DEFORMATION BANDS AND FLUID-FAULT INTERACTION IN NON-WELDED TUFFS

Fault-zone structures in non-welded tuffs are more similar to those found in porous sandstones than those found in low porosity rock. Faulting of low porosity rock typically results in a relatively fine-grained fault core surrounded by a fracture-rich damage zone. The fracture can be considered the fundamental building block of this type of fault zone. In contrast, high porosity sedimentary rocks and poorly lithified sediments fail by a combination of grain-size reduction through cataclasis, particulate flow, and/or pore collapse. The structures produced by these processes include deformation bands in sands and sandstones - structures that decrease bulk saturated permeability. We show that other high porosity geomaterials, such as non-welded tuffs of the Bandelier Tuff, New Mexico, USA, deform by similar mechanisms. Faults in glassy, non-welded, relatively unaltered tuffs form a zonal architecture in which mesoscopic fractures are absent. Instead, deformation bands dominate the fault-zone structure. Evidence of compactive cataclasis through distributed microfracturing of volcanic glass, pumice, and phenocrysts is found in the deformation bands. Fault-parallel alignment of phenocrysts and pore space is developed through grain rotation and destruction of glass walls surrounding vesicles in pumice fragments. Although cataclasis demonstrably reduces porosity and pore size in the deformation bands, the destruction of walls separating pores may increase the connectivity of pore space. The effect of these competing processes on permeability and permeability anisotropy is being investigated. Locally, alteration of fault-zone material to clays records fluid-fault interaction. In the absence of such alteration, reduction in pore and grain size may result in a mode of failure transition and development of a through-going slip surface, a process similar to that described in porous sandstone. Iron and tin oxides are locally precipitated along one slip surface, recording post-faulting fluid flow.