Reconstructing the thermal history of the Morondava Basin, Madagascar, after Gondwanan breakup as resolved through detrital zircon (U–Th)/He and U–Pb geochronology

Abstract

Formation of the Phanerozoic basins of Madagascar coincided with the initial stages of Permian break-up of Gondwana. The largest of these, the Morondava Basin, records the tectonic history of rifting, drifting, and uplift of the Malagasy terrane over the last 300 Myr. Herein, we have applied detrital zircon U–Pb geochronology and (U–Th)/He low-temperature thermochronology to resolve the thermal history of the basin as Madagascar separated from Africa and subsequently India. Coarse-grained siliciclastic samples were taken along two transects parallel to the Morondava River in the central basin, including from the Permo-Triassic failed rift basin and from post-Jurassic passive margin sequences. Detrital zircon U–Pb age distributions are defined by common Neoarchean and Neoproterozoic populations which suggests input from the basement terranes of the Malagasy Central Highlands (Antananarivo domain). Samples also contain Paleo-to Mesoarchean and Cambrian populations likely associated with metamorphic terranes amalgamated during the Pan-African orogeny. Single crystal zircon (U–Th)/He dates are 503 ± 17 Ma to 40 ± 1 Ma with effective uranium (eU) values ranging from 35 to 1760 ppm, which exhibit a strong negative date-eU relationship that indicates partial resetting of the He systematics in zircon throughout the basin. Inverse and forward thermal history models from the data reveal separate thermal histories for strata on either side of basin-dividing Illovo Fault. Under an elevated geothermal gradient, temperatures did not exceed 180 °C either during post-Gondwanan burial of Jurassic rift strata or during Late Cretaceous burial of Early Cretaceous passive margin strata. Intrusion of Turonian-age dikes and sills related to the Marion hot spot likely promoted subtle yet widespread heating. The time of latest cooling during the Paleogene recorded in the basin is poorly constrained by the detrital zircon thermochronology.