Rodrigo Riquelme

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.

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.

Cenozoic tectonic evolution in the Central Andes in northern Chile and west central Bolivia: implications for paleogeographic, magmatic and mountain building evolution

A review of available stratigraphic, structural, and magmatic evolution in northernmost Chile, and adjacent Peru and Bolivia shows that in this region: (1) compression on the Paleogene intra-arc during the middle Eocene Incaic phase formed the NNE-SSW-oriented Incaic range along the present-day Precordillera and Western Cordillera, and (2) post-Incaic tectonic conditions remained compressive until present, contrasting with other regions of the Andes, where extensional episodes occurred during part of this time lapse. A late Oligocene–early Miocene peak of deformation caused further uplift. The Incaic range formed a pop-up structure bounded by two thrusts systems of diverging vergencies; it represented a major paleogeographic feature that separated two domains with different tectonic and paleogeographic evolutions, and probably formed the Andean water divide. This range has been affected by intense erosion and was symmetrically flanked by two major basins, the Pampa del Tamarugal and the Altiplano. Magmatic activity remained located along the previous Late Cretaceous–early Eocene arc with slight eastward shift. Further compression caused westvergent thrusting and uplift along the western Eastern Cordillera bounding the Altiplano basin to the east by another pop-up shaped ridge. Eastward progression of deformation caused eastvergent thrusting of the Eastern Cordillera and Subandean zone.

Erosion in the Chilean Andes between 27°S and 39°S: tectonic, climatic and geomorphic control

Abstract The effect of mean precipitation rate on erosion is debated. Three hypotheses may explain why the current erosion rate and runoff may be spatially uncorrelated: (1) the topography has reached a steady state for which the erosion rate pattern is determined by the uplift rate pattern; (2) the erosion rate only depends weakly on runoff; or (3) the studied catchments are experiencing different transient adjustments to uplift or to climate variations. In the Chilean Andes, between 27°S and 39°S, the mean annual runoff rates increase southwards from 0.01 to 2.6 m a−1 but the catchment averaged rates of decadal erosion (suspended sediment) and millennial erosion (10Be in river sand) peak at c. 0.25 mm a−1 for runoff c. 0.5 m a−1 and then decrease while runoff keeps increasing. Erosion rates increase non-linearly with the slope and weakly with the square root of the runoff. However, sediments trapped in the subduction trench suggest a correlation between the current runoff pattern and erosion over millions of years. The third hypothesis above may explain these different erosion rate patterns; the patterns seem consistent with, although not limited to, a model where the relief and erosion rate have first increased and then decreased in response to a period of uplift, at rates controlled by the mean precipitation rate.

Long-term denudation rates from the Central Andes (Chile) estimated from a Digital Elevation Model using the Black Top Hat function and Inverse Distance Weighting: implications for the Neogene climate of the Atacama Desert

A methodology for determining long-term denudation rates from morphologic markers in a Digital Eleva- tion Model (DEM) is checked by a comparative study of two drainage basins in the Precordillera of the Central Andes. In both cases the initial confi guration of an incised pediment surface has been restored by using two different methods: the Black Top Hat (BTH) function and the Inverse Distance Weighting (IDW) interpolation. Where vertical incision and hillslope erosion are recorded, the IDW appears to be the most adequate to reconstitute the pediment surfaces. Conversely, where only vertical incision is observed, the BTH describes more precisely the former pediment surfaces and it is easier to solve. By subtracting the DEM from the reconstructed marker we calculated an eroded volume, and estimated its uncertainty by considering Root Mean Square Error (RMSE) and DEM grid error. For the last ~10 Myr we obtained long-term denudation rates of 7.33±1.6 m/Myr in the San Andres drainage basin and 13.59±1.9 m/Myr in the El Salado drainage basin. These estimations are largely in agreement with other reported estimates of long-term denudation rates in the Atacama Desert. Comparison with long-term denudation rates reported in a wide range of climatic regimes suggests that our estimates cannot be explained by the current rainfall in the Precordillera. However they could be explained by a rainfall similar to that reported 40 km to the east in the Puna. This suggests that during the time span concerned the geomorphologic evolution of the study area, this evolution is dominated by an orographically controlled rainfall pattern. The preserved pediment surface and the small long term denudation rates determined in this study also indicate that the Precordillera was never reached by humid tropical air masses and precipitation as currently observed in the Altiplano during the summer months.

A genetic model based on evapoconcentration for sediment-hosted exotic-Cu mineralization in arid environments: the case of the El Tesoro Central copper deposit, Atacama Desert, Chile

Although the formation of exotic-Cu deposits is controlled by multiple factors, the role of the sedimentary environment has not been well defined. We present a case study of the El Tesoro Central exotic-Cu deposit located in the Atacama Desert of northern Chile. This deposit consists of two mineralized bodies hosted within Late Cenozoic gravels deposited in an arid continental environment dominated by alluvial fans with sub-surficial ponded water bodies formed at the foot of these fans or within the interfan areas. Both exotic-Cu orebodies mostly consist of chrysocolla, copper wad, atacamite, paratacamite, quartz, opal, and calcite. The most commonly observed paragenesis comprises chrysocolla, silica minerals, and calcite and records a progressive increase in pH, which is notably influenced by evaporation. The results of stable isotope analyses (δ13C and δ18O) and hydrogeochemical simulations confirm that evapoconcentration is the main controlling factor in the exotic-Cu mineralization at El Tesoro Central. This conclusion complements the traditional genetic model based on the gradual neutralization of highly oversaturated Cu-bearing solutions that progressively cement the gravels and underlying bedrock regardless of the depositional environment. This study concludes that in exotic-Cu deposits formed relatively far from the source, a favorable sedimentary environment and particular hydrologic and climatic conditions are essential to trap, accumulate, evapoconcentrate, neutralize and saturate Cu-bearing solutions to trigger mineralization. Thus, detailed sedimentological studies should be incorporated when devising exploration strategies in order to discover new exotic-Cu resources, particularly if they are expected to have formed relatively far from the metal sources.