Descripción: This paper describes for the first time in the study area the lithology, structure and different metamorphic events that have affected the basement in the eastern margin of Brazo Huemul of Nahuel Huapi Lake. These rocks have been classified as protomilonitic paragneisses and they were correlated to the Colohuincul Complex. A brittle - ductile shear zone with an orientation of N14°W (similar to the regional trend) has been interpreted as a deep angle reverse fault (74°SW), that put metamorphic basement rocks in tectonic contact with volcanic/ volcaniclastic rocks from Ventana Formation. In agreement with the mineral assemblages observed (quartz - chlorite - biotite - garnet (almandine) - albite) it was possible to determine a sedimentary pelitic protolith that was affected by regional metamorphism under high temperature green schist facies (biotite - garnet zone) conditions. © 2006 Asociación Geológica Argentina.

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: The Chancho-Co hill is the main positive feature in the caldera del Agrio domain with the only exception of the Copahue volcano. Its development is lin- ked to a series of NE-trending thrusts with a general vergence to the SE, which affects Late Pliocene successions gathered in Las Mellizas Formation. The main topographic break in this fan of reverse faults coincides with the Copahue Fault. This structure uplifted Late Pliocene sequences over younger unconsolidated fluvial and coluvial deposits. The detailed study of these sequences allowed identifying at least two periods of activity for the Copahue Fault. This fault, as well as the other neigh- bor thrusts, cuts the Copahue lavas to the south showing the precise geometry of the Chanchó-Co uplift. Finally, the inver- sion of Paleogene depocenters at depth determined from gravimetric studies is proposed to explain the deeper geometry of the neotectonic structures in the area.

Descripción: A process of stream piracy caused by the active retrograde erosion of Limay Chico stream - a tributary of Limay river- is described. It is located in the southern part of headwaters of the Alicurá drainage basin, tributary of Collon Cura river. It is supposed that the increment of the headward erosion is related to the postglacial climatic conditions and a coetaneous deepening of the Limay river channel, a process expressed by modern and lower erosion terraces. Moreover the retrograde erosion was enhanced by the special location of the Limay Chico valley along an important line of structural weakness, the Limay Chico fault. The fluvial capture and the consequent diversion of the Alicurá basin upper part showed previous favorable conditions: 1) the topographic lower disposition of Limay Chico stream and their base level; 2) its steeper slope and the location of Limay Chico valley along a zone of structural and litologicalk weakness; 3) transverse arrangement of the Alicurá river's headwaters; 4) its gentle slope and location of their basin and base level at a topographic higher level. Several wind gaps were recognized and were interpreted two probable elbows of capture and barbed pattern. Also the increment of discharge of Limay Chico stream caused in their basin local phenomena of static rejuvenation. It is suppose a close genetic relation between the landslides observed along the steep fault scarp promoted by the Limay Chico Fault and the postglacial climatic change and the local erosional deepening of Limay Chico valley. The slumps characterized by the coalescence of their scan, generated the irregular and ruged landscape of the western side of Limay Chico valley. On the eastern valley slope the slumps reach minor expression. Longitudinal profils and slopes of the analized streams are included.

Descripción: Quartzitic sandstones of the sierras Bayas Group in Sierras Septentrionales of Buenos Aires are studied. Outcrops of these deformed rocks are observed at sierras Bayas and Barker localities, where quartzitic sandstones from Precambrian units are present. These rocks are compared with similar fault breccias in Sierras Australes of Buenos Aires, which were develop in quartzitic rocks of several Paleozoic units of this region. At both regions, fault zones are subvertical layers with acute contacts on the wall rocks. An original sedimentary protolith is recognized in textures and structures preserved in all samples of Sierras Bayas. Sedimentary textures at Sierras Australes are obliterated by penetrative deformation in greenschists facies methamorphism and also show a metaquartzitic protolith. In both analyzed regions, similar cataclastic textures are observed. All these breccias and microbreccias would be associated with a major extensional tectonic event of Jurassic - Cretaceous age, which could be related to the origin of Colorado and Salado extensional basins during the opening of the South Atlantic Ocean.

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: The first rock avalanche to be identified in the southern Sierras Pampeanas is described here and its origin analysed. It is located in the northern sector (Cordon El Realito) of the San Luis fault scarp, which marks the western slope of the Sierra Grande de San Luis. It is suggested that the avalanche was triggered by seismic schock, a hypothesis supported by the seismotectonic and geologic conditions of the region as well as by the records of historical seismicity and by the recognition of modern fractures, scarplets and springwater spots in the piedmont area. Several other geological factors also support this suggestion, such as joint density, main shear zone location, attitude of the internal structure of granitoids and other nearby gravitational movements. The primary landforms of the avalanche landscape strongly suggest that the event was a recent one. The estimated speed is about 190 km/hour. © 1997 Asociacón Geológica Argentina.

Descripción: Small outcrops of biotitic-amphibolic schists and amphibolites found in two localities suggest that the Cushamen Formation is host rock of the Central Patagonian Batholith in Gastre. The characteristics of these rocks were investigated using structural, petrographic, magnetic fabric, and garnet chemical-zonation studies. Garnet is observed as a late product of contact metamorphism. Structural data from the studied area and from other localities in the region are in conflict with models that invoke the occurrence of uniform, large-scale dextral displacements along the Gastre lineament in Mesosoic times. Rather, the data suggest that the Gastre granitoids record heterogeneous low-temperature deformation of likely Late Triassic age. This low-temperature deformation appears as small, localized outcrops, and we suggest that it is related to the accommodation of the successive magma batches that built the Central Patagonian Batholith.

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.