Characterizing Deformation in Fold-and-Thrust Belts over Tectonic to Human Timescales by Leveraging Dense Field, Geophysical, and Seismological Datasets: Results from Case-Studies in the Canadian Rockies and San Joaquin Valley, California

Robert Welch, Loyola Marymount University
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Abstract

Throughout much of the 19th and 20th centuries, our understanding of fold-and-thrust belts was derived from classic field locations. Field work in these locations allowed us to better understand the geometry and development of fold-and-thrust belts over tectonic-scale timescales and served as the basis for methods of balanced cross-section construction. The seismic reflection method and quantitative fault-related folding theories further advanced these studies, extending them to tectonically active fold-and-thrust belts where modern seismology and other methods provide insights into their behavior over human timescales. My research seeks to span these approaches by developing methods that increase the amount of quantitative information we gather from field exposures, which can be directly integrated with modern geophysical constraints to develop more comprehensive understandings of these settings and the processes that govern them. To illustrate this approach, we establish a 3D geometric and kinematic description of the Front Ranges of the Canadian Rockies and the South-Central San Joaquin Valley of California to evaluate and expand upon fundamental paradigms that define our understanding of thin-skinned fold-and-thrust belts.

Our observations and analysis support many of the fundamental conclusions of previous work. However, in the Canadian Rockies, our analysis allows us to quantitatively define zones with a pervasive, multimodal distribution of bed dips. These are separated by axial surfaces or hinge zones in the backlimbs of thrust sheets, rather than the previous hypothesis of listric backlimb geometries. These observations, coupled with the geologic map patterns, note the presence of multiple regional basal detachments over a ~950 m zone. They also identify out-of-sequence thrusting throughout the region.

In the San Joaquin Valley, in an active fold-and-thrust belt, we combine our surface observations with dense subsurface geophysical datasets and seismicity, including an ongoing sequence of moderate-magnitude earthquakes. Together, these data illuminate the major fault ramps and detachments, showing consistency in the broad geometry of the basal and upper detachments. However, structural style changes notably along strike. The thrust ramps that comprise this system ruptured in a series of earthquakes in the 1980's. Our analysis across the length of this fold-and-thrust belt provides evidence of late Pleistocene deformation over a much larger area, including detachments and additional thrust ramps that support a much larger and more complex series of potential earthquake sources.

These insights, together with our semi-automated geologic data collection methods, have applications for investigating ancient and active fold-and-thrust belts worldwide.

Session

Deformation in the upper crust