Technical Note 20/4

LEG 173
RETURN TO IBERIA:
Rift-to-Drift Processes Within the Ocean-Continent Transition West of Iberia

Modified from Manuscript by Bob Whitmarsh Based on Proposal 461
Submitted By T.J. Reston, M-O Beslier, G. Boillot, D.S. Sawyer, and R.B. Whitmarsh
Staff Scientist: Paul Wallace
Co-Chief Scientists: Marie-Odile Beslier & R.B. Whitmarsh
Abstract

The Galicia Bank and Iberia Abyssal Plain segments of this margin were drilled during ODP Legs 103 and 149 and have been extensively studied geophysically. Leg 149 determined landward and oceanward limits to the ocean-continent transition (OCT) off western Iberia by drilling an east-west transect of holes. However, only one of these holes penetrated basement in a region that is probably typical of the OCT, at the latitude of the southern Iberia Abyssal Plain. This site, Site 900, cored 56 m of fine- to coarse-grained gabbro that experienced synrift dynamic 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 deep cutting 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. A sequel to Leg 149, Leg 173 will drill a small number of holes to basement on basement highs, mainly within the ocean-continent transition (OCT), to characterize the OCT, to test the simple-shear lithospheric extension hypothesis for the lower-plate (?) margin, to determine the extent of synrift magmatism, and to determine the existence and nature of the first-formed oceanic crust.

To Leg 173 Proposed Site Information

To Leg 174A

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