The Oligo-Miocene Tectonic Mode Switch: From a Brief Period of Widespread Extension to the Final Closure of the Neuquén Basin

Abstract

Geological observations in the Neuquén Basin indicate a Late Oligocene to Early Miocene episode of extension followed by an abrupt shift towards regional compression. However, the reasons behind this brief extensional episode and the Oligo-Miocene tectonic mode switch are not fully understood. Through the aid of numerical modelling, it has been shown that after a period of limited subduction in Early Palaeogene times, the penetration of Nazca’s slab tip into the mantle transition zone in Late Oligocene times resulted in the renewal of effective subduction due to the effect of the slab pull force. This renewed subduction consists of an initial stage of higher trench hinge retreat and steep slab dips, leading to extension and mantelic upwelling processes east of the trench. Then, the natural evolution of the slab produces a deceleration of roll-back and shallowing of the subduction angle once it reaches the lower mantle, resulting in horizontal shortening. These results indicate that the effect of the slab pull force is a potential responsible for the OligoMiocene tectonic mode switch, causing the opening of a series of intra-arc basins and widespread magmatism partially devoid of arc-like components followed by an increasing influence of the slab in the magmatic arc and the final closure of the Neuquén Basin.