congresos y reuniones científicas
Links between magmatic arc behavior and upper plate deformation in the Malargüe Andes
Congreso; VI LASI The physical geology of subvolcanic systems - laccoliths, sills and dykes; 2019
Institución organizadora:
CONICET-AGA-University of Oslo
The Neogene volcanic arc in the Malargüe fold- and-thrust belt (35ºS) has experienced an expansion towards the foreland since the late early Miocene and a retreat to its current position along the Argentina-Chile drainage divide in the Plio-Quaternary (citas).The expansion towards the foreland coincides roughly in time and space with the advance of contractional deformation and uplift (Mescua et al., 2014). However, a precise chronology of magmatic activity and its relationship with deformation/stress state in the upper plate has not been presented. In this presentation we will review recent data on: (i) absolute dating of volcanic arc rocks using modern methods (U/Pb, Ar/Ar); (ii) recent geochemical analysis of volcanic arc rocks; and (iii) timing of deformation/uplift events obtained from fault-slip data and thermochronological studies. The location of the volcanic front through time is usually interpreted as a proxy for the subduction angle. We propose that the deformation in the upper plate has also an important role in the location of magmatic activity in continental subduction systems, through (1) the stress state in the upper plate, (2) crustal thickness variations and (3) the activity of faults that act as pathways for magma migration. We document that following the onset of contraction at ~20 Ma in the westernmost sector of the belt, the arc expanded at ~18 Ma to the inner sector of the Argentinean Andes, and volcanism was never interrupted in this sector until the late Pleistocene (Sruoga et al., 2008; 2009). Until 13 Ma, this magmatism has geochemical characteristics intermediate between arc and retroarc (tholeiitic) volcanism. These volcanic rocks are deformed and folded. Since 13 Ma, volcanic rocks in the inner sector with clear arc signature and steeper REE patterns are not deformed but only tilted in fault blocks, coincident with crustal thickening and migration of contraction to the external sector of the Malargüe fold-and-thrust belt.  The eastward advance of deformation and uplit was accompanied by arc expansion during the prolonged contractional period that produced the current orogenic front of the Malargüe fold-and- thrust belt in the late Miocene. Syn-contractional volcanics along the external sector of the belt span the 13-5 Ma period (Nullo et al., Horton et al.)The easternmost volcanic edifices with calk-alkaline geochemistry and arc signature were emplaced at 5-4 Ma, 200 km east of the current arc. During this same period, a well established volcanic front seems to be developed at Risco Plateado, only 30 km east of the current arc. This volcanic front remained active until the early or Mid-Pleistocene (~ 1 Ma). Limited thermochronological data (U-Th/He on apatite) suggest out-of-sequence exhumation took place in the inner Malargüe fold-and-thrust belt at around 4 Ma (Bande et al., 2019), indicating that the Risco Plateado arc was coeval with active thrusting which may have channeled magma ascent. This period also records contraction and uplift in the easternmost Malargüe fold-and-thrust belt along the orogenic front (Silvestro et al., 2005).  The retreat of arc volcanism to its present position along the Argentina-Chile divide took place   in the middle Pliocene (after 1 Ma). This coincides with a change of the stress state in this region from  contraction (registered only in pre-Pliocene rocks) to strike-slip, which activated major dextral N-S faults. This is recorded in fault slip-data and in the seismicity, as determined by focal mechanisms. These deep faults likely act as fluid pathways allowing a focused ascent of magma across the crust. We conclude that the Neogene behaviour of the arc shows an expansion coinciding with upper-plate contraction and crustal thickening. Between 16 and 5 Ma volcanism was distributed in a ~100 km-wide region, but a clear volcanic belt cannot be recognized. After 5 Ma, the arc was still wide, but a volcanic front with N-S trend developed along the Risco Plateado sector, coincident with the reactivation of thrusts in this area. After 1 Ma, the arc retreated to its current position favored by the development of N-S crustal strike-slip faults that act as magma pathways.