THE ROLE OF FRICTIONAL STRENGTH

[1] At a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc. In this paper, we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins. In numerical models, we vary thickness of a serpentinized layer in the mantle wedge (15 to 25 km) and the frictional strength of both the sediments and serpentinized mantle (friction angle 1° to 15°, or static friction coefficient 0.017 to 0.27) to control the amount of frictional coupling between the plates. With plastic strain weakening in the lithosphere, our numerical models can attain stable subduction geometry over millions of years. We find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths. In the case of low sediment and serpentinite friction, the subduction interface has a shallow dip, while the subduction zone develops an accretionary prism, a broad forearc high, a deep forearc basin, and a shallow trench. In the high friction case, the subduction interface is steep, the trench is deeper, and the accretionary prism, forearc high and basin are all absent. The resultant free-air gravity and topographic signature of these subduction zone models are consistent with observations. We believe that the low-friction model produces a geometry and forearc structure similar to that of accretionary margins. Conversely, models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin. We find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

Subduction dynamics as revealed by trench migration

International audienceNew estimates of trench migration rates allow us to address the dynamics of trench migration and back-arc strain. We show that trench migration is primarily controlled by the subducting plate velocity V-sub, which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: V-t = 0.5V(sub) - 2.3 (in cm a(-1)) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending

Segmentation-based approaches for characterising plant architecture and assessing its plasticity at different scales

International audiencePlants are modular organisms that develop by the repetition of elementary botanical entities or constructional units through the three main and fundamental morphogenetic processes of growth, branching and reiteration (Barthélémy and Caraglio, 2007). Repetition of these entities induces gradual or abrupt changes in their characteristics. These characteristics are quantified through several variables, called the entity attributes. On the one hand, differences between entities reflect different stages of differentiation in the meristems (Nicolini and Chanson, 1999), which are ordered in time and correspond to the notion of physiological age of meristems (Barthélémy and Caraglio, 2007). The changes of one attribute through the plant structure are referred to as morphogenetic gradient. On the other hand, part of the entity differences can be imputed to environmental factors. Based on this botanical model of plant functioning, our approach jointly relies on categories of entities with similar characteristics, a description of those within-category characteristics and the category topological organisation within the plant. This constitutes a useful summary of the plant architecture, which is the basis for 1) highlighting hidden regularities in plant structures, for a better understanding of the gradients and organisation rules; 2) proposing and validating ecophysiological hypotheses, and new sampling protocols; 3) adopting a powerful paradigm for modelling plant development. In this work, the entity categories are identified using a hidden Markov tree (HMT) model, in which local dependencies only ( i.e. interactions between connected entities) are accounted for. This is not sufficient to accurately describe the topological distribution of the entities within the plant, and the changes of the plant topology. This is why various complementary methods and models (among which edit distance algorithms and sequence analysis using Markovian models), performed at different scales, are used in our approach to provide a more detailed description of the architecture, and to assess how various controlled factors affect architecture plasticity

Exploring morphogenetical gradient variability using hidden Markov tree models in young individuals of the tropical species Symphonia globulifera (Clusiaceae)

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Analysis of Cecropia sciadophylla Morphogenesis Based on a Sink-Source Dynamic Model

International audienceAlthough there is an increasing number of models simulating the functional and structural development of trees at organ scale, few of them can be fully calibrated, evaluated and validated. A major obstacle resides in the intrinsic complexity of trees due to their high stature, large number of organs and long life span that limits the possibilities of experimental work and the access to measurement data. This is why 'model plants' such as the neotropical genus Cecropia are of great interest. This genus has a simple architecture and some qualities that allow collecting exhaustive datasets at the organ scale. In this paper, we evaluate the GreenLab model on data recorded on 11 individuals measured in 2007 in French Guiana. The branching and flowering patterns are analyzed using an index of trophic competition

Analysing the effects of local environment on the source-sink balance of Cecropia sciadophylla: a methodological approach based on model inversion

International audienceContext : Functional - structural models (FSM) of tree growth have great potential in forestry, but their development, calibration and validation are hampered by the difficulty of collecting experimental data at organ scale for adult trees. Due to their simple architecture and morphological properties, " model plants " such as Cecropia sciadophylla are of great interest to validate new models and methodologies, since exhaustive descriptions of their plant structure and mass partitioning can be gathered. * Aims : Our objective was to develop a model-based approach to analysing the influence of environmental conditions on the dynamics of trophic competition within C. sciadophylla trees. * Methods : We defined an integrated environmental factor that includes meteorological medium-frequency variations and a relative index representing the local site conditions for each plant. This index is estimated based on model inversion of the GreenLab FSM using data from 11 trees for model calibration and 7 trees for model evaluation. * Results : The resulting model explained the dynamics of biomass allocation to different organs during the plant growth, according to the environmental pressure they experienced. * Perspectives : By linking the integrated environmental factor to a competition index, an extension of the model to the population level could be considered

Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data

Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as ‘opportunistic’ since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts

Oscillations in Functional Structural Plant Growth Models

International audienceThe dynamic model of plant growth GreenLab describes plant architecture and functional growth at the level of individual organs. Structural development is controlled by formal grammars and empirical equations compute the amount of biomass produced by the plant, and its partitioning among the growing organs, such as leaves, stems and fruits. The number of organs initiated at each time step depends on the trophic state of the plant, which is evaluated by the ratio of biomass available in plant to the demand of all the organs. The control of the plant organogenesis by this variable induces oscillations in the simulated plant behaviour. The mathematical framework of the GreenLab model allows to compute the conditions for the generation of oscillations and the value of the period according to the set of parameters. Two case-studies are presented, corresponding to emergence of oscillations associated to fructification and branching. Similar alternating patterns are commonly reported by botanists. In this article, two examples were selected: alternate patterns of fruits in cucumber plants and alternate appearances of branches in Cecropia trees. The model was calibrated from experimental data collected on these plants. It shows that a simple feedback hypothesis of trophic control on plant structure allows the emergence of cyclic patterns corresponding to the observed ones