FAST AND FURIOUS ULTRACATACLASITES AND PSEUDOTACHYLYTES FROM CATASTROPHIC LANDSLIDES

Large-scale, catastrophic landslides constitute one of the most extreme cases of high-velocity and high strain rate deformation of rocks. A wide range of fault rocks form during these exceptional mass wasting events, including cataclasites, breccia, ultracataclasite and even pseudotachylytes (frictional melts). They also share some petrographic similarities and complexity with impact-generated cataclastic rocks. The deformation mechanisms involved in these materials attest of conditions ranging from low-temperature hypervelocity deformation, fluidization, vaporization, to high-temperature frictional melting.

The kinematic analysis of these rocks is, in principle, simplified because the transport direction (downward) and sense of shear (normal) are perfectly known. Thus these rocks offer exceptional opportunities to develop new structural tools that could be applied to other settings where the kinematics is unknown.

Recent investigations on three landslide pseudotachylytes (Tsergo Ri, Nepal; Markagunt, Utah; and Köfels, Austria) show that frictional melts formed as a result of high velocity, gravity-driven slides. The magnetic fabric (anisotropy of magnetic susceptibility or AMS) of these materials shows remarkable consistency at the scale of a a few centimeters. The magnetic properties of the pseudotachylytes suggest that the ferromagnetic carriers formed through the breakdown of ferromagnesian minerals in the host rock. These ferromagnetic minerals underwent plastic deformation at high temperature and acquired a shape preferred orientation under viscous flow conditions in a silicate melt. The AMS therefore reflects the frozen downward direction of slip during the gravity slide. The obliquity of the AMS fabric with respect of the sliding surface further indicates a normal sense of shear. The oblate symmetry of the magnetic fabric is consistent with a dominantly simple shear deformation mechanism with low vorticity.

Further investigations of the AMS symmetry and vorticity will be needed to evaluate how the specific characteristics of a gravity slide (e.g., normal stress, velocity) affect magnetic fabric parameters and fabric consistency.