Abstract
Abstract Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the <250 °C thermal history of Precambrian basement over the past >1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to <200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ∼3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ∼80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity.
Highlights
The Precambrian-Cambrian crystalline basement–sedimentary contact, or the Great Unconformity (GU), represents a prominent erosion surface observable across North America (Marshak et al, 2017) and globally (Ronov et al, 1980)
The zircon (U-Th)/He (ZHe) data set is composed of 37 single zircon grain aliquots from 7 sample locations (∼5 grains from each sample) that span river miles (RM) 80–236 (Fig. 1; measured downstream from Lees Ferry; see Table S1 and the Supplemental Material text for analytical methods)
Differences in effective uranium (eU) concentration among zircon crystals with the same time-temperature (t-T) history are proportional to differences in radiation damage, which results in a range of crystal-specific He diffusivities due to the damage-diffusivity relationship (Guenthner et al, 2013)
Summary
The Precambrian-Cambrian crystalline basement–sedimentary contact, or the Great Unconformity (GU), represents a prominent erosion surface observable across North America (Marshak et al, 2017) and globally (Ronov et al, 1980). Researchers have debated whether the erosion processes that created the GU are related to the Rodinia supercontinent cycle (Timmons et al, 2005; DeLucia et al, 2018; Flowers et al, 2020) or Cryogenian glaciations (Keller et al, 2019). The timing of these weathering processes has broad implications for resolving the GU’s association with key changes in oceanic and atmospheric chemistry (Husson and Peters, 2017), and subsequent biomineralization during the Cambrian explosion (Peters and Gaines, 2012). This highlights that the duration of the GU varies depending on the ages of local strata (e.g., the sub-Tonto Group unconformity has between 200 and 1300 Ma of time “missing”)
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