Structure of the Southern Canadian Rocky Mountain Foreland and Western San Joaquin Basin Fold-and-Thrust Belts

Abstract

This thesis investigates the geometry and kinematic evolution of fold-and-thrust belts from tectonic to human timescales in the Front Ranges of the Southern Canadian Rocky Mountains and the South-Central San Joaquin Valley of California. Findings advance the field’s capabilities for developing internally consistent 2D and 3D models of both ancient and active fold and thrust belts and have important implications for earthquake hazards in California. In chapter one, we investigate the geometry and kinematic evolution of the Southern Front Ranges of the Canadian Rockies, and orogen that formed over tens of millions of years associated with subduction of the Pacific Plate and accretion of tectonic terrains to western North America. Due to its spectacular field exposures, this mountain belt has long served as a prototypical example of a thin-skinned fold-and-thrust belt. Nevertheless, many questions remain unanswered regarding its detailed structure and kinematic evolution. In conjunction with 10 m and 1 m/pixel digital elevation models, high-resolution satellite imagery enables unparalleled quantitative definition of the fold geometry and bedding-fault relationships in the Front Ranges. Using these fundamental quantitative relationships, we develop a series of new, kinematically viable cross sections and 3D models that both reinforce, and in some cases overturn, previous paradigms about this fold and thrust belt. Traditionally, the region is thought to contain a single basal detachment and develop through a break forward sequence of thrusting. Our analysis shows that this fold and thrust belt is underlain by multiple basal detachments and developed through a complex sequence of thrusting that fundamentally controls the map patterns expressed within the Front Ranges. The methods we develop for remote data collection and analysis can be applied to develop and refine interpretations of fold and thrust belts worldwide. Chapters two and three investigate the geometry and seismic hazards associated with Central California's active Southern San Joaquin Fold and Thrust Belt. In Chapter 2, we develop an internally consistent structural model for the system of blind-thrust faults and their overlying, en-echelon anticlines. Portions of these faults generated a series of Mw 5.5-6.5 earthquakes in the 1980s, yet the activity and earthquake potential of along-strike and down dip segments of the system remain uncertain. Our model reveals that the fold changes along strike from a structural wedge to a composite growth fault-bend fold. The wedge locally consumes much of the slip on the underlying blind thrust fault, whereas the fault-bend folds send slip eastward into the basin on an upper detachment surface. Consistent with this, we observe distinct variations in the eastern extent of deformation in the basin. Specifically, slip into the basin on the upper detachment is manifest in both fault-related folds and additional thrust ramps. To the west and at depth, these fault segments sole to two primary basal detachment levels that extend beneath the Temblor Ranges. In chapter three, we couple the structural model from chapter two with groundwater wells to determine the activity of various segments of the Southern San Joaquin Fold and Thrust Belt. We find that sediments of late Pleistocene age are folded through active axial surfaces above each of the major blind-thrust segments. Our observation indicates that the entire system of ramps and detachments in this fold and thrust belt are active. Based on the size of the fault system, it is capable of generating Mw ≈7.7 earthquakes. We additionally identify new active thrust faults in the basin that have the potential to generate damaging earthquakes.