Strain and permeability gradients traced by stable isotope exchange in the Raft River detachment shear zone, Utah

Abstract

Combined geochronological and stable isotope data of quartzite mylonite from the footwall of the Raft River detachment shear zone (NW Utah, USA) reveal that an important phase of ductile deformation and infiltration of meteoric water in the shear zone occurred in Miocene time. 40 Ar/ 39 Ar release spectra are complex, and plateau ages decrease systematically from 31.1 ± 0.8 Ma at the top to 20.2 ± 0.6 Ma at the bottom of the quartzite mylonite section, capturing a segment of the ~40-15 Ma geochronologic record that has been documented regionally and is likely related to partial to total overprinting of Eocene white mica 40 Ar/ 39 Ar ages in the Miocene. Hydrogen stable isotope values of syn-kinematic muscovite range from-123 ‰ to-88 ‰ and suggest that meteoric water infiltrated the detachment shear zone during mica (re)crystallization and mylonite development. Bulk stable isotope analyses from fluid inclusions in quartz support a meteoric origin for the fluid (low D/H and 18 O/ 16 O ratios). Quartz and muscovite oxygen isotope analyses show varying degrees of 18 O depletion, suggesting spatially variable time-integrated interaction of meteoric fluids with recrystallizing shear zone minerals. The overall pattern of D/H and 18 O/ 16 O ratios indicates that fluids were channelized along restricted layers or shear zones within the deforming detachment system. The variability in 18 O/ 16 O ratios of both quartz and muscovite and the fluid-rock isotopic exchange results can be explained by variations in the shear zone permeability (confined versus diffuse flow) along with strain variations along the transport direction (from flattening to constriction).

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

“…For the three minerals analyzed, oxygen isotope values fall within a narrow 2‰ range, with 10.2‰ < δ 18 O Qtz < 11.9‰, 4.1‰ < δ 18 O Bt < 5.7‰, and 5.3‰ < δ 18 O Hbl < 7.0‰, similar to values reported by Kerrich and Rehrig (). Such values are somewhat depleted compared to typical metamorphic rock compositions (Taylor & Sheppard, ), and are similar to δ 18 O values reported for other MCCs (Holk & Taylor, ; Holk et al, ; Mulch et al, , ; Gébelin et al, , ; Gottardi et al, , ; Quilichini et al, , ). Using the calibrations by Bottinga and Javoy () and the temperatures estimated by geothermometry (Table ), we estimate that the δ 18 O value of the fluid present during fluid–rock exchange ranges from 7.3 to 9.4‰.…”

“…We propose that the progressive incisement of the DSZ is caused by progressive migration of the deformation front toward lower structural levels, a concept proposed in other detachment shear zones (Thor‐Odin Dome (Mulch et al, ), Raft River Mountains (Gottardi et al, )), but particularly well preserved in the PMDSZ. The strength of the lithosphere is generally modeled by a ductile power laws for the lower crust, and is a function of differential stress, strain rate, temperature, grain size, fluid activity, and deformation mechanisms of the rheologically significant mineral species (Brace & Kohlstedt, ; Kusznir & Park, ).…”

Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah)

“…Oxygen isotope exchange thermometry at Clear Creek Canyon [ Gottardi et al ., ] indicates a steep geothermal gradient with temperatures increasing from 345 ± 25°C at the top to 485 ± 20°C near the base of the 100 m thick RRDSZ and a diverse pattern of fluid flow and fluid‐rock interaction that responds to changes from flattening to constrictional strain along the shear zone [ Gottardi et al ., ].…”

Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

“…Hbl+Scp) in the shear zone as compared to the undeformed host rock (Figure 10b), and even in the ultramylonite as compared to the mylonite, indicates a positive feedback between increasing strain and increasing fluid flow. This could have occurred due to a strain-induced rise in permeability (Fusseis, Regenauer-Lieb, Liu, Hough, & De Carlo, 2009;Gottardi et al, 2015).…”