Reconstruction of the Phanerozoic tectono-thermal history of central and southern Madagascar, based on fission track thermochronology

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Three fission track (FT) sections running from the western margin of the basement to the eastern continental margin of central Madagascar give titanite FT ages ranging between 483±33 Ma and 266±13 Ma and apatite FT ages varying between 403±33 Ma and 79±5 Ma. Calculated cooling rates from combined titanite and apatite FT data range between 12.5 °CMa-1 and 0.4 °CMa-1 and provide evidences for two episodes of accelerated cooling in the working area. The first phase, recorded between the Itremo region and the Angavo shear zone (AGSZ) at ~420-330 Ma reached cooling rates up to 12.5 °CMa-1. The second cooling phase is documented at the present western basement/basin contact during the ~370-260 Ma period with cooling rates up to 10 °CMa -1. The older cooling event is most probably related to temporally occurring phases of advective heating and fluid circulation during the Silurian until the Early Carboniferous. In contrast, cooling along the present western basement/basin contact is related to high denudation pre-dating and synchronous with deposition of the Karoo Supergroup in the Morondava basin. FT data suggest a brittle reactivation along Cambrian ductile high-strain zones predating the opening of the Karoo rift basins. Apatite FT ages from samples located between the western basement margin and the AGSZ were affected by the intracontinental Karoo rifting between Madagascar and East Africa, lasting from the Early Permian to the Early Jurassic. Additionally, the current distribution of brittle structures in this area was set up during this time. The Late Jurassic opening of the Somali and Mozambique basins had only a limited influence on the denudation dynamics in the study area. Nevertheless, structural data document the active tectonics during this time. Along the eastern coast, the Cretaceous Madagascar-India break-up affects the apatite FT system of samples within a ~60 km wide stripe along the eastern coastline. Here, apatite FT data and modelled T-t (time-temperature) paths indicate a Cretaceous cooling. Cooling was associated with a denudation amount of ~4 km crustal section. Around and east of the AGSZ the orientation of brittle structures and thermally nearly unaffected Devonian basement titanite FT ages argue for a limited tectono-thermal impact of the Marion hot spot during the Madagascar-India break-up.