173 Scientific Prospectus



This site lies between Sites 900 and 899 where coring reached basement during Leg 149, on the crest of a 7-km-wide north-south basement high crossed by three E-W lines and one N-S line. The high appears to be a fault block bounded to the west by an oceanward-dipping normal fault and underlain by a sub-horizontal reflector (detachment fault?), 1.5 s below the crest of the high, that intersects the top of basement south of the site. The basement cores will serve to discriminate among the variety of models postulated for the development of the OCT.


The N-S crossing of this site, plus a further E-W crossing of the broad high on which it lies, reveal that the high has a trend somewhat west of north and shoals quite rapidly northward; the high could conceivably represent the southward continuation of the Vasco da Gama Seamount south of the Iberia Abyssal Plain. The objective of the site is to determine the westward edge of continental crust and to establish the petro-structural evolution of the continental crust and the spatial relationships between upper lithospheric levels (continental crust) and deeper levels (gabbros of Site 900).


This site lies on a tilted fault block of probable continental crust that is capped by tilted transparent syn- or prerift sediments. The plan is to drill to the top of the transparent layer and then to core down to the underlying basement. The objectives of the site are the same as those at Site IBERIA08A.


The seismic data indicate that the high on which this site is situated has an elliptical plan view slightly elongated in a N-S direction. The principal objective of the site is to drill down through possible upper crust or early synrift or prerift sediments to a strong extensive intrabasement reflector (H), interpreted to be an extensional synrift tectonic feature (detachment fault?). The H reflector outcrops a short distance to the west of the sites in the vicinity of Site 900. We expect the gabbro cored there to underlie H. Two other nearby seismic lines that also image H are currently being pre-stack depth migrated.


This site lies over an elongated margin-parallel basement ridge that lies about 15 km east of the crest of the J magnetic anomaly and 20 km west of the peridotite ridge. Here, attempts to model the magnetic anomalies by seafloor spreading predict the presence of ca. 130 Ma oceanic crust. The objective of the site is to sample the early-formed oceanic crust. Samples from this site are expected to provide definitive evidence of the oceanic nature of the crust immediately west of the peridotite ridge and may enable the seafloor-spreading model to be verified. They will also yield the, possibly unusual, chemistry of the thin crust formed by the earliest magma-starved seafloor spreading and provide valuable petrological information about initial melt production following continental break up at a nonvolcanic margin.

Site 901

This site, already drilled and cored during Leg 149, lies over a tilted fault block capped by a layer of tilted transparent shallow-water sediments of Late Jurassic (Tithonian) age. It exhibits an almost identical geometry on seismic profiles to Site IBERIA08B and the scientific objectives are the same. Because Site IBERIA08B lies west of the major, deeply penetrating north-northeast fault bounding Site 901 this latter site has a lower priority.

The following priorities have been assigned to the sites.

Priority 1

Site IBERIA07A, the most oceanward site within the OCT, will sample basement on a topographic high, which is apparently bounded to the west by a normal fault and underlain by a strong sub-horizontal reflector. This will test the simple-shear extension model for the upper lithosphere and may even reveal an upper mantle exposure. Sites IBERIA09A/9B (either 9A or 9B, but not both, will be drilled) will transect the H seismic reflector, and thereby test the simple-shear lithospheric extension model, and assess the lateral extent of the possibly synrift gabbro basement rocks sampled at Site 900. Sites IBERIA08A/IBERIA08B/901 (one site only) will be drilled to confirm the continental nature of the basement and its approximate original level in the crust, to thereby limit the landward edge of the OCT. In addition, all of the above sites will potentially provide evidence to assess the contribution and lateral extent of synrift magmatism.

Priority 2

Site IBERIA10A will be drilled to demonstrate the existence, age, and chemical nature of the early-formed oceanic crust 20 km oceanward of the peridotite ridge.

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