Regional Geology of the Goban Spur Continental Margin

Abstract

All available geophysical data have been integrated with the Leg 80 drilling results to produce a regional geological synthesis of the Goban Spur passive continental margin. The thin sediment sequence overlying basement is divided into three seismostratigraphic sequences. The lowest of these, sequence 3, occurs in fault-bounded basins, often wedgeshaped in cross-section. Intraformational reflector geometry shows that faulting and tilting of the underlying basement occurred simultaneously with the deposition of formation 3. The upper two sequences are draped over the basement topography and postdate any large-scale fault movement. The early history of the margin, the rift phase, was characterized by the formation of an "en-échelon" series of margin-parallel normal faults, most of which downthrow to the west, toward the incipient ocean. No evidence of fractures perpendicular to the margin has been found. 3 Estimates of upper-crustal thinning, derived from the geometry of fault blocks, are in the range 1.03 to 2.20. They show no relationship with whole-crustal extension values derived from gravity modeling, indicating that the uniform extension model of passive-margin formation cannot be applied to Goban Spur. Evidence from one drill site (549) indicates that the rift phase had a duration of about 15 m.y. However, the age of the rifting-to-spreading transition derived from Site 549 is between 2 and 4 m.y. greater than the age derived from Site 550, drilled on the ocean crust. A possible explanation is that extension within the upper continental crust was taken up by dyke injection, particularly within the rift axis, during the later stages of rifting. Syn-rift volcanic activity, identified from seismic profiles and magnetic-anomaly modeling, may be evidence for this phenomenon. At the end of rifting, a wide zone of fault-bounded basement highs and sedimentary basins, with a relief of ± 1 km relative to sea level, existed east of Pendragon Escarpment. West of the escarpment lay a narrow trough, with water depths up to 2 km. Ocean crust began to accrete in this deep-water area in the middle Albian. The post-Albian structural history of the margin has been dominated by thermal subsidence.

Biostratigraphical correlation of the Campanian–Maastrichtian boundary: Lägerdorf–Hemmoor (northwestern Germany), DSDP Sites 548A, 549 and 551 (eastern North Atlantic) with palaeobiogeographical and palaeoceanographical implications

“…The isotherms were constructed assuming a boundary horizon between the warmer, near-surface water an colder, deep water at 200-500 m depth (Schonfeld, Sirocko & J0rgensen, 1991). The palaeobathyrnetry after Graciansky & Bourbon (1985) was corrected after Masson, Montadert & Scrutton, (1985). The palaeotemperatures are expressed in degrees Celsius.…”

Thinning of the Goban Spur continental margin and formation of early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies

“…Adjacent to Goban Spur, the oldest recognized magnetic anomaly is anomaly 34 ( Masson et al 1985), located about 115 km west of the ocean-continent transition. The oceanic crust located between anomaly 34 and the margin was created during the long Cretaceous period of normal geomagnetic polarity, at approximately its present latitude (Dyment & Arkani-Hamed 1994).…”

“…It crosses the ocean-continent transition in the vicinity of the westernmost topographic feature of the continental margin (Fig. l), as revealed by gravity modelling and seismic data displaying characteristic oceanic basement hyperbolae west of this area ( Masson, Montadert & Scrutton 1985). A narrow, high-amplitude ( 500 nT peak-to-peak) magnetic anomaly is observed between *Now at: Laboratoire de Mesures en Forage (ODP), IMT-Technopole de Chateau-Gombert, 13451 Marseille Cedex 20, France.…”

Early Eocene palaeoceanography and palaeoclimatology of the eastern North Atlantic: stable isotope results for DSDP Hole 550

“…Palaeodepth estimates, based on backtracking from an empirical subsidence curve for the North Atlantic (Tucholke & Vogt 1979), suggest that during the early Eocene water depths at Site 550 were 3800--4000 m (see also Masson et al 1985). This indicates that Site 550 is one of the deepest recovered across the early Eocene, worldwide, and offers the opportunity to trace more directly deep water circulation patterns, in contrast to most earlier studies that have been limited to intermediate and upper deep water modes of circulation (e.g.…”