STRUCTURALLY TRAPPED ULTRAHIGH-TEMPERATURE METAMORPHISM IN THE CORE OF THE EAST-AFRICAN OROGENY, SOUTHERN MADAGASCAR

An increasingly favored tectonic model for ultrahigh-temperature metamorphism is the accumulation of radiogenic heat in orogenic plateaus. In this model, sustained overthickening of the crust in continental collisions (60–70 km, such as central Tibet) results in an increase in the conductive geothermal gradient due to heat produced by the decay of U, Th, and K. The magnitude and rate of heating is higher in crust with elevated concentrations of these radioactive elements (especially if they reside in the middle to lower crust). In such models, heating usually continues until extensional collapse or erosional thinning of the plateau. Although this model works well, it is challenged by the need to sustain a relatively static, hot, overthick crust, without collapse, lateral escape, or erosion for ≥50 Ma, because radiogenic heat production is slow.

We present new thermobarometry and monazite petrochronology across Ediacaran–Cambrian (ultra)high-temperature metamorphic terranes of southern Madagascar for which the orogenic plateau model has been proposed as part of the terminal collision of the East African Orogeny. We use our data to establish a revised paleogeographic, temporal, and structural correlation of the orogen, which is now fragmented among Africa, Madagascar, India, Sri Lanka, and Antarctica. We argue that the southern Madagascar (ultra)high-temperature terranes were structurally confined in a vertical, lithosphere-scale, transpressional shear system during orogenesis, while thrust nappe and tectonic escape systems operated elsewhere. By limiting exhumation and lateral thinning, this structural trapping may have facilitated greater accumulation of radiogenic heat, thereby producing longer-lasting and higher-temperature (higher-T/P) metamorphism than would have otherwise been possible.