GSA Connects 2021 in Portland, Oregon

Paper No. 53-8
Presentation Time: 2:30 PM-6:30 PM


CEESAY, Roheyatou M.1, DELGADO, Daliedmarie2, KUMAR, Namitha3, LANKER, Sabrina L.3, PRICE, Daniel4, WAIS, Paul5, WELSH, Josie6, ALLEN, Joseph7, REYNOLDS, Aislin8 and SHAW, Colin8, (1)Department of Geology, Macalester College, 1600 Grand Ave, Saint Paul, MN 55105, (2)Department of Geology, University of Puerto Rico Mayaguez Campus, P.O. Box 9000, Mayaguez, PR 00681, (3)Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48104, (4)Department of Geology and Geography, West Virginia University, Morgantown, WV 26508, (5)Department of Earth and Atmospheric Sciences, SUNY - College at Oneonta, 108 Ravine Parkway, Oneonta, NY 13820, (6)Earth and Environmental Sciences, University of Minnesota, 116 Church St SE, Suite 150, Minneapolis, MN 55455, (7)Department of Physical and Environmental Sciences, Concord University, Athens, WV 24712, (8)Department of Earth Sciences, Montana State University, P.O. Box 173480, Bozeman, MT 59717

The earthquake barrier model provides a framework for understanding fault propagation and geometry, but it is unclear what geologic features may act as earthquake barriers in natural systems. Observations of pseudotachylyte-bearing faults in the 25-km-long Homestake shear zone (HSZ) fault system suggest metamorphic fabric plays an important role in impeding or promoting earthquake rupture. We use high resolution UAS-derived orthophotographs to generate maps with remarkable detail of fault and country rock fabric geometries. Country rock in the HSZ consists of folded gneisses and schists showing variable degrees of transposition into a dominant NE trending foliation. Field mapping defined two fabric domains, one with fully transposed host rock and a second with partially transposed host rock and foliation oblique to the pseudotachylyte-bearing faults in the shear zone. Injection veins and damage zones show a pronounced asymmetry on dextral faults indicating a northeast-directed rupture propagation. Field observations also indicate that ruptures frequently terminate in the oblique fabric domains. Earthquake barriers are known to slow or stop rupture propagation. To evaluate the influence of foliation on rupture velocity, we employ methods based on experimental results that show a relationship between injection vein divergence angle and fault roughness to slip velocity. This study was conducted through an NSF Research Experience for Undergraduates (REU) program using largely open source tools including StraboSpot2 and Strabo Tools to collect and compile detailed observations keyed to digital outcrop images and QGIS to analyze our UAS photogrammetry image orthorectified using Agisoft Metashape.