37. Gondwanaland Dispersal and the Early Formation of the Indian Ocean

Deep Sea Drilling results in the Indian Ocean and mapping of Mesozoic magnetic anomalies allow construction of a model for the plate system in the early Indian Ocean. In spite of the fact that the concept of Gondwanaland, the late Paleozoic supercontinent of the Southern Hemisphere, has been extant for over 50 years (Wegener, 1929; DuToit, 1937), the details of the exact configuration of its component continents and the dispersal of these continents via plate motions with time, remain largely speculative. Recent interpretations of marine magnetic anomalies in the Indian Ocean together with Deep Sea Drilling Project (DSDP) results (Figure 1), put sufficient restraints on the dispersal of the Gondwana continents that it is possible to construct a plate model for the early formation of the Indian Ocean. Major insights in this area have been provided by McKenzie and Sclater (1971) and Falvey (1972a, b). McKenzie and Sclater (1971), using Cenozoic marine magnetic anomalies, accurately reconstructed the relative position of the Gondwana continents in the Late-Cretaceous/early-Tertiary period. Falvey (1972b) presented some novel ideas on the configuration of the Mesozoic plate system in the eastern part of the Indian Ocean. Some particular questions which are left unanswered by these studies are: (1) where was India located in Gondwanaland, against west Australia (yeevers et al., 1971; Crawford, 1969) or against Antarctica (DuToit, 1937; Falvey, 1972B); (2) how did the Gondwana continents disperse in time, to arrive at the relative positions that McKenzie and Sclater (1971) determined? Because the breakup and dispersal of Gondwanaland covered a time span including a midto Late-Cretaceous period of nonreversal of the earth's magnetic field (85-111 m.y. as suggested by Larson and Pitman, 1972), sea-floor chronology prior to the earliest McKenzie and Sclater reconstruction (Late Cretaceous or younger) cannot be deduced from marine magnetic anomalies unless sea floor older than about 110 m.y. can be found. Therefore, the early history of the Indian Ocean can only be determined by deep drilling in sea floor thought to be older than Late Cretaceous (i.e. Cloniacian, anomaly 32 of Heirtzler et al., 1968). Using DSDP data and Cretaceous and Tertiary magnetic 'Woods Hole Oceanographic Institution, Contribution No. 3249. Now at Department of Geological Sciences, University of California at Santa Barbara, Santa Barbara, California. The Gondwana continents include: South America, Africa, India (with Ceylon), Madagascar, Australia, and Antarctica, plus microcontinents such as the Seychelles. anomalies mapped off west Australia, a model is suggested which requires India to fit against Antarctica, for Gondwanaland to disperse first in the eastern Indian Ocean and extreme southern Atlantic, followed by dispersal in the western Indian Ocean and equatorial regions of the South Atlantic. This model should only be considered an outline sketch of early dispersal. -Ultimately, a more rigorous story should be constructed with determinations of poles of rotation, finite rotations, and paleomagnetic latitudes. Deep Sea Drilling Project holes (von der Borch, Sclater, et al., 1972; Luyendyk, Davies, et al., 1973; Heirtzler, Veevers, et al., 1973b) in the eastern Indian Ocean (Wharton Basin) along with magnetic lineations recently charted, can be interpreted to show that the seafloor isochrons here run east-west and that the sea floor is oldest in the south (Sclater and Fisher, in press, and Figure 2). The oldest known sea floor in the Indian Ocean is Late Jurassic (Oxfordian) and is found northwest of Australia in the North Australian Basin (Heirtzler et al., 1973a). The sea floor here is older than that in the southern Wharton Basin, implying that rifting and spreading occurred here first. East-westtrending marine magnetic anomalies have been mapped in the northern Wharton Basin by Sclater and Fisher (in press) and Lowrie et al., (1972), and in the North Australian Basin by Falvey (1972b). These anomalies are early Tertiary in the northern Wharton Basin, and evidently Early Cretaceous in the North Australia Basin. Earlier, Falvey (1972b) apparently misidentified this latter set of anomalies as Cenozoic, but the drilling results imply they are older. Attempts to model this sequence using Larson and Pitman's (1972) Mesozoic time scale and various combinations of spreading rates and skewness, θ (Schouten and McCamy, 1972) have not yet been successful, and there is some doubt as to whether these anomalies are related to sea-floorspreading processes. The distribution of isochrons in Figure 2 indicates that eastern India was most likely against Antarctica in Gondwanaland as suggested earlier by DuToit (1937), McElhinney (1970), Falvey (1972b), and Smith and Hallam (1970), and that the dispersal plate system was initiated first off northwest Australia, followed by spreading between India and Antarctica. Figure 3a shows an east-west-trending ridge off northern Australia and India (?) in the Late Jurassic. This ridge may have