New Mexico Geological Society Annual Spring Meeting — Abstracts

Characteristics of faults in ignimbrite sequences are of interest for a variety of reasons, including their potential impact on fluid flow and contaminant transport. SmallI displacement faults in the Bandelier Tuff of Los Alamos, New Mexico, display a range of deformation characteristics that are linked to petrophysical characteristics, such as porosity and degree of welding. The two basic structural features found among the ignimbrite units are shear fractures and deformation bands. The primary control on which of these features forms in response to deformation is degree of welding (cohesion of volcanic ash). Low porosity welded units deform by transgranular fracture, resulting in shear fractures that are similar to well studied faults in low porosity crystalline rock. In contrast, high porosity, non-welded units deform by catacastic grain crushing and pore collapse within deformation bands. Deformation bands have been described previously only in high porosity sandstones and poorly lithified sediments. Our investigation of the impact of variations in welding and protolith porosity on defonnation characteristics strengthens the established link between porosity and mode of deformation in sedimentary rocks, and identifies welding as a critical component in this link. Through this research, we hope to develop criteria by which we can predict whether fractures or deformation bands will form in a given ignimbrite. Such criteria are necessary to develop accurate hydrologic models in ignimbrites since fractures and defonnation bands affect porosity and permeability in very different ways. This has been illustrated in sedimentary rocks, where the presence of fractures increases fault-zone porosity and permeability, and deformation bands reduce porosity and saturated permeability.