Deep Crustal Evolution, Continental Deformation, and the Roots of Faults from Seismology and Xenoliths

Vera Schulte-Pelkum
Fri, April 27, 2018, 10:30am - 11:30am

Video Broadcast

Host: Thorsten Becker

Abstract:
I will present two studies that combine seismic and xenolith data to investigate continental assembly, evolution of deep crustal structure, and deformation along faults and their ductile roots of the North American continent.

The first study uses EarthScope and pre-existing active source seismic data and xenolith studies to map the distribution of high-velocity lower crust, indicating mafic or garnet-bearing material, across the U.S. and assess its relationship to proposed emplacement and loss mechanisms such as under- and intraplating, collision, extension, heating, cooling, hydration, and delamination. Thin layers of high-velocity crust related to regional processes are found scattered throughout the continent. Thicker layers in large areas are found in the central and eastern U.S. in areas with thick crust, bounded roughly by the Rocky Mountain Front, which cuts across Proterozoic assembly provinces. Hence, the modern north-south first-order contrast in structure may reflect garnet growth with aging of continental crust in much of the central and eastern U.S., while conditions in the western U.S. are unfavorable for growth and maintenance of thick layers of high-velocity garnet-bearing lower crust. We find areas with differences between the seismically defined Moho and the petrological crust-mantle boundary.

The second project combines laboratory data from xenoliths and anisotropic receiver functions to map fault zones and shear zones in the lithosphere. We developed a method to image contrasts in azimuthal anisotropy, dipping rock fabric, and dipping isotropic contrasts based on azimuthally varying conversions in receiver functions. Unlike shear wave splitting, azimuthally varying P to S conversions provide a large-amplitude, robustly observable signal even for small (few percent) contrasts in anisotropy in thin (few km) shear zones and provide depth resolution. Strikes from receiver functions typically align with surface fault traces in tectonically active regions, with depths of the converters extending into the ductile regime. Interpretation of observed seismic anisotropy requires knowledge of underlying symmetry systems, which are not well determined for the crust. We analyze a compilation of whole rock elasticity tensors from ultrasound and microstructural laboratory measurements. Our collection contradicts the commonly made assumption of elliptical hexagonal anisotropy; we observe that crustal anisotropy deviates from the elliptical case with increasing strength of anisotropy, which changes inferred anisotropy amplitudes and orientations in receiver function and surface wave studies. Our observations suggest that reactivation of inherited structures may play a significant role in present day deformation.