Abstract Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3‐D numerical models of elasto‐plastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized or diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber's crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest how natural observations may be used to constrain the mode of failure occurring underneath a volcano. We discuss several natural examples in this context.
Research Article| December 01, 1996 Anelasticity explains topography associated with Basin and Range normal faulting R. Hassani; R. Hassani 1Laboratoire de Géophysique et Tectonique, Montpellier, URA CNRS-UM2 5573, France Search for other works by this author on: GSW Google Scholar J. Chéry J. Chéry 1Laboratoire de Géophysique et Tectonique, Montpellier, URA CNRS-UM2 5573, France Search for other works by this author on: GSW Google Scholar Author and Article Information R. Hassani 1Laboratoire de Géophysique et Tectonique, Montpellier, URA CNRS-UM2 5573, France J. Chéry 1Laboratoire de Géophysique et Tectonique, Montpellier, URA CNRS-UM2 5573, France Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (12): 1095–1098. https://doi.org/10.1130/0091-7613(1996)024<1095:AETAWB>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. Hassani, J. Chéry; Anelasticity explains topography associated with Basin and Range normal faulting. Geology 1996;; 24 (12): 1095–1098. doi: https://doi.org/10.1130/0091-7613(1996)024<1095:AETAWB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Topography associated with normal faulting in the Basin and Range (western United States) is usually modeled as a flexure of a broken elastic plate. However, modeled effective rigidities are usually 100 times lower than the rigidity deduced from upper-crustal thickness. This discrepancy may be related to a significant anelastic deformation, which we explore through numerical modeling. Because experimental rock rheology evidences a pressure-dependent yield stress beyond the elastic limit in crustal rocks, we made a finite element model that accounts for such a crustal rheology and also for the frictional behavior of an embedded high-angle fault.Resulting topography after extension is similar to that obtained from a thin elastic plate model; however, the corresponding strain pattern differs. First, the footwall rotates in a rigid fashion over a width of 15 km that matches the typical size of uplifted Basin and Range blocks. Second, hanging-wall subsidence results from a significant horizontal extension and rotation of the crust. We suggest that plastic deformation in the deep part of the footwall could trigger the development of a new high-angle fault with a fault spacing that matches Basin and Range structure. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
While the Central Andes (Peru, Bolivia) exhibit a high mean elevation, a large orogenic belt, a significant shortening and the lack of any exotic mesozoic material, the northern Andes (Ecuador, Colombia) are marked by a moderate mean elevation, a narrow shape, a low shortening amount and the presence of large Cretaceous oce-anic terranes accreted to the western margin. The aim of this work is to propose a model for the orogenic buildup of the Andes of Ecuador, which differs from that commonly proposed for the Central Andes, and takes into account the geological evolution and struc-ture of this part of the chain.
Abstract Gravity gliding implies rigid translation of a body down a slope where displacements are parallel to a tilted detachment plane. Although large‐scale gravity gliding is commonly observed offshore, under conditions of high fluid overpressure and abundant upslope sedimentary supply, its occurrence on land is debated. We investigate the mechanical feasibility of such a process as well as the role of fluvial incision and sedimentation down the slope in the initiation of the gliding. We use a two‐dimensional (2‐D) finite element model combined with a 2‐D failure analysis approach. The numerical models simulate the deformation and provide quantitative estimates of the failure criteria at the head and toe of the overburden. Analytical solutions approximate the numerical results by taking into account the fluvial incision and sedimentation, the internal friction angle, and the thickness and length of the overburden. Our models are based on a field example in the Andean foothills of Argentina, where gravity gliding of a 1000 m thick section is suspected above a crustal‐scale anticline. The incision and sedimentation reduce and strengthen, respectively, the downslope resistance to contractional failure. The critical slope at which the gliding is initiated is reduced by fluvial incision and increased by sedimentation. We show that tectonic uplift may lead to large‐scale gravity gliding on land where the overburden thickness is less than 2000 m. Incision facilitates and localizes the frontal shortening. Incision greater than 1000 m may trigger gliding for overburden up to 4000 m thick, while sedimentation thicker than 1000 m inhibits gliding. These results show that thin‐skinned onland gravity gliding could be common in tectonically active regions where incision is important.
The Eocene tectonic evolution of the easternmost Caribbean Plate (CP) boundary, i.e. the Lesser Antilles subduction zone (LASZ), is debated. Recents works shed light on a peculiar period of tectonic duality in the arc/back-arc regions. A compressive-to-transpressive regime occurred in the north, while rifting and seafloor spreading occurred in Grenada basin to the south. The mechanism for this strong spatial variation and its evolution through time has yet to be established. Here, using 3-D subduction mechanical models, we evaluate whether the change in the trench-curvature radius at the northeast corner of the CP could have modulated the duality. We assume asymmetrical CP boundaries at the north (from east to west: oblique subduction to strike-slip) and at the south (subduction-transform edge propagator-like behavior). Regardless of the imposed trench curvature, the southern half of our modeled CP always undergoes a NW-to-W-oriented extension due to the tendency of the southernmost part of the South-America slab to rollback. In contrast, the tectonic regime in the northeast corner of the CP depends on the trench-curvature radius. A low radius promotes transtension-to-transpression, with a NE-oriented compressive component of the principal stress. A high trench-curvature largely reduces the compressive component and promotes an extensive regime similar to that in the south. We thus propose that an initially low-curvature radius of the NE-LASZ triggered the tectonic N-S duality in the Eocene and led to an ephemeral period of transpression/compression at the north, although an additional mechanism might have been required to locally enhance compression.
Two‐dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere. The subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence. The lithosphere is assumed to be underlain by an in viscid asthenosphere. Different rheological laws have been considered for the lithosphere, including elasticity and elastoplasticity. The modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters: the density contrast between lithosphere and asthenosphere (Δρ = ρ L ‐ ρ A ) and the coefficient of friction along the subduction plane. Varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc‐back arc system. Extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction, while negative Δρ values and/or high friction leads to a compressional regime. The coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0.1. The results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions.
