33 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

This session explores key structural processes that govern the temporal, mechanical, and rheological evolution of crustal/lithospheric extensional and strike-slip systems with a focus on novel field observations, microstructural and analytical methodologies, and numerical modeling.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Map-Scale Riedel Shears as Kinematic Indicators of Left-Lateral Displacement on the Foil Fault, Ozarks Plateau, Southern Missouri, USA

Paper number: 33-11
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

     The trace of the Foil Fault in northeastern Taney County and northwestern Ozark County in southern Missouri runs approximately 18 miles (30 km) in length, trending WSW-ENE. It stretches from Hercules Glades Wilderness Area in the Mark Twain National Forest to near Foil, Missouri at the eastern tip. An erosional remnant of lower to middle Mississippian (Kinderhookian and lower Osagean) strata follows the curvilinear crest of a ridge that crosses the fault but roughly follows a SW-NE course. Glade Top Trail is a gravel road that follows the ridge. Where the Foil Fault cuts the ridge, topographic elevations on the sub-Mississippian unconformity below the crest of the ridge show vertical displacement along the trace of the fault, approximately 40 feet (12 m), with the upthrown side to the north.

     The late Kenneth C. Thomson, Missouri State University, mapped the geology of many of the 7.5-minute quadrangles in this area and noted clusters of subparallel photo-lineaments in lower Ordovician strata, below the sub-Mississippian  unconformity, on either side of the fault. The same features are prominent on lidar imagery (3dep hillshade) on The National Map. We interpret these features as a series of Riedel (R) shears, mostly fractures that indicate left-lateral movement along the Foil Fault. The Skyline Ridge Fault north of the Foil Fault, a short reverse fault with minimal displacement, may be a Riedel conjugate shear (R'). It is notable that Mississippian strata are cut by the Foil Fault and offset vertically but photo-lineaments are not expressed in Mississippian strata on the ridgetops.  Rather, the latter are found in the lower Ordovician dolomitic strata in glade areas where soils tend to be thinner. Commonly, fractures appear to be lined with cedar and stunted post oak trees in NAIP imagery. Within the context of pervasive strike-slip faulting in the Ozarks, the interpretation of Riedel shears fits within the structural framework that previous investigators have recognized.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413431

© Copyright 2025 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.