Fylla Bank: structure and evolution of a normal-to-shear rifted margin in the northern Labrador Sea

Cenozoic seafloor spreading between Greenland and North America is generally considered a major right‐stepping ridge‐transform‐ridge system between NW–SE trending spreading ridge segments in the Labrador Sea and Baffin Bay. The ridges were linked by N–S/NNE–SSW trending transform motions in the Davis Strait, in particular expressed by the ∼1000‐km‐long Ungava Fault Zone. Fylla Bank, part of the southern West Greenland continental margin, is located in the northernmost Labrador Sea at the transition between the normal and shear rifting regimes of the Labrador Sea and Davis Strait. As such, the Bank may be compared with the Demerara Plateau, part of the French Guinea‐Northeast Brazil continental margin. Seismic reflection interpretations presented in this study show that Fylla Bank is situated above an extensive basin complex, herein referred to as Fylla Structural Complex, which contains an up to 5‐km‐thick Cretaceous–Cenozoic sedimentary succession above an inferred pre‐Cretaceous basement. Seismic mapping of basement structures show that the complex is dominated by NNW‐/NW‐striking rift basins in its southern part and NNE‐striking rift basins in its northern part. The rift basins are interpreted to be the result of an initial late‐Early Cretaceous rift phase, which mainly resulted in the formation of the NNW‐/NW‐striking structures, and a subsequent early Campanian rift phase, mainly resulting in the formation of large NNE‐striking rotated fault blocks. Resumed rifting in the early Cenozoic deepened the NNE‐striking rift basins. The NNE‐oriented structures have previously been interpreted to initiate during the latest Cretaceous. However, this study suggests that they initiated transfer faults already during the late‐Early Cretaceous rift phase and possibly correlate with along‐strike discontinuities in oceanic crust in the Labrador Sea to define margin segmentation in southern West Greenland, including the borders of Fylla Bank. A structural‐kinematic model presented here thus suggests that the Cretaceous–Cenozoic poly‐phase rifting to some extent was controlled by pre‐existing crustal fabric. Combined with an interpreted interplay between normal stresses in the Labrador Sea and oblique‐shear stresses in the Davis Strait, this resulted in a very complex structural‐tectonic evolution and the formation of several distinct structural styles. The seismic interpretations are supported by maps of the Moho topography and crustal thickness which were compiled from results of pseudo‐3‐D gravity modelling. The maps show minimum crustal thicknesses (11 km) and maximum Moho uplifts in areas where the NNW‐/NW‐ and NNE‐striking structures interact. Moreover, a strong correlation is found between Moho topography, crustal attenuation, rift‐enforced thermal uplift and erosion, and post‐rift subsidence in the area. This is interpreted to be a result of thermally controlled basin dynamics.