Crustal deformation and fluid flow at the brittle-ductile transition zone (BDTZ) are closely related and reciprocally dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We present the results of a multi-scale and multi-technique study that allowed us to reconstruct the temporal variations in fluid pressure, temperature, and bulk composition of the fluids that mediated deformation and steered strain localization within a strike-slip fault originally active at the BDTZ under overall ductile conditions. The studied fault zone is hosted in the Paleoproterozoic Svecofennian basement of SW Finland. The fault core is characterized by two synkinematic and laterally continuous quartz-chlorite veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Meso- and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation. Geochemical data from fault-minerals and the fluid inclusion study indicate that the two distinct quartz types precipitated from different fluid batches with a fluid bulk salinity in the 0-11 wt% NaCleq range. Quartz-chlorite and sphalerite-stannite geothermometry indicates that the temperature of the fluids involved in the deformation evolved through time from > 350 °C during Qtz I precipitation to < 300 °C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Initial, fluid-mediated embrittlement of the crystalline basement at the BDTZ generated a diffuse network of joints and/or hybrid/shear fractures in the damage zone; subsequent strain accommodation led to more localized deformation within the fault core.
Fluid-mediated, brittle-ductile cyclicity at seismogenic depth: Fluid record and deformation history of a fault system of the Svecofennian basement in SW Finland
Marchesini B.
;Garofalo P. S.;Viola G.
2019
Abstract
Crustal deformation and fluid flow at the brittle-ductile transition zone (BDTZ) are closely related and reciprocally dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We present the results of a multi-scale and multi-technique study that allowed us to reconstruct the temporal variations in fluid pressure, temperature, and bulk composition of the fluids that mediated deformation and steered strain localization within a strike-slip fault originally active at the BDTZ under overall ductile conditions. The studied fault zone is hosted in the Paleoproterozoic Svecofennian basement of SW Finland. The fault core is characterized by two synkinematic and laterally continuous quartz-chlorite veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Meso- and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation. Geochemical data from fault-minerals and the fluid inclusion study indicate that the two distinct quartz types precipitated from different fluid batches with a fluid bulk salinity in the 0-11 wt% NaCleq range. Quartz-chlorite and sphalerite-stannite geothermometry indicates that the temperature of the fluids involved in the deformation evolved through time from > 350 °C during Qtz I precipitation to < 300 °C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Initial, fluid-mediated embrittlement of the crystalline basement at the BDTZ generated a diffuse network of joints and/or hybrid/shear fractures in the damage zone; subsequent strain accommodation led to more localized deformation within the fault core.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
