Christian Sue

Quaternary erosion-induced isostatic rebound in the western Alps

The high elevation and deep incision of the Alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates. Normal faulting and horizontal extension, however, dominate current tectonic activity, and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations. Here we test the idea that enhanced Quaternary erosion of the Alps and isostatic compensation of the mass removed can account for the distribution of present-day geodetically measured rates of vertical movement in the western Alps. Using geophysical relief and Kuhlemanns estimated average erosion rate for the Alps, we quantify the spatial distribution of erosion and the volume of eroded rock, respectively. From these, we obtain a map of rock eroded within a given time span. The calculated isostatic response of the Alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the Alpine lithosphere reaches a maximum of [~]500 m since 1 Ma in the inner Swiss Alps, and vertical movement extends across the entire belt, including peri-Alpine basins. Assuming a steady erosion rate since 1 Ma, this rebound accounts for half of the measured vertical motion of 1.1 mm/yr in the southern Valais. Thus, a significant fraction ([~]50%) of the present-day vertical movement results from the isostatic response to enhanced erosion during Plio-Quaternary time

Widespread extension in the core of the western Alps revealed by earthquake analysis

The western Alps are an active collision belt whose current stress field is inhomogeneous [Muller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision-related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break-off, interfere with the boundary forces driven by the ongoing Europe-Africa convergence.

Mongolian summits: An uplifted, flat, old but still preserved erosion surface

In Gobi Altay and Altay, Mongolia, several flat surfaces, worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m, suggests disruption of a single large-scale surface. New thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during Jurassic time. The Gobi Altay and Altay flattopped massifs are tectonically and geomorphologically unique. Their preservation for ~150 m.y. implies that no further tectonic movements occurred before the onset of the last deformation episode, 5 ± 3 m.y. ago. It also suggests that very low erosion rates were maintained by a dry climate over millions of years.

La denudation tectonique de la zone ultradauphinoise et l'inversion du front brianconnais au sud-est du Pelvoux (Alpes occidentales); une dynamique miocene a actuelle

In the western Alps, to the southeast of the Pelvoux massif (Champsaur-Embrunais-Brianconnais-Queyras transect), the Brianconnais zone consists of the southern tip of the Zone Houillere and small nappes of Mesozoic sediments, emplaced during the Eocene in HP-LT metamorphic conditions. During the Oligocene this tectonic pile was thrusted onto a late Eocene to early Oligocene flexural basin, deformed in low grade metamorphic conditions and belonging to the Ultradauphine zone. This major thrust, called here CBF [Chevauchement Brianconnais Frontal: Tricart 1986] represents the boundary between the external and the internal zones of the western Alps. It contains thin tectonic lenses of Subbrianconnais origin, so that the Brianconnais Front and the Penninic Front almost merge. Late Alpine extension. - We have recently discovered that the CBF was subsequently reactivated as an extensional detachment. This major negative inversion is associated with widespread extension in the internal (Brianconnais and Piemont) zones, resulting in multiscale normal faulting. Current field work in the Queyras area shows that this brittle multitrend extension is a continuation of the ductile extension that accompanied the exhumation of blue-schist bearing metamorphic units. Along the same transect, the external (Ultradauphine) zone was not affected by late-Alpine extension. This is still the present situation: to the east of the aseismic Pelvoux massif, the CBF bounds the Brianconnais seismic arc, the activity of which may be the continuation of the late-Alpine extension. At the scale of the western Alpine arc, active extensional-transtensional tectonics dominate in the internal zones while compressional uplift affects the external zone. In this contrasted stress field, the thrust-fault zone between internal and external arcs plays a major role of decoupling that can be demonstrated in several sites between the area analysed here and the Central Alps, including along the Ecors profile. Contribution of thermochronology. - In this paper, we compare apatite fission track (FT) ages from both sides of the inverted CBF to the southeast of the Pelvoux massif. In the hangingwall of the CBF, two ages were obtained from magmatic intrusions within the Zone houillere, close to Briancon. They are compared to recently published ages from the Champsaur Sandstones unit in the footwall of the CBF, along the same transect.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Origin of the current stress field in the western/central Alps: role of gravitational re-equilibration constrained by numerical modelling

Abstract We interpret the strain and stress fields of the western/central Alpine arc on the basis of 2.5D finite element modelling and a recent seismotectonic synthesis. Models have fixed boundary forces and different crustal geometries, so that they respond to buoyancy forces (variations in gravitational potential energies). The seismotectonic regime, characterized by orogen-perpendicular extension in the high topographic core of the belt and local orogenperpendicular compressional/transpressional deformation in the external zones, appears to be very close to the modelled gravitational regime. Rotation of Apulia has a minor effect on the current strain or stress fields of the Alpine realm. Nevertheless, it could help to explain the orogen-parallel dextral faulting that is observed all along external zones, from the northern Valais to the Argentera external crystalline massif. Our results highlight the consequences for the Alpine realm of ongoing convergence between the African and European plates. Our interpretation is that collision is no longer ongoing and that buoyancy-driven stresses dominate the present-day geodynamics of the western/central Alps.

The Relation Between Neogene Denudation of the Southernmost Andes and Sedimentation in the Offshore Argentine and Malvinas Basins During the Opening of the Drake Passage

The Neogene orogenic growth of the Southern Patagonian Andes has been related to the approximation and collision of a series of segments of the Chile seismic ridge, which separates the Antarctic and Nazca plates, against South America. The compiled thermochronological data consistently indicates an eastward moving trend of exhumation, were uplift of the western basement domain occurred from ~34 to 15 Ma, and was followed by denudation of the basement front and the fold and thrust belt between ~20 and 5 Ma. There has been an assumption that tectonic growth in southern Patagonia ended in late Miocene times, largely based on the top age of the molasse deposits of the Santa Cruz Formation, spanning from ~22–19 to 14 Ma. There is, however, multiple thermochronological evidence that exhumation in the hinterland continued profusely, with large volumes of rock denudated rapidly between ~15 and 5 Ma, and steadily since ~7 Ma. However, continental sedimentation rate was very low in the Magallanes–Austral Basin of the Southernmost Andes after 14 Ma, an effect produced by the dynamic uplift of Patagonia. Contrastingly, the upper Miocene–lower Pliocene constitutes an aggradational period very well developed in the offshore Argentine continental margin. We propose that the great volumes of sediments produced by Miocene–Pliocene denudation of the Southernmost Andes bypassed Patagonia and reached the Argentine and Malvinas basins, where they were accommodated in thick sequences with high sedimentation rates. Those sediments were distributed along the Southern Atlantic margin by sub-Antarctic currents, which propagated into the Argentine continental margin during the deepening of the Drake Passage. The sediments were probably funneled through gargantuan fluvial and glacifluvial W–E systems, similar to those preserved in Patagonia from the last glaciation, and axially through the Fuegian Andes foothills toward the offshore basins.

Wrenching Tectonism in the Southernmost Andes and the Scotia Sea Constrained from Fault Kinematic and Seismotectonic Overviews

The geodynamics of the study area includes subduction, orogen dynamics, and major transcurrent tectonics at a complex transform margin in the southern ocean. We present a synthesis of the fault system associated to major fault zones in the Fuegian Andes and the Scotia Sea and review the available kinematic databases from microtectonic measurements. Our synthesis of six independent fault kinematic studies is coherent with a very consistent shortening direction-oriented NE–SW in the Southernmost Andes. The stability of the stress pattern and orientation of the shortening axes on a more regional scale involving the Scotia Sea indicate a steady E–W to NNE–SSW σ1/shortening direction since middle Eocene times. This observations reflect that the global left-lateral motion between Antarctica–Scotia–South America plates circuit is the main driving force for the entire area and in particular for the Southernmost Andes, over the last ~45 Ma. Both the observed short-term geodetic and the long-term geological slip rates of the Magallanes–Fagnano fault system from Tierra del Fuego are moderate (~5 mm/year), and the expected time span between major M8 earthquakes would be around 10 kyr. Yet the time between the two most recent large earthquakes was about 70 years. Such a great discrepancy suggests a complex mechanics on the Magallanes–Fagnano fault system, leading to complex recurrence time history for the characteristic earthquake (M ≥ 7).

From ocean formation to mountain evolution in alpine-type orogens

The 12th Alpine Workshop, organized by Christian Sue, Gianreto Manatschal and Raymond Cirio, was held in the Montgenèvre resort, located next-door to one of the bestpreserved ocean floor sequences exposed in the Western Alps, known as the Chenaillet ophiolite. As all the previous bi-annual series of Alpine workshops this conference was again held in a friendly and stimulating atmosphere. Before introducing the collection of papers received after the conference we take the opportunity to mention the locations and dates of the previous conferences. The first Alpine Workshop conference was held in Grenoble (France) in 1993, and was followed by Basel (Switzerland) in 1995, Oropa (Italy) in 1997, Tübingen (Germany) in 1999, Obergurgl (Austria) in 2001, Sopron (Hungary) in 2003,Opatja (Croatia) in 2005, Davos (Switzerland) in 2007, Cogne (Italy) in 2009, Saint-Florent (Corsica, France) in 2011, Schladming (Austria) in 2013, and Montgenèvre-Briançon (France) in 2015. In 2013, this informal series of conferences acquired an official character, being held under the auspices of the European Geosciences Union for the first time under the name ‘‘EmileArgandConference onAlpineGeological Studies’’, in honour of the famous Swiss geologist, Emile Argand (1879–1940), who first linked plate tectonics and dynamics of orogeny (e.g. Argand 1924). The aim of the conferences is to promote the discussion of new data and interpretations based on studies addressing the structural, tectonic, metamorphic and sedimentary evolution of the Alps, and adjacent Mediterranean Alpine-type orogens, in an informal and friendly atmosphere by bringing senior researchers together with young scientists, including numerous PhD students. A first group of papers arising from the 12th Alpine Workshop focuses on a topic that recently received a lot of attention by the international community, including oil companies working in off-shore areas of passive margin environments. Such environments are characterised by highly extended continental crust grading into adjacent oceanic areas formed along slow spreading (Atlantic) type ridges, the fossil equivalents ofwhich are exposed in theAlps. The paper byM. E. Epin, G. Manatschal and M. Amann analyses well preserved structures still inherited from rift-related tectonism in an area of intense Alpine deformation related to the subsequent closing ofAlpine Tethys. Their case study carried out in the Err and Platta nappes of southeastern Switzerland, representing an ocean-continent transition, focuses on the discussion as to how much rift-inheritance controls the architecture of a collisional orogen such as the Alps. They point out that retro-deformation of Alpine shortening inevitably results in a template of non-layer cake rift structures in the internal most parts of a collisional orogen. N. Incerpi, L. Martire, G. Manatschal and S. M. Bernasconi also present a study carried out in the same area; they specifically focus on hydrothermal fluid circulation in preand syn-tectonic sediments associated with detachment faults in order to improve our knowledge on thermal regimes in hyperextended continental margins. Their study reveals the complexity of the evolution of Preface to Part 1 of the Swiss Journal of Geosciences, Volume 110, Number 2. This part consists of a collection of papers presented at the 12th Alpine Workshop (third EGU-sponsored Emile Argand Conference on Alpine Geological Studies) held in MontgenèvreBriançon (France) on 13–19 September 2015.

Orogen-parallel brittle extension as a major tectonic imprint in the Neogene evolution of the south-western Alpine arc

We present a new analysis of the late Alpine brittle deformation in the southern branch of the western Alpine arc, focusing on the stack of internal metamorphic nappes east of the Argentera external crystalline massif. The regional-scale fault network is dominated by a NW–SE-striking right-lateral fault system that follows the general curvature of the arc and controls the overall morphology of the area. A second fault set strikes N–S and is mainly represented by normal faults which accommodate orogen-parallel extension. Structural analysis and paleostress tensors derived from inversion of fault-slip data reveal a complex pattern of deformation involving extensional and strike-slip deformation events. The orogen-parallel extension previously described in the internal zones at the east of the Pelvoux massif is confirmed further south, and we show that it is combined with right-lateral strike-slip deformation that increases in intensity towards the south-west. The stability of the minimum stress axis (σ3) direction suggests that extensional and transcurrent regimes are contemporaneous and highlights regional interferences between inner brittle extension, parallel to the strike of the belt, and the strike-slip strain field driven by the counterclockwise rotation of the Apulia–Adria plate. The curved geometry of the belt constrains the direction of extension and the coupling relationship between the internal and external Alpine zones. We propose that the Neogene tectonic history is a result of the unique curved tectonic architecture of the south-western Alps with respect to the rotation of the Apulia–Adria indenting plate. The southern tip of the western Alpine arc represents a transitional zone between extension in the inner chain and strike-slip/compression in the outer parts.