Titanite and apatite fission track analyses on basement rocks of central-southern Madagascar: constraints on exhumation and denudation rates along the eastern rift shoulder of the Morondava basin

Abstract

Titanite and apatite fission-track (FT) thermochronology from basement rocks in central-southern Madagascar reveals a protracted post Late Neoproterozoic/Early Cambrian history of extensional tectonism, denudation and sedimentation. Titanite FT ages range between 379 ± 38 and 276 ± 17 Ma and apatite FT ages vary between 379 ± 19 and 150 ± 8 Ma. Combined titanite and apatite FT data from the western palaeo margin of Madagascar suggest denudation rates of ∼200–100 mMa −1 during Carboniferous times. The Late Neoproterozoic/Early Cambrian Ejeda shear zone was probably reactivated during this time. In contrast, for the same period denudation rates inland are ∼110–25 mMa −1. During Permo-Triassic rifting, areas that previously underwent fast denudation were buried by sedimentary cover up to ∼4.5 km. At this time, a graben developed along the transcontinental Bongolava–Ranotsara shear zone (BRSZ). Graben faults are exposed at the northeastern graben shoulder. Identical titanite and apatite FT ages close to the BRSZ indicate rapid cooling associated with fluid circulation during Early Permian times. The initial Gondwana break-up during Middle Jurassic times and the drift of Madagascar along the Davie transform fault did not significantly influence the FT data and had only minor geomorphic impact in the study area. Only the far southwestern part of the island is characterised by a higher degree of denudation (max. ∼3.5 km) during Early Jurassic times. Early Cretaceous and Cenozoic volcanic activity affected the apatite FT data from southern Madagascar. Modelled time–temperature ( T– t) paths argue for a reheating of samples from southern Madagascar to temperatures of ∼60–80 °C during the times of magmatism, before final cooling to surface temperatures.