What is the geometry of faults and shear zones that accommodate continental extension? This is a simple, yet unresolved question with important geodynamic implications about the nature of extensional processes. In this seismological experiment, the PIs will deploy very densely spaced EarthScope FlexArray stations across an active normal fault to construct a receiver function image that traces the fault through the entire crust.
Geologic mapping and active-source reflection seismics indicate the presence of different modes of upper crustal extension that include high-angle planar faults, listric faults and low-angle detachment faults. In pure shear extension, faults are rooted in the mid-crust at subhorizontal decollements that mark the transition from localized brittle deformation in the upper crust to pervasive ductile flow in the lower crust and often faults are assumed to curve into them. The discovery of low-angle detachment faults that brought metamorphic mid-crustal rocks to the surface led to the very different simple shear model where extension in the lower crust and upper mantle occurs along narrow ductile shear zones. Slip along shallow dipping surfaces, however, has not been observed for large earthquakes, which indicate planar, steeply dipping faults. Some large earthquakes seem to rupture into the lower crust. This is intriguing because rupture into the lower crust likely leaves a structural imprint. However, high-resolution (high-frequency) reflection seismic surveys, which provide the most detailed images of fault geometry, have only penetrated into the uppermost crust. This may be due to loss of energy from random scatterers or that the fault zone is not sharp enough to be imaged by high-frequency waves.
The centerpiece of this project is a one-year deployment of a linear array of 33 three-component seismometers across an active, large-throw normal fault -- the Pleasant Valley, NV, fault that ruptured in 1915 in a magnitude 7-7.5 earthquake. The use of small station spacing and lower frequency energy from natural sources will allow the PIs to produce a receiver function image showing the fault geometry across the entire crust and address basic questions about continental extension: 1) are faults in the brittle upper crust planar or listric, 2) do faults extend into the lower crust as localized shear zones or do they root in a mid-crustal decollement, and 3) is crustal-scale faulting deflecting the Moho and affecting the upper mantle?
Resolving the geometry of continental normal faults is a globally relevant question with far reaching scientific and societal consequences. Crustal-scale fault geometry is important to construct realistic geodynamic models for continental extension. Fault geometry strongly affects earthquake hazard and is particularly relevant for the densely populated eastern and western Basin and Range boundaries (Salt Lake City, Reno) and globally for places like Italy and the entire Aegean region.
