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Eric Ferre, Southern Illinois University (United States)
Olivier Galland, Physics of Geological Processes, University of Oslo (Norway)
Thomas Kalakay, Rocky Mountain College (United States)
Domenico Montanari, Centro di Eccellenza per la Geotermia di Larderello (Italy)
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In both extensional and compressional zones, plutonic bodies appear to concentrate along major fault zones. Along continental arcs, the ascent and intrusion of felsic magma is thought to be controlled by extentional tectonics and structures, despite the overall contractional setting of such orogens. To address this apprent contradiction, we propose that active shallowly-inclined, dip-slip reverse shear zones can play a significant, yet often underestimated, role in guiding felsic magma towards the surface.
A short review of six representative synkinematic granites, of different ages, all emplaced along thrusts is presented: the Early Proterozoic Chilimanzi granites, Zimbabwe Craton, the Neoproterozoic Rahama granite, Nigeria-Brazil Transaharan Belt, the Palaeozoic Wyangala granites, Lachlan Fold Belt, Australia, the Late Cretaceous granites of Southwest Montana, USA, the Late Cretaceous to Miocene granites of the Hidaka Belt, Hokkaido, Japan, and the Miocene granites, High Himalaya, Tibet. Our review highlights that most of the considered granite bodies exhibit a flat-lying laccolithic shape, contrasting with the commonly assumed inverted teardrop shape inferred from the diapiric ascent model. In addition, those intrusions exhibit structural and chronological relationships with the thrusts faults and associated structures, suggesting that such structures played a major role in the transport and emplacement of the granites.
In order to understand the mechanisms of granite intrusion along thrusts, we resorted to two distinct laboratory experimental studies. The scaling of the considered experimental techniques were different, i.e. one simulated low-viscosity magma and the other higher viscosity magma. Both of them consisted of a bed of granular media shortened by a piston, while a visco-plastic fluid was injected at constant flow rate at the base of the bed. Although the experimental techniques were different, they yielded similar geometries. Thrust faults accommodated the shortening, and the compressional state of stress favoured horizontal transport of the model magma into a sill. When the model magma reached a thrust fault, it flowed upward by following the fault. The experiments also provided evidence that the emplacement of the model magma partly controlled the deformation pattern.
The geological observations and experimental results show that magma can be transported and intruded along thrust faults and ramps. We infer that thrusts represent significant weak heterogeneities that control the emplacement of magma in the continental crust. These results have wide implications for the understanding of magma transport, and show that melts can transport laterally over significant distances. In addition, our results are useful to understand the plumbing systems of active volcanoes built up in thrusting tectonic settings.
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