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Burkhard, Martin

Nom
Burkhard, Martin
Affiliation principale
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Ancien.ne collaborateur.trice
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https://libra.unine.ch/handle/123456789/34638

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  • Publication
    Accès libre
    Inversion tectonics, interference pattern and extensional fault-related folding in the Eastern Anti-Atlas, Morocco
    (2008)
    Robert-Charrue, Charles
    ;
    Burkhard, Martin 
    The Anti-Atlas belt of Morocco is a Variscan chain which appears as a huge anticlinorium oriented NE-SW. In the internal part of this structure, the actual relief shows the basement cropping out as inliers (Piqué 2001) or boutonnières. The cover, a thick pile of Paleozoic sediments up to 12 km thick, is gently folded and of low grade metamorphism in its lower level. The lack of major décollement, of a deformation front or thrust-fault makes the Anti-Atlas an unusual type of belt, which does not fit with classic schemes. The Anti-Atlas has been considered to be a thick skinned fold belt, with the crystalline basement involved in the horizontal shortening and where the folding of the cover fits to a “buckle fold” mode (Burkhard et al. 2006). This structural style is determined by two key parameters: the total thickness of Paleozoic cover series and the relative abundance of shale vs. competent marker beds.
    The Eastern Anti-Atlas has particular features which are not found elsewhere in the Anti-Atlas. Its location at the intersection between the NW-SE Ougarta chain and the ENE-WSW main body of the Anti-Atlas, produces an egg-box interference pattern. The significance of minor folds and thrusts in the competent beds and their particular orientation is examined. The fact that the cover in the Eastern Anti-Atlas is only 6 km thick, which changes the shale vs competent beds ratio, influences its structural style. The feature that distinguishes the most Eastern Anti-Atlas is the presence of large E-W normal faults affecting the whole structure and creating extensional fault-related folding in the cover.
  • Publication
    Accès libre
    Quantification of strain rate in the Western Alps using geodesy: comparisons with seismotectonics
    (2008)
    Delacou, Bastien
    ;
    Sue, Christian
    ;
    Nocquet, Jean-Mathieu
    ;
    Champagnac, Jean-Daniel
    ;
    Allanic, Cécile
    ;
    Burkhard, Martin 
    The contrasted seismotectonic regime of the Western Alps is characterized by radial extension in the high chain, combined with local compressive areas at the foothill of the belt, and everywhere occurrence of transcurrent tectonics. Here, we compare this seismotectonic regime to a large-scale compilation of GPS measurements in the Western Alpine realm. Our analysis is based on the raw GPS database, which give the measured velocity field with respect to the so called “stable Europe”, and an interpolated velocity field, in order to smooth the database on a more regular mesh. Both strain rate and rotational components of the deformation are investigated. The strain rate field shows patch-like structure, with extensional areas located in the core and to the North of the belt and compressional areas located in its periphery. Although the GPS deformation fields (both raw and interpolated) are more spatially variable than the seismotectonic field, a good qualitative correlation is established with the seismotectonic regionalization of the deformation. The rotation rate fields (both raw and interpolated) present counterclockwise rotations in the innermost part of the belt and a surprising continuous zone of clockwise rotations following the arc-shape geometry of the Western Alps along their external border. We interpret this new result in term of a counterclockwise rotation of the Apulia plate with respect to the stable Europe. This tectonic scheme may induce clockwise rotations of crustal block along the large strike-slip fault system, which runs in the outer part of the belt, from the Rhône-Simplon fault to the Belledonne fault and Southeastward, to the High-Durance and Argentera fault.
  • Publication
    Accès libre
    Illite crystallinity patterns in the Anti-Atlas of Morocco
    (2008)
    Ruiz, Geoffrey M. H.
    ;
    Helg, Urs 
    ;
    Negro, François 
    ;
    Adatte, Thierry 
    ;
    Burkhard, Martin 
    The low-grade metamorphism of the sedimentary cover of the Moroccan Anti-Atlas is investigated using Illite crystallinity (IC) method. More than 200 samples from three key areas (southwestern, central and eastern Anti-Atlas) have been taken from a maximum of different stratigraphic levels and have been analysed. The metamorphism is of low to very low degree throughout the southern flank of the Anti-Atlas. It increases from northeast to southwest. Whereas in the eastern Anti-Atlas diagenetic and anchizonal IC-values are predominant, in the western and central Anti-Atlas also epizonal IC-values are found. In every respective area the IC improves with stratigraphic age. At the scale of the entire Palaeozoic Anti-Atlas basin the IC correlates best with estimated paleo-overburden. However, burial metamorphism cannot be the cause even though considering missing sedimentary pile of Late Carboniferous age. The ‘abnormal’ paleo-geothermal gradient of 43–35 °C/km we evidenced for the Carboniferous is a true one, and has to be related to a basement sequence enriched in heat producing elements such as series of the West African Craton.
  • Publication
    Accès libre
    Structural pattern of the western Las Cañadas caldera (Tenerife, Canary Islands) revealed by audiomagnetotellurics
    (2008)
    Coppo, Nicolas
    ;
    Schnegg, Pierre-André 
    ;
    Falco, Pierik
    ;
    Costa, Roberto 
    ;
    Burkhard, Martin 
    The local and regional structural pattern of volcanic edifices strongly controls the space distribution of electrical resistivity. Here we report on the structural context of the western part of the Las Cañadas caldera of Tenerife (LCC) thought to have initiated the formation of the caldera. Using a new dataset of 11 audiomagnetotelluric tensors we emphasize the resistivity distribution of Ucanca caldera and propose a major revision of its extension. We find that Ucanca caldera has a limited westwards extent and that El Cedro sector is a depression margin of the caldera. According to the extent of hydrothermalized rocks at the base of the LCC wall and the distribution of Pico Teide – Pico Viejo vents, we constrain the location and size of Ucanca caldera. The interpretation of these results also constrains the extension of the Icod Valley and proposes a headwall located below the Pico Teide – Pico Viejo Complex.
  • Publication
    Accès libre
    Aseismic deformation in the Alps: GPS vs. seismic strain quantification
    (2007)
    Sue, Christian
    ;
    Delacou, Bastien
    ;
    Champagnac, Jean-Daniel
    ;
    Allanic Cecile
    ;
    Burkhard, Martin 
    Neotectonics of the Western and Central Alps is characterized by ongoing widespread extension in the highest zones of the chain and transcurrent/compressive tectonics at the external limits of the belt. The overall geodetically measured deformations also indicate extension across the Western Alps. There is a good qualitative coherency between seismotectonic and geodetic approaches. Here we attempt to quantify the seismic part of the deformation. The seismic strain is compared to the deformation derived from geodesy. In sub-areas of homogeneous seismic stress/strain, we computed the total seismic moment tensor and related strain tensor. This study provides new quantitative elements about the ongoing geodynamic processes in the alpine belt. The important discrepancies obtained between seismic strains and geodetically-measured deformations raise the issue of aseismic deformation in the Alps, which could be related to elastic loading, creeping and/or a slower ductile-style deformation.
  • Publication
    Accès libre
    First results of cosmogenic dated pre-Last Glaciation erratics from the Montoz area, Jura Mountains, Switzerland
    (2007)
    Graf, Angela A.
    ;
    Strasky, Stefan
    ;
    Ivy-Ochs, Susan
    ;
    Akçar, Naki
    ;
    Kubik, Peter W.
    ;
    Burkhard, Martin 
    ;
    Schlüchter, Christian
    The Jura Mountains are a perialpine chain bending from the SW to the NW of Switzerland. Alpine ice reached only marginal areas of this chain during the Last Glacial Maximum (LGM) but remaining erratic boulders of alpine origin indicate older glaciations of unknown age, tentatively correlated either with the classical Rissian or with the Most Extensive Glaciation. Presented here are the first results of dating these pre-LGM boulders by using cosmogenic radionuclides (10Be) and noble gases (21Ne). Both data sets are in good agreement within 1σ error, with radionuclide apparent exposure ages ranging from 60 to 107 ka, and noble gas apparent ages from 73 to 123 ka. Considering the importance of erosion on older boulders, the age of the erratics corresponds most probably to Marine Isotope Stage 6. These results are encouraging for further studies on pre-LGM erratic boulders in the mid-latitudes.
  • Publication
    Accès libre
    Tectonics of the Anti-Atlas of Morocco
    (2006)
    Burkhard, Martin 
    ;
    Caritg, Séverine
    ;
    Helg, Urs 
    ;
    Robert-Charrue, Charles
    ;
    Soulaimani, Abderrahmane
    Tectonique de l'Anti-Atlas marocain. L'Anti-Atlas est revu et examiné sous l'angle de sa signification géodynamique comme bassin paléozoïque et comme chaîne plissée paléozoïque. Le raccourcissement est accommodé par le plissement polyharmonique de la couverture, avec une nette implication du socle. Aucun système significatif de chevauchement ni duplex ne s'est développé. L'Anti-Atlas est un bassin intracratonique fortement inversé plutôt qu'une partie de la marge passive de la Paléotéthys. L'inversion doit dater du Carbonifère tardif/Permien précoce. La direction du raccourcissement a changé au cours du temps depuis une direction NW–SE vers une direction nord–sud et peut-être même NE–SW, ce qui conduit à la formation de figures d'interférences de plis en dômes et bassins aux échelles allant de 100 m à 10 km., The Anti-Atlas is reviewed and examined in the light of its geodynamic significance as a Palaeozoic basin and fold belt. Shortening is accommodated by polyharmonic buckle folding of the cover in a thick-skinned fashion without the development of any significant thrust/duplex systems. The Anti-Atlas is heavily inverted deep intracratonic basin, rather than a former passive margin of the Palaeo-Tethys Ocean. Inversion took place in Late Carboniferous to Early Permian times. Main shortening directions changed from NW–SE to north–south and maybe NE–SW through time, leading to the development of dome and basin patterns on scales from 100 m to 10 km.
  • Publication
    Accès libre
  • Publication
    Accès libre
  • Publication
    Accès libre
    Origin of the current stress field in the western/central Alps : role of gravitational re-equilibration constrained by numerical modeling
    (2005-05-20)
    Delacou, Bastien
    ;
    Sue, Christian
    ;
    Champagnac, Jean-Daniel
    ;
    Burkhard, Martin 
    We interpret the strain and stress fields of the western / central Alpine arc on the basis of 2.5D finite element modelling and a recent seismotectonic synthesis. Models have fixed boundary forces and different crustal geometries, so that they respond to buoyancy forces (variations in gravitational potential energies). The seismotectonic regime, characterized by orogen-perpendicular extension in the high topographic core of the belt and local orogen-perpendicular compressional / transpressional deformation in the external zones, appears to be very close to the modelled gravitarional regime. Rotation of Apulia has a minor effect on the current strain or stress fields of the Alpine realm. Nevertheless, it could help to explain the orogen-parallel dextral faulting that is observed all along external zones, from the northern Valais to the Argentera external crystalline massif. Our results highlight the consequences for the Alpine realm of ongoing convergence between the African and European plates. Our interpretation is that collision is no longer ongoing and that buoyancy-driven stresses dominate the present-day geodymamics of the western / central Alps.