U‐Th Dating of Syntectonic Calcite Veins Reveals the Dynamic Nature of Fracture Cementation and Healing in Faults. Issue 22 (23rd November 2019) - Williams, Randolph T.
Abstract: Fracture cementation is an important control on the recovery of prefailure levels of permeability and strength in faults. The timescales 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 to 0.80 mm/ka and exhibit a power function correlation with minimum fracture apertures. We argue that this correlation is the result of crystal growth in a transport‐limited system, where cementation rates were proportional to rates of postseismic fluid flow in individual fractures. We further argue that such transport‐limited, flux‐dependent cementation necessarily leads to a heterogeneous distribution of permeability and strength recovery as fluids migrate through fault‐zone fracture networks. These heterogeneities may influence rupture propagations pathways and the continual development of fault‐zone architecture/complexity. Plain Language Summary: Fractures formed during earthquakes greatly increase the permeability of fault zones while decreasing their strength, thereby exerting a significant influence on the cycle of earthquakes in the crust. Permeability and strength, however, experience partial recovery during interseismic times, a process referred to as fault or fracture "healing." Precipitation of minerals within fractures is commonly cited as an important control on healing, but rates of this process are largely unknown. We present the results of a novel sampling approach which allowed us to estimate the rates of fracture cementation in a seismogenic fault in New Mexico, USA. Our results show that cementation rates are highly variable among individual fractures. Rates of fracture cementation are strongly correlated with the aperture (or width) of the original fracture during healing. We conclude that cementation rates that are dependent on rates of fluid flow through individual fractures following earthquakes, with the widest fractures accommodating more fluid flow and rapid healing. This suggests healing of faults during interseismic periods is spatially heterogeneous and continuously migrates as fracture networks evolve. Key Points: U‐Th dating was used to estimate the timescales of calcite cementation and healing/sealing in fault‐localized fractures Results suggest that cementation rates are "flux dependent" and proportional to rates of postseismic fluid flow through fractures This dependency likely leads to heterogeneous distributions of postseismic recovery of permeability and strength in fault damage zones