DIAGENETIC CONTROLS ON FAULT-ZONE ARCHITECTURE AND PERMEABILITY STRUCTURE IN NORMAL FAULTS: TEMPORAL CONSTRAINTS ON THE DISTRIBUTION, RECURRENCE, AND DURATION OF FAULT-ZONE FLUID MIGRATION

We present results of field, microstructural, geochemical, and geochronological analyses designed to assess diagenetic controls on the development of fault-zone architecture and permeability structure over time in the Loma Blanca fault zone, Socorro Basin, New Mexico. Our data show that both the type and distribution of deformation structures formed in response to fault slip changed as syntectonic cementation changed sand to sandstone. Initial deformation in poorly lithified sands of the hanging wall damage zone was accommodated by particulate flow and deformation-band formation. This deformation resulted in a foliation defined by the alignment of elongate sand grains sub-parallel to the fault. Microstructural analyses show that the damage zone was locally up to 10 m wide during this phase of faulting. Subsequent calcite cementation stiffened and strengthened the damage zone, and led to the formation of mesoscopic extension fractures during later slip events. Stable isotope analyses of calcite veins filling extension fractures show δ¹³C values as high as +6.00‰, suggesting depressurization and degassing of CO₂ charged fluids following fault slip and fracture formation. These veins are restricted to a narrow zone no more than 1 – 5 m from the principal slip surface. They exhibit crack-seal microstructures, recording repeated episodes of fracture opening and sealing. Thus, syntectonic cementation resulted in a transition from continuous fluid flow in a relatively wide, foliation-dominated damage zone to episodic fluid flow in a relatively narrow, fracture-dominated damage zone. Fault-zone permeability structure was therefore controlled by the properties of the “active” fault damage zone (i.e., the fraction of the total fault damage zone actively accommodating deformation and fluid flow at a given time). U-series analyses of calcite veins provide constraints on the recurrence and duration of post-failure fluid migration. Preliminary results show a well defined periodicity of fault slip, fracturing, and fluid flow (recurrence interval 73 ± 16 ka). Fractures sealed over a comparatively short time scale of 16 ± 4 ka. Collectively, our results provide quantitative constraints on the spatial and temporal distribution of damage zone fluid flow during the development of normal faults in extensional basins.