Estanislao Godoy

The control of pre-existing extensional structures on the evolution of the southern sector of the Aconcagua fold and thrust belt, southern Andes

Abstract The Aconcagua fold and thrust belt, located in the Andean mountains at 32°30′ to 34°S, has been described as a classic model of a thin-skinned thrust belt. However, new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple Mesozoic extensional phases, overprinted by structural inversion, as well as thin- and thick-skinned tectonics. Two major superimposed extensional structural styles have been identified for the Mesozoic characterized by distinctly oriented stress fields. A key role in the evolution of this part of the fold and thrust belt was played by a Late Triassic to Early Jurassic depocentre and by Late Jurassic block faulting. Shortening was accommodated by a combination of inversion of pre-existing normal faults, development of footwall short cuts and both thin and thick-skinned thrusting. Synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults, with further shortening along newly formed thin-skinned thrust faults. The geometry of thin-skinned fault systems is controlled by the architecture of the rift basin, competent footwalls forming barriers to the lateral propagation of detachments.

Late Paleozoic K/Ar ages of blueschists from Pichilemu, Central Chile

Two crossite concentrates and one blueschist whole rock were analyzed by the K/Ar method. These samples belong to the high/intermediate pressure Western Series of the Chilean metamorphic basement and, in this area, are intruded by a small monzonite body. Ages obtained were 211 m.y. and 329 m.y. for the mineral concentrates and 211 m.y. for the whole rock. Discussion based on crystal size as a factor for retention of40Ar during localized re-heating of the metamorphic rocks due to the monzonitic intrusion leads to the acceptance of 329 m.y. as the minimum age of crossite crystallization. This age agrees with the whole rock Rb/Sr limiting reference isochrons (273–342 m.y.) previously obtained for the metamorphic basement of Central Chile which did not include samples of the present area. This age provides the first evidence of a Paleozoic blueschist assemblage in the eastern Pacific border and would suggest the existence of a Late Paleozoic subduction zone along the western margin of South America.

Late Oligocene-Early Miocene compressional tectosedimentary episode and associated land-mammal faunas in the Andes of central Chile and adjacent Argentina (32-37°S)

Abstract A reassessment of the geologic and land-mammal fossil evidence used in attribution of a tectosedimentary episode in the Andes between 32 and 37°S to the Middle Eocene “Incaic tectonic phase” of Peru indicates that the episode occurred during Late Oligocene-Early Miocene times(~ 27-20 Ma). From west to east, three structural domains are recognized for this time span in the study area: a volcanic arc (Chile); a thin-skinned, E-verging fold-thrust belt (Cordillera Principal, Chile-Argentina border strip); and a foreland basin (Argentina). Initiation of thrusting in the Cordillera Principal fold-thrust belt produced the coeval initiation of sedimentation in the foreland basin of adjacent Argentina. This onset of foreland deposition postdates strata bearing a Divisaderan Land Mammal Age fauna (i.e. ~ 35-30 Ma) and is marked at ~ 36°30′S by the base of the “Rodados Lustrosos” conglomerates, which are conformably overlain by sedimentary rocks containing a Deseadan Land Mammal Age fauna (i.e. ~ 29-21 Ma). Geologic relationships between the thick volcanic Abanico (Coya-Machali) and Farellones formations also demonstrate that this tectosedimentary episode practically ended at ~ 20 Ma at least in the volcanic arc, and was therefore roughly coeval with the major tectonic crisis (~ 27-19 Ma) known in northwestern Andean Bolivia some 1500 km to the north. This strongly suggests that a long, outstanding tectonic upheaval affected at least an extended 12–37°S segment of the Andean margin of South America during Late Oligocene and Early Miocene times.

The Puelche Volcanic Field: extensive Pleistocene rhyolite lava flows in the Andes of central Chile

Un remoto campo volcanico, ubicado en las escarpadas nacientes de los rios Puelche e Invernada (35,8°S), constituye el principal agrupamiento de lavas rioliticas cuaternarias conocido hasta la fecha en la Zona Volcanica Meridional de los Andes. El Campo Volcanico Puelche pertenece a un cinturon intra-arco de centros magmaticos siliceos que se extiende, por lo menos, sobre 140 km en direccion norte-sur. Esta faja yace significativamente mas al este que el frente volcanico pero, a pesar de ello, considerablemente mas al oeste que los campos volcanicos extensionales intraplaca representados por los basaltos alcalinos de la pampa argentina. Se distingue en el un cuerpo hipabisal riodacitico (70,5% SiO2) con biotita del Pleistoceno temprano, 11 unidades eruptivas de lava riolitica de biotita de alto contenido en potasio (71,3-75,6% SiO2) y cuatro unidades eruptivas de andesitas basalticas (53,9-54,9% SiO2), cuyos conductos cortan a algunas de las riolitas. Los contactos basales de las lavas rioliticas y sus infrayacientes precursores piroclasticos estan generalmente cubiertos por derrubios, pero la erosion glacial ha expuesto, de manera espectacular, sus estructuras de flujo, zonaciones litologicas internas y algunos de sus alimentadores. El espesor de las lavas rioliticas varia entre 75 y 400 m. Discordancias de borde de acantilado y relaciones de relleno de valle entre las 11 unidades rioliticas muestran que la secuencia eruptiva abarco al menos un episodio glacial, el cual acentuo el relieve local. La falta de rasgos asociados a contacto con hielo sugiere, sin embargo, que todas las erupciones tuvieron lugar durante intervalos no-glaciales, probablemente entre 400 ka y 100 ka. Erosion glacial post-eruptiva redujo a las riolitas a varios remanentes aislados que, en conjunto, cubren 83 km2 y representan un volumen actual de 21 km3. Consideraciones basadas en las pendientes, espesores de las lavas y la paleotopografia sugieren, sin embargo, que tanto el area como el volumen original fueron aproximadamente tres veces mayores. El contenido en fenocristales de las riolitas varia entre 1 y 12%, con plagioclasa >> biotita > oxidos de FeTi en todas las unidades y escasa anfibola pero conspicua en las rocas menos siliceas. Las andesitas basalticas (pobres a ricas en fenocristales), son de quimismo variable. Muestran grandes diferencias en sus porcentajes de fenocristales de clinopiroxeno, olivino, plagioclasa y cuarzo, este ultimo en xenocristales. Destaca la bimodalidad del campo volcanico, desconociendose unidades eruptivas cuaternarias con contenidos en SiO2 entre 55 y 70%. Las composiciones en elementos mayores y trazas tanto de las rocas maficas como siliceas son, sin embargo, tipicas de series de arco en margen continental, y no de series intracontinentales. La falta de unidades eruptivas intermedias y las diferencias en la isotopia de Sr entre las rocas maficas y las siliceas sugieren que las riolitas fraccionaron, mas bien, a partir de un magma parental hibrido que, en forma continua, a partir de un magma basaltico. Estas riolitas podrian contener mayores contribuciones en fundidos parciales de la corteza superior que los productos siliceos de los centros del frente volcanico, ubicados 30 km mas al oeste.

Barren Miocene granitoids in the Central Andean metallogenic belt, Chile: Geochemistry and Nd-Hf and U-Pb isotope systematics

Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were undertaken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32° and 34°S. The studied granitoids yield resolvable crystallization ages of 11.3±0.1 Ma (Cerro Meson Alto massif), 10.3±0.2 Ma (La Gloria pluton), 14.9±0.2 Ma/14.9±0.1 Ma (Yerba Loca stock) and 11.2±0.1 Ma/14.7±0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32°S, slightly younger mineralized porphyries at Rio Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.512758- 0.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial eHf-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CHUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from Rio Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic arc front migration.

Evolution of shallow and deep structures along the Maipo-Tunuyán transect (33°40'S): From the Pacific coast to the Andean foreland

Abstract We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

The age and constitution of Cerro Campanario, a mafic stratovolcano in the Andes of Central Chile

La edad y formacion del cerro Campanario, un estratovolcan mafico de la Cordillera de Talca. El Cerro Campanario, un destacado hito natural en la divisoria continental cerca de Paso Pehuenche, es el remanente dejado por erosion glacial de un estratovolcano mafico mas joven de lo supuesto anteriormente. Tal como lo indican los analisis 40Ar/39Ar y K-Ar de sus bastante uniformes lavas de andesitas basalticas, ricas en olivina y plagioclasa, su edificio de 10-15 km3 crecio rapidamente hacia fines del Pleistoceno medio (150-160 ka).

The north-western margin of the Neuquén Basin in the headwater region of the Maipo drainage, Chile

Abstract Volcanic and pyroclastic rocks of intermediate composition, dating from Kimmeridgian time with peak ages during both the early–middle Tithonian and the late Hauterivian, characterize several localities of the north-western margin of the Neuquén basin along the main Andean Range at 33–34°S. The latter is thought responsible for the desiccation of the basin during Aptian–Albian time. The thick Tithonian submarine lavas at the Volcán valley (33°30′S), on the other hand, coincide spatially with the only pass-way recognized at that time in the Chilean Coast Range. This coincidence may be related to an isolated relic rift-like structure that coexisted with the regional intra-arc extensional setting during Tithonian time. Continental-scale rifting in the southern Andes has been recognized only for the Late Triassic–Early Jurassic period.

Enseñando Geología a los niños - Guillermo Chong D.

A new shallow multichannel seismic survey was carried out in the Llancanelo Lake region (Southern Mendoza Province, Argentina) in order to depict the major Neogene sedimentary-volcanic sequences and regional evolution of a tectonic-volcanic basin located in a key region at the eastern foot of the Andes cordillera. This research is settled on early studies that comprised seismic works reaching depths of 600/700 m as well as geoelectric and electromagnetic surveys reaching the uppermost 80-100 m of the sequences. The processing of the reflection seismic data enabled to obtain a stacked section useful for a preliminary geological interpretation. The obtained results indicated the presence of three major sedimentary units with increasing volcanic (basaltic layers) intercalations with depth that accommodate to the geometry of the depocenter. The entire sequence encompasses most of the Neogene. The previous research set the methodological basis for performing more detailed shallow seismic surveys and allowed defining the strategies for obtaining a better constrained geological model. Seismic transects located north of the lake were densified by adding new profiles. Deep seismic and borehole data from the industry were included in this study in order to provide a regional stratigraphic structure of the Cenozoic sequences and to contribute to the knowledge of the final stages of evolution of the Llancanelo basin.The Cuyania stratigraphic record of the Pygodus serra Zone, and the E. robustus and E. lindstroemi subzones, is discussed in this contribution. Three classical sections have been sampled for conodonts in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The Ponon Trehue and La Cantera formations start with conglomerates and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section, exhibits a massive sandstone bed with carbonate nodules between the black shale. The key conodonts recovered indicate a late Darriwillian age for the bearing strata, these conodonts specimens from the Ponon Trehue region are compare with those from the Precordillera. Finally, the studied clastic sedimentary successions that record this biozone have been compared, concluding that eustacy did not control the sedimentary change in Cuyania for this time interval.In this contribution we present new specimens of Litopterna recovered during the last decade in Quebrada Fiera (Mendoza Province, Argentina), whose fossiliferous sediments, currently recognized as the base of Agua de la Piedra Formation, are assignable to Late Oligocene (Deseadan South American Land Mammal Age). Two remains mentioned in the first publication on this locality were neither detailed nor described, and they have not been located in the corresponding repository. The new material consists of postcranial fragmentary remains (astragali, calcaneum, and metapodials) of Macraucheniidae (Cramaucheniinae) and an incomplete upper molar (M3) of Proterotheriidae (Proterotheriinae). These few remains of litopterns contrast with the abundance of notoungulates at Quebrada Fiera. A comparative study was carried out with material from Patagonia (Argentina) and taxa recorded in Bolivia and Peru for the same temporal interval. The specimens of Cramaucheniinae are assigned to Coniopternium andinum and the molar of Proterotheriinae to cf. Lambdaconus suinus. This contribution allows us to extend the geographical range of Coniopternium, filling the gap between the Patagonian and lower latitude localities (Bolivia and Peru) in which this genus was found. The record of L. suinus in Quebrada Fiera expands the geographical range of this species outside from Patagonia.ABSTRACT. The aquatic sloth, Thalassocnus, is one of the most intriguing linage of mammal knew from the southern pacific coast of South America during the late Neogene. It was initially recognized in Peru, but recent paleontological surveys also demonstrate its presence in Chile. However, the determination of how many species of Thalassocnus were in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (7-7.5 Ma), Bahia Inglesa Formation (Atacama Region, northern Chile), which shows affinities with Thalassocnus. Morphological comparisons with others forms from Chile and Peru allow us to attribute the CPUC/C/557 specimen to T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahia Inglesa, Horcon and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and a younger species than T. natans (probably T. yaucensis). Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of this extraordinary and enigmatic clade of organisms.The uppermost Cretaceous continental deposits of the Neuquen Basin (west-central Argentina) have yielded a high diversity vertebrate assemblage, including numerous dinosaur species. The Neuquen Group is the richest dinosaur-bearing unit of the basin and comprises the Rio Limay, the Rio Neuquen and the Rio Colorado subgroups. In this group, there is abundance of isolated, disarticulated or partially articulated sauropods and theropods. However, little is known about the taphonomic history of fossil assemblages. In this study, dinosaur remains from the Rio Neuquen Subgroup (Plottier Formation) found in the Cerro Guillermo area in southern Mendoza are examined. The investigation of fossil occurrences within the study area revealed the existence of different taphonomic modes, from isolated bones until partially articulated skeletons preserved in floodplain and channel settings. SEM-EDS analysis showed the substitution of hydroxyapatite by francolite in the bone microstructure. The presence of fluorine –in one of the cases– suggested a link between the elemental composition and depositional environments: floodplain and fluvial channel. The survey of the vertebrate accumulation types and their sedimentary context allowed documenting a wide range of processes responsible for their genesis, operating within a fluvial-dominated environment. This contribution represents an holistic approach about taphonomic history of Cretaceous dinosaurs for assessing the differential preservation of fossil assemblages in fluvial environments.

La edad y formacion del cerro Campanario, un estratovolcan mafico de la Cordillera de Talca.

El Cerro Campanario, un destacado hito natural en la divisoria continental cerca de Paso Pehuenche, es el remanente dejado por erosion glacial de un estratovolcano mafico mas joven de lo supuesto anteriormente. Tal como lo indican los analisis 40Ar/39Ar y K-Ar de sus bastante uniformes lavas de andesitas basalticas, ricas en olivina y plagioclasa, su edificio de 10-15 km3 crecio rapidamente hacia fines del Pleistoceno medio (150-160 ka).