SEVIER-BELT EXHUMATION IN CENTRAL UTAH CONSTRAINED FROM COMPLEX ZIRCON (U-TH)/HE DATASETS: RADIATION DAMAGE AND HE INHERITANCE EFFECTS ON PARTIALLY RESET DETRITAL ZIRCONS

Zircon (U-Th)/He dating is a potentially useful tool for describing Cretaceous exhumation in the western thrust sheets of the Sevier belt, especially as in situ exhumation constraints are lacking in many locations. Such constraints provide a timing framework, which can be used to address large-scale tectonic problems in the North American Cordillera. Unfortunately, the hanging walls of many western thrust sheets carry sedimentary units that have experienced variable amounts of burial and exhumation. Zircons from these units have been partially reset after deposition and are difficult to interpret due to inherited pre-depositional dates and kinetic behaviors of grains from diverse sources. We present several complex zircon He datasets from sedimentary rocks that have been heated to temperatures near the zircon He partial retention zone, leading to several types of date-effective uranium (eU) correlations caused by relationships between pre-depositional inherited age, radiation damage, and He diffusion kinetics. Our samples were collected along three sub-vertical transects in mountain ranges in central Utah. Each range lies in the hanging wall of one of three major thrust sheets that compose part of the Charleston-Nebo Salient. Zircons from the Stansbury and Oquirrh Mts. show large date variation that can be partially understood with a radiation damage-based model for He diffusion in zircon. We combine the output from this model with a new approach for understanding partially reset datasets that relies upon the concept of an “inheritance envelope.” For the Stansbury transect, this approach yields inconclusive results; some aspects of the model generate inheritance envelopes that match the real dataset, while others do not. But time-temperature (tT) constraints from inheritance envelopes in the Oquirrh Mts. transect suggest a pulse of exhumation beginning at either 110 or 100 Ma. The final transect from the Wasatch Range near Provo, UT is relatively simple and here we document a pulse of exhumation at 100 Ma. For the Oquirrh and Wasatch Range, these direct measurements of thrust sheet tT histories offer insight into the evolution of the central Utah Sevier belt. The methods of zircon He date interpretation presented here may also prove useful for understanding exhumation in other portions of the Sevier belt.