Controls on Fault-Zone architecture in poorly Lithified Sediments, Rio Grande Rift, New Mexico: Implications for Fault-Zone Permeability and fluid flow

We have studied the geometry and continuity of structures and diagenetic features of a normal growth fault in poorly lithified sediments. Fault-zone width and complexity vary spatially with the grain-size distribution of faulted beds. The fault zone is narrow and structurally simple where it cuts either thick beds with >20% clay and silt, or thin beds that alternate between >20% and ≤20% clay and silt. Where the majority of beds juxtaposed by the fault are ≥80% sand and gravel, and clay beds are thin and rare, the fault zone is wide and structurally complex. In all cases, the fault zone can be divided into three architectural elements. The core includes the primary slip surface(s) and a nearly continuous clay smear 0.3 - 32 cm wide. It is flanked by structurally and lithologically heterogeneous mixed zones, which include material derived from adjacent sediments during fault movement. Mixed zone sediments vary from little deformed to well foliated, tectonically mixed material within which bedding has been destroyed. The mixed zones are bound by damage zones, within which deformation was confined to minor faults and folds. Grain-size and structural variations among these elements lead us to conclude that they have hydrologie significance. In addition, the fault zone is preferentially cemented with respect to adjacent sediments. We use degree of cementation as a proxy for fluid flux, and patterns of cementation as a record of paleo-flow pathways. Extensive sparry calcite cement is typically confined to coarse-grained sediments in the hanging wall (basinward) mixed zone. Steeply plunging, elongate patterns of cement are interpreted to record subvertical groundwater flow at the time of precipitation. As regional flow is inferred to have occurred roughly from the margins to the center of the basin at the time of cementation, these relationships indicate a combination of crossfault and subvertical, fault-parallel flow.