Syn-convergence flow inside and at the margin of orogenic plateaus: Lithospheric-scale experimental approach

International audienceThis study investigates three-dimensional flow modes of orogenic plateaus by means of physical modeling. Experiments consist of shortening two contiguous lithospheres of contrasting strength, one being a weak plateau-type lithosphere and the other a strong craton-type lithosphere. The lateral boundaries are either totally confined or allow escape toward a lateral foreland on one side. Two synconvergence flow regimes are distinguished, which are governed by the balance between the gravity potential and the strength of the plateau crust and the resistance of its lateral foreland. The first regime implies transversal (orogen-normal) injection of plateau lower crust into the collision zone as a result of confinement of the plateau by an increasingly stiffer lateral boundary. As a precursor mechanism to channel flow, transversal injection responds to downward thickening of the plateau crust that is forcedly extruded into the orogenic wedge. The second regime is that of collapse-driven lateral escape of the plateau. This regime is established after a threshold is attained in the interplate coupling in the collision zone, which allows the gravity potential of the plateau to overcome the resistance of its lateral boundary. Under the collapse-driven escape regime (orogen parallel), such as that governing Tibet during the last 13 Ma, most of the convergence in the plateau and the top and rear of the collisional wedge is transformed into lateral flow and extension

Orocline and syntaxes formation during subduction and collision

International audiencepresent work investigates the formation of curved ranges and syntaxes with scaled laboratory experiments. We simulated subduction and collision processes comparable to India-Asia configuration involving a continental upper plate and a subducting plate composed of an oceanic lithosphere and a continental indenter. The experiments reveal that the shape of the mountain range (concave, straight, or convex) and the development of syntaxes are controlled by the subduction interface, the buoyancy number (Fb) of the upper plate (i.e., thickness and viscosity), and the boundary conditions. Four end-members regimes of indentation can be defined depending on the range shape and dynamics of the upper plate. The curvature of the range is convex toward the subducting plate with syntaxes for a weak subduction fault and concave without syntaxes for a strong subduction fault. Convex curvature and syntaxes form by overthrusting of upper plate material on the subducting plate, which is faster at the center than at the extremities. They are associated with a rather flat slab (underthrusting) during continental collision. Low-Fb experiments show less pronounced curvatures associated to thickening comparable to the early stages of the India-Asia collision. In contrast, a thick and weak upper plate (high Fb) leads to gravity collapse that increases the amplitude of the curvature and lateral escape, similar to the late evolution of the Himalaya-Tibet system. Important lateral decoupling on the sides of the indenter enhances the indentation and produces sharper syntaxes

Continental delamination: Insights from laboratory models

One of the major issues of the evolution of continental lithospheres is the detachment of the lithospheric mantle that may occur under certain conditions and its impact on the surface. In order to investigate the dynamics of continental delamination, we performed a parametric study using physically scaled laboratory models. The adopted setup is composed of a three-layers visco-elastic body (analog for upper crust, lower crust, lithospheric mantle) locally thickened/thinned to simulate a density anomaly (lithospheric root) and an adjacent weak zone, lying on a low viscosity material simulating the asthenosphere. The results emphasize the interplay between mantle flow, deformation, surface topography and plate motion during a three-phases process: (1) a slow initiation phase controlled by coupling and bending associated with contraction and dynamic subsidence, (2) lateral propagation of the delamination alongside with extension and a complex topographic signal controlled by coupling and buoyancy, while poloidal mantle flow develops around the tip of the delaminating lithospheric mantle, and (3) a late phase characterized by a counterflow that triggers retroward motion of the whole model. A semiquantitative study allows us to determine empirically two parameters: (1) an initiation parameter that constrains the propensity of the delamination to occur and correlates with the duration of the first stage, (2) a buoyancy parameter characterizing the delamination velocity during late stages and therefore its propensity to cease. Finally, we point out similarities and differences with the Sierra Nevada (California, USA) in terms of topography, deformation and timing of delamination.Peer reviewe

Pre-, syn- and post-breakup evolution of northern South America from apatite fission track data and inverse thermal history modelling

International audiencePhanerozoic evolution of the Guiana Shield in northern South America is poorly known and remains an important question for source-to-sink studies. As part of the Source to Sink project TOTAL R&D-BRGM-INSU, this study presents a suite of 40 new apatite fission track analysis results from French Guiana, Surinam and north Brazil. Inverse thermal history modelling reveals a marked, albeit poorly constrained, period of relatively high temperatures (temperatures > 110 °C) during the Jurassic (200-150 Ma), i.e. prior to the generally accepted timing of the Equatorial Atlantic Ocean opening (150-110 Ma). Lower temperatures are recorded on the southerm flank of the Amazonas Basin and a little portion of the Surinam interior. All data reveal then a protracted phase of cooling starting from ~150 Ma and representing the main event during the Mesozoic and Cenozoic for French Guiana and Surinam basement. A second thermal event is nevertheless recorded after the deposition of Upper Jurassic and Lower Cretaceous detrital sedimentary rocks on the northern flank of the Amazonas Basin. Maximum temperatures for this event (80-100 °C) are reached during the middle Cretaceous (115-90 Ma). Implications of these new results for the Guiana Shied evolution and the Equatorial Atlantic opening are discussed

Pre-, syn- and post-breakup evolution of northern South America from apatite fission track data and inverse thermal history modelling

International audiencePhanerozoic evolution of the Guiana Shield in northern South America is poorly known and remains an important question for source-to-sink studies. As part of the Source to Sink project TOTAL R&D-BRGM-INSU, this study presents a suite of 40 new apatite fission track analysis results from French Guiana, Surinam and north Brazil. Inverse thermal history modelling reveals a marked, albeit poorly constrained, period of relatively high temperatures (temperatures > 110 °C) during the Jurassic (200-150 Ma), i.e. prior to the generally accepted timing of the Equatorial Atlantic Ocean opening (150-110 Ma). Lower temperatures are recorded on the southerm flank of the Amazonas Basin and a little portion of the Surinam interior. All data reveal then a protracted phase of cooling starting from ~150 Ma and representing the main event during the Mesozoic and Cenozoic for French Guiana and Surinam basement. A second thermal event is nevertheless recorded after the deposition of Upper Jurassic and Lower Cretaceous detrital sedimentary rocks on the northern flank of the Amazonas Basin. Maximum temperatures for this event (80-100 °C) are reached during the middle Cretaceous (115-90 Ma). Implications of these new results for the Guiana Shied evolution and the Equatorial Atlantic opening are discussed

Superimposed Rifting at the Junction of the Central and Equatorial Atlantic: Formation of the Passive Margin of the Guiana Shield

International audienceThe passive margin of the Guiana Shield formed at the junction of the Central and Equatorial Atlantic Oceans that developed successively by a complex rifting process that achieved the final dispersal of Western Gondwana. Yet, the resulting spatial distribution of crustal thinning along the margin remains to be mapped and its controlling parameters deciphered. We used subsurface data to map the variability of crustal thinning along the margin. We show that the margin segments width primarily depends on their obliquity to rifting extension direction. The necking domain of the transform/oblique margin segments is much narrower (<100km) than divergent segments that include hyper-extended or distal margin domains as well (200–300km). Moreover, for a similar obliquity, the width of margin segments resulting from the Central Atlantic rifting are wider than those resulting from the Equatorial Atlantic rifting. This is primarily due to the higher obliquity and rate of the latter. Additionally, along the Central Atlantic margin segments, thinning was accommodated by lower-crustal ductile deformation, the thick syn-rift in-fill was associated with magmatism and a thicker than average oceanic crust. In contrast, the Equatorial Atlantic margin segments show little ductile deformation, reduced clastic syn-rift infill and thinner than average oceanic crust. These observations suggest that the lithosphere affected by the Central Atlantic rifting was warmer than that affected by the Equatorial Atlantic rifting. Finally, the two-step thinning in the overlapping area of the two rift systems individualized a promontory of thinned continental crust that remained as the Demerara Platea

The sediment routing systems of Northern South America since 250 M

International audienceThe present study is a contribution to the understanding of continental-scale source-to-sink sedimentary systems over geological time scales. The northern half of South America is taken as a case study to explore the relations between (i) continental tectonic boundary conditions (active / passive margin, orogenic activity) and intracontinental deformation, (ii) the spatial distribution of clastic sediment sources and sinks over the continent and its margins and (iii) the configuration of the main sediment routes. To do that, the evolving surface configuration of northern South America is investigated through a series of 10 paleo-geological maps compiled for key periods of the Meso-Cenozoic. The maps display (i) sedimentation areas and their depositional environments, (ii) sediment transport routes based on paleocurrent measurements and provenance studies, (iii) magmatic occurrences, (iv) active faults, (v) denudation/burial histories deduced from low-temperature thermochronology studies and (vi) areas of extensive Cenozoic lateritic cover.Three main successive continental-scale regimes of the source-to-sink systems are documented. An early erosional regime until ca. 125 Ma is attested to by a major sedimentary hiatus over a long-lasting asthenosphere-supported eroding superswell at the scale of northern South America and northwestern Africa. Sediments were routed from the superswell to the proto-Andean marginal basins, the Central Atlantic rifts/rifted margin basins and the Saharan cratonic basin. After ca. 125 Ma, a cratonic regime of mixed erosion/deposition established and maintained until the onset of the Andean orogeny at ca. 85 Ma. During this period, sediment routes from eroding cratonic domains to the proto-Andean margin basins were episodically closed by inherited Paleozoic structures (i.e., “arches”), whilst sediment fluxes to cratonic or rifted margin basins were partitioned by Atlantic rift shoulders / marginal upwarps of evolving amplitude that formed transient continental divides. After ca. 85 Ma, an Andes-dominated regime installed, in which Andean retro-foreland basins became the main sinks for orogenic sediment fluxes, while exchanges with cratonic basins were controlled by the topographic expression of foreland basins forebulges. Under that regime, cratonic domains mainly acted as by-pass zones for clastic sediments, and their extensive Paleogene and Lower Neogene lateritic covers argue for limited clastic exports from areas exposed to erosion. Very-long wavelength (x 1000 km) asthenospheric support, long-wavelength (x 100 km) lithospheric-scale deformation / vertical movements and climate-controlled erosion processes interacted to regulate cratonic sediment source-to-sink regimes of northern South America since 250 Ma

Correcting laser scanning intensity recorded in a cave environment for high-resolution lithological mapping: A case study of the Gouffre Georges, France

International audienceActive remote sensing by laser scanning (LiDAR) has markedly improved the mapping of a cave environment with an unprecedented level of accuracy and spatial detail. However, the use of laser intensity simultaneously recorded during the scanning of caves remains unexplored despite it having promising potential for lithological mapping as it has been demonstrated by many applications in open-sky conditions. The appropriate use of laser intensity requires calibration and corrections for influencing factors, which are different in caves as opposed to the above-ground environments. Our study presents an efficient and complex workflow to correct the recorded intensity, which takes into consideration the acquisition geometry, micromorphology of the cave surface, and the specific atmospheric influence previously neglected in terrestrial laser scanning. The applicability of the approach is demonstrated on terrestrial LiDAR data acquired in the Gouffre Georges, a cave located in the northern Pyrenees in France. The cave is unique for its geology and lithology allowing for observation, with a spectacular continuity without any vegetal cover, of the contact between marble and lherzolite rocks and tectonic structures that characterize such contact. The overall accuracy of rock surface classification based on the corrected laser intensity was over 84%. The presence of water or a wet surface introduced bias of the intensity values towards lower values complicating the material discrimination. Such conditions have to be considered in applications of the recorded laser intensity in mapping underground spaces. The presented method allows for putting geological observations in an absolute spatial reference frame, which is often very difficult in a cave environment. Thus, laser scanning of the cave geometry assigned with the corrected laser intensity is an invaluable tool to unravel the complexity of such a lithological environment