COUPLED PB ISOTOPE AND AR THERMOCHRONOMETRY FROM SINGLE DETRITAL K-FELDSPAR GRAINS: A NOVEL METHOD TO CONSTRAIN THE TIMING AND RATES OF GEOLOGIC EVENTS AND PROCESSES (Invited Presentation)

Widespread focus upon detrital zircon for carrying out geochronologic and geochemical studies of sediment provenance neglects important complimentary information recorded by modal phases such as K-feldspar. For example, ⁴⁰Ar/³⁹Ar analysis of detrital K-feldspar is capable of providing detailed information regarding the thermal evolution of basement source regions. Moreover, incorporation of Pb into K-feldspar allows its Pb isotope composition to be used as a provenance tool. To date, comprehensive integration of Pb isotopic and ⁴⁰Ar/³⁹Ar thermochronologic detrital K-feldspar data has never been attempted. We have developed a reasonably high-throughput and high-precision LA-ICP-MS analysis protocol to generate statistically meaningful detrital K-feldspar Pb isotope datasets. Interpretation of Pb isotope data as ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb is amenable to two-dimensional Kolmogorov-Smirnoff statistical tests. Following the measurement of Pb isotope composition, the same single detrital K-feldspars are analyzed by either total fusion or ramp-heating ⁴⁰Ar/³⁹Ar methods to reveal the thermal evolution of the source region. Full integration of Pb isotopic composition with ⁴⁰Ar/³⁹Ar thermal history data is capable of providing unprecedented insights into the spatial and thermal history of the source region. To highlight the broad applications of this new methodology, we apply coupled Pb isotope and ⁴⁰Ar/³⁹Ar measurements from single detrital K-feldspar grains from Eocene strata from the southern Sierra Nevada forearc basin to: (1) demonstrate Eocene drainage patterns varied spatially in response to changing deformation of the southern California margin; and (2) assess variability in basement thermal history in response to spatial differences in shallow subduction history. Detrital K-feldspar Pb isotope distributions of the southernmost Sierra Nevada forearc reveal extensive drainage networks in southern California that transitioned from extra-regional to localized sources during the Eocene-Miocene. In contrast, forearc strata situated further north are dominated by local Sierra Nevada sources only. ⁴⁰Ar/³⁹Ar thermochronology from both areas reveal rapid cooling between 85-75 Ma throughout the entire southern Sierra Nevada/western Mojave region.