IMPLICATIONS FOR SYN-FAULTING FLUID FLOW FROM MACROSCOPIC FAULT ROCK TEXTURES AND GEOCHEMISTRY: THE SAN YSIDRO FAULT, ALBUQUERQUE BASIN, NEW MEXICO

The evolution of modern extensional basins is accompanied by faulting in poorly lithified, siliciclastic sediments at low pressure and temperature. Recent work has shown that such faulted sediments deform by plastic-like micromechanisms that are distinct from their counterparts in brittle rock. However, a full understanding of macroscale deformation, geochemical and hydrological processes associated with this type of faulting are lacking.

The San Ysidro normal fault in the NW Albuquerque basin of the Rio Grande rift is a spectacularly exposed, representative, and well-studied intrabasin fault. It preserves evidence for syn-rift, fault-related processes that have implications for the present hydrogeology of the basin. Macroscopic fault-rock textures in dismembered and ‘smeared' sand beds include deeply incised tool marks and convolutions that record the movement of large asperities when the sediment was unconsolidated and saturated. Fault core clays envelop sand grains and pebbles and infill convoluted spaces along the margins of the fault core where it is corrugated.

Geochemical data from fault and host rocks indicate that the major geochemical signature associated with the fault is mineralogical and caused by mechanical entrainment of clay into the core with little geochemical reaction. Rare Earth and other elemental data from these clays depart little from their host rock composition or average crustal abundances.

These geochemical data and textural observations suggest initial strain accommodation in the fault core was dominantly plastic and occurred at near-surface temperature and pressure below the paleo-groundwater table. Such deformation contrasts with brittle faulting in rock, in that little new syn-deformational permeability was created within the fault core preventing significant fluid flux and thus neomineralization. This implies that faults of this type were poor conduits for mineral-bearing fluids or the migration of hydrocarbons during deformation. As well, these processes likely cause such intrabasin faults to act as partial barriers to fluid flow today. Permeability and mapping data shows pervasive clay-rich cores with nearly six orders of magnitude lower permeability than the host sediment leading to the possibility of aquifer compartmentalization under pumping stresses.