A Comparison Between Modelling of Coulomb Stress and Field Observations of Off-Fault Strain around Pseudotachylyte Fault Veins, Norumbega Fault System, southern Maine
Catherine Ross, McGill University
Christie Rowe, McGill University
Mark Swanson, University of Southern Maine
Static stress changes caused by fault motion may be of significant magnitude around fault bends, ends, and intersections, and have been shown to partially explain aftershock distributions aftershocks (Poliakov et al., 2002). In the brittle-ductile transition zone, these stress concentrations may be relaxed after earthquakes by ductile flow. Small-scale deformation features adjacent to pseudotachylyte-filled fault veins may record deformation in response to static stress changes in the wallrock caused by slip on non-planar or intersecting fault surfaces.
Starting from an existing pseudotachylyte map of the Fort Foster Brittle Zone in Kittery, Maine (Swanson, 2013) we mapped the deformation structures in detail at several selected sites of the outcrop surface. High-resolution photographs and field measurements were taken where pseudotachylyte fault veins bend and there was associated near-fault small-scale deformation features. A ~3 m2 area of outcrop was selected for Coulomb3 stress modelling. It was observed that the general deformation pattern between two pseudotachylyte veins was characterized by a simple pattern: when one fault bends away from the other, the deformation in between the two is characterized by mm- to cm-scale pseudotachylyte injection veins; where the faults are parallel to each other, the deformation style is characterized by ductile features such as tight isoclinalkink and bend drag folds.
The pseudotachylyte photo-mosaic was then used as a basemap in Coulomb3 software in order to build an idealized fault model. The rake and slip of the earthquakes which formed the pseudotachylytes are not known, so we assumed that the shear zone is primarily dextral, and we use the average scaling of normal earthquakes to make estimates of the slip. The predicted stress change orientation and magnitude distribution was were produced by Coulomb3. We compare verify the stress change distributions from the models to the distribution of small near-fault structures mapped in the field. The distribution of stresses are similar to the distribution of shortening and extensional strain measured in the wallrock, therefore we conclude that static stress changes were accommodated plastically in the compressional region where the two faults propagated parallel to each other, and in a brittle manner in the extensional region behind the rupture tip in the post-seismic time interval. This suggests that damage occurs over the entire fault by more than just the propagating tips and also suggests that the type of deformation is heavily influenced by the geometry and heat diffusion of the pseudotachylytes.