LINKING TECTONIC EVOLUTION WITH FLUID HISTORY IN HYPER-EXTENDED RIFTED MARGINS: EXAMPLES FROM THE FOSSIL ALPINE AND PYRENEAN RIFT SYSTEMS, AND THE PRESENT-DAY IBERIA RIFTED MARGIN

Polyphase detachment faults are responsible for extreme crustal thinning and mantle exhumation. These structures are intimately linked with the evolution of distal parts of magma poor rifted margins. During the evolution of detachment faults, fluid-rock interaction plays an important role changing the chemical and physical properties of rocks, and consequently it has influence in the strain localization and structural evolution of the margin. Those changes are best indicated by hydration reactions in the continental and exhumed mantle domains, and by the pervasive cementation and precipitation of quartz and calcite along the fault zones.

Although the chemical and mineral reactions are well known, it is still unclear to what extent these reactions lead to changes in the overall rheology of the extending lithosphere and how they can affect the thermal evolution of the hyper-extended rifted margins. In order to answer to these questions it is important to understand the origin, timing, pathways and composition of the fluids generated during rifting. These questions can be addressed in well-known hyper-extended rift systems of the Alpine Tethys margins exposed in the Alps, the Mauléon basin in the Western Pyrenees and the Deep Iberia margin drilled and seismically imaged offshore Portugal. All of these rift settings show evidence for detachment systems associated with hyper-extension and mantle exhumation.

The aim of this ongoing study is to characterize the fluid signature in hyper-extended domains in magma-poor rifted margins. The study of different sites that experienced different degrees of compressional and metamorphic overprint enables us to compare results and to define the general importance of fluid systems in the development of hyper-extended rift systems.

The first results show that in all three geological settings fluid percolation can be recognized in fault rocks linked to the detachment systems. Evidence for the presence of fluids comes from the analyses of hydration reactions in fault zones. In the Alps the major and trace elements show a gain in elements typical from mantle rocks. In the Pyrenees, microstructural studies show that detachment faulting crossed a range of crustal depths providing constraints on the depths of fluid migration.