NEOPROTEROZOIC NAPPES AND SUPERIMPOSED FOLDING OF THE ITREMO GROUP, WEST-CENTRAL MADAGASCAR

The Precambrian geology of west-central Madagascar is newly interpreted in light of recent field observations, structural analysis, and U-Pb geochronology. The bedrock of the area consists of (1) late Archean migmatitic gneiss and schist, (2) medial Proterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on (1, above), (3) Neoproterozoic (~1000 Ma-720 Ma) plutonic rocks emplaced into both (1) and (2, above), and (4) latest Neoproterozoic to medial Cambrian (~570-520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons into units (1), (2), and (3). Our new observations and isotopic ages provide important constraints on models describing Gondwana's amalgamation. (1) Archean gneisses and medial Proterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (2) This continental fragment (both basement and cover) was extensively invaded by arc-related magmas prior to the onset of regional metamorphism and deformation in the period from ~1000 Ma to ~720 Ma. (3) Continental collision related to Gondwana's amalgamation began after ~720 Ma and before ~570 Ma, with thermal effects that lingered on to ~520 Ma. The oldest structures produced during this collision are km-scale fold- and thrust-nappes with SE-directed vergence. They resulted in the km-scale inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence warm rocks were thrust over cool rocks thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of W-dipping continental convergence. (4) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold forms. The geometry and orientation of these younger upright folds is consistent with E-W horizontal shortening within a sinistral transpressive regime along the Ranotsara and related shear zones of south Madagascar, and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.