A wealth of seafloor spreading data is available for most parts of the North Atlantic Ocean, but near the continental margins they become more and more difficult to justify as unequivocal signals of oceanic crustal accretion 1–5. This problem is particularly prominent for studies of the relative motions of plates bearing Greenland and North America, which formed oceanic basins in the Labrador Sea and its smaller neighbour Baffin Bay (Figs. 1 and 2)1,2,6,7. From the basin margins, the earliest signs of rifting are interpreted from diverse alkaline igneous intrusions that are dated to Late Triassic or early Jurassic times8,9. Younger intrusions have been related to Kimmeridgian (~ 150 Ma) and Early Cretaceous (~ 140 − 133 Ma) extension9. Sedimentation in fault-bounded basins started in the Early Cretaceous, and gave way to regional thermal subsidence at a poorly-constrained later in the Cretaceous10.
The majority of modern plate kinematic models for the region rely heavily on magnetic anomaly identifications from the Labrador Sea1,2,6,7,11 (Fig. 3). The consensus from magnetic anomaly studies (e.g.12,13) is that steady-state seafloor spreading was certainly underway by chron C26 (59 Ma) at the latest (Figs. 2 and 3), although many authors cite a slightly older age of C27 (62 Ma; e.g.2). During that period, the North Atlantic region was visited by large-volume magmatism related to the Iceland Plume, whose products at the continental margins are expressed in seismic reflection data as seaward dipping reflectors (SDRs; Fig. 2;14,15). This event not only accompanied the onset of steady-state seafloor spreading in the Labrador Sea, it was swiftly followed at ~ 56 Ma by a fundamental change in the orientation of plate divergence related to the onset of seafloor spreading between Greenland and Eurasia2. The youngest consistently-identifiable magnetic reversal isochron in the Labrador Sea is C20 (42 Ma), but a broad swath of seafloor lies between it and the mid-ocean ridge. Using North Atlantic magnetic isochrons near the South Greenland Triple Junction, this seafloor can be attributed to ongoing spreading until extinction of the ridge at ~ 33 Ma16.
Outside the Labrador Sea and the C27-C13 period, understanding is less evenly shared. Weak magnetic anomaly lineations marginward of C27 have been related to sources in thin igneous crust formed at distributed sites within broad continent-ocean transition zones (see17–19). It remains open to question whether such settings might produce dateable linear magnetic reversal isochrons like those over thicker (~ 7 km) standard oceanic crust and, thus, whether published interpretations of the weak anomalies as reversal isochrons as old as C33 1,20 can be seen as reliable (~ 79 Ma; Fig. 2). Davis Strait connects the Labrador Sea and Baffin Bay regions, and is host to two large N-S trending transform faults (see, Fig. 3) which are thought to have acted as sheared continental margin segments during Paleocene and Eocene times21–25. The affinity of the crust in the floor of Davis Strait is uncertain21,26,27. To the north, wide-angle seismic profiles 23,25,28,29 and a clear extinct median valley 30 leave little doubt about the absence of continental crust in Baffin Bay. Beyond this, the basin’s plate tectonic history is understood by comparison to the Labrador Sea’s owing to the lack of strong linear magnetic anomaly signals 28,31,32. This has proved difficult to reconcile with the widespread evidence for Eocene convergent tectonics in and around Baffin Bay2,29,33,34.
Here we revisit the region’s tectonic evolution by closure of its plate circuit. Using seafloor spreading data (magnetic anomaly isochrons of undisputed oceanic origin and fracture zone traces) from the North Atlantic and Arctic Ocean we produce two independent models describing 1) Eurasian–North American (EUR-NAM) plate divergence and 2) Greenland–Eurasian (GRN-EUR) divergence (Fig. 2), since as early as mid-Cretaceous times. Focusing on the late Cretaceous – Eocene time period, we generate a Greenland–North America (GRN-NAM) plate model by adding the GRN-EUR and EUR-NAM rotations (Fig. 1). In this way, we produce a framework solution for the Labrador Sea—Baffin Bay region that is insensitive to conflicting interpretations of the region’s geological and geophysical data sets. Using this framework, we show that continental breakup between the North American and Eurasian plates in the Labrador Sea dates to no earlier than C33 (74 Ma). We suggest it is possible this breakup occurred when the plate boundary propagated through a set of abandoned rift basins produced during earlier Cretaceous distributed extension. Breakup was followed by the formation of broad continent-ocean transition zones until, at C27 (61 Ma), the onset of NE-SW-oriented oceanic accretion at a stable mid-ocean ridge. Soon after its formation, the ridge was adopted as the site of NNW-SSE-oriented divergence between the North American and Greenland plates. The ridge retained its orientation in spite of the large (70–80°) accompanying change in plate divergence direction. We suggest it is possible that, in response, large normal faults along the median valley started to act in oblique slip, unlike at active mid ocean ridges of similar length where spreading obliquity nowhere exceeds 45°. Our model suggests very little oceanic crust would have been generated in Davis Strait in pre-Paleocene times, and that afterwards it became a site of transpressional deformation. Well-known compressional structures in Baffin Bay are a result NW-SE oriented compressive phase during the Middle to Late Eocene (C21 – C13; 46–33 Ma) that followed the cessation of plate divergence at C21 (46 Ma).