Research
Architecture and Rheology of the Deep Seismogenic Zone
Geological observations of the architecture and rheology of continental transform faults provides critical information about the physical processes and mechanical properties of the seismogenic zone, which aid in the interpretation of seismological and geodetic data, and tests the applicability of experimental rock deformation laws to naturally deformed materials. The goals of this project are to collect detailed observations from outcrop to microscopic scales to characterize the geological processes that control seismic and aseismic slip modes in continental transform faults. Geological observations of this nature fill a gap in our understanding of the processes and mechanisms that accommodate transient aseismic slip detected by geophysical methods, which improves our understanding of factors contributing to the seismic potential of active faults. These observations also inform us of the rheology of lithospheric fault zones and the mechanisms that localize strain in the crust.
Diagram showing the relationships between geological and geophysical obsertations of slip modes along continental transform faults.
Pseudotachylyte - earthquake-generated melt - within an ultramylonitic shear zone.
Fault System and Shear Zone Geochronology
The timing of crustal deformation is primarily determined through geochronology of deformed, cross-cutting or overlying rock units, however such constraints are relative and do not directly date deformation. During ductile deformation, chronometrically important accessory phase minerals (i.e., apatite and titanite) may experience crystal plastic strain, which may generate fast diffusion pathways for radiogenic Pb to escape the crystal lattice. This process effectively resets the U-Pb clock at the timing of deformation. I am exploring the occurrence of this phenomena and its applicability to directly date ductile deformation using an integrated microstructural (EBSD) and petrochronological approach (LA-ICP-MS U-Pb + TE). Check out Odlum, Levy et al. (2022) to see some of our recent findings.
BSE image of deformed apatite grain within recrystallized pseudotachylyte.
EBSD Mis2Mean map showing dislocation microstructure within apatite.
Deformation-Metamorphism Feedbacks during Orogenic Collapse
Orogenic collapse is a fundamental part of the orogenic cycle, and contributes to the thermal and mechanical structure of the lithosphere, and the differentiation of continental crust. Scientists have long debated the mechanisms by which deep crust is exhumed during collapse, especially in the North American Cordillera, where many orogenic collapse models were first developed. I have worked to elucidate the evolution from orogenic construction to collapse in the US Cordillera through integration of geological mapping, macro- and microstructural analysis, and petrochonology. Through this detailed analysis I have found metamorphism exerted a first-order control on the development of mid-crustal shear zones during the Eocene-Oligocene ignimbrite flare-up. These mid-crustal fabrics were captured by normal faults during widespread Middle Miocene Basin and Range extension leading to exhumation of deep crustal rocks in the metamorphic core complexes. These results highlight the important of (de)coupling between the upper and lower crust during orogenic collapse, and the influence of mid-crustal fabrics on the kinematics of upper crustal deformation. See Levy et al. (2023) to see the full story.
Schematic cross sections showing (A) Eocene–Oligocene and (B) Miocene development of the Ruby Mountains–East Humboldt Range in the North American Cordillera.