CLUMPED-ISOTOPE THERMOMETRY ALONG THE MORMON PEAK DETACHMENT

We present stable and clumped-isotope data from breccias and gouges along the Mormon Peak detachment, a low-angle normal fault in southeastern Nevada developed within Paleozoic carbonate rocks. These results expand on previous work, (1) to better characterize the deformation of the footwall, and (2) to further explore the presence of anomalously hot samples near the slip surface. For variably cataclasized footwall samples within 10 cm of the fault, we observe a spatial trend in temperature and isotopic ratios. Samples of host rock and breccia clasts consistently record temperatures and isotopic ratios comparable to the unaltered host rock, which are 100 to 130°C, δ¹³C of -2 to 2‰ (VPDB), and calculated pore fluid δ¹⁸O of 2 to 7‰ (VSMOW). These are similar to values expected for burial diagenesis in a Paleozoic shelf environment. Closer to the detachment surface, these values all decrease, with temperatures of crystallization along the fault locally below 20°C, δ¹³C of -5 to -6‰, and calculated pore fluid δ¹⁸O of -11 to -8‰. These values suggest precipitation from soil waters, indicating thorough near-surface fluid infiltration along the fault system late in its history. The trend probably reflects mixing of host rock and new material that precipitated in the more permeable fault zone as deformation occurred. Contrary to the cooling trend, several samples record precipitation temperatures much higher than that of the host rock, including a sample of cataclasite within the footwall c. 10 cm below the detachment. This sample has an oxygen and carbon isotopic composition similar to that of the host rock, as does a band of unaltered host rock within it. The fine-grained breccia matrix temperatures are, however, about 60°C warmer (c. 190°C). Because of the lack of change in δ¹⁸O between clasts and matrix, we interpret this sample to have received its “excess heat” not from hydrothermal fluids (as is the case for many coarsely crystalline void fill samples located above the detachment, which are derived from meteoric waters), but from local frictional heating, and possibly calcining reactions. A marked change in temperature without any change in oxygen and carbon isotopic ratios is also observed in a gouge sample from the detachment surface, showing a range of temperatures from 40°C to 210°C across 1 cm of gouge.