Diagenetic Controls on the Evolution of Fault-Zone Mechanical Properties and Permeability Structure: Loma Blanca Fault Zone, Socorro Basin, NM

Abstract

Fault-zone diagenesis can greatly impact both the mechanical and the hydrologic properties of fault zones. Conceptual models of fault-zone architecture and permeability structure typically consider damage zones to be defined by fault-related fractures, which increase the potential for episodic fault-parallel fluid transport. However, faults in poorly lithified sands and high porosity sandstones are mechanically weak, and generally unable to produce, or sustain, open transgranular fractures. In these materials, deformation is accomplished by particulate flow and/or deformation band formation. Diagenesis can facilitate a transition to rock that can fracture, but only once compaction and/or cementation have sufficiently strengthened the damage zones. We utilize U-Pb and U-series geochronology in addition to δ18O, δ13C, 87Sr/86Sr and trace element analyses of fault-zone calcite cements to constrain: 1) the timing of the transition from pre-lithification particulate flow and deformation band formation to post-lithification fracturing during progressive cementation, and 2) the duration and recurrence interval of post-lithification fracture generation and diagenetic sealing events in the Loma Blanca fault, Socorro Basin, NM. Field and microstructural evidence indicate that the Loma Blanca fault initiated in poorly lithified sand. Calcite cementation progressively strengthened the damage zone, at which point pre-lithification particulate flow fabrics and deformation bands were overprinted by post-lithification fractures. Multi-generational, crack-seal calcite veins filling post-lithification fractures indicate that fluid flow in the Loma Blanca was episodic during this phase of fault history. δ18O and δ13C analyses reveal distinct trends in isotopic composition as a function of deformation style, where early pore-filling cements are isotopically lighter than late-stage calcite veins. Late-stage calcite veins have δ13C values as high as +6.00‰, likely indicating rapid CO₂ degassing during fluid flow and cementation following fault slip. 87Sr/86Sr values suggest a constant fluid source throughout the diagenetic history. Variations in isotope and trace element geochemistry are therefore predominantly the result of variations in fluid temperature. U-series analyses of late-stage calcite veins show a protracted history of fracture generation, fluid flow, and sealing and provide insight into the temporal scales of fault-zone mechanical evolution and fluid migration.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015