ALBERT

Description: We assessed tectonic controls on the spatial and temporal distribution of fault zone flow pathways in the Rio Grande rift (New Mexico, USA) by using fault zone calcite cements as a geochemical record of syntectonic fluid flow. Cement 18 O, 13 C, and 87 Sr/ 86 Sr values indicate that older, large-displacement master and basin-margin faults were cemented by more isotopically evolved basinal brines than younger intrabasin faults. These data suggest that diagenetic fluids in basin-bounding faults equilibrated predominantly with downdip Paleozoic carbonates. In contrast, intrabasin faults transmitted fluids from shallow stratigraphic sources. This pattern of flow pathways is linked to the systematic distribution of sediments and faults that record rift evolution, which dictated spatial and temporal variations in fault zone architecture and permeability structure. Our results indicate that the depths from which fluids can be transported in active rift basins ultimately depend on both tectonically mediated variations in the grain size of syntectonic sediments entrained in fault damage zones and fault displacement magnitude.

Description: Diagenesis demonstrably changes both the mechanical and hydrologic properties of sediments. However, the effect of these changes on the distribution and types of structures that develop in fault zones over time, and their impact on fault-zone fluid flow has not previously been systematically investigated. We explored the impact of diagenesis on the mechanical and hydrologic properties of initially unlithified sand cut by a syndepositional normal fault in an extensional basin. Field, microstructural, and geochemical data document a diagenetic record of initially continuous fluid flow through a hanging-wall damage zone up to 10 m wide. In this zone, initial deformation via particulate flow is recorded by a foliation defined by a grain shape preferred orientation, preserved by subsequently precipitated, pore-filling calcite cement. A transition to episodic fluid flow is demonstrated by calcite veins that crosscut the previously cemented, foliated sandstone within relatively narrow (≤5 m from the fault core) segments of the older damage zone. Unlike the widespread record of particulate flow, veins are restricted to the vicinity of a mapped relay zone between overlapping fault segments, and an inferred, partially covered relay zone. Vein microstructures record repeated fracture opening and sealing. Veins in breccia zones have 13 C values as high as +6.0, suggesting degassing of CO 2 - and/or CH 4 -charged fluids. These data collectively suggest that relatively early damage zone cementation strengthened and stiffened the foliated damage zone, affecting the localization of brittle fractures and fluid flow in relay zones. Our results highlight systematic changes in the character and locus of deformation and fluid flow during the development of normal faults and provide a basis for predicting how these structures may act to trap or transmit fluids during the development of extensional basins.