Embryonic Ocean Research Articles

Geochemical dichotomy of Tethyan oceanic mantle and the supercontinent connection: Insights from Os isotopic signatures of mantle peridotites from Naga Hills ophiolite

Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ∼ 5–20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218–0.1266) and γOs values of −3.73 to −0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative γOs values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.

The Merluza Graben: How a Failed Spreading Center Influenced Margin Structure, and Salt Deposition and Tectonics in the Santos Basin, Brazil

AbstractThe relative timing between crustal extension and salt deposition can vary spatially along passive margin salt basins as continents unzip, or as the locus of extension shifts toward the embryonic ocean spreading center. Determining the relative timing of salt deposition, rifting, and seafloor spreading is often problematic due to the diachronous nature of rifting, the ability of salt to fill pre‐existing topography, and the subsequent flow and deformation of that salt. We here use 2D PSDM seismic data and structural restorations to investigate the Merluza Graben, a large rift‐related depocentre located in the southern, most proximal part of the Santos Basin, Brazil, along‐strike of a failed spreading center, the Abimael Ridge. The graben is defined by up to 3.5 km of base‐salt relief along its basinward‐bounding fault and internal base‐salt horsts that are up to 1 km high. This compartmentalizes deformation, producing intra‐graben extensional and contraction salt structures, ramp‐syncline basins, and expulsion rollovers, resulting in a remarkably different salt‐tectonic structural style to that seen in the adjacent areas. We also conduct structural restorations to analyze the spatial and temporal evolution of salt‐tectonic structural styles and the relationship this has to potential prolonged crustal extension in the Merluza Graben. This approach further constrains local variations in the relative timing of rifting and salt deposition, and the impact this has on salt tectonics along the margin. The results of our study can be applied to better understand the tectono‐stratigraphic development of other salt‐bearing rifted margins.

Geochronological and geochemical constraints on the origin of the Yunzhug ophiolite in the Shiquanhe–Yunzhug–Namu Tso ophiolite belt, Lhasa Terrane, Tibetan Plateau

The formation of the Shiquanhe–Yunzhug–Namu Tso ophiolite mélange zone (SNMZ) within the Lhasa Terrane, Tibetan Plateau, is key to understanding the Mesozoic tectonic evolution of this terrane, which remains controversial. We show that the Yunzhug ophiolite in the central segment of the SNMZ formed at ~150Ma, based on U–Pb dating of zircons from a gabbroic sample in a well-developed sheeted dike complex. Geochemically, these mafic rocks are dominated by E-MORB-type compositions, along with minor amounts of rocks with P-MORB-type compositions. The samples also exhibit high εNd(t) values and lack negative Nb and Ta anomalies. Data for all the samples plot within the MORB array on a Th/Yb–Nb/Yb diagram. Therefore, these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean, and not in a back-arc basin as has been previously assumed. Taking into account the regional geology, we propose that the Yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the Lhasa Terrane, which provides new insights into the Jurassic tectonic evolution of the Lhasa Terrane.

Study of Conrad and Shaban deep brines, Red Sea, using bathymetric, parasound and seismic surveys

Red Sea was formed where African and Arabian plates are moving apart. Each year the plates drift about 2.5cm farther apart, so that the Red Sea is slowly but steadily growing hence known as the next coming ocean simply an embryonic ocean. It is characterized by the presence of many deep fractures, located almost exactly along the middle of the Sea from northwest to southeast. Theses fractures have steep sides, rough bottom and brines coming up form on the bottom. Brine deposits are the result of subsurface magmatic activity. They are formed in graben structure as shown by the bathymetric, parasound and seismic studies in the investigated area.

A Jurassic oceanic core complex in the high-pressure Monviso ophiolite (western Alps, NW Italy)

The eclogite-facies Monviso ophiolite in the western Alps displays a complex record of Jurassic rift-drift, subduction zone, and Cenozoic collision tectonics in its evolutionary history. Serpentinized lherzolites intruded by 163 ± 2 Ma gabbros are exposed in the footwall of a thick shear zone (Baracun shear zone) and are overlain by basaltic lava flows and synextensional sedimentary rocks in the hanging wall. Mylonitic serpentinites with sheared ophicarbonate veins and talc-and-chlorite schist rocks within the Baracun shear zone represent a rock assemblage that formed from seawater-derived hydrothermal fluids percolating through it during intra-oceanic extensional exhumation. A Lower Cretaceous calc-schist, marble, and quartz-schist metasedimentary assemblage unconformably overlies the footwall and hanging-wall units, representing a postextensional sequence. The Monviso ophiolite, Baracun shear zone, and the associated structures and mineral phases represent core complex formation in an embryonic ocean (i.e., the Ligurian-Piedmont Ocean). The heterogeneous lithostratigraphy and the structural architecture of the Monviso ophiolite documented here are the products of rift-drift processes that were subsequently overprinted by subduction zone tectonics, and they may also be recognized in other (ultra)high-pressure belts worldwide.

Depth to Curie temperature across the central Red Sea from magnetic data using the de-fractal method

Abstract The central Red Sea rift is considered to be an embryonic ocean. It is characterised by high heat flow, with more than 90% of the heat flow measurements exceeding the world mean and high values extending to the coasts — providing good prospects for geothermal energy resources. In this study, we aim to map the depth to the Curie isotherm (580 °C) in the central Red Sea based on magnetic data. A modified spectral analysis technique, the “de-fractal spectral depth method” is developed and used to estimate the top and bottom boundaries of the magnetised layer. We use a mathematical relationship between the observed power spectrum due to fractal magnetisation and an equivalent random magnetisation power spectrum. The de-fractal approach removes the effect of fractal magnetisation from the observed power spectrum and estimates the parameters of depth to top and depth to bottom of the magnetised layer using iterative forward modelling of the power spectrum. We applied the de-fractal approach to 12 windows of magnetic data along a profile across the central Red Sea from onshore Sudan to onshore Saudi Arabia. The results indicate variable magnetic bottom depths ranging from 8.4 km in the rift axis to about 18.9 km in the marginal areas. Comparison of these depths with published Moho depths, based on seismic refraction constrained 3D inversion of gravity data, showed that the magnetic bottom in the rift area corresponds closely to the Moho, whereas in the margins it is considerably shallower than the Moho. Forward modelling of heat flow data suggests that depth to the Curie isotherm in the centre of the rift is also close to the Moho depth. Thus Curie isotherm depths estimated from magnetic data may well be imaging the depth to the Curie temperature along the whole profile. Geotherms constrained by the interpreted Curie isotherm depths have subsequently been calculated at three points across the rift — indicating the variation in the likely temperature profile with depth.

Asymmetric generation of oceanic crust at the ultra‐slow spreading Southwest Indian Ridge, 64°E

We describe topographic, gravity, magnetic, and sonar data from a Southwest Indian Ridge spreading segment near 64°E, 28°S. We interpret these to reveal crustal structure, spreading history, and volcanic and tectonic processes over the last 12 Myr. We confirm that the crust is some 2 km thicker north of the ridge axis, though it varies along and across axis on scales of ∼10 km and 4 Myr. The plate separation rate remained approximately constant at 13 ± 1 km Myr−1, but half‐spreading rates were up to 40% asymmetric, varying between faster‐to‐the‐north and faster‐to‐the‐south on a 4 Myr timescale. Topography shows a dominant E–W lineation normal to the N–S spreading direction. This is superficially similar to faulted abyssal hill terrain of the Mid‐Atlantic Ridge (MAR), but inferred fault scarps are 3–4 times more widely spaced and have greater offsets. Conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center. They have temporal spacings of ∼4 Myr and are thought to reflect episodic melt focusing along an otherwise melt‐poor ridge. Additionally, there are places, mainly on the southern plate, where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the MAR near 13°N. There is generally more extrusive volcanism on the northern plate and more tectonism on the southern one. Extrusive volcanism has propagated westward from the segment center since 2 Ma. The FUJI Dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block, capped by extrusive rock near the segment center, exposing gabbro via a detachment fault over the Dome and probably exposing deeper crust or upper mantle farther west near the segment end. Magnetic anomalies are continuous along this block. We suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge‐transform intersections.

MANTLE XENOLITHS FROM HARRAT UWAYRID (SAUDI ARABIA): METASOMATIC PHENOMENA, THERMAL HISTORY AND LI-BE-B CHARACTERISTICS

Mantle xenoliths investigated within this study are from Harrat Uwayrid, a mid Miocene to Recent volcanic field in the northwestern part of the Arabian plate. The Red Sea region offers the chance to study the formation of a new embryonic ocean, where continental rifting begins to convert into seafloor spreading. Volcanic activity related to the opening of the Red Sea started about 32 Ma B.P. and continued to recent time. Seafloor spreading in the Red Sea Region approximately commenced at 5 Ma B.P. All investigated xenoliths can be divided into spinel-peridotites (lherzolites, harzburgites, wehrlites) and spinel-pyroxenites (clino- and orthopyroxenites, websterites) and furthermore subdivided into four groups by their XCr [=Cr/(Cr+Al)] content of spinel. Some of these rocks contain additional pargasites in textural equilibrium with the other faces. In addition, most of the xenoliths show also different secondary phases (pargasite, olivine, clinopyroxene, apatite) as result of metasomatic overprints with fluids or melts. One websterite contains Ba-rich minerals like Ba-phlogopite (BaO: 11-13 wt%), Ba-pargasite (BaO: 0.8 wt%) and celsian (BaO: 30-33 wt%). Temperatures and pressures of the samples are in the range of 800 to 1100 °C (2-pyroxene-thermometer; Brey and Kohler 1990) and pressures vary between 10 and 20 kbar (Ca-in-Ol-barometer; Kohler and Brey 1990). Within the thermo barometric data of well-equilibrated xenoliths it was possible to construct a continental model geotherm (heat flow: 80-90 mW/m2) for the mantle beneath Harrat Uwayrid, which is in good accordance to other volcanic fields in this region (McGuire 1987, 1988; Al-Mishwat and Nasir 2004). The elevated heat flow is due to a young heating event of the lithospheric mantle in the north, which has not reached the surface so far. Also it can be suggested that the lithosphere in this region is attenuated. Li, Be and B were studied using the SIMS (secondary ion mass spectrometry, university of Heidelberg). Our results show that the partitioning of Li, Be and B is independent of temperature and pressure. But it appears associated with the kind and degree of metasomatic overprint and xenolith composition. Especially clino- and orthopyroxene of some spllherzolites show a decrease in their Li content from core to rim because of metasomatic reaction with a Li-depleted melt. Be and B display no zoning at all, because of slower diffusion rates. Partitioning of Li, Be and B in cpx vs. ol, opx vs. ol and cpx vs. opx follows a linear trend for all types of xenoliths. Deviation from this trend could be caused by a supply or removal of Li, Be and B without re-equilibration. Li and B are both fluid and melt mobile, Be is nearly immobile in fluids but mobile in melts. Hence fluid-influenced xenoliths are enriched in Li and B, but not in Be. This makes Li, Be and B excellent tracers to detect the kind of metasomatic agents.

REVISITING THE LANZO PERIDOTITE (NW-ITALY): 'ASTHENOSPHERIZATION' OF ANCIENT MANTLE LITHOSPHERE

In this paper, we compiled previous investigations and summarize recent results to show that the Lanzo peridotite records a sequence of deep lithospheric igneous events that largely obliterated the previous subcontinental history. Field, microtextural and trace element data indicate the progressive evolution of an igneous system that forms above an upwelling asthenosphere, most probably during the transition from a rifted system to the establishment of an (ultra-) slow spreading ridge of the Piedmont Ligurian ocean. Pervasive porous flow of melt forming ‘reactive’ spinel peridotites followed by plagioclase peridotite is later replaced by focused porous flow, reflected by a system of replacive harzburgite and dunites. Cooling of the Lanzo peridotite leads to the interstitial crystallization of clinopyroxene and plagioclase, and finally to the formation of gabbro dikes. Thus, the Lanzo peridotites may indicate that extension-driven ‘asthenospherization’ of lithospheric mantle is an important process in the development of embryonic ocean basins.

Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust–mantle relationships

Many mantle peridotites exhumed along ancient and present-day magma-poor passive continental margins, along (ultra-) slow spreading ridges and fracture zones are plagioclase-bearing and generally too fertile to be the residue of partial melting processes alone. Likewise, the associated gabbroic and basaltic rocks are not a priori genetically linked to the underlying mantle rocks. Trace element and Nd isotopic studies in the eastern Central Alps peridotites in eastern Switzerland and northern Italy provide evidence for near-fractional melting and depletion at high pressure in Permian time followed by refertilization of subcontinental mantle by ascending melts at low pressure in Jurassic time. These results suggest regional-scale modification of ancient subcontinental mantle by melt infiltration and melt–rock reaction during incipient opening of oceanic basins. The similar Nd isotopic composition of plagioclase peridotite (ϵNd 160: 7.4–10.6) and associated mafic crust (ϵNd 160: 7.3–9.6) indicates that the liquids, which reacted with the peridotites derived from similar N-MORB type mantle sources. Plagioclase peridotites in magma-poor passive margins may predominantly form as a consequence of diffuse porous flow of melt in the thermal boundary layer above an upwelling asthenosphere and probably represent modified ancient subcontinental mantle. Thus, plagioclase peridotites exhumed in passive margins and possibly in (ultra-) slow spreading ridges may represent magma-poor periods where liquids stagnate in the thermal boundary layer and react with the surrounding peridotites. Once the effects of conductive heat loss dominate over advection of heat from below, diffuse porous flow of melt becomes less important and might be replaced by the formation of gabbro bodies.

Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow

A series of recent papers have indicated that reconstructed bulk compositions of abyssal peridotites define chemical correlations, namely increasing FeO tot and decreasing SiO 2 with increasing MgO, which cannot be produced by simple extraction of partial melts. However, no general consensus exists on the reliability of these trends, because they could be artifacts of the adopted calculations, and on the origin of abyssal peridotites. Specifically, it has been inferred that abyssal peridotite compositions are consistent with combined histories of partial melting and subsequent melt migration which caused either olivine addition, or dissolution and precipitation reaction, by equilibrium porous flow [Niu et al., Earth Planet. Sci. Lett. 152 (1997) 251–265; Asimow, Earth Planet. Sci. Lett. 169 (1999) 303–319]. We report combined bulk-rock and mineral chemical data for ophiolitic peridotites from the Erro-Tobbio (ET) Unit (Voltri Massif, Ligurian Alps), which represent lithosphere remnants of the Jurassic Ligurian Tethys embryonic ocean. These peridotites include sp-lherzolites and sp-harzburgites, and display overall depleted geochemical signature. However, comparison between bulk rock and corresponding mineral compositions reveals that these rocks cannot be residues of simple (equilibrium or fractional) melt extraction. Mineral compositions are similar in all the samples. By contrast, the bulk rock compositions define striking correlations, i.e. increasing FeO tot, Ni, Co, and decreasing Al 2O 3, SiO 2, CaO, Sc, Cr, Yb N, with increasing MgO: the MgO–FeO tot and MgO–SiO 2 correlations are similar to those recognized in abyssal peridotites. Thus, the ET peridotites provide evidence that the above trends are indeed consistent with similar variations in on-land peridotites; also, these trends cannot simply result from progressive melt depletion, because constituent minerals in the different ET samples have rather uniform composition. Calculated bulk modes indicate that the observed chemical variations are coupled to systematic modal changes, namely decrease in cpx and opx, and increase in olivine, at increasing bulk MgO. The ET peridotites also display decrease of the cpx/opx ratio at increasing bulk MgO, and this argues against a process of simple olivine addition. By contrast, some peculiar bulk-mineral compositional features – e.g. (i) nearly constant olivine Mg* [=Mg/(Mg+Fe tot)] values, at increasing modal olivine, (ii) opposite bulk Cr and Ni correlations, (iii) bulk Cr and Yb decrease, and parallel Ni increase, strikingly correlated with progressive modal cpx decrease and concomitant modal olivine increase – are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions). Bulk-rock and mineral chemistry data in the ET peridotites thus indicate that the major element correlations inferred by Niu et al. for the abyssal peridotites are reliable, and most likely result from a combined history of partial melting and melt interaction by reactive porous flow.

Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece): new constraints for the early oceanization phases of the Neo-Tethyan Pindos basin

Abstract The Middle Unit of the central-northern Argolis Peninsula, in NE Peloponnesus (Greece), is composed of several tectonic slices, locally including intact sequences of mafic volcanic rocks topped by radiolarian cherts. Although some of these sequences are Jurassic in age, many of them display a Triassic age based on biostratigraphical evidence. The petrological studies presented in this paper indicate that the Triassic volcanic rocks were generated in a mid-ocean ridge setting, and that they represent the oldest remnants of the Pindos oceanic crust so far recognized in the Subpelagonian zone. On the basis of immobile trace element analyses, two chemically distinct groups of Triassic lavas can be recognized in the various volcanic sequences. One group is represented by transitional-type mid-ocean ridge basalts (T-MORBs) displaying moderate light rare earth element (LREE) enrichment, and incompatible element abundances very similar to those observed in present-day T-MORBs. The other group exhibits a range of characteristics typical of many normal-type MORBs: that is, variable LREE depletion and flat N-MORB normalized patterns of incompatible element abundance. Moreover, many geochemical characteristics indicate that the various N-MORB type volcanic sequences originated from chemically distinct (heterogeneous) sub-oceanic mantle sources. Analogous to similar basalts from ophiolitic mélanges of the Dinaride-Hellenide belt, the T-MORBs from the Argolis Middle Unit are interpreted as having originated from a primitive mantle source variably enriched by an ocean-island basalt (OIB)-type component. In contrast, the contemporaneous occurrence of N-MORBs implies that, during the Mid-Late Triassic, oceanic spreading of the Pindos basin had already reached, at least in some sectors, a quasi-steady state involving only sub-oceanic mantle sources and their partial melt derivatives. Our model for the Triassic opening of the Pindos oceanic basin and its related tectonomagmatic evolution is largely supported by comparison with the Red Sea embryonic ocean, a modern analogous setting.

Sr, Pb Isotopes and REE Analyses of Five Cores of the Red Sea: An Insight into Hydrothermal Input

The formation of the Red Sea was initiated by the separation of the Arabian and Nubian plates and is generally considered to be an embryonic ocean which offers the opportunities to study the transition from a continental to an oceanic rift. Between 16 and 20~ the oceanic crust is continuous, further to the North the axial trough becomes discontinuous and only some isolated deeps contain metalliferous sediments. Hydrothermal fluids flow and could react with the important Miocene evaporite deposits, the recent sediment deposits and the young oceanic crust. The resulting fluids overflow in the deeps and allow the formation of hot, salty, concentrated and reducing brine pools. Sr, Pb isotopic compositions and REE patterns have been determined on sediments and interstitial waters from five cores of the Red Sea sampled during the RedSed cruise in september 1992. Four of these cores were retrived from four deeps located on the axial graben (Atlantis II, Thetis, PortSoudan and Suakin), and one at 10 km from the axial trough. This study yields new constraints on the contribution of the different end-members (biogenic, detritic, basaltic and evaporitic), on the formation of the metalliferous sediments, and on the impact and magnitude of hydrothermalism in these various deeps. The strontium isotopic results of sediment interstitial waters show values similar to those deep water of each site. They are equilibrated with the present brines or with the overlying water for the deep without brines. This argues for water circulation towards the sedimentary column. These waters do not give the Sr isotopic signature corresponding to that present at the time of the deposit. The results obtained on the salt-free sediments trace various sources. Atlantis II deep contains the most metalliferous muds with individualization of sulphide and oxide layers. The sediments of Atlantis II deep present the lowest Sr isotopic composition (0.70702 < 87Sr/86Sr < 0.70817; Fig. 1), a major positive Eu anomaly (Eu/Eu* max = 19.25) and high LREE EOST/CGS CNRS-UMR 7517, 1, rue Blessig, F-67084 Strasbourg cedex, France

New evidence and solution to the Maden complex controversy of the Southeast Anatolian orogenic belt (Turkey)

The volcanosedimentary units of Late Mesozoic-Tertiary age that outcrop in the Southeast Anatolian orogenic belt are commonly referred to as the Maden complex. There is a long-lasting controversy over its definition, age, stratigraphic and structural position, and the origin, and thus, the orogenic evolution. To solve this problem, large strips across the Southeast Anatolian orogenic belt have been studied extensively, and different rock groups which were regarded previously as the Maden unit have been differentiated. Their major characteristics and differences have been identified. The Maden unit sensu stricto is here redefined as a volcanosedimentary succession of Middle Eocene age representing a short-lived back-arc basin which reached the stage of an embryonic ocean. Presently, the Maden group occurs mainly within the lower nappe stack of the nappe zone of the Southeast Anatolian orogen. It rests stratigraphically on an amalgamated nappe package consisting of the different metamorphic tectonic units and, in turn, is overlain tectonically by the upper nappe units.

Crustal structure of rifted continental margins: Geological constraints from the proterozoic rocks of the Canadian shield

Rifts associated with possible Proterozoic continental margins and embryonic ocean basins are characterized by an association of tholeiitic volcanics that have chemical characteristics of MORB's, flysch-like sedimentary rocks and extensive gabbro sill swarms comagmatitic with the volcanic rocks. These rifts are similar in stratigraphy and crustal structure to modern rifts such as the Gulf of California. The similarity of these Proterozoic rifts to modern rifts indicate that the process(es) of rifting on the Earth may have been persistent for the past 2.0 Ga. A variety of other Proterozoic rifts are also present in the Canadian Shield, and workers in the Precambrian must examine each rift on an individual basis in order to determine the tectonic setting of the rift being studied.

Espirito Santo--Pattern of Brazilian Marginal Basins: ABSTRACT

The Espirito Santo basin lies in the eastern continental margin of Brazil, as a member of an assemblage of Cretaceous/Tertiary basins correlatable by close stratigraphic and structural analogies. It is considered as the most complete model of this group of basins, and can serve as a pattern for a study of their origin and evolution. The basin originated by Early Cretaceous tectonism that fractured the original cratonic mass into elongated grabens bounded by normal faults. Immature fluviolacustrine sediments were deposited in this tectonically active rift valley. End_Page 601------------------------------ Sea-floor spreading caused the subsequent widening of the basin, and allowed the first marine transgression that formed a long and narrow embayment--the primitive South Atlantic Ocean. Here, due to restrictive conditions, an evaporitic section was deposited. The combined effect of the enlargement of this embryonic ocean by eastward migration of the Mid-Oceanic Ridge since Late Cretaceous time, and basin tilting due to detumescence of the pre-rift arching, determined the deposition of a thick, predominantly marine section with deltaic sedimentation near the Doce River area, and of shallow-platform carbonates in the central and eastern parts of the basin. This upper marine section, although only weakly structured by the progressively decreasing reactivation of the Early Cretaceous faulting, underwent adiastrophic movements by growth faulting and halokinesis. Late Cretaceous to middle Eocene submarine volcanic activity built up the framework of a 30,000-sq km accretion to the preexisting continental shelf. The process of origin and evolution, and the structural and stratigraphic characteristics of the Espirito Santo basin, cause consideration of the basin as potentially petroliferous, and warrant the exploration program that is being carried out. The accumulated knowledge of the Espirito Santo basin, that reflects the regional situation of the whole Brazilian continental margin, affords an important clue to the study of continental drift of South America and Africa. End_of_Article - Last_Page 602------------