HR: 09:00h
AN: T51F-05 INVITED    [Abstracts]
TI: The Crustal Section Exhumed by Oceanic Detachment Faults
AU: * Tucholke, B E
EM: btucholke@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Behn, M D
EM: mbehn@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Canales, J
EM: jcanales@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Xu, M
EM: minxu@mit.edu
AF: MIT/WHOI Joint Program in Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
AU: Buck, W R
EM: buck@ldeo.columbia.edu
AF: Division of Marine Geology and Geophysics, Lamont-Doherty Earth Observatory, Palisades, NY 10964, United States
AU: Lin, J
EM: jlin@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AB: Although normal faults are ubiquitous on mid-ocean ridges, they develop larger offsets and thus expose oceanic core complexes at slower-spreading ridges because the average amount of melt accreted at the spreading axis is reduced and tectonic extension consequently increases. The longest-lived (detachment) faults slip for as long as 1-2 m.y. and exhume hundreds of square kilometers of lower ocean crust and upper mantle in the fault footwalls. To maintain isostatic equilibrium, these footwalls roll over and form megamullions that are characterized by domed shapes and by large, enigmatic, fault-surface corrugations (mullions) as well as striations that parallel fault-slip direction. Over the past decade, dozens of megamullions have been identified on mid-ocean ridges spreading at slow to intermediate rates, and these features offer tremendous potential for study of the internal structure, lithologic architecture, and alteration of the oceanic lithosphere. Because detachment faulting suggests extreme tectonic extension, we expect to see little manifestation of magmatism in these tectonic windows. However, recent studies show that at least some megamullions form in association with emplacement of large gabbro bodies and thus with apparently elevated magmatism. Here we present geological and geophysical data, together with numerical modeling results, to show that long-lived detachment faults are likely to form megamullions only when ca. 30-50 percent of total extension is accommodated by magmatic accretion. Under these conditions magmatism may focus unevenly along the spreading axis and create an irregular brittle-plastic transition where detachments are rooted, thus explaining the initiation of large fault corrugations. If magmatism persists, it may continue to focus toward the original injection points, thus tending to orient gabbro bodies parallel to footwall corrugations. In the absence of continuing magmatism, corrugations may still be formed by the cool hanging-wall mold. The morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: 2007 Fall Meeting