FEEDBACKS BETWEEN MAGMATISM AND STRESS AND STRAIN FIELDS IN THE MALARGüE FOLD AND THRUST BELT, SOUTHERN CENTRAL ANDES (34º-36ºS)

The Malargüe fold and thrust belt is a thick-skinned belt formed during the subduction-related Andean orogeny. The main phase of deformation took place since the early Miocene (~20 Ma), and defomation advanced towards the foreland, reaching the present location of the orogenic front near the Miocene-Pliocene boundary. The magmatic arc records an expansion towards the foreland from 20 to 2 Ma, associated to an evolution from an immature arc with tholeitic affinities to a mature arc with calkalkaline geochemistry developed over thick crust. Previous works have linked this structural and magmatic evolution to an episode of slab shallowing, interpreting that the expansion of the magmatic arc is related to a change in the location of slab dehydration. This implies that magmas traversed the crust vertically. In contrast, we propose that the ascent of magmas is strongly controlled by crustal structures, and that the location of magmatism on the surface is not a reflection of the location of slab dehydration. We will analyze the current stress field in the orogenic front and the inner orogen based on fault-slip and oilwell data, and discuss its control on the ability of magmas to rise through the crust. We will explore the relationship between these parameters during the Miocene history of the belt, and propose an integrated model in which a compressional stress field hinders vertical magma ascent through the crust, resulting in the use of low-angle active faults and detachments as pathways. Horizontal shortening thickens the crust, further difficulting magma ascent. This produces an expansion towards the foreland that accompanies the migration of deformation. The Plio-Quaternary focusing of the arc in the inner part of the orogen observed in the Malargüe fold and thrust belt is the result of the activation of sub-vertical crustal strike-slip faults, evidenced in kinematic data of faults and in seismic activity in the current arc region. This conforms ideal pathways for vertical magma ascent. In this way, we explain the coupled evolution of the magmatic arc and fault activity in the Malargüe fold and thrust belt as a result of the dynamics of the orogenic system, with no need for changes in slab dip.