Effects of permeability fields on fluid, heat, and oxygen isotope transport in extensional detachment systems

Abstract

[1] Field studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large-scale fault-zone architecture. We conduct continuum-scale (i.e., large-scale, partial bounce-back) lattice-Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields as well as buoyancy-driven flow on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between meteoric fluid and rock. Results show that fluid migration to middle to lower crustal levels is fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones, while lower ratios allow leakage of the fluids from the fault into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser extent, temperature distributions (disturbance of the geothermal field by~25 C). Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show d 18 O depletion concentrated along the fault and shear zones, similar to field data.

Fluid–Rock Interaction and Strain Localization in the Picacho Mountains Detachment Shear Zone, Arizona, USA

“…These results are similar to stable isotope data from the South Mountain (Smith et al ) that also suggest that the fluid present during ductile deformation had a metamorphic/magmatic composition. However, they are significantly different than other noted δ 2 H values found in other North American MMCs, where fluid composition has been estimated as −140 to −120‰ in the Thor Odin Dome, ~−115‰ in the Kettle Dome, −120‰ in the Ruby Mountains, and −113‰ in the Snake Range for example (see Gottardi et al, , and references therein). In the following, we propose a model that reconciles observed argon, stable isotope, and geothermometry results observed in the PMDSZ.…”

“…Fluid–rock interaction in a detachment system associated with the formation of a metamorphic core complex (modified after Gottardi et al, ; Wintsch & Yeh, ).…”

New parametric implementation of metamorphic reactions limited by water content, impact on exhumation along detachment faults

“…However, it cannot be excluded that this transition from the asymmetric mode to a symmetric spreading mode might not be real due to the abnormally strong near-surface thermal gradients predicted by the models that do not take into account near-surface cooling due to hydrothermal circulation. We suggest therefore that hydrothermal processes, so far neglected in our approach as well as in all of the previous numerical models, can lead to a convective cooling at the surface that would result in significant strengthening of the exhumed material when it arrives to the surface, which would cancel or attenuate the later symmetric phase (Famin et al, 2004;Gottardi et al, 2011;Gottardi et al, 2013).…”

“…Yet, several authors have shown that during active extension, surface-derived fluids can penetrate in the brittle upper crust through the arrays of normal faults that spread down to the detachment footwall (Famin et al, 2004;Gottardi et al, 2011;Gottardi et al, 2013;Morrison and Anderson, 1998;Mulch et al, 2006). Hence, the meteoric fluid down-flows can mix up with the metamorphic fluid circulation in the brittle-ductile mylonitic shear zones (Ferry, 1994;Fusseis and Handy, 2008;Manchtelow and Pennacchioni, 2004;Marquer and Burkhard, 1992;Oliot et al, 2014).…”

The fate of fluid inclusions during high‐temperature experimental deformation of olivine aggregates

“…This low‐viscosity phase is collected in the shears where it builds an interconnected fluid network that leads to fluid flow. Results from these olivine + water torsion experiments may also be exportable to other phases, such as quartz, in which aqueous fluid flow occurs during dislocation creep deformation, as documented extensively in extensional mylonite zones [e.g., Person et al ., ; Gottardi et al ., ; Gottardi and Teyssier , ].…”