The Damage Structure of the Gole Larghe Fault Zone Inferred From Laboratory Measurements and Modeling of Physical and Hydraulic Properties

Abstract:

The Gole Larghe Fault Zone is an exhumed ancient fault zone located in the tonalitic Adamello batholith in the southern Italian Alps. The presence of pseudotachylytes (solidified melt created during rapid frictional sliding) shows that it has been seismically active before exhumation. Previous studies of meso- and microscale fracture densities and orientations showed that the damage structure of the fault zone is highly symmetric with respect to the fault core and that several sub-zones can be distinguished. These structural data are now linked to laboratory measurements of ultrasonic wave velocities and permeabilities on samples taken along a 1.3 km long transect perpendicular to the fault strike to further characterize its damage structure, fluid flow properties, and sealing history. Additionally, we aim to link the various physical properties of the damaged rocks by modeling crack densities, orientations and aspect ratios.

Laboratory ultrasonic wave velocities and permeabilities correlate systematically with the microfracture density across the Gole Larghe Fault Zone: P-wave velocity is high in the wall rocks, where microfracture density and permeability are low, and low in the damage zone, where microfracture density and permeability are high. In the core zone, microfractures were pervasively sealed by syntectonic mineral precipitation (mainly epidote and K-feldspar), leading to high P-wave velocities and low permeability. The alteration by sealing poses a limitation to the application of the modeled properties: as the model treats rocks with sealed microfractures as intact, the physical properties might be consistently modeled, but they lose their relation to the initial damage state.