Diagenetic Controls on Normal Fault-Zone Architecture Determine the Episodicity of Fault-Zone Fluid Migration

Abstract

We investigated the role of diagenesis in the evolution of fault-zone architecture and permeability structure by field, microstructural, geochemical, and geochronological analyses of calcite cements in the Loma Blanca fault zone, Socorro Basin, NM. Our results show that syntectonic cementation changed both the type and distribution of structures formed in response to fault slip. These changes produced a shift from continuous to episodic fault-zone fluid flow. Deformation of poorly lithified sands initially took place via particulate flow, producing a foliation defined by elongate sand grains in a damage zone that was locally up to 10 m wide. Subsequent calcite cementation increased the mechanical competency of the damage zone. Calcite cement is not distributed continuously along strike, and records the locations of channels of high fluid flux. Cements in these channels reduced permeability sufficiently to facilitate development of fluid overpressures, recorded by cross-cutting calcite-filled extension fractures (veins) and breccia. Veins and breccia are found only at two locations along strike, indicating further localization of fluid flux following cementation of host sands. Veins and breccia, confined to a volume within 5 m of the fault core, include crack-seal microstructures – evidence of repeated fracture opening and sealing events inferred to record fault-valve behavior. Thus, syntectonic cementation resulted in a transition from continuous fluid flow in a relatively wide damage zone to episodic fluid flow in a narrower damage zone. All cement δ18O values are similar, demonstrating that fluid temperature remained relatively constant despite fundamental changes in deformation mechanisms and fluid flow pathways. Stable isotope analyses of veins from breccia zones, however, show δ13C values as high as +6.0‰. These values record depressurization of CO₂-charged brines during post-failure fluid migration, providing further evidence of fault-valve behavior. U-series dating of calcite veins constrains the recurrence and duration of episodes of fluid migration. Dates for distinct slip and fluid flow events suggest a recurrence interval of ∼70 ka whereas dates across individual veins span ∼16 ka, showing that fractures seal on shorter time scales. Collectively, our results constrain the role of diagenesis in determining the spatial and temporal distribution of fault-zone fluid flux during the development of a basin-margin normal fault in an extensional basin.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016