Quantifying stresses and strains from the Jura Arc, and their usefulness in choosing a deformation model for the region
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In a study of the Jura arc, focusing on the aspects of finite strain, displacements and differential stresses, we have produced new data and models to explain the formation of the Jura arc and suggest that these models may be applicable as a classification for other arcs around the world. The classification of arcs has been greatly simplified to just three end member models. These end members are defined by the mechanisms of formation of the arcs. The different mechanisms imply different displacement vector and finite strain patterns. Additionally, all models involve some degree of passive rotation of material within the arc. We point out that previous classifications overlap, are contradictory or incorrect. New finite strain data recorded in the Molasse Basin from calcite twinning shows tiny strains (ca. 0.1%). These strains are an order of magnitude (at least) less than those suggested by folding (>1%) across the basin. The orientation data from finite strain analysis shows a divergent pattern of maximum shortening axes across the Swiss Molasse Basin. This mirrors the pattern from within the Jura to some degree. The shortening axes are very often oblique to fold axes trends. The strain data fits very well with a modelled "Primary Arc". Displacements have been estimated relative to a fixed line on the edge of the Internal Jura. This relative displacement field shows a near constant orientation over a small area of the Jura (ca. 50x40km). There is no strong lateral displacement gradient across the region which we take to be due to the small size of the zone studied. Importantly, our results suggest that there is no arc parallel extension (arc parallel strains are very close to 0%). This rules against both "spreading" and classical "orocline" models of formation for the Jura arc. Plan view strain models have been used successfully to show the consequences of different types of transport parallel simple shear on a region. Without some differential transport of a lower boundary of a region (developing strong simple shear) we do not reproduce the arcuate strain trajectories we find from arcuate mountain belts. We suggest that the strain features such as stylolites or twins are formed by the first increments of strain across a region and record a punctual strain state. The full finite deformation of the region, even in plan view, is accommodated by faulting and folding. transport parallel simple shear models remain a subset of "Primary Arcs" and this model is favoured for the formation of the Jura in the light of model results. Differential stresses across the Molasse Basin have been measured using different palaeopiezometric techniques based on calcite twinning. We find a large difference between results for identical samples depending on the technique used and suggest that this is due to the conditions under which the piezometers were originally tested and calibrated. we also suggest that on the basis of mechanical consideration and present day measures, we can favour the lower values over the higher ones.
Thèse de doctorat : Université de Neuchâtel, 2000 ; 1511
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