Paper No. 2
Presentation Time: 9:15 AM


BROWNE, Cassandra, University of Texas at Austin, Institute for Geophysics, J.J. Pickle Research Campus, Bldg. 196, 10100 Burnet Rd. (R2200), Austin, TX 78758, HAYMAN, Nicholas W., Institute for Geophysics, University of Texas, 10100 Burnet Rd, Bldg 196, Austin, TX 78758, MARONE, Chris, Geosciences, Penn State University, 503 Deike Building, University Park, PA 16802 and KAPROTH, Bryan M., Dept. of Geological Sciences, Penn State University, State College, 16802,

Fault and fracture systems along Mid-Ocean Ridges act as conduits for hydrothermal fluid flow and accommodate subaxial subsidence and tectonic extension. Useful data bearing on ocean crustal faulting come from seafloor images and samples of East Pacific Rise (EPR)-spread crust exposed in the “tectonic windows” Hess and Pito Deeps. Additional data stem from mapping and sampling of the Troodos Ophiolite in Cyprus and Miocene basalt sections exposed in Iceland.

Microstructural analyses of basaltic fault rocks, and the particle size distribution (PSD) of these materials, provide important insight into ocean crustal fault mechanics. In most tectonic faults PSD tends to approximately follow a power-law, reflecting incremental fracturing of grains within shear zones. In contrast, ocean crustal faults tend to have more complex (Weibull Distribution) PSD, best visualized with lognormal plots wherein the PSD approximately follows two lognormal distributions separated by a point of maximum curvature around 1 cm. This 1-cm characteristic length scale is interpreted as a result of a change in deformational mechanism from fluid controlled fracture to grain reduction controlled primarily through shear faulting.

Both the distinctive PSD, and the igneous and hydrothermal mineralogy of oceanic fault rocks are expected to lead to mechanical behavior that differs from continental faults. To further explore these differences, variably chlorite- and epidote-rich fault rocks were subjected to friction experiments at the Penn State Rock Mechanics Laboratory. The friction experiments show unaltered basalt and epidote are velocity weakening, meaning they have a propensity for unstable, potentially seismic slip. Icelandic gouge and chlorite breccia are velocity strengthening, meaning the slip on these rocks tends to be stable, potentially leading to aseismic creep.

A reasonable model for ocean crustal faulting of the upper, basaltic oceanic crust therefore holds that a characteristic length scale of ~ 1 cm controls the deformational mechanism of fault and fracture systems. The mechanical model of evolution of fault and fracture provides a geologic constraint on temperature fluctuation and earthquake studies and offers a possible explanation of the initiation and evolution of the brittle-deformation in the oceanic crust.