28 Geologic Mapping in the West (Posters)
This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.
Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E
Neotectonic Mapping of Normal Faults in Low-strain Settings: Fault Scarps or Buttress Unconformities?
Paper number: 28-8
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM
Authors
-
George PharrisNPS, Yosemite National Park, El Portal, California
Abstract
Geomorphic slip rate studies provide estimates of fault activity by quantifying the amount of accrued offset recorded by surfaces of variable age. In extensional settings, time bracketing of accrued offset is complicated by hanging-wall aggradation, whereby the hanging wall of a normal fault is buried by footwall colluvium, aeolian deposits, or other depositional materials (e.g., McCalpin et al., 2021). In low-strain settings, such as the Basin and Range and Rio Grande rift physiographic provinces of western North America, the underrecognized impact of hanging-wall aggradation may result in widespread underestimates of Quaternary slip rates, masking potential hazard. Conversely, the tendency for fault scarps to be mapped as “certain” contacts across surfaces of variable age–without considering the impact of hanging-wall aggradation–may result in overinterpretation of fault offset through time.
Here we provide case studies from the North American southwest where burial of fault scarps obscures “true” fault offset. We propose that mapping at neotectonic scales (e.g., ~1:5000) must reflect offset variation in surfaces of variable age, rather than the common treatment of scarps as continuous and “certain” at all scales. Neotectonic slip rate studies should consider when normal fault scarps are more effectively envisioned as buttress unconformities, rather than fault scarps with surficial offset, to accurately communicate seismic hazard to stakeholders. Ultimately, estimation of fault activity rates requires both paleoseismological trenches and slip rate analyses to fully quantify the spatiotemporal hazard along Quaternary normal faults in low-strain settings.
Geological Society of America Abstracts with Programs. Vol. 58, No. 4
doi: 10.1130/abs/2026RM-410675
© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.