Elliptic calderas in the Ethiopian Rift: control of pre-existing structures

The Ethiopian Rift is characterized by several Quaternary calderas. Remote sensing and field analyses were used to investigate the regional structural control on three calderas (Fantale, Gariboldi, Gedemsa) in the axial part of the rift. These calderas are located along the Wonji Fault Belt (WFB), a zone of Quaternary NNE-SSW normal faults and extensional fractures. The three calderas show E-W elongation and major E-W vent alignments, oblique with regard to the mean NW-SE extension direction. No significant evidence of E-W tectonic structures has been found near the calderas, the only relevant systems being those of the WFB. Conversely, left-lateral E-W-trending faults are present at the rift borders and on the Nubia and Somalia plateaus, implying a predominant pre-rift activity. The E-W fractures were partly reactivated during rifting, possibly controlling the development of the magma chambers. Thus, the E-W elongation of the calderas would be the surface expression of such a control, rather than the result of regional extension. An evolutionary model on the role of different structures on magmatism at different crustal levels within the rift is proposed

Structural control on Late Miocene to Quaternary volcanism in the NE Honshu arc, Japan

Volcanological and structural field data are used to define the tectonic control on the N-S volcanic arc of NE Honshu (Japan) since Late Miocene. During Late Miocene-Pliocene, bimodal products were mainly erupted from along-arc and NE-SW aligned and elongated calderas. The deformation pattern mostly consisted of N-S dextral faults and subordinate NE-SW extensional structures, produced by NE-SW compression. This pattern, due to the indentation of the Kuril sliver, is similar to that of oblique convergence settings. Magma rose and extruded along NE-SW areas of localized extension created by the dextral faults. These extensional areas were uncoupled with regard to those, ~E-W trending, inferred to have focused the rise of melts from the subducting slab in the mantle. During Quaternary, a larger amount of andesite was mainly erupted from along-arc and ~E-W aligned and elongated stratovolcanoes. The deformation pattern mostly consisted of N-S thrust faults and subordinate ~E-W extensional structures, produced by ~E-W compression, resulting from orthogonal convergence due to the variation in the absolute motion of the Pacific Plate. The ~E-W extensional structures are the shallowest expression of ~E-W trending hot mantle fingers, suggesting mantle-crust coupling for the rise of magma. Such a coupling ensures: (a) higher extrusion and (b) mixing between a deeper mafic and a shallower felsic magma, generating the andesites. The significantly larger volumes (per Ma per 200 km of length of the arc) of magma erupted during Quaternary show that pure convergence conditions do not necessarily hinder the rise and extrusion of magma

Three-dimensional instantaneous mantle flow induced by subduction

We conduct three-dimensional subduction experiments by a finite element approach to study flow around slabs, which are prescribed based on a transient stage of upper mantle subduction from a laboratory model. Instantaneous velocity field solutions are examined, focusing on the toroidal vs. poloidal components as a function of boundary conditions, plate width, and viscosity contrast between slab and mantle. We show how the slab-to-mantle viscosity ratio determines the strength of toroidal flow, and find that the toroidal flow component peaks for slab/mantle viscosity ratios !100, independent of slab width or geometry

Three-dimensional instantaneous mantle flow induced by subduction

We conduct three-dimensional subduction experiments by a finite element approach to study flow around slabs, which are prescribed based on a transient stage of upper mantle subduction from a laboratory model. Instantaneous velocity field solutions are examined, focusing on the toroidal vs. poloidal components as a function of boundary conditions, plate width, and viscosity contrast between slab and mantle. We show how the slab-to-mantle viscosity ratio determines the strength of toroidal flow, and find that the toroidal flow component peaks for slab/mantle viscosity ratios ~100 independent of slab width or geometry.PublishedL08304JCR Journalreserve

Scaling laws of dynamic topography and uplift-rate in 3D spherical geometry

International audienceThermo-chemical convection in the Earth's mantle is thought to significantly affect the topography of the lithosphere. However, the comparison of such dynamic topography as obtained from convection models with the observed, non-isostatic topography remains complicated, both because of uncertainties about crustal structure and mantle flow estimates. Here, we focus on the latter and evaluate the role of lateral and radial viscosity variations for topography estimates. We report the magnitude of dynamic topography and uplift rates from a set of numerical computations of mantle convection in regional sections of a spherical annulus using the finite element software CitcomS. We strive to establish scaling laws of dynamic topography and uplift rate as a function of the rheology and Rayleigh number. We test both Newtonian and non-Newtonian rheologies with temperature-dependent viscosities. The dimensions of the Stokes equation suggest that both the uplift rate and the dynamic topography can be described by the Frank-Kamenetskii parameter for temperature dependent viscosity, the average viscosity, and the Rayleigh number, but with different exponents. We test the validity of this approach

Predicting trench and plate motion from the dynamics of a strong slab

The motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle. These models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation. Here, using a recent compilation of a global data set, we found that the motion of trenches (either advancing or retreating with respect to upper plates) scales with their corresponding subducting plates motion. Based on simple experimental tests, we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships. We deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle