lived juxtaposition of variably fertile mantle that dates back to the Paleoproterozoic.

For the Charlie-Gibbs fracture zone, which represents the northern boundary of the zone of fracture-free seafloor between Newfoundland and Iberia, the fracture zone alignment shows a closer match with Appalachian trends (Williams, 1984), suggesting more recent tectonic inheritance than the Newfoundland-Azores fracture zone. In this case, less of a lithospheric mantle contrast would be expected on either side of the transform (Thomas, 2014), perhaps explaining why the Charlie-Gibbs fracture zone lies within a broader zone of consistently magma-poor Atlantic rifting that surrounds the whole island of Newfoundland and only transitions to magma-rich rifting farther north in the Labrador Sea (Keen et al., 2012; Welford, 2024).

While slow seafloor spreading is expected to generate a stable series of closely spaced en echelon ridge-transform segments due to variations in along-strike mantle melting (Martinez and Hey, 2022), instead, the Mid-Atlantic Ridge between Newfoundland and Iberia is relatively straight, and the resulting seafloor is fracture free and smooth. While smooth seafloor is typically attributed to exhumed peridotites at ultraslow seafloor spreading ridges (Cannat et al., 2006), the slowly spreading Mid-Atlantic Ridge is floored by oceanic crust with clear magnetic anomalies (Srivastava et al., 1990). From a melt perspective, this observation implies that the underlying lithospheric mantle has been uniformly strong and chemically depleted since at least the North Atlantic breakup (Martinez and Hey, 2022). Analysis of serpentinized mantle peridotites along the Newfoundland margin, exhumed adjacent to the earliest oceanic seafloor, supports the presence of depleted mantle in this region, inferred to have been inherited from Appalachian or older accretionary orogenesis (Müntener and Manatschal, 2006).

The tectonic evolution of the southern North Atlantic follows from multiple rifting and orogenic cycles, possibly spanning as far back as the Paleoproterozoic. The persisting influence of lithospheric structures that are almost two billion years old, manifesting as fracture-free seafloor within the modern southern North Atlantic, suggests that inheritance plays an even greater and long-lived role in subsequent tectonism than previously argued. Similarities in the orientations of the repeated Wilson cycles experienced in this part of the North Atlantic, along with similarities in their rifted margin geometries, luckily preserve the evidence for inheritance. While tracking ancient lithospheric inheritance in regions that have experienced more complicated and more variably oriented tectonism may well be impossible, the influence of Proterozoic sutures and scars on subsequent tectonism may be just as important in those regions as they were for the evolution of the southern North Atlantic despite remaining hidden.

This research has been supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant). Peter Klitzke, Brendan Murphy, and an anonymous reviewer are thanked for their constructive comments.