Tectonic Development and Metallogeny of Madagascar with Reference to the Fracture Pattern of the Indian Ocean

摘要

Madagascar is apparently a large block of continental crust intermittently uplifted as a complex horst between two subsiding depressions within the system of north-northeast–trending fractures of the Indian Ocean floor. A set of major east-trending fractures intersect the Precambrian basement rocks. These fractures are probably of Precambrian age, but later movements occurred along them. Because of the continuing movement, these fracture zones are topographically detectable as a system of evenly spaced low areas followed by river valleys that cut the sedimentary cover of western Madagascar. The Mahanoro-Tsiribihina fracture zone in the middle of the island is the most significant. Its latitude roughly corresponds to that of a segment of the east-trending Rodriguez fracture zone which intersects the crest of the Mid–Indian Ocean (Mid-Oceanic) Ridge east of long. 63° E. An axis of magmatic reactivation in central Madagascar corresponds to a deep-seated Precambrian fracture zone along which magmatic processes occurred at different times. Endogenous ore deposits of Precambrian and Paleozoic age are related to the intersection of the east-trending fracture zones with the axis of magmatic reactivation and lines parallel to this axis. The mineralization associated with Cretaceous intrusive rocks exhibits another pattern of distribution. Madagascar essentially occupies its original place. Furthermore, India was not derived from the western Indian Ocean by continental drift. Instead, a continuous landmass, referred to as the "Malagasy-Mascarene subcontinent," existed in the region between the eastern coast of present-day Africa and the Mid-Oceanic Ridge. This interpretation does not contradict the derivation of India from the central part of the Indian Ocean between 75 and 55 m.y. B.P. as suggested by McKenzie and Sclater. If the above two interpretations are correct, disintegration of a continental mass as well as continental drift have contributed to the origin of the Indian Ocean. Key Words: structural geology • economic geology • Madagascar • Malagasy-Mascarene subcontinent • sea-floor spreading • continental disintegration