40Ar/39Ar Thermochronology of Sedimentary Basins Using Detrital Feldspars: Examples from the San Joaquin Valley, California, Rio Grande Rift, New Mexico, and North Sea

Abstract

40Ar/39Ar age spectrum analysis of low temperature K-feldspars generally reveals age gradients indicative of the time the sample cooled between 200°C to 100°C. Reheating the feldspar above ≈ 120°C for geologically significant intervals sets up a radiogenic 40Ar (40Ar*) concentration gradient characteristic of diffusion loss that can yield information related to the original cooling age and the timing and thermal intensity of the later event. Detrital microcline (and other K-feldspar) in sedimentary rocks can be used in this way to monitor the thermal evolution of a basin because accumulated 40Ar* is lost in response to rising temperature. Applications of this approach to deep drill-hole samples from the San Joaquin Valley, California, Rio Grande Rift, New Mexico, and North Sea basin demonstrate the utility of this method in providing thermal and temporal constraints for geodynamic models. Improvements in analytical methods promise to reduce present limitations on interpretation attributable to sample heterogeneity.