Phanerozoic Morphotectonic Evolution of the Zimbabwe Craton: Unexpected Outcomes From a Multiple Low‐Temperature Thermochronology Study

Abstract

The fragmentary Phanerozoic geological record of the anomalously elevated Zimbabwe Craton makes reconstructing its history difficult using conventional field methods. Here we constrain the cryptic Phanerozoic evolution of the Zimbabwe Craton using a spatially extensive apatite (U‐Th‐Sm)/He (AHe), apatite fission track (AFT), and zircon (U‐Th)/He (ZHe) data set. Joint thermal history modeling reveals that the region experienced two cooling episodes inferred to be the denudational response to surface uplift. The first and most significant protracted denudation period was triggered by stress transmission from the adjacent ~750–500 Ma Pan‐African orogenesis during the amalgamation of Gondwana. The spatial extent of this rejuvenation signature, encompassing the current broad topographic high, could indicate the possible longevity of an ancient topographic feature. The ZHe data reveal a second, minor denudation phase which began in the Paleogene and removed a kilometer‐scale Karoo cover from the craton. Within our data set, the majority of ZHe ages are younger than their corresponding AHe and AFT ages, even at relatively low eU. This unexpectedly recurrent age “inversion” suggests that in certain environments, moderately, as well as extremely, damaged zircons have the potential to act as ultra‐low‐temperature thermochronometers. Thermal history modeling results reveal that the zircon radiation damage accumulation and annealing model (ZRDAAM) frequently overpredicts the ZHe age. However, the opposite is true for extremely damaged zircons where the ZHe and AHe data are also seemingly incompatible. This suggests that modification of the ZRDAAM may be required for moderate to extreme damage levels.

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…In addition, the low‐temperature sensitivity and radiation damage influences on the apatite He system make it ideal for quantifying the timing, magnitude, and extent of low (<2 km) amplitude deposition and denudation episodes in cratonic regions, even when the deposited rocks have been completely eroded from the rock record. Apatite He date‐eU relationships inform long‐term thermal histories linked to these surface processes (e.g., Ault et al, , ; Ault, Flowers, & Bowring, ; Flowers, ; Flowers et al, ; Guenthner et al, ; Mackintosh et al, ). Figure presents apatite He, AFT, and zircon He data patterns from across the interior of continental North America.…”

“…Apatite, zircon, and titanite date-eU relationships (e.g., Figure 3) are a powerful tool for reconstructing protracted thermal histories characteristic of ancient bedrock (Ault et al, 2009(Ault et al, , 2013Ault et al, 2018;Baughman & Flowers, 2018;DeLucia et al, 2018;Flowers, 2009;Flowers et al, 2007;Guenthner et al, 2013Guenthner et al, , 2014Guenthner et al, , 2017Johnson et al, 2017;Mackintosh et al, 2017;Orme et al, 2016;Powell et al, 2016;Weisberg et al, 2018). Damage-diffusivity patterns can be predicted given a thermal history or inverted to derive a thermal history using HeFTy or QTQt.…”

Influence of Rift Superposition on Lithospheric Response to East African Rift System Extension: Lapur Range, Turkana, Kenya

“…Guenthner et al () suggested that the correlation is a product of intragrain differences in accumulated radiation damage in zircons. In certain cases, such variations result in varying He diffusion rates (or He retentivity) between aliquots, effectively lowering the temperature range of the PRZ for grains with significant radiation damage (e.g., Johnson et al, ; Mackintosh et al, ). The age‐eU relationship observed in shallower Lapur basement samples is interpreted as indicating age dispersion as the product of variations in He diffusivity related to different amounts of α radiation damage accumulation.…”

Long-term reactivation and morphotectonic history of the Zambezi Belt, northern Zimbabwe, revealed by multi-method thermochronometry

“…By employing AFT, apatite (U-Th-Sm)/He (AHe) and zircon (U-Th)/He (ZHe) thermochronometry, we reconstruct the thermal history of a sample through a range of potentially ~30-220 °C (equivalent to <1-8 km assuming a typical mobile belt geothermal gradient of 25 °C/km (Nyblade et al, 1990) and surface temperature of 20 °C). Indeed, the different temperature sensitivities of the AHe, AFT and ZHe systems-typically ranging from ~30-90 °C (Flowers et al, 2009), ~60-110 °C (Gleadow et al, 2002) and <50-220 °C (Guenthner et al, 2013;Johnson et al, 2017;Mackintosh et al, 2017), respectively-provide insight into different portions of the morphotectonic history.…”

“…the He partial retention zone (PRZ) (Wolf et al, 1998), controlled by numerous grainspecific factors, including the amount of accumulated radiation damage (Guenthner et al, 2013;Shuster et al, 2006), grain size (Reiners et al, 2004;Reiners and Farley, 2001) and parent nuclide heterogeneity (Farley et al, 2011;Hourigan et al, 2005). In both systems, radiation damage is the main control on the temperature sensitivity range such that the He PRZs of individual grains can vary from ~30-90 ºC for the AHe system (Flowers et al, 2009;Gautheron et al, 2009) and <50-220 ºC (Anderson et al, 2017;Guenthner et al, 2013;Johnson et al, 2017;Mackintosh et al, 2017) for the ZHe system. Some other factors affect the measured parent-daughter ratio and in turn, the measured AHe and ZHe ages.…”