Descripción: The Atuel depocentre corresponds to a Late Triassic - Early Jurassic NNW-trending subbasin, located in the northern sector of the Neuquén basin. Based on pre-existing stratigraphical data and present structural analysis we propose that the Atuel depocentre is bounded by the presence of two NNW-trending major normal faults, named Alumbre and La Manga. These faults are inferred to have controlled the development of two west-facing half-grabens: the Río Blanco, a completely emerged half-graben, and the western Arroyo Malo, a completely submerged half-graben. The structural model presented here is based on the assumption that both, the basement structural grain and the regional extension direction, exerted a first-order control in the development and evolution of the Atuel depocentre. During the early stage of rifting (pre-Rhaetian - Middle Hettangian) the pre-Triassic Alumbre and La Manga faults reactivated in an oblique mode. During the second episode of rifting, both Alumbre and La Manga faults continued to play, while WNW-trending normal fault developed in order to accommodate the strain inside both half-grabens. The third extensional event began with an abrupt marine rise inside the Arroyo Malo half-graben during late Middle Hettangian, as a result of the last displacement of the Alumbre fault, and finished with an abrupt marine drop associated with the desactivation of the La Manga fault.

Descripción: Triassic clastic and volcanic rocks from the Precordillera were deposited in the Cuyana rift basin filling half-graben systems. Contractional/transpressional Andean tectonics leads to the almost complete inversion of some portions of the basin which resulted in present-day isolated, structurally controlled outcrops of these Triassic rocks. In the Southern Precordillera both the degree of Neogene tectonic inversion and structural compexities are variable. At the regional scale, these variations in Andean deformation are related to first order anisotropies like the Cuyana basin borders and previous shear zones of Permian age (San Rafael orogenic phase). This paper focuses on the kinematic analysis done in the Cerro Manantial thrust sheet area (Cordón San Bartolo, central sector of the South Precordillera) where tectonic inversion was not that strong and the influence of oblique strain zones is practically null. Four sets of extensional/ transtensional faults were recognized affecting Triassic sedimentary rocks of the El Cielo Formation (Uspallata Group) at the Quebrada El Salto. Fault displacements are of decimetric to metric scale. Mesoscopic kinematic indicators (en-échèlon tensional gashes, Riedel shear fractures, sigmoidal fractures) were measured. Once Andean deformation was restored, a NNE direction for the Triassic extension was determined. Considering a northern branch of the Cuyana basin trending NNW (Az. 150°) and oblique to the direction of extension (Az. 35-40°), a sinistral strike-slip component could be inferred for this portion of the basin.

Descripción: The Gondwana margin contained some Triassic basins that together constitute a regionally northwest-trending extensional system. The Cuyana rift Basin is internally composed of a family of hemigrabens filled with thick piles of sedimentary clastic and epiclastic rocks that can reach more than three thousand meters. In particular the Rincón Blanco sub-basin, one of the northernmost depocenter of this rift, is bounded by a linked through-going normal fault that usually displays an en-échèlon map view. Along strike existence of discrete depocenters and alternation of sedimentary wedges of different types suggest a linkage origin for some of them separated by a transfer zone. The infilling was strongly controlled by tectonics which in term produced distinctive features along the whole sedimentary sequence. An immediate consequence of this latter is that the architecture of the fill resulted from the geometry and the displacement of the bounding normal faults. Using lithology and structural data the infilling was subdivided into packages of genetically linked units bounded by regional extended surfaces. Hence, three depositional sequences or tectono- stratigraphic units separated by regional unconformities have been recognized. They were interpreted as a result of a major reactivation of the extensional system that could have evolved along strike as segments of fault that linked together and/or as laterally propagating faults. Using these basic concepts it was possible to reconstruct the geometry and the history of the infilling of the east margin of the hemigraben, buried under several backthrusts. Additionally, it could be possible to separate a sequence of rock, the Marachemill Unit, from the Rincón Blanco Group and to understand their tectonosedimentary relationships.

Descripción: A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Cordoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano) magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E (A95=3.3, K=35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.

Descripción: This thematic map describes the principal features of the landscape, a relative sequence of its geomorphic evolution, and arrives to important conclusions about its regional stratigrapby. The regional landscape is composed and polycyclic. The geoforms are related to different exogenous processes (glacial, fluvial and mass-wasting). An endogenous process (volcanism) formed the more distinctive features; a great tectonic-volcanic caldera and the preglacial Copahue stratovolcano. Some minor and youngest volcanic forms were formed during postglacial times. A previous fluvial cycle was replaced by glacial morphogenesis (valley glaciation) during the Quaternary, due a climatic intervention. A huge and very thick (500-800 m) ice-blister was lodged into the caldera. The ice-blister exceded the caldera's edges forming several glacial diffluences and giving an important and additional supply to neighbouring glacial valleys (Hualcupén and Trocoman), out of to the depression. The Pleistocene glaciation shows two local cycles of volcanic eruptions: pre- and post-glacial. The origin of many local lakes was related to glacial exharation. Others lakes have different genesis: by blocking slide, crater-lake and glacial exharation associated with important structural weakness (Caviahue Lake related to the Trapa-Trapa lineament or Caviahue graben). During postglacial times important rock-avalanches occurred in the Hualcupén valley: the Cajón Chico and Compul rock-avalanches were related to seismic shocks. Previously, their accumulations were interpreted as terminal moraines of a second glaciation. A fluvial rejuvenation phase was the consequence of a blocking of Agrio River by the Agrio volcanic flows, during the Holocene. After that, the river eroded this obstacle restablishing its previous base level. The Agrio cascade represents the upstream present position of the consequent rejuvenation wave. © 2005 Asociación Geológica Argentina.