Comparison of Continental Margins of Eastern North America at Cape Hatteras and Northwestern Africa at Cap Blanc

Abstract

The opposing continental margins of Cape Hatteras and Cap Blanc appear broadly symmetric with respect to (1) distribution of physiographic provinces, with the exception of the lower continental rise hills which have not been observed off Cap Blanc, (2) early and middle Paleozoic, Mesozoic, and Cenozoic stratigraphic frameworks, (3) late Precambrian, Paleozoic, and Mesozoic tectonic frameworks, with certain qualifications, and (4) certain offshore residual magnetic anomalies, including a positive anomaly associated with the continental shelf edge and the boundary between smooth and rough magnetic fields. The third point must be qualified because (a) the absence of late Paleozoic (post-Devonian) strata in the Mauritanides fold belt of northwest Africa, as opposed to their i volvement in the Appalachian fold belt of eastern North America, limits resolution of the respective orogenic movements and (b) the existence of a system of Triassic fault-block basins parallel with the northwest African continental margin, corresponding to the system in eastern North America, has not been well documented. The opposing continental margins appear broadly asymmetric with respect to Cenozoic tectonic frameworks. Sedimentary strata underlying the coastal plain, continental terrace, and continental rise near Cape Hatteras are predominantly undeformed, except by superficial gravitational displacement processes acting above Horizon A (Late Cretaceous-Eocene). Sedimentary strata underlying the corresponding physiographic provinces off Cap Blanc are deformed by deep structural processes including (1) compressional folding related to Alpine diastrophism, (2) tensional faulting along west-northwest fracture trends, coincident with eastward projections of fracture zones which cross the Mid-Atlantic Ridge, and along inferred north-northeast fracture trends which regionally parallel the continental m rgin, (3) volcanism, at least of Oligocene to Holocene age, concentrated along the fracture zones, and (4) diapirism, probably produced by rock salt deposits beneath the continental terrace and continental rise correlative with Late Triassic and Jurassic evaporites in the northwest African coastal basins. Mesozoic and Cenozoic mean rates of subsidence and sequences of gross lithology generally correlate between the opposing continental margins. Similarities in the stratigraphic records of the opposing continental margins and the adjacent ocean basin indicate that the continental margins have behaved as if vertically, as well as horizontally, coupled to the ocean basin. Mesozoic and Cenozoic mean subsidence rates of the opposing continental margins (1-9 cm/1,000 year) are about 10-3 of inferred mean spreading rates of the intervening sea floor (1-4 cm/year). The mean subsidence rates vary in unison with the independently inferred mean spreading rates, to a first approximation. The limited data suggest that the epeirogenic subsidence of the opposing continental margins and the inferred spreading of the intervening sea floor are related genetically. The apparent symmetry in space and time of tensional rifting, orogenic compression, and epeirogenic subsidence on the opposing continental margins is consistent with a hypothesis of sea-floor spreading. These findings imply that geologic conditions relevant to the occurrence of petroleum can be predicted between the opposing continental margins.