Ocean Drilling Program, United States

Rock samples from Sites 897 and 899 were examined to determine the origin of the early Cretaceous lithologic complex recovered on the top of basement highs in the Iberia Abyssal Plain. Samples studied belong to sedimentary rocks, serpentinized peridotite breccias, metabasites, and mafic rocks. The sedimentary rocks correspond to reworked elements of previously consolidated pelagic and turbidite facies and to unconsolidated pelagic marly sediments. Nannofossil assemblages from the sedimentary rocks indicate a similar early Aptian age for this lithologic complex at the two sites. Ultramafic rocks present in breccias (boulders, clasts, and fragments) vary in composition, and range from peridotite and serpentinized peridotite to altered serpentinite. Chlorite-rich metabasites are minor components in the breccia. Mafic rocks are mainly associated with sediments and occur at both sites. They comprise basaltic lava, undeformed microgabbros, and sheared amphibolite. A polyphase deformation that resulted in cataclastic fabrics affected the serpentinized peridotite breccias. Cataclastic lineation, flow, and foliation structures are present in the breccia fabrics and, consequently, the serpentinized peridotite breccia is considered a tectonic cataclasite. The igneous-sedimentary complex is interpreted as olistostrome(s) that accumulated in a basin-plain setting. Breccia intervals are interpreted as olistoliths or blocks from former cataclastic breccias. The olistostrome, involving sedimentary, ultramafic, and mafic rocks, is thought to be derived from a lower Aptian marginal ridge related to a probable transform fault boundary (the Figueiro Transform Fault), between the Galicia Margin and the Iberia Abyssal Plain at that time. The marginal ridge could have exposed serpentinized peridotites, serpentinized peridotite cataclastic breccias, mafic rocks, and older sediments to the seafloor. Cataclastic breccias and other fault rocks included in the olistostrome are interpreted as mainly originating from wrench tectonics in the transform fault zone, which probably also involved hydrothermal activity and seismicity. The activity of the Figueiro Transform Fault at the Iberia Abyssal Plain/Galicia Margin boundary can be restricted to the time between the development of the transitional crust (about 130 km wide) and early stages of seafloor spreading in the Iberia Abyssal Plain, and the breakup of the Galicia Margin; that is, from about late Hauterivian (129 Ma) to latest Aptian-early Albian (about 110 Ma). Post-rift normal faulting, which probably occurred at any time after the Late Aptian and before the latest Cretaceous or Paleogene, affected the crust at the ocean/continent transition of the Iberia Abyssal Plain and led to fault-bounded basement highs, on top of which the Aptian olistostromes were encountered.