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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B05403, doi:10.1029/2004JB003253, 2005

Discrete element simulations of gravitational volcanic deformation: 2. Mechanical analysis

Julia K. Morgan

Department of Earth Science, Rice University, Houston, Texas, USA


Patrick J. McGovern

Lunar and Planetary Institute, Houston, Texas, USA


Abstract

Particle dynamics simulations of granular piles constructed upon planar substrates with different shear strengths model the gravitational deformation of natural volcanoes. Results reveal direct correlations among pile morphology, layer stratigraphy, deformation structures, and stress field, dependent upon the shear strength along the base of the pile. Piles constructed above a strong, cohesive substrate that prevents basal slip develop outward dipping layers through particle avalanching down angle of repose slopes. In the absence of basal cohesion, slip occurs along a basal décollement if the topographically induced shear stresses exceed the basal shear strength. For friction coefficients decreasing from 0.3 to 0.1, fault planes migrate progressively inward, defining a transition in deformation mode from shallow slumping to deep-seated landsliding and, finally, to full gravitational spreading. The variations in deformation mode reflect the dependence of internal fault geometry on basal strength in accordance with critical Coulomb wedge theory. In order for slip to occur both internal and basal failure criteria must be met simultaneously. Topographically induced shear stresses increase outward along the base of the pile, enabling décollement sliding once the basal failure criterion is met. Normal faults arise from the inside edge of the slipping décollement, accommodating downward and outward displacements of the flanks. Surface slopes evolve to maintain equilibrium between gravitational driving and basal resisting stresses. This simple mechanical understanding of gravitational deformation of granular piles provides insight into the controls on modes of volcanic deformation, providing a predictive tool for understanding volcanic evolution and hazards in many settings.

Received 23 June 2004; accepted 14 January 2005; published 12 May 2005.

Keywords: gravitational spreading; volcanotectonics; discrete element method (DEM); Coulomb rheology.

Index Terms: 3070 Marine Geology and Geophysics: Submarine landslides; 3037 Marine Geology and Geophysics: Oceanic hotspots and intraplate volcanism; 6225 Planetary Sciences: Solar System Objects: Mars; 8020 Structural Geology: Mechanics, theory, and modeling; 8122 Tectonophysics: Dynamics: gravity and tectonics.

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Citation: Morgan, J. K., and P. J. McGovern (2005), Discrete element simulations of gravitational volcanic deformation: 2. Mechanical analysis, J. Geophys. Res., 110, B05403, doi:10.1029/2004JB003253.