Fluid flow compartmentalization during thrusting: A petrological and geochemical approach in the Southern Pyrenees.

Abstract

A well exposed thrust in the Southern Pyrenees was selected to characterize the transfer of fluids along and across a fault zone and to determine the thrust behavior as conduit or barrier to fluid migration. The studied thrust juxtaposes a Cenomanian-Turonian carbonate sequence in the hanging wall against a Coniacian carbonate succession in the footwall. Structural data, and petrological and geochemical analyses applied to calcite veins and host rocks, reveal different diagenetic histories between the footwall and the hanging wall. Such differences are interpreted to result from the low permeability of the foliated cataclasite. Within the footwall, three fracture sets (F1 to F3) and a chaotic breccia have been recognized. F1 to F3 fractures were cemented by three generations of calcite cement, Cc1 to Cc3, respectively, whereas the breccia was cemented by Cc1 and Cc2. Results suggest that during the development of fractures F1 to F3, the fluid regime changed from percolating meteoric waters, at temperatures around 50 ºC, to upward migrated formation fluids, at 110 ºC. Contrary to the footwall, the hanging wall was only affected by a crackle breccia cemented by calcite cement Cc4, which is the same cement recognized on the main thrust plane. Cc4 precipitated from formation waters migrating upward along the thrust plane, at 95 ºC. Therefore, during deformation, the studied thrust acted as a transversal seal and as a longitudinal path for fluids.