Strain pattern and late Precambrian deformation history in southern Madagascar

Abstract

This paper examines the thermo-mechanical evolution of the lower crust, in Precambrian times, with an example from southern Madagascar. The finite strain pattern is derived from the study of satellite images complemented by field structural analysis. The finite geometry reflects the superposition of two distinct finite strain patterns, D1 and D2. The geodynamic significance of the D1 event remains unclear. However, the D2 finite strain pattern is partitioned between a network of kilometric vertical shear zones and folded domains, and is consistent in the Mozambique belt with east–west shortening in a transpressive regime.The metamorphic conditions associated with the two finite strain fields show that rather uniform, high temperatures were ubiquitous in southern Madagascar. By contrast, regional pressure differences were controlled primarily by the major shear zones (D2). Chronological constraints obtained from monazite (U–Th–Pb electron microprobe dating) indicate ages of 590–530 Ma for the D1 event and 530–500 Ma for the D2 event. The two distinct strain fields D1 and D2 are continuous in time.The late pan-African transpressional tectonic regimes, contemporaneous with granulitic facies metamorphic conditions, resulted in strong strain partitioning between shear zones and folded domains. The D2 transpression was efficient and pervasive enough to exhume pieces of the lower crust. The amount of exhumation was controlled by the D2 strain gradient (amount of pure shear with respect to simple shear).