Manuel Roda

Numerical simulations of an ocean‐continent convergent system: Influence of subduction geometry and mantle wedge hydration on crustal recycling

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2D finite element thermo-mechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5 and 10 cm/y, slab angles of 30o, 45o and 60o and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. TPmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips that does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable pre-collisional mechanism for burial and exhumation of continental crust.

Integration of natural data within a numerical model of ablative subduction: a possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence.

The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions

Abstract Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

Pressurized groundwater outflow experiments and numerical modeling for outflow channels on Mars

The landscape of Mars shows incised channels that often appear abruptly in the landscape, suggesting a groundwater source. However, groundwater outflow processes are unable to explain the reconstructed peak discharges of the largest outflow channels based on their morphology. Therefore, there is a disconnect between groundwater outflow processes and the resulting morphology. Using a combined approach with experiments and numerical modeling, we examine outflow processes that result from pressurized groundwater. We use a large sandbox flume, where we apply a range of groundwater pressures at the base of a layer of sediment. Our experiments show that different pressures result in distinct outflow processes and resulting morphologies. Low groundwater pressure results in seepage, forming a shallow surface lake and a channel when the lake overflows. At intermediate groundwater pressures, fissures form and groundwater flows out more rapidly. At even higher pressures, the groundwater initially collects in a subsurface reservoir that grows due to flexural deformation of the surface. When this reservoir collapses, a large volume of water is released to the surface. We numerically model the ability of these processes to produce floods on Mars and compare the results to discharge estimates based on previous morphological studies. We show that groundwater seepage and fissure outflow are insufficient to explain the formation of large outflow channels from a single event. Instead, formation of a flexure-induced subsurface reservoir and subsequent collapse generates large floods that can explain the observed morphologies of the largest outflow channels on Mars and their source areas.

Catastrophic ice lake collapse in Aram Chaos, Mars

Hesperian chaotic terrains have been recognized as the source of outflow channels formed by catastrophic outflows. Four main scenarios have been proposed for the formation of chaotic terrains that involve different amounts of water and single or multiple outflow events. Here, we test these scenarios with morphological and structural analyses of imagery and elevation data for Aram Chaos in conjunction with numerical modeling of the morphological evolution of the catastrophic carving of the outflow valley. The morphological and geological analyses of Aram Chaos suggest large-scale collapse and subsidence (1500 m) of the entire area, which is consistent with a massive expulsion of liquid water from the subsurface in one single event. The combined observations suggest a complex process starting with the outflow of water from two small channels, followed by continuous groundwater sapping and headward erosion and ending with a catastrophic lake rim collapse and carving of the Aram Valley, which is synchronous with the 2.5 Ga stage of the Ares Vallis formation. The water volume and formative time scale required to carve the Aram channels indicate that a single, rapid (maximum tens of days) and catastrophic (flood volume of 9.3 × 104 km3) event carved the outflow channel. We conclude that a sub-ice lake collapse model can best explain the features of the Aram Chaos Valley system as well as the time scale required for its formation.

Tectono-metamorphic map of the Mont Morion Permian metaintrusives (Mont Morion—Mont Collon—Matterhorn Complex, Dent Blanche Unit), Valpelline—Western Italian Alps

Abstract Please click here to download the map associated with this article. The presented map displays the structural and metamorphic evolution of lithotypes from pre-Permian to present. We distinguish pre-Permian rocks (e.g., amphibolite, biotite-bearing gneiss and acid granulite) preserved as roof pendants (i.e., xenoliths) within Permian intrusives. Permian intrusives and hosted xenoliths are then re-equilibrated during Alpine evolution, producing coronitic to mylonitic metaintrusives, due to meter to kilometer-scale fabric gradients, and associated white mica-, glaucophane-bearing gneiss. The map also shows the traces of the superimposed foliations and the fold axial planes. The traces are distinguished on the basis of their relative chronology and mineralogical support. This information, reported on a single map, allows us to reconstruct the successive stages of this fragment belonging to the African plate continental crust, from the pre-Alpine extension, recorded by granulite- to amphibolite-facies xenolits, to the Permian intrusive phase (e.g., Mont Morion, Mont Collon and Matterhorn intrusives) lasting with the Alpine subduction-collision related evolution. The Mont Morion, part of the Mont Morion-Mont Collon-Matterhorn Complex of the Dent Blanche unit, may be interpreted as a multi-stadial Alpine km-scale shear zone, where Permian intrusive rocks are transformed into white mica chlorite-bearing or glaucophane-bearing gneisses along high-strain horizons (100 m-thick), while within low-strain cores (100- to 1000 m-thick), meta-intrusives preserve igneous features and xenoliths of amphibolites, acid granulites and biotite-bearing gneisses. In this paper, an outcrop tectono-metamorphic map (1:10,000 scale) is presented, based upon fieldwork at 1:5,000 together with an interpretative map (1:15,000 scale), in which three dimensional relationships are described, and micro- to mesoscopic fabric types are shown, corresponding to finite strain states recorded by rocks.

Thermo-mechanical numerical model of the transition from continental rifting to oceanic spreading : the case study of the Alpine Tethys

We develop a two-dimensional thermo-mechanical numerical model in which the formation of oceanic crust and serpentinite due to the hydration of the uprising mantle peridotite has been implemented, with the aim of discussing the behaviour of the lithosphere of the Alps and Northern Apennines during the transition from continental rifting to ocean spreading of the Alpine Tethys. The predictions of the model are compared with natural data related to the Permian–Triassic high-temperature – low-pressure (HT-LP) metamorphism affecting the continental lithosphere and data from the Jurassic P–T evolution of the oceanic lithosphere from the Alps and the Northern Apennines. Our analysis indicates that a thinned continental crust, an ocean–continent transition zone and an oceanic lithosphere characterize the final structure of the system in a poor magma rift pre-Alpine configuration. We also find that mantle serpentinization starts before crustal break-up and that denudation occurs before ocean spreading. The mantle denudation starts several million years before the gabbros/basalt formation, generating an ocean–continent transition zone from the passive continental margin to the oceanic lithosphere of size 160–280 km. The comparative analysis shows that the extension of a hot and weak lithosphere, which promotes the development of hyperextended Alpine margins, better agrees with the natural data. Finally, our comparative analysis supports the hypothesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start in a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture.

Taking advantage of petrostructural heterogeneities in subduction-collisional orogens, and effect on the scale of analysis

Since the beginning of the last century, tectonic history of polyphase metamorphic tectonites of orogenic basement complexes is often related to primary links with metasediments, of presumably known origin, and location of their original basins. However such history is worth to be compared with results of an alternative, independent investigation that pursues: i) an objective reconstruction of the evolutionary steps modifying the lithostratigraphic setting and of its deformation-metamorphism interactions during plate-scale events, and ii) a privileged reconstruction of the rock memory for the structural and metamorphic correlation of crystalline basement units. Interpretative merging of data gathered from these affine rock properties made interpretations of orogenic zones more actualistic and based on recognition of tectonic trajectories of units through evolving geodynamic contexts. In this account a refinement of the analytical approach to inferring deformation and metamorphic paths and constructing geological histories of basements in axial zones of orogenic belts is presented and examples are synthesized from the Western Alps and the Canadian Cordillera, based on detailed structural and lithostratigraphic mapping in harmony with macro- and micro- structural techniques of analysis, are reported from the two belts.

Structural and metamorphic evolution during tectonic mixing: is the Rocca Canavese Thrust Sheet (Italian Western Alps) a subduction-related mélange?

In the Sesia-Lanzo Zone (SLZ), the subunit Rocca Canavese Thrust Sheet (RCT) is characterised by a mixture of mantle- and crust-derived lithologies and can be a good candidate to be a former subduction-related melange of the Austroalpine domain. The unit consists of metapelites, metagranitoids, metabasics, metagabbro and serpentinised lherzolite lenses from metre to hundred-metre size.According to the literature, PT peak conditions for all lithologies are 0.8-1 GPa at 300-400°C, in lawsonite-blueschist facies conditions recorded during the Alpine subduction. However recent work describes different mineral assemblages for the metamorphic peak, separating rocks with lawsonite from those with jadeite. Therefore, we refined the meso and microstructural analysis of the tectonic slices of RCT and we performed a detailed thermo-barometry of different metamorphicstages in order to quantify Alpine peak conditions and P-T-d-t paths, and to test whether the RCT represents a subduction-related melange. We focus on metagabbros, Jd-bearing and Lws-bearing glaucophanites since they have the most suitable chemistry allowing to reconstruct the complex evolution of the mixing. Metagabbros and Jd-bearing glaucophanites experienced a D1a metamorphic stage characterised by a pressure of 1.3-1.8 GPa and temperature of 450-550°C, in eclogite facies condition. Lws-bearing glaucophanites experienced a D1b metamorphic stage at a temperature

Morphological Expressions of Crater Infill Collapse: Model Simulations of Chaotic Terrains on Mars

Martian chaotic terrains are characterized by deeply depressed intensively fractured areas that contain a large number of low-strain tilted blocks. Stronger deformation (e.g. higher number of fractures) is generally observed in the rims when compared to the middle regions of the terrains. The distribution and number of fractures and tilted blocks are correlated with the size of the chaotic terrains. Smaller chaotic terrains are characterized by few fractures between undeformed blocks. Larger terrains show an elevated number of fractures uniformly distributed with single blocks. We investigate whether this surface morphology may be a consequence of the collapse of the infill of a crater. We perform numerical simulations with the Discrete Element Method and we evaluate the distribution of fractures within the crater and the influence of the crater size, infill thickness and collapsing depth on the final morphology. The comparison between model predictions and the morphology of the Martian chaotic terrains shows strong statistical similarities in terms of both number of fractures and correlation between fractures and crater diameters. No or very weak correlation is observed between fractures and the infill thickness or collapsing depth. The strong correspondence between model results and observations suggests that the collapse of an infill layer within a crater is a viable mechanism for the peculiar morphology of the martian chaotic terrains.