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

LOBO, CONSTANZA; GUILLOT, MAURICIO GONZÁLEZ; GHIGLIONE, MATÍAS C.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2022

0895-9811

Between Viedma and San Martín lakes, in an areaof 1200 km2 that partly covers the Southern Patagonian cordilleraand the Miocene-Pliocene extra-Andean basaltic plateaus, mafic dike swarms cropout which have not been studied previously. As a result of mapping on satelliteimages and in the field more than 1500 dikes were identified. The dikes cropout in a zone between rear-arc calc-alkaline intrusions to the west andtholeiitic-alkaline plateau basalts to the east, within the internal andexternal fold-thrust belt. In order to understand the genetic relationshipbetween the dikes, other igneous units in the region and the structures of theAndean orogeny, a petrological study (including petrography and geochemistry)and a structural analysis of the dike orientations were carried out. The dikeswere emplaced close to the surface (<1 km). Three predominant compositionswere recognized: basalts, which predominate, followed by tephrites andandesites, the latter restricted to a single sector. We propose preliminarily thefollowing correlations: (i) the dikes of tephritic composition with the RíoCarbón Essexite (Eocene), (ii) the andesites with the Chaltén Adakite (Miocene)and (iii) the basaltic dikes with the Pliocene basaltic plateaus. All theseunits are related in the literature to different stages of ridge subduction andslab window development. The dikes present variable, but not random, preferentialorientations, with no relation with composition. Our analysis suggests thatdike emplacement was controlled by previous anisotropies in a heterogeneous andvery shallow crust. The anisotropy in the internal fold-thrust belt wasachieved mainly during Eocene and/or Late Cretaceous contraction, whereas that in the external fold-thrust belt wasdeveloped during a less significant Miocene compressive event, thus resultingin a more homogeneous dike orientation. The vertical disposition of the dikes indicateshorizontal stretching, as previously proposed for the last stages of Andeangrowth.