U-series Dating of Syntectonic Calcite Veins Constrains the Time Scales of the Elements of the Seismic Cycle in an Intraplate Normal Fault

Abstract:

The link between fluid overpressure and the earthquake cycle has been documented through previous studies of vein arrays associated with faults and carefully designed experimental work. In the interseismic period, elevated pore fluid pressure (overpressure) will reduce the effective fault-normal stress, weakening the fault and promoting seismic rupture. Fractures produced during faulting will serve as fluid migration pathways until they are sealed by either collapse or precipitation of cement. Following sealing, pore fluid pressure is inferred to progressively increase until it reaches a level sufficient to start the cycle again.

Though the rock record of this overpressure-driven seismic cycle is clear, the timescales of the different elements of the cycle have not been quantified. We have addressed this problem by dating calcite veins in the hanging wall damage zone of the Loma Blanca fault zone of the Socorro Basin, Rio Grande rift, New Mexico. These veins exhibit crack-seal microstructures that record repeated episodes of fracture opening, fluid migration, and fracture sealing, suggesting a prolonged history of recurrent seismicity and post-failure fluid migration. Stable isotope analyses of these veins reveal distinct fluid chemistries associated with individual fluid migration events. Carbon isotope values as high as +6.00‰ suggest depressurization and degassing of CO2 charged fluids, supporting the interpretation that fracturing was associated with fault slip. Preliminary U-series dating of calcite veins show a well-defined periodicity of fault slip and fracture formation, with a slip recurrence interval of approximately 73 ± 17 ka, consistent with previous studies of other faults in the Rio Grande rift. Analyses of cements deposited during single crack-seal events record sealing times of approximately 16 ± 4 ka. These results suggest that the time required to re-establish sufficient pore fluid pressure for failure following sealing of damage zone fractures may be ca. 50 ka. Our results provide the first quantitative constraints on the timescales of fluid overpressure development, fault slip, and fracture sealing in a seismogenic fault zone, and may provide insight into the seismic cycle of intraplate fault zones in other extensional tectonic settings.