GSA 2020 Connects Online

Paper No. 32-1
Presentation Time: 5:30 PM

THE FOLD ILLUSION: THE ORIGINS AND IMPLICATIONS OF OGIVES ON SILICIC LAVAS


WHITTINGTON, Alan1, ANDREWS, Graham D.M.2, KENDERES, Stuart M.3, ISOM, Shelby Lee4, BROWN, Sarah R.5, PETTUS, Holly D.6, COLE, Brenna G.7 and GOKEY, Kailee2, (1)Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, (2)Department of Geology & Geography, West Virginia University, 98 Beechurst Avenue, Morgantown, WV 26506, (3)Department of Geological Sciences, University of Missouri, 101 Geology Building, Columbia, MO 65211; Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, (4)Department of Geology and Geography, West Virginia University, Brooks Hall, 98 Beechurst Ave, Morgantown, WV 26506, (5)Department of Geology & Geography, West Virginia University, 98 Beechurst Avenue, Morgantown, WV 26506; West Virginia Geological and Economic Survey, 1 Mont Chateau Rd, Morgantown, WV 26508, (6)Department of Geology & Geography, West Virginia University, 98 Beechurst Ave., Morgantown, WV 26506, (7)Geology and Geography, West Virginia University, Morgantown, WV 26505

Folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first-order criteria for identifying ancient lavas on Earth and other planetary bodies. However, the presence of surface-folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western United States and Sardinia, Italy. We interpret the ridges and troughs on their upper surfaces, typically referred to as ‘ogives’ or ‘pressure ridges’, as fracture-bound structures rather than folds. We report on the absence of large-scale, buckle-style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures. We report viscosity data and results of stress analyses that preclude folding (ductile deformation in compression) of the upper surface of silicic lavas at timescales of emplacement (weeks to months). Therefore, analysis of fold geometry (wavelength, amplitude, etc.) is erroneous, and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior. The presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses, but relating to their tensile strength, not viscosity.