Predictive models for the deep geometry of a thick-skinned thrust matched to crustal structure: Wind River Range, western USA

The ability of models designed to use near-surface structural information to predict the deep geometry of a faulted block is tested for a thick-skinned thrust by matching the surface geometry to the crustal structure beneath the Wind River Range, Wyoming, USA. The Wind River Range is an ∼100-km-wide, thick-skinned rotated basement block bounded on one side by a high-angle reverse fault. The availability of a deep seismic-reflection profile and a detailed crustal impedance profile based on teleseismic receiver-function analysis makes this location ideal for testing techniques used to predict the deep fault geometry from shallow data. The techniques applied are the kinematic models for a circular-arc fault, oblique simple-shear fault, shear fault-bend fold, and model-independent excess area balancing. All the kinematic models imply that the deformation cannot be exclusively rigid-body rotation but rather require distributed deformation throughout some or all of the basement. Both the circular-arc model and the oblique-shear models give nearly the same best fit to the master fault geometry. The predicted lower detachment matches a potential crustal detachment zone at 31 km subsea. The thrust ramp is located close to where this zone dies out to the southwest. The circular-arc model implies that the penetrative deformation could be focused at the trailing edge of the basement block rather than being distributed uniformly throughout and thus helps to explain the line of second-order anticlines along the trailing edge of the Wind River block. Key points: (1) The circular-arc fault model and the oblique-shear model predict a lower detachment for the Wind River rotated block to be ∼31 km subsea, consistent with the crustal structure as defined by teleseismic receiver-function analysis. The thrust ramp starts where this zone dies out. (2) The kinematic models require distributed internal deformation within the basement block, probably concentrated at the trailing edge. (3) The uplift at the trailing edge of the rotated block is explained by the circular-arc kinematic model as a requirement to maintain area balance of a mostly rigid block above a horizontal detachment; the oblique-shear model can explain the uplift as caused by displacement on a dipping detachment.

Geomorphic regulation of floodplain soil organic carbon concentration in watersheds of the Rocky and Cascade Mountains, USA

Mountain rivers have shown the potential for high organic carbon (OC) storage in terms of retaining OC-rich soil. We characterize valley bottom morphology, floodplain soil, and vegetation in two disparate mountain river basins: the Middle Fork Snoqualmie, in the Cascade Mountains, and the Big Sandy, in the Wind River Range of the Rocky Mountains. We use this dataset to examine variability in OC concentration between these basins as well as within them, at multiple spatial scales. We find that although there are some differences between basins, much of the variability in OC concentration is due to local factors, such as soil moisture and valley bottom geometry. From this, we conclude that local factors likely play a dominant role in regulating OC concentration in valley bottoms, and that inter-basin trends in climate or vegetation characteristics may not translate directly to trends in OC storage. We also use analysis of OC concentration and soil texture by depth to infer that OC is input to floodplain soils mainly by decaying vegetation, not overbank deposition of fine, OC-bearing sediment. Geomorphology and hydrology play strong roles in determining the spatial distribution of soil OC in mountain river corridors.