Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone

31p.International audienceThe initial propagation of the Western Alpine orogen was directed northwestward, as shown by basement-involved and Mesozoic sedimentary cover compressional structures and by the early foreland basins evolution. The crystalline basement of the Dauphine zone recorded three shortening episodes: pre-Priabonian deformation D1 (coeval with the Pyrenean-Provence orogeny), and Alpine shortening events D2 (N-NW directed) and D3 (W-directed). The early Oligocene D2 structures are trending sub-perpendicular to the more recent, arcuate orogen and are interfering with (or truncated by) D3, which marks the onset of westward lateral extrusion. The NW-ward propagating Alpine flexural basin shows earliest Oligocene thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Gravity enhanced submarine erosion gave birth locally to steep submarine slopes overlain by kilometric-scale blocks slid from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features indicate a N-NW-ward directed propagation, consistent with D2 in the Dauphine foreland. The Internal zones represent the paleo-accretionary prism developed during this early Alpine continental subduction stage. The early buildup has been curved in the arc and rapidly exhumed during the Oligocene collision stage. Westward extrusion and indenting by the Apulian lithosphere allowed the modern arc to crosscut the western, lateral termination of the ancient orogen from similar to 32 Ma onward. This contrasted evolution leads to propose a palinspastic restoration taking in account important northward transport of the distal passive margin fragments (Brianconnais) involved in the accretionary prism before the formation of the Western Alps arc

Cooling, exhumation and topographic evolution in continental magmatic arcs : an integrated thermochronological and numerical modelling approach : example from North Cascades (U.S.A.) and the Motagua fault zone (Guatemala)

Cette thèse cible l'étude de la structure thermique de la croûte supérieure (<10km) dans les arcs magmatiques continentaux, et son influence sur l'enregistrement thermochronologique de leur exhumation et de leur évolution topographique. Nous portons notre regard sur deux chaînes de montagne appartenant aux Cordillères Américaines : Les Cascades Nord (USA) et la zone de faille Motagua (Guatemala). L'approche utilisée est axée sur l'utilisation de la thermochronologie (U-Th-Sm)/He sur apatite et zircon, couplée avec la modélisation numérique de la structure thermique de la croûte. Nous mettons en évidence la variabilité à la fois spatiale et temporelle du gradient géothermique, et attirons l'attention du lecteur sur l'importance de prendre en compte la multitude des processus géologiques perturbant la structure thermique dans les chaînes de type cordillère, c'est à dire formées lors de la subduction océanique sous un continent.This thesis focuses on the influence of the dynamic thermal structure of the upper crust (<10km) on the thermochronologic record of the exhumational and topographic history of magmatic continental arcs. Two mountain belts from the American Cordillera are studied: the North Cascades (USA) and the Motagua fault zone (Guatemala). I use a combined approach coupling apatite and zircon (U-Th-Sm)/He thermochronology and thermo-kinematic numerical modelling. This study highlights the temporal and spatial variability of the geothermal gradient and the importance to take into account the different geological processes that perturb the thermal structure of Cordilleran-type mountain belts (i.e. mountain belts related to oceanic subduction underneath a continent)

Evolution du refroidissement, de l'exhumation et de la topographie des arcs magmatiques actifs : exemple des North Cascades (USA) et de zone de faille Motagua (Guatemala)

This thesis focuses on the influence of the dynamic thermal structure of the upper crust (<10km) on the thermochronologic record of the exhumational and topographic history of magmatic continental arcs. Two mountain belts from the American Cordillera are studied: the North Cascades (USA) and the Motagua fault zone (Guatemala). I use a combined approach coupling apatite and zircon (U-Th-Sm)/He thermochronology and thermo-kinematic numerical modelling. This study highlights the temporal and spatial variability of the geothermal gradient and the importance to take into account the different geological processes that perturb the thermal structure of Cordilleran-type mountain belts (i.e. mountain belts related to oceanic subduction underneath a continent).Cette thèse cible l'étude de la structure thermique de la croûte supérieure (<10km) dans les arcs magmatiques continentaux, et son influence sur l'enregistrement thermochronologique de leur exhumation et de leur évolution topographique. Nous portons notre regard sur deux chaînes de montagne appartenant aux Cordillères Américaines : Les Cascades Nord (USA) et la zone de faille Motagua (Guatemala). L'approche utilisée est axée sur l'utilisation de la thermochronologie (U-Th-Sm)/He sur apatite et zircon, couplée avec la modélisation numérique de la structure thermique de la croûte. Nous mettons en évidence la variabilité à la fois spatiale et temporelle du gradient géothermique, et attirons l'attention du lecteur sur l'importance de prendre en compte la multitude des processus géologiques perturbant la structure thermique dans les chaînes de type cordillère, c'est à dire formées lors de la subduction océanique sous un continent

Relief evolution above Patagonian slab window inferred from low-temperature thermochronology: subduction or climate?

International audienceThe formation and evolution of relief in subduction-related orogens result from a variety of processes acting at different scales of time and space. The interplay between tectonics and erosion (river incision, glacial erosion. . . ) is generally the principal contributor to the relief development. However, Earth's surface topography is also shaped by mantle convection, the latter generally producing a low amplitude, long-wavelength deflection of the surface as a response to the distribution of density anomalies in the mantle. For regions where mantle dynamics may change rapidly, e.g. in subduction zones where slab windows form, the signal of dynamic topography may also be variable in time and space, and exert an important control on landscape evolution, but this issue has been poorly addressed so far. Patagonian is one of the few regions on Earth where a slab window is currently developing. The arrival at trench of the Chile Ridge separating the Nazca and Antarctic plates at the latitude of 54 S ca. 16 Ma ago and the westward motion of South America led to the intermittent migration toward the north of the associated triple junction and the progressive enlargement of the Patagonian slab window, which is clearly identified on tomographic images as a low seismic velocity anomaly in the upper mantle. The contribution of slab-window-related dynamic topography in the topographic evolution of the Patagonian Cordillera has generally not been considered mainly because local flexural and isostatic adjustments due to tectonics and erosion obscure the dynamic topography signal. In particular, glaciations recorded by the oldest glacial till preserved in South America, played an important role in shaping the Andean landscape as early as ca. 5-7.4 Ma. In this study, we combine low-temperature thermochronology apatite (U-Th)/He data and semi-analytical modeling of dynamic topography to investigate the role of slab window and climate on cooling/heating history and relief evolution in the Patagonian Cordillera. In particular, we discuss a new thermochronological dataset consisting in 22 samples divided into four elevation transects. Sampling sites were chosen at the same distance from the trench (250-300 km), on the leeward eastern side of the orogen, for latitudes ranging between 45 S and 48 S to detect a potential northward migration of the thermal signal associated with the northward migration of the slab window. We show that history of heating and cooling for this region of the southern Andes compares well with the time-evolution of slab window and that present-day latitudinal topographic variations cannot be explained by climate alone but require an additional support by dynamic topography

Dynamic topography control on Patagonian relief evolution as inferred from low temperature thermochronology

International audienceWe combine low-temperature thermochronology apatite (U-Th)/He data and semi-analytical modeling of dynamic topography to investigate the role of slab window and climate on cooling/heating history and relief evolution of the Patagonian Cordillera. In particular, we discuss a new thermochronological dataset consisting in 22 samples divided into four elevation transects. Sampling sites were chosen at the same distance from the trench (250-300 km), on the leeward eastern side of the orogen, for latitudes ranging between 45°S and 48°S to detect a potential northward migration of the thermal signal associated with the northward migration of the slab window. We show that history of heating and cooling for this region of the southern Andes compares well with the northward migration history of slab window. In particular, a phase of heating is recorded at 15-10 Ma to the south and at ≤5 Ma to the north, preceding by ∼5 Ma the opening of the slab window beneath Patagonia, followed by a phase of rapid cooling and denudation to the south, with values as high as 650 m/Myr between 5 and 3 Ma. We also show that present-day latitudinal topographic variations require a support by dynamic topography associated with slab window

Relief evolution above Patagonian slab window inferred from low-temperature thermochronology: subduction or climate?

International audienceThe formation and evolution of relief in subduction-related orogens result from a variety of processes acting at different scales of time and space. The interplay between tectonics and erosion (river incision, glacial erosion. . . ) is generally the principal contributor to the relief development. However, Earth's surface topography is also shaped by mantle convection, the latter generally producing a low amplitude, long-wavelength deflection of the surface as a response to the distribution of density anomalies in the mantle. For regions where mantle dynamics may change rapidly, e.g. in subduction zones where slab windows form, the signal of dynamic topography may also be variable in time and space, and exert an important control on landscape evolution, but this issue has been poorly addressed so far. Patagonian is one of the few regions on Earth where a slab window is currently developing. The arrival at trench of the Chile Ridge separating the Nazca and Antarctic plates at the latitude of 54 S ca. 16 Ma ago and the westward motion of South America led to the intermittent migration toward the north of the associated triple junction and the progressive enlargement of the Patagonian slab window, which is clearly identified on tomographic images as a low seismic velocity anomaly in the upper mantle. The contribution of slab-window-related dynamic topography in the topographic evolution of the Patagonian Cordillera has generally not been considered mainly because local flexural and isostatic adjustments due to tectonics and erosion obscure the dynamic topography signal. In particular, glaciations recorded by the oldest glacial till preserved in South America, played an important role in shaping the Andean landscape as early as ca. 5-7.4 Ma. In this study, we combine low-temperature thermochronology apatite (U-Th)/He data and semi-analytical modeling of dynamic topography to investigate the role of slab window and climate on cooling/heating history and relief evolution in the Patagonian Cordillera. In particular, we discuss a new thermochronological dataset consisting in 22 samples divided into four elevation transects. Sampling sites were chosen at the same distance from the trench (250-300 km), on the leeward eastern side of the orogen, for latitudes ranging between 45 S and 48 S to detect a potential northward migration of the thermal signal associated with the northward migration of the slab window. We show that history of heating and cooling for this region of the southern Andes compares well with the time-evolution of slab window and that present-day latitudinal topographic variations cannot be explained by climate alone but require an additional support by dynamic topography

Climatic control on the location of continental volcanic arcs

International audienceOrogens and volcanic arcs at continental plate margins are primary surface expressions of convergent plate tectonics. Although it is established that climate affects the shape, size, and architecture of orogens via orographic erosion gradients, the ascent of magma through the crust and location of volcanoes along magmatic arcs have been considered insensitive to erosion. However, available data reveal westward migration of late-Cenozoic volcanic activity in the Southern Andes and Cascade Range where orography drives an eastward migration of the topographic water divide by increased precipitation and erosion along west-facing slopes. Thermomechanical numerical modeling shows that orographic erosion and the associated leeward topographic migration may entail asymmetric crustal structures that drive the magma ascent toward the region of enhanced erosion. Despite the different tectonic histories of the Southern Andes and the Cascade Range, orographic erosion is a shared causal mechanism that can explain the late-Cenozoic westward migration of the volcanic front along both magmatic arcs

Slab flattening, magmatism, and surface uplift in the Cordillera Occidental (northern Peru)

International audienceThe impact of subduction processes on surface uplift and relief building in the Andes is not well understood. In northern Peru, we have access to a modern flat subduction zone (3°–15°S) where both the geometry and timing of the flattening of the slab are well constrained. Some of the highest Andean peaks, the Cordillera Blanca (6768 m) and the Cordillera Negra (5187 m), are located just above the Peruvian flat slab. This is a perfect target to explore the impact of slab flattening and associated magmatism on Andean topography and uplift. We present new apatite (U-Th)/He and fission-track data from three vertical profiles in the Cordillera Blanca and the Cordillera Negra. Time-temperature inverse modeling of the thermochronologi-cal data suggests that regional exhumation in the Cordillera Occidental started at ca. 15 Ma, synchronous with the onset of subduction of the Nazca Ridge and eastward movement of regional magmatism. We propose that ridge subduction at 15 Ma and onset of slab flattening drove regional surface uplift, with an important contribution of mag-matism to relief building in the Cordillera Occidental