HR: 11:05h
AN: T11E-04    [PDF]
TI: Rolling Hinge Formation of S and Overlying Crustal Blocks at the Galicia Rifted Margin
AU: * Sawyer, D S
EM: dale@rice.edu
AF: Rice University, MS-126 6100 Main St., Houston, TX 77005 United States
AU: Pappano, P
EM: pappano@rice.edu
AF: Rice University, MS-126 6100 Main St., Houston, TX 77005 United States
AU: Henning, A T
EM: ahenning@rice.edu
AF: Rice University, MS-126 6100 Main St., Houston, TX 77005 United States
AB: The last stage of continental breakup in the Galicia Bank Basin and Iberia Abyssal Plain appears to have involved "rolling hinge" style extension in forming the S detachment and the allocthonous continental blocks that overly S. We support this assertion using seismic examples from the Iberia Seismic Experiment, conducted in 1997 by U.S., German, Portuguese, and Spanish scientists, using the R/V Ewing's 4 km streamer and deep penetration airgun array. Profile ISE-5 is a north/south strike line across the Galicia Bank basin and over the area where S is well developed and studied. S is clearly seen, but is not the relatively simple, laterally continuous, reflector seen in dip lines. It is offset by one or more north-dipping mantle-penetrating faults and its reflection character changes at several points along the profile. Fault bounded blocks interpreted to consist of continental crust, prerift sediments, and postrift sediments overly S. In one location it appears that S may be directly overlain by postrift sediment. In strike cross-section, the continental blocks have a "pod-like" shape, thick (1-3 km) in the center and pinching out to both sides. The continental crustal section over S varies laterally and consists of a stack of one to three of these overlapping pods. In dip-section these blocks are bounded top and bottom by down-to-the-west normal faults that sole into S. We suggest that this geometry is consistent with margin formation as a rolling hinge detachment. The detachment had two major parts. To the west, the detachment fault dipped at high angle to the west into the upper mantle. The roughly horizontal detachment, imaged as the reflector S, is the continuation of this fault to the east. The upper mantle rocks forming the footwall of the detachment fault were pulled to the east, out from under the hanging wall along the high angle portion of the fault. They were then flexed and rotated so that the detachment became roughly horizontal. As substantial extension was accommodated by this system, blocks of continental crust and sediment were periodically transferred from the hanging wall to the footwall. They were then carried, as on a conveyor belt, to the east. In this system, a given area of the hanging wall might be covered by one or several allocthonous blocks, or might be left entirely uncovered. The later would be a zone of exhumed continental mantle. We believe that the pod-like blocks we image along profile ISE-5 are produced in this way. They are in a sense, "flakes" of continental crust plucked from the hanging wall. The reflection character of S varies along strike because different portions of the fault have moved at different times and are covered by different hanging wall material. The continuous segments of S, like the allocthonous crustal blocks, have greater dip extent than strike extent. The relative rates of extension and of plucking crustal blocks from the hanging wall controls the thickness of the crust over the detachment.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8010 Fractures and faults
DE: 8105 Continental margins and sedimentary basins
SC: Tectonophysics [T]
MN: 2003 Fall Meeting