A petrological and structural assessment of mafic dike swarms in the Patagonian fold-thrust belt during seismic ridge collision

Abstract

Between Viedma and San Martín lakes, in an area of 1200 km2 that partly covers the Southern Patagonian cordillera and the Miocene-Pliocene extra-Andean basaltic plateaus, mafic dike swarms crop out which have not been studied previously. As a result of mapping on satellite images and in the field more than 1500 dikes were identified. The dikes crop out in a zone between rear-arc calc-alkaline intrusions to the west and tholeiitic-alkaline plateau basalts to the east, within the internal and external fold-thrust belt. In order to understand the genetic relationship between the dikes, other igneous units in the region and the structures of the Andean orogeny, a petrological study (including petrography and geochemistry) and a structural analysis of the dike orientations were carried out. The dikes were emplaced close to the surface (<1 km). Three predominant compositions were recognized: basalts, which predominate, followed by tephrites and andesites, the latter restricted to a single sector. We propose preliminarily the following correlations: (i) the dikes of tephritic composition with the Río Carbón Essexite (Eocene), (ii) the andesites with the Chaltén Adakite (Miocene) and (iii) the basaltic dikes with the Pliocene basaltic plateaus. All these units are related in the literature to different stages of ridge subduction and slab window development. The dikes present variable, but not random, preferential orientations, with no relation with composition. Our analysis suggests that dike emplacement was controlled by previous anisotropies in a heterogeneous and very shallow crust. The anisotropy in the internal fold-thrust belt was achieved mainly during Eocene and/or Late Cretaceous contraction, whereas that in the external fold-thrust belt was developed during a less significant Miocene compressive event, thus resulting in a more homogeneous dike orientation. The vertical disposition of the dikes indicates horizontal stretching, as previously proposed for the last stages of Andean growth.