CRUSTAL FLUID FLOW ON A GRAND SCALE: THE ORIGIN OF THRUST FAULT FLUIDS, NORTHERN UTAH

The motion of thrust sheets is thought to be facilitated by high pore-fluid pressure along the decollement. Syntectonic fluids acquire a chemical and isotopic signature indicative of the rocks through which they pass. These chemical signatures should be preserved in minerals precipitated within the deformation zone as fault-induced fractures are filled and recrystallized. Therefore, by analyzing the isotopic and chemical composition of vein minerals in thrust faults, it should be possible to identify the origin and pathways of thrust fault fluids.

The Willard and Ogden thrust faults are located in northern Utah. These stacked thrust sheets formed during the Sevier orogeny and display brittle deformation in the form of breccia and cataclasite. Undeformed host, breccia clasts, and syntectonic vein samples along both thrust zones were analyzed for their Sr, O, C, Nd, and Pb isotopic compositions, as well as for rare earth element concentrations.

Analyses of the undeformed host and breccia clasts yield Sr isotope ratios ranging from Cambrian seawater to slightly radiogenic values (0.70957 to 0.71492), and oxygen isotope ratios ranging from -4.4 to -10.3‰ (PDB). Analyses of carbonate veins reveal radiogenic Sr ratios between 0.71871 and 0.72607, and oxygen ratios between -8.5 and -28.9‰ (PDB). The oxygen isotopic composition of the veins in the Ogden thrust are about 10‰ lighter than the overlying Willard thrust. REE concentrations in the basement rocks are considerably higher than in Paleozoic sediments and Precambrian metasediments of the Willard thrust sheet, and can therefore be used as a tracer for thrust fault fluid flow.

Based on the radiogenic Sr isotope ratios in the veins, it appears that fluids were forced from the underlying crystalline basement into the thrusts as deformation occurred. The light oxygen isotopic compositions of the veins indicate elevated temperatures and help to constrain the deformation temperatures. The extremely light delta ¹⁸O values in veins of the Ogden thrust indicate a different path for fluid flow or higher deformation temperatures in the Ogden fault zone than in the Willard fault zone.