| Fault zone structure over a wide range of scales strongly influences earthquake mechanics. In
this work we quantify the hierarchical structure, fracture network characteristics, and velocity
structure of the seismic Gole Larghe Fault Zone (GLFZ) in the Italian Alps using a range of
digital fieldwork techniques and experimental facilities. The GLFZ is c.500m thick and
accommodates a total displacement of c.1000m, distributed amongst hundreds of first- and
second-order oblique-slip cataclasite- and pseudotachylyte-bearing faults. The GLFZ was
exhumed from 8-10km depth, nucleated in an area of pre-existing joints in the Adamello
batholith, and records thousands of seismic ruptures. The continuous, glacier-polished
nature of the exposures allows systematic data and sample collection at scales of
centimetres to kilometres. Main results to date are: 1) Joints outside the GLFZ formed
predominantly at temperatures >500Ë C during cooling of the pluton, whilst cataclasite- and
pseudotachylyte-bearing fault strands within the GLFZ were active at 200-300Ë C. The
transition from jointed ’wall rock’ to ’fault zone’ is marked by an abrupt increase in
macroscopic fracture density; 2) Second-order faults inside the GLFZ are strongly
clustered around first-order faults. However, in around 70% of cases, second-order
faults are asymmetrically distributed on the northern side of first-order faults. This
damage asymmetry is not explained by lithological variation, but may reflect the fact
that propagating earthquake ruptures preferentially follow one of the boundaries
between a pre-existing joint cluster and relatively intact host rock; 3) P-wave velocities
measured on 37 samples collected along a c.300m profile across the wall rock and fault
zone increase from 3500-4500m/s in the wall rock to 5000-6000m/s inside the
fault zone. Preliminary microstructural observations indicate that the increase in
P-wave-velocities probably correlates with an increase in microfracture density.
Microfractures are often sealed with epidote and K-feldspar, which may explain the
observed velocity increase. The above field observations suggest that the GLFZ is
markedly different from other seismic fault zones, where fracture density increases
exponentially towards the fault zone. The relationship between P-wave-velocities and
microfractures also highlights the importance of fluid-rock interaction processes during the
seismic cycle, resulting in induration and effective healing processes due to fluid flow. |