Weakness of serpentine minerals revealed by friction experiments under low and high temperature conditions.

Serpentines are important constituents of fault rocks and mélanges in a large variety of tectonic settings, including some major plate-boundary structures such as the San Andreas fault. Many of these structures are considered frictionally weak on geological and geophysical basis (i.e. µ<<0.6). However, previous studies report that the strength of serpentines is not low enough to explain such fault weakness, with the possible exception of chrysotile at room T. Instead, the presence of talc, clays and elevated fluid pressures is commonly invoked to solve the "weak fault" conundrum. Here we report the frictional strength of mineralogically-controlled gouges of serpentine (lizardite, chrysotile/polygonal serpentine and antigorite) under both room and hydrothermal conditions (T up to 170 0C). Experiments were performed in direct shear configuration using a biaxial machine (HP-HT Laboratory, INGV Rome, Italy) and a triaxial apparatus with an external furnace (Rock Mechanics Laboratory, Durham University, UK). The sliding strength of lizardite and chrisotile/polygonal (the typical association in retrograde serpentinites and in several natural shear zones) is lower than previously reported (µ<0.2) and scarcely affected by temperature changes for T<200°. Interestingly, these results are in agreement with the fault strength inferred for the central segment of the San Andreas fault where abundant serpentinites are present. Our observations, together with field evidence from natural shear zones, suggest that serpentine-rich faults may significantly contribute to the weakness of major faults throughout the brittle upper crust.