DIAGENETIC HISTORY OF CALCITE CEMENTED NORMAL FAULTS: CONSTRAINTS ON FLUID SOURCES AND FAULT SEAL IN THE RIO GRANDE RIFT, NM

Approaches to evaluating fault seal typically consider only variations in host-rock lithology and throw, neglecting the effects of fault zone diagenesis. This is in part due to difficulties in constraining the fluid sources and pathways responsible for fault zone cementation. As cement volume is directly related to fluid flux, chemistry, and temperature, prediction of the basin scale distribution of fault zone cementation requires better knowledge of controls on regional flow pathways and how these pathways evolved during basin development. To address this problem, we have conducted field, petrographic, and geochemical analyses on faults of the Rio Grande rift of New Mexico selected to encompass a wide range of structural settings, displacement magnitudes, and temporal relations to rift history. We present preliminary data concerning the textural and geochemical variations within, and structural context of, fault-zone cements, which constrain diagenetic and fluid source histories for these faults. Field observations suggest that diagenesis was synchronous with deformation and that cement content is controlled in part by each fault’s structural setting within the basin. Fault zone deformation transitions from initial particulate flow and deformation band generation to failure along discrete fractures as cement content increases. Several generations of fault cement have been documented at both outcrop and thin section scales, and these have been correlated with the structural evolution of the analyzed faults where possible. By characterizing the texture and geochemistry of cements within the structural context of a fault zone we are able to delineate a diagenetic and fluid flow history for individual faults in the Rio Grande rift. Establishment of fluid flow histories for individual faults in the rift is a fundamental first step in examining the basin-scale controls on fault zone fluid flow and cementation.