At the brittle-ductile transition, repeating cycles of frictional and viscous deformation can be controlled by multiple fluid pulses channelled in the fault zones. Faults exhumed from the brittleductile transition zone and exposed in the Olkiluoto high-grade nuclear waste repository were studied combining field and microstructural observations with fluid inclusions and mineral pair geothermometry on synkinematic minerals. Faulting initiated as a consequence of a first event of fluid overpressure (Pf > 210 MPa) with the formation of a diffuse network of joints and hybrid–shear fractures. Cyclical brittle and ductile shearing followed, triggered by repeated hydrofracturing induced by a fluid pressure up to 210 MPa under overall ductile environmental conditions, demonstrated by mutually overprinting veining, crystal-plastic deformation and cataclasis. Hydrofracturing and their association with pseudotachylite-bearing faults suggest that fluid-mediated deformation may represent the record of the seismic cycle in the studied fault system. Fluid pulses also enhanced the latest reactivation of the fault system under fully brittle conditions at lower pressure (140-180 MPa) and temperature (≤ 300° C). LA-ICP and EBSD analyses on authigenic pyrite also indicate that fluid flow enhanced the net transport of a range of heavy elements through the fault zone (e.g. Co, Cu, Sn, Ag, Pb).
The role of fluids on strain localization at seismogenic depth: a case study from brittle-ductile faults from Olkiluoto Island, SW Finland
Marchesini B.
;Viola G.;
2020
Abstract
At the brittle-ductile transition, repeating cycles of frictional and viscous deformation can be controlled by multiple fluid pulses channelled in the fault zones. Faults exhumed from the brittleductile transition zone and exposed in the Olkiluoto high-grade nuclear waste repository were studied combining field and microstructural observations with fluid inclusions and mineral pair geothermometry on synkinematic minerals. Faulting initiated as a consequence of a first event of fluid overpressure (Pf > 210 MPa) with the formation of a diffuse network of joints and hybrid–shear fractures. Cyclical brittle and ductile shearing followed, triggered by repeated hydrofracturing induced by a fluid pressure up to 210 MPa under overall ductile environmental conditions, demonstrated by mutually overprinting veining, crystal-plastic deformation and cataclasis. Hydrofracturing and their association with pseudotachylite-bearing faults suggest that fluid-mediated deformation may represent the record of the seismic cycle in the studied fault system. Fluid pulses also enhanced the latest reactivation of the fault system under fully brittle conditions at lower pressure (140-180 MPa) and temperature (≤ 300° C). LA-ICP and EBSD analyses on authigenic pyrite also indicate that fluid flow enhanced the net transport of a range of heavy elements through the fault zone (e.g. Co, Cu, Sn, Ag, Pb).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
