Architecture of African Rifts with Special Reference to the Brazilian Marg In"
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Revtsta Brasi leira de Geocie ncias 18(3):312. setembro de 1988 ARCHITECTURE OF AFRICAN RIFTS WITH SPECIAL REFERENCE TO THE BRAZILIAN MARG IN" 8.R. ROSENDAHL"" The Ease African Rift is composed of zones of linked similar polarity half- graben are often platforms and fluvial half-graben. The key to interpreting seismic data from such clastics may enter these half-graben across such platforms, as areas is recognizing that linking geometries have a direct well as fro m their shoaling sides. bearing on rift basin morph ology. Dip profil es across These concepts are very useful in analyzing rifts such as overlapping, opposing half- graben show two half- graben that the Reconcavo Basin. The style of linking determines the loci face each other. This creates an apparen t full-graben with of fluvial clastic input, which bears on the distribution of some sort of complex axial structure, usually antiformal in reservoi r rocks. Also, the nature of interference character. Such structures have been termed " interference accommodation zones is dynamic - these features evolve with accommodation zones" (INA Z) because they accommodate the adjacent depocenters, usually remaining high relative to the subsidence of the facing half-graben. In some cases, the the depocenters. The stratigraphic and hydr aulic accommoda tion can involve transverse, strike-slip, and consequences of this, combined with the scale o f these compressional tectonics, although the overall stress regime is features and the fact that they merge into platfo rms, make extensional. Depending upon the geometry, INAZ's can be them primary targets for petroleum explora tion in rifts. The very large feat ures and they o ften terminate in platforms, Agua Grande field in the Reconcavo Basin is an exce llent which serve as the access routes for fluvial clastics entering example. On the ot her side of the coin are the isolation rift basins with this type of geometry. Dip profiles across accommodation zones, which usually are poor synrift non-overlapping, oppo sing half-graben, show two exploration targets. Another value o f these riftin g concepts is half-graben facing away from each other. They are usually that they offer new insight into the progression from separated by backbones of relatively unsubsided country rock, continental rift to juvenile ocean basin to passive continental termed "isolation accommodation zones" (ISA Z). These can margin. For example, facing half-graben arrangements, hence be sites of considerable strike-slip faulting. Unlike INAZ's, INA Z's, are a necessary step toward successful rifting, which which usually are not modern-day barriers to fluvial sediment probably begins with juvenile half-graben expressions and dispersal, ISAZ's often act as nearly complete dams ends with the success of one spreading center and throu ghout the history of a rift. Fluvial clastic input to abandonment of one or more others , A related application is half-graben linked in this fashion is mainly from the shoaling the realization that synunetric conjugate rift margins are an or ramping sides. Althou gh it is common for one sense of unlikely outcome of ripping the Tanganyika or Malawi rifts polarity to dominate in African rift zones, there is never an apart. Instead, asymmetry should rule. Most of the rifled exclusive direction of asymmetry and usually no more than terrain probably is left on only one of the margins, in a two adjacent half-graben display the same polarities. This pattern that alternates between the margins along strike. The contracts with the Triassic rift basins of the Eastern U.S., CabindalCampos case is an excellent example. where all but a few half ~ graben have their border fault systems on the west. The difference probably relates to reoccupation of Paleozoic thrust planes in the Triassic case, and lack of one systematic low-angle thrusts in the old, cold, MANUSCR ITO 542 and brittle crust of pre-rifted East Africa. The areas between Recebide em 08 de Janeiro de 1988 • Abstract. Paper presented at the II Simp6sio sobre a Bvolucao do Atlantica Sui. Rio deJaneiro, July 23-25. 1987 .. Project Probe, Duke Univ., Durham, N.C.,U.S.A. Revista Brasileira de Geociencles 18(3):3 13, setembro de 1988 PHYSICAL MODELLING OF EARLY CRETACEOUS CONTINENTAL BREAKUP BETWEEN SOUTH A M ERI CA AND AFRICA' PETER SZATMARI" and JOSE R. AIRES" On the basis of seismic reflection profiles and field and the bottom of the dish outside are uniformly covered, observation of fault kinematics, Szatmari et al. (1985, 1987) before the start of the experiment, by silicone and sand. modified the early Cretaceous pole position proposed by During the experiments, rotation rates of 10"3 to 10"4 degree Rabinowitz & Labrecque to lie near Sao Luis, Maranhao, on per hour are maintained from several hours to several days. the Equatorial Margin of northeasten Brazil. The new pole The experiments effectively recreate ' many of the position. for pre-Aptian time, is in the interior of northeastern structural feactures of South America's Eastern and Brazil, near Lavras da Mangabeira, Ceara, at 38.9°Wand 6.9°S. Equatorial Margins. A structural high forming west .of the The effect of an Euler pole situated within the separating pole along the Equatorial Margin correlates with the continents has been little studied either theoretically or Ferrer- Urbano Santos Arch, while a platelet breaking away experimentally. Such effects include reactivation of ductile from the South American continent corresponds to the wrench zones in the Precambian basement, especially near the Northeast Brazilian Microplate. Along the Eastern Margin, pole, by horizontal movement of crustal and lithospheric betweeen Salvador and Recife, a complex systems of wrench blocks. This movement' opens up isolated rifts and creates faults and tensional fractures form, which correlate extremely local compressive structures between the laterally dislocated well with the fault patte rn of the Sergipe-Alagoas basin. and slightly rotated blocks. Several of the. strike slip faults penetrate deep inland, The present study tests these effects by physical modeling recreating known inland faults and grabens. following techniques developed by Cobbold and coworkers. This study shows the value of physical modeling in the Two layers are used: a ductile lower layer made of a special analysis of complex tectonic problems. silicone and representing the lower crust and portions of the mantle lithosphere; and an upper layer made of sand and representing the upper crust. In the experiments, a thin acrylic sheet cut to the shape of eastern South America is rotated in a MANUSCRITO: 543 round dish about the proposed pole. Both the rotating sheet Reeebldo em 08 de J aneiro de 1988• • Abstract. Paper presented at the II Simpdsio sobre a Evolucao do AtlAntica Sui, Rio de Janei ro. July 23~ 25. 1987. •• Petrob ravcenpes. Cid. Universitaria, Quadra 7. I1ha do Fundao, CEP 2 1912, Rio de Janeiro, RJ. Brasil Revista Brasileira de Geocienci es 18(3):314. setembro de 1988 THE SOU TH ATLANT IC RIFTING' roxo CLAUDIO J. CONCEI<;Ao" , PEDRO VICTOR ZALAN'" and SVEN WOLFF'" The history of the South Atlantic ocean has its origin in the formation of the Sao Paulo Plateau and the origin of basic the Jurassic, nearly 200 Ma. when the African and South magmatism of the Parana Basin. American continents began their separating process. From A tentative comparison will be between some observed this process, know n as the rifting phase. developed nearly facts along the evolution of the Atlantic ocean and its possible always in a tensional tectonic environment, a framew ork was causes in the rifting phase. Consequently, proposals will arise defined that has influenced the entire geology of this ocean. for an explanation of the limits of the evaporitie basin on the A model of evolution for the rifting phase is proposed in west coast, depositional limits of the basins; uplifting of the this paper. which includes the acumulation stanges of energy Serras do Mar and Mantiqueira in the Tertiary, etc... accompanied by crustal stretching (ductile behavior), followed Finally, several evidentes related to the age of the first by stages of energy release through faults (britt le behavior). manifestations of the rifting of the South Atlantic will be The possible sedimentary environments involved in earch of discussed. these stages will also be discussed; The importance of the large crustal discontinuites in the development of the rifting, acting or not as energy release zones, is anot her topie to be mentioned. Emphasis will be MANUSCRITO: 544 placed on the Curitiba-Maringa fault zone, its influence on Recebfdc em 08 de Janeiro de 1988. * Abstract. Paper presented at the II Simpdsio sobre a Bvolucao do Atlanrico Sui, Rio de Janeiro, July 23-25, 1987 . ** PetrobnWCenpes. Cid. Universitaria, Quadra 7, lIha do Fundao. CEP 21912. Rio de Janeiro. RJ, Brasil. *** PetrobnislDepex, Av, Republica do Chile, ns 65, CE P 20035 . Rio de Janeiro. RJ, Brasil. T HE NORT HEASTERN BRAZIL A ND GABO N BAS INS : A DOUBLE RIFTING SYSTEM ASSOCIATED W ITH M U LT IPLECRUSTAL DETACHM ENT SURFACES AUGUSTO C. M. CASTRO Jr." Analysis of structural stratigraphic and gravimetric data Early Cretaceous reconstruct ions of the Sout h Atlantic from the Reconcavo, Tucano, and Sergipe-Alagoas basins in Ocean. nort heaste rn Brazil and the Gabon basin in Africa shows that Interpretation of the regional geologic sett ing suggests that these basins originated in a double rifting system associated the double rifting system and the point of final continental with multiple crustal detachment surfaces, and that the rupture are controlled by the pre- existing struc tural direction of dip of the detachment surfaces was reversed at orientation of the tectonic provinces in the Precambrian the Vaza-Barris tra nsfer fault. This geometty is in agreement basement.