EVIDENCE FOR FAULT-VALVE BEHAVIOR IN ULTRACATACLASITES OF CARBONATE-HOSTED NORMAL FAULTS, CENTRAL APENNINES, ITALY

We examine fault precipitates from three carbonate-hosted normal faults of the Central Apennines, Italy, to show how deformation features on the scales of mm to cm retain evidence for fault-valve behavior and possible co-seismic fluid flow along slip surfaces. Mm-wide zones of ultracataclasites in fault core samples exhibit textures expected of comminuted carbonate host rock, but XRF chemical map data from Mg-rich dolomite-hosted samples show that these zones have little to no Mg. Additionally, the contact between the low-Mg ultracataclasites and the surrounding high-Mg fault breccia is sharp, and photomicrographs show a thin zone of calcite separates them. We interpret the low-Mg ultracataclasites and other texturally similar ultracataclasites to be the product of coeval calcite precipitation and comminution along fault slip surfaces during seismic events. The textural evidence for this is threefold: (1) The low-Mg ultracataclasites are materially distinct from the surrounding comminuted fault materials and do not include any survivor clasts, suggesting that strain was localized to this mm-wide zone during comminution; (2) Several of the ultracataclasites in both primary and secondary fault cores exhibit cm-scale injection structures into the surrounding fault rock, indicating significant fault overpressures; (3) Fine-grained carbonates texturally similar to the low-Mg ultracataclasites are also found in 2-3 cm-wide damage zone veins that show little to no lateral offset, indicating the materials were comminuted elsewhere and mobilized to their current position. Clumped isotope measurements of these ultracataclastic veins and of the low-Mg ultracataclasites yield higher temperatures (~45 °C) than those measured in most other fault materials (10 – 25 °C). The low-Mg ultracataclasite isotopic values (𝛿¹³C_{carbonate-vpdb} = -3.5 ‰; calculated 𝛿¹⁸O_fluid-vsmow = -3.7 ‰) appear to characterize a fluid endmember present in many fault core and damage zone samples. The ultracataclastic veins seem to represent precipitation from a different fluid isolated to the damage zone (𝛿¹³C_{carbonate-vpdb} = +1.7 ‰; 𝛿¹⁸O_fluid-vsmow = +3.9 ‰). These results show that carbonate ultracataclasites preserve a record of fault fluid flow during overpressurization events.