Deciphering orogenic evolution

International audienceDeciphering orogenic evolution requires the integration of a growing number of geological and geophysical techniques on various spatial and temporal scales. Contrasting visions of mountain building and lithospheric deformation have been proposed in recent years. These models depend on the respective roles assigned to the mantle, the crust or the sediments. This article summarizes the contents of the Special Issue dedicated to 'Geodynamics and Orogenesis' following the 'Réunion previous termdesnext termprevious termSciencesnext termprevious termdenext termprevious termlaprevious termTerre' 2010 conference held in Bordeaux, France. Further, based on the example of the Western Alps-Mediterranean domain we emphasize the possibility to integrate long and short term, plate- to sample-scale, datasets in order to constrain orogenic evolution

Combination of Numerical Tools to Link Deep Temperatures, Geological Structures and Fluid Flow in Sedimentary Basins: Application to the Thermal Anomalies of the Provence Basin (South-East France)

International audienceIn the Provence basin, south-eastern France, more than 230 Bottom Hole Temperature (BHT) data have been compiled and corrected for transient disturbances to provide a thermal model of this Mesozoic to Cenozoic sedimentary basin. The thermal gradient of the area averages 29.9°C/km (32.5°C/km in all France), but some places show gradients reaching 36°C/km or 22°C/km. To characterize thermal anomalies, a three-dimensional model of the temperatures was built between the surface and 5km depth, allowing us to elaborate sets of thermal maps and cross-sections. The newly identified temperature anomalies may reach temperature difference up to 40°C at 3km depth through the basin. After attempting to find correlations between thermal anomalies and large scale features (Moho depths, sediment cover thickness), it appears that fluid circulation may better explain locations, amplitudes and wavelengths of thermal anomalies along faulted zones. In fact, spatial evolution of anomalous cold/warm zones follow directions of main faulted zones. In addition, it is shown that the account of a depth-dependent permeability allows the superimposition of positive and negative thermal anomalies. Away from permeable zones, thermal anomalies should be explained by conductive processes, among which heat refraction due to thermal conductivity contrasts may be significant. In particular, anisotropy of thermal conductivity of clayey formation is shown to enable the development of thermal anomalies similar to those observed between permeable zones. Evolution of fluid circulation in faulted zones (involving enhanced vertical heat transfer) combined with thick anisotropic sediments (involving enhanced horizontal heat transfer) may explain complex thermal patterns deduced from present-day temperature measurements

Diachronous exhumation of HP-LT metamorphic rocks from southwestern Alps: evidence from fission-track analysis

International audienceNew fission-track ages on zircon and apatite (ZFT and AFT) from the southwestern alpine paleo-accretionary wedge document a contrasting cooling history from east to west. In the eclogitic Monviso ophiolites, the ZFT ages are 19.6 +/- 0.8 Ma and the AFT ages are 8.6 +/- 1.7 Ma. In the HT-blueschist eastern Queyras, ZFT ages range from 27.0 +/- 1.5 Ma to 21.7 +/- 1.6 Ma and AFT ages from 14.2 +/- 2.0 to 9.4 +/- 1.1 Ma. In the LT-blueschist western Queyras, ZFT ages are between 94.7 +/- 3.1 Ma and 63.1 +/- 2.9 Ma and AFT ages are between 22.2 +/- 1.6 and 22.6 +/- 1.5 Ma. The Chenaillet ophiolite yields ages of 118.1 +/- 3.7 Ma on ZFT and of 67.9 +/- 8.5 Ma on AFT. These new FT data combined with petrological and geochronological constraints record a diachronous exhumation in the paleo-accretionary wedge during subduction and collision

How many subductions in the Variscan orogeny? Insights from numerical models

We developed a 2D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence, both followed by continental collision, aiming to better understand the evolution of the Variscan belt. Three models with different velocities of the first oceanic subduction have been implemented. Striking differences in the thermo-mechanical evolution between the first subduction, which activates in an unperturbed system, and the second subduction, characterised by an opposite vergence, have been enlighten, in particular regarding the temperature in the mantle wedge and in the interior of the slab. Pressure and temperature (P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central (FMC). The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC, while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models. Analysing the agreement of both double and single subduction models with natural P-T estimates, we observed that polycyclic models better describe the evolution of the Variscan orogeny

The Monviso ophiolitic massif (Western Alps), a section through a serpentinite subduction channel.

The exhumation of subducted lithosphere requires a mechanically weak zone at the interface between the subduction plane and the rigid overlying mantle peridotites with a viscosity greater than 10 20 Pa.s. At shallow depths (< 40-50 km) blueschists are exhumed in accretionary wedge along the interface between the subducting plate and the overriding plate (Platt, 1993). At greater depth, serpentinites plays the role of mechanically weak layer in cool continental subduction and act as the lubricant and produce a return flow for the exhumation of eclogitic rocks. The close association of serpentinites and eclogites in the Monviso massif (Western Alps) allow to discuss the concept of subduction serpentinite channel. We propose that the Monviso ophiolitic massif corresponds to a section of a 50 km long serpentinite channel in where eclogitic blocks were exhumed between 60 and 45 Ma and ended whEuropean continental margin was involved in the southeast dipping subduction zone

Deciphering large-scale superposed fold systems at shallow crustal levels in collision zones: insights from the Marguareis Massif (southwestern Alps)

We present and discuss the results of a field-based approach including accurate geological mapping and micro- to map-scale structural analysis to highlight the finite strain pattern recorded in Marguareis Unit, a massif deformed at shallow crustal levels at the boundary between Maritime and Ligurian Alps. We describe superposed tectonic structures developed under low-grade metamorphic conditions during the Alpine collision and nowadays exceptionally well recorded in the area of interest. We demonstrate that the structural frame of the Marguareis Unit results from superposition of fourfold systems, later segmented, but without significant displacements, by brittle faults

HP-LT rocks exhumed during intra-oceanic subduction: the example of the Escambray massif (Cuba).

High-Presssure metabasites embedded in a serpentinite or metasedimentary matrix from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origine and the evolution of the Northern Carribean boundary plate during the Cretaceous, in a global subduction context. Geochemical analyses (major, trace elements and isotopes) of the high pressure rocks show that they could be partially derived from the Cretaceous calc-alkaline arc described in Central Cuba, these were probably incorporated in the subduction zone by tectonic erosion. The High-Pressure rocks record a prograde path from the epidote bearing amphibolite facies to the barroisite bearing eclogite facies (P = 19 ± 2 Kbar, T = 590 ± 90 °C). These metabasites show evidence of retrogression starting from the glaucophane bearing eclogite facies to the lawsonite bearing blueschist facies. Therefore, these HP/LT rocks are characterized by a counter-clockwise cooling P/T path, which can be explained by the exhumation of HP rocks while the subduction was still active. Concordant geochronological data (Rb/Sr and Ar/Ar) suggest that the main exhumation of HP/LT rocks from the Sancti Spiritus dome occurred 70 Ma ago by top to SW thrusting. The retrogressed trajectory of these rocks, means that the northeast subduction of the Farallon plate continued after 70Ma. The final exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematic