LAUSR

The Mormon Peak and Heart Mountain detachments are carbonate-­‐hosted low-­‐angle faults of Tertiary age in the western United States that formed in the uppermost continental crust. Both faults were active during regional explosive volcanism, with a magmatic center of approximately 30 km distant in the case of the Mormon Peak detachment, and directly within the area of exposure of the Heart Mountain detachment. We present results from 137 carbonate clumped-­‐isotope thermometric analyses within fault rocks related to the Mormon Peak and Heart Mountain detachments. We collected breccias, veins, gouges, and other fault rocks predominantly from within ~1 meter of the detachments. Our results suggest the breccias and gouges are mixtures of (1) host rock, (2) authigenic or vein material, and (3) in some cases includes material that is frictionally heated during faulting. The majority of fault rocks are depleted in δ18O and cold relative to the host rock, indicating the addition of material that precipitated from meteoric water under ambient conditions. However, a few samples preserve temperatures of over 250 °C, which based on textural and geochemical criteria are difficult to explain other than by frictional heating during slip. The primary contrast between the two faults is the greater variation and higher ambient temperatures associated with the Mormon Peak detachment (c. 25 to 165 °C), and evidence for circulation of meteoric fluids up the fault plane from depths of at least 4 km. Surprisingly, despite its much closer association with magmatism, ambient temperatures along the Heart Mountain detachment are general lower and show much less variation (40-­‐90 °C). In both