Fault–valve action and vein development during strike–slip faulting: An example from the Ribeira Shear Zone, Southeastern Brazil

Abstract

Fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of Neoproterozoic–Cambrian age in the Ribeira River Valley area, southeastern Brazil. Geometric and microstructural constraints indicate that foliation–parallel and extensional veins were formed during dextral strike–slip faulting. Both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates, and were crystallized in the presence of CO 2–CH 4 and H 2O–CO 2–CH 4–NaCl immiscible fluids following unmixing from a homogeneous parental fluid. Contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels. Foliation–parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values (319–397 °C and 47–215 MPa), which is compatible with predicted fluid pressure cycle curves derived from fault–valve action. Growth of extensional veins occurred in shallower structural levels, under pressure fluctuating from near hydrostatic to moderately subhydrostatic values (207–218 °C and 18–74 MPa), which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone. Fluid immiscibility and precipitation of quartz in foliation–parallel veins resulted from fluid pressure drop immediately after earthquake rupture. Fluid immiscibility following a local pressure drop during extensional veining occurred in pre-seismic stages in response to the development of fracture porosity in the dilatant zone. Late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity (0.2 to 44 wt.% equivalent NaCl) H 2O–NaCl–CaCl 2 fluid inclusions trapped in healed fractures mainly in foliation–parallel veins, which also exhibit subordinate H 2O–NaCl–CaCl 2, CO 2–(CH 4) and H 2O–CO 2–(CH 4)–NaCl fluid inclusions trapped under subsolvus conditions in single healed microcracks. Recurrent circulation of aqueous–carbonic fluids and aqueous fluids of highly contrasting salinities during veining and post-veining stages suggests that fluids of different reservoirs were pumped to the ruptured fault zone during faulting episodes. A fluid evolution trending toward CH 4 depletion for CO 2–CH 4–bearing fluids and salinity depletion and dilution (approximation of the system H 2O–NaCl) for aqueous–saline fluids occurred concomitantly with decrease in temperature and pressure related to fluid entrapment in progressively shallower structural levels reflecting the shear zone exhumation history.