Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through lidar topographic analysis Glenn D. Thackray*, David W. Rodgers, and David Streutker Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA ABSTRACT surface from raw point data, and rasterizes High-resolution lidar data reveal a prominent latest Pleistocene–Holocene scarp on the those data to resolutions as fi ne as 1 m. Sawtooth fault (central Idaho, United States). The fault scarp marks 55–65 km of the range In the areas of lidar coverage, we analyzed the front, and may comprise two segments. The scarp is 4–9 m high in latest Pleistocene glacial bare-earth DEM data sets and associated digital landforms (11–14 ka) and 2–3 m high in Holocene alluvial landforms, implying 2–3 postglacial hillshade images, SPOT satellite imagery, aerial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest photographs, and surfi cial geologic maps in that active faulting may have migrated northward during Pleistocene time. Detailed compari- ArcGIS 9.2 (ESRI, 2006). We extracted topo- sons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and fi eld surveys graphic profi les across the fault scarp from the indicate that the bare-earth lidar data document the fault scarp morphology accurately and raw (vegetated) and bare-earth lidar topographic allow for detailed fault analysis where fi eld evaluation is diffi cult. The documentation of Holo- data sets, and analyzed those profi les in Micro- cene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across soft Excel. We profi led the fault scarp in selected central Idaho, and that the fault contributes to seismic hazards. areas of lidar coverage using a laser rangefi nder and refl ector, or via transit leveling. In areas INTRODUCTION and metamorphosed Paleozoic wall rocks and lacking lidar coverage, we extended mapping of Detailed topographic analysis using data roof pendants (Fisher et al., 1992). The adjacent the scarp through detailed examination of stereo from lidar (light detection and ranging) has basin contains sediments of estimated Neogene aerial photographs and other data, coupled with revolutionized mapping of active faults in areas to Quaternary age, overlain by middle and late fi eld investigation in selected areas. of dense vegetative cover, resulting in the iden- Pleistocene glacial landforms. Deglaciation tim- tifi cation of previously unknown, active faults ing near the range front was estimated from 10Be RESULTS (e.g., Johnson et al., 2004; Hunter et al., 2011) exposure dating of moraine boulders (11.3 ka; and enhancing understanding of known faults Sherard, 2006) and from radiocarbon dating Fault Scarp Characteristics (e.g., Harding and Berghoff, 2000). Highly of lake sediment (14 ka; Thackray et al., 2004; Our analysis indicates a previously unmapped detailed topographic data sets and accurate bare- Mijal, 2008). Thus, we use a range of 11.3– topographic lineament along the range front. earth digital elevation models derived through 14 ka for the Pleistocene deglacial landscape This lineament cuts latest Pleistocene–Holo- digital vegetation removal make lidar an ideal disrupted by the Sawtooth fault scarp. Holocene cene landforms (Fig. 1B), and we interpret it tool for locating fault scarps. Here we document alluvial landforms (undated, assumed age 5 ka) as a fault scarp. We identifi ed a continuous previously unknown latest Pleistocene–Holo- locally incise Pleistocene landforms. scarp in several areas on our northern lidar data cene offset on the Sawtooth fault, a major range- The Sawtooth fault has previously been set, spanning 10 km of range-front length. The bounding normal fault in central Idaho (United inferred along the range front based on the scarp is generally a single-strand feature and States), with important implications for regional extreme relief (1300 m) and abrupt, linear lies within 1 km of the topographic range front. tectonic processes. We highlight the utility of topographic change from valley to mountain. It is best developed in latest Pleistocene (post– lidar analysis, not only for identifying young The fault is broadly inferred to have been 14 ka) glacial landforms. On our southern lidar fault scarps, but also for digital evaluation of active during late Quaternary time (Geomatrix data set (5 km of range-front length), the scarp fault characteristics in remote wilderness areas Consultants, Inc., 1989; Breckenridge et al., is discontinuous, and clearly recognized in where terrain and access hinder detailed study. 2003; Crone and Neier, 2003). However, latest only a few locations. In only one location, the We fi nd that lidar analysis can be as effective as Pleistocene–Holocene fault scarps remained observed scarp lineament is broken and exhib- detailed fi eld study in the initial evaluation of unidentifi ed despite decades of surface geo- its a possible left-stepping, en echelon offset. fault characteristics and hazards. logic work along the forested, topographically Stream channels that incise directly across the complex range front (Williams, 1961; Thack- scarp exhibit no clear lateral offset. SETTING AND CONTEXT ray et al., 2004; Sherard, 2006). Between and beyond the areas of lidar cov- Located in the northeastern corner of the erage, the fault scarp is represented on aerial Basin and Range province in central Idaho, LIDAR ACQUISITION AND photographs as lineaments of vegetation, water- the east-dipping Sawtooth fault separates the PROCESSING courses, and topography. These features coincide Sawtooth Mountains from a structural basin This study employed lidar data comple- with the imaged scarp where the lidar data exist, comprising the Sawtooth Valley and Stanley mented by aerial photograph analysis and fi eld and can be mapped at various levels of confi - Basin (Figs. 1A and 1B). The Basin and Range mapping. We utilized two lidar data sets, col- dence for 65 km along the Sawtooth range front. is a 750-km-wide extensional tectonic province lected in 2005 by Airborne 1, Inc. (Fig. 1A). Field surveys at fi ve locations covered by with Miocene–Holocene fault activity. Its north- Data were processed to bare-earth (last return) lidar imagery and at six additional locations eastern corner includes several active faults that digital elevation models (DEMs) using meth- confi rm that the fault scarp exists, continuously have produced Pleistocene to Holocene ruptures, ods described in detail by Streutker and or discontinuously, along at least 55 km of including the A.D. 1983, M 6.9, Borah Peak Glenn (2006). The two data sets cover an area range-front length, from Stanley Lake to Pettit earthquake (Fig. 1C). The Sawtooth Mountains >100 km2 and include fi rst- and last-pulse data Lake, and may extend an additional 10 km south expose Cretaceous granodiorite, Eocene granite, with small point footprints (~30 cm). The bare- (Fig. 1A). At Fishhook Creek (Fig. 1B), the scarp earth DEM processing assumes a semi-open is characterized by a single, 4.1–6.5-m-high *E-mail: [email protected]. canopy, iteratively interpolates the ground step in late Pleistocene valley-bottom glacial GEOLOGY, June 2013; v. 41; no. 6; p. 639–642; Data Repository item 2013182 | doi:10.1130/G34095.1 | Published online 16 April 2013 GEOLOGY© 2013 Geological | June Society 2013 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 639 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/41/6/639/3544297/639.pdf by 19774 on 02 April 2020 topographic profi les across the scarp at 25 loca- A 115º W B tions (8 fi eld checked) in the Redfi sh Lake lidar coverage area (Fig. 1A), using the profi ler tool in ArcGIS 9.2 (ESRI, 2006) on bare-earth lidar Stanley Lake DEMs (see Table DR1 in the GSA Data Reposi- Stanley Basin tory1). On 16 of those profi les, the scarp is dis- Stanley cernible and readily measured, while on the Fishhook valley remaining 9 profi les, the scarp height is ambigu- ous in complex topography. The 16 topographic Fig. 2 profiles profi les in which the scarp is readily discernible Fig 1b document down-to-the-east vertical offset of 6.3 ± 1.5 m (4.1–8.7 m, n = 12) in Pleistocene land- forms (11.3–14 ka), and 3.0 ± 0.5 m (2.5–3.5 m, Redfish Lake n = 4) in Holocene alluvial landforms. The scarp heights do not show a statistically signifi - cant trend with distance along the 10 km fault 0 500 m length, and height measurements are elsewhere Sawtooth Valley too sparse to determine trends over longer scarp lengths. The inferred slip rate, assuming range- Hell Roaring Creek front deglaciation at 14 ka, is 0.5 ± 0.1 mm/yr (60° assumed fault dip) to 0.9 ± 0.2 mm/yr (30°). 44º N To assess the accuracy of topographic pro- 0 5 km fi les derived from the bare-earth DEMs, we compared profi les derived from (1) the bare- Pettit Yellowbelly Lake Lake earth DEM, (2) the raw (vegetated) DEM, and Fault scarp, observed (3) our fi eld-surveyed profi les. On an example Fault scarp, inferred profi le at Fishhook Creek (Figs. 1B and 2), the raw lidar DEM and the corresponding bare- Lidar coverage earth DEM yield a scarp height estimate within 0.1 m of each other. A corresponding topo- graphic profi le measured with a laser range- C fi nder documents scarp height within 0.2 m of 45º N that derived from the bare-earth lidar profi le. However, our laser rangefi nder profi le uniquely documented a shallow depression (graben?) at the base of the scarp. n Snake EasterRiver Plain DISCUSSION 115º W Implications for Lidar-Assisted Fault Figure 1. A: Hillshade image of Sawtooth Mountains range front (Idaho, United States), de- Analyses rived from 10 m digital elevation model.