Leg 173 Scientific Report


ODP Leg 173 was devoted to investigating the nature and evolution of the basement within the ocean-continent transition (OCT) zone of the west Iberia continental margin to understand modes of lithospheric extension and to test models of the tectonic and igneous processes that accompanied this extension. Five relatively deep holes (terminating between 630 and 960 mbsf) were drilled into basement highs along an east-west transect begun during Leg 149. The degree to which the six major scientific objectives of this leg were met is briefly outlined below.

Objective 1. To sample acoustic basement, principally within the OCT, to characterize the tectonic and magmatic processes that dominate the transition from continental rifting to oceanic crustal formation in space and time.
Acoustic basement was sampled at all five sites drilled during the leg; however, at only three of these sites (Sites 1067, 1068, and 1070) were igneous/metamorphic basement rocks reached. Nevertheless, the acoustic basement samples from all the sites contribute in some way to the characterization of those tectonic and magmatic processes that dominate the transition from continental to oceanic crust in space and time (see below). For example, samples of igneous basement from Sites 1067, 1068, and 1070 will provide important geochronologic, geochemical, petrologic, and structural constraints about the age, origin, and tectonometamorphic evolution of the OCT. Fault gouge and breccias encountered at Sites 1068 and 1070 will help to characterize details of the synrift tectonic activity. Mineralogical and structural variations in the serpentinized peridotites (Sites 1068, 1070) will be used to document the nature and evolution (e.g., partial melting, mode of extension) of the mantle during the transition from continental rifting to incipient oceanic crustal formation. Moreover, the occurrence of mantle rocks at this latitude of the OCT demonstrates that the mantle is exposed, at least sporadically, over a width of more than 100 km.

Objective 2. To constrain models of continental thinning and breakup by drilling (1) through a major synrift tectonic contact (detachment) on the east side of the basement high on which Site 900 has already been drilled (Iberia-9) and (2) a basement high 14 km further west bounded by westward-dipping normal faults (Iberia-7A).
The results from Sites 1067, 1068 (both Iberia-9), and 1069 (Iberia-7A) will have implications for tectonic activity on a broad scale, particularly regarding hypotheses that involve simple shear as a stretching mechanism of the lithosphere. The intense ductile shear deformation in the upper part of the cored sections of the amphibolite (Site 1067) suggests the existence of a major shear zone in rocks that are now at the top of the basement. The hypothesis that the upper H reflector is a major low-angle fault is not enhanced by the results from Sites 1067 and 1068; the reflector may in fact represent the crust/mantle boundary. It proved impossible for logistic and technical reasons to reach the H reflector under Site 1067; hence, Site 1068 was drilled in an offset location to sample the rocks (serpentinized peridotite) beneath the reflector. The presence of upper continental crust under Site 1069 was established fairly conclusively. Thus, any tectonic hypothesis for development of the OCT must also now take into account, in addition to the presence of mantle rocks at Site 1068, the likelihood that Site 1069 lies over an isolated fault block of continental crust.

Objective 3. To determine the role and extent of synrift magmatism in the OCT.
Evidence of possible synrift melting within the OCT was acquired at Sites 1067 and 1068. At Site 1067, amphibolites had been intruded by tonalite gneiss and meta-anorthosite and appear to be part of a ~550-m-thick sequence overlying the upper H reflector, which may represent the crust/mantle boundary. Clasts of similar material, together with metagabbro, were cored at Site 1068 overlying serpentinized peridotite. These rocks display a heterogeneous and locally intense ductile deformation that suggests they may have played an important role in localizing deformation during the formation of the margin.

Objective 4. To sample acoustic basement beneath Site 901, or at a site 20 km farther west (Iberia-8B), to confirm the existence of continental crust and to determine the approximate crustal level from which it came.
Site 1065 (Iberia-8B) attempted to sample continental crust on a major tilted fault block. Unfortunately acoustic basement was discovered to consist of lithified claystones that could be cored only very slowly and no clear crystalline basement reflector could be discerned beneath them. Nevertheless the earliest sediments indicate shallow-water deposition in the Middle-Late Jurassic, which is a strong argument for there being continental crust beneath the site.

Objective 5. To sample early-formed oceanic crust, 20 km west of the peridotite ridge that marks the landward limit of oceanic crust.
Site 1070 (Iberia-10A) sampled serpentinized peridotite intruded by several thin (1 to 4 cm wide) gabbroic veins and a 4-m-thick intrusion of pegmatitic gabbro. These cores may represent early formed oceanic seafloor. However, no evidence of upper oceanic crust, either lavas or sheeted dikes, was found. It may be that the thermal state of the lithosphere at this time was too cold to supply enough partial melting to generate typical upper oceanic crust. The oldest microfossils above basement indicate a late Aptian (115 Ma) age somewhat younger than the age (125 Ma) predicted by modeling seafloor-spreading anomalies.

Objective 6. To investigate the early sedimentary history of the rifted margin.
Mesozoic sediments were cored in three out of the four holes drilled within the OCT zone (Sites 1065, 1068, and 1069). These sediments provide glimpses of the early history of the west Iberia margin. For example, at Site 1065 Middle-Late Jurassic basal sediments are pre-rift while the Early to Late Cretaceous sediments of Sites 1068 and 1069 record events on the margin during or immediately after breakup. Many basement highs are capped by breccias that directly or indirectly reflect the tectonic events that accompanied the late stages of rifting.

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