Strain pattern and late Precambrian deformation history in southern Madagascar

This paper examines the thermo-mechanical evolution of the lower crust, in Precambrian times, with an example from southern Madagascar. The finite str ain pattern is derived from the study of satellite images complemented by f ield structural analysis. The finite geometry reflects the superposition of two distinct finite strain patterns, D1 and D2. The geodynamic significanc e of the D1 event remains unclear. However, the D2 finite strain pattern is partitioned between a network of kilometric vertical shear zones and folde d domains, and is consistent in the Mozambique belt with east-west shorteni ng in a transpressive regime. The metamorphic conditions associated with th e two finite strain fields show that rather uniform, high temperatures were ubiquitous in southern Madagascar. By contrast, regional pressure differen ces were controlled primarily by the major sheer zones (D2). Chronological constraints obtained from monazite (U-Th-Pb electron microprobe dating) ind icate ages of 590-530Ma for the D1 event and 530-500 Ma for the D2 event. T he two distinct strain fields D1 and D2 are continuous in time. The late pa n-African transpressional tectonic regimes, contemporaneous with granulitic facies metamorphic conditions, resulted in strong strain partitioning betw een shear zones and folded domains. The D2 transpression was efficient and pervasive enough to exhume pieces of the lower crust. The amount of exhumat ion was controlled by the D2 strain gradient (amount of pure shear with res pect to simple sheer). (C) 1999 Elsevier Science B.V. All rights reserved.