Luis V. Dimieri

Control en la magnitud de desplazamiento de rumbo del Sistema Transformante Fagnano, Tierra del Fuego, Argentina

Constraints on the magnitude of strike-slip displacement along the Fagnano Transform System, Tierra del Fuego, Argentina. New fi eld data, specially the discovery of the contact between Upper Cretaceous-Paleocene rock units in a previously unexplored area in southeast Tierra del Fuego, allowed us to estimate the amount of strike-slip displacement accommodated by the Fagnano Transform System (FTS). This stratigraphic contact is exposed both to the north and south of the FTS and it is deformed by a NW-SE compressive regional structure that forms part of the Fuegian thrust-fold belt, called the Cerro Piramide-Cerro Malvinera Thrust (PMT). The compressive structure is laterally offset by the FTS. The precise mapping of the PMT and the Upper Cretaceous-Paleocene contact shows a horizontal offset of ca. 48 km along the transform system in the Isla Grande de Tierra del Fuego. A maximum age of ~7 Ma for the begin- ning of the strike-slip in this sector is estimated by combining the obtained offset with published data on the slip-rate along the FTS in Tierra del Fuego. This Late Miocene age coincides with the creation of the divergent plate boundary between Sandwich and Scotia Plates, which has been proposed as responsible for the beginning of the strike-slip activity between South America and Scotia Plates.

Tectonic wedge geometry at Bardas Blancas, southern Andes (36 °S), Argentina

Abstract A tectonic wedge at the mountain front of the Cordillera Principal, Mendoza province, Argentina, shows some geometrical features that depart from examples described elsewhere. This wedge underthrusts the roof sequence beneath a blind thrust. The wedge is deformed by imbricate backthrusting that branches into the hangingwall ramp of the leading horse. This backthrust system contributed to the uplift and shortening of the mountain front.

Progressive deformation of a Coulomb thrust wedge: the eastern Fuegian Andes Thrust-Fold Belt

Abstract Time-calibrated balanced-cross sections of the eastern Fuegian Thrust–Fold Belt reveal many complex pro- and retro-vergent structures, rooted at the base of Cretaceous and within Paleocene rocks. These structures involve the unconformity-bounded syntectonic sequences of the Austral foreland basin, and accommodate a minimum shortening of c. 41.8 km. The complex kinematics of the thrust–fold belt are recorded by: (1) propagation of the basal décollement into the foreland, and forward-directed thrusting during the Ypresian; (2) out-of-sequence thrusting in the Lutetian; (3) subsidence and sedimentation from the Late Lutetian to the Oligocene; (4) backthrusting during the Oligocene; and (5) a renewed stage of forward-directed thrusting between the latest Oligocene and the Early Miocene, probably related to accretion below the sole fault in the hinterland. This thrust sequence is interpreted as the result of critical Coulomb wedge behaviour during the first stage of thrust–fold belt expansion, with accretion of new material that led to a taper decrease. The subsequent period of internal deformation corresponds to a subcritical stage, during which backthrusting accommodates significant shortening (c. 15%). After growth and taper increase, the last period of forward thrusting at the wedges front marks the inception of a new critical stage.

Cenozoic contractional tectonics in the Fuegian Andes, southernmost South America: a model for the transference of orogenic shortening to the foreland

The Fuegian thrust-fold belt has been subjected to significant shortening during the Cenozoic. Although contemporaneous shortening and uplift was also recognized in the Fuegian Andes central belt (hinterland), previous studies stated that most of that deformation developed out-of-sequence with the thrust-fold belt and hence did not contribute to its shortening. Therefore, no suitable geometric and kinematic model has been proposed for the mechanism that links deformation in both domains. Here we address the style and timing of the younger (Late Cretaceous-Paleogene) structures of the central belt and the structural evolution of the thrust-fold belt, based on published and own data. We reinterpret the style of the central belt structures, proposing a new model in which basement thrusting in the central belt caused all the shortening in the foreland cover. We postulate that the basement was involved in a regional-scale duplex whose roof thrust was the decollement of the thin-skinned thrust-fold belt. Therefore, the total slip transferred through the roof thrust accounted for the shortening in the cover rocks. The basement duplex incorporated the underthrust crust from the footwall of the orogenic wedge through forward propagation, in sequence with the thrust-fold belt, from the Late Cretaceous to the Miocene.

Geometric analysis and timing of structures on mafic–ultramafic bodies and high grade metamorphic rocks in the Sierras Grandes of San Luis Province, Argentina

Abstract The study area comprises a portion of the Sierras Grandes of San Luis Province in west-central Argentina. This sector is composed of high-grade granulite facies metamorphic rocks, mainly gneisses and migmatites, with intercalated igneous mafic–ultramafic bodies. These bodies occur as discontinuous lenses along a narrow central belt concordant with the general NE–SW structural trend. The granulite facies metamorphism reached by this central area has been ascribed to the intrusion of the mafic–ultramafic bodies into an amphibolite facies metamorphic sequence. On both sides of the mafic–ultramafic bodies, a decrease of metamorphic grade to amphibolite facies is apparent. Numerous pegmatitic bodies intruding the metamorphic complex can be found elsewhere and, towards the western limit of the study area, La Escalerilla metagranite constitutes one of the larger granitic exposures of the Sierras. The results of the structural analysis, done along cross-sections transverse to the mafic–ultramafic belt, allow us to establish a structural evolution for this large geological feature. The described deformational events can be correlated with the tectonic framework postulated for the southern Pampean Ranges.

Microstructural and geochemical analysis of Paleoproterozoic pseudotachylytes in Río de la Plata craton, Tandilia belt, Argentina

The current study focuses on the analysis of pseudotachylytes from the Azul Megashear Zone, located in the Rio de la Plata craton, central Argentina. This shear zone is a strip of mylonitic rocks of more than 40km in length and with a maximum width of 2.5km. Glassy and microcrystalline pseudotachylytes are hosted by Paleoproterozoic gneiss mylonite, granitoid mylonite, granulite mylonite and striped gneisses. The pseudotachylytes occur as fault and injection veins characterized by sharp contacts with no gradation into the mylonites and show meltorigin features such as partially melted quartz and feldspar clasts with embayed rims and flow structures. Their composition is similar to that of the rocks in which they are injected. In those cases involving banded host rocks, slight chemical differences can be observed in the pseudotachylytes mainly in terms of their fabric and mineralogy. Most pseudotachylytes have textures deriving from post-solidification ductile deformation, but others bearing mylonite-like features are related to viscous flow prior to complete solidification. There are indications that some of these pseudotachylytes were formed during the ductile regime, others being generated in the brittle regime during shear zone reactivation following exhumation or uplift. The occurrence of several pseudotachylytegenerating events indicates intermittent aseismic/seismic slip at different crustal levels throughout this shear zone.

Thrust-fold belt kinematics and orogenic growth in Tierra del Fuego, Argentina: implications of backthrusting for critical Coulomb wedge development

Los braquiopodos retzidinos son una fraccion menor de las faunas devonicas de la CordilleraCantabrica (Norte de Espana). Aparte de un par de formas raras, impublicadas, del Praguiense delDominio Palentino y del Emsiense inferior del Astur-Leones, proximas al genero Rhynchospirina, ellinaje alcanzo su maximo de diversidad en la parte superior del Emsiense, con dos especies del generoRetzia, R. adrieni y R. cf. prominula, Cooperispira subferita y, quizas, una forma impublicada dePlectospira. El grupo no es conocido en el resto del Devonico y reaparece en el Pensilvaniense con algunasformas del genero Hustedia. En este trabajo se propone un nuevo taxon de la Familia Retziidae,Argovejia n.gen., de la parte final del Emsiense superior de Asturias y Leon, constituido por su especietipo,A. talenti n.sp. y, quizas, por las formas del Emsiense superior del Macizo Armoricano (Francia)Retzia haidingeri var. armoricana y Retzia haidingeri var. dichotoma.The Ronda Depression is filled by Neogene sediments on the boundary between Subbeticreliefs, with NE-SW structural trends, and the frontal Subbetic Chaotic Complexes. The folding stylein the Subbetic Units of Western Betics is strongly controlled by the rheology of the rocks: thick andmassive beds of Jurassic limestones over Triassic marls and gypsum with plastic behaviour. Main deformationstructures in the sedimentary infill of the Ronda depression are simultaneous box folds withNNE-SSW and WNW-ESE trends that only affect its southwestern part. This distribution of folds isa consequence of the inherited fold trend that affected the basement during Early Burdigalian age.

Tectonic Evolution of the Chos Malal Fold‐and‐Thrust Belt (Neuquén Basin, Argentina) From (U‐Th)/He and Fission Track Thermochronometry

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.

The Structure of the Southern Central Andes (Chos Malal Fold and Thrust Belt)

The Chos Malal fold and thrust belt, formed during the Andean orogeny, is characterized by the involvement of both the Paleozoic basement and Mesozoic strata of the Neuquen Basin into the deformation. Two detailed structural cross sections, built based on previous field mapping, new subsurface interpretations, and seismic and borehole data, allow characterizing the structural style of this orogenic belt. A close interaction between large thick-skinned structures (first order) and complex thin-skinned structures (second, third, and fourth order), related to the presence of multiple detachments in the sedimentary cover, is recognized. The largest thrusts form basement-involved duplex structures, with a lower detachment located at a depth of about 12–14 km and an upper detachment in the Jurassic evaporites of the Auquilco Formation. Displacement transmitted by these basement sheets in the inner zone of the Chos Malal fold and thrust belt produces a wide region of thin-skinned deformation, which contains second-order fault-bend folds that transfer deformation to the overlying Agrio Formation shales (Early Cretaceous) giving rise to third-order folds and thrusts involving this unit. In the outer zone, the basement-involved thrusts have less displacement and form monoclines and a complex thin-skinned deformation restricted to the deformation front, possibly caused by buttressing effect exerted by the overlying Miocene volcanic sequences. This impediment in forward deformation leads to an important out-of-sequence faulting, whose displacement is compensated by a passive-roof backthrust along the Cretaceous evaporites of the Bajada del Agrio Group forming a triangle zone. Second-order anticlines under this triangle zone, where the seismic data are of low quality, constitute important hydrocarbon oil fields such as El Porton and Filo Morado. Understanding the close relationship between the structures of different order cropping out in the inner zone of the Chos Malal fold and thrust belt is important to interpret the subsurface structures forming hydrocarbon oil fields in the outer zone as well as to identify other complex structures that may lead to new exploration opportunities. Restitution of the structural cross sections allowed calculating a tectonic shortening for this region in the order of 22–25 km (16–18%), higher than estimated by previous authors who generally simplified the thin-skinned deformation and considered the tectonic inversion of normal faults as the main mechanism of deformation in this orogen.