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A paleomagnetic study performed in Plio-Pleistocene volcanic rocks of two localities in the Southern Central Andes is presented. The region corresponds to the transition zone between the Central and the Patagonian Andes. In the latter, deformation is almost exclusively controlled by the dextral-strike slip Liquiñe-Ofqui Fault Zone, that extends from ∼47 °S to ∼38 °S and accommodates the deformation generated by the oblique subduction of the Nazca Plate below South America. To the north of ∼36 °S, deformation is mainly contractional. Our study was carried out in the transition zone between both tectonic regimes. Sampling consisted of 31 paleomagnetic sites distributed in two localities: the Loncopué (∼38.0 °S, 70.7 °W) and Andacollo (∼37.2 °S, 70.8 °W) areas. We sampled Pliocene and Pleistocene volcanic rocks with ages younger than 5 Ma. After stepwise demagnetization and principal component analysis, site mean remanence directions were computed. Both localities show non-significant clockwise rotations of 1.7° ± 14.4° (Loncopué) and 8.1° ± 8.4° (Andacollo). Although the presently available database does not support significant tectonic rotation of the Andacollo lavas, preliminary results obtained from upper Miocene volcanics in a nearby area indicating ∼20° of clockwise rotation, encourage further studies to improve the resolution of the paleomagnetic data to determine if crustal block rotations have been, or still are, part of the Andean deformational processes. A revision of previously published paleomagnetic data in the northern Patagonian Andes suggests that, against original claims, no significant tectonic rotations associated to the Liquiñe-Ofqui Fault Zone, between ∼39 and 37.5 °S, can be unambiguously demonstrated.

Depósitos clásticos de la Formación Rancahué (Oligoceno Superior), que constituye parte del registro volcánico y sedimentario de la cuenca oligocena-neógena de Aluminé (Cordillera de los Andes, 70°W y 39°S), permitieron analizar los fenómenos de interacción sedimentaria en ambientes volcánicos continentales y caracterizar los mecanismos de preservación de restos de troncos asociados a sucesiones aluviales volcano-sedimentarias. En estos depósitos, se ha identificado un bosque petrificado de tierras altas con 33 ejemplares preservados en posición de vida, con Nothofagoxylon como elemento dominante, desarrollado bajo condiciones climáticas templadas y húmedas. En el área de estudio, la Formación Rancahué se caracteriza por la estrecha interrelación lateral y secuencial entre tres asociaciones de facies. La asociación A se compone de aglomerados y conglomerados gruesos monomícticos (con fenoclastos de basaltos) acumulados en ambientes proximales de altas pendientes (taludes y en el pie de taludes subaéreos) como producto de ¹debris floods' o de flujos hiperconcentrados, avalanchas, deslizamientos y flujos de detritos. Estos depósitos tienen procedencia local y se vinculan con un cuerpo volcánico basáltico que generó un importante relieve intracuencal. La asociación B consiste en conglomerados, sabulitas, areniscas conglomerádicas y areniscas gruesas y muy gruesas con variadas estructuras primarias y se asigna a un sistema fluvial con desarrollo regional, baja canalización y carga trac-cional, con áreas de aporte en la cordillera de los Andes. La asociación C se compone de facies clásticas de textura fina, acumuladas esencialmente por decantación en ambientes subaéreos y subácueos, con el periódico aporte de lluvias de cenizas volcánicas, y con niveles en los que se aprecia la formación de suelos. Los depósitos fluviales incluyen restos de troncos transportados que muestran el poder erosivo de las corrientes de flujo rápido que los acumularon. El bosque petrificado arraigó en depósitos de la asociación de facies C y quedó preservado en los suprayacentes aglomerados y conglomerados de la asociación A. Se interpreta que los troncos fueron sepultados por los flujos gravitacionales que operaron en abanicos aluviales y coluviales de la zona riparia hidrogeomórfica y que se caracterizan por una tasa de sedimentación alta y un discreto poder erosivo por su naturaleza laminar o quasi laminar. The siliciclastic deposits of the Rancahué Formation (Oligocene, Aluminé Basin), formed in the eastern side of the Andean Cordillera (70°W y 39°S) are studied to evaluate the relationship among different depositional systems in volcanic settings and to disclose the processes that contributed to the preservation of a forest developed at the foot of a volcanic relief. In the alluvial volcano-sedimentary record of the Rancahué Formation an upland petrified forest with 33 logs in life position was identified. This flora, with Nothofagoxylon as the main taxon, was developed under temperate and humid climatic conditions. Three facies associations were defined in the study region. Facies association A was accumulated in subaerial talus and foot-talus slopes, and is composed of monomictic (basaltic) boulder and cobble conglomerates formed by debris floods, hyperconcentrated flows, avalanches, landslides and debris flows. The source of these sediments is local and is related to the growth of an intrabasinal basaltic volcanic edifice. Facies association B consists of cross-stratified, plane bedded and massive conglomerates, pebbly sandstones and coarse-grained sandstones accumulated in a regional braided fluvial system with source areas located in the Andean Cordillera. Facies association C is characterised by fine-grained sediments accumulated in subaerial and subaqueous environments, with periodic contribution of ash falls and development of palaeosols. The forest rooted in deposits of facies association C, and the logs preserved in life position are located within the overlying coarse-grained volcaniclastic deposits of facies association A. The burial and preservation of this vegetation was favoured by the highly aggrading rhythm and comparative low erosive power of the gravitational flows that operated in alluvial and colluvial fans of the riparian hydrogeomorphic zone. Conversely, abundant transported and broken trunks included in the deposits of facies association B clearly indicate the strong erosive power of the currents of the associated braided fluvial system.

Depósitos clásticos de la Formación Rancahué (Oligoceno Superior), que constituye parte del registro volcánico y sedimentario de la cuenca oligocena-neógena de Aluminé (Cordillera de los Andes, 70ºW y 39ºS), permitieron analizar los fenómenos de interacción sedimentaria en ambientes volcánicos continentales y caracterizar los mecanismos de preservación de restos de troncos asociados a sucesiones aluviales volcano-sedimentarias. En estos depósitos, se ha identificado un bosque petrificado de tierras altas con 33 ejemplares preservados en posición de vida, con Nothofagoxylon como elemento dominante, desarrollado bajo condiciones climáticas templadas y húmedas. En el área de estudio, la Formación Rancahué se caracteriza por la estrecha interrelación lateral y secuencial entre tres asociaciones de facies. La asociación A se compone de aglomerados y conglomerados gruesos monomícticos (con fenoclastos de basaltos) acumulados en ambientes proximales de altas pendientes (taludes y en el pie de taludes subaéreos) como producto de ʻdebris floodsʼ o de flujos hiperconcentrados, avalanchas, deslizamientos y flujos de detritos. Estos depósitos tienen procedencia local y se vinculan con un cuerpo volcánico basáltico que generó un importante relieve intracuencal. La asociación B consiste en conglomerados, sabulitas, areniscas conglomerádicas y areniscas gruesas y muy gruesas con variadas estructuras primarias y se asigna a un sistema fluvial con desarrollo regional, baja canalización y carga traccional, con áreas de aporte en la cordillera de los Andes. La asociación C se compone de facies clásticas de textura fina, acumuladas esencialmente por decantación en ambientes subaéreos y subácueos, con el periódico aporte de lluvias de cenizas volcánicas, y con niveles en los que se aprecia la formación de suelos. Los depósitos fluviales incluyen restos de troncos transportados que muestran el poder erosivo de las corrientes de flujo rápido que los acumularon. El bosque petrificado arraigó en depósitos de la asociación de facies C y quedó preservado en los suprayacentes aglomerados y conglomerados de la asociación A. Se interpreta que los troncos fueron sepultados por los flujos gravitacionales que operaron en abanicos aluviales y coluviales de la zona riparia hidrogeomórfica y que se caracterizan por una tasa de sedimentación alta y un discreto poder erosivo por su naturaleza laminar o quasi laminar., The siliciclastic deposits of the Rancahué Formation (Oligocene, Aluminé Basin), formed in the eastern side of the Andean Cordillera (70°W y 39°S) are studied to evaluate the relationship among different depositional systems in volcanic settings and to disclose the processes that contributed to the preservation of a forest developed at the foot of a volcanic relief. In the alluvial volcano-sedimentary record of the Rancahué Formation an upland petrified forest with 33 logs in life position was identified. This flora, with Nothofagoxylon as the main taxon, was developed under temperate and humid climatic conditions. Three facies associations were defined in the study region. Facies association A was accumulated in subaerial talus and foot-talus slopes, and is composed of monomictic (basaltic) boulder and cobble conglomerates formed by debris floods, hyperconcentrated flows, avalanches, landslides and debris flows. The source of these sediments is local and is related to the growth of an intrabasinal basaltic volcanic edifice. Facies association B consists of cross-stratified, plane bedded and massive conglomerates, pebbly sandstones and coarse-grained sandstones accumulated in a regional braided fluvial system with source areas located in the Andean Cordillera. Facies association C is characterised by fine-grained sediments accumulated in subaerial and subaqueous environments, with periodic contribution of ash falls and development of palaeosols. The forest rooted in deposits of facies association C, and the logs preserved in life position are located within the overlying coarse-grained volcaniclastic deposits of facies association A. The burial and preservation of this vegetation was favoured by the highly aggrading rhythm and comparative low erosive power of the gravitational flows that operated in alluvial and colluvial fans of the riparian hydrogeomorphic zone. Conversely, abundant transported and broken trunks included in the deposits of facies association B clearly indicate the strong erosive power of the currents of the associated braided fluvial system., Centro de Investigaciones Geológicas

The previous analysis of the prehistoric slide phenomena in the main Andes of Neuquén Province in Argentina (36o-38oS), triggered the study of those located south of this la-titude. Therefore a new set of mass wasting deposits has been identified, whose trigger factors and corresponding tectonic framework, where they are located, differ from the northern slides. Their study was performed in three specific areas: Aluminé, Chapelco and Arroyo Limay Chico. Earth flows predominate in relation to slumps. The only exception is the Chapelco rock avalanche. Their occurrence seems to be related to the presence of Limay Chico member of the Caleufú Formation, whose lithology (expanding clays), particular rheology before water saturation, and particular disposition underlying basaltic lava and conglomeradic plains were conditioning factors. The trigger factor of the Present and prehistoric earth flows has been intense orographic precipitations. Water saturation reduced friction and cohesion in materials, incrementing water pore pressure and producing slope instabilities. Rotational slides are related to the slope basal erosion produced by lateral river migration and additionally to precipitations. Slope orientation has been important in determining location of main slides. Lateral slope release through the flanks of glacial valleys due to debutressing because of the glacial retire in the area, has produced instability and rock fracturation that led to slide phenomena. The Chapelco rock avalanche is particularly related to copious precipitations during postglacial times. Thus sliding main trigger factors south of 38oS differ from those pro-posed for the northern area where earthquakes dominated. Their specific age is still unknown, although their geomorphologic analysis allow to constraint a postglacial age for all these occurrences. However, temporal relationships are lacking for the specific case of the arroyo Limay Chico, beyond the glaciated area. Fil:Folguera, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.