Bedrock and Surficial Geologic Map of the Red Rock 7.5’ Quadrangle, Beaverhead County, Southwestern Montana

The Red Rock 7.5 minute quadrangle, located in Beaverhead County, southwestern Montana, spans the Red Rock River Valley, an extensional graben formed between the Tendoy mountain front and the western flank of the Blacktail-Snowcrest uplift (Fig. 1). Notable landmarks within the quadrangle include the Clark Canyon Reservoir (Bureau of Reclamation dam number MT00569) located in the northwest area of the quadrangle and Interstate 15 which runs northwest-southeast through the quadrangle. The highest elevations in the map area are located within the Tendoy Mountains and the Red Rock Hills and are underlain by Paleozoic and Cenozoic bedrock. From these points, broad alluvial fans grade down to the Red Rock River Valley. The quadrangle contains about 3,000 ft of relief. Mapping of the Red Rock quadrangle was done at a scale of 1:12,000 and was compiled at a scale of 1:24,000. Field work was completed in the summer of 2005 in collaboration with the mapping of the adjacent Briggs Ranch and Kidd quadrangles (Figs. 1 and 2). This strategy allowed for the comparison of structure and stratigraphy across quadrangle boundaries and provided a regional context for the mapping of each quadrangle. This new mapping complements previous mapping of the Monument Hill quadrangle (Newton and others, 2005), Dixon Mountain quadrangle (Harkins and others, 2004b), Caboose Canyon quadrangle (Harkins and others, 2004a), and Dell quadrangle (Aschoff and Schmitt, 2005) and collectively provides new detailed mapping and analysis of a portion of the Red Rock River Valley from Lima to the Clark Canyon Dam (Figs. 1 and 2). This report includes a map and cross section for the Red Rock quadrangle as well as a discussion of the stratigraphy and structure of the map area

Bathymetric Signatures of Submarine Forearc Deformation: A Case Study in the Nankai Accretionary Prism

Abstract Large earthquakes and tsunamis in subduction zone forearcs occur via slip on the shallow plate boundary and upper plate faults, but the locations, geometries, and slip histories of these faults can be difficult to constrain in regions with minimal subsurface geophysical and stratigraphic data. Here, we test a new approach to quantify the submarine seafloor geomorphic response to forearc deformation in order to identify structures that contribute to active deformation, to interpret their geometry and kinematics, and to evaluate their relative rates, magnitudes, and timing of deformation. We develop a workflow that uses filtered bathymetric digital elevation models, where long wavelength topography has been removed, to isolate the slope, relief, curvature, ridgelines, and trough lines associated with faults, fault‐related folds, and slope failures. We apply these methods to the Kumano region of the Nankai accretionary prism, southeastern Japan, where existing constraints on fault geometry, kinematics, and deformation history allow us to both evaluate the efficacy of our approach and to identify the lateral continuity of deformation processes. Our bathymetric analyses yield a high‐resolution tectono‐geomorphic map of active structures and reveal along strike variations in strain accumulation and out‐of‐sequence deformation. These metrics also demonstrate the importance of a strike‐slip fault system at the seaward edge of the Kumano Basin as a primary structure that accommodates deformation and partitions strain in the Nankai forearc. These results show the utility of using a submarine tectono‐geomorphic approach to evaluate active deformation in forearcs, particularly in regions with limited geophysical and core data