Bridging earthquakes and mountain ranges in the Santa Cruz Mountains restraining bend with mechanical modeling, geologic reconstructions, and thermochronology

Deformation in Earth's crust accumulates during and in between earthquakes to build Earth's mountain ranges and to produce signatures of geologic deformation preserved in the rock record. As this deformation accumulates through time, rheological properties imparted by the protracted geologic history of Earth's deforming crust control the resultant distribution and magnitude of rock uplift, exhumation, and erosion that conspire to shape the morphology of Earth's surface. This dissertation investigates how the rheological properties of Earth's crust influence the accumulation of this deformation through time, and, conversely, how expressions of time-integrated deformation of Earth's crust may reveal insight into the operant rheological and geophysical properties that are difficult to measure in-situ. To address these questions, I quantify and model deformation, rock uplift, and exhumation surrounding restraining bends in strike-slip fault systems. Deformation that occurs during and in between individual earthquakes is dominantly elastic, but Earth's mountain ranges host geologic structural features, such as faults and folds, that demonstrably record the accrual of inelastic deformation over the course of millions of years. This apparent rheological discrepancy highlights a persistent major challenge in the Earth Science community, which seeks to clarify the connection between individual earthquake cycles and the mountains that they build. Chapter 1 directly addresses this long-standing problem by linking and unifying observations of deformation that span timescales ranging from decades to millions of years. In this contribution, my co-authors and I created a coupled tectono-geomorphic model that predicts rock uplift, exhumation, topographic relief, erosion rates, and horizontal surface velocities surrounding the Santa Cruz Mountains restraining bend (the SCM bend) in the San Andreas fault, near San Francisco, CA, USA. Chapter 1 shows that incremental irrecoverable deformation incurred during dominantly elastic earthquake cycles accumulates to produce the inelastic deformation we observe in the rock record. Results suggest that, during an individual earthquake cycle, the majority of inelastic deformation occurs in between major earthquakes along the San Andreas fault, as opposed to during the earthquakes themselves. The tectonic-geomorphic models in Chapter 1 generally reproduce records of rock uplift, exhumation, and erosion in the Santa Cruz Mountains (SCM), but the distribution of these quantities in the natural SCM setting is far more complex than that captured in these models. In Chapter 2, my co-authors and I combine low-temperature apatite (U-Th)/He thermochronology with 3D geologic reconstructions to quantify the distribution of rock uplift and exhumation throughout the SCM southwest of the San Andreas fault. Results suggest that deformation and uplift have preferentially accumulated in a relatively weak lithotectonic terrane embedded within a complex and heterogeneous transform plate boundary. Chapter 2 shows that the protracted geologic history and resultant lithostratigraphic structure of the crust influence the localization of strain and uplift along the plate boundary as deformation accumulates. Chapters 1 and 2 illustrate that SCM-site-specific distributions of rock uplift and exhumation provide insight into the rheological properties of the crust surrounding the SCM bend. In Chapter 3, I utilize these diagnostic metrics, and use a suite of generalized restraining bend models to infer operant fault frictional strength for restraining bends around the world based on inferred distributions of rock uplift and exhumation in the natural settings. Model results show that deformed and uplifted crust advects into and through restraining bends when fault frictional strength is low, and impounds upwind of the restraining bend when fault frictional strength is high. These results also suggest that the propagation of strike-slip faults from the tips of restraining bends also appears indicative of moderate to high fault frictional strength. This chapter illustrates that geologic observations of deformation may be integral to constraining geophysical parameters, like the frictional strength of faults, that may be challenging to measure directly

Keywords: Geological and Environmental Sciences.
Advisor: Hilley George E.
Graham S. A. (Stephan Alan)
Shuster David L.