70-3 Connecting the Timing, Rate, Extent, and Elevation of the Lewis Thrust System with its Foreland Basin: Insights from a New Balanced Cross Section and Implications for Dynamic Topography

Abstract:

The North American Cordillera in Montana and southwest Canada retains significant elevation and relief 50 Ma after shortening ceased, and it represents one of the most comprehensively studied orogenic systems on earth. Despite this, longstanding questions about the Lewis thrust (LT), a major thrust sheet within the Cordillera, remain. In particular, the amount of shortening associated with the LT, the magnitude of sedimentation associated with that movement, the age the LT initiated, and its easternmost extent, are all unknown. Though the LT overlies 75 Ma sedimentary rocks, providing a younger age constraint, the timing of thrust initiation remains unconstrained. The magnitude of sedimentation associated with the Lewis and later thrusts is unknown due to significant erosion of foreland basin material. Additionally, these foreland basin rocks currently sit at over 1 km in elevation, despite originally being deposited at sea level. In order to address these questions, we present a new balanced cross-section which spans from the Washington-Idaho border to the Bearpaw Mountains in central Montana. This cross-section expands the section of Fuentes et al. (2012) 300 km westward of the Rocky Mountain trench, and increases the magnitude of shortening in the system from 136 to 300 km. Flexural-kinematic modeling links shortening along the LT to the deposition of synorogenic foreland basin sediments, beginning with the Kootenai Formation at 125 Ma, and ending with lower Wasatch Formation deposition at approximately 56 Ma. New cross section interpretations kinematically link a proposed duplex beneath the Plains Anticline in western Montana to thrusting in the Sawtooth Range. Modeling the system produces a significant thickness of foreland basin material that must be eroded in order to accurately reproduce the surface geology in the Sawtooth Range and the foreland basin. Models quantify a potential thickness range greater than 4–6 km in the foreland basin at the location of the Oxy Larson 1-1 well (25 km east of the deformation front), which indicates greater than 3–5 km of overlying foreland basin sediment has been removed. In the Sawtooth Range, model results suggest removal of at least 5–8 km of overburden. The magnitude and timing of this uplift and associated erosion, constrained by thermochronometric data, can be been used to quantify the influence of mantle-driven processes on the formation of present-day topography.

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