USING QUANTITATIVE GEOMORPHOLOGY TO ASSESS QUATERNARY FAULTING ON CROWLEY’S RIDGE, NEW MADRID SEISMIC ZONE, ARKANSAS

The New Madrid Seismic Zone (NMSZ) is a region of intraplate seismicity in the central Mississippi Valley with a history of large (M6-8) earthquakes. Historical seismicity, geomorphic evidence (e.g., paleoliquefaction features, ponding of rivers, warped fluvial terraces) of prior earthquakes, and rare fault scarps led most previous investigators to focus on the eastern NMSZ. Here, we focus on Crowley’s Ridge, a 300-km-long north-south trending ridge of uplifted Cretaceous, Eocene, and Plio-Pleistocene sedimentary rocks in the western NMSZ. Previous seismic reflection data show primarily steeply-dipping normal and minor reverse faults bounding 20- to 40-km-long segments of Crowley’s Ridge, but the timing of most recent faulting and lateral extent of these faults is unknown. To assess the recency (10⁵-10⁶ yr timescale) of deformation, we use landscape-scale (km-scale) quantitative geomorphic analyses, such as patterns of relief, slope, hypsometry, river profiles, and the drainage network, on a 10-m digital elevation model (DEM), in addition to targeting several key sites with possible offset or deformed surfaces to quantify deformation of <15 ka braidplains and fluvial terraces (10⁴ yr) using 0.5-m and 1-m lidar DEMs. Variations in ridge and drainage basin asymmetry, drainage basin relief, hypsometric integral and curves, and changes in the drainage network pattern from north to south along Crowley’s Ridge indicate evidence of Pleistocene tectonic uplift. A 1-m lidar DEM of terraces spanning the St. Francis water gap, near the northern end of Crowley’s Ridge, reveals low magnitudes (<1.5 m) of uplift and warping since the last glacial maximum: either post-15-ka deformation rates are low (<0.1 mm/yr) or modern fluvial deposition is high enough to mask it. In contrast, small scarps (<2 m) visible in 1-m lidar along the margins of southern Crowley’s Ridge cut post-15-ka surfaces and provide evidence of recent, but low-rate (<0.2 mm/yr) faulting. These preliminary results suggest there may be some evidence of late Quaternary faulting along Crowley’s Ridge, which could have important implications for seismic hazard modeling efforts in this region.