A concerted effort through EarthScope is currently underway to better understand seismicity along the San Andreas Fault, which poses serious threats to society from large-magnitude earthquakes. The San Andreas Fault Observatory at Depth features an instrumented drill hole, allowing characterization of the rocks, fluids, kinematics, and stress orientations and magnitudes within the fault zone to depths of approximately 3 km. These studies are designed partly to investigate the strength of the San Andreas Fault, which is critical for seismic forecasting. In conjunction with these studies it is important that we locate and study appropriate exhumed examples of similar faults at the surface today. One of the very few surface occurrences of an analog for the San Andreas Fault is the Norumbega Fault System (NFS), which cuts across the entire State of Maine and is the field site for this study. Through the NFS study we hope to clarify the value of near-surface stress measurements on the San Andreas Fault as indicators for the strength of the fault. This work is intended to support the dissertation of a female PhD student, and several undergraduate student theses. Our close cooperation with the Maine Geological Survey, and our involvement with K-12 Earth Science teachers in Maine, lends additional societal relevance to the work.
Due to the large elastic moduli of solid Earth materials elastic stresses are transmitted over great distances and depths within the Earth. For this reason, the stress orientations and magnitudes measured near Earths surface provide only a partial picture of the state of stress on active faults. For a more complete picture, we must understand the state of stress on faults at greater depths, particularly near the frictional-to-viscous transition where the strongest part of the crust occurs. For this reason we are focusing on the deeply exhumed NFS. To provide relatively tight constraints on the stress tensor the field locality should contain irrefutable evidence for coseismic rupture (pseudotachylyte), and be characterized by: (a) tightly constrained kinematic boundary conditions, (b) microstructures amenable to estimates of differential stress and mean kinematic vorticity, and (c) lithological (rheological) heterogeneity providing natural variability in the stress and vorticity estimates. In such a system, 3D numerical experiments can provide best-fit solutions for the field-derived stress and vorticity estimates, and in doing so solve for the principal and Cartesian stresses. Our work in the NFS involves field mapping, microstructural analysis using optical and electron-beam techniques, and 3D numerical modeling that is tightly constrained by the field-derived data. Our primary aims are to: (a) systematically evaluate the microstructural variation across the strain gradient in the fault zone mylonites, (b) use microstructural measurements to calculate temperatures, differential stresses and mean kinematic vorticity numbers for numerous samples across the field area, (c) use these calculated values and 3D numerical experiments to help constrain the stress tensor at the time when this mylonite zone was overlain by a seismically active fault, and (d) extend our numerical modeling to the surface to explore how kinematic boundary conditions and stresses at depth influence active faults above.
Major earthquakes along tectonic plate boundaries originate along faults many kilometers beneath the Earth’s surface. Direct observations of these earthquake source regions along active faults (e.g., the San Andreas fault system in California) are impossible due to their great depths. However, ancient fault systems that have been deeply eroded over millions of years of geologic time provide opportunities to study the inaccessible roots of currently active fault systems. One such example of a deeply eroded fault system is the Norumbega fault system in south-central Maine. The rocks currently exposed at the surface along this ancient fault system were once deep beneath an active system of faults comparable in scale to the present day San Andreas. Studies of these ancient deeply eroded rocks thus provide an opportunity to make direct observations of earthquake source regions. Information obtained from detailed studies of this fossilized fault system provides a unique opportunity to understand processes currently occurring at earthquake source depths along active fault systems. This study is focused on locating and studying a relatively rare fault rock type called pseudotachylyte. This rock forms during high magnitude earthquake events when frictional heat along the fault melts the surrounding rocks. The pseudotachylyte is essentially solidified frictional melt that formed along an ancient fault at the earthquake source region. It typically appears as very fine-grained black veins within otherwise normal looking host rocks (see attached figures). Detailed studies of this pseudotachylyte along the presently inactive Norumbega fault system in Maine can help with the understanding of processes occurring deep beneath active fault zones such as the San Andreas in California. Work associated with this study has involved field mapping of the spatial distribution of pseudotachylyte-bearing faults in south-central Maine. Three separate ancient inactive faults containing pseudotachylyte were identified and numerous samples were collected for more detailed laboratory studies. These laboratory studies involved detailed microscopic observations, geochemical studies, and geochronology (dating the formation of the pseudotachylyte and thus the age of the earthquake activity that produced the frictional melts). Two undergraduate students at Middlebury College were involved in these studies and the work formed the basis of their undergraduate theses. Our detailed studies of pseudotachylyte, in conjunction with partnered colleagues at the University of Maine, have revealed the following: (1) Previously formed pseudotachylyte can be significantly modified by later deformation along the fault zone and transformed into thin layers of another fault rock called ultramylonite. This ultramylonite can be found in great abundance along one of our faults and we speculate it may have weaken the deeper levels of the overlying fault zone and altered the temporal and spatial distribution of later seismic activity. (2) Along one of the faults the pseudotachylyte is not altered by later events and likely represents the end of seismogenic activity along this portion of the Norumbega fault system. Pseudotachylyte geochronology ("age dating") suggests this occurred around 240 million years ago in south-central Maine. The older modified pseudotachylyte likely formed more than a hundred million years earlier – suggesting this was a very long lived fault system. (3) Another portion of the fault system mapped during this project contains little to no pseudotachylyte. It is possible that this finding indicates more aseismic creep (ductile flow) along this portion of the fault system. This relatively "continuous flow" without frictional melting likely reflects differences in the strength of the rocks in this particular region.
