ÉVOLUTION THERMOMÉCANIQUE DE LA CROÛTE INFÉRIEURE DU SUD DE MADAGASCAR

Abstract : This work is devoted to the characterization of the thermo-mechanical evolution of the lower crust from the example of Madagascar. The finite strain pattern of Southern Madagascar is derived from the study of satellite images complemented by structural analуsis on the field (foliation, stretching lineation trajectories and kinematics). The strain is partitioned between blocks (dome-and-basin folded domains) surrounded by ductile shear zones. These geometries reflect the superposition of two distincts finite strain pattern D1 and D2. D2 finite strain pattern reworked the sub¬horizontal D1 structures giving rise to a network of anastomozed vertical shear zones (15-25 km wide and> 1000 km long). It is consistent with East-West shortening in a transpressive regime. We determined the metamorphic conditions associated with the two finite strain fields. Estimates have been realized for various lithologies by using several methods (TWEEQ software and usual calibrations). Results show that rather uniform high-temperature conditions can be found everуwhеrе in Southern Madagascar. Ву contrast, there are regional pressure differences controlled by the major shear zones, which in turn control the movements of the crustal blocks. Thus the transpressive D2 strain field is responsible for exhumation of deep-seated rocks. D1 and D2 both occurred under granulite facies conditions. Chronological constrains obtained from monazite (U-Th-Pb) indicate ages at 580-550 Ma for Dl and 520-500 Ma for D2. In order to characterize the rheological behavior of granulites in Madagascar, we carried out a study of microstructures, microfabrics and deformation mechanism of quartz- & feldspar-bearing rock. Our results demonstrate that the continental lithosphere, despite its high composition heterogenеitу, is rheoligiсally homogeneous. Furthermore, the granoblastic textures of granulites are not only annealing textures but thеу are also textures characteristic of high-stained rocks at high-temperature while viscositу cintrasts between phases are very low. Finallу, the pan-African origenic cycle (580-500 Ma) in Madagascar is presented at the scale of the Gondwana super-continent where the D2 deformation stage results from the convergence of cratons. The latter constitute major heterogeneities within the continental lithosphere which control the repartition of the radiochronologic data (diachronous ages), the P-T data (pressure differences), and the strain partitioning (simple shear vs. pure shear, and strain gradient). Ву integrating these direct observations as well as indirect observations derived from seismology and gravimеtry, we infer that the vertical shear zones are deeply rooted in the mantle. This lithospheric anisotropy is associated with fluid transfers allowing heat advection in the orogenic mots responsible for the development and persistent granulite facies metamorphism during tens of million years.
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Études Approfondies, Jean-Emmanuel Martelat. ÉVOLUTION THERMOMÉCANIQUE DE LA CROÛTE INFÉRIEURE DU SUD DE MADAGASCAR . Planète et Univers [physics]. Université Blaise Pascal Clermont-Ferrand II, 1998. Français. ⟨tel-01622712⟩

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