Carbonate pseudotachylytes: evidence for seismic faulting along carbonate faults

The Canalone Porta fault, developed within dolomitic and marly limestones in the Grigna Massif (Southern Alps, Italy), shows two types of reddish veins: one parallel to the principal slip plane (fault-veins) and the other intruding the fault host rock (injection-veins). The veins have a dominant carbonate composition and show a microtexture with millimetre-sized clasts embedded in a fine-grained matrix. This matrix is composed of micrometre-sized globular clusters of micrometre- to nanometre-sized calcite and dolomite crystals, bound together by K-bearing aluminosilicate glass. Field and petrological data suggest that this carbonate-rich matrix represents an undercooled melt-bearing assemblage produced at T≈700°C and P≈0.1-0.2GPa. The Canalone Porta veins are therefore carbonate pseudotachylytes, i.e. the friction-induced melting product of carbonate rocks. Herein, we discuss the scarcity of these rocks in general and criteria to find and recognize them along relatively deep fault zones.