Gérard Hérail

Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: Example from the foreland thrust system of the southern Bolivian Subandean Zone

The Subandean Zone of Bolivia is a foreland fold and thrust belt which forms the eastern edge of the central Andes mountains. Between 19°S and 22°S latitude, the construction of five balanced cross sections shows that the N-S trending Subandean Zone is characterized by the existence of passive roof duplexes. These complex structures can be distinguished by the lithotectonic unit within which duplexing occurs. The five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes. The sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes. Apparently, these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units. While a passive roof duplex was developing, the sole thrust stuck and the major horizontal displacements were then transferred either to out-of-sequence thrusts or to a new sole thrust, in a deeper detachment horizon. Therefore each passive roof duplex would correspond to the orogenic front of the Andean range at one very particular time in the history of the Subandean Zone of southern Bolivia. From south to north, the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front. Available data do not permit us to explain completely this phenomenom.

Tectonic rotations within the Bolivian Altiplano : Implications for the geodynamic evolution of the central Andes during the late Tertiary

Paleomagnetic results from 61 sites in Tertiary red beds and volcanic rocks from the Bolivian Andes allow quantification of tectonic rotations within the Altiplano domain. A total of 16 sites were also obtained in lower Miocene ignimbrites that cover the forearc of the north Chilean Andes near Arica. In the southern Altiplano (Lipez region) a local clockwise rotation of up to 38° is recorded in lower Miocene volcanic rocks. Farther north, near the Salar de Uyuni, there is no evidence of significant rotations. Counterclockwise rotations are observed in the Northern Altiplano. The largest counterclockwise rotation (28°) is found in Eocene-Oligocene red beds (Tiwanaku Formation) along the eastern border of the Corque syncline. Middle Miocene sediments located within the center of the Corque basin record a counterclockwise rotation of only 10°. About 200 km north of the Corque basin, Eocene-Oligocene sedimentary rocks near the locality of Viacha record only 10° counterclockwise rotation. Paleomagnetic results in lower Miocene ignimbrites from the forearc near the Arica bend do not show evidence for late Cenozoic rotation of the forearc. These new results suggest that the 20° to 25° counterclockwise rotation of the southern Peruvian forearc occurred prior to the Miocene. Anisotropy of magnetic susceptibility (AMS) data show a magnetic fabric mostly controlled by bedding, but slight AMS lineations are found almost parallel to the major regional structural trends such as fold axes. The apparent relationship between tectonic rotations, AMS lineations, and structural trends suggests that rotations occured after or during the final stage of folding. Paleomagnetic data obtained in Paleozoic rocks show evidence of clockwise rotation of the southern sub-Andean ranges. Tectonic rotations during the Neogene are mostly localized on the eastern side of the central Andes. The curvature of the occidental margin near Arica was likely acquired prior to the last stage of Andean deformation. Although the magnitude of the rotations may vary from one locality to the other, there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south. A large-scale regional mechanism is needed to explain this pattern of tectonic rotations within the central Andes. We propose a model in which along-strike segmentation of the Andean foreland and indentation of the Altiplano by a curved forearc are the major factors controlling tectonic rotations within the Altiplano, Eastern Cordillera, and sub-Andean ranges.

A geomorphological approach to determining the Neogene to Recent tectonic deformation in the Coastal Cordillera of northern Chile (Atacama)

The large (≈10000 km2) and local-scale ( 300 m) sedimentary succession was deposited east of the AFS. The succession fills previously deep paleovalleys. And it consists of gravel, so-called “Atacama Gravels”, which passes laterally into fine-grained playa related deposits near the AFS. We interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the Precordillera due to the activity on AFS. A pedimentation episode followed sediment deposition and is locally strongly re-incised by the main modern-day river valleys draining the Precordillera. Incision may result from either regional uplift of the forearc, and/or from more localized activity on the AFS. Furthermore, Recent (Quaternary?) tectonic activity on the AFS has been observed which is consistent with a localized relative uplift of the crustal block west of the AFS.

Slope and climate variability control of erosion in the Andes of central Chile

Climate and topography control millennial-scale mountain erosion, but their relative impacts remain matters of debate. Confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long-term erosion. However, there is a lack of data documenting these erosion factors. Here we report suspended-load measurements, derived decennial erosion rates, and 10 Be-derived millennial erosion rates along an exceptional climatic gradient in the Andes of central Chile. Both erosion rates (decennial and millenial) follow the same latitudinal trend, and peak where the climate is temperate (mean runoff ~500 mm yr ‐1 ). Both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of ~0.55 m/m. The comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0% in the humid zone to more than 90% in the arid zone. Our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity.

Gold grain morphology and composition as an exploration tool: application to gold exploration in covered areas

The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; >300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.

Strike-slip faulting, thrusting and related basins in the Cenozoic evolution of the southern branch of the Bolivian Orocline

Abstract The evolution of the Cenozoic deformation of the Cordillera Oriental and of the contact zone with the Altiplano in southern Bolivia is well documented in the Tupiza, Nazareno and Estarca basins. The tectonic evolution started at about 29 Ma. The period between 29 and about 22-21 Ma is marked by development of a pull-apart basin related to NS-trending left-lateral strike-slip faulting. During this period, initial deposition consisted of andesitic lavas and detrital sediments (breccias; matrix-supported red conglomerates; sands and silts…) of the Catati and Tupiza Formation, deposited in alluvial fan and flood plain environments. From 21 to 20 Ma, the tectonic setting evolved to NS-oriented thrusts, which had a dextral component. This event caused the Tupiza basin to evolve into a full-ramp basin, and triggered the development of two piggy-back basins: Nazareno and Estarca. Both basins record detrital deposition (Nazareno Formation) in an alluvial fan environment. At 10-9 Ma, deformation in this area stopped allowing peneplanation during which time the San Juan de Oro erosional surface was formed and the fluvial conglomerates of the Oploca Formation were deposited. This segment of the Andes was then tectonically transported to the east, while uplift due to thrusting continued in the eastern Andean front and in the Subandean zone.

Magnetostratigraphy of the Miocene Corque basin, Bolivia: , Implications for the geodynamic evolution of the Altiplano during the late Tertiary

A magnetostratigraphic study of a thick Miocene continental red bed sequence (Totora Formation) located in the north central Altiplano (the Corque syncline) yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the Altiplano. Paleomagnetic results obtained in 653 samples from a composite 4.5 km thick red bed sequence across the Corque syncline indicate that most of the Totora sequence was deposited in the middle Miocene from 14 to 9 Ma with high sedimentation rates up to 970 m/Myr from 12 to 9 Ma. The high rate of infill of the Corque basin demonstrates active subsidence of the northern Altiplano during the middle Miocene. Deformation ceased in the northern Altiplano domain only at the end of the Miocene. Two major tuff beds within the sequence have been previously dated by 39 Ar/ 40 Ar of sanidine minerals [Marshall et al., 1992]. The excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by Cande and Kent [1995]. The numerous paleomagnetic results obtained in middle Miocene sediments within the Corque syncline, a structure which can be traced for more than 100 km along its axis, demonstrate that the Corque syncline rotated counterclockwise by 10.8°±2.9° since 9 Ma as a consequence of the internal deformation of the Altiplano. Inclination flattening of 17° is recorded in these red bed sediments. This result provides additional evidence that detrital magnetizations, especially in red beds, should be taken with caution when estimating paleolatitudes or long-term variations of the geomagnetic field.

Sedimentación sintectónica asociada a las estructuras neógenas en la Precordillera de la zona de Moquella, Tarapaca (19°15'S, norte de Chile).

En el norte de Chile (18-20°S), el borde occidental del alto Plateau andino (Altiplano) fue alzado por un sistema de fallas inversas de alto angulo y pliegues asociados. Este se desarrollo a lo largo de la Precordillera, que corresponde a la unidad morfoestructural que conecta al Altiplano, al este, y la Depresion Central, al oeste. En la zona de Moquella (19,3°S), ese sistema estructural esta representado por una flexura (Flexura Moquella) y depositos sintectonicos asociados. Esta flexura fue causada por una falla inversa, ciega y con vergencia al oeste, que estuvo activa, al menos, entre los 21 Ma y 16 Ma. Acomodo un total aproximado de 700 m del alzamiento del borde occidental del Plateau andino, contribuyendo de esta manera a casi un 20% de su alzamiento. Los depositos sintectonicos, de 300-400 m de espesor, corresponden a una sucesion sedimentaria y volcanica neogena, denominada Formacion Latagualla, expuesta en la Depresion Central hasta el borde occidental de la Precordillera. Esta formacion registro parte de la deformacion y la erosion neogena del borde occidental del Plateau andino a esta latitud. Las evidencias de su caracter sintectonico corresponden a su geometria en estratos de crecimiento asociados a la flexura. Ademas, sobre el eje de la Flexura Moquella se produjeron deslizamientos de grandes dimensiones que tambien han sido reconocidos mas al norte, en Arica, indicando ser un fenomeno caracteristico de este borde de los Andes.

THE FIRST CENOZOIC MAMMAL FAUNA FROM THE CHILEAN ALTIPLANO

Despite its richness, South Americas Cenozoic mammal record is strongly biased geographically towards a small portion of the continent, mainly Patagonia. The rapidly growing list of major Cenozoic localities at lower latitudes thus marks a significant advance in our understanding of mammalian evolution in South America (see summaries in Flynn and Swisher, 1995; Flynn and Wyss, 1998). This paper reports the discovery of the first Cenozoic mammal fauna known from the Chilean Altiplano. It is part of an ongoing collaborative effort to sample faunas of a variety of ages across a large latitudinal transect of western South America, the goal of which is to elucidate temporal patterns of provincialism within the continent, as well as the tectonic and uplift history of the Andes Mountains and associated paleoenvi

Late Miocene–Holocene canyon incision in the western Altiplano, northern Chile: tectonic or climatic forcing?

Abstract: Major fluvial incision (600–1000 m) affecting the Coastal Cordillera and Central Depression of northern Chile is analysed to evaluate supposed coeval uplift of the Altiplano and/or climatic changes in the Atacama Desert. The timing of the beginning of incision is constrained by the age of deposition of the Central Depression top. In the north (18–19°S), this top corresponds to fluvial gravels accumulated between 11.9 ± 0.6 Ma and 8.3 ± 0.5 Ma, which are genetically related to semiarid climate and to an eastward poorly dissected parallel drainage network that developed between 15.0 ± 0.6 and 11.2 ± 0.6 Ma; thus, gravel deposition ended at 11.9–11.2 Ma. To the south (19–20°S), the Central Depression top corresponds to c. 6 Ma alluvial deposits. Stratigraphically determined canyon ages and knickzone locations indicate that southward dissection began later and/or developed under a regime of lower erosion capacity owing to drier climate. Vertical incision rate evolution is compatible with eastward knickzone migration. Dissection required a considerable altitude difference between ancient and present-day river base levels, which was achieved predominantly by basin infill on an already partially elevated bedrock. Therefore subsequent incision would have been triggered by local semiarid climatic periods rather than by contemporaneous surface uplift. Exoreic canyons occur when climatic conditions in the catchments are arid–semiarid whereas endoreism is developed when these conditions in catchments are hyperarid.