173 Scientific Prospectus
As outlined above, Leg 149 largely succeeded in determining the oceanward and landward bounds of the OCT in the southern Iberia Abyssal Plain. The principal problem now is to investigate the nature of the basement within the OCT itself, to relate it to the general problems of modes of lithospheric extension and rift-to-drift processes, and to test aspects of the models for these tectonic and igneous processes. Leg 173 will attempt to achieve the following objectives:
Determine the role of detachment tectonics in the evolution of the margin. This will be done by drilling through a seismic reflector, interpreted as a major tectonic contact or detachment, on the east side of the high on which Site 900 has already been drilled. This new site (two alternative sites, IBERIA09A/9B have been proposed) will also enable determination of the kinematics of deformation and tectono-metamorphic evolution of deep lithospheric levels during a rifting episode and will help to assess the lateral extent of the Site 900 mafic rocks. Another site (IBERIA07A) will be drilled on the westernmost basement high associated with a westward-dipping normal fault, to test the prediction that simple-shear, as a thinning mechanism of the lithosphere, led to the exposure of low-level continental crust (or even uppermost mantle) at this point of the OCT .
- Sample acoustic basement, principally within the OCT, to characterize the tectonic and magmatic processes that dominate the transition from continental to oceanic crust in space and time (see Sites 901, IBERIA07A, IBERIA08A/8B, IBERIA09A/9B, IBERIA10A).
Determine the role and extent of synrift magmatism in the OCT crust, which is inferred to exist from the new magnetic anomaly chart and other data. Use isotope data to determine the petrogenetic origin and dates of original crystallization and subsequent metamorphism of igneous rocks (see Sites 901, IBERIA07A, IBERIA08A/8B, IBERIA09A/9B) and to characterize synrift underplated material (Site IBERIA09A/9B).
Sample acoustic basement beneath Site 901 or Sites IBERIA08A/8B to confirm predictions of the existence of continental crust, determine the approximate level in the crust from which it came, thereby setting an unequivocal landward limit to the OCT, and to define geometrical relationships between deep and shallow lithospheric levels (see Sites 901, IBERIA08A/8B).
Sample the early-formed oceanic crust, as this would complete the whole transect from continental to oceanic crust. Its presence is inferred only by geophysical observations. Samples from this site (Site IBERIA10A) are expected to provide definitive evidence of the oceanic nature of the crust immediately (20 km) 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 and geochemical information about initial melt production (c.f. Site 900 gabbro) following continental break up at a nonvolcanic margin (see Site IBERIA10A).
To 173 Drilling Strategy
173 Table of Contents