Leg 173--Return to Iberia

Co-Chiefs: Bob Whitmarsh and Marie-Odile Beslier	Cruise Dates: 21 April-16 June, 1997
Staff Scientist: Paul Wallace	Operations Superintendent: Michael Storms

The Galicia Bank and Iberia Abyssal Plain segments of the west Iberia margin were drilled during Legs 103 and 149 and have been extensively studied geophysically. Leg 149 determined landward and oceanward limits to the ocean-continent transition (OCT) of the crust off western Iberia by drilling an east-west transect of holes. However, only one of these holes penetrated basement between these limits. This site, Site 900, cored 56 m of fine- to coarse-grained gabbro that had experienced synrift dynamic re-crystallization under granulite facies conditions at 136.4 ± 0.3 Ma, according to ⁴⁰Ar/³⁹Ar dating. Geophysical data clearly show that the OCT has magnetic and seismic velocity properties that are in some sense transitional between continental and oceanic crust. Multichannel seismic reflection profiles, one of which has been recently reprocessed, strongly indicate that, although the eastern (landward) part of the OCT is dissected by deeply penetrating normal faults and low-angle detachments, these die out westward (oceanward) into a region of smoother basement that lacks significant intrabasement reflectors and is of uncertain origin. Leg 173 will drill a small number of holes to basement, on basement highs mainly within the OCT, to complete the Leg 149 transect. These holes will characterize the OCT, test models of lithospheric (crustal) extension, determine the extent of synrift magmatism, and examine the nature of the oldest oceanic crust.

In the Iberian Abyssal Plain Leg 149 defined landward and oceanward limits to the crustal OCT, where the transition zone is defined as the region between a margin-parallel peridotite ridge marking the landward edge of ocean crust and the most seaward tilted fault block of continental crust. However, only one hole (Hole 900A) penetrated basement in the 130-km-wide region between these limits. Leg 173 is a sequel to Leg 149, and it will enable (1) drilling and coring of a well-imaged major detachment fault (an analogue to the S-reflector), (2) recovery of more rift-related igneous material (e.g., gabbro) and its host rock (mantle), continental crust or slow spreading oceanic crust, (3) testing of the nature of the topographic high between Site 900 and the most landward known serpentinite basement outcrop, and, finally, (4) sampling of the oldest oceanic crust. These observations, together with the improved quality and quantity of seismic images, will allow the addressing of the modes of breakup of the lithosphere, the timing and nature of melt generation from the mantle during the final stages of rifting and, the nature and age of early-formed "normal" oceanic crust. The planned drilling will also add to knowledge of the early sedimentary history of the rifted margin.

Objectives

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.
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 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 will be drilled on the western-most 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.
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 and to characterize synrift underplated material.
Sample acoustic basement beneath Site 901 or Sites IBERIA 08A/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 define geometrical relationships between deep and shallow lithospheric levels.
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 non-volcanic margin.