New Mexico Geological Society Annual Spring Meeting — Abstracts

Faults in ignimbrites are of interest for a variety of reasons, including their potential impact on fluid flow and transport through vadose zones in places like Los Alamos, NM, and Busted Butte, NV. We have investigated petrophysical controls on deformation in small-displacement faults in these areas. Fractures and deformation bands are found at both sites. The primary controls on which structure forms in an ignimbrite unit are the number and strength of grain contacts in the groundmass, which are inversely proportional to porosity and directly related to degree of welding and postdepositional crystallization. Low porosity, welded units deform by trans granular fracture; high porosity, glassy, non-welded units deform by cataclasis and pore collapse within deformation bands. Moderately high porosity, non-welded units with postdepositional crystallization deform by either cataclasis with associated pore collapse, or by fracture, depending on local variations in clast size and nature of crystallization.

Clast-and pore-size reduction in deformation bands is significant, producing indurated zones of clay-sized material. Preferential wetting, inferred to promote alteration and cementation, is observed in many deformation bands. Many of these bands are also locally rich in smectite and/or cemented by calcite. We therefore interpret variably altered fault-zone material as evidence of preferential fluid flow through deformation bands in the vadose zone, which we infer to be the result of enhanced unsaturated permeability due to pore-size reduction in deformation bands. This is significant for models of fluid flow and transport, since most assume matrix-dominated flow through non-welded ignimbrites, disregarding potential effects of smallI
displacement faults.