Late Cenozoic stress field evolution in the southern limit of Puna plateau (26º30?-27º40? S)

QUIROGA, R.; GIAMBIAGI, L.B.; WAGNER, F. ; FUENTES, G.; MARDONEZ, D.; ECHAURREN, A.; MESCUA, J.F.; PEÑA, M.; NORAMBUENA SOTO, J.P.; STRECKER, MANFRED R.

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina y UNPSJB

We present a structural analysis using 86 reduced paleostress tensors that document the spatiotemporal evolution of the stress fi eld since the last 24 my across the southern margin of the Puna Plateau in transition with the Frontal Cordillera, Fiambalá basin and Sierra Pampeanas areas (26º30?-27º40? S, Fig. 1). The paleostress tensors were obtained using the Wintensor software (Delvaux and Sperner 2003), analyzing ~1000 fault-slip data measured throughout the study area and compiled from Quiroga et al. (2021). The software generates the stress tensor by separating mechanically-compatible, homogeneous data subsets (e.g., Delvaux and Sperner 2003). We compare the compression directions from the new paleostress analysis with field observations that can help identifying the age of the diff erent deformational events, considering the absolute and relative ages of deformed and non-deformed stratigraphic units and cross-cutting relations between the analyzed faults. To elucidate the regional implication of this analysis, we combine the resultswith regional structural mapping, forward modeling of three regional sections using Andino 3D software, and U-Pb geochronology. The results show that in the Western border between the Puna and Cordillera Frontal an E-W compression started after the late Oligocene and continued until the middle Miocene, inferred from 1) A new U-Pb age of 11,8 ± 0,4 Ma determined from the two youngest zircons, taken from a subhorizontal layer without any evidence of contractional deformation, that covers the deformed late Eocene-early Oligocene rocks. 2) Because these structures are non-aff ected by normal fault documented during the Oligocene (Mpodozis etal. 2020), this compressive regime is later than the early Oligocene. The structures related to this event are aff ected by a strike-slip to transpressive tectonic regime, generating a NW-SE system of high-angle strikeslip faults with sinistral displacement (Fig. 1). In the central border between Puna Plateau and Fiambalá basin, we document an E-W compression that started in the Miocene and continued until Pliocene times, related to the development of a N-S-striking fault-and-thrust belt, with east vergence. The compression shifted from E-W to N-S after the middle Pliocene (~3 my), aff ecting the northern area of the structural system developed in the previous stage. In the Eastern border of the Plateau (Puna Plateau-NW Sierras Pampeanas) the compression was active since the Miocene during almost the entire period but started to be synchronous, since the early-middle Pliocene (~5), with N-S compression and strike-slip deformation inferred from contractional structures aff ected by strike-slip and reverses faults (Quiroga et al. 2021). In the high-altitude areas of this region (PV, Fig. 1), reverse structures aff ecting the middle to upper Miocene sequences suggest an E-W compression and strike-slip faults deformation followed this event (Fig. 1). Thecompressive and strike-slip deformation are previous to the N-S extension documented in the area since ~5 Ma (e.g Schoenbohm and Strecker 2009, Montero et al. 2010). On a regional scale, we associate the reduced paleostress tensors to three orogenic stages involved with the evolution of the Central Andes at this latitude: (1) The E-W compression documented from late Oligocene to middle Miocene in the area, is interpreted to be related to a fi rst stage of the Andean construction,thickening and growth of the crust and topographic uplift; (2) The onset of a strike-slip regime since~11 Ma in the western border of the Puna Plateau and ~5 Ma in its eastern border, which started earlier, but very close to (3) the extensional regime previously documented since ~5 Ma, in the Puna Plateau. These last two events refl ect a transitional stress fi eld due to the increase of vertical stress (shift from minimum vertical stress (σ3) in the compressive regime in the stage (1), to an intermediate vertical stress (σ2) in a strike-slip regime in the stage 2) during the thickening of the crust and an increment of gravitational potential energy. The transitional stage was the passage from the compressional regime related to the orogenic construction, to the extensional regime associated with the orogenic collapse experienced in the high areas of the Puna Plateau. The documented N-S extension aff ecting the highest areas was synchronous to the N-S compression identifi ed in this work, aff ecting the lower areas beyond the Plateau, as a result of the boundary eff ect and transference of gravitational potential energy from the high areas to lower areas during the uplift of the plateau.