Discrete Element Simulations of Volcanic Spreading: Implications for the Structure of Olympus Mons

Introduction: Large basaltic volcanoes, such as Olympus Mons on Mars, and the Hawaiian volcanoes on Earth, are thought to be subject to large scale slope failure and flank collapse, which scatter landslide and avalanche debris (a.k.a., aureole deposits) about the base of the edifice [1], [2], [3], [4], [5]. Volcano growth may also be accompanied by gravitational relaxation of the edifice, referred to as volcanic spreading. This deformation is accommodated by outward flank displacement along a low-angle décollement near the base of the edifice, and thrust faults that surface at the distal edge of the volcano [6], [7]. Volcanic spreading is well documented in Hawaii [8], [9], and accounts for extensive anticlinal ridges and frontal scarps that border the submarine toes of the flanks [10], [11], [12]. A prominent scarp at the distal edge of Olympus Mons, may have a similar origin as the terrestrial flanking benches [7], [13], a hypothesis that can be tested by dynamic modeling of volcano deformation. Here, we carry out numerical simulations using the Discrete Element Method to explore the consequences of landsliding and volcanic spreading. The resulting models compare favorably to morphologic features noted on Olympus Mons, suggesting the importance of volcanic spreading in this setting. Background: The enormous Olympus Mons measures 600-800 km in diameter, and reaches up to 23 km in height [13]. A steep scarp, up to 10 km high, encircles much of the lower flanks of the volcano [13]. Although the frontal scarp occurs at a remarkably constant distance from the summit, it is locally embayed, where outboard, well defined aureole lobes radiate from the base of the edifice. The association of aureole deposits and the scarp re-entrant suggests that the former derived from the collapse of the lower bench [13]. The volcanic flanks above the frontal scarp commonly exhibit concave up profiles, with the steepest slopes below the summit, and nearly horizontal slopes just behind the perimeter scarp [13]. Concentric terraces upon the upper flanks are interpreted to mark thrust faults at the downslope edges of slumps [14]. Similarly, shallow slopes above the basal scarp may reflect uplift, folding, and rotation above thrust faults at the base of edifice [6], as interpreted in Hawaii [6], [10], [11]. Continuous small scale detachment from the outer slopes of the frontal ridge would maintain the steep frontal scarp. Occasional larger slope failures may breach the lower flanks, spreading debris in front of the broken edifice to form the aureole lobes. Discrete Element Method: The discrete element method (DEM) simulates an assemblage of discrete particles that interact with each other according to fundamental contact physics [15], [16]. Contact forces are summed for each particle, and an finite-difference approach is used to solve Newton's equations of motion for each particle. Several contact laws can be used, yielding a range of material rheologies. For these initial simulations, we use a Hertz-Mindlin contact theory, wherein particles respond elastically to normal forces at their boundaries, and contact shear forces are limited by interparticle sliding friction μ [16]. In our 2D simulations, round particles enhance particle rolling, limiting assemblage strength [16]; In order to attain realistic shear strengths, particle rotations are restricted throughout the volume. Energy is dissipated at the contacts by viscous or force damping [15].