Static versus dynamic fracturing in shallow carbonate fault zones

Abstract

Moderate to large earthquakes often nucleate within and propagate through carbonates in the shallow crust. The occurrence of thick belts of low-strain fault-related breccias is relatively common within carbonate damage zones and was generally interpreted in relation to the quasi-static growth of faults. Here we report the occurrence of hundreds of meters thick belts of intensely fragmented dolostones along a major transpressive fault zone in the Italian Southern Alps. These fault rocks have been shattered in-situ with negligible shear strain accumulation. The conditions of in-situ shattering were investigated by deforming the host dolostones in uniaxial compression both under quasi-static (strain rate ∼10−5 s−1) and dynamic (strain rate >50 s−1) loading. Dolostones deformed up to failure under low-strain rate were affected by single to multiple discrete extensional fractures sub-parallel to the loading direction. Dolostones deformed under high-strain rate were shattered above a strain rate threshold of ∼120 s−1 and peak stresses on average larger than the uniaxial compressive strength of the rock, whereas they were split in few fragments or remained macroscopically intact at lower strain rates. Fracture networks were investigated in three dimensions showing that low- and high-strain rate damage patterns (fracture intensity, aperture, orientation) were significantly different, with the latter being similar to that of natural in-situ shattered dolostones (i.e., comparable fragment size distributions). In-situ shattered dolostones were thus interpreted as the result of high energy dynamic fragmentation (dissipated strain energies >1.8 MJ/m3) similarly to pulverized rocks in crystalline lithologies. Given their seismic origin, the presence of in-situ shattered dolostones can be used in earthquake hazard studies as evidence of the propagation of seismic ruptures at shallow depths.