Comportement non-standard des prismes d'accrétion tectonique (Implications structurales)

Les marges actives et leurs prismes d'accrétion montrent une grande diversité de géométries et de types de déformation interne. Des paramètres, tels que le taux de convergence et la sédimentation dans la fosse, sont les principaux facteurs déterminant s'il y a ou non accrétion. Les mécanismes de déformation interne sont encore peu connus, ainsi que la manière dont ils sont affectés par des facteurs de perturbation tels que la présence d'hétérogénéités internes, la sédimentation dans le prisme lui même ou le chevauchement des ride océaniques. Le but principal de ce travail est donc de mieux appréhender comment les facteurs de déformation interne des prismes d'accrétion sont affectés par des facteurs perturbants. Un comportement standard a été défini afin de comparer le comportement de prismes d'accrétion avec un modèle au fonctionnement idéal. Il a été montré que la résistance des couches internes ductiles par rapport à celle des couches fragiles, contrôle le style structurel à l'intérieur de prisme, particulièrement au-dessus des couches ductiles. Au contraire, le profil général du prisme n'est que peu affecté par ce paramètre. Le dépôt d'une plate-forme progradante au-dessus des écailles imbriquées du prisme produit l'arrêt de leur activité et le déplacement du front de déformation. La présence d'un niveau de décollement interne induit le découplement de la plate-forme par rapport aux structures compressives sous-jacentes. Ce phénomène produit à son tour le développement des failles normales sur la plate-forme et une augmentation du raccourcissement au-dessus du détachement.Active margins and their accretionary prisms show an important diversity of geometries and internal deformation patterns. Parameters like the convergence rate and sedimentation at the trench are the primary controls dictating whether there is accretion or not. Little is known about the internal deformation mechanisms and how are they affected by perturbation factors such as internal heterogeneities, sedimentation and oceanic seamounts. The main goal of this study is to understand how the internal deformation mechanisms on accretionary prisms are affected by these perturbation factors. A standard behaviour has been defined in order to compare the behaviour of an accretionary prisms with an ideal model of functioning. It has been shown that internal ductile layers and their relative strength with respect to the fragile layers, control the structural style inside the accretionary prism, particularly above the ductile layer. On the contrary the overall profile of the prism is little or no affected by this parameter. The deposition of a prograding platform above the imbricated thrust sheets of the prism induces the arrest of their activity and the translation of the deformation front. The presence of an internal detachment level enhance the decoupling between the platform and the underlying compressional structures. This induces in turn the development of normal faulting on the platform and the increase of the shortening above the detachment.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

From neogene thin-skin to recent thick-skin deformation in Haiti fold-and-thrust belt (Western Hispaniola)

International audienceHaiti lies along the North Caribbean plate boundary, marked by two major left-lateral strike-slip faults. A southwest-verging Trans-Haitian fold-and-thrust belt (FTB), mostly affecting tertiary carbonate platform, developed in central Haiti between the two faults, south of the Massif du Nord and the San Juan–Los Pozos Fault Zone, as a result of north-south shortening. The large NW-SE antiformal Gonâve Island and Ridge mark the southwest termination of the FTB as it encountered the Southern Peninsula and Enriquillo-Plantain Garden Fault Zone. We show that this collision resulted in a tightening of the deformed wedge and thrust faults rooted deeper into the crust, leading to a transition from thin-skinned to thick-skinned tectonics in the Pliocene. These short- (3–6 km) and long- (15–30 km) wavelength folding deformations are temporally distinct in the north and more concomitant in the south, where compressional thin-skinned deformation is still active. The thick-skinned shortening was mostly expressed during the Quaternary, as shown by the formation of folds and uplift of terraces between 1.7 Ma and today. This thick-skinned detachment is characterised by a stronger imprint of strike-slip component, dragging the FTB to a more E-W direction in its northwest and southeast extremities. Understanding of the fault zone geometry of the recent 2010 Haiti earthquake (M = 7.2) could be reconsidered in the light of this tectonic transition

How transpressive is the northern Caribbean plate boundary?

International audienceTranspressive deformation at the northern Caribbean plate boundary is accommodated mostly by two major strikeslipfaults, but the amount and location of accommodation of the compressional component of deformation is still debated.We collected marine geophysical data including multi-beam bathymetry and multichannel seismic reflection profiles alongthis plate boundary around Hispaniola, in the Jamaica Passage and in the Gulf of Gonâve. The data set allows us to imagethe offshore active strike-slip faults as well as the compressional structures. We confirm that the Enriquillo-Plantain-GardenFault Zone (EPGFZ) in the Jamaica Passage has a primary strike-slip motion, as indicated by active left-lateral strike-sliprelatedstructures, i.e.: restraining bend, asymmetrical basin, en echelon pressures ridges and horsetail splay. Based ontopographic cross-sections across the EPGFZ, we image a very limited compressional component, if any, for at least thewestern part of the Jamaica Passage. Toward the east of the Jamaica Passage, the fault trace becomes more complex and weidentify adjacent compressional structures. In the Gulf of Gonâve, distributed folding and thrust faulting of the most recentsediments indicate active pervasive compressional tectonics. Estimates of shortening in the Jamaica Passage and in the Gulfof Gonâve indicate an increase of the compressional component of deformation towards the east, which nonetheless remainsvery small compared to that inferred from block modelling based on GPS measurement

The northern Caribbean plate boundary in the Jamaica Passage: structure and seismic stratigraphy

International audienceMultibeam bathymetry data and multichannel seismic reflection profiles have been collected at the end of 2012 along the Enriquillo-Plantain-Garden Fault Zone (EPGFZ) in the Jamaica Passage, between Jamaica and Hispaniola. Analysis of the data set reveals the tectonic evolution and the stratigraphic complexity of the northern Caribbean boundary. Stratigraphic correlations with previous marine and on land studies are proposed to place the identified seismic sequences in their regional tectonic history. Two distinct crustal domains are interpreted. Typical stratigraphic sequences for the rifted blocks of the Eastern Cayman Trough margin are identified in five basins of the Jamaica Passage, highlighting the eastward limit of the Cayman Trough margin. These inherited basins are deformed and folded during a first phase of compression that could correspond to the regional tectonic rearrangement recorded in the early Miocene (about 20 Ma). A distinct crustal domain that we propose to relate to the Carib Beds (Caribbean typical reflectors A′′,B′′ and V) is identified in the southern part of the Jamaica Passage, indicating that the Caribbean Large Igneous Province could extend up to the extreme northeast part of theLower Nicaragua Rise. The left-lateral EPGFZ currently cuts acrosstwo pre-existing basins, the Morantand Matley basins. During the activity of the EPGFZ, these basins are deformed and folded indicating a second phase of compression. In contrast, the Navassa basin, located in the middle of the Jamaica Passage, results from the strike-slip motion of the EPGFZ and is interpreted as an asymmetricalbasin bordered by the EPGFZonly on its northern sid

Evolution of fault permeability during episodic fluid circulation : evidence for the effects of fluid–rock interactions from travertine studies (Utah–USA).

International audienceFaults are known to be important pathways for fluid circulation within the crust. The transfer properties along faults can evolve over time and space. The Little Grand Wash and Salt Wash normal faults, located on the Colorado Plateau, are well known examples of natural CO2 leakage systems from depth to surface. Previous studies dated and established a chronology of CO2-enriched fluid source migration along the fault traces and linked the aragonite veins observed close to Crystal Geyser to CO2-pulses. However, multiple circulation events recorded along a given fault segment deserve to be studied in minute detail in order to unravel the chronology of these events, precipitation processes and associated mechanisms. A combination of structural geology, petrography, U/Th dating, oxygen and carbon isotope analysis were used to study the fault related CO2-enriched paleo-circulations in order to build a conceptual model of CO2-circulation along the faults. This study resulted in the precise descriptions of the features attesting CO2-enriched fluid circulation by a characterization of their relationship and architecture at the outcrop scale. These features are witnesses of a large range of circulation/sealing mechanisms, as well as changes in fluid chemistry and thermodynamic state of the system, providing evidence for (i) the evolution of the fluid through a pathway from depth to the surface and (ii) different cycles of fault opening and sealing. Large circulation events linked with fault opening/sealing are observed and calibrated in nature with millennial circulation and sealing time-lapses. Numerical modelling indicates that such sealing timescale can be explained by the introduction of a fault sealing factor that allows modifying permeability with time and that is calibrated by the natural observations