International audienceA sharp increase in deposited sediment volume since Pliocene times has been observed worldwide and in particular around the European Alps. This phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing. Observation of in-situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates, and to determine whether this phenomenon is widespread or localized at active tectonic structures. We exploit the unique density of fission-track ages in the Western European Alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13.5 and 2.5 Ma. Our novel technique is based on the association of isoage contours with age–elevation relationships. It uses map-view interpolation, enabling a spatio-temporal analysis of exhumation rates over the entire Western Alps. The resulting exhumation histories reconstructed for eight areas of the Western Alps display strong similarities in timing and rates with orogen-wide average denudation rates inferred from sediment volumes. This consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates. We conclude that exhumation rates in the Western Alps have increased more than twofold since Late Miocene times. This increase may have been locally modulated by the distinct response of different tectonic units

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[1] Several recent studies proposed an important increase in exhumation rate in the western European Alps since circa 5-4 Ma. In order to assess potential spatial differences in exhumation histories, we present new apatite fission track (AFT) and apatite (U-Th)/He (AHe) ages from the central Aar Massif (Guttannen area, Switzerland) and the western Lepontine Dome (Formazza area, Italy). Internal U/Th zoning in apatites explains alpha-ejection-corrected AHe ages that are older than the corresponding AFT ages in this study. A qualitative interpretation of AFT and AHe ageelevation relationships suggests a two-phase (9-7 and 5-3 Ma) exhumation scenario affecting the central Alps, with a stronger expression of the Pliocene signal in the Formazza area. However, a quantitative evaluation of exhumation scenarios using the 3-D heat equation solver Pecube highlights the existence of several other likely scenarios, casting doubt on the validity of a qualitative interpretation of the age-elevation relationships. In Formazza, scenarios suggested by quantitative modeling include continuous denudation at a rate of $750 m/Ma and a one-step exhumation rate change from 300 to 1000 m/Ma at 5 Ma. In Guttannen, they include continuous denudation at a rate of $400 m/Ma with valley deepening and two periods of higher exhumation rate (increasing from 300 to 700 m/Ma repeatedly at 9-7 and at 5-3 Ma). Contingent upon further flexural isostatic modeling, the magnitude of exhumation recorded in the axial region of the Alps since circa 5 Ma does not appear sufficient to solely explain the denudation recorded in the North Alpine Foreland Basin. Citation:

In this paper, we merge more than 200 new apatite and zircon (U-Th)/He analyses and 21 apatite fi ssion-track analyses from 71 new samples with previous published thermochronologic data using the same systems to understand the growth and largescale kinematics of the central Andes between 21°S and 28°S. In general, minimum dates decrease and the total range of dates increases from west to east across the range. Large variations in thermochronometer dates on the east side refl ect high spatial gradients in depth of recent erosional exhumation. Almost nowhere in this part of the Andes has Cenozoic erosion exceeded ~6-8 km, and in many places in the eastern half of the range, erosion has not exceeded 2-3 km, despite these regions now being

The relative intensity of tectonic and climatic forcing in the western European Alps has been a matter of debate since the recognition of a signifi cant increase in denudation rates over the past few million years. We address this question by quantitatively correlating the spatial pattern of long-term exhumation rates with those of potential short-term tectonic, climatic, and morphologic variables. We fi nd that present-day rock-uplift rates (as measured by geodesy relative to a specifi c reference point) and mean elevation are correlated with long-term exhumation rates, whereas relief, present-day precipitation, discharge, stream power, and released seismic energy are not, or are only weakly, correlated. We attribute the lack of correlation between long-term exhumation and precipitation to a strong temporal variability in climate and erosional processes during Pliocene-Pleistocene time. The correlations among present-day rock-uplift rates, present-day elevations, and long-term exhumation rates suggest that rockuplift rates have been sustained for millions of years, consistent with rock-uplift rates being the isostatic response to crustal unloading. The lack of a correlation of the released seismic energy with either rock uplift or long-term exhumation denies active tectonics supporting evidence. Po basin Penninic frontMolasse basin S0012-821X(03)00031-1.

In recent years, longitudinal changes on the thin/thick-skinned tectonic styles of the Central Andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen, and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the Peru-Chile trench. The search for paleo-climate records is therefore crucial for our understanding of the history of the Central Andes. In this paper, we focus on the Atacama Gravels, an extensive blanket of Miocene continental deposits filling a Neogene paleo-valley system along the southern Atacama Desert in northern Chile. An east-west transect, between Pedernales and Chañaral (26º30'S), enabled us to carry out a sedimentological and tectonic study of the Atacama Gravels, based on logging and field observations along the Rio Salado canyon. New 39 Ar-40 Ar ages obtained on intercalated and overlying ignimbrites date the beginning of the Atacama Gravels sedimentation at around the Oligocene-Miocene boundary and cessation of sedimentation in the late Miocene. Thirteen lithofacies, included within five facies associations (A1 to A5) were identified. Depositional environments vary from proximal alluvial fan (A1, A2) in the Precordillera through ephemeral fluvial (A3, A4) to distal playa lake (A5) in the Coastal Cordillera. No evidences of syn-sedimentary deformation have been found, showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes, and climate change appears to be the dominant controlling factor of sediment preservation. A progressive change from semi-arid towards hyper-arid climatic conditions during the Miocene, led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the Oligocene.

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