Transtension y transpresion del Jurasico Medio-Superior al Cretacico Inferior durante la construccion del arco magmatico en Chile central: evidencia a partir de enjambres de diques maficos.

Christian Creixell,Paulina Vasquez,Miguel Angel Parada,Diego Morata,Cesar Arriagada,Carlos Perez De Arce

doi:10.5027/andgeov38n1-a04

Abstract

The Middle-Late Jurassic mafic dike swarms of central Chile between 33o and 33o45'S register the tectonic activity of the contemporaneous arc 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 arc. The Middle-Upper Jurassic and Lower Cretaceous 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 arc at this latitude. Middle to Upper Jurassic dike swarms (Concon 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 emplacement under sinistral transtension. During this stage, dikes acquired a fabric characterized by magnetic foliation clockwise oblique to dike trend and gently plunging lineations. This stage was followed rapidly by dike emplacement under sinistral transpression, with associated steeply plunging lineations in the dikes, reverse shear zones in the country rocks and local occurrence of horizontal mafic dikes. On the other hand, the Lower Cretaceous 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 emplacement correlates with changes in the geochemical composition of the dikes, suggesting a strong coupling between tectonics and nature of magma supplies in the arc. Finally, the current results show that the Mesozoic evolution of the Coastal Batholith of central Chile occurred 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 Jurassic times.

Highlights

Subduction zones concentrate crustal deformation caused by convergence of subducting and overriding plates
The Middle-Upper Jurassic and Lower Cretaceous mafic dike swarms in the Coastal range of central Chile has been studied through field observations, geochronology and anisotropy of magnetic susceptibility (AMS) to document the tectonic and magmatic evolution of the Jurassic-Cretaceous arc at this latitude
Field data and AMS fabrics indicate that the Concon Mafic Dike Swarm (CMDS) and Cartagena Mafic Dike Swarm (CrMDS) were emplaced in two stages, a first sinistral-transtension, followed by sinistral transpression

Summary

Introduction

Subduction zones concentrate crustal deformation caused by convergence of subducting and overriding plates. The Mesozoic arc of the central Andes exposed along the Coastal Ranges of north and central Chile has proved to be an exceptional site to describe and understand upper-plate deformation that occurred during Mesozoic subduction along the western margin of Gondwana. This simple model does not apply, if there are pre-existing fractures that can open following the σ3 direction but are not oriented perpendicular to σ3 In these cases, dike orientation cannot be considered directly as paleostress indicators and a more careful analysis of emplacement mechanisms, mainly studying emplacement fabrics, is needed to understand how dikes are related to the stress state of the crust during magma intrusion (e.g., Delaney et al, 1986; Blumenfeld and Bouchez, 1988; Féménias et al, 2004; Clemente et al, 2007). The study of emplacement mechanisms of dike swarms has been proved as can be an efficient tectonic tool to understand the evolution of ancient magmatic arcs (e.g. Glazner et al, 1999)

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