STRUCTURAL DEVELOPMENT, MECHANICAL EVOLUTION, AND HYDROLOGIC SIGNIFICANCE OF FAULTS IN POORLY LITHIFIED SEDIMENTS OF THE RIO GRANDE RIFT, CENTRAL NEW MEXICO

Fault zones in poorly lithified sediments of the Tertiary Sante Fe Group in the Albuquerque basin, New Mexico, USA, differ significantly from brittle faults formed in lithified sedimentary and crystalline rocks. Undeformed Santa Fe Group sediments are intermediate between soil and sedimentary rock in their elastic properties. Macroscopic fractures are absent within the studied fault zones. The fault cores are composed of foliated clay flanked by structurally and lithologically heterogeneous mixed zones, in turn flanked by damage zones. Mixed zones exhibit macroscopically ductile structures formed by sediment disaggregation and penetrative particulate flow, accompanied to varying degrees by cataclasis in sandy sediments. These structures are overprinted by deformation bands, indicating a temporal and mechanical shift to localized cataclasis. The observations are consistent with the inferred consolidation history of the faults and host sediments, as well as predictions from critical state soil mechanics theory. The mode of cataclastic grain fracture differs from that in lithified rocks. It is controlled by mineralogy and relative grain strength. Particle size measurements indicate that progressive deformation produces a particle size distribution that can be described by a power-law model, characterized by low D values (1.7 – 2.1). This indicates a preponderence of large particles with respect to a distribution produced by cataclasis by constrained comminution (D ~ 2.6). Equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments have little potential to act as vertical-flow conduits and have a much greater effect on horizontal flow than faults with fractures. This hydrologic contrast stems from the differences in characteristic structures and deformation mechanisms between faults in unlithified sediments and sedimentary rocks, and highlights the dissimilar responses of these materials to tectonic deformation.