Continuous Thermal Histories from MDD Modeling of 40Ar/39Ar K-feldspar Analyses and Applications to Extensional Tectonics

Martin Wong, Colgate University
Phillip Gans, University of California, Santa Barbara
Peter Zeitler, Lehigh University
Bruce Idleman, Lehigh University
Damien Roesler, Colgate University

Most tectonic processes impart a thermal signature on the crust, which has made thermochronology a critical tool for constraining the timing, magnitude, rate and spatial patterns of tectonic deformation. Most thermochronometers have a single closure temperature and therefore record cooling through one point along a T–t path of a sample. Thus, reconstruction of a complete thermal history requires the application of multiple thermchronometers and interpolation of a cooling history between point constraints, which may lead to inaccurate thermal histories. An ideal thermochronologic system would instead record a continuous thermal history over a broad range of temperatures.

One of the best opportunities to extract continuous thermal histories is the use multiple diffusion domain (MDD) modeling of 40Ar/39Ar K–feldspar diffusion data. 40Ar/39Ar K–feldspar age spectra typically show increasing ages with higher temperature step–heating experiments, suggesting the presence of multiple diffusion domains, each with a characteristic closure temperature. Domain modeling combined with inversion of the 40Ar/39Ar data can be used to constrain a set of thermal histories that satisfy the measured age spectrum. For most K–feldspar samples, MDD modeling can constrain the continuous thermal history of a sample from ~300–150°C, which is applicable to a wide range of tectonic processes and therefore represents a potentially powerful technique. However, remaining uncertainties about whether this technique yields geologically meaning thermal histories continues to hinder a broader application of this method to new geologic problems.

Here we present an overview of the MDD method and its assumptions. We then present two field-based case studies at tilted normal fault blocks in the Basin and Range, the Grayback block in AZ and Gold Butte block in NV, that document that the MDD approach is capable of producing high precision and geologically meaningful continuous thermal histories that are well calibrated with other thermochronometers. Finally, we show how MDD thermal models can be applied to investigate the timing, magnitude and rate of extensional exhumation, constrain paleo-geothermal gradients, and the initial dip of low–angle normal faults. These results document the potential utility of the MDD approach to addressing a wide spectrum of tectonic questions.