Browsing by Author "Roquer, Tomas"

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    Decoding the state of stress and fluid pathways along the Andean Southern Volcanic Zone
    (2023) Perez-Estay, Nicolas; Ruz-Ginouves, Javiera; Perez-Flores, Pamela; Sielfeld, Gerd; Roquer, Tomas; Cembrano, Jose
    Decoding means decrypting a hidden message. Here, the encrypted messages are the state of stress, fluid pathways, and volcano tectonic processes occurring in volcanoes of the Andean Southern Volcanic Zone (SVZ). To decode these messages, we use earthquake focal mechanisms, fault slip data, and a Monte Carlo simulation that predicts potential pathways for magmatic and hydrothermal fluids. From this analysis, we propose that SVZ volcanoes have three end-member stress patterns: (i) Stress-A, a strike-slip regime coupled with the regional far-field tectonic stress; (ii) Stress-B, an extensional regime that may be promoted by volcanic edifice loading and upward pressure due to magma inflation occurring within the upper brittle-crust; and (iii) Stress-C, a local and transient fluid-driven stress rotated similar to 90 degrees from Stress-A. Notoriously, Stress-C pattern was observed in most volcanoes with historical eruptions. We propose that volcanoes presenting Stress-B are attractive geothermal targets, while Stress-C could be used as a predicting signal for impending eruptions.
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    Multi-scale flow structure of a strike-slip tectonic setting: A self-similar model for the Liquine-Ofqui Fault System and the Andean Transverse Faults, Southern Andes (39-40 degrees S)
    (PERGAMON-ELSEVIER SCIENCE LTD, 2022) Roquer, Tomas; Arancibia, Gloria; Crempien, Jorge G. F.; Mery, Domingo; Rowland, Julie; Sepulveda, Josefa; Veloso, Eugenio E.; Nehler, Mathias; Bracke, Rolf; Morata, Diego
    The flow structure of a brittle crustal volume is defined by the multi-scale geometric and hydraulic properties of its fracture meshes. The length density distribution n(L,l) and the transmissivity distribution K(L,l) control the hydrologic scaling, where l is fracture length and L is the system size. The flow structure might display at most three key hydrologic scales: the connection scale, above which flow is focused in few critical paths; the channeling scale, above which flow is distributed in several paths; and the homogenization scale, above which permeability approaches a constant value. According to these scales, the hydrological structure could be distributed or clustered, thus having a clear impact in geothermal exploration campaigns and reservoir modeling. In this work, we determine the multi-scale flow structure for the Liquine-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF) in the Southern Andes, by establishing the hydrologic scaling they follow. Using fractal statistics, we integrated geological data at the regional, meso-and micro-scale, including image analysis from X-ray microtomography. Our results suggest a self-similar, dense network with n(L,l)similar to l(-a) and a = 2.6-2.9, from the regional scale where the LOFS and ATF interact to the meso-and micro-scale within highly fractured areas of the LOFS. Scaling models are constrained by the length distribution, and other power-law functions reflecting the geometric arrangement of fractures, as well as the spatial distribution of superficial geothermal occurrences. Thus, we expect the hydrologic scaling to depend on the transmissivity distribution. Lognormal transmissivity distribution yields a permeability increase with scale, from the connection to the homogenization scales; whereas power-law transmissivity distribution yields a permeability increase from the connection scale without a limiting value. Approximations of the connection scale are around 10(-3)-10(0) m; the channeling scale, around 100-104 m; and if the homogenization scale exists, it should be equal or greater than 10(3)-10(4) m. Finally, the results presented here could to define the internal architecture of fracture meshes in fault-controlled fluid flow, and be used to select an appropriate hydrologic model according to the analyzed scale. Therefore, these findings must be taken into consideration in future geothermal prospecting, modeling and exploitation.
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    Regolith-hosted rare earth genesis under humid climates in the Coastal Cordillera of the Central Andes
    (2023) Bustos, Nicolas; Marquardt R., Carlos; Roquer, Tomas; Vergara, Paulina
    Regolith-hosted rare earth element deposits (RH-REE) have been recently found in Chile, nonetheless their ore formation controls, which include temperate to arid climate, intrusive rocks with weatherable REE minerals, and gentle slopes or pediplains, among others, differ from the most common deposits in Asia. This work addresses mineralization and exploration methods for RH-REE deposits across the Chilean Coastal Range. The studied prospect is associated with late Carboniferous I-type tonalitic intrusions. Mineralogical analysis (TIMA-X) indicates that allanite and monazite are the main REE-bearing minerals in the host rock. The regolith in the study area is up to 60 m deep and contains an exchangeable-REE concentration up to 2000 ppm, as determined by portable X-ray fluorescence (pXRF) and ICP-MS. We were able to recognize that REE-adsorption takes place both in preserved and partially eroded regoliths. Progressive weathering of these regoliths causes the pedolith/saprolith limit to be dynamic and accumulate REE due to the breakdown of allanite and monazite. Our data shows that the search for pediplains on suitable lithologies, mapping of regolith profiles, and their analysis via pXRF by quantifying yttrium, appears to be a suitable strategy for early exploration of RH-REE deposits in the Chilean Coastal Range.
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    Selective reactivation of inherited fault zones driven by stress field changes: Insights from structural and paleostress analysis of the Pocuro Fault Zone, Southern Central Andes (32.8 degrees S)
    (PERGAMON-ELSEVIER SCIENCE LTD, 2022) Taucare, Matias; Roquer, Tomas; Heuser, Gert; Perez-Estay, Nicolas; Arancibia, Gloria; Yanez, Gonzalo; Viguier, Benoit; Figueroa, Ronny; Morataa, Diego; Daniele, Linda
    This study aims to explain the selective reactivation of normal faults during the Andean orogeny at the Southern Central Andes western flank. We conducted a structural mapping and paleostress field reconstruction in the regional-scale Pocuro Fault Zone (PFZ) at 32.8 degrees S. Results reveal that the architecture of the PFZ results from at least two deformation phases, each revealing an individual progressive and gradual evolution. The earliest deformation phase is recorded by two similar to NS-striking normal faults involving a 5 km wide damage zone characterized by quartz-laumontite and calcite veins that were developed under an extensional regime with a WNW-ESE-trending sigma 3-axis. The latest deformation phase is recorded by one NS-striking reverse-dextral fault with goethite-hematite syn-tectonic precipitation and two NW-striking reverse-sinistral faults. Reverse faults were developed under a compressional/transpressional regime characterised by an ENE-WSW-trending sigma 1-axis with a sigma 2-/sigma 3-axis permutation. From a geophysical data reassessment, we inferred that observed faults in the surface within the PFZ are regional-scale deep-seated structures. Considering previous geochronological data, we correlated the earliest and latest phases with the Abanico Basin extension (middle Eocene-early Miocene) and its subsequent inversion (Miocene). Given the neotectonic evidence (geomorphic markers and deformation of unconsolidated deposits), the latter phase likely remains active. Quartz-laumontite cementation of the fault core's cataclastic material promotes mechanical strengthening leading to negative feedback for the reactivation of inherited normal faults as reverse ones. Conversely, the concentration of fractures in the damage zone between the normal faults promotes mechanical weakening resulting in a preferential area for the propagation of reverse fault during the compressive/transpressional phase.