Structure of thinned continental crust across the Orphan Basin from a dense wide-angle seismic profile and gravity data

Abstract

We present a 500-km long, 2-D P-wave velocity model across the Orphan Basin, offshore NE Newfoundland, Canada, from Flemish Cap to the Bonavista Platform, formed using refraction and wide-angle reflection data from 89 ocean-bottom seismometers. This layered model builds on a recent traveltime tomography result using additional constraints from coincident multichannel seismic reflection and gravity data plus borehole logs from three wells. The model shows (i) post-rift Tertiary (velocities ∼1.7–3.5 km s−1) and (ii) both post-rift and syn-rift, Cretaceous and Jurassic sediments (∼4–5 km s−1), deposited within an eastern and a western sub-basin that are separated by a major basement block. The existence of Jurassic sediments indicates a pre-Cretaceous rifting phase in the eastern sub-basin, and possibly in the western sub-basin. However, there is no evidence that Triassic sediments are widespread across the Orphan Basin. Two upper crustal sublayers and one lower crustal layer are defined by differences in velocities (5.4–6.1, 6.1–6.5 and 6.3–7.1 km s−1, respectively) and vertical velocity gradients (mean = 0.14, 0.10 and 0.05 s−1, respectively). Crustal thinning is asymmetric across the Orphan Basin. Within the eastern sub-basin, continental crust beneath Flemish Cap (∼32 km thick; β ∼ 1.1) thins westward into a 35-km-wide zone of hyperextended crust (<10 km thick; β > 3.4) beneath an 11-km-deep sedimentary basin. Within the western sub-basin, the Bonavista Platform crust (∼32 km thick) thins eastward into a 116-km-wide zone of hyperextended crust. Two zones of thicker crust (β = 2–3.5) exist within the central section, with muted topography within the eastern part and large basement highs in the western part, separated by the eastward dipping White Sail Fault (WSF). The zone to the east of the WSF displays higher velocities in the lower crust than to the west. This can only be explained by a lateral ductile flow across the zone boundary. By combining the two upper crustal sublayers into one, we define the full crustal thicknesses of the upper and lower crusts as 12 and 22 km, respectively. The extension and thinning factors of these two layers are calculated across the basin. Discrepancies between upper crustal thinning and lower crustal thinning are common but only produce a small mass deficit (∼7 per cent or 1.5 km) in the lower crust. Structural connections are shown between the Rockall Trough and the West Orphan Basin and between the Porcupine Basin and the East Orphan Basin in that a wider hyperextended western basin is paired with a narrower eastern basin by a middle zone of thicker crust. In contrast to the Rockall Trough and the Porcupine Basin, serpentinized mantle is not observed in the East Orphan Basin where hyperextended crust is observed (βmax ∼ 8.5). One possible cause is that the restricted size of the basin and its location adjacent to Flemish Cap may have permitted a heavier supply of sediment to cover the basement early during its extension. Such a cover would inhibit the flow of water into the crust and thus leave the mantle unchanged.