Strain and stress field evolution in the Andean orogenic front between 35-36ºS and its link with fluid migration

BARRIONUEVO, M.; MESCUA, J.; GIAMBIAGI, L.; SURIANO, J.; DE LA CAL, HERNÁN; SOTO, JORGE LUIS; STAHLSCHMIDT, E.

Montgenevre

Congreso; YORSGET Young Researchers in Structural Geology and Tectonics; 2018

TecTask, University of Leeds

The role of inherited structures in the development of the fold-and-thrust belt has been longrecognised in the Southern Central Andes. Here we present our work focused in thedeformation front of this part of the Andes (35-36ºS) to analyse the interaction betweenforelandward migration of the thrust front and previous normal structures but taking intoaccount its role in the fluid migration depending on the stress-field variations.In this area, the Malargüe fold-and-thrust-belt developed as a thick- and thin-skinned beltfrom the Late Cretaceous until present-day. The main phase of deformation started in theMiocene (~20 Ma) and the deformation front progressed eastward into the foreland inconcomitance with the expansion of the magmatic arc in the same direction. Thedeformation in this belt involves previous rift structures of the Triassic-Cretaceous NeuquénBasin, one of the most important oil-producing basins in Argentina, that were reactivatedunder contraction as reverse and strike-slip faults, while new Andean faults both thickskinnedas thin-skinned were created. Many dykes and sills were intruded during the LateMiocene into the Mesozoic sedimentary sequence of the Neuquén Basin near thedeformation front of this belt. These igneous bodies are potential reservoirs for hydrocarbonsas it is shown in other parts of the basin.Based on subsurface information (seismic data, oil-well logs, minifrac tests) as well assurface data (fault slip data, dyke and sill analysis) we hypothesise on how the stress-statechanges and its influence in the faults that could act as flow paths for magmatic fluids as wellfor hydrocarbons. We identified three main orientations in the Mesozoic normal faults thatbounded previous rift-related depocenters, a NNW set, a NNE set and a NW to WNW set.The first and second set (NNW and NNE) were inverted during Andean contraction probablydue to their orientation, whilst the NW to WNW set does not appear to be inverted as reversefaults but with a strike-slip kinematic. This relies on the fact that these faults have the samemean orientation of strike-slip faults which affects quaternary deposits along with the meanorientation of dykes which present strike-slip faulting.Using previous published data of our group as well as other?s authors, we propose a modelwhich relates the progression of the deformation to the foreland and the stress-state changeaccording to the location in the fold-and-thrust belt. In this model during the first stage (17-10Ma) the present-day front was on a foredeep position while the main deformation waslocated to the west and some of the normal faults started to be inverted. Subsequently thedeformation progressed to the east and in the second stage (10-7 Ma) this area was under acompressive to strike-slip stress state. Under this regime, thin- and thick-skinned andeanfaults developed together with the reactivation of NW to WNW normal faults as strike-slipfaults. These strike-slip structures were suitable for magma migration and many of the dykesand sills were emplaced following this trend. Finally, in the third stage (7-1 Ma), while thedeformation continued its migration to the east, pre-andean NNW faults were invertedalongside with thin-skinned reverse and strike-slip faults that were developed under acompressive to strike-slip stress state which continues currently according to our fault-slipdata and oil-well minifrac tests. Hydrocarbon migration gets favoured under this regimeusing the strike-slip faults as fluid conducts.