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XVII Reunión de Tectónica: Resúmenes
Abstract
Este libro contiene los resúmenes de trabajos presentados en la XVII Reunión de Tectónica, llevada a cabo en la ciudad de La Rioja, Argentina, entre los días 3 y 5 de septiembre de 2018. Estudiantes y profesionales de Ciencias de La Tierra presentan y discuten los avances científicos originales alcanzados o en desarrollo sobre temáticas relacionadas a la tectónica y geología estructural a diversas escalas, desde la microestructuras hasta la arquitectura litosférica. Las áreas temáticas incluyen: evolución de basamentos cristalinos (deformación y adición magmática), tectónica preandina, tectónica Andina, modelado y geofísica, neotectónica, y análisis estructural aplicado y riesgo sísmico (geotecnia, minería, sistemas petroleros riesgo sísmico).
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In the southern central Andes at 37-38°S latitude, the Chos Malal fold-and-thrust belt (FTB), which results from the Late Cretaceous closure of the Neuquén Basin, has generated increasing interest because of its potential for hydrocarbon exploration. Using detailed field mapping, seismic reflection, and well data analysis, we have produced balanced cross sections, which combined with apatite and zircon (U-Th)/He, and fission track thermochronology from samples distributed along the FTB, bring new constraints on the chronology of the structural development of the Chos Malal FTB. Fully reset samples obtained from the Early Jurassic rocks at the bottom of the sedimentary sequence exposed in the Cordillera del Viento, a major basement-involved hinterland structure, permit to quantify its cooling rate from 5.4 ± 4.1 and 3.8 ± 3.2 °C/Ma between 70 and 55 Ma down to between 2.0 ± 1.3 and 1.3 ± 0.9 °C/Ma after 55 Ma until the present. Detrital apatite fission track ages from Late Jurassic and Early Cretaceous sandstones reveal that tectonically driven exhumation through basement-involved thrusting has occurred at ~15-7 Ma in both the inner and outer sectors of the FTB. Finally, the cooling and exhumation of the Las Yeseras-Pampa Tril basement-involved anticlines at the mountain front at ~9-7 Ma, slightly younger than previously assumed, suggests a normal sequence of faulting propagation. Our proposed thermostructural model of the Chos Malal FTB contributes to a better understanding of the tectonic evolution of this segment of the Andes.
La configuración actual de la Precordillera Oriental, documenta la evolución geológica y los distintos eventos tectonomagmáticos acaecidos en el margen activo del supercontinente de Gondwana desde el Proterozoico hasta la actualidad. Si bien el magmatismo neopaleozoico-mesozoico inferior está ampliamente difundido en varias provincias geológicas del centro-oeste de Argentina y su evolución petrológica y tectónica es bastante conocida, los antecedentes respecto a la presencia de cuerposígneos en la Precordillera de San Juan son escasos. La presente contribución aporta datos para la caracterización geológico-petrólógica de los cuerpos traquíticos de edad triásica aflorantes en el cerro La Chilca y genéticamente emparentados con la Traquita La Flecha.
Early Paleozoic basement of the eastern North Patagonian Massif includes low- and high grade metamorphic units, which consist mainly of alternating paraderived metamorphic rocks (mostly derived from siliciclastic protoliths) with minor intercalations of orthoderived metamorphic rocks. In this contribution we provide a better understanding of the tectonic setting in which the protoliths of these units were formed, which adds to an earlier suggested idea. With this purpose, we studied the metasedimentary rocks of the low-grade Nahuel Niyeu Formation from the Aguada Cecilio area combining mapping and petrographic analysis with U-Pb geochronology and characterization of detrital zircon grains. The results and interpretations of this unit, together with published geological, geochronological and geochemical information, allow us to interpret the sedimentary and igneous protoliths of all metamorphic units from the massif as formed in a forearc basin at ~520-510 Ma (Nahuel Niyeu basin). It probably was elongated in the ~NW-SE direction, and would have received detritus from a proximal source area situated toward its northeastern side (present coordinates). The basin migth be related to an extensional tectonic regime. Most likely source rocks were: (1) 520-510 Ma, acidic volcanic rocks (an active magmatic arc), (2) ~555->520 Ma, acidic plutonic and volcanic rocks (earlier stages of the same arc), and (3) latest Ediacaran-Terreneuvian, paraderived metamorphic rocks (country rocks of the arc). We evaluate the Nahuel Niyeu basin in an autochthonous position of the eastern North Patagonian Massif, adding to the discussion of the origin of Patagonia.
Mineral systems associated with extensional fault systems in continental environments and not related to magmatic activity involve various deposit models that can be grouped into a single system which would plausibly explain the source and chemical composition of fluids, the formation depth, the source of heat and the structural history within a common geologic setting.
Mineral deposits related to this tectonic setting are those described within the detachment-related model, including massive replacements, stockworks and veins of Cu and Fe oxides (with or without Au), polymetallic sulfide veins, barite and/or fluorite veins and stratabound and vein Mn deposits. This paper reviews the abovementioned deposits as well as others related to this tectonic setting, such as Se-rich polymetallic deposits, Almadén type Hg deposits, U-Ni-Co-As-Ag polymetallic deposits with subordinated Bi-Cu-Pb-Zn (five element deposits), simple polymetallic Pb-Ag-Zn deposits, and the members of the IOCG clan (including the Au-Ag detachment-related deposits) involved in the Cu-Fe (-Au) model deposits.
All these mineral deposits are the result of fluid motion unrelated to magmatism in areas of thermal anomalies. These fluids collect certain elements producing a district mineralogical specialization with inhomogeneities in the distribution of mineralization types along the region affected by rifting. Fluid homogenization temperatures and salinities range between 60° and 430 °C, and 0 and 27 equivalent wt% NaCl, respectively. The O, S and D isotope composition is consistent with basinal poral fluids derived from meteoric waters under different P-T conditions in an active faults environment. Pb isotopes suggest that Pb derives from a mixture of rocks with a long period of residence in the upper crust and rocks deformed along repeated orogenic cycles with a contribution of Pb from the lower crust and even the mantle. Fluid flow along fault planes leads to different types of hydrothermal alterations depending on P-T conditions, particularly propylitization and low temperature potassium metasomatism.
All such deposits can be found in two different geotectonic environments of metallogenic interest involving extensional faults associated with detachment zones in depth; i.e., 1) continental extension in a back-arc environment and 2) extension with rift development and generation of oceanic crust in a passive continental margin. Although the economic importance of this group of deposits is mainly related to industrial minerals, non-magmatic IOCG deposits could significantly augment the economic potential of this setting. The rift setting itself, without relation to detachments, is favorable for concentrating metals in stratabound deposits such as SEDEX ores. Additionally, the development of deposits directly related to magmatic activity contributes to the economic interest of this environment.
The Sierras Pampeanas of Argentina have been used as a classic case study to understand the processes and mechanisms involved in thick-skinned deformation; however, the history of exhumation and uplift of these ranges remains enigmatic. This study presents new thermochronology and field observations from the Sierra de Velasco, one of the highest relief ( > 4 km) and least studied mountains in the Sierras Pampeanas. Advances in the annealing and diffusion kinetics of the apatite fission track and (U-Th-Sm)/He systems provide the tools to interpret a data set with a complex Carboniferous to Miocene cooling history. Our results show that rocks sampled across 2 km of structural depth have experienced protracted exposure to temperatures at or above ~60 °C from ca. 320 to 120 Ma. The well-constrained thermal history allows us to identify subsequent thermal perturbations including an elevated geothermal gradient during Cretaceous rifting, late Cretaceous- Paleocene cooling due to isothermal relaxation and/or exhumation, and accelerated Miocene exhumation. Quantitative estimates of the minimum rock overburden suggest that 1.0 ± 0.8 km of rock has been eroded from the Sierra de Velasco since the early Miocene. Although a low geothermal gradient (≤25 °C/km) may support the exhumation of Sierra de Velasco coincident with the onset of flat-slab subduction in the late Miocene, we suggest inherited paleotopography existed before the Miocene and possibly since the Paleozoic. Final cooling and exhumation beginning in the early to middle Miocene, concurrent with the onset of flat slab subduction, contributed to the topography observed today, but cannot explain the entirety of the modern relief. We propose that a history of long-lived topography may be extrapolated throughout the Sierras Pampeanas region.
Geothermal fields in subduction-related orogens are closely linked to areas characterized by young magmatic and tectonic activity, both in arc- and back-arc settings. The spatio-temporal interaction of Quaternary volcanic complexes with regional extensional and transtensional structures might favor a hydrothermal circuit between meteoric water and magmatic fluids. This study encompasses a kinematic analysis of fault structures from the high-enthalpy system located at the western flank of the Domuyo volcano in Argentina. An analysis of remote sensing data was applied to detect regional lineaments, lineament density, and to identify fracture patterns possibly associated with the different deformational stages documented in the area. These results were combined with detailed fracture analysis and kinematic study of mesoscale faults, as well as existing geological, structural, and geophysical data. The integration suggests that the fluid dynamics of the Domuyo geothermal field are directly conditioned by pre-existing basement structures that were reactivated as normal faults during Pliocene-to-Quaternary times. Furthermore, the intensely fracture late Triassic - early Jurassic units are interpreted as the potential level for the reservoir. The fault reactivation was likely associated with the accommodation of regional extension along pre-existing fault structures, and locally enhanced by hydrothermal effects of the Domuyo geothermal field.
We report a study integrating 13 new U–Pb LA-MC-ICP-MS zircon ages and Hf-isotope data from dated magmatic zircons together with complete petrological and whole-rock geochemistry data for the dated granitic rocks. Sample selection was strongly based on knowledge reported in previous investigations. Latest Devonian–Early Carboniferous granite samples were collected along a transect of ~ 900 km, from the inner continental region (present-day Eastern Sierras Pampeanas) to the magmatic arc (now Western Sierras Pampeanas and Frontal Cordillera). Based on these data together with ca. 100 published whole-rock geochemical analyses we conclude that Late Devonian–Early Carboniferous magmatism at this latitude represents continuous activity (ranging from 322 to 379 Ma) on the pre-Andean margin of SW Gondwana, although important whole-rock and isotopic compositional variations occurred through time and space. Combined whole-rock chemistry and isotope data reveal that peraluminous A-type magmatism started in the intracontinental region during the Late Devonian, with subsequent development of synchronous Carboniferous peraluminous and metaluminous A-type magmatism in the retro-arc region and calc-alkaline magmatism in the western paleomargin. We envisage that magmatic evolution was mainly controlled by episodic fluctuations in the angle of subduction of the oceanic plate (between flat-slab and normal subduction), supporting a geodynamic switching model. Subduction fluctuations were relatively fast (ca. 7 Ma) during the Late Devonian and Early Carboniferous, and the complete magmatic switch-off and switch-on process lasted for ~ 57 Ma. Hf TDM values of zircon (igneous and inherited) from some Carboniferous peraluminous A-type granites in the retro-arc suggest that Gondwana continental lithosphere formed during previous orogenies was partly the source of the Devonian–Carboniferous granitic magmas, thus precluding the generation of the parental magmas from exotic terranes.
The Nenquén Group was deposited during a period dominated by the Cretaceous Greenhouse and can be divided in three cycles correlated with large-scale changes in the evolution of the Andean foreland basin. The filling of the Neuquén Group is constituted by a complete cycle and two incomplete cycles of underfilled-overfilled, separated by first-order discontinuities assigned to the uplift of the Agrio fold-and-thrust belt during the Chasca/Catequil, Mid Ocean Ridge (CCMOR) collision, coinciding with first-order climatic changes within the Cretaceous greenhouse cycle.
The Candeleros Formation in the base of this group was deposited in late underfilled conditions, showing prominent forebulge zones. It is demonstrated that during the Albian, with the cratonward migration of the uplifting forebulge zones, the axis of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed. On top, the Huincul Formation was deposited in an overfilled period without orogenic load, while the Cerro Lisandro Formation was deposited in early underfilled conditions with orogenic load. The Río Neuquén Subgroup started with a late underfilled period (Portezuelo Formation -second-order discontinuity), and after wards the Plottier Formation was deposited in an overfilled period without orogenic load. Finally, the Río Colorado Subgroup was deposited under late and early underfilled conditions (Bajo de la Carpa and Anacleto Formations respectively).
The Guacha Corral shear zone (GCSZ) is represented by mylonites that were developed under amphibolites facies conditions from migmatitic protoliths. In this contribution, geophysical, petrological and structural data were combined to determine the 3D geometry of the GCSZ. New gravimetric, magnetometric and structural studies, along an E-W profile, were integrated with existing magnetotelluric and seismological data from a representative regional database of the Eastern Sierras Pampeanas. The zonation of different fabrics across the GCSZ suggests that the pre-existing heterogeneities of the protoliths played a key role in governing the degree of metamorphism of different regions. The low gravity anomalies observed in the GCSZ suggest a transitional boundary zone between the migmatitic and mylonitic domains, where highly deformed shear bands are interspersed with undeformed rocks, presenting gradual contacts. The mylonites in this shear zone show a considerably reduced density when compared to the migmatite protoliths. The density of the rocks gradually increases with depth until it reaches that of the protolith. These changes in the gravity values in response to density changes allowed us to infer a listric geometry at depth of the GCSZ. Low gravity anomalies in the profiles, in regions where high density rocks (migmatites) outcrop at the surface, were modeled as buried granitic plutons.