Tectonic implications of exposure of lower continental crust beneath the Iberia abyssal plain, Northeast Atlantic Ocean; geophysical evidence

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doi: 10.1029/2000TC900016
Author(s): Whitmarsh, R. B.; Dean, S. M.; Minshull, T. A.; Tompkins, M.
Author Affiliation(s): Primary:
University of Southampton, Southampton Oceanography Centre, Southampton, United Kingdom
University of Wisconsin at Madison, United States
Volume Title: Tectonics
Source: Tectonics, 19(5), p.919-942. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0278-7407 CODEN: TCTNDM
Note: In English. 108 refs.; illus., incl. sects., 3 tables, sketch maps
Summary: We present new seismic data from a basement high beneath the southern Iberia Abyssal Plain that appears, from the latest Ocean Drilling Program (ODP) results and indirectly from various seismic observations, to consist of lower continental crustal material. This is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks. We infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting. Our results lead to the following new observations: low-angle detachment faults, previously reported along the east-west Lusigal-12 profile across the southern Iberia Abyssal Plain, penetrate both the 3-10 km thick continental crust and, unusually, the uppermost mantle. Processing of new multichannel seismic reflection profiles across the basement high on which ODP Sites 900, 1067, and 1068 are located reveals that the high is bounded by a previously unsuspected steep, landward dipping normal fault on its east flank. Basement cores from the above ODP sites also reveal that the high is not capped by the previously predicted early syn-rift sediment but by rocks from the lower continental crust. These unexpected observations are incorporated into a new tectonic model of the development of this part of the west Iberia margin which is also consistent with other geophysical observations. A novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3-6 km by a currently poorly understood process. The model implies three stages of deformation: (1) lithospheric extension, principally by symmetric pure shear, which leads to a ∼10 km thick crust in which the lower crust is largely absent over the axial zone and the crust-mantle boundary forms a shear zone, (2) further thinning and then dissection of the most distal continental crust by seaward dipping, low-angle normal faults, (3) inception of a high-angle, landward dipping normal fault that offsets the tectonized crust-mantle boundary and uplifts the lower crust to the crest of the basement high on which ODP Sites 900, 1067, and 1068 were drilled. Copyright 2000 by the American Geophysical Union.
Year of Publication: 2000
Research Program: ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Atlantic Ocean; Boundary conditions; Continental crust; Crust; Geophysical methods; Geophysical profiles; Geophysical surveys; Iberian abyssal plain; Leg 149; Leg 173; Lower crust; Mantle; North Atlantic; Northeast Atlantic; ODP Site 1067; ODP Site 1068; ODP Site 900; Ocean Drilling Program; Reflection methods; Rift zones; Seismic methods; Seismic profiles; Surveys; Tectonics; Thickness; Upper mantle
Coordinates: N444000 N445000 W0113000 W0115000
Record ID: 2001008807
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.