3D numerical modelling of the Wilson cycle: structural inheritance of alternating subduction polarity

Alternating subduction polarity along suture zones has been documented in several orogenic systems. Yet the mechanisms leading to this geometric inversion and the subsequent interplay between the contra-dipping slabs have been little studied. To explore such mechanisms, 3D numerical modelling of the Wilson cycle was conducted from continental rifting, breakup and oceanic spreading to convergence and self-consistent subduction initiation. In the resulting models, near-ridge subduction initiating with the formation of contra-dipping slab segments is an intrinsically 3D process controlled by earlier convergence-induced ridge swelling. The width of the slab segments is delimited by transform faults inherited from the rifting and ocean floor spreading stages. The models show that the number of contra-dipping slab segments depends mainly on the size of the oceanic basin, the asymmetry of the ridge and variations in kinematic inversion from divergence to convergence. Convergence velocity has been identified as a second-order parameter. The geometry of the linking zone between contra-dipping slab segments varies between two end-members governed by the lateral coupling between the adjacent slab segments: (1) coupled slabs generate wide, arcuate linking zones holding two-sided subduction; and (2) decoupled slabs generate narrow transform fault zones against which one-sided, contra-dipping slabs abut.

SOBRE A EVOLUÇÃO TECTÔNICA DO ORÓGENO ARAÇUAÍ-CONGO OCIDENTAL

Caracterizada, há 30 anos atrás, como cinturão de dobramentos brasilianos que limitaria o Cráton do SãoFrancisco pelo sudeste e sul, a Faixa Araçuaí é hoje entendida como parte do domínio metamórfico externodo Orógeno Araçuaí-Congo Ocidental. Este componente da grande rede orogênica do Gondwana Ocidentalque, na África, compreende a Faixa Oeste-Congolesa e, no Brasil, a Faixa Araçuaí e terrenos adjacentes aleste, possui uma série de atributos singulares. Contornado pelo Cráton do São Francisco-Congo e contíguo, asul, ao sistema orogênico Ribeira exibe, em mapa, uma forma em ferradura e vergências centrífugas, o que aprincípio sugere uma evolução essencialmente ensiálica. Os estudos realizados no Orógeno Araçuaí mostram,entretanto, que geração e consumo de assoalho oceânico constituem fases da sua evolução, como tambémo são vários pulsos de volumosa produção de magmas graníticos a partir de fontes tanto mantélicas, quantocrustais. Analisado do ponto de vista tectônico, o Orógeno Araçuaí-Congo Ociental pode ser subdividido emdez compartimentos, os quais desempenharam papéis distintos no curso de sua história. Dada a sua naturezaconfinada e as funções cinemáticas desempenhadas pelas peças do seu arcabouço, postulou-se a hipótese,ora em fase de teste, de que o Orogeno Araçuaí-Congo Ocidental tenha evoluído a partir de uma baciaparcialmente assoalhada por crosta oceânica - a Bacia Macaúbas, iniciada por volta de 880 Ma - através ummecanismo que lembra a operação de um quebra-nozes. Ou seja, as peças cratônicas do São Francisco e doCongo, articuladas por meio de riftes interiores, mover-se-iam em sentidos opostos por forças de colisõesem suas margens e promoveriam o fechamento da bacia mediterrânea precursora. Ao evento colisionalprincipal, que se desencadeou por volta de 580 Ma, sucederam as fases de escape lateral da porção sul e decolapso gravitacional. Antevê-se que a continuidade do estudo da porção brasileira desta feição orogênica,que constitui um excepcional laboratório natural, trará respostas para muitas questões ainda em aberto nãosó sobre esta, mas também sobre as cadeias de montanhas de um modo geral.Palavras-chave: Faixa Araçuaí, Orógeno Araçuaí-Congo Ocidental, Evento Brasiliano-Panafricano,Neoproterozóico, Gondwana Ocidental. ABSTRACT: The Araçuaí Belt, portrayed by Almeida (1977) as a brasiliano orogenic domain developed along thesoutheastern margin of the São Francisco Craton, is now viewed as part of the external zone of the socalled Araçuaí-West Congo Orogen. This orogen, which also encompasses the West Congo Belt of Africaand the terrain between the Araçuaí Belt and the Brazilian continental margin, exhibits a whole series ofpuzzling features. Confined to a tongue-shaped enclave between the São Francisco and Congo cratons,the Araçuaí-West Congo Orogen involves, besides Neoproterozoic ophiolites, a large volume of plutonicrocks including subduction-related granites. Its evolution is thus associated with ocean floor spreading andsubduction, processes difficult to reconcile with its confined nature. From a tectonic perspective, the Araçuaí-West Congo Orogen can be subdivided in ten compartments, which played distinct rules in the course of itsdevelopment. Considering the peculiar setting it formed and knowing the kinematic function of the mainstructures, the tectonic evolution of the Araçuaí-West Congo Orogen can be best explain by a model thatinvolves the closure of a basin partially floored by oceanic crust – the Macaúbas basin, iniciated around880 Ma - through a mechanism that resembles the operation of a nutcracker. The São Francisco and Congocratons, like pincers of a nutcracker, and articulated along interior rifts (the Pirapora, Paramirim and Sanghaaulacogens), rotated against each other, compressing the Macaúbas basin that lay in-between. The drivenforces for the closure are probably triggered by collisions along the margins of the São Francisco-Congo plateduring the final assembly of West Gondwana. The main collisional stage around 580 Ma was followed by thelateral escape of the southern portion of the orogen and gravity collapse. We anticipate that the continuationof the study of the Brazilian or Araçuaí portion of the Araçuaí-West Congo Orogen, which correspond to anextraordinary natural lab, will bring solution not only for the enigmas presented by this peculiar orogen, butalso for questions related to the anatomy and development of mountain belts in general.Keywords: Araçuaí belt, Araçuaí-West Congo orogen, Brasiliano-Pan African event, Neoproterozoic,, WestGondwana

Titanium geophysics: The application of induced polarization to sea‐floor mineral exploration

Titanium is abundant in the Earth's crust, but it can be economically extracted from only a limited group of minerals, principally rutile [Formula: see text] and ilmenite [Formula: see text], both found mainly in fossil beach‐complex placer deposits. Both minerals have only a weak magnetic susceptibility, insufficient to permit correlation between magnetic surveys and known titanium‐rich deposits. However, ilmenite shows an unusually strong induced‐polarization (IP) response, whereas the IP response of rutile is relatively weak. IP spectral signatures for ilmenite acquired in laboratory and field settings are also distinctly different from those of other polarizing materials, for instance pyrite. A nonfloating, towed‐streamer IP system was designed and deployed in surveys off the coasts of Virginia and Georgia. When the cable lies on the sea floor, calculations indicate that only about 8 percent of the injected current actually finds its way into the underlying sediment. Partly because of this high transmitted‐current to injected‐current ratio, a stationary‐streamer IP noise envelope of about 2–4 milliradians (mrad) phase shift and a towed‐streamer noise envelope of 4–6 mrad were measured. Two surveys were undertaken, one of which covered about 30 traverse km of the Atlantic continental shelf (ACS) and crossed two vibracore sites where geologic control could be obtained. Many IP anomalies were observed, with some ranging as high as 20+ mrad; about one‐third of the shallow bathymetric lows (probable paleochannels) showed anomalous IP results. Modeling suggests that these anomalies may have been caused by significant heavy‐mineral placer bodies containing as much as 20 percent ilmenite. Identification of an anomaly in towed‐streamer (conventional IP) mode data will probably be posibble only if the deposit contains more than 1 to 2 weight percentage of ilmenite. Attempts to use the spectral IP method to identify ilmenite directly with the marine IP streamer in a stationary sampling mode gave equivocal results. Although the spectral IP method appears to work well on land, in the marine application the ilmenite was present in only small quantities at the vibracore sites investigated, and the resulting weak signal was partially masked by the noise present in the IP measurement. To make spectral measurements, the streamer must be positioned accurately over a polarizing source. This task is difficult, because the deposits tend to be made up of numerous discrete, kilometer‐long bodies, perhaps no more than 50 m wide and usually only 5 to 15 m thick. Such deposits cannot be adequately tested by a vibracore survey designed to sample every 300 or 1000 m, even if the survey is augmented with a high‐resolution seismic profile. This work suggests that the large IP response of ilmenite may permit rough quantification of sea‐floor placer mineral sources from a shipborne platform while the ship is in motion. Polarizing mineral species might even be identified by using spectral IP measurements. Applications of the technology to identifying other marine mineral deposits, such as smokers at ocean‐floor spreading centers and cobalt‐rich manganese crusts, are logical extensions of this research.

Sri Lanka in Gondwanaland and the evolution of the Indian Ocean

SummaryThe present position of Sri Lanka and its past fit in Gondwanaland are investigated in relation to the development of the Indian Ocean. The 3 stages in the evolution of the Indian Ocean are shown to be controlled by fundamental NW, NS, and NE Precambrian lineament directions in Sri Lanka and South India. These older lineaments were rejuvenated in the Mesozoic–Tertiary to form oceanic transform faults which determined the phases of Indian Ocean floor spreading.

Continental displacement and expansion of the earth during the Mesozoic and Cenozoic

Most reconstructions of Pangaea, the early Mesozoic supercontinent, assume an Earth of modern dimensions. Such reconstructions produce major geometric and Earth of modern dimensions. Such reconstructions produce major geometric and geological fit inconsistencies particularly in areas such as the Arctic, Caribbean, Mediterranean, and southeast Asia and Indonesia. The ocean floor spreading history of these regions and the adjacent oceans indicates that they have grown by areal expansion since their initiation. In contrast, the various reconstructions of Mesozoic and Cenozoic stages which assume an Earth of constant dimensions, require that these regions, either initially or during their development, should contract in area. The geological evidence from the continental margins and from the Earth’s oceans does not support the amount of subduction, either in whole or in part, required by the constant dimension hypothesis. It is shown that an exact fit of the various continental fragments together to reform Pangaea, which agrees with the geometric and geological matches, is obtained when the value of the Earth’s surface curvature is increased to the point at which the diameter of the globe is 80 % of its current mean value. This corresponds in time to the late Triassic-early Jurassic. It is asserted that the early Upper Jurassic to Recent ocean floor spreading data now available, displayed here in maps, also demonstrate progressive global expansion commensurate with an increase in diameter of 20 % of the Earth’s current mean value. Series of maps employing a zenithal equidistant projection are used to illustrate stages in the inferred development of certain regions during the Mesozoic and Cenozoic according to the ocean floor spreading data. The global expansion deduced from the geometric requirements of the spreading data in these maps permits a much more straightforward reading of the development of ocean basins and associated displacement of continents; one which accords with the field evidence. The inconsistencies seen in constant dimensions reconstructions do not arise. The results are summarized in outline hemisphere maps for which a new cartographic projection has been developed.