TRANSITION FROM CATACLASITES TO PSEUDOTACHYLYTES AND CONSEQUENCES ON THE MESOSCOPIC STRUCTURE OF A “STRONG-TYPE” SEISMOGENIC FAULT

Mature major faults are characterized by high values of displacement/fault thickness resulting by localized repeated slip along the same weakness horizon. A contrasting end-member is a fault where successive slip increments migrate to different discrete shear zone thus resulting in a fault zone with a low displacement/fault thickness ratio (strong fault?). An example of this second type of fault is studied in this work. The Gole Larghe Fault Zone is an exhumed paleoseismic source crosscutting the Adamello tonalites (Italian Southern Alps). Fault rocks are an association of indurated cataclasites and pseudotachylytes produced at 6-8 km depth and 250-300 ^oC. At the base of the Lobbia Glacier, in a 1 km² glacially polished outcrop, the Fault Zone accommodates 1000 m of dextral strike slip displacement over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of cataclastic horizons (fault_1-3). Faults_1-2 nucleated on a pre-existing set of pervasive cooling joints. Faults₃ form a network of minor shear fractures newly produced during slip on faults_1-2. Within each cataclastic horizon pseudotachylytes usually occur at the host rock/cataclasite boundary and mark the last deformation event. In cataclasites, approaching pseudotachylytes: i) the fractal number of the clast size distribution increases, and ii) the mean clast size decreases. The matrix of cataclasites contains abundant K-feldspar+epidote and minor chlorite. Pseudotachylytes and cataclasites have a similar chemical composition and are enriched in Loss On Ignition, K, Rb, Ba, U and Fe₂O₃ compared to host rock suggesting fluid-rock interaction during deformation. Faults_1-2 accommodate seismically a relevant part of slip and single faults within the Fault Zone were not active at the same time. Migration of slip to new deformation horizons is related to the induration of fault rocks during slip by precipitation of K-feldspar+epidote. Progressive cementation of the fault rock assemblage enhanced elastic loading in the adjacent host rock, which lastly overcome the fault rock strength leading to seismic sliding and friction-induced melting. Pseudotachylytes welded the active fault and displacement migrated to nearby joints, thickening the fault zone.