COG3

Causes and consequences of dynamic uplift of Madagascar and its surroundings

 

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. These observations suggest that some component of Malagasy topography is supported by sub-plate mantle convective processes. In the first part of this talk I examine a suite of new geologic, geomorphic and thermochronologic observations which all suggest Madagascar has experienced 400 - 1000 m of Neogene uplift. Eocene marine limestones containing nummulites are found at 400 - 700 m in elevation. Inverse modeling of both apatite fission track and apatite helium thermochronologic data, which was sampled during a recent field campaign, indicates that a pulse of erosion began in Neogene times. This increase in denudation is consistent with a few hundred meters of Neogene long-wavelength uplift. Implications arising from the Quaternary and ongoing history of dynamic support are then explored by examining a transect of Quaternary sea-level markers from the northern Malagasy coastline, a passive margin which should be stable. I present a suite of radiometric dating and surveying results for three terrace levels, marking three interglacial periods which crop out sporadically along 80 km of coastline. The elevation of the upper (undated) terrace decreases from 33.8 m to 29.5 m over a distance of 35 km. An intermediate terrace has an average radiometric age of 130.7 +/- 13.2 ka (i.e. MIS~5e) and its elevation decreases from 9.3 m to 2.8 m over a distance of 80 km. The record of the lower terrace is fragmentary and consists of beach rock containing rare corals with ages of 1.6 - 3.8 ka. The spatial gradient of the MIS~5e marker is inconsistent with glacio-isostatic adjustment calculations. Instead, I propose that variable elevations of this marker around Madagascar at least partly reflect spatial patterns of dynamic topography generated by sub-plate mantle convection.