Modification of δ D values in eastern Nevada granitoid rocks spatially related to thrust faults
Stable isotope data have been determined for 13 Mesozoic and Tertiary plutons in eastern Nevada and nearby Utah. In the southern Snake Range of eastern Nevada, where relations are best exposed and have been most intensively studied, δ D, δ 18 O, and apparent K-Ar ages depend on proximity to the Snake Range decollement. Where stresses resulting from late movement on the decollement have caused cataclasis of Oligocene (37 Ma) granitoid rock, δ 18 O, δ D, and K-Ar age values as low as −2.5‰, −155‰, and 18 Ma, respectively, have been determined. Where there has been no cataclasis, δ 18 O values of Jurassic, Cretaceous, and Oligocene granitoid rocks are apparently unaffected, but both δ D values and K-Ar ages have been modified for distances of tens of meters below the decollement. Results similar to those in the southern Snake Range have been observed in other eastern Nevada granitoid rocks spatially related to regional thrust faults, as in the Kern Mountains, the Toana Range, and the northern Egan Range. In each of these areas cataclasis or deformation of granitoid rocks has resulted in lowered δ 18 O, δ D, and K-Ar age values. Where there has been no cataclasis or deformation, δ 18 O values are unaffected, but both δ D and K-Ar age values have been lowered by stresses resulting from postcrystallization movement along overlying thrust faults. Many of the plutons discussed have not been deeply eroded, and spatially related thrust faults crop out. Where thrust faults are not in evidence and the granitoid rocks give δ D values lower than about −130‰ along with spuriously low K-Ar age results, modification of the δ D and K-Ar age values may have been caused by stresses related to late movement along an overlying (now eroded) thrust fault.