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Leg 103

The Galicia Margin

Galicia Margin is a starved passive ocean margin with a thin sediment above acoustic basement. Rift structures control the present-day morphology. The continental basement is broken by normal, possibly listric, faults into narrow, elongate tilted blocks trending north and dipping gently east, forming a series of half grabens. On the western, uplifted side of some blocks, basement and possibly pre-rift sedimentary rocks crop out. Leg 103's primary objective was an investigation of rifting, subsidence, and sedimentation on this margin and its relationship to initiation and progressive opening of the adjacent North Atlantic, and of the transition between continental and oceanic crust and evolution of the more thickly sedimented conjugate margin of North America. Five sites (Site 637 to Site 641) were cored during Leg 103.

At three sites (Sites 638, 639, and 641), located on the deep western edge of the margin, coring was conducted through the "break-up unconformity" and then 1200 m of Lower Cretaceous (Albian-Valanginian) to Upper Jurassic (Tithonian) rocks to the top of basement. The occurrence of Lower Cretaceous turbidites beneath the seismic reflector, previously interpreted as being the top of the possible pre-rift carbonate platform, extends the syn-rift period on Galicia Margin back to at least the Valanginian. Tithonian limestones, resting either directly on basement or on a thin basal conglomerate overlying the basement, implies that this region remained a structural high until late in the Jurassic.

Drilling near the boundary between oceanic and continental crust resulted in the penetration of 74 m of upper mantle peridotite (Site 637). This peridotite could be part of a Hercynian ophiolitic slab included in a tilted block of the continental crust or, more likely, it was emplaced during late rifting of the margin as a result of the stretching of the lithosphere.

A deep and continuous seismic reflector, S, which marks the top of acoustic basement on the outermost part of the margin, had previously been interpreted as the brittle/ductile transition in continental crust where listric normal faults merge at depth. The discovery of Lower Cretaceous syn-rift turbidites in the acoustically incoherent unit above reflector S indicates that the reflector actually marks a level near the original depositional contact between the syn-rift sediments and the underlying carbonate platform and/or the crystalline basement rock, now transformed into a detachment-fault surface.

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