INSIDE A WRINKLE RIDGE: STRUCTURAL INVESTIGATION OF AN EARTH ANALOGUE IN THE YAKIMA FOLD AND THRUST BELT, WA

Thrust fault-related landforms are observed on many solar system bodies, including Earth. These landforms exhibit a wide range of planforms and morphologies, from linear map patterns and asymmetric topography to sinuous, segmented map patterns showing alternating asymmetry of ridges. Among such landforms, so-called wrinkle ridges are the most morphologically complex. Thus, understanding the three-dimensional structural complexity that produces such map patterns and morphologies is important for developing kinematic models of uplift and for deducing the strain accommodated by the developing structure. Various techniques like elastic dislocation modeling, observing structural exposures in planetary imagery, and comparing thrust fault-related landforms to Earth analogues have been used to investigate the underlying structural geometry of the ridges. However, studies of Earth analogues, identified by asymmetric profiles and parallel alignment of ridges, have been limited to the presence of surface breaking thrust faults and the dip of those faults at outcrops, leaving the complicated subsurface structure unexplored. We investigate the structure of a ridge within the Yakima Fold and Thrust Belt (YFTB), central Washington. Saddle Mountain is a ~70 km E-W-oriented ridge of deformed Columbia River Flood Basalt that diverges westward into two separate ridges, Manastash Ridge and Boylston Mountain. The research presented here synthesizes 384 field measurements of basalt flow contacts, 10 well logs (three over 3 km deep), three ~10 km seismic profiles, a 10m digital elevation model, and geologic maps to produce a 3D model of the interior structure the ridge. This model was developed using Midland Valley’s MOVE Structural Geology Software. Ten cross sections were produced and interpolated to show changes in fault depth, linkage, shape, orientation, and resulting deformation across the ridges. Faults dip more steeply at depth than anticipated and often link with back thrusts, and structural geometry transitions from symmetric to asymmetric fault systems from east to west. This detailed model illuminates the complexity of the structural geometry within a terrestrial “wrinkle ridge”.