Denudational and thermal history of the Early Cretaceous Brandberg and Okenyenya igneous complexes on Namibia's Atlantic passive margin

摘要

The Early Cretaceous Brandberg and Okenyenya igneous complexes provide ideal locations to quantify the postbreakup denudation history of the continental margin of central Namibia. Apatite fission track analysis has been applied to vertical sample profiles extending over 1-2 km of relief to constrain the denudation history of this region. An advantage of vertical profiles is that they allow the inference of the paleogeothermal gradient directly from the data. An independent, data-consistent thermal gradient estimate is essential to determine the amount of denudation as well as to resolve phases of slow and accelerated denudation. We obtained independent paleogeothermal gradient estimates for the two vertical profiles from 26 ± 32℃ to 23 ± 16℃. The uncertainties are large but are considered pessimistic. The best fit values agree well with each other, and the present-day gradients of 22℃. The total amount of denudation since the Late Cretaceous has been estimated as 5 km in the Brandberg area and 4 km for the Okenyenya area. The data from the two vertical profiles imply a phase of rapid exhumation in the Late Cretaceous between 80 and 60 Ma, at rates between 0.2 and 0.125 km/m.y. These decline rapidly in the Early Tertiary to a maximum denudation rate between 0.023 and 0.015 km/m.y. The denudation chronology inferred from the new apatite fission track data is consistent with the observation of a major influx of clastic sediment to offshore basins about 50 m.y. after continental breakup. The study shows that vertical fission track profiles provide key insights into postbreakup tectonic and geomorphic processes in the Central Damara Zone in Namibia, where no Phanerozoic sedimentary record has been preserved.