Basin-forming processes and the deep structure of the Campos Basin, offshore Brazil

Papers Basin-forming processes and the deep structure of the Campos Basin, offshore Brazil W. U. Mohriak Petroleo Brasileiro S.A., D e p a r t m e n t of Exploration, Avenida Chile 65-S. 1301 20035, Rio de J a n e i r o - R J , Brazil

R. Hobbs BIRPS Group, Bullard Laboratories, University of Cambridge, UK

J. F. Dewey Department of Earth Sciences, University of Oxford, Oxford, UK

Received 26 June 1989; revised 19 November 1989; accepted 20 November 1989 This paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins. The events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors: shear zones, thin layers of intrusive rocks, underplated material, gneissic banding, fluids and heterogeneities in the crust and mantle. Several types of sedimentary basins have been recognized with the aid of deep seismic reflection data: basins formed along active master faults; basins formed along inactive faults; basins with no fault control; basins formed by low angle detachment faults; and basins formed by pervasive pure shear, or an approximately pure shear, where the lower crust has been locally extended by a different amount than the upper crust. The mechanisms of basin forming processes recognized in deep seismic profiles are: (a) simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle; (b) thinning of the lower crust and mantle by ductile processes, convective mechanisms or flow in the mantle and the upper crust by faulting, resulting in an overall pure shear; and (c) thinning of the lower crust associated with phase changes in the mantle or non-conservation of mass. Two end-member models of basin formation involving lithospheric stretching may be recognized in the South Atlantic: simple shear stretching, where only the upper crust is involved in the process and mantle uplift is offset relative to the basin; and pure shear stretching, where uplift of the mantle vertically balances the basin fill. The first mechanism has been suggested for the onshore Tucano Basin. The data obtained for the offshore Campos Basin suggests regional lithospheric stretching and crustal thinning with Moho uplift compensating for sediment accumulation in the basin depocentre. Complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the Campos basin. Keywords: basin formation; lithospheric stretching; Campos Basin, Brazil; seismic reflection profiles

Introduction Among the geophysical methods that have been used in constraining geological models of the deep structure of sedimentary basins, gravity is probably most intensively used (Karner and Watts, 1981; Rabinowitz, 1982). However, it does not determine a definitive geological model. There is an inherent non-uniqueness in modelling sedimentary basins because the distribution of densities and the geometry of the structures in the lower crust are not known, and different configurations might fit the observed anomalies (Hutchinson et al., 1983). Nonetheless, by assuming a realistic density distribution in the crust, it has been possible to constrain geological models and to infer the deep structure of several basins (Talwani, 1976; Keen et al., 1982; Klitgord and Berhrendt, 1979; Hutchinson, 1982; Keen, 1982). The method indicates that passive margin basins show variable crustal sections along the margin, 0264-8172/90/020094-29 $03.00

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Marine and Petroleum Geology, 1990, Vol 7, May

that the relationship between sediment accumulation and thinning of the crust is complex (Beaumont et al., 1982; Dillon et al., 1982) and that some of the basins seem to have a high density layer in the lower crust, whereas others do not (Grow, 1981; Keen, 1982; White et al., 1987). With the advent of deep seismic reflection data processing in the last decade, geodynamic models can be constrained by imaging the lower crust underlying sedimentary basins. A global perspective on the deep structure of mobile belts and sedimentary basins is now possible, given the amount of data that has accumulated during the last ten years. However, some questions are still unresolved, particularly the geometry of major faults, the accommodation of strain in the lower crust and upper mantle, the effects of stretching the lithosphere heterogeneously, and the relationship between shear zones, sedimentary basins

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et a l. attraction of deep seismic profiling is that it combines a and deep seismic reflectors. high resolution with the possibility of imaging the lower These are but a few of the major issues raised in the crust and upper mantle. geological literature during the past few years as a One of the most important conclusions derived by result of processing seismic reflection lines to ten or the use of the deep seismic reflection method in more seconds two-way travel time (TWT). This paper profiling the continental lithosphere is that the Moho analyses some of the published data and discusses may not be an immutable and fiat marker horizon possible factors giving rise to deep seismic reflectors. insensitive to stresses iri the crust, and lacking memory The deep structure of sedimentary basins is used as a for past tectonic events. It seems that very often its guideline to a classification based on crustal geometry topography and magmatism may fade away with time, and basin-forming processes. but, in several instances, the Moho responds to stress The Campos Basin, the most prolific oil province by active deformation, particularly in extensional offshore Brazil, is discussed to outline some of the most regimes. There is some evidence that even in diagnostic features of its structural style and tectonic compressional regimes, deformation may also involve evolution. This paper presents some of the recently the rupturing of the mantle. In a recent paper published acquired evidence and discusses possible models that could explain the observations obtained by the by the ECORS Group (ECORS Pyrenees team, 1988), exploratory activity in the South Atlantic, including it is shown that the deep seismic reflection lines can effectively constrain the myriad of tectonic hypotheses borehole data, gravity, industry seismics and deep that have been proposed for the Pyrenees mountain seismic profiles. belt by several generations of geologists. Coincident seismic reflection and refraction surveys Models for the evolution of sedimentary basins provide an improved understanding of the continental lithosphere. In several regions, it has been shown that The following mechanisms have been suggested to the refraction Moho (corresponding to the region account for the observed subsidence in passive margin where there is a rapid increase in seismic velocity from basins: creep in the lower crust, from elevated land about 6-7 km/s to 8.0-8.5 kin/s) coincides with an masses toward the oceanic crust (Bott, 1971); deep array of parallel reflectors in the lower crust crustal metamorphism, associated with phase changes (Klemperer et al., 1986: Klcmpcrer and the BIRPS in the lower crust (Falvey, 1974); and thermal group, 1987) usually occurring lit time-depths of about contraction mechanisms, following thinning of the crust 7-12 s TWT. either by uplift and erosion (Sleep, 1971; Sleep and The lower crust Characteristically shows strong Snell, 1976), necking of the crust (Artemjev and events and, particularly in extensional regions, they are Artyushkov, 1971), dyke intrusion (Royden et al., predominantly laminated and can be correlated with 1980), or instantaneous fithospheric stretching refraction velocities of about 6.5-7.3 km/s (Mooney (McKenzie, 1978). Several modifications to the latter and Brocher, 1987). Deep seismic reflectors are the model have been introduced to account for specific result of strong impedance contrasts. Their origin is observations in some basins, in particular, two-layer probably associated with onc or a combination of the stretching mechanisms (Royden et al., 1980), finite following causes. flexural rigidity to compensate isostatically the sedimentary loading (Karner and Watts, 1981) and Simple shear in the crust along discrete faults finite rifting times (Cochran, 1983). The models based on thermal contraction seem to The inherited structural grain from ancient tectonic comply with most of the observations in passive margin events is often imaged in the seismic profiles (see, for basins, and a characteristic feature in these basins is example, Brewer et al., 1983). This has been proved by thinning of the crust, as suggested by gravity and diverse boreholes drilled in regions where deep seismic magnetic modelling and substantiated by seismic data. reflectors extend into the upper crust, as, for example, in the Wind River region (Smithson et al., 1979). The Flannan thrust in the northern United Kingdom Deep seismic reflectors (Smythe et al., 1982) probably corresponds to a very deep upper mantle event associated with simple shear. Reflection seismic profiling is the most efficient tool The high reflectivity of the shear zones observed in the employed by the petroleum industry in the search for seismic lines might be caused by magmatic intrusions hydrocarbon accumulations. It provides a reliable along the fault plane, brecciation and mylonitization, image of the structural and stratigraphic relationships fluid percolation, or by hydration of the original rocks. in the top layers of sedimentary basins. Industry seismic sections have time depths of only 5 or 6 s TWT, and, usually, they do not allow the definition of Mid-crustal shears associated with heterogeneous intra-basement structures because the intra-basement crustal stretching upper crustal portion of the lithosphere is usually largely devoid of coherent seismic reflections, although This possibility, as described by Reston and Blundell this may be due to acquisition techniques being (1987), is associated with the common observation of inappropriate for the purpose. The acquisition and dipping reflectors in the slightly stretched regions of processing of continuous seismic reflection lines to at platforms adjacent to basins where major extension has least 1(t s TWT has contributed to a better occurred. The reflectors might correspond to dilatant understanding of tectonic processes associated with Riedel-type shears :formed when the crust is deep structures. The interplay between deep structure, heterogeneously stretched, so that the lower crust is basin formation and surface geology has been studied stretched by a larger factor than the upper crust. with the aid of several geophysical methods (in However, these reflectors are most often imaged in particular gravity and magnetic surveys), but the main platform and intracratonic regions, where the seismic Marine and Petroleum Geology, 1990, Vol 7, May

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Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. crust cannot be explained by this hypothesis alone, quality is rather poor and strongly affected by several because the impedance contrast caused by diffraction events. compositional variation, phase changes or anisotropy in Layering p r o d u c e d by pervasive shear igneous rocks corresponds to only minor values of the reflection coefficient (about 0.01-0.04). Only if there is Oblique layering is commonly observed in the lower considerable hydration in these rocks, as a result of crust in the region beneath the Gulf of Maine fluid percolation along faults, for example, would the (Hutchinson et al., 1987; Hutchinson et al., 1988), an coefficient be high enough (Warner and McGeary, area affected by Palaeozoic compressional events and 1987). Mesozoic extension, and in the south-east coast of the USA (Pratt et al., 1987). These events are probably associated with shear zones in the lower crust and Crustal profiles of sedimentary basins probably related to the Palaeozoic compressional phase. The regional Moho is flat and apparently Gravity and magnetic methods have been employed by truncates the dipping events, suggesting that it several workers to model the deep crustal structure post-dates the observed structures, and is possibly beneath sedimentary basins. The North Sea has been associated with igneous intrusions in the boundary studied by, among others, Donato and Tully (1981) and between the mantle and the crust. Zervos (1987); in the United States, several regions have been studied, including the Eastern Coast Underplating in the lower crust (Hutchinson et al., 1982; Hutchinson et al., 1983) and the Rio Grande Rift in the Basin and Range Province One of the most important consequences of thinning (Daggett et al. 1986). Several regions in the South the lithosphere is the change in pressure that occurs Atlantic have been studied, particularly the S/io Paulo adiabaticalb,, resulting m partial melting of Plateau (Talwani, 1976: Kowsman et al., 1982; harzburgites and other mantle rocks. These melts can Guimarfies et al., 1982) and the south-western coast of migrate upwards, mix with crustal rocks and either Africa (Rabinowitz and LaBrequc, 1979). These extrude or become trapped into the lower crust as workers have all pointed out the almost ubiquitous massive underplated bodies (Cox, 1980; Furlong and mantle compensation for the low density sediments, Fountain, 1986). There is growing evidence that there although with different degrees of Moho uplift and is exchange of material between the upper mantle and crustal structure. Gravity modelling usually requires an the crust during certain geological processes (Keen et input of lower crustal densities intermediate between al., 1982: Keen, 1985: Serpa and Voogd, 1987). The the mantle and the crust: this is also a viable solution in occurrence of thin htyered intrusions near the the modelling of refraction data. The Red Sea crust-mantle boundary and underplated masses have (Cochran, 1983), the North Sea (Sclater and Christie, been suggested bv several workers (see, for example, 198(1: Barton and Wood, 1984: Zcrw~s, 1987; Furlong and Fountain, 1986: kase, 1986; Serpa and Klemperer, 1988) and the Rio Grande Rift (Sinno el Voogd, 1987: ttauser et al., 1987: White etal., 1987" al., 1986) are typical examples of basins characterized Keen and Voogd, 1988). by an underlying mantle uplift according to models based on potential methods and refraction. Gneissic banding Deep seismic reflection is the only method that has Rifts arc formed mainly in areas of older orogenic allowed imaging of the deep crust and upper mantle events, and the pervasive banding of igneous rocks is a underlying several sedimemarv basins throughout the feasible explanation for the laminated appearance of world. The controlling structural clenTents arc imaged the lower crust in regions subjected to differential with a good resolution in somc regions, and it has been melting and recrystalization processes during orogenic shown that some of these [cattlres cxtcnd into the events (Meissner and Wever, 1986). upper mantle. Several normal maslcr faults that bound sedimentary troughs have bccn demonstrated to Fluids under high pressure correspond with earlier thrusts oI a previous orogenic This possibility has been discussed by Brown et at. event (e.g. the Outer Isles "thrust, Brewer and Smythc, (1987) and several other workers (see, for example, 1984). Fyfe, 1986). This hypothesis is associated with the Figure 1 shows different typcs of basins associated occurrence of mid-crustal bright spots that look with different lithosphere responses that can be extremely flat (over part of their length), which is observed with the deep seismic reflection method. similar to that imaged in gas fields with a horizontal Figure I(A) is typical of basins lormed along active gas-water contact. However, any gases that could master faults dipping either towards the continent or occur at this range of depths (15 km or more) would be towards the ocean. The basin prolilc, on a crustal scale, in a supercritical state. Another possibility is that they corresponds to a half-graben where sediments are might represent surface fluids (water or methane) deposited syn-tectonically during the rift phase, and the trapped along a low angle fault or detachment during equivalent seismic reflectors are rotatcd towards the the subduction process (Nelson et al., 1987), or major fault plane. faults that could have been used as ducts during fluid A classical example of this kind of basin in the migration (Nelson, I986). Southern Jeanne d'Arc basin, in the (~rand Banks, Eastern Canada (Voogd and Keen, 1987: Keen and Different lithological composition or degrees o f Voogd, 1988), Deep reflection profiles show a master alteration o f a lithologic type fault extending to Moho depths, although no offset in This possibility is based on the field occurrence of lower the Moho is noticed. The interpreted seismic lines crustal rocks with different theological properties. indicate rapid crustal thinning from the platform However. the bright reflectors observed in the lower towards the depocentre of tlhe basin. The mechanism of 96

Marine and Petroleum Geology, 1990, Vol 7, May

Basin-forming processes of the Campos Basin, Brazil." W. U. Mohriak et a l.

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Figure 1 Schematic diagram showing different types of crustal profiles and sedimentary basins. (A) Basins associated with active, deep-penetrating master faults; (B) basins in which major faults are observed but do not control the subsidence; (C) no major fault control; (D) basins associated with low angle detachments; (E) basins associated with crustal thinning and Mohouplift; and (F) the model envisaged for the Campos Basin, with a veneer of sediments deposited late in the basin evolution, onlapping a region of almost unstretched crust. The Moho uplift underlying the depocentre shows a gentle topography. Antithetic faulting is characteristic of the region where the gradient of crustal thinning changes rapidly

basin formation is associated with simple shear in the crust (ahmg discrete faults) and, probably, penetrative pure shear underneath the depocentre, where the Moho is not clearly imaged. Possible detachments in the lower crust or at Moho depths might have controlled the extensional process of these basins (Tankard and Welsinki, 1987). Another outstanding example of such a class of sedimentary basins is provided by the North Lewis or Minch basin, in the north-western British Isles (Sea of Hebrides). Deep seismic reflectors control the sedimentation in relatively small troughs, with block rotation and reverse drag clearly seen in the seismic sections, an apparent Moho offset by the master fault (Pcddy. 1984), and the existence of deep events in the mantle (the Elannan Thrust). This type of basin can also bc recognized in shallow seismic sections showing active control of faulting on the sedimentation, as. for example, in the Basin and Range Province (Anderson et al., 1983). Figure 1 (B) shows basins formed along master faults with no apparent activity during sedimentation. This model probably applies to basins like the Plymouth Bay

Basin (BIRPS and E C O R S , 1986; Bois el al., 1986; Bois et al., 1987; Cheadle et al., 1987) where deep seismic reflectors apparently bound the sedimentary trough but no major fault control or block rotation in the sedimentary succession is involved, and the basin geometry corresponds to a more or less circular thermal sag. Although the seismic reflectors corresponding to the sedimentary layers are usually fiat-lying, some sections show a marked asymmetry of the basin profile, and some fault control may be present, as suggested by the theoretical model developed by Williams and Vann (1987), which ascribes broad hanging wall synclines to planar faults in which llhe main displacement occurs at the centre of the fault, and the strain dies away towards the surface and the lower crust. Figure I(C) shows basins formed with no apparent fault control. This type corresponds with an intracratonic or interior sag basin, as, for example, the Paris basin in France ,and the Parami Basin in Brazil. There is no clear Moho uplift, and no detachment is observed. Some of them seem to be underlain by a rift, such as the Michigan basin (although the linkage between a rift several hundreds of million years older than the main subsidence phase has not been clarified). while in others, such as the Panami Basin, magmatic processes seem to have been fundamental in their development. These basins are more or less circular in shape, and the subsidence is probably thermal in origin (see. for example. Haxbv c: a/., 1976). The earliest sedimentary deposits outcrop towards the limits of the basin. Figure I(D) shows basins formed along low' angle master faults that detach in the lower crust or mantle. This type of basin can bc considered as an end-member of crustal extension, because it does not result in substantial thinning of the mantle lithosphere under the depocentre of the basin. This model is not supported by seismological evidence (earthquake modelling) which suggests that most faults are planar and have a shallow focus (Jackson, 1987). The Basra and Range province offers several examples of these faults (Wernicke, 1981 Allmendinger et al., 1983) which apparently detach at a depth of about 10-15 km and then move aseismically. The Sevier Desert Basin can be considered as the prototype of this feature. The model developed by Wernicke (1985) implies balancing upper crustal extension by a mantle uplift offset relative to the basin depocentre. The Tucano-Jatob~i basin in Brazil (Ussami et al., 1986) might correspond to a similar situation, although no dccp seismic evidence is awtilable in the area Figure I(E) shows basins formed bv pervasive pure shear in the lithosphere, x~ith crustal thinning. This model can be applied to several passive margin and intracratonic basins w!here no detachment is observed. Some of them seem to bc associated with massive underplating (e.g. Hatton Bank, see White el al., 1987a), whereas others are characterized by great crustal thinning without substantial volcanic extrusion (e.g. the North Sea, see Klemperer, 1988: Pinet et al., 1987a). The Parentif; basin in F'rance provides a spectacular example of such thinning, which probably occurs at the expense of the lower crust (Pinet el al. 1987b; Moretti and Pinet, 1987: Marillier et al., 1988). The North Sea shows a mantle uplift underlying the main depositional trough (Ziegler, 1975" Klemperer, 1988). Two-way travel time decp seismic sections show M a r i n e a n d P e t r o l e u m G e o l o g y , 1990, V o l 7, M a y

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Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. limit of the basin. Post-rift tectonism is also observed in a gentle uplift in time, but when the section is this region. converted to depth, using proper seismic velocities, an upwarping in the Moho is observed, with a magnitude similar to that in the Parentis basin (from about 30 to Contrasting extension basin-forming processes in 20 kin). Thinning of the crust is mainly accommodated the South Atlantic by homogeneous stretching, vertically balancing the Two end-member processes of extension can be depocentre of the Viking Graben. Although, at scales identified for the formation of sedimentary basins: of the order of 10-20 km, the rift is highly asymmetric, simple shear (Wernicke, 1985), and pure shear with a typical half-graben profile, the mantle can be (McKenzie, 1978; Coward, 1986). The structural and characterized by a more or less symmetrical and stratigraphic diversity observed in the continental regional uplift, at scales of about 100 km (Klemperer, margin of Brazil is the result of a complex geological 1988). Alternative models involving low angle evolution during which the subsidence pattern can be detachments in the lower crust and mantle have also linked to extensional processes affecting the been proposed for the North Sea (Beach et al., 1987). lithosphere. Although a number of geological elements Figure I(F) shows the proposed model for the are reflected as a surface expression of deep structural Campos Basin. The diagram shows a very rapid features that underlie the basins, the general rule for thinning of the crust towards one end of the plate, and a the Brazilian passive margin is that the deep structure is gradual and smooth Moho uplift from the depocentre masked by the carapace of sediments associated with towards the oceanic crust. Localized upper crust the high input of detritus derived from the highlands heterogeneities, igneous intrusions or secondary, onshore, in particular, in the south-east coast, where a smaller mantle upwarps in the less extended region, characteristic mountain range is present near the towards the continent, are also possible elements used borders of the Campos and Santos basins. to explain some characteristics of the available data. All offshore basins bordering the continental margin Syn-rift antithetic normal faulting predominates in the of the South Atlantic (Figure 2) show evidence of both region where the gradient of crustal thinning changes rifting and crustal attenuation prior to break-up. rapidly, whereas synthetic normal faulting Subsequent thermal subsidence, evolving over a time predominates towards the centre of the basin. Only interval of more than 100 My, is characterized by a thermal phase sedimentation is observed in the region progressively prograding shelf with thick turbiditic and of unthinned to slightly thinned crust at the western

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98

Marine and Petroleum Geology, 1990, Vol 7, May

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. Atlantic rift basins. Basically, in Early Cretaceous deltaic clastics overlying a shallow-water limestone platform. times, the only tectonic movement that can be In contrast, the stratigraphical and gravity data of the demonstrated is the divergent motion between South onshore Tucano and Jatobfi basins, north of the America and Africa, with crustal thinning and vertical Rec6ncavo Basin in north-east Brazil (see insert map of isostatic adjustments. The model for the Campos Basin Figure 2), suggest that up to 7 km of non-marine assumes stretching of the lithosphere, an initial, sediments were deposited during the rifting stage of the fault-controlled phase of subsidence, and a subsequent South Atlantic, with no apparent extension of the lower phase of thermal subsidence (Mohriak et al., 1987; crust beneath these basins and no subsequent thermal Mohriak, 1988). The regional mantle uplift compensates for the observed subsidence. A general subsidence (Ussami et al., 1986). The Tucano and thinning of the crust, probably by pure shear in the Jatobi basins are characterized by a large negative lower crust, and with some contribution of faulting in Bouguer anomaly (-150mgal amplitude); gravity the upper crust, is indicated by several methods modelling attributed this mass deficiency to the density (Mohriak, 1988), and will be discussed more fully in the differential between sediments and the crust. The following sections. absence of isostatic compensation suggests that they formed by upper crustal extension without complementary extension of the lower crust and Deep structure and regional profiles of the passive uppermost mantle, and no upwarping of the Moho margin basins in the South Atlantic underlying the Tucano basin. The stratigraphic evidence obtained mainly from In the last two decades, several authors have proposed industry wells is also a potentially sensitive indicator of a variety of models for the deep structure along the temporal development of subsidence and provides regional profiles in the South Atlantic passive margin an important parameter to constrain the models of (see for example, Ewing et al., 1969; Nairn and Stehli, basin formation. The subsidence history of Tucano and 1973; Ponte, 1973; Ponte and Asmus, 1976; Leyden, Jatobi basins differs radically from the subsidence in 1976; Mascle and Renard, 1976). Notwithstanding all the other basins along the Brazilian coast, where two the scientific and industrial research in the region, the distinct phases of tectonic subsidence may be boundary between the continental and the oceanic recognized. In Tucano and Jatoba, about 5 km of crust is not known in detail in the Campos Basin, and non-marine Upper Jurassic to Neocomian sediments the type of crust underlying the sedimentary sequence were deposited over a period of about 20 My, and there in the slope and rise is still a matter of debate. Cande is no evidence of any further significant subsidence. A and Rabinowitz (1979) suggested that two negative mechanism for the linked development of these magnetic anomalies observed in the Silo Paulo Plateau onshore and offshore basins was proposed by Ussami, (Santos Basin) might be associated with sea-floor et al., (1986), along the lines suggested by Wernicke spreading, so that oceanic crust of different ages would (1985), in which upper crustal extension in the onshore probably occur in a large area of the Santos and region is connected to, and balanced against, deeper Campos Basin. Rabinowitz and LaBreque (1979) also lithospheric extension beneath the incipient passive suggested the occurrence of oceanic crust underlying margin by intracrustal detachments. The proposed the salt province. mechanism assumes an inhomogeneous, essentially However, there is the following evidence indicating upper crustal stretching without major sub-crustal that the crust underlying the depocentre of the Campos involvement, which results in a short-lived subsidence basin is continental. First, granitic basement rocks have with little or no thermal recovery of the lithosphere. been sampled in some boreholes, as, for example, However, there is another important structural element RJS-94 and CST-001 (which drilled through more than that controls the framework of the Tucano-Jatobil 500 m of basalt overlying the Precambrian basement). basin. The fault system shows opposite dips in different Secondly, sparse refraction data in the Silo Paulo sectors of the basin, with transverse features or Plateau indicated seismic velocities of 5.6-6.5 km/s accommodation zones perpendicular to the fault for the basement (Leyden, 1976). Thirdly, structural trends, in a fashion similar to models proposed for the maps for the basement underlying the sedimentary African Rifts (see, for example, Bosworth, 1985; basin show the same, N E - S W trend as observed in Bosworth, 1987: Rosendahl, 1987; and Castro, 1987). the onshore Mantiqueira Province, composed of Contrasting with the tectonic evolution of these Precambrian rocks. Fourthly, rift sequence sediments onshore basins, the Campos Basin, offshore Rio de are present in deep waters, as shown by the Janeiro (Figure 2), is a rift that evolved into a passive high-amplitude reflectors characteristic of the margin. In the 1971)s, the uplift model was used Neocomian coquinas. Fifthly, as an analogue basin, the extensively to explain the tectonic evolution of several plateau of Pernambuco and Rio Grande do Norte, Brazilian basins (Estrella, 1972; Ponte, 1973). Asmus offshore of the north-east region of Brazil, has and co-workers advocated the model for the Campos provided samples of Precambrian gneisses, identical to basin in several papers (e.g. Asmus and Porto, 1972; those onshore. Kowsman et al. (1982) pointed out that Asmus and Ponte, 1973; Asmus, 1975; Ponte and the magnetic maps in the Silo Paulo Plateau, south of Asmus, 1976: Asmus and Ferrari, 1978; Asmus and the Campos Basin, also show the N E - S W trend Porto, 1980: Asmus, 1982). An essential requisite of observed in the Precambrian continental region, the model was hot-spot activity causing domal uplift confirming that conlinental crust is underlying the and block faulting towards the axis, in a similar marginal sedimentary' basins. These lines of evidence situation to that envisaged by Sleep (1971) and Sleep suggest a probable continental origin for the basement and Snell (1976). underlying Campos and Santos Basin, at least as far as However the geological data now available do not the present day slope. Moreover, the post-Apian give support to extensive crustal erosion in the South Cretaceous pole of rotation for the opening of the Marine and Petroleum Geology, 1990, Vo] 7, May

99

B a s i n - f o r m i n g p r o c e s s e s o f t h e C a m p o s Basin, B r a z i l : W. U. M o h r i a k et al.

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Figure 3 Free-air anomaly map of the south-east margin of Brazil (simplified from Rabinowitz and Cochran 1979). Minus signs represent negative anomalies; plus signs represent positive anomalies. A regional transect C-C' extends towards the Brazil Basin, which is underlain by oceanic crust. A composite seismic section based on line 38-RL-250 extends from the north-west region of the basin to the external limit of the salt diapir province, which probably coincides with the boundary between continental and oceanic crust

South Atlantic is located in the North Atlantic (Rabinowitz and LaBreque, 1979), implying that spreading rates h)r the northern basins should be smaller than the rates for basins close to the equator of rotation. The existence of oceanic crust in the area of the depocentre of the Campos Basin (batymetries less than 500 m) would imply a spreading rate faster than the spreading rate observed in the southern part of the South Atlantic, where the magnetic anomalies are well constrained. The northern and southern parts of the Brazilian eastern margin show different topographic and bathymetric profiles, particularly contrasting gradients, and the tendency for the slope to be narrow and abrupt towards the north-east of Brazil (Moody et al., 1979), where the oceanic crust is much closer to the coastline than in the south. In the south-east, the shelf-edge south of the Vit6ria-Trindade Ridge (Columbia Seamount chain, Espirito Santo Basin) is farther away from the coast, particularly in the Santos Basin. The salt province is also much wider in the southern Santos and Campos than in the northern basins. In the North, the coastline is oblique to regional E - W features, while in the South, particularly in the region of the Campos Basin, there is a tendency for the bathymetry to follow the coastline and ancient Precambrian structures, verging from a N E - S W direction to a more northerly trend in front of the Silo Tom6 Low. The depocentres of the sedimentary basins in this region follow the same trend, with an abrupt change of orientation in the region of the Cabo Frio Homocline (Kumar et al., 1979). 100

Marine and Petroleum

Geology,

1990, Vol 7, M a y

Rabinowitz and Cochran (1979) presented a regional map with free-air gravity anomalies bordering the continental margin of Brazil. A simplified version, contoured at 25 regal intervals, showing the anomalies in south-east Brazil, is presented in Figure 3. This map, associated with the sediment isopachs in the same area (Kumar et al., 1979), shows that the gravity anomalies and the sedimentary isopachs do not correlate for the different basins in the continental margin of Brazil. Regionally, the most striking gravity anomalies are associated with the Columbia Seamount Chain (Vitdria-Trindade Ridge), the Abrolhos Bank, and an alternation of localized positive and negative anomalies probably associated with the sedimentary basins that occur onshore and offshore. Positive anomalies are observed in front of the Sergipe-Alagoas, Jequitinhonha (Bahia Sui), Campos, and the southern part of the Pelotas Basin. The Santos Basin shows no positive anomaly resembling the anomalies observed in the above basins, except for an area in the region of the Floriandpolis Platform, which has a reduced sedimentary thickness. Regional isopach maps show a general and progressive thinning of the clastic sequence towards deeper waters, although an increase in salt thickness is observed in the seismic lines. Figure 4 shows the total sediment thickness variation along line C - C ' (see location in Figure 3) cross-plotted against the backstripped basement depth. The sediment loading was removed from the total sediment thickness by simple Airy isostasy unloading, to show the basement behaviour along the profile. There is a general thinning

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Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. SECTION

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Figure 4 Regional transect C-C' in the South Atlantic, showing the total sedimentary thickness, the gravity anomaly profile and the external limit of the salt province

of the clastic sedimcntary sequence from the depoccntre of the basin toward a region more or less close to the external limit of the salt, with a marked increase in sedimentary thickness beyond that point (Lobo and Ferradaes 1983). The gravity anomalies (free-air and Bouguer), and the external limit of the salt, which possibly coincides with the boundary between continental and oceanic crust, are also plotted. Generally, the total sedimentary thickness decreases progressively as the palaeobathymetry increases, and the basement becomes shallower, although the structural behaviour is much more complicated than the simple tapering-off profile shown above. A real section, extending from the continental margin towards the oceanic crust, is shown in Figure 5. The composite section shows the half-graben to the west of the Badejo High (near boreholc RJS-27 of the seismic line 38-RL-25()) a basement-involved roll-over anticline (near borchole RJS-178 of the seismic line 48-RL-341), the Late Tertiary offlapping section, erosional events beyond the slope, horizontal sediments in the rise and the increased halokinetic activity in deep waters. The section also shows that in the middle of the external region, where the total sedimentary thickness is greatly reduced, there is a substantial increase in the salt thickness, possibly reaching a thickness of about 5 km locally, Huge salt diapirs (exceeding 10 km in diameter) arc observed in the middle of the rise, and several evacuation grabens and peripheral synclines are associated with salt mobility. The clastic section seems to pinch out towards the external limit of the salt province (seismic lines 52-RL-238 and 66-RL-181. Just beyond this line, there is a sudden increase in thickness, and deep water reflection seismic lines show that the reflectors are almost horizontal, with no perturbation at all (see detail of seismic line 66-RL-18 in Fig,ure 6). It is

assumed that they were deposited on oceanic crust, and that their tectonic environment is different from the stretched continent crust that underlies the Campos Basin. A similar feature is observed in the Angola Basin, as shown in Figure 7 (top), which also shows the refraction velocities in a transect extending from the Brazilian to the African coast (bottom). It is apparent in the refraction transect that the lithospheric stretching in the Silo Pau/o Plateau is distributed over a much wider area than in the African side of the rift, suggesting a marked asymmetry of rupturing by emplacement of the oceanic crust closer to the African coastline in this part of the South Atlantic (Mohriak, 19881. A large positive gravity anomaly is identified at the western border of the depocentre of the Campos Basin. The free-air anomaly becomes negative towards the shelf-edge, reaching values as low as -50 regal, and then, towards the Brazil Basin, it levels off, approaching a stabilization around 'values between 0 and -25 mgal. The crustal structure of the basins in the South Atlantic is not known :in detail, although some studies indicate that, for some basins, there is marked thinning of the crust (Ewing et al., 1969: Leyden, 19761. Seismic refraction data is sparse in the region, but the lines available in the Silo Paulo Plateau suggest a normal crust thickness near the coastline and a gradual thinning towards the oceanic crust /(see Figure 7 (bottom)]. Kowsman ei: al. (1~)82) prepared a Bougucr map for the Sio Paulo Plateau area, showing different sectors along the margin. The same strong gravity gradients in the region of Campos basin and the Florianopolis Platform ;are evident, and in the region of the Santos basin the gradients are characterized by the gradual increase in the Bougucr anomaly. Guimaries et al. (1982) presented a gravit} model for the southern part of the plateau, suggesting thinning of the continental crust (with mean density 2.84 g/cm 3) and a transitional crust (density 2?)4 g/cm ~) underlying the deep water region of llhe Santos Basin. The gravity anomalies can be used to deri~e a general crustal model of the line C - C ' which is shown in Fi#ureS. Although a rough approximation (large contour interval, sediments with one density only) the model suggests a rapid thinning of the crust in the platfl~rm region, with a maximum gradient of mantle uplift near the depocentre of the basin.

Deep structure of the Campos Basin This section will discuss thc deep structure of the Campos Basin, based on more detailed gravity models, backstripping, and deep seismic reflection lines. A Bouguer map of the Campos Basin (Figure 9) shows a conspicuous N E - S W alignment and, in the region of the Silo Tome Low, an E - W perturbation in the potential field is apparent. An outstanding positive gravity anomaly is observed in the area of the Badejo High (anomaly B). Other local positive anomalies also occur in the northern part of the basin, such as anomalies I, E and J, which are aligned in the same NE trend of the regional trend of the basin. C and D are local negative Bouguer anomalies trending approximately E - W , and ',hey are associated with local depocentres in the Silo Tomd Low, in the northern part of the basin. The negative anomaly H is associated with a small graben of possible Tertiary age, unrelated to the M a r i n e and P e t r o l e u m G e o l o g y , 1990, Vol 7, M a y

101

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Figure 7 Top: regional profiles across the offshore Angola Basin, showing the salt province and the proposed boundary between the continental and oceanic crust (from Mascle and Renard 1976). Bottom: schematic diagram showing seismic velocities and crustal profile derived from refraction data across the South Atlantic (from Leyden 1976) Neocomian rift phase of the Ca~pos Basin. The Bouguer map of Campos Basin shows that the low density sediments in the depocentre of the basin do not produce strong negative gravity anomalies. On the contrary, we observe huge gravity highs indicating that the resultant anomaly is actually being overcompensated by a mass excess at depth (Mohriak and Dewey, 1987). This section will discuss deep structure features observed in two seismic lines crossing the north-west region of the basin: lines 203-RL-76 and 203-RL-83. The main structural elements in the region are shown in Figure 2 and Figure 3. They correspond to the Pre-Aptian limit of the basin, which corresponds to a

hinge line or fault that confines the Neocomian sedimentation to the offshore region; the Silo Tom~ Fault system, of Tertiary age; and the Badejo High, which is characterized by steep faults dipping towards the continent. The N E - S W aligments of the western border of the Badejo High is interrupted in the area of the Tertiary fault; apparently, the northern segment of the high occurs to the west of the wells RJS-192 and RJS-79. The geological interpretation of the seismic lines 203-RL-76 and 203-RL-83 are shown in Figures 10 and 11, respectively. Both sections show the important system of half-grabens formed in the rift phase (Neocomian), and the regional distribution of the

Marine and Petroleum Geology, 1990, Vol 7, May

103

Basin-forming processes of the Campos Basin, Brazil." W. U. Mohriak et al. i;

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sedimentary sequences, which are accommodated in the basin as a result of structural and stratigraphic combinations (Mohriak et al., 1987). Figure 12 shows the raw data and a seismic interpretation of the western part of the regional line 203-RL-76. No sediments older than Tertiary are observed westwards of the pre-Aptian limits of the basin. The sediment accumulation in that area is probably associated with a thermal sag that is distributed more regionally than the rift-sag, to flexural effects, or to post-Neocomian uplift (Mohriak, 1988), The feasibility of a genetic model for a sedimentary basin has to comply with the stratigraphic evolution of the sediment carapace and ultimately, with the underlying structures that control the subsidence. The deep structure of the Campos Basin was completely unknown until recently, and the mechanism of basin formation was derived from theoretical models that involved crustal uplift as the effective basin-forming process (see, for example, Campos et al., 1974). As discussed earlier, these models are not adequate to explain the sedimentary pattern in the basin and other geological constraints. Although the deep structure is not usually imaged in the shallow sections employed in hydrocarbon exploration, even shallow seismic sections (5 to 6 s TWT) may contribute to the understanding of the intra-basement structures that occur in the Campos Basin. Again we turn to the area of the Silo Tomd Low, where very strong intra-basement reflectors have been mentioned in several internal reports by Petrobrfis. These reflectors show considerable consistency in the seismic sections in the area, and have been mapped in the north-western region between the pre-Aptian hinge line, the Tertiary fault and the western border fault of the Badejo High (Lobe el al., 1983). The original map was based on the interpretation of a seismic grid of industry lines 2 - 4 km apart. The dip lines are oriented N W - S E and E - W , and the strike lines are oriented N E - S W and N - S . A typical example of a seismic line with an intrabasement reflector is shown in Figure 12.

104

Marine and Petroleum Geology, 1990, Vol 7, May

An array of dipping reflectors is observed between 4 and 6 s T W T between boreholes RSJ-058 and the pre-Aptian hinge line. They cannot be explained as multiples because they are dipping against the inclination of the sedimentary layers. The origin and cause of these reflectors are virtually unknown, and their depth of occurrence (about 10 km) prevents any possibility of sampling by industrial drilling. Three possible hypotheses have been devised (Mohriak, 1988): thin layers of intrusive rocks in the Precambrian crust, shear zones with mylonitization along the antithetic faulting that is characteristic of that area, and roll-over anticlines, which have been observed to involve the basement in the upper crust, might also involve the deeper crust. The structural map of these features (Figure 13) shows that the apex occurs at a depth of about 9 - t 0 km and extends for more than 30 km in a N - N E / S - S W direction, with a width of about 10-15 km. Although the general trend of this feature is probably N E - S W , following the regional structures and the gravity anomalies, the poor quality of the seismic data between the northern sector (wells RJS-094 and RJS-225) and the southern sector (wells RJS-177 and RJS-150) precludes the continuation of the anomalous reflectors in the area of the wells RJS-077 and RJS-058. The intracrustal reflectors observed in the north-west area of the basin are probably associated with even deeper crustal features, formed during the crustal thinning caused by the lithospheric stretching that resulted in the rifting of the South American and African continents (Mohriak and Dewey, 1987). The Bouguer map also shows a positive anomaly (anomaly I, Figure 9), that can be attributed to deep causes. The magnetic map shows a very large positive anomaly extending in an E - W direction. This area of the basin (Figure 13) is anomalous in many ways, particularly in terms of the structures present. Major normal faults in the basin usually show a N - S to N E - S W trend, but, besides this universal direction, faults with E - W

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak 42

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industry boreholes are projected along this line. (b) Seismic and geological interpretation of the seismic section 203-RL-76. The Pre-Aptian limit of the basin is positioned at point 8 of the :ransect, to the west of borehole RJS-094. The western portion of the line is characterized by antithetic and synthetic faulting, giving rise to asymmetric grabens filled with Neocomian sediments. A marked instability occurs towards the shelf edge, where huge roll-overs associated with listric faults, detaching on salt, are observed

Figure 10(a) Seismic section 203-RL-76, a regional transect in the basin extending for about 170 km in a NW/SE direction, from the north-west area of the basin to deep waters. Several

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Basin-forming processes of the Campos Basin, Brazi/: W. U. Mohriak et al.

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Figure 11 Seismic and geological interpretation of the seismic section 203-RL-83 and a south-west extension towards the onshore region, where the first stratigraphic well (CST-0]) was drilled, near Cape S~o Tom6, in 1958. The Pre-Aptian hinge line is positioned between point 10 of the section and the CST-01 well. A characteristic antitheticfault controls the Neocomian sedimentary trough to the west of the Badejo High direction have been mapped in the north-western cross-plotted with the seismic line 203-RL-76, shows at region (Lobo et al., 1983). Several half-grabens the western limit of the line (points 0 to 10) a rapid bounded by antithetic faults are characteristic of this increase in the gravity anomaly from about 0 regal to area. The downthrown blocks show maximum offsets about 50 regal in less than 30 km. Towards the ocean, (reaching hundreds of meters) with dip-slip toward the the anomaly flattens and even decreases slightly at continent, and the syn-rift sediments in these about point 25. From this to about point 55, the asymmetric troughs may reach 3-5 kin. The only anomaly shows a gentle increase, and then decreases example of Tertiary diastrophic tectonism in the basin again towards the eastern limit of the line. We observe in the transect across the Campos Basin is also in this region. The largest earthquake offshore south-east Brazil, reaching a magnitude of 4.9, (regional seismic line 203-RL-76) that, once the effects occurred near this region, in 1972 (Assump~fio et. al., of the sediments are removed, the dominant feature 1980). Also, temperature measurements in logging throughout the whole basin is a long wavelength gravity anomaly with a large, positive amplitude. operations (Roos and Pantoja, 1978) indicate that the geothermal gradient is higher than 30°C/km in this The gravity field in the north-western part of the region, while the basin shows a mean geothermal Campos basin may be modelled to express changes in gradient of about 20°C/kin. Moreover, the Campos crustal thickness, assuming a simple distribution of basin is characterized by production of oil about 20-30 ° density in the crust, and a higher density for mantle API, and once more an anomaly is present in the area, rocks. The gravity modelling is usually handicapped by with the production of condensate and gas by the well lack of near-surface control and lack of knowledge of RJS- 150. the densities in the upper part of the crust. This problem has been alleviated in this study by a detailed The pattern of subsidence and gravity anomalies, sectioning of the basin into layers of different densities, coupled with the tectonic evolution, suggest that based on industry borehole data. This allows modelling stretching of the lithosphere, with crustal thinning, may of the deep structure of the basin, attributing gravity be responsible for the origin and development of the anomalies to the high density contrast between the Campos Basin (Mohriak et al., 1987). The study of the mantle and the crust, after backstripping the deep structures in the north-west area of the basin sedimentary layers (see Zervos, 1987). This difference included gravity modelling, backstripping, and deep in density is postulated to be approximately known, as seismics (Mohriak and Dewey, 1987). The gravity most wells that reached basement, either volcanic or anomaly profile (Figure 14 - - top) digitized and

Marine and Petroleum Geology, 1990, Vol 7, May

107

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109

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al.

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Figure 15 Deep seismic reflection line 203-RL-76: (a) uninterpreted and (b) with an interpretation. This profile corresponds to the same part of the basin transect shown in Figure 12. Intra-crustal dipping reflectors extending from 4.5 to 6.0 s TWT in the western half of the section are also observed in industry seismic lines. An array of strong, sub-parallel reflectors is observed between 9.3 and 7.0 s in the western half of the section. The base of these reflectors may correspond to the boundary between the crust and the mantle. Approximate vertical exaggeration is 2.5:1

Precambrian granitic rocks, have shown densities that are around 2.8 g/cm ~, which is acceptable as a mean value for an homogeneous continental crust (see Smithson et al., 1981). The mantle density is usually taken as 3.3 g/cm 3. The gravity model (Mohriak and Dewey, 1987) 110

Marine and Petroleum

Geology,

1990, V o l 7, M a y

suggests a very rapid thinning of the crust from 32 to 20 km in less than 40 km, and a possibly flexural downsagging of the Moho below the depocentre of the basin. The resultant gravity anomaly, obtained by subtracting the effect of the sediments from the observed variation, has been modelled in terms of a

NW

Basin-forming processes of the Campos Basin, BrazE." 14/,U. Mohriak et al. 8E 1 - R J S - 9 4 225

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broad Moho uplift (due to lithospheric stretching and crustal thinning) extending from the coastline to deeper waters, with a long wavelength topography (Figure 14 bottom). The region with the rapid changes in the gradient of crustal thinning coincides with the anomalous area of the S~o Tome Low already described. Several seismic lines in the Campos Basin have been processed to 10 s TWT, with the purpose of understanding some of the deep structures of the basin. This paper discusses the deep seismic reflectors observed in seismic line 203-RL-76. Bacoccoli and

-

-

Aranha (1984) suggested that these features might correspond to a mantle uplift at 30 km depth. Other possibilities include a very large and deep magmatic intrusion of mafic rocks with high density, and underplating as the result of partial melting of mantle rocks during the stretching phase (Furlong and Fountain, 1986; White et al., 1987. The north-western part of the deep seismic line 203-RL-76 (Figure 15, compare with the shallow section in Figure 12, and geological interpretation of the sedimentary sequences in Figure 10) shows a conspicuous array of deep reflectors that have been Marine and Petroleum Geology, 1990, Vol 7, May

111

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. SE£TION 2 0 3 - R L - 7 6 WITH CRUSTAL DENSITY = 2.90 g/cc DISTANCE (kin)

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interpreted as a Moho uplift near the Pre-Aptian limit of the basin (Mohriak and Dewey, 1987). The dome-like appearance of the structure on the seismic line is probably caused by a depression of the Moho towards the deeper parts of the basin, possibly associated with sedimentary loading. The abundant recurrence of laminated reflector zones that appear above the domal structure are probably associated with one or a combination of factors capable of causing deep seismic reflectors, as listed in this paper. We suggest that they might be associated with middle and lower crust hetcrogeneities marking the transition zone between the seismically transparent brittle upper layer and the ductile lower layer of the crust at the time of rifting. The shallower reflections at about 10 km depth are also characterized by an array of strong, subhorizontal to inclined reflections, for which we feel inclined to postulate a probable magmatic origin as thin layered intrusions or shear zones associated with the antithetic faults of the rift phase. Towards the centre of the basin, the deep seismic reflections seem to lose their strong contrast as coherent signals, possibly due to absorption in the upper crust. This phenomena has also been observed in other deep seismic reflection lines throughout the world (see Hobbs et al., 1987). The sediment isopach in the north-western part of the basin does not correspond vertically with crustal thinning, according to the gravity modelling and the interpretation of the deep seismic line. A substantial thinning is modelled between the unstretched crust at the origin and point 1(I of the transect, as shown in 112

Figures 10 and 14, but, in this region, only a thin veneer of post-rift sediments can be identified.

Deep structure modelling with synthetic seismic sections Given the non-unique nature of gravity modelling, a change in crustal or mantle densities will affect the resultant model (see, for example, Hutchinson et al., 1983). In the preceding models, a density of 2.80 g/cm 3 was employed as a mean crustal density. However, the density of the lower crust is not known, and it may vary substantially if magmatic intrusion or underplating has occurred in a sedimentary basin. The effects of changing the mean crustal density to a value slightly larger (2.90 g/cm 3) is illustrated in "Figure 16, which also indicates crustal thinning similar to the previous models, but the apex of the mantle upwarp in the north-western limit of the line would occur in a slightly higher position, at a depth of about 18 kin, and the overall crustal thinning would be slightly larger. Migrated seismic sections are excellent tools to study particular geological models because the processing tends to represent dipping events in their true position and correct dips. An interactive process of generating synthetic seismic sections based on geological models derived from gravity modelling is useful to quantify variations in the deep structure and to compare with the real seismic data. The migration of the western part of the deep seismic reflection line 2(13-RL-76 was performed by using a ray.-tracing program (Warner, 1987; Raynaud, 1988) that is applied to smoothed

Marine and Petroleum Geology, 1990, Vol 7, May

Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak digitized reflectors. The original line interpretation of the seismic section is shown in Figure 17 (bottom), and the depth-migrated section is shown in Figure 17 (top). A mean crustal velocity of 6.0 km/s and a mantle velocity of 8.0 km/s was employed throughout the section. The velocities of sedimentary layers range from 2 to 4 km/s. Assuming the Moho to correspond with the lower boundary of the array of reflectors observed in the section, from about 9 to 7 s T W T , the depth model indicates a mantle upwarp from about 30 km in the NW extremity of the line, to about 20 km at a point corresponding to the distance of 40 km, and a downwarp to about 25 km at a point 60 km from the origin. We also observe that some reflectors caused by diffraction (particularly in the western limb of the array of reflectors) are collapsed during the process. Different geological models are obtained by modelling the gravity data assuming changes in mean crustal density. The models, input to the AIMS seismic package, are plotted with no vertical exaggeration in the following figures, and they are used to derive synthetic seismic sections by using rock velocities ranging from 2 to 4 km/s in the sedimentary section, 6 km/s for basement rocks and 8 km/s for mantle rocks. The contrasts between different gravity models can be weighted qualitatively when the synthetic seismic section is c o m p a r e d with the real data. The geological model assuming densities of 2.8 and 3.3 g/cm" for the crust and mantle, respectively (Figure 18), indicates a rise in the mantle from about 32 km at the origin, to about 22 km at the point where x = 40 km. A sw~thetic section generated with the above model results in the mantle upwarp from l0 s at the

et a l.

origin, to 8 s at x = 40 kin, which is to() low when c o m p a r e d with the real seismic section. The model with crustal density 2.9 g/cm 3 and mantle density 3.3 g/cm 3 results in a mantle upwarp of 7.0 s when x = 50 km, but the origin now is at 11 s (Figure 19). This suggests that the density contrast is acceptable for the region near the apex of the doming, but too low for the region with normal crustal thickness. The model with crustal density 2.8 g/cm ~ and mantle density 3.4 g/cm 3 implies a relativelv small mantle uplift (the Moho would always occur below 2(1 km of depth) and results in a mantle upwarp of ,~.5 s at x = 40 km, and the origin is at 10.5 s (Figure 20). This modelling suggests that the ideal density model would require variaNe densities in the crust, with a lower density differential between the crust and mantle near the apex of the upwarping. This might be a reasonable possibility if there is interlayering of crust and denser rocks in the lower crust, or underplating. However, given the uncertainties of the density and even velocity distribution in the lower crust, a simple model with the mean density of 2.8 g/cm ) for the crust and 3.3 g/cm 3 for the mantle seems satisfactory as a first approximation.

Backstripping versus deep seismic reflectors versus gravity modelling The backstripping method (Watts and Ryan, 1976) may be used to estimate the depth to the mantle by analysing the tectonic subsidence and assuming that the crust was thinned by a factor g during the stretching (see, for example, Sawyer, 1985). Backstripped wells in

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Figure 17 (a) Depth-migrated seismic line line 203-RL-76 (north-west part), with no vertical exaggeration; (b) line interpretation of the north-west part of seismic line 203-RL-76. Velocities employed are 2 - 4 km/s for the sedimentary section, 6 km/s for crustal rocks and 8 km/s for mantle rocks. Possible diffractions that occur between 10 and 20 km from the origin (7-8 s TWT) tend to collapse in the depth section Marine and Petroleum

Geology,

1990, V o l 7, M a y

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Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. the Campos Basin indicate an increase in the stretching factor from values slightly greater than 1.0 in the western region of the basin to values slightly smaller than 2.0 towards its depocentre, indicating a relatively small lithospheric thinning. Figure 21 shows the crustal thinning variation for the transect 203-RL-76 (from about 30-32 km to about 15 km near the depocentre, applying the technique to several boreholes,along the profile, depth-converting the array of reflectors observed in the deep seismic line, and modelling the gravity anomalies in the Bouguer map), shows good agreement, between estimates derived from totally independent methods (Mohriak and Dewey, 1987).

Differential thinning in crustal blocks? An intriguing observation of blocks with different crustal thinning results from the application of the simple crustal and mantle densities as above to the north-west region of the Campos Basin. The Bouguer anomaly along the seismic line 203-RL-83 shows a gentle decrease (of the order of 20-30 mgal), following a steep gradient near the Pre-Aptian limit of the basin. The gravity modelling (see Mohriak and Dewey, 1987) required a thickening of the crust at the extremity of the line, resulting in two thinned crustal blocks separated by almost unthinned crust, in a situation that resembles boudinage. However, the characteristics of the gravity anomaly suggests that the amplitude of the first Moho upwarp might be affected by lateral structures, such as basement highs, and the gravity anomaly could be alternatively associated with upper crust denser rocks or complex density distribution (Mohriak and Dewey, 1987). To evaluate this effect in other lines, the gravity model of a composite line based on the seismic section 38-RL-250, a dip regional transect in the centre of the basin, parallel to line 203-RL-76, was extended as far as the outcrop of the Precambrian rocks in the onshore region, near the city of Campos. Figure 22 shows the gravity (free-air anomaly, derived from the Bouguer map plotted at 1 mgal interval) data for the composite section extending from the coastline to the deep water region, and the crustal behaviour modelled with the simple density distribution in the crust. The section shows a gradual increase in the gravity field, from about-20 mgal near the western edge of the basin, where the Precambrian crystalline rocks outcrop, to a local gravity high (about 20 mgal), 40 km to the east. There is then a characteristic, rapid decrease in the gravity anomaly from the coastline to an intermediate block with 0 mgal at point 25 (50 km from the origin), near the Pre-Aptian limit of the basin (point 26). The gravity anomaly increases rapidly from the Pre-Aptian limit towards the Badejo High (located between points 40 and 47 of the section), and the maximum value of the anomaly (about 70 mgals) coincides with the apex of the High. Beyond the Badejo High (east of point 47), both the gravity and the magnetic anomaly seem to be smoothed. There is a gradual decrease in the free-air gravity anomaly to about 0 mgal at the eastern edge of the section, 170 km away from the outcrop of the Precambrian basement. The regional gravity profile was modelled by backstripping the sedimentary layers with known density, and assuming a mean crustal density of 2.8 g/cm. The section shows different gradients of

crustal thinning: a gradual rise in the Moho from the region where the Precambrian outcrops (unthinned crust assumed to be at 32 kin) towards an onshore point where the Moho seems to rise gently to a local maximum of about 25 km (point 20, about 40 km from the origin of the extended composite line - - see Figure 22) and then there is an abrupt decrease in the gravity anomaly. Eastwards of this anomaly, in the offshore region, local gravity lows (anomalies C and D in Figure 8) correspond to deep troughs in the Silo Tom6 Low, but after removal of the gravity effect due to the low density sediments, a mantle uplift is necessary to explain a residual positive gravity anomaly and the presence of a thick section of syn-rift and post-rift sediments. This local gravity low cannot be explained by an increase in thickness of low density sediments alone. Deep causes are needed and we may postulate a local thickening of the crust, extending from the western limit of sediment accumulation towards a point midway between the Pre-Aptian limit of the basin and the western limit of the Badejo High. Structural maps in the region where the Moho gradient is abrupt show characteristic block faulting in the upper crust, with assymmetric half-graben filled by Neocomian sediments. The mantle rises rapidly in the region where the major trough adjacent to the Badejo High is bounded by an antithetic fault (between points 38 and 40). Between this point and the Pre-Aptian limit of the basin, a major crustal roll-over (hinged high) is observed. From the Badejo High, towards deep water regions, there is an almost imperceptible gradient in the Moho topography. Crustal thinning is implied, nonetheless, by the thicker sedimentary sequence and increased bathymetry. Two structural compartments are suggested to occur in the deep crust: one sector, beyond the Badejo High, is characterized by thinned crust and ahnost flat Moho. Geologically, it coincides with the occurrence of thick, almost unperturbed sediments of the Neocomian rift-phase, which is affected by predominantly synthetic faults with relatively small offsets. The existence of high quality source rocks in this sequence, and adequate geothermal history, associated with effective migration pathways to younger reservoirs in the basin, were favourable to both the generation and accumulation of hydrocarbons. The other sector in the deep crust corresponds with variable gradients of crustal thinning, with rapid perturbations in the Moho topography. Geologically it coincides with a region of the basin characterized by abundant asymmetric half-graben and block-rotated sedimentary sequences. This model assumes, that the mean crustal density is kept constant at 2.8g/cm 3 but, obviously, local intrusions with higher density contrasts or complex density distribution in the crust would affect the gravity profile and result in similar anomalies. Keeping the model as simple as possible, the very gentle crustal thinning (the Moho is rising at an angle of only 10-15 °) would be indicative of thinning in areas with only a veneer of Tertiary sedimentation, as indicated by the seismic lines. The absence of syn-rift sediments in this region indicates that a localized shoulder uplift was present during the stretching or that sedimentary loading in the depocentre induced flexural subsidence in the margin much later in the basin history. The modelling suggests that beyond the first mantle upwarp, the crust is thickened again, and the transition

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Figure 22 (a) Gravity data for the composite line based on seismic section 38-RL-250 and extending to the Precambrian outcrops near Campos, the largest city in the region. (b) Gravity model assumes a mean crustal density of 2.8 g/cm 3. A substantial crustal thinning is suggested by the rapid increase of the gravity anomaly from the origin to the Pre-Aptian limit of the basin (point 20). In this part of the transect, only a thin veneer of post-rift sediments is present. The region between the hinge line and the Badejo High (points 40-47) is possibly characterized by rapid changes in crustal thickness, while, towards deep waters, the crust has been extended more homogeneously. The crust is affected by extensional faulting during the rift phase, and particularly in the western part of the line, there is a marked antithetic fault control on the syn-rift half graben filled with Neocomian sediments. Neocomian source rocks with excellent potential for hydrocarbon generation are present eastwards of the Badejo High in this section. Adequate thermal maturation and efficient pathways for hydrocarbon migration to younger reservoirs have made the Campos Basin the most prolific basin in Brazil

from the unthinned block to the main mantle upwarp would occur very abruptly. This might be associated with an effective greater resistance to stretching in the western part, during the beginning of the rifting, and a subsequent relocation of the rift axis to a more easterly position, where the rheoiogical properties would be more favourable, or to differential thinning of the crust during lithospheric stretching. It is apparent in the gravity modelling and seismic interpretation that thinning of the crust is not vertically balanced by sediment accumulation in the western border of the basin. The gravity model suggests a substantial thinning (from 32 to about 25 kin) as the Bouguer anomaly increases from -20 to 20 mgal in about 40 km along the composite transect based on the seismic line 38-RL-250. However, there is only a very thin carapace of post-rift sediments extending from the Pre-Aptian hinge line to the Precambrian outcrops near Campos.

Summary and conclusions The deep seismic reflection method has significantly contributed to the understanding of the origin of sedimentary basins and other geological features. The events observed at lower crustal depths in seismic reflection data correspond to impedance contrasts

associated with one or a combination of the following factors: shear, thin layers of intrusive rocks, underplated material, gneissic banding, fluids and heterogenities in the crust and mantle. Several types of sedimentary basin can be recognized with the aid of deep seismic reflection data. Among them we recognize basins formed along active master faults, basins formed along inactive faults, basins with no fault control, basins formed by low angle detachment faults, basins formed by pervasive pure shear, or an approximately pure shear, with the lower crust extended by a different amount than the upper crust. Two end-member models of basin formation involving lithospheric stretching are possible: the pure stretching model, where uplift of the mantle vertically balances the sedimentation, and the simple shear stretching, where only the upper crust is involved in the process. The Campos and the Tucano-Jatobfi basins seem to represent the two processes of basin formulation in the South Atlantic. Seismic data and gravity modelling indicate that the passive margin basins in the south-east region of Brazil have a reduced crustal thickness in the depocentre, with a regional uplift from about 30 km in the onshore region to less than 20 km in the depocentre, and probably rising to less than 10 km towards the oceanic crust. The boundary between the continental crust and

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Basin-forming processes of the Campos Basin, Brazil: W. U. Mohriak et al. AIImendinger, R. W. et al. (1987) Overview of the Cocorp 40° N the oceanic crust is not known in detail, but it possibly Transect, Western United States: the fabric of an orogenic coincides with the external limit of the salt province. belt Geol. Soc. Am. Bull. 98, 308-319 This would indicate that crustal extension in the Anderson, R. E., Zoback, M. L. and Thompson, G. A. (1983) Implications of selected subsurface data on the structural Brazilian Santos and Campos basins is distributed over form and evolution of some basins in the northern Basin and a much wider area compared with the African side Range Province, Nevada and Utah Geol. Soc. Am. Bull. 94, counterpart basins. Indeed, although the basins across 1055-1072 the South Atlantic belong to the same family of Artemjev, M. E. and Artyushkov, E. V. (1971) Structure and extensional basins, they are not mirror images of each isostasy of the Baikal Rift and the mechanism of rifting J. Geophys. Res. 76, 1197-1211 other. Important discrepancies in their tectonic Asmus, H. E. and Porto, R. (1972) Classifica~ao das bacias evolution are observed when studied in detail. sedimentares brasileiras segundo a tect6nica de placas The gravity anomaly maps show different patterns Anais do XXVI Congresso Brasilelro de Geologia, SBG, for basins with different tectonic evolution after the Bel@m, 2, 67-90 Asmus, H. E. and Ponte, F. C. (1973) The Brazilian marginal break-up of Pangaea. The Campos Basin is basins, In: The Ocean Bas/ns and Margins." The South characterized by a positive Bouguer anomaly indicating Atlantic (Eds A. E. Nair and F. G. Stehli) Plenum Press, New a high density mass at depth compensating for the low York, USA, Vol. 1 87-132 density of the sediment carapace. Gravity modelling Asmus, H. E. (1975) Controle estrutural da deposiq~o mesozoica suggests a general thinning of the crust, with a small nas bacias da margem continental brasileira Revista Brasileira de Geoci#ncias 5, 160-175 gradient in the onshore region and increasing rapidly at Asmus, H. E. and Ferrari (1978) Hipotese sobre a causa do the western limit of the Bajedo High. Different tectonismo Cenozoico na regi~o sudeste do Brasil Serie compartments in sedimentary basins may be associated Projeto Remac 4, 75-88 with different crustal profiles. This study indicates a Asmus, H. E. and Porto, R. (1980) Diferenqas nos estagios iniciais da evolu(~ao da margem continental brasileira: gradual thinning from the unstretched region to the possiveis causas e implica(~6es ,4nais do XXXI Congresso Pre-Aptian limit of the basin, an abrupt change in Brasileiro de Geo/ogia, SBG, Cambori0, 1,225-239 crustal thickness towards the Bajedo High (possibly a Asmus, H. E. (1982) Significado geotect6nico das fei(~6es hinge zone) and a gentle, smooth Moho topography in estruturais das bacias marginais brasileiras e areas the depocentrc of the Campos Basin, and suggests the adjacentes ,4nais do XXXII Congresso Brasileiro de Geologia, SBG, Bel@m, 4, 1547-1557 possibility of crustal boudinage or differential thinning Assumpqao, M. et al. (1980) Sismicidade do Sudeste do Brasil in cruslal blocks during lithospheric stretching, ,4nais do XXXI Congresso Brasileiro de Geologia, SBG, although localized upper crust heterogenities, igneous Camboriu, 2, 1075-1092 intrusions or different terrancs might also give rise to Barton, P. and Wood, R. (1984) Tectonic evolution of the North the observed gravity anomalies. Sea basin: crustal stretching and subsidence Geophys. J. R. ,4stron. Soc. 79, 987-1022 Large basement-involved antithetic normal faulting Bacoccoli, G. and Aranha, L. G. F. (1984) Evoluq~o estrutural seems to predominate in the region where crustal Fanerozolca do Brasil meridiona/ Relatorio Interno thickness changes rapidly, while towards the Petrobras, Depex, Rio de Janeiro, Brazil depocentrc of the basin, the majority of Beach, A., Bird, T. and Gibbs, A. (1987) Extensional tectonics and crustal structure: deep seismic reflection data from the North basement-involved normal faulting shows small offsets Sea Viking Graben, In: Continental Extensional Tectonics and synthetic dips. "The gravity modelling and seismic (Eds M. P. Coward, J. F. Dewey and P. L. Hancock) Geol. Soc. data also suggest that thinning of the crust does not Spec. Pub. 28, 467-476 correspond vertically with svn-rifl sedimentation in the Beaumont, C., Keen, C. E. and Boutilier, R. (1982) On the north-western part of the Cantpos Basin. The gravity evolution of rifted continental margins: comparison of models and observations 1for the Nova Scotian margin: models suggest substantial thinning of the crust from Geophys. J. R. ,4stron. Soc. 70, 667-715 the Precambrian outcrops towards the Pre-Aptian BIRPS and ECORS (1986) Deep seismic reflection profiling hinge line. However, only post-rift sediments, between England, France and Ireland J. Geol. Soc. London associated with a very late phase of thermal subsidence, 143, 45-52 Bois, C. et al. (1986) Deep seismic profiling of the crust in the are observed in this onshore region.

Acknowledgments We gratefully acknowledge Petrobrfis" support for this study. We thank the board of directors and the Department of Exploration Superintendent, Dr Milton R. Franke, for permission to publish this paper, which was written during a PhD programme at the University of Oxford. The AIMS seismic package was used at the BIRPS Group, Cambridge University, on the Bullard Laboralories VAX l 1/750, to obtain synthetic seismic sections from geological models. We thank Dr Dave Smythe from Glasgow University for kindly providing the line-migration ray-tracing programme. We also thank I)r S. Klemperer for his comments and helpful suggestions. We are grateful for the constructive suggestions made by the two referees.

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