TECTONIC CONTROLS ON FAULT-ZONE FLOW PATHWAYS IN THE RIO GRANDE RIFT: INSIGHTS FROM C, O, SR, AND CLUMPED ISOTOPE ANALYSES OF SYNTECTONIC CALCITE CEMENT

We assessed tectonic controls on the spatial and temporal distribution of fault-zone flow pathways in the Rio Grande rift, NM, USA, by using fault-zone calcite cements as a geochemical record of syntectonic fluid flow. Faults were selected based on their structural position within the rift. Cement δ¹⁸O and δ¹³C values indicate that older, large-displacement master and basin-margin faults were cemented by more isotopically evolved basinal brines than younger intrabasin faults. Strontium isotope and trace-element analyses further suggest that diagenetic fluids in these basin-bounding faults equilibrated predominantly with down-dip Paleozoic carbonates, with minor input from interbedded calcareous shales and/or Proterozoic crystalline basement rocks. In contrast, intrabasin faults transmitted meteoric fluids from shallow stratigraphic sources. Clumped isotope analyses reveal cement precipitation temperatures of ~10 – 20 ºC in all faults. These data suggest that fluid ascent rates in master and basin-margin faults were sufficiently slow as to allow equilibration with the geothermal gradient, and aid in the reconstruction of fluid chemistry.

The documented pattern of flow pathways is linked to the mechanical and textural properties of the host rock, which dictated spatial and temporal variations in fault-zone architecture and permeability structure. Specifically, large displacement master and basin-margin faults juxtapose hanging wall damage zones of sheared coarse-grained sediments and footwall damage zones with a significant extent of fractured pre-rift units, providing effective conduits from deep stratigraphic levels. Smaller displacement intrabasin faults served as barriers to flow through older, low-permeability lacustrine and playa deposits localized nearer basin centers, but shuttled water either upward or downward through younger, axial fluvial sands. Such tectonically mediated variations in the grain size of syntectonic sediments entrained in fault damage zones and fault displacement magnitude are often predictable during the development of extensional basins. Therefore, our results provide a fundamental first step toward predicting regional fault-zone flow patterns in extensional tectonic settings.