Extensional detachment systems separate hot footwalls from cool hanging
walls, but the degree to which this thermal gradient is the product of
ductile or brittle deformation or a preserved original transient
geotherm is unclear. Oxygen isotope thermometry using recrystallized
quartz-muscovite pairs indicates a smooth thermal gradient (140 degrees
C/100 m) across the gently dipping, quartzite-dominated detachment zone
that bounds the Raft River core complex in northwest Utah (United
States). Hydrogen isotope values of muscovite (delta D-Ms similar to-100
parts per thousand) and fluid inclusions in quartz (delta D-Fluid
similar to-85 parts per thousand) indicate the presence of meteoric
fluids during detachment dynamics. Recrystallized grain-shape fabrics
and quartz c-axis fabric patterns reveal a large component of coaxial
strain (pure shear), consistent with thinning of the detachment section.
Therefore, the high thermal gradient preserved in the Raft River
detachment reflects the transient geotherm that developed owing to
shearing, thinning, and the potentially prominent role of convective
flow of surface fluids.