Istituto di Scienze Marine     
Loreto M. F., Tinivella U., Ranero C. R. Evidence for fluid circulation, overpressure and tectonic style along the Southern Chilean margin. In: Tectonophysics, vol. 429 (40271) pp. 183 - 200. Elsevier Science Bv, 2007.
The southern Chilean convergent margin, between 50 degrees and 57 degrees S, is shaped by the interaction of the three main plates: Antarctic, South America and Scotia. North of 53 degrees S, the convergence between Antarctic and South America plates is close to orthogonal to the continental margin strike. Here, the deformational style of the accretionary prism is mainly characterized by seaward-verging thrusts and locally by normal faults and fractures, a very limited lateral extension of prism, a very shallow dip (similar to 6 degrees) decollement, and subduction of a thick and relatively undeformed trench sedimentary sequence. South of 53 degrees S, convergence is oblique to the margin, locally, the trench sediments are proto-deformed by double vergence thrusting and the front of the prism grows through landward-verging thrusting. The decollement is sub-horizontal and deep, involving most of the sediment over the oceanic crust in the accretionary process, building a comparatively wide and thicker prism. A Bottom Simulating Reflector is present across the whole prism to the abyssal plan, suggesting the presence of gas in the sediments. The analysis of P- and S-wave velocity reflectivity sections, derived by amplitude versus offset technique (AVO), detailed velocity information and the velocity-derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre-stack depth migrated seismic lines, aiming to explain the relation between fluid circulation and tectonics. Accretion along double vergence thrust faults may be associated with the presence of overpressured fluid, which decreases the effective shear stress coefficient along the main decollement and within the sediments, and modify the rheolgical properties of rocks. The presence of an adequate drainage network, represented by interconnected faults and fractures affecting the entire sedimentary sequence, can favour the escape of pore fluid toward the sea bottom, while, less permeable and not faulted sediments can favour fluid accumulations. Gravitational and tectonic dewatering, and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench. The results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation. (c) 2006 Elsevier B.V. All rights reserved.
DOI: 10.1016/j.tecto.2006.09.016
Subject subduction

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