Testing the detachment fault refrigeration hypothesis at high-spatial resolution by SIMS oxygen isotope analysis.

Claudia I. Roig

Abstract: Metamorphic Core Complexes (MCC’s) are the focus of large-scale tectonics research in the Basin and Range Western US, as well as a natural laboratory for understanding the effects of crustal extension, and the role of fluid dynamics in places with complex thermal histories. Extensional detachment systems, like the Oligocene-Miocene Whipple detachment fault (WDF), are important orogen-scale structures for fault-controlled hydrothermal activity and fluid-assisted deformation processes that occur within an actively exhuming detachment system. Oxygen isotopes are useful in preserving records of variation in time-temperature-fluid conditions and thus present an avenue to study detachment system dynamics. This study aims to assess the extent of rock-fluid interaction at the grain-scale, and how it influenced cooling and deformation along the WDF. The grain-scale oxygen isotope record will be targeted using Secondary Ion Mass spectrometry (SIMS) oxygen isotope analysis to measure δ¹⁸O in quartz and epidote mineral pairs, since these are a good δ¹⁸O thermometery pair and are affected by fluids. Quartzofeldspathic mylonites were collected along a 50-m transect parallel to the WDF transport direction to assess the spatial relation of temperature gradients and fluids to the main detachment surface. Preliminary backscattered electron imaging of epidote porphyroclasts with REE-rich cores in the mylonites display textural and chemical features consistent with modification by syn-deformational hydrothermal reactions. Intragrain oxygen isotope data will be compared to electron microprobe data to track fluid chemistry and assess the roles of deep crustal vs. shallow crustal/meteoric fluids on WDF deformation. Future work will include the use of oxygen isotope geospeedometry to model intragrain (core-to-rim) δ¹⁸O variations in mineral grains that have shown potential to record thermal diffusion zoning during cooling across a range of temperatures.

Time: April 21st (10:30)

Advisor: Chloe Bonamici

Co-Authors: Chloe Bonamici

Stream: Zoom

Email: roig@wisc.edu