HR: 1340h
AN: T23B-1415    [Abstracts]
TI: Faults of subaxial origin exposed in the Hess and Pito Deep Rift walls: implications for hydrothermal systems and seismicity of axial regions.
AU: * Hayman, N W
EM: hayman@ig.utexas.edu
AF: Institute for Geophysics, University of Texas, J.J. Pickle Research Campus,10100 Burnet Rd., Bldg. 196, Austin, TX 78757, United States
AU: Karson, J A
EM: jakarson@syr.edu
AF: Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244-1070, United States
AB: The rift walls adjacent to Hess and Pito Deep Rifts (HDR and PDR) provide tectonic windows into ocean crust that was accreted and spread from the EPR ~1 mya at >110 mm/yr (HDR), and ~3 mya at >140 Ma/yr (PDR). Faults are exposed in each area that: strike parallel to the ridge axis and abyssal hill lineaments, bound tilted dikes and lavas, are cut by dikes, have growth-relationships with lavas, and are rich in hydrothermal minerals. Thus, the faults developed within the intrusive and eruptive center of the EPR in the presence of high- temperature fluids. The faults contain cataclastic units that have bulk-rock variations above igneous compositions of up to 6 wt% MgO and 20 wt% SiO₂. Mechanically comminuted gouge preserves relatively unaltered compositions, likely having sealed the faults to hydrothermal fluid flow. Differences between the two localities abound. In the HDR exposures, >10 m wide damage zones rich in <1 cm-wide chlorite and actinolite veins surround rare, <1 m-wide faults. In contrast, the PDR exposures contain narrow damage zones localized around multiple splays of well-defined fault planes and multiple bands of cataclastic and gouge material, all with mutually crosscutting relationships with networks of >1 cm-wide quartz- (and locally epidote-) rich veins. The multiple increments of deformation, fault slip, and hydrothermal fluid flow occurred within the subaxial center, before possible reactivation by abyssal hill faults. This environment is largely aseismic in the modern EPR. The patterns of deformation and alteration of oceanic crust therefore require a model of faulting that depends less on the strain-weakening evolution intrinsic to seismogenic faults, and more on the coupling of hydrothermal processes and distributed cracking.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 8000 STRUCTURAL GEOLOGY
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting