Contrasting Mesozoic evolution across the boundary between on and off craton regions of the South African plateau inferred from apatite fission track and (U‐Th‐Sm)/He thermochronology

Abstract

The timing and mechanisms involved in creating the elevated, low‐relief topography of the South African plateau remain unresolved. Here we constrain the thermal history of the Southwest African plateau since 300 Ma by using apatite fission track (AFT) and (U‐Th‐Sm)/He (AHe) thermochronology. Archean rocks from the center of the Kaapvaal Craton yield AFT ages of 331.0 ± 11.0 and 379.0 ± 23.0 Ma and mean track lengths (MTLs) of 11.9 ± 0.2 and 12.5 ± 0.3 µm. Toward the southwest margin of the craton and in the adjacent Paleozoic mobile belt, AFT ages are significantly younger and range from 58.9 ± 5.9 to 128.7 ± 6.3 Ma and have longer MTLs (>13 µm). The range of sample AHe ages complements the AFT ages, and single‐grain AHe ages for most samples are highly dispersed. Results from joint inverse modeling of these data reveal that the center of the craton has resided at near‐surface temperatures (<60°C) since 300 Ma, whereas the margins of the craton and the off‐craton mobile belt experienced two discrete episodes of cooling during the Cretaceous. An Early Cretaceous cooling episode is ascribed to regional denudation following continental breakup. Late Cretaceous cooling occurs regionally but is locally variable and may be a result of a complex interaction between mantle‐driven uplift and the tectonic setting of the craton margin. Across the entire plateau, samples are predicted to have remained at near‐surface temperatures throughout the Cenozoic, suggesting minimal denudation (<1 km) and relative tectonic stability of the plateau.

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

“…These LTT results reiterate that particularly in cratonic settings where zircons and apatites have experienced significant diffusional loss of helium and annealing of tracks due to prolonged residence at low temperatures, the measured apparent ages do not hold any particular geological meaning (Wildman et al, ). In this section, an inverse thermal history modeling approach is used to extract geological significance from this complex LTT data set and generate the most plausible thermal history for the measured LTT ages and MTL data, as well as attempt to account for the effects of individual grain chemistry, accumulated radiation damage, etc., on the resultant individual LTT age and thus thermal history.…”

“…Low‐temperature thermochronology (LTT) studies of basement rocks have provided an invaluable tool for deciphering the cryptic Phanerozoic histories of cratonic settings (e.g., Ault et al, ; Flowers & Kelley, ; Lorencak et al, ; Wildman et al, ). The strength of the LTT method lies in its ability to quantitatively constrain the timing and rates of processes that disrupt the thermal regime of the upper crust over geological timescales (e.g., Reiners et al, ).…”

“…Previous published work in southern Africa has largely focused on sampling across the Great Escarpment and passive continental margin of South Africa and Namibia with the dominantly AHe and AFT data revealing different aspects of its Mesozoic and Cenozoic evolution (Brown et al, , , ; Brown, ; Cockburn et al, ; Flowers & Schoene, ; Gallagher & Brown, ; Green et al, ; Kounov et al, , ; Luft et al, ; Raab et al, , ; Tinker et al, ; Wildman et al, , ) (Figure a). Very few studies have investigated the morphotectonic histories of the cratonic interior regions of southern African, and the spatial coverage of those that have is limited (Belton & Raab, ; Beucher et al, ; Brown, ; Brown et al, , ; Flowers & Schoene, ; Raab et al, ; Stanley et al, ; Wildman et al, ) (Figure a). This is significant because as shown by recent work (Stanley et al, ; Wildman et al, ), the cooling and denudation response to horizontal tectonic‐derived stresses as well as vertical mantle‐derived stresses of the cratonic interior can differ markedly to that of the off‐craton (or cratonic margin) regions.…”

“…Very few studies have investigated the morphotectonic histories of the cratonic interior regions of southern African, and the spatial coverage of those that have is limited (Belton & Raab, ; Beucher et al, ; Brown, ; Brown et al, , ; Flowers & Schoene, ; Raab et al, ; Stanley et al, ; Wildman et al, ) (Figure a). This is significant because as shown by recent work (Stanley et al, ; Wildman et al, ), the cooling and denudation response to horizontal tectonic‐derived stresses as well as vertical mantle‐derived stresses of the cratonic interior can differ markedly to that of the off‐craton (or cratonic margin) regions.…”

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

“…Southern Africa, a continental shield residing at a mean elevation of~1 km (Nyblade & Robinson, 1994) is widely cited as an example of dynamically supported topography (Gurnis et al, 2000;Lithgow-Bertelloni & Silver, 1998). Combined AFT and apatite He data from Archean rocks of the Kaapvaal craton and off-craton Paleozoic mobile belts suggests Early Cretaceous cooling and unroofing focused on craton margins with comparatively limited erosion in the cratonic interior (Wildman et al, 2017). Apatite He thermochronometry from kimberlites document a Cretaceous eastward wave of erosion and imply disparate lithospheric responses to deep mantle processes on and off craton (Stanley et al, 2015).…”