U-TH DATING OF SYNTECTONIC CALCITE VEINS REVEALS THE DYNAMIC NATURE OF FRACTURE CEMENTATION AND HEALING IN FAULTS

Fracture cementation is an important control on the recovery of pre-failure levels of permeability and strength in faults. The time scales of this process, however, are almost entirely unknown from direct analysis of the rock record. We report U-Th dating results that quantify rates of fracture cementation in syntectonic calcite veins from the Loma Blanca fault, New Mexico, USA. Measured rates vary from ~0.05 – 0.80 mm / ka and exhibit a power function correlation with minimum fracture apertures. We argue that this correlation is indicative of calcite precipitation that was transport-limited during post-seismic fluid flow. We further argue that such transport-limited crystal growth leads naturally to a heterogeneous distribution of fracture cementation in fault damage zones, which occurs preferentially within the highest transmissivity portion of the larger fracture network. As these preferential fracture-flow pathways become occluded with cements, fluid flux redistributes through other paths, leading to continuously variable rates of crystal growth both within and between individual fractures. For this reason, individual fractures need not be expected to seal monotonically through time, but could instead experience order-of-magnitude increases or decreases in sealing rate depending on their geometric properties (e.g. aperture, length/width, orientation) and position within a continually evolving network. We propose that this process likely results in significant variations in the degree and time scales of fault-rock healing during interseismic periods, which may influence propagation pathway(s) of subsequent ruptures and the continual development of fault-zone architecture / complexity.