legends.htm

Lithospheric Evolution of Gondwana East from Interdisciplinary Deep Surveys (LEGENDS)

General Background;

Systematic exploration of the continental lithosphere by deep seismic reflection profiling over the past 20 years has revolutionized our view of the deep crust and upper mantle. Spectacular though these results have been, the surveys have been limited largely to National Programs in those countries with the financial resources to mount such expensive geophysical initiatives. Thus, while major networks of deep seismic profiles now span North America, Europe, Japan and Australia/New Zealand, much of the world's continents remain unsampled. Although recent multinational efforts have produced important deep geophysical transects of such key targets as the Himalayas/Tibet (INDEPTH), the Urals (URSEIS) and the Andes (ANCORP), most of Asia, Africa, South America and Antarctica remain terra incognita in terms of modern, high resolution deep seismic imaging. From a geological perspective, perhaps the largest expanse of unexplored continental lithosphere lies in those fragments that were once part of the supercontinent of Gondwanaland. With the exception of Australia, which pioneered the use of deep reflection surveying since the 1960’s, few seismic profiles exist to delineate the gross structure, much less the details, of continental architecture of this region which played such an important historical role in the development of plate tectonic theory.

We suggest that it is now time to consider a comprehensive program of deep geophysical surveys, cored by seismic reflection profiling, to probe Gondwanaland. Geological and geo-chemical studies of the rocks in East Africa, Madagascar, India, Sri Lanka, Australia and East Antarctica now provide a firm basis for framing geotectonic questions that can be addressed by such surveys. New seismic technologies make surveys is previously remote areas much more practical. Furthermore, the present-day dispersal of the fragments of Gondwanaland make many of these geological problems accessible to marine deep seismic profiling, which is considerably less expensive than similar surveys on land. The East African Orogen (also known as Mozambique belt) is the site of ocean closure and continental or terrane collision in the Neoproterozoic, when the Congo-Kalahari blocks converged with India and East Antarctica to form Gondwanaland. The site of the collision zone is defined by a broad belt, several hundred kilometers wide, of granites, gneisses and high-grade metamorphic rocks involved in the collision, which is analogous to the Cenozoic collision between India and Asia. Deep erosion has exposed at surface rocks of the collision zone to middle and lower crustal depths, yet the region is still underlain by an apparently full thickness of continental crust.

Scientific Target;

Now we propose to conduct seismic and other geophysical experiments from the cratonic parts of East Africa through the collision zone to cratonic India, then followed to continue into East Antarctic Archaean craton. Madagascar is the keystone in the tectonic profile because the suture between Indian and Congo-Kalahara cratons potentially runs through the island. There are several models for where the suture might lie, and a real possibility exists that there may be more than one branch of the Neoproterozoic Mozambique ocean that closed during the collision. Also, we need to investigate whether major crustal boundaries recognized by geological mapping extend to depth and continue across the Gondwanan fragments. Gondwanaland reconstructions place the western margin of India adjacent to Madagascar which, in turn, lies adjacent to Somalia, Kenya and Tanzania. A complete traverse through the collision zone can be made at the latitudes of Dar-Es-Salaam in Tanzania, Antananarivo. and Tulear in Madagascar and Panjim and Banglore in southern India, finally concluding with surveys onto Napier Complex of Antarctica. This profile, ca. 2,500 km long in the Gondwanaland reconstruction, crosses from undeformed ca. 3 Ga old cratons on either end, though a broad zone of ca. 2.5 Ga continental crust heated and partially melted in the late Neoproterozoic and across the site of the now disappeared Neoproterozoic Mozambique Ocean.

As for East Antarctica, a project to study the "Structure and Evolution of the East Antarctic Lithosphere (SEAL) " has already been underway since the 1996-1997 austral summer season within the framework of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys have focused on deep seismic refraction/wide-angle reflection probing of the East Antarctic craton. The ultimate goal of SEAL is to obtain a whole crustal section of the various geological terrains, from Western Enderby Land to Eastern Dronning Maud Land and ranging in age from Archean to early-Paleozoic. In the austral summer seasons of 2000 and 2002, deep seismic probings were conducted on an ice sheet in the northern Mizuho Plateau of the Lützow-Holm Complex (LHC). The LHC experienced regional metamorphism in the late-Proterozoic to Paleozoic, and was under compressional stress perpendicular to the thermal axis during metamorphism in 500 Ma. At 150 Ma extention due to break-up of the Gondwana super-continent in conjunction with Antarctica/Australia-India rifting induced uplift of mantle material and underplating of the LHC. The SEAL transect thus constitutes an important lead element for deep seismic exploration of Gondwana. Synthesis of these results with the proposed program of systematic seismic exploration of neighboring elements of Gondwana should offer major new insights into how continental lithosphere both amalgamates and breaks up, with important implications for both continental tectonics and natural resource assessment.