DuBois
David L.
DuBois
David L.
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ArticleHeterogeneous seismic velocity structure of the upper lithosphere at Kane oceanic core complex, Mid-Atlantic Ridge(American Geophysical Union, 2009-10-10) Xu, Min ; Canales, J. Pablo ; Tucholke, Brian E. ; DuBois, David L.The Kane oceanic core complex (OCC) is a large, corrugated megamullion that was formed by a long-lived detachment fault at the axis of the Mid-Atlantic Ridge adjacent to Kane Fracture Zone between 2.1 and 3.3 Ma. We use refracted arrivals recorded along a 6-km-long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the OCC. Results are presented in high-resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion. The models show large lateral variability in P wave velocity within the upper ∼0.5–2.0 km of the lithosphere, and these variations correlate to first order with observed variations in lithology, documented by in situ basement samples and seafloor morphology. Lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions, serpentinized peridotites, and basalts at scales of a few kilometers to ∼10 km. Serpentinized peridotites appear to dominate the central and older parts of the OCC. High-velocity gabbros are consistently (but not exclusively) present closer to the termination of the Kane detachment fault and toward the ends of the OCC. The structure of the lithosphere exhumed by the Kane detachment fault is far from the standard ophiolite-based Penrose model, and it does not show segment-centered magmatism that is commonly interpreted at slow spreading ridges. If the gabbros exhumed toward the termination of the OCC were emplaced deep (∼10 km) beneath the spreading axis, they may have constituted a weak zone that focused initiation of the Kane detachment fault. Alternately, as the OCC footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply, changes in mantle fertility, or decompression melting. Late stage volcanism is clearly associated with a major high-angle normal fault that cuts the detachment surface; this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall. The shape of the large-scale corrugated morphology of the OCC is nearly invariant in the dip direction across major changes in basement lithology, indicating that once established, the form of the Kane detachment fault was highly resistant to modification.
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Working PaperTranscription of 9-track tapes to CD-ROM(Woods Hole Oceanographic Institution, 1999-12) Bolmer, S. Thompson ; DuBois, David L. ; Hoskins, Hartley ; Sass, Warren J.The WHO! Marine Seismic community had about two thousand 9-track magnetic tapes in storage. The data stored were from marine experiments dating from the 1970's to 1990 and from computer models. The experiments included observations made with seafloor instruments and sondes in boreholes. These tapes were created on Digital Equipment Corporation (DEC) V AXNMS and other older computer operating systems. The tapes needed to be read and put into a format that contemporary UNIX operating systems could read. This report documents the logistics of transcribing this information onto CD-ROM. A search was made to find the best way to read and rewrite these 9-track tapes to new media that would have a longer shelflife and be readable on today's UNIX variants. As a result of this search, an outside company was hired to do these conversions. The tapes that were readable from this first group of 381 tapes selected for transcription are now on twenty-five CDROMs.
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ArticleSeismic evidence for large-scale compositional heterogeneity of oceanic core complexes(American Geophysical Union, 2008-08-06) Canales, J. Pablo ; Tucholke, Brian E. ; Xu, Min ; Collins, John A. ; DuBois, David L.Long-lived detachment faults at mid-ocean ridges exhume deep-seated rocks to form oceanic core complexes (OCCs). Using large-offset (6 km) multichannel seismic data, we have derived two-dimensional seismic tomography models for three of the best developed OCCs on the Mid-Atlantic Ridge. Our results show that large lateral variations in P wave velocity occur within the upper ~0.5–1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology, and we use these correlations to show that gabbros are heterogeneously distributed as large (tens to >100 km2) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along-isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults. This suggests enhanced magmatism during the late stage of OCC formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults, and it therefore may explain eventual abandonment of detachments that form OCCs. Detailed seismic studies of the kind described here, when constrained by seafloor morphology and geological samples, can distinguish between major lithological units such as volcanics, gabbros, and serpentinized peridotites at lateral scales of a few kilometers. Thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced.