Causes and Consequences of Neogene Dynamic Uplift of Madagascar

Madagascar is characterized by elevated topography, positive long wavelength free-air gravity anomalies and slow sub-plate shear wave velocity anomalies. Offshore, oceanic residual depth measurements range from +0.5 to +1.5 km. Eocene marine limestones at elevations of more than 450 m and emergent Quaternary coral reef deposits suggest that uplift began during the Cenozoic Era and has continued until recent times. Here, the timing of uplift is constrained to Neogene times with a suite of new thermochronologic analyses. Apatite fission track analyses were carried out for 18 new samples from northern Madagascar, a subset of which were also analyzed using the apatite helium technique. These data were augmented by reanalyzed, published samples. Apatite fission track ages are 200-250 Ma with mean track lengths of 12 μm. Apatite He ages are highly dispersed in samples from the highlands (i.e. AHe age >150 Ma) but a narrower, younger range of 30-60 Ma is found on the coastal lowlands. Inverse thermal history modeling of thermochronologic data reveals a markedly increased regional cooling rate in Neogene times, particularly on the low coastal plains. Inferred cooling requires around 1 km of denudation since the start of the Neogene Period. We compare these uplift and denudation estimates to predictions of dynamic uplift which are calculated by converting shear wave velocities within a 150 ± 150 km thick asthenospheric channel to temperature, yielding potential temperature, T_p = 1380 ± 30^oC. This range of temperatures is in agreement with independent estimates from Neogene basaltic samples (T_p = 1400 ± 100 ^oC). Isostatic uplift resulting from this asthenospheric temperature anomaly is estimated and agrees closely with oceanic residual depths, the magnitude of denudation, and the elevation of marine limestones. We conclude that Madagascar has undergone regional uplift of between 400-900 m during Neogene times in response to the arrival of a hot asthenospheric temperature anomaly.