Alfred Kroener

SHRIMP dating of magmatic zircons from granitoid gneisses and charnockites of the Trivandrum and Nagercoil Blocks in the granulite terrane of southernmost India yielded well-defined protolith emplacement ages between 1765 and ca. 2100 Ma and also document variable recrystallization and/or lead-loss during the late Neoproterozoic Pan-African event at around 540 Ma. Hf-in-zircon and whole rock Nd isotopic data suggest that the granitoid host rocks were derived from mixed crustal sources, and Hf–Nd model ages vary between 2.2 and 2.8 Ga. A gabbroic dyke, emplaced into a charnockite protolith and deformed together with it, only contained metamorphic zircon whosemean age of 542.3±4.0 Mareflects the peak of granulite–facies metamorphism during the Pan-African high-grade event. The Sm–Nd whole-rock isotopic system of several granitoid
samples dated in this study was significantly disturbed during granulite–facies metamorphism, most likely due to a CO2-rich fluid phase. Whole-rock Nd model ages are consistently older than zircon-derived Hf model ages. The Trivandrum and Nagercoil Blocks are reinterpreted as a single tectono-metamorphic terrane predominantly consisting of Palaeoproterozoic granitoids interlayered with supracrustal rocks that must be older than ca. 2100 Ma. Ductile deformation, migmatization and anatexis have obliterated the original rock relationships. These blocks probably have their counterpart in the Highland Complex of neighbouring Sri Lanka and in the high-grade Palaeoproterozoic terrane of southern Madagascar. We speculate that the southern Indian
khondalites may have their counterparts in the khondalite belt of the North China Craton.

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on
their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the
lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold
and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain
concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the
initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.

The Kaoko belt belongs to a Neoproterozoic belt system of southwestern Africa, which probably resulted from collision between the Congo Craton and the Rio de la Plata Craton. The prevailing rock types, evolved at the western margin of the Congo Craton, are of metaigneous and metasedimentary origin. We focused on an area around a large-scale major structural discontinuity, the Puros Lineament, separating the Western from the Eastern Kaoko Zone. The aim of this study is to understand the evolution of this part of the Kaoko ...

Abstract Ion microprobe zircon ages, a Nd model age and Rb single bond Sr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. ...

Classic models of orogens involve a Wilson cycle of ocean opening and closing with orogenesis related to continent-continent collision. Such models fail to explain the geological history of a significant number of orogenic belts throughout the world in which deformation, metamorphism and crustal growth took place in an environment of on-going plate convergence. These belts are termed accretionary orogens but have also been refereed to as non-collisional orogens, Pacific-type orogens, Turkic-type and exterior orogens. Accretionary orogens evolve in generally curvilinear belts comprising dominantly mafic to silicic igneous rocks and their sedimentary products and accumulated largely in marine settings. They are variably deformed and metamorphosed by tectono-thermal events aligned parallel to, and punctuating, facies trends. Accretionary orogens form at sites of subduction of oceanic lithosphere and consist of magmatic arcs systems along with material accreted from the downgoing plate and eroded from the upper plate. Deformational features include structures formed in extension and compressive environments during steady-state convergence (arc/backarc vs. accretionary prism) that are overprinted by short regional compressive orogenic events. Orogenesis takes place through coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat slab subduction, and rapid absolute upper plate motion over-riding the downgoing plate. The Circum-Pacific region provides outstanding examples of accretionary orogens. The Pacific formed during breakup of Rodinia in the Neoproterozoic and has never subsequently closed, resulting in a series of overall ocean-ward younging orogenic systems that have always faced an open ocean, yet have been the sites of repeated tectono-thermal events and continental growth. Accretionary orogens have been active throughout Earth history. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products and include Archean greenstone belts, the Paleoproterozoic Birimian orogen (W. Africa), the Arabian-Nubian shield (Pan African) and Paleozoic orogens in Asia.

Systematic exploration of the continental lithosphere by deep seismic reflection pro- filing over the past 20 years has revolutionized our view of the deep crust and upper mantle. From a geological perspective, perhaps the largest, scientifically coherent ex- panse of unexplored continental lithosphere is represented by those fragments that were once part of the supercontinent of Gondwanaland. With the exception of Aus- tralia and, to some degree, India, relatively few deep reflection profiles exist to delin- eate the gross structure, much less the details, of the continental architecture of this geologically pivotal region. Independent initiatives such as the KRISP refraction ex- periments in East Africa and the recent Kaapvaal Broadband seismic investigation in southern Africa are a good start. However, we suggest that it is now time to consider a more comprehensive program, perhaps along the lines EUROPROBE, to systemat- ically probe Gondwana along transects cored by deep reflection profiling. Geological and geochemical studies of the rocks in East Africa, Madagascar, India, Sri Lanka, Australia and East Antarctica, for example, now provide a firm basis for framing im- portant geotectonic questions that can be addressed by an integrated program of sur- veys that sample those fragments. New seismic technologies have made surveys in remote areas 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. In this presentation we will describe LEGENDS (Lithosphere Evolution of Gondwana East from iNterdisciplinary Deep Surveys), an attempt to build upon both the expe- rience of previous deep seismic programs and the geologic perspectives promoted by the IGCP to probe one of the last major frontiers in deep seismic exploration.