A transient fault-valve mechanism operating in upper crustal level,Sierras Pampeanas, Argentina

MARÍA SILVIA JAPAS; NILDA URBINA; PATRICIA SRUOGA; JOSÉ MATÍAS GARRO; OSCAR IBAÑES

JOURNAL OF GEODYNAMICS

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2016 vol. 101 p. 142 - 154

0264-3707

tLocated in the Sierras Pampeanas (the broken-foreland of the Pampean flat slab segment in the southernCentral Andes), the Cerro Tiporco volcanic field shows Neogene hydrothermal activity linked to migrationof arc-magmatism into the foreland. Late Neogene deposits comprise epithermal vein systems emplacedin Precambrian?Early Palaeozoic igneous-metamorphic basement, Late Miocene sedimentary rocks andEarly Pliocene volcaniclastic rocks. Mineralization consists of calcareous onyx, aragonite and calciteveins as well as travertine deposits. Onyx and aragonite occur as fill of low-displacement nearly ver-tical reverse-sinistral faults striking NW, and nearly horizontal dilatant fractures. The latter consist ofload-removal induced fractures affecting the igneous-metamorphic rocks, as well as bedding planes inthe Late Miocene sediments. The presence of veins recording multiple fracture episodes and crack-and-seal growth of veins suggests relatively low differential stress and supralithostatic fluid pressure, aswell as cyclic changes in pore pressure and high mineral-deposition/fracture-opening ratio. These con-ditions support a mechanism of fault-valve behaviour during onyx and aragonite vein emplacement. Thefault-valve mechanism involves fractures associated with impermeable barriers between environmentswith different fluid pressure. Faulting generated an appreciable directional permeability triggering fluidmigration from the highest to the lowest pressure region, with subsequent deposition and sealing thatstarted a new pressurization-faulting-sealing cycle. Late aragonite and calcite veins suggest a change inkinematics indicating the onset of tectonic-load conditions.