Microstructure and frictional properties of sheared calcite speleothems: natural vs. experimental investigation

Abstract:

Several alpine caves in Austria preserve evidences related to active faulting, such as broken and scratched speleothems. Here, in order to better understand fault slip behavior and related potential earthquake hazards, microstructures of experimentally deformed speleothems are presented and compared with naturally deformed ones.

Speleothems are monomineralitic rocks precipitated in caves, composed of columnar centimeter-scale calcite crystals with strong growth orientation. In order to better study the origin and evolution of deformation in faulted speleothems we performed sliding experiments using a rock deformation biaxial apparatus. In order to recreate the faulting conditions observed in Austrian caves, speleothems were cut into rectangular blocks and sheared against each other, with long growth axes of calcite perpendicular to the shearing direction. The experiments were performed under room conditions, sliding velocity in the range of 0.001-0.01 mm/s, and constant effective normal stress of 3 MPa.

The mechanical data show fairly high friction coefficient (0.7-0.95) accompanied by the production of calcite-rich fault gouge which displays Riedel shears within a foliated cataclasite and drastic grain size reduction (nano-scale). The transition from the fault gouge towards the undeformed crystals is characterized first by a series of in situ jigsaw puzzle fracturing, then dense mechanical twin network, which is decreasing in its intensity away from the gouge (i.e. principal slip surface).

The similarity between laboratory induced and naturally formed microstructures reinforce the tectonic interpretation of the damaged speleothems. Detailed microstructure investigations, including electron backscattered diffraction technique combined with electron microprobe and cathodoluminescence, are on the way to help distinguishing between seismic slip and/or aseismic creep.