Maarten J. de Wit

Rocas Verdes ophiolites, southernmost South America: remnants of progressive stages of development of oceanic-type crust in a continental margin back-arc basin

Abstract The Mesozoic Rocas Verdes are a group of mafic igneous complexes in the southernmost Andes. They consist of pillow lavas, dykes and gabbros, interpreted as the upper parts of ophiolites formed along mid-ocean-ridge-type spreading centres that rifted the southwestern margin of the Gondwana continental crust, during the onset of spreading in the South Atlantic, to form the mafic igneous part of the floor of a back-arc basin behind a contemporaneous convergent plate boundary magmatic arc. Mafic dykes and gabbros intrude older continental lithologies along both flanks of the Rocas Verdes, and these same leucocratic country rocks are engulfed in the Rocas Verdes mafic complexes. These relations indicate that the Rocas Verdes ophiolites formed in place and are autochthonous. Zircon U/Pb ages, as well as both chemical and lithostructural characteristics of these ophiolite complexes, suggest that the Rocas Verdes basin formed by ‘unzipping’ from the south to the north, with the southern part beginning to form earlier, and developing more extensively, than the northern part of the basin. The Rocas Verdes ophiolites contain a wealth of information about progressive stages of continental rifting during back-arc basin formation, magmatic and metamorphic processes along mid-ocean-ridge-type spreading centres, and as analogues to Archaean greenstone belts.

Variations in the degree of crustal extension during formation of a back-arc basin

Abstract Ophiolite complexes in southern Chile represent the remnants of the mafic portion of the floor of a Cretaceous back-arc basin which widened markedly from north to south over a length of 600 km. Detailed field and geochemical studies of ophiolites in the northern (Sarmiento complex) and southern (Tortuga complex) extremities of the originally wedge-shaped back-arc basin floor, indicate significant north—south differences in the mode of emplacement of basaltic magmas into the pre-existing continental crust, during the formation of the basin. In the northern narrow extremity of the original basin, mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust. In the southern wider part of the original basin, mafic magmas appear to have been emplaced at a localized oceanic-type spreading centre. The observed north—south variations resulted in formation of back-arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic. These variations are interpreted as representing different stages of evolution of a back-arc basin which formed due to a subtle interplay between subduction induced back-arc mantle convection and the release of stress across the convergent plate boundary, possibly related to ridge subduction. Prior to the release of stress, heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in South America. With the release of stress, mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism. Initially, basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks. Progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic-type spreading centre. Observations in southern Chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back-arc convection, and hence the mechanism of emplacement into the crust of mafic mantle melts. The various lithologies observed in southern Chile could also be expected to form during the opening phase of major ocean basins and to currently underlie Atlantic-type continental margins.

Effective elastic thickness of the continental lithosphere in South Africa

We have estimated the effective elastic thickness of the continental lithosphere beneath South Africa using the coherence technique. This involves (1) estimating the coherence between Bouguer gravity anomalies and topography in the spectral domain and (2) comparing the coherence with that predicted by an elastic plate model that flexes under loads placed on, within and beneath the lithosphere. The depth to the base of this elastic layer which gives the best root-mean-square fit between estimated and predicted coherence is the effective elastic thickness (Te). Two major tectonic provinces, namely, the Archean Kaapvaal Craton and the Mesoproterozoic Namaqua-Natal Mobile Belt (which together form the Kalahari Craton in southern Africa), are found to have Te values of 72 km and 38 to 48 km, respectively. There is indication from the coherence data that over the Kalahari Craton, topographic features with equivalent wavelengths less than ∼200–300 km are supported by the rigidity of the lithosphere while features with wavelengths greater than 700 km are compensated. It is implied from the present findings that each of the two South African tectonic provinces can be considered as separate coherent domains. On the basis of geotherms from both provinces, the effective elastic thicknesses obtained point to a lithospheric basal temperature of about 600°C. Geologic and geophysical considerations suggest that the contrast in flexural rigidity of the lithosphere between the two provinces can be attributed to the combined effects of compositional and thickness differences of the lithosphere, as well as variation in present-day asthenospheric heat flow. Our Te estimates of the Namaqua-Natal Mobile Belt are similar to those of the Mesoproterozoic Grenville belt, corroborating tectonic models which suggest that these two belts may once have been part of the same mobile belt. In contrast, the Te estimate of the Kaapvaal Craton fall below average global Archean cratonic values; the reasons for this are not clear.

The onset of interaction between the hydrosphere and oceanic crust, and the origin of the first continental lithosphere

Abstract New continental crust forms above subduction zones through the recycling of hydrated oceanic lithosphere. The most efficient process known for oceanic lithosphere hydration takes place at the submerged mid-ocean ridges where the lithosphere is young and warm, and cools through hydrothermal convection. Such mid-ocean ridge hydrothermal interactions were operative at least as far back as 3.5–3.8 Ga. The apparent absence of preserved continental crust older than 4.0 Ga may reflect the absence of hydrothermal interaction before that time. This model requires that prior to about 4.0 Ga mid-ocean ridges stood above sea level. Our calculations show, however, that on a plate-tectonic early Earth with substantially less continent, realistic higher heat flow, and a volume of sea water similar to that of today’s ocean, Archaean mid-ocean ridges would have remained below sea level. Only with a substantial reduction of surface water would Earth have been able to recycle dry oceanic lithosphere, and thus prevent the present day style of continental crust formation. A 30% reduction of surface water is required to elevate early Earth’s ridges above sea level. This excess water may have been stored in nominally anhydrous minerals of the mantle. Early Earth’s mantle may have released a significant proportion of its initial water only gradually through convective overturn of the oceanic floor. Given realistic ocean-floor creation rates, it would have taken roughly 500 Ma to process the early Earth’s mantle through a MORB generation event if only the upper mantle was involved and considerably longer if whole mantle convection was involved. The inefficiency of water extraction during this process is illustrated by the amount of water apparently present in the source regions for present-day MORB. In this scenario, the Hadean-Archaean transition may mark the time when Earth changed its style of cooling from one dominated by heat exchange directly to the atmosphere to one dominated by heat exchange with the hydrosphere, which still buffers Earth’s heat loss today.

Kaapvaal Craton special volume- An introduction

Kaapvaal Craton Special issue dedicated to three outstanding South African Earth Scientists Rod Green Eddie Kostlin Jock Robey In recognition for their pioneering work and lengthy commitment to unraveling the structure and evolution of the Kaapvaal Craton and the Geology of South Africa. Without their generous and unselfish support, the Kaapvaal Craton Projects would not have happened. ( A ) Total field magnetic coverage of southern Africa, original compiled at 1: 1 000 000. This map is a contribution from Anglo American Corporation Limited, made available for use throughout the duration of the Kaapvaal Craton Project through the offices of E. Kostlin and P. Mostert. ( B )Geological map for southern Africa with main lithostratigraphic units, merged and modified after Key and Ayres (2000) for Botswana; Martini et al. (2002) for South Africa, Lesotho and Swaziland; Van Wyk (1980) for Namibia; Thieme and Johnson (1977) for Zambia; and Stagman (1984) for Zimbabwe. Chronostratigraphic interpretations by Jelsma and Thiart at CIGCES, Department of Geological Sciences, University of Cape Town (2002), and Jelsma at De Beers GeoScience Centre, Exploration Division (2003–2004). Outline of the Southern Africa Craton after De Wit and Thiart. The Kaapvaal Craton Project was a massive six-year collaborative effort by Earth science colleagues from South Africa and the USA to better understand processes that formed Archean continental lithosphere, and to track the subsequent evolution of remnants of such ancient continents, known as cratons, using the rock and mineral record of southern Africa. The project was in progress between 1996 and 2002, but its conception goes back much further than that. In some ways, early collaborations between South African and American geoscientists as far back as the Upper Mantle Projects of the 1960s, and the Special Geodynamics Projects during the 1970s and 1980s were important stepping-stones towards a “big picture” project. The discovery of …