Reconstructing Deep-Time Burial and Erosion Histories Using (U-Th)/He Data Patterns, Geologic Constraints, and Thermal History Modeling: an Example from the Kaapvaal Craton, Southern Africa

Abstract:

Deciphering deep-time thermal histories is particularly challenging given the potential for Phanerozoic thermal overprinting and the need to untangle an integrated history of radiation damage accumulation and annealing and He ingrowth and diffusion. Here, we combine titanite and zircon (U-Th)/He (THe, ZHe) thermochronology with geologic constraints from the Kaapvaal craton, southern Africa to decipher two distinct phases of craton-wide burial coincident with Rodinia and Pangea supercontinent assembly. The Mesoproterozoic basin was eroded during Neoproterozoic-Paleozoic time such that the thermal imprint of its former extent can only be detected with thermochronology.

THe and ZHe dates vary from 1187 to 135 Ma and 997 to 32 Ma, respectively. Both are negatively correlated with effective U concentration, consistent with lower He retentivity at higher radiation damage dose. The oldest THe dates, combined with volcanic rocks that document the basement was at the surface at ~1.4 Ga, require heating to ≥130°C at ~1.0-1.2 Ga to cause complete He loss from the titanites, which we attribute to widespread burial of the craton during Rodinia supercontinent assembly. Spatially variable ZHe data patterns can be explained by geologically-reasonable differences in Phanerozoic Karoo basin burial during Pangea assembly and subsequent erosion.

We exploit independent geologic and geochronologic constraints on the thermal history and use inverse modeling in HeFTy to demonstrate that THe and ZHe results are compatible with significant Mesoproterozoic burial and spatially variable Phanerozoic burial and exhumation. We then apply forward modeling to a) document the relationship between the temperature and duration of burial for complete He loss from titanite and thus THe date resetting, b) show the data are less sensitive to the details of the pre-1.4 Ga and pre-300 Ma exhumation histories than to peak burial temperatures and durations, and c) estimate the extent to which the zircon radiation damage accumulation and annealing model (ZRDAAM) may overestimate damage annealing and conclude that inaccuracies in the ZRDAAM do not change our first-order geologic interpretations.