IMAGING PSEUDOTACHYLYTE FROM SOURCE TO SINK

Pseudotachylyte-bearing fault zones provide a wealth of information about the evolution of brittle fault zones during individual earthquakes. Extensive frictional melting requires slip rates that are unique to earthquakes. Rapid quench times of the relatively thin melt veins freeze in structural geometries soon after slip ceases. The geometry of pseudotachylyte veins thus reflects the physical and thermal conditions of the fault zone during and immediately after an earthquake.

We have combined a variety of methods in order to characterize the geometry of fault and injection veins from the Gole Larghe fault zone (Italy), an exhumed strike-slip fault zone in tonalite of the Adamello batholith. Field and photographic measurements characterize the geometry of veins exposed on outcrop faces at scales from 10^-3 to 10 m while high-resolution x-ray computed tomography (microXCT) characterizes 3D geometry of pseudotachylyte vein cores from 10^-5 to 10^-1m.

The microtopography of fault vein surfaces (contact between pseudotachylyte and wall rock, extracted from microXCT data) is highly correlated with wall rock mineralogy. Specifically, biotite is recessed up to 10^-4m below the average surface elevation, an effect that we attribute to preferential melting under disequilibrium conditions. The average relief of biotite varies between the two sides of the fault at a single location and is not obviously correlated with present structural position along the fault (i.e. extensional or contractional bends). The microtopography of fault veins thus likely reflects the integrated thermo-mechanical history of a given patch of wall rock.

In contrast, surfaces of injection veins show biotite in both positive and negative relief across a single vein, consistent with mechanical opening with little to no local melting. This observation provides support for models that use injection vein opening to estimate near fault stress and strength variations; however, other inelastic processes as well as fluid flow effects may also affect final injection vein geometries.