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Andean geology

On-line version ISSN 0718-7106

Abstract

CREIXELL, Christian et al. Transtensión y transpresión del Jurásico Medio-Superior al Cretácico Inferior durante la construcción del arco magmático en Chile central: evidencia a partir de enjambres de diques máficos. AndGeo [online]. 2011, vol.38, n.1, pp.37-63. ISSN 0718-7106.

The Middle-Late Jurassie mafic dike swarms of central Chile between 33°and 33°45'S registerthe tectonic activity of the contemporaneous are represented by the Coastal batholith. These dike swarms evidence alternate episodes of transtension and transpression across NW-striking structures, which controlled the construction of the magmatic are. The Middle-Upper Jurassie and Lower Cretaeeous mafic dike swarms in the Coastal range of central Chile has been studied through field observations, geochronology and AMS (anisotropy of magnetic susceptibility) to document the tectonic and magmatic evolution of the Jurassic-Cretaceous are at this latitude. Middle to Upper Jurassie dike swarms (Concón and Cartagena mafic dike swarms) were emplaced between 163 and 157 Ma, along NW-SE to WNW-ESE-striking host fractures, registering a first stage of magma emplaeement under simstral transtension. During this stage, dikes acquired a fabric characterized by magnetic foliation clock wise oblique to dike trend and gently plunging lineations. This stage was followed rapidly by dike emplaeement under sinistral transpression, with associated steeply plunging lineations in the dikes, reverse shear zones in the country rocks and local oceurrence of horizontal mafic dikes. On the other hand, the Lower Cretaeeous El Tabo dike Swarm was emplaced along tensile fractures that do not register shear displacements along their walls. These dikes were emplaced at shallower crustal levels along tensile fractures.Within each dike swarm, differences in the tectonic style of emplaeement correlates with changes in the geochemical composition of the dikes, suggesting a strong coupling between tectonics and nature of magma supplies in the are. Finally, the current results show that the Mesozoic evolution of the Coastal Batholith of central Chile oceurred in several stages of deformation and not under a simple scenario of extensional tectonics. This magmatism and deformation were strongly controlled by NW- to WNW-striking fractures. These structures are spatially correlated with regional-scale Cenozoic lineaments that are oblique to the orientation of the Andean orogen. We suggest that these lineaments are features inherited at least from Jurassie times.

Keywords : dike swarm; Magma emplacement; Magnetic fabrics; Transtension; Transpression; Jurassie; NW-SE structures.

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