Halocinese e tectônica de rafts na bacia emersa de Benguela (Angola): dados de campo, modelagem física e restauração estrutural

Este trabalho foi feito com dados de campo da Bacia de Benguela, na costa sul de Angola. Nesta área aflora uma plataforma carbonática de idade albiana, análoga às formações produtoras de petróleo de mesma idade na Bacia de Campos. Duas unidades principais compõem a seção albiana na área: uma unidade inferior caracterizada por carbonatos de águas rasas com intercalações de conglomerados e arenitos na base, totalizando 300 m de espessura total, e uma unidade superior formada por sedimentos carbonáticos de águas mais profundas com espessura de 100 m. Esta seção se apresenta em dois domínios estruturais. O norte é caracterizado por uma plataforma de cerca de 5 km de largura, 35 km de comprimento na direção NE e que se encontra próxima ao embasamento. No domínio sul, essa plataforma se apresenta em fragmentos de menos de 10 km de comprimento por até 5 km de largura, alguns deles afastados do embasamento. Dado que há evaporitos sob essa plataforma, fez-se este trabalho com o objetivo de se avaliar a influência da tectônica salífera na estruturação das rochas albianas. A utilização de imagens de satélite de domínio público permitiu a extrapolação dos dados pontuais e a construção de perfis topográficos que possibilitaram a montagem das seções geológicas. Os dados de campo indicam que a estruturação na parte norte, onde há menos sal, é influenciada pelas estruturas locais do embasamento. Já na parte sul, onde a espessura de sal é maior, a deformação é descolada das estruturas do embasamento. Foi feita uma seção geológica em cada domínio e a restauração dessas duas seções mostrou que a unidade albiana na parte sul sofreu uma distensão de 1,25 (125%) e a de norte se distendeu em 1,05 (105%). Essa diferença foi causada por uma maior espessura original de sal na parte sul, que fez com que a cobertura albiana se distendesse mais em consequência da fluência do sal mergulho abaixo, em resposta ao basculamento da bacia em direção a offshore. A intensidade da deformação na parte sul foi tal que os blocos falhados perderam os contatos entre si, formando rafts. Este efeito foi reproduzido em modelos físicos, indicando que a variação na espessura original de sal tem grande influência na estruturação das rochas que o cobrem. As variações de espessura de sal são controladas por zonas de transferência do embasamento e estas mesmas estruturas controlam os limites dos blocos que sofrem soerguimento diferencial no Cenozoico.

鄂尔多斯盆地西缘中生代构造与地层分析及盆地演化研究

Through the detailed analyses of Mesozoic tectono-stratigraphy and basin formation dynamic mechanism and the styles of different units in the western margin of Ordos Basin(Abbreviated to "the western margin"), while some issues of the pre-Mesozoic in the western margin and central part of Ordos Basin also be discussed, the main views and conclusion as follows: 1. There are three types of depositional systems which are related with syndepositional tectonic actions and different tectonic prototype basins, including: alluvial fan systems, river system (braided river system and sinuosity river system), lacustrine-river delta system and fan delta system. They have complex constitutions of genetic facies. For the tectonic sequence VI, the fan sediments finning upper in the north-western margin and coarse upper in the south-western margin respectively. 2. In order to light the relationship between basin basement subsidence rate and sediment supply and the superposed styles, five categories of depositional systems tracts in different prototype basins were defined: aggrading and transgressive systems tracts during early subsidence stage, regressive and aggrading systems tracts during rapid subsidence, upper transgessive systems tracts during later subsidence stage. Different filling characteristics and related tectonic actions in different stages in Mesozoic period were discussed. 3. In order to determined the tectonic events of the provenance zones and provenance strata corresponding to basins sediments, according the clastics dispersal style and chemical analyses results of sediments in different areas, the provenance characteristics have been described. The collision stage between the "Mongolia block" and the north-China block may be the late permian; The sediments of Mesozoic strata in the north-western margin is mainly from the Alex blocks and north-Qilian Paleozoic orogeny, while the south-western margin from Qinling orogeny. The volcanic debris in the Yan'an Formation may be from the arc of the north margin of north-China block, although more study needed for the origin of the debris. The provenance of the Cretaceous may be from the early orogeny and the metamorphic basement of Longshan group. 4. The subsidence curve and subsidence rate and sedimentary rate in different units have been analyzed. For different prototype basin, the form of the subsidence curves are different. The subsidence of the basins are related with the orogeny of the basins.The beginning age of the foreland basin may be the middle Triassic. The change of basement subsidence show the migration of the foredeep and forebulge into the basin. The present appearance of the Ordos basin may be formed at the late stage of Cretaceous, not formed at the late Jurassic. 5. The structure mode of the west margin is very complex. Structure transfer in different fold-thrust units has been divided into three types: transfer faults, transition structures and intersected form. The theoretic explanations also have been given for the origin and the forming mechanism. The unique structure form of Hengshanpu is vergent west different from the east vergence of most thrust faults, the mechanism of which has been explained. 6. In Triassic period, the He1anshan basin is extensional basin while the Hengshanbu is "forland", and the possible mechanism of the seemingly incompatible structures has been explained. First time, the thesis integrate the Jurassic—early Cretaceous basins of west margin with the Hexi corridor basins and explain the unitive forming mechanism. The model thinks the lateral extrusion is the main mechanism of the Hexi corridor and west margin basins, meanwhile, the deep elements and basement characters of the basins. Also, for the first time, we determine the age of the basalt in Helanshan area as the Cretaceous period, the age matching with the forming of the Cretaceous basins and as the main factor of the coal metamorphism in the Helanshan area. 7. The Neoprotterozoic aulacogen is not the continuation of the Mesozoic aulacogen, while it is another new rift stage. In the Paleozoic, the Liupanshan—southern Helanshan area is part of the back-arc basins of north Qilian ocean. 8. The Helanshan "alacogen" is connected with the north margin of north China block, not end at the north of Zhouzishan area like "appendices". Also, I think the upper Devonian basin as the beginning stage of the extensional early Carboniferous basins, not as a part of the foreland basins of Silurian period, not the collision rift. 9. The controlling factor of the difference of the deformation styles of the north-west margin and the south-west margin is the difference of the basements and adjacent tectonic units of the two parts.

O graben de Palestina: contribuição à estratigrafia e estrutura do estágio rifte na bacia do Araripe, nordeste do Brasil

The Palestina Graben is one of the NE-trending asymmetric grabens of the Araripe Basin. This basin rests on the precambrian terrains of the Transversal Zone, Borborema Province, immediately to the south of the Patos Lineament. It is part of the Interior Basins province of Northeastern Brazil, being related to the fragmentation of the Gondwana supercontinent and the opening of the South Atlantic ocean. The Palestina Graben trends NE-SW and presents an asymmetric geometry, controled by the NW extensional eocretaceous strain. The graben borders display distinct geometries. The SE border is a flexural margin, characterized by the non conformity of the eopaleozoic Mauriti Formation (the oldest unit of the basin) overlying the crystalline basement, but also affected by normal faults with small displacements. On the opposite, the NW border is continuous and rectilinear, being marked by normal faults with major displacements, that control the general tilting of the layers to the NW. In this sense, the Mauriti Formation is overlain by the Brejo Santo, Missão Velha (which also occurs in the Brejo Santo-Mauriti horst, to the NW of the fault border) and Abaiara formations, the latter restricted to the graben. The interpretation of available gravity data and a seismic line indicates that the main fault has a variable dip slip component, defining two deeper portions within the graben, in which the sedimentary column can reach thicknesses of up to 2 km. Regarding to the stratigraphy of Araripe Basin in the study area, the sedimentary package includes three distinct tectonosequences. The Paleozoic Syneclisis Tectonosequence is composed by the Mauriti Formation, deposited by a braided fluvial system. The Jurassic Tectonosequence, whose tectonic setting is still debatable (initial stage of the Neocomian rift, or a pre-rift syneclisis ?), is represented by the Brejo Santo Formation, originated in a distal floodplain related to ephemeral drainages. The Rift Tectonosequence, of neocomian age, includes the Missão Velha Formation, whose lower section is related to a braided to meandering fluvial system, outlining the Rift Initiation Tectonic Systems Tract. The upper section of the Missão Velha Formation is separated from the latter by a major unconformity. This interval was originated by a braided fluvial system, overlain by the Abaiara Formation, a deltaic system fed by a meandering fluvial system. Both sections correspond to the Rift Climax Tectonic Systems Tract. In the area, NE-trending normal to oblique faults are associated with NW transfer faults, while ENE to E-W faults display dominant strike slip kinematics. Both NE and E-W fault sets exhibit clear heritage from the basement structures (in particular, shear zones), which must have been reactivated during the eocretaceous rifting. Faults with EW trends display a dominant sinistral shear sense, commonly found along reactivated segments of the Patos Lineament and satellyte structures. Usually subordinate, dextral directional movements, occur in faults striking NNW to NE. Within this framework bearing to the Palestina Graben, classical models with orthogonal extension or pull-apart style deserve some caution in their application. The Palestina Graben is not limited, in its extremeties, by E-W transcurrent zones (as it should be in the case of the pull-apart geometry), suggesting a model close to the classic style of orthogonal opening. At the same time, others, adjacent depocenters (like the Abaiara-Jenipapeiro semi-graben) display a transtensional style. The control by the basement structures explains such differences

Evidências da herança geotectônica pré-cambriana na geração da barcia potiguar: um estudo geofísico multdisciplinar

The 3D gravity modeling of the Potiguar rift basin consisted of a digital processing of gravity and aeromagnetic data, subsidized by the results of Euler deconvolution of gravity and magnetic data and the interpretation of seismic lines and wells descriptions. The gravity database is a compilation of independent geophysical surveys conducted by several universities, research institutions and governmental agencies. The aeromagnetic data are from the Bacia Potiguar and Plataforma Continental do Nordeste projects, obtained from the Brazilian Petroleum Agency (ANP). The solutions of the Euler Deconvolution allowed the analysis of the behavior of the rift main limits. While the integrated interpretation of seismic lines provided the delimitating horizons of the sedimentary formations and the basement top. The integration of these data allowed a 3D gravity modeling of basement topography, allowing the identification of a series of internal structures of the Potiguar rift, as well intra-basement structures without the gravity effect of the rift. The proposed inversion procedure of the gravity data allowed to identify the main structural features of the Potiguar rift, elongated in the NE-SW direction, and its southern and eastern faulted edges, where the sedimentary infill reachs thicknesses up to 5500 m. The southern boundary is marked by the Apodi and Baixa Grande faults. These faults seem to be a single NW-SE oriented fault with a strong bend to NE-SW direction. In addition, the eastern boundary of the rift is conditioned by the NE-SW trending Carnaubais fault system. It was also observed NW-SE oriented faults, which acted as transfer faults to the extensional efforts during the basin formation. In the central part of the residual anomaly map without the gravity effect of the rift stands out a NW-SE trending gravity high, corresponding to the Orós-Jaguaribe belt lithotypes. We also observe a gravity maximum parallel to the Carnaubais fault system. This anomaly is aligned to the eastern limit of the rift and reflects the contact of different crustal blocks, limited by the eastern ward counterpart of the Portalegre Shear Zone

Caracterização das falhas de transferência na porção norte da Bacia de Santos a partir da integração de dados geológicos e geofísicos

Structures transverse/sub-transverse to the shoreline have been identified and characterized on the precambrian basement outcrop on the continent adjacent to the northern Santos Basin. These structures were analyzed from images of digital elevation model SRTM 90m by extracting NW-SE lineaments that intersect the NE-SW foliation. The lineaments were selected, classified into 48 segments that extend toward offshore, and correlated with basin structures. In the basin 25 2D seismic sections, 12 well logs and data from potential methods were interpreted, identifying the key stratigraphic levels and the major structures. Structural maps of each horizon were generated. Six transfer faults (FTs) were recognized and named FT-1 to FT-6, whose extensions correspond to continental lineaments named FC1 to FC6. The FTs are related to the basin deformation and evolution. In seismic sections, these faults have lateral slip in flower structures, displacement inversions from normal at the top to reverse at the base, abrupt changes in thickness or even disappearance of the seismic reflectors. The structural map of the Basement and Top of the Rift shows control of some depocenters by faults and displacements in some areas. The maps of potential methods indicate that there are pronounced anomaly shifts in some areas, associated with FTs. Some seismic sections indicate reactivation of FTs when they intersect horizons from the basement until the most recent layers. The 3D integration of data facilitated the observation of the FT extensions in the continent discontinuity.

Falhas de transferência da porção norte da Bacia de Santos interpretadas a partir de dados sísmicos: sua influência na evolução e deformação da bacia

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

From point defects to plate tectonic faults

Understanding and explaining emergent constitutive laws in the multi-scale evolution from point defects, dislocations and two-dimensional defects to plate tectonic scales is an arduous challenge in condensed matter physics. The Earth appears to be the only planet known to have developed stable plate tectonics as a means to get rid of its heat. The emergence of plate tectonics out of mantle convection appears to rely intrinsically on the capacity to form extremely weak faults in the top 100 km of the planet. These faults have a memory of at least several hundred millions of years, yet they appear to rely on the effects of water on line defects. This important phenomenon was first discovered in laboratory and dubbed ``hydrolytic weakening''. At the large scale it explains cycles of co-located resurgence of plate generation and consumption (the Wilson cycle), but the exact physics underlying the process itself and the enormous spanning of scales still remains unclear. We present an attempt to use the multi-scale non-equilibrium thermodynamic energy evolution inside the deforming lithosphere to move phenomenological laws to laws derived from basic scaling quantities, develop self-consistent weakening laws at lithospheric scale and give a fully coupled deformation-weakening constitutive framework. At meso- to plate scale we encounter in a stepwise manner three basic domains governed by the diffusion/reaction time scales of grain growth, thermal diffusion and finally water mobility through point defects in the crystalline lattice. The latter process governs the planetary scale and controls the stability of its heat transfer mode.

Micropolar flow in a porous channel with high mass transfer

Two dimensional flow of a micropolar fluid in a porous channel is investigated. The flow is driven by suction or injection at the channel walls, and the micropolar model due to Eringen is used to describe the working fluid. An extension of Berman's similarity transform is used to reduce the governing equations to a set of non-linear coupled ordinary differential equations. The latter are solved for large mass transfer via a perturbation analysis where the inverse of the cross-flow Reynolds number is used as the perturbing parameter. Complementary numerical solutions for strong injection are also obtained using a quasilinearisation scheme, and good agreement is observed between the solutions obtained from the perturbation analysis and the computations.