173 Scientific Prospectus


Figure 1. Bathymetric chart of the west Iberia margin; contours at 200, 500, 1000, 1500 m, etc. Existing DSDP/ODP sites are shown by black dots. Inset is an expanded plot of the boxed area at 40° 40'N showing old (circles) and proposed (triangles) drill sites and tracks of seismic reflection profiles used to create the composite cross-section in Figure 2.

Figure 2. Composite seismic time section created from the track segments of, from west to east, the Sonne, JOIDES Resolution, and Lusigal profiles shown in Figure 1. The irregular surface is the top of the acoustic basement. Basement was sampled at Sites 897, 899, and 900 during Leg 149. The nature of basement (oceanic, transitional, extended continental) is indicated; at Site 901 it is inferred. Proposed Sites IBERIA07A, IBERIA08A/B, IBERIA09A/B, and IBERIA10A are also indicated. IBERIA07A lies slightly to the north of this profile.

Figure 3. Part of a new reduced-to-the-pole magnetic anomaly chart of the whole west Iberia margin produced in collaboration with the Atlantic Geoscience Centre, Dartmouth, Nova Scotia, Canada (Miles et al., 1996). The chart was made from a 5-km-gridded data set and is contoured at 25 nT. The main chart is based on over 400,000 sea-surface observations which were corrected to remove the effects of secular variation, high geomagnetic activity, spurious tracks, and systematic cross-over errors. Greater confidence in the quality of the resulting dataset allowed the use of the small contour interval. Leg 149 drilled to basement in this area (white dots); proposed sites are shown by squares. The data were reduced to the pole to clarify many features. Major linear trends in the anomalies are picked out by bold lines. The chart clearly shows the strong positive J anomaly which appears just west of 13° W and south of 41° 30'N. Between Site 897 and the continental shelf (~9° 15'W) and north of 41° N other less strong positive anomalies are associated with the shallow regions of Galicia Bank. South of ~41° N the chart can be divided into three distinct zones (bounded by the two broad N-S black bands) based on the character of the anomalies.

Figure 4. East-west migrated multichannel seismic reflection profile Lusigal-12 through Sites IBERIA08A, IBERIA09A/B, 900, and 901 (Fig. 1). The lower profile is an interpretation of basement reflections seen in the upper profile. Synrift I sediments are marked by close diagonal ruling; synrift II sediments are marked by coarse ruling. H is a detachment fault controlling extension during synrift I phase; L is a listric fault active during the synrift II phase. Pre-rift units are probably present in fault block FB, but are not identified. The magnetic anomaly profile (top) was computed from the data set used to produce the magnetic anomaly chart in Figure 3.

Figure 5. Cartoon incorporating available drill core and geophysical evidence into a single integrated model for the evolution of the southern Iberia Abyssal Plain margin (Krawczyk et al., 1996). As high level extension was accommodated along a detachment fault, upwelling of the asthenosphere in response to "pure shear'' extension at depth caused melting and intrusion of lower crust into the transition zone. Intrusion probably continued during subsequent block-faulting which dissected the lower plate to the detachment fault. Drilling during Leg 173 will test this model by investigating the nature of crust in the transition zone and testing the detachment hypothesis.

Figure 6. Sketches a to d describe the progressive stretching of a four-layer brittle-ductile continental lithosphere leading to mantle exhumation at a rifted margin (Brun and Beslier, 1996). The lowermost sketch is a schematic interpretative geologic cross-section of the Iberia Abyssal Plain based on the model in the rest of the figure.

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