Muriel Gerbault

Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code PARAVOZ to study the effects of rheological differences between the upper and lower crust in the fore-arc, on localisation of deformation. Results show that a relatively weak lower crust leads to wide and homogeneous thickening of the crust, associated to an increase of the western component of descendent crustal flow. A weak lower crust consequently achieves lower topography than the opposite case, and a better transmission of stress and deformation to the west of the growing topography. Furthermore, testing the thermal perturbation corresponding to an active-arc, shows an important control over the down-west flow of the lower crust, at isotherms 400-500'C around 40 km depth. High temperatures rise the brittle-ductile transitions, favor decoupling of the upper and lower crust, consequently diffusing deformation and smoothing the topography. Rheology and temperature control the thickness and width of the competent layers, therefore determining the Eastward or Westward vergence of modeled fault structures. Finally, we propose that a plateau-like deformation might be controlled by a weak lower crust, independent of the thermal weakening produced by arc-magmatism, while a stronger lower crust will produce a more Puna-like mountain building.

We present here a model applied to the Pacific plate for a mechanism governing plate motion related to the plate geometry and kinematics. We start from the observation that from the Kermadec Tonga trench to the Easter microplate, a group of recent and presumed non-deep Pacific hotspots forms a wide east-west channel, and hypothesize that this is not a coincidence. We develop plane strain numerical models of an area corresponding to the Pacific plate from the mid-oceanic ridge to the subduction zone under the Australian plate, with differential velocities applied on the northern and southern part of the plate because of absolute trench motions. Our 2D models indicate a shear band, associated to a change from compressional stresses to the south to tensional stresses to the north, which develop after 10 Myr between the Australian plate corner and the Easter microplate. We propose that the South Central Pacific (SCP) intraplate volcanism is related to this process, and may represent the first step of a future plate re-organization, which will eventually break the Pacific plate in a southern and a northern plate due to intraplate stresses. Lithospheric extension associated with a fertile mantle source is necessary for the presence hotspots when these are not related to a deep mantle plume. To fully explain the SCP volcanism, we show that there is no relation between present-day SCP and the old Northwestern Pacific volcanism, except that it was created over the same mantle region before 70Ma, which represents the very fertile mantle needed.

Internal contrasts in strength are responsible for lithospheric buckling. These are quantified by comparing the Indian Ocean data to two-dimensional visco-elasto-plastic numerical models where the material properties depend on temperature and pressure. The central Indian Basin is ...

The development of the Alpine mountain belt has been governed by the convergence of the African and European plates since the Late Cretaceous. During the Cenozoic, this orogeny was accompanied with two major kinds of intraplate deformation in the NW-European foreland: (1) the European Cenozoic Rift System (ECRIS), a left-lateral transtensional wrench zone striking NNE-SSW between the western Mediterranean Sea and the Bohemian Massif; (2) long-wavelength lithospheric folds striking NE and located between the Alpine front and the North Sea. The present-day geometry of the European crust comprises the signatures of these two events superimposed on all preceding ones. In order to better define the processes and causes of each event, we identify and separate their respective geometrical signatures on depth maps of the pre-Mesozoic basement and of the Moho. We derive the respective timing of rifting and folding from sedimentary accumulation curves computed for selected locations of the Upper Rhine Graben. From this geometrical and chronological separation, we infer that the ECRIS developed mostly from 37 to 17 Ma, in response to north-directed impingement of Adria into the European plate. Lithospheric folds developed between 17 and 0 Ma, after the azimuth of relative displacement between Adria and Europe turned counter-clockwise to NW–SE. The geometry of these folds (wavelength = 270 km; amplitude = 1,500 m) is consistent with the geometry, as predicted by analogue and numerical models, of buckle folds produced by horizontal shortening of the whole lithosphere. The development of the folds resulted in ca. 1,000 m of rock uplift along the hinge lines of the anticlines (Burgundy–Swabian Jura and Normandy–Vogelsberg) and ca. 500 m of rock subsidence along the hinge line of the intervening syncline (Sologne–Franconian Basin). The grabens of the ECRIS were tilted by the development of the folds, and their rift-related sedimentary infill was reduced on anticlines, while sedimentary accumulation was enhanced in synclines. We interpret the occurrence of Miocene volcanic activity and of topographic highs, and the basement and Moho configurations in the Vosges–Black Forest area and in the Rhenish Massif as interference patterns between linear lithospheric anticlines and linear grabens, rather than as signatures of asthenospheric plumes.