Papers by Muriel Gerbault
Fractures et contraintes autour d'une chambre magmatique en surpression par modelisation thermo-m... more Fractures et contraintes autour d'une chambre magmatique en surpression par modelisation thermo-mecanique Directeur de thèse Supervisor Muriel GERBAULT 04 92 94 26 27
Physics of The Earth and Planetary Interiors, 2009
The Chilean Andes extend northsouth for about 3000 km over the subducting Nazca plate, and show e... more The Chilean Andes extend northsouth for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on firstorder tectonic features. Here, rheological parameters are tested with numerical models of a subduction zone incorporating slabpull and upper plate convergence, which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and forearc crust rheology, and arc temperature, on the development of superficial nearsurface faulting as well as viscous shearzones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first order by upper plate weakness, and to a second order by interface strength. In cases of a strong interface, a weak forearc crust is dragged downward by "tectonic erosion", a scenario for which indications are found along the northern Chilean margin.
Diverse mechanical processes involved in build-up and segmentation of the Andes have been address... more 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 relat... more 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.
Earth and Planetary Science Letters, 2008
Physics of The Earth and Planetary Interiors, 2009
Earth and Planetary Science Letters, 2008
The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. N... more The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. Noting that this volcanism is present from the Kermadec Tonga trench to the Easter microplate and forms a wide east-west channel, we propose to explain its occurrence in relation to the Pacific plate geometry and kinematics. We construct 2D numerical models of stress and strain within the Pacific plate using its velocity field and boundary conditions. The 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 Central Pacific 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. Present-day intraplate volcanism would define break up spots of the future border.
Earth and Planetary Science Letters, 2000
Internal contrasts in strength are responsible for lithospheric buckling. These are quantified by... more 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 ...
Geochemistry Geophysics Geosystems, 2008
This paper presents an interdisciplinary study of the Northern Chile Double Seismic Zone. First, ... more This paper presents an interdisciplinary study of the Northern Chile Double Seismic Zone. First, a high resolution velocity structure of the subducting Nazca Plate has been obtained by a new doubledifference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth and the two seismic planes are 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated to this DSZ are deduced by using current thermalpetrologicalseismological models, and are compared to pressuretemperature conditions provided by a numerical thermo mechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes are related to fluid releases associated to metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within highP (> 2.4 GPa) and low T (< 570°C) jadeitelawsonite blueschists and, at greater depth (> 130 km), lawsoniteamphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated to serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (~2530 vol.% antigorite, ~010% vol. % brucite and ~410 vol. % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental forearc, thermomechanical models indicate that both seismic planes delimit the innerslab compressional zone around the 400°C (±50°C) isotherm. Lower plane earthquakes are predicted to occur in the slabs flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upwards from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip extension characteristics. Numerical tests indicate that innerslab compression is not only favored by the slab's thermal structure such as plate age. i) A weak ductile subduction channel, and ii) a cold mantle forearc both favor innerslab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. iii) Decreasing the radius of curvature of the slab broadens the depth of innerslab compression, whereas iv) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if indeed DSZs contour innerslab compression, they cannot only be linked to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.
Journal of Structural Geology, 1998
Geophysical Research Letters, 1999
Large-scale periodic undulations within the oceanic and continental lithospheres revealed by a nu... more Large-scale periodic undulations within the oceanic and continental lithospheres revealed by a number of observations, are often treated as compressive instabilities, i.e. lithospheric buckling or folding. These undulations are normally associated with intensive faulting, which raises questions on the role of faulting in the folding process, and even on the possibility of folding in highly faulted media. In this study, we demonstrate that folding can "survive" faulting and that both processes may develop concurrently, so that faulting may serve as a mechanism of folding in the brittle domain. We support this hypothesis by direct numerical modeling. The results are compared with the data on three most prominent and well-known cases of the oceanic and continental folding-like deformation ("Indian Ocean" type, "Western Gobi (central Asia)" type and "Central Australian" type). We demonstrate that under reasonable tectonic stresses, folds can develop from faults cutting through the brittle parts of the lithosphere. The predicted wavelengths and nite growth rates are in agreement with the observations. We also show that within a continental lithosphere with thermotectonic age greater than 400 My, either a bi-harmonic mode (two superimposed wavelengths, crustal and mantle one) or a coupled mode (mono-layer deformation) of inelastic folding can develop, depending on the strength and thickness of the lower crust.
International Journal of Earth Sciences, 2007
The development of the Alpine mountain belt has been governed by the convergence of the African a... more 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.
Geophysical Journal International, 2002
Crustal thickening in an oblique continental collision, such as in the South Island of New Zealan... more Crustal thickening in an oblique continental collision, such as in the South Island of New Zealand, necessarily involves deformation processes in three dimensions (3-D). We have investigated the role played by the strength of the lower crust using simplified 3-D mechanical models. These models show that crustal thickening occurs away from the area of maximum compression, along an axis inclined to the plate boundary (about 10 • -20 • to the plate boundary in the case of the South Island), and perpendicular to the convergence direction. Furthermore, the specific geometry of the relatively old and strong Australian lithosphere versus the Pacific lithosphere also controls the location of crustal thickening. These conditions could explain the observed mismatch between the locations of maximum elevation and minimum gravity in South Island, New Zealand, as a consequence of decoupled deformation owing to low-viscosity lower crust.
Journal of Geophysical Research, 2003
1] Compression of the entire continental lithosphere is considered using twodimensional numerical... more 1] Compression of the entire continental lithosphere is considered using twodimensional numerical models to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The numerical scheme incorporates brittle-elastic-ductile rheology, heat transfer, surface processes, and fault localization. Models are based on the central section of the New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. The results are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located $60-80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located $100-130 km east of the Alpine Fault as a result of shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for mantle lithosphere strength of the order of 200 MPa and an intermediate rate of strain softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but the amplitude of the crustal root is too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in the Southern Alps. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; 8159 Tectonophysics: Rheology-crust and lithosphere; 9355 Information Related to Geographic Region: Pacific Ocean; KEYWORDS: mechanical modeling, rheology of the lithosphere, strain localization, decoupled crust and mantle, continental collision, Southern Alps Citation: Gerbault, M., S. Henrys, and F. Davey, Numerical models of lithospheric deformation forming the Southern Alps of New Zealand,
Journal of Structural Geology, 2008
Reverse faults in northern Chile have formed 20-300 m high scarps that contain open fractures whi... more Reverse faults in northern Chile have formed 20-300 m high scarps that contain open fractures which occur in a zone of 20-1600 m wide. Two-dimensional numerical models were used to explore the geometrical and mechanical parameters needed to produce extension within a bulk contractional regime. All of the mechanical models show the same structure as the field: a concentration of cracks predominantly at the top, rather than on the forelimbs of the scarps. In the field case extension begins as soon as a discrete scarp forms; with progressive shortening the scarp height increases producing a broadening of the zone in extension. The numerical models show that this broadening stabilizes when a maximum in the scarp height is reached. To produce concentration of the extension on top of the scarps, the reverse fault needs to be weak (f w 10 ). The models suggest that distribution of this extensional zone depends on the cross-sectional geometry of the fault and on the location of the detachment at depth. The main mechanism that produces extension on the top of the reverse scarp is stretching of the topographic surface by folding of the hanging wall at the tip of the fault zone.
Tectonophysics, 2005
The relatively low elevation and thick crust in the Altiplano, in comparison to the higher elevat... more The relatively low elevation and thick crust in the Altiplano, in comparison to the higher elevation, but thinner crust in the Puna plateau, together with geophysical data, suggest that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the Altiplano. Excess density in the Altiplano mantle could create differential horizontal stress in the order of 25 MPa between both lithospheric columns. Numerical models accounting for pressure and temperaturedependent rheology show that such stress can induce horizontal ductile flow in the lower crust, from the Puna towards the Altiplano. With a minimum viscosity of 10 19 Pa.s, this flow reaches 1 cm/yr, displacing more than 50 km of material within 5 Ma. If the lower crust viscosity is smaller, the amount of orogenyparallel lower crustal flow can be even greater. Such a mechanism of channel flow may explain that different amounts of crustal material have been accomodated by shortening in the Altiplano and in the Puna. Because of the strength of the brittle upper crust, this channel flow does not necessitate large amounts of suface deformation (except vertical uplift), making it difficult to detect from the geology.
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Papers by Muriel Gerbault