Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern Madagascar, with implications for Gondwana studies

Southern Madagascar comprises a complex Precambrian terrain of high-grade m etamorphic rocks with a history of polyphase deformation and metamorphism. Two prominent N-S trending late Neoproterozoic ductile shear zones, the Amp anihy and Vorokafotra shears, each with projected strike length of > 450 km and between 10 and 20 km in width, crosscut the region. A third set of en echelon shears forms part of the early Paleozoic Ranotsara Shear Zone that cuts the basement in a NW-SE direction over a combined strike length of > 4 00 km. The host rocks of these shears comprise paragneisses (metasediments) with detrital zircons ranging in age between 720 and 1900 Ma. A felsic lay er, interpreted as a metavolcanic rock, gives a date of 722+/-1 Ma. Remnant s of late Archean orthogneisses in the central part of the study area may r epresent basement to the paragneisses. Four episodes of deformation and met amorphism have been recognized on the combined basis of field observations, petrogenesis, and U/Pb analyzes of zircons, monazites, sphenes, and rutile s. Two episodes of early simple shear deformation (D-1 and D-2) at midcrust al levels occurred between 627 and 647 Ma, during which northeast verging r ecumbent sheath folds and ductile thrusts were formed and peak prograde met amorphism reached 7-12 kbar at 750 degrees -900 degreesC. Early prolate min eral fabrics (L-1/L-2) are preserved in massif-type anorthosite bodies and their marginal country rocks. D-1 occurred between 630 and 647 Ma, while D- 2 occurred at 627-628 Ma. This was followed by a 10-15 Myr period of static , annealing metamorphism until 609-614 Ma when bulk shortening (D-3) took p lace. D-2 and D-3 are coaxial but are separated in time by leucocratic dyke s that intruded between 610 and 620 Ma. D-3 was focused zonally, forming th e prominent N-S shear zones between 607 and 609 Ma; its oblate strain resul ted in a strong composite D-2/D-3 fabric defined by subvertical S-tectonite s and subhorizontal intersection lineations. A variety of post-D-3 pegmatit es accompanied similar to 85 Myr of relatively static annealing and metasom atic/metamorphic mineral growth, during which numerous occurrences of phlog opite, uranium, and rare earth elements formed. A continuum of concordant m onazite dates suggests that this thermal event is part of an extended perio d of low-pressure (3-5 kbar) charnockite-producing processes between 520 an d 605 Ma. The continuum, however, appears to be punctuated at similar to 58 0, 550, and 520 Ma. Deformation (D-4) recorded within the Ranotsara Shear Z one overlaps with the youngest parts of the regional metamorphic conditions between 520 and 550 Ma. Prevailing low-pressure, high-temperature amphibol ite-granulite facies rapidly gave way to greenschist facies conditions betw een 490 and 530 Ma, as is evident from overlapping ages of zircon, monazite , sphene, and rutile. We conclude that D-1 to D-3 represents a period of 40 Myr of compressional deformation that we interpret to be related to collis ional events during the amalgamation of Gondwana. The first part of the the rmal continuum between 550 and 605 Ma reflects similar to 55 Myr of slow co oling and annealing at midcrustal levels, while the onset of the last episo de, between 520 and 530 Ma, heralds accelerated exhumation accompanied by e xtensional tectonics between 490 and 520 Ma. We believe that this postcolli sional time span represents a prolonged period of evolution of a Tibetan-st yle plateau into an Aegean-style extensional terrain. This similar to 100 Myr event in southern Madagascar is similar to that rec orded throughout large sectors of the East African Orogen between ca. 500 a nd 600 Ma. We believe that this type of postconvergent thermotectonism best represents the original definition of "Pan-African" [Kennedy, 1964], which in today's terminology equates with "postorogenic extensional collapse" [D ewey, 1988], or "destabilization of an orogen" [Lipps, 1998]. Kennedy's Pan -African was widespread throughout the interior a supercontinent, when Gond wana's periferal margins were subjected to far-field tensional forces. This suggests that neither gravitational collapse of the Pan-African-Braziliano Orogens nor delamination were the sole or even the dominant driving forces for the postconvergent extension.