Sébastien Carretier

Field examples of strike‐slip fault terminations in Mongolia and their tectonic significance

Deformation at the ends of large intracontinental strike-slip faults that do not simply link other major structures often involves rotations about a vertical axis. We use earthquake slip vectors, surface rupture in earthquakes, and geomorphology to examine the ends of three major strike-slip faults in Mongolia. In these places a simple pattern is seen, consisting of a thrust fault on one side, with a displacement that decreases away from the strike-slip fault, consistent with local rotational deformation. Strike-slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area, for example, from dominantly strike-slip to dominantly dip-slip, while still accommodating the overall deformation required by larger-scale regional motions. Such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis, which may be detected paleomagnetically. The kind of strike-slip fault termination described here may have consequences for how large strike-slip fxaults evolve and grow and for the variation in displacement along their length.

Land-Level Changes Produced by the Mw 8.8 2010 Chilean Earthquake

The 2010 Mw 8.8 Chilean earthquake ruptured ~500 kilometers and vertically displaced over 3 meters. We observed vertically displaced coastal and river markers after the 27 February 2010 Chilean earthquake [moment magnitude (Mw) 8.8]. Land-level changes range between 2.5 and –1 meters, evident along an ~500-kilometers-long segment identified here as the maximum length of coseismic rupture. A hinge line located 120 kilometers from the trench separates uplifted areas, to the west, from subsided regions. A simple elastic dislocation model fits these observations well; model parameters give a similar seismic moment to seismological estimates and suggest that most of the plate convergence since the 1835 great earthquake was elastically stored and then released during this event.

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.

Specific suspended sediment yields of the Andean rivers of Chile and their relationship to climate, slope and vegetation

Abstract Using daily suspended sediment and water discharge data, we calculated the current mean annual runoff and Specific Suspended Sediment Yield (SSY) for 66 mountainous and piedmont catchments in Chile. These catchments are located from the extreme north of Chile to Southern Patagonia and cover an exceptionally wide range of climates, slopes, and vegetation. The SSY ranges mainly between 0 and 700 t km-2 year-1 with some exceptions as high as 1780 t km-2 year-1. The SSY increases between the extreme north and 33°S and then decreases toward the south. Sediment and water discharge north of 33°S occur mainly during summer. Farther south the contribution of winter precipitation increases and predominates. When the SSY database is correlated with topographic, climatic and vegetation indices, it is found to correlate significantly with runoff and mean slope only. In order to concentrate on erosion processes in the mountain range, 32 mountainous catchments were selected along a strong north–south SSY gradient between 27°S and 40°S. From north to south, SSY increases strongly with runoff and then decreases, even while runoff keeps increasing. In catchments where SSY is low, although runoff is high, the mean slope is less than 40% and the vegetation cover is greater than 8%. For the other catchments, runoff variations explain 67% of the variance in sediment yields. Thus, SSY seems to be controlled by vegetation cover and slope thresholds. In addition, SSY also correlates with glacier cover. However, a correlation between SSY and seismicity, although possible, is ambiguous. Citation Pepin, E., Carretier, S., Guyot, J. L. & Escobar, F. (2010) Specific suspended sediment yields of the Andean rivers of Chile and their relationship to climate, slope and vegetation. Hydrol. Sci. J. 55(7), 1190–1205.

Contrasting mountain and piedmont dynamics of sediment discharge associated with groundwater storage variation in the Biobío River

Supply and transport of sediment in catchments involve processes with fundamental consequences for river management, land use, and the prediction of climate-driven sediment fluxes. In the present study we addressed spatial variability in the water routes through the surface and subsurface of a catchment and the suspended sediment discharge (Qs) over a mountain-piedmont system. We analyzed daily suspended sediment concentration (Cs) and water discharge (Q) measurements at stations located in different topographic settings (mountain and piedmont) in the Biobio River basin (southern central Andes, 37–39°S). In steep catchments, the Q versus Qs relationship has a marked seasonal hysteresis. In the piedmont, Qs is proportional to Q, with no seasonal hysteresis. The contrast in the hysteresis pattern between catchments with different topographies is explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table is near the surface. In the mountains, groundwater storage results in large seasonal variations in the proportion of Q that flows at the surface and transports sediment from the hillslopes, producing hysteresis. By separating the total Q into two components (direct discharge, Qd and base flow, Qb), we observed the response of Qs to the fraction of water that quickly leaves the catchment after a rainfall event (Qd). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Qs into a linear relationship with Qd over the entire catchment and lead us to propose that sediment mobilization to the river along the Biobio catchment is primarily controlled by overland flow. Our findings highlight the importance of an adequate hydrological model for understanding the erosion and transport processes of a catchment, and which can be applied to other natural and modeled mountain-piedmont systems.

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.

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.