The physico-chemical processes operating along natural faults in carbonate rocks during seismic slip remain poorly constrained. At depths greater than 1–2 km, typical of seismogenic zones, thermal decomposition of carbonates (i.e, CO2 release via decarbonation) is generally not expected based on petrological experiments. In contrast, carbonate melting is thermodynamically allowed at temperatures exceeding ∼650 °C in the presence of water. Nevertheless, natural examples recording frictional carbonate melting along faults are still rare and not fully understood. The Pietra Grande thrust in the Brenta Dolomites (Southern Alps, Italy) offers an exceptional case to investigate such processes. This exhumed fault in carbonate rocks preserves evidence of seismic slip deformation in the form of carbonate pseudotachylyte. The associated suite of fluidized fault rocks displays sharp-walled injection veins and large melt pockets. The very fine-grained matrix is compositionally homogeneous and consists of micrometre- to nanometre-sized calcite crystals, coarsening into calcite grains larger than 50 μm with dolomite exsolutions, along with minor cryptocrystalline to amorphous K-bearing aluminosilicates. Using the reconstructed compositions of the coarse carbonate grains prior to dolomite exsolution and considering the burial depth of the host sedimentary sequence, we estimated a temperature of ∼675 °C at 60 MPa. These conditions are consistent with carbonate melting under water-saturated conditions. However, the isotopic compositions of host rocks and pseudotachylytes suggest a more complex process. A first stage of decarbonation likely occurred during initial heating, accompanied by a pressure drop due to fault dilatancy. In a subsequent step, pressure recovered toward the lithostatic value predicted at depth, thereby suppressing further decomposition by expanding the stability field of carbonates. At this point, melting became possible, provided that free water is available. Thus, the presence or absence of water is a critical factor in determining whether deep carbonate faults produce pseudotachylytes or instead undergo subsolidus cataclastic deformation.
Can frictional melting occur in carbonate-hosted faults during seismic slip? Evidence from the Pietra Grande thrust (Dolomites, Italy)
Giacomo Prosser;
2026-01-01
Abstract
The physico-chemical processes operating along natural faults in carbonate rocks during seismic slip remain poorly constrained. At depths greater than 1–2 km, typical of seismogenic zones, thermal decomposition of carbonates (i.e, CO2 release via decarbonation) is generally not expected based on petrological experiments. In contrast, carbonate melting is thermodynamically allowed at temperatures exceeding ∼650 °C in the presence of water. Nevertheless, natural examples recording frictional carbonate melting along faults are still rare and not fully understood. The Pietra Grande thrust in the Brenta Dolomites (Southern Alps, Italy) offers an exceptional case to investigate such processes. This exhumed fault in carbonate rocks preserves evidence of seismic slip deformation in the form of carbonate pseudotachylyte. The associated suite of fluidized fault rocks displays sharp-walled injection veins and large melt pockets. The very fine-grained matrix is compositionally homogeneous and consists of micrometre- to nanometre-sized calcite crystals, coarsening into calcite grains larger than 50 μm with dolomite exsolutions, along with minor cryptocrystalline to amorphous K-bearing aluminosilicates. Using the reconstructed compositions of the coarse carbonate grains prior to dolomite exsolution and considering the burial depth of the host sedimentary sequence, we estimated a temperature of ∼675 °C at 60 MPa. These conditions are consistent with carbonate melting under water-saturated conditions. However, the isotopic compositions of host rocks and pseudotachylytes suggest a more complex process. A first stage of decarbonation likely occurred during initial heating, accompanied by a pressure drop due to fault dilatancy. In a subsequent step, pressure recovered toward the lithostatic value predicted at depth, thereby suppressing further decomposition by expanding the stability field of carbonates. At this point, melting became possible, provided that free water is available. Thus, the presence or absence of water is a critical factor in determining whether deep carbonate faults produce pseudotachylytes or instead undergo subsolidus cataclastic deformation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
