A titanite fission track profile across the southeastern Archæan Kaapvaal Craton and the Mesoproterozoic Natal Metamorphic Province, South Africa: evidence for differential cryptic Meso- to Neoproterozoic tectonism

Abstract

Titanite fission track data are presented from samples taken at regular intervals along a ∼ 250 km long traverse from the ca 1.1 Ga Natal Metamorphic Province into the Archæan Kaapvaal Craton, southeastern South Africa. The titanite fission track traverse was intended to distinguish any differences in low-temperature (280°C) cooling trajectories between the two provinces and to identify, so far unrecorded, exhumation events. Titanite fission track age data in four groupings were obtained, namely at 1100 ± 200, 670 ± 100, 520 ± 50 and 170 ± 15 Ma. In addition to these main groupings, the older group contains a few single-grain ages of Palæoproterozoic age in the northernmost craton sample, from near Piet Retief. Samples from the craton, and the northernmost sample from the Natal Metamorphic Province, show a concentration of Grenville age ( ca 1100 Ma) single-grain ages. They reveal a cooling event that indicates that the craton margin was affected during the Mesoproterozoic to a larger extent then previously thought. Possibly, the dates reflect the timing of differential uplift of the rigid craton in response to loading by the over-thrusting of the Tugela nappes some 200 km to the south during the Mesoproterozoic collision event of the Natal Metamorphic Province. Alternatively, the data may suggest that the Tugela nappes extended far further north than previously thought, and the dates represent uplift from their unroofing. The age group of 670 ± 100 Ma is interpreted to represent mixed ages, with some titanite grains having predominantly Grenville-age age components and others having mainly Pan-African age components. Generally, the intensity of Pan-African overprint increases to the south and east. The southern craton samples and all samples from the Natal Metamorphic Province show a concentration of Pan-African single-grain ages of ca 520 ± 50 Ma. These dates are considered to represent fairly rapid uplift of the western foreland during the Pan-African collision of East and West Gondwana, which took place far to the east. Finally, samples taken from the northeastern part of the traverse give Jurassic dates, taken to represent thermal resetting during the Karoo volcanic event as a result of the break-up of Gondwana. The titanite fission track data presented thus aim to demonstrate the power of medium- to low-closure temperature geochronometer to reveal and distinguish younger, largely unseen but nevertheless important, tectonic events.