CONTROLS ON FAULT-ZONE ARCHITECTURE AND CEMENTATION IN POORLY CONSOLIDATED SANDS AND GRAVELS: LOMA BLANCA FAULT, RIO GRANDE RIFT, NEW MEXICO

The Loma Blanca fault is a moderate-displacement (<60 m) normal fault that cuts poorly consolidated Plio-Pleistocene fluvial sands and gravels of the Ancestral Rio Grande River. Cementation along the strike of the fault is highly variable. Where well developed, cement occurs preferentially on the hanging wall side of the fault and attains a fault-perpendicular thickness of up to eight meters. Cementation in these areas is continuous and extends for distances of greater than 200 meters along strike. The degree of cementation correlates strongly with the mean grain size of the host sediments. Where the host sediments are coarse sand and gravel the fault is always well cemented; where the host sediments are fine and very fine sand cementation is absent or present as isolated concretions. Calcite-lined fractures and breccia zones are abundant in well-cemented areas and occur within a few meters of the main slip surface. Calcite commonly does not seal either fractures or breccia pores, indicating that deformation-related porosity remained open during and after cementation. Crosscutting relationships indicate at least two principal stages of fracture development. Deformation bands and macroscopically ductile shear zones are locally abundant in sand-dominated portions of the outcrop. Clay is present locally as isolated pods up to a few 10s of centimeters in length associated with major slip surfaces. Such pods were produced by entrainment of fluvial mud balls and rip-up clasts. The Loma Blanca fault differs from previously studied normal faults in the rift in two ways: the abundance of fractures, which are largely absent in the other faults, and the absence of a clay-rich core, which characterizes the other faults. Preliminary interpretation of fault-zone evolution is as follows: (1) initial faulting of stratified sands and gravels, including formation of deformation bands and shear zones; (2) preferential cementation of coarse-grained hanging-wall sediments due in part to relatively high fluid flux; and (3) absence of clay core zone prevents deformation from being localized, resulting in multiple generations of fractures in cemented hanging wall. Fault-zone permeability changed dramatically through time as a result of the above sequence.