Romanche Fracture Zone Research Articles

Seismic geomorphology of the deepwater sediment dispersals systems in the drift phase of the Mundaú sub-basin, equatorial margin, Brazil

The Ceará Basin, located in the Brazilian Equatorial Margin, represents an exploratory frontier situated in a complex structural framework. The basin observed a dynamic interplay of extensional, transtensional, and transpressional regimes, registered by several sedimentary intervals and unconformities poorly documented by wells, especially at the deeper portions. In this study, based on 3D seismic and well data from the Mundaú Sub-Basin, we interpreted the main seismic sequences of the drift phase to build a Relative Geological Time (RGT) model, based on a semi-automatic seismic traces correlation matched with the well top formations. A seismic attribute analysis from key seismic surfaces extracted from the RGT, highlights the main sediment dispersal routes revealing a significant change after the Mid-Eocene, once sedimentation overpassed a topographic barrier created by fault reactivations, in a transpressional regime, associated with the Romanche Fracture Zone. The integration of seismic and well data affords a deeper understanding of basin dynamics, revealing how sedimentary processes respond to shifts in tectonic stress in this portion of the basin.

Cretaceous to Recent tectono‐sedimentary history and subsidence of the Barreirinhas, Ceará and Potiguar Basins, Brazilian Equatorial Margin

AbstractStratigraphy along the Brazilian Equatorial Margin is a crucial guide to the geodynamic history of rifting of Pangea and formation of the South Atlantic Ocean. Understanding the evolution of the Brazilian Equatorial Margin, which intersects the Saint Paul and Romanche Fracture Zones on the western margin of South Atlantic Ocean, is also key for reconstructing eustatic histories and natural resource exploration. In this study, we quantify the stratigraphic and subsidence histories of three sedimentary basins—Barreirinhas, Ceará, Potiguar—that sit within the margin. Stratigraphy was mapped using ca. 900‐line‐km of two‐dimensional seismic data. Biostratigraphic and check‐shot data from 23 wells drilled on the continental shelf, slope and in the distal parts of these basins were used to date and depth‐convert stratigraphy. Check‐shot data were also used to parameterise compaction. The mapped stratigraphy was backstripped to calculate subsidence histories for the basins. Subsidence curves were decompacted, water‐loaded and corrected for palaeo‐water depths using biostratigraphic data from well reports. The mapped stratigraphy of the Barreirinhas and Ceará Basins and theoretical subsidence curves indicate that stretching factors did not exceed 1.6. These values suggest that these basins can be regarded as failed rifts. In contrast, more distal stratigraphy mapped in the Potiguar Basin to the south indicates that it stretched by a factor of 5–6. Calculated subsidence histories indicate that this basin formed primarily because of Cretaceous rifting and Cretaceous to Recent post‐rift thermal sag, with amplitudes governed by the amount of initial stretching.

Antarctic Bottom Water in the Vema Fracture Zone

AbstractA section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41° W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3°С to 1.6°C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (∼30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows.

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Oceanographic data from measurements in deep-water channels of the Atlantic Ocean were analyzed. Data from two different methods were compared: CTD/LADCP measurements from research vessels and data from autonomous mooring stations. The comparison showed that discrepancies between the data obtained by two different methods can significantly exceed the instrumental accuracy of the instruments. This result highlights the importance of correct selection of mooring station locations in conditions of significant spatial gradients in measured characteristics. The results of the analysis showed that throughout the Antarctic waters pathway, the measurement differences are of the same orders of magnitude. Therefore, the conclusions can be valid for all abyssal channels with intense AABW flows.

Application of deep learning for seismicity analysis in Ghana

We present the characterization of regional seismicity in Ghana by processing the Ghana Digital Seismic Network (GHDSN) data set recorded between September 2012 and April 2014, implementing deep learning (DL). Local earthquakes are detected in this dataset using EQTransformer, a DL model with a hierarchical attentive mechanism (HAM) for simultaneous earthquake detection and P- and S-phase picking. A Conservative Strategy (CS) is devised to detect the missing phases and to associate the detected phases to circumvent the false-negative issue of EQTrans-former processing low signal-to-noise ratio (SNR) seismograms. We performed a joint inversion by grid search in 1D velocity model space and simultaneous inversion for the hypocentral parameters, incorporating 559 detected arrival times (292 P and 267 S phases). The results obtained by velocity inversion contain thicknesses of 1, 13, 8, 13, and 10 km, from the surface to a depth of 45 km, with Vp = 5.9, 6.1, 6.3, 6.5, 6.9, and 7.2 km/s, respectively. The updated velocities for the first and last layers are 6% and 26% and the Vp/Vs (=1.70) is 3.03% higher than the previously reported values. A total number of 73 earthquakes with a local magnitude of 2.5 < Ml < 3.9 are located, comprising four main clusters of events, showing a high correlation with the mapped faults zones. The hypocentral depth distribution is mainly in the range of 7-15 km, confined to the upper crust in the region. No specific seismic activity in the eastern branch of Coastal Boundary Fault (CBF) and the continuation of Romanche Fracture Zone (RFZ) in the study period was observed, casting further doubt on the activity of this branch and the hypothesis of stress transfer from RFZ to southern Ghana. The results reinforce the intraplate nature of the tectonic activities in the region. Finally, an updated seismic catalog up to April 2022 is presented for Ghana by incorporating all reported catalogs and combining the newly detected events.

Multiple Abyssal Jets Flowing Into the Vema Deep, Romanche Fracture Zone

AbstractStrong acceleration of abyssal flows in narrow deep‐water channels and fracture zones is a key feature of bottom circulation in the Atlantic Ocean. In the Equatorial Atlantic, these bottom currents transport Antarctic Bottom Water (AABW) over the Mid‐Atlantic Ridge from west to east. The main pathway for Antarctic waters in this region is the Romanche Fracture Zone. The deepest point of this fracture zone is the Vema Deep; its maximum depth based on the published data is 7,856 m. This deep basin is filled by waters of Antarctic origin overflowing a sill in a narrow channel. During the expedition on the R/V Akademik Ioffe (August 2022), we revealed a strong flow in the middle part of the Romanche Fracture Zone and collected new data on thermohaline and kinematic structures of this gravity current. Our survey is the first observational evidence of the intense flow into the Vema Deep. At the sill, the flow splits into branches flowing through three distinct channels of intricate configuration located in the southern transform valley of the fracture zone. The northern channel is proved to be the main pathway of the coldest and densest bottom waters to the Vema Deep. We also found that vertical structure of the flow is presented by two individual jets, namely the deep and bottom jets. The total transport of AABW through the Romanche Fracture Zone at this location was estimated at 1.40 Sv; the velocities exceeding 10 cm/s were found at depths greater than 5,000 m.

Continental-scale structural heritage from rift extension to postrift inversion: Implications for the central Brazilian Equatorial Margin evolution

The link between major basement anisotropies and basin evolution is crucial for understanding the role of tectonic inheritance on continental margins. However, it is still a matter of debate how and why it happens, especially in the Equatorial Atlantic, where the interplay between Precambrian fabric and the shearing of major oceanic fractures has been studied. Here, we investigate the offshore Ceará Basin and its basement in the central part of the Brazilian Equatorial Margin to (1) map basement anisotropies underneath marginal basins and show how Precambrian crustal domains influenced basin evolution; (2) determine the deformation style of reactivated basement faults; and (3) assess the interplay between reactivated faults and the oceanic Romanche Fracture Zone. The combined geological and geophysical data show that reactivation of NE-SW to ENE-WSW continental-scale ductile shear zones that acted as crustal weakness zones started in the Early Cretaceous under a normal stress field. Stress field inversion occurred in the Barremian during the continental breakup when the South American plate started to became an independent intraplate area under a strike-slip stress regimes. The shear zones arrest at the E-W-striking Romanche Fracture Zone, which marks the continental-oceanic boundary. The major fault offsets occurred along the Transbrasiliano Shear Zone, which represented a collisional boundary between paleocontinents in the Neoproterozoic. Fault reactivation also occurred in the eastern boundary of the Ceará Terrace marginal ridge along the Transbrasiliano Shear Zone, uplifted during tectonic inversion. We conclude that fault reactivation mainly occurred between contrasting Precambrian terrains separated by ductile shear zones. Additionally, the folds associated with the Romanche Fracture Zone were mainly generated along reactivated faults that affected syn- to postrift units on the Equatorial Margin.

The Barreirinhas Basin internal architecture and the evidence of transform movements along the Romanche Fracture Zone, Brazilian Equatorial Margin

The Barreirinhas Basin, located in the central part of the Brazilian Equatorial Margin, was originated by Pangea breakup during Early Cretaceous time. The Romanche Fracture Zone played an essential role during the entire tectono-stratigraphic evolution of the Barreirinhas Basin. This fracture zone divided the basin in two sub-basins to the south and north with distinct structural style printed on the basin internal geometry. This study is aimed to investigate how the tectonic and sedimentary evolution in the onshore, shallow and deep-water Barreirinhas Basin, were influenced by the Romanche Fracture Zone. Eight 2D seismic sections, subjected to seismic attributes to enhance the seismic stratigraphic horizons and structural features, and supported by five well profiles, allowed analyzing the basin infill, major faults, and the spatial relationship with the fracture zone. The basin evolution and the tectono-stratigraphic architecture are marked by structures from rift to drift stages. The rift sequences are composed of four seismic units (U1-U4) delimited on top by the horizon R4. The sag and drift sequences are composed of six seismic units (U5-U10) delimited between horizons R4-R10. The structural architectures are dominated by transtensional regimes, such as negative flower faults, tilted blocks, and wipeout zones, whereas transpressional tectonics is also present by reverse faults and positive flowers, which are fingerprints of Romanche Fracture Zone. Especially in the southern sub-basin, the transform tectonic activity deformed the entire basin from Aptian to Holocene. These structures correspond to those in Africa conjugate basins, where recurrent dextral strike-slip plate motions caused transtensional and transpressional deformations in the sedimentary pile since separation of South America and Africa. Asymmetric basin architecture, intensity and asynchrony of tectonic regimes distinguish the Atlantic Equatorial basins.

Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments.

The Romanche fracture zone influences the segmentation of the equatorial margin of Brazil

This study addresses the crustal segmentation of the Brazilian equatorial margin (BEM) controlled by the Romanche fracture zone (RFZ). It uses a combination of satellite and marine gravity anomalies and seismic reflection data to constrain the continent-ocean crust transition. We propose a simplified evolution model of the BEM that involves dextral strike-slip plate motion along the RFZ in an oblique margin transform setting, which caused differential extension of a 31 km thick continental crust and the subsequent generation of a 5.0–10 km thick oceanic crust in the early Cretaceous. As a result, the trend of the coastline is controlled by two main directions: NW-SE and E-W. Two NW-SE-oriented segments, which are related to the oblique spreading centers, formed during the initial generation of oceanic crust adjacent to the margin. The western segment exhibits an ~160-km-wide hyperextended crust orthogonal to the shoreline, with a constant thickness of 10 km. In contrast, the eastern segment exhibits a 90-km-wide continental crust and a 13 km-thick oceanic crust. The central E-W-oriented segment is related to the RFZ and presents a sharp and steep boundary between the continental and oceanic crusts. The breakup geometry and shoreline orientations and the subsequent oceanic spreading centers are oblique (~40°) to the transform. In the Santonian (~83.5 ± 8 Ma), when the transform margin reached its passive stage, isochron 34 became orthogonal to the transform. At present, the RFZ forms well-defined ridges in the equatorial Atlantic that rise to ~2.5 km above the surrounding basins. These ridges and a series of NW-SE-oriented seamounts acted as a topographic barrier and prevented the deposition of a significant volume of sediment sourced from the continent. In addition, neotectonic reactivation of the fracture zone with a major normal component along the continental-oceanic lithospheric boundary (COB) indicates that the RFZ is a long-lived fault.

Preliminary Investigation of Seismicity in Parts of North Central Nigeria, Using High Resolution Aeromagnetic Data

Qualitative analysis of Aeromagnetic data of parts of north central Nigeria had been carried out with the aim to delineate seismic prone areas. The study area is bounded with latitude 9. 00o to 10. 00oN and longitude 7.00o to 9.00oE with an estimated total area of 24,200 km2. Vertical derivatives and upward continuation filters were used to enhances long wavelength anomalies which could give preliminary information about the magnetic structures present in the study area. The total magnetic intensity map shows both positive and negative anomalies with susceptibility ranging from 33487.7 nT to 33800.9 nT. The high magnetic susceptibilities dominated in the basement region around the north-eastern and north-western parts of the study area which corresponds to Naraguta, Jemma and Kafanchan area. Based on the geology of the area this is attributed to granite, schist and migmatite rocks. The low magnetic values are made of sediment deposition also dominates the south-western part of the study area, corresponding to Abuja and Gitata. The area with magnetic susceptibility values ranging between 33506.6 nT and 33653.8 nT indicates alluvium deposite around Bishini and Kachia area. Lineament from First Vertical Derivative trend in the northeast-southwest and east-west directions, which is an extension from Romanche Fracture Zone. Majority of this lineament trends in the same directions as the Romanche Fault Line which continues at 25 km and 30 km into n inferred closure within the study area which is trending along north-west direction. This, closure occurs probably because of the existence of the paleo fracture zone (Romanche Fracture Zone) within the study area. At 40 km and at 80 km a regional trend in the NE-SW direction in (porphyritic) basement rocks. At the northeast and southeastern part of the area which is made up of basement complex that corresponds to areas around Gitata, Kafanchaan and Jemma are prone to tectonic activities while the southwestern part of the area around Abuja is seismic free. It is suggested that those lineaments identified, most especially at the southeastern part could be the reason for the shaking of the subsurface which result into earth tremors.

Tectono-sedimentary evolution and petroleum systems of the Mundaú subbasin: A new deep-water exploration frontier in equatorial Brazil

The Brazilian equatorial margin (BEM) evolved in response to transform motion between Brazil and Africa. In 2012, Petrobras drilled the Pecem well in the Mundau subbasin (Ceara Basin) of the BEM to record the first deep-water oil discovery in the region. This work investigates the deep-water evolution of the Mundau subbasin focusing on its structural and sedimentary evolution, and characterizes the petroleum systems in this new exploration frontier. For such purposes, poststack seismic reflection, borehole, and geochemical data were used. Three tectono-stratigraphic sequences representing synrift (Mundau Formation), transitional (Paracuru Formation), and drift strata (Ubarana Formation) were divided into seven seismic units. Different tectonic domains were interpreted: proximal, distal, and Romanche Fracture Zone. Typical structures of transform margins, such as marginal ridges and marginal plateaus, were not identified in the Mundau subbasin. Instead, the subbasin was predominantly deformed by transtensional movements. The Mundau and Paracuru Formations are mature within the oil window, whereas the Ubarana Formation is immature. Main reservoir intervals consist of approximately 1-m (∼3.28-ft)-thick intercalations of sandstone between shales, siltstones, and marls. The seal rocks comprise shales in the Ubarana Formation, whereas the hydrocarbon trap is related to an unconformity and a normal fault. This work concludes that the Paracuru Formation is the main source and reservoir in the deep-water Mundau subbasin, effectively comprising a Paracuru–Paracuru petroleum system. The results have significant implications for petroleum exploration in the BEM by proposing a developed transitional petroleum system in the distal parts of northeastern Brazil.

Recent Seismotectonic Stress Regime of most Seismically active zones of gulf of Guinea and its Kinematic implications on the adjoining sub-Sahara west African region

The Tectonic stress regime of Gulf of Guinea region has been studied by stress tensor inversion analysis for the area bounded by lati− tude −10.0 0 S to 4.0oN and longitude −25 0 W to −11.0 0 E. A total of one hundred and four focal mechanism solutions, pertaining to the earthquake events which have occurred in this region, were used for this study. In order to decipher the stress pattern of this region, we have divided the region into four fracture zones namely Romanche, Chain−Romanche, Charcot and Ascension fracture−zones based on seismicity clustering, tectonics and available focal mechanism solutions. The seismicity pattern indicates that none of the nearby coun− tries on the border line between the west Africa region and the gulf of Guinea is devoid of seismicity. Simultaneously, the stress tensor inversion in the four subzones of investigation indicates different types of stress orientations. All of these zones are characterized by vary− ing principal axial directions. The orientation of the principal axial direction along Romanche, Chain−Romanche, Charcot and Ascen− sion fracture−zones are along NE−SW, NE−SW, ENE−WSW and ESE−WNW respectively. The stress tensor inversion results indicate that Chain−Romanche, Charcot and Ascension fracture zones are characterized by extensional stress regime while Romanche fracture zone is characterized by strike−slip stress regime respectively. These patterns imply sea floor spreading activities in and around the mid oceanic ridges and the general orientation of the extensional stress regime is found towards the continents, along the line of migration / progression of earthquakes.

The distribution of neodymium isotopes and concentrations in the eastern tropical North Atlantic

Dissolved neodymium (Nd) and its radiogenic isotope composition (143Nd/144Nd, expressed as εNd) belong to the key parameters of the international GEOTRACES program, which aims to investigate the processes controlling the distribution of trace elements and their isotopes in the global ocean. We present Nd isotope and concentration ([Nd]) data from eleven full depth water column profiles from the eastern (sub)tropical Atlantic Ocean between 2°N and 29°N and from the Romanche Fracture Zone sampled during Meteor cruise M81/1 (GEOTRACES cruise GA11) in February/March 2010.The εNd signatures range from −12.9 in upper North Atlantic Deep Water (UNADW) at the equator to −8.1 in the mixed layer near the Canary Islands. Nd concentrations range from 11.9 pmol/kg observed within the Equatorial Undercurrent to 40.2 pmol/kg in Antarctic Bottom Water (AABW) in the Romanche Fracture Zone. Large variations in surface water Nd isotope compositions (−12.7 ≤ εNd ≤ −8.1) and concentrations (15.7 pmol/kg ≤ [Nd] ≤ 27.7 pmol/kg) are caused by partial dissolution of Saharan dust particles between 2°N and 13°N and volcanic island weathering (Canaries) between 25°N and 29°N. In contrast to dust inputs, which predominantly affect the Nd concentrations and isotope compositions of surface waters and underlying South Atlantic Central Water (SACW), contributions originating from the Canary Islands affect the Nd isotope composition of the entire surrounding water column without significantly altering Nd concentrations, thus confirming the concept of boundary exchange in its strictest sense.The results confirm that the composition of lower North Atlantic Deep Water (LNADW, εNd = −12.1) in the abyssal plains of the eastern North Atlantic is exclusively set by the mixing ratio of LNADW and Antarctic Bottom Water (AABW). Upper North Atlantic Deep Water (UNADW), in contrast, is characterized by εNd signatures between −12.7 and − 12.0 between 2°N and 10°N, whereas further north it is clearly affected by admixture of Mediterranean Water (MW) and radiogenic inputs from the Canary Islands and likely also the Cape Verde Islands.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Structure of the Deep Spillway in the Western Part of the Romanche Fracture Zone

AbstractStructure of the deep spillway was studied on the basis of conductivity‐temperature‐depth, lowered acoustic Doppler current profiler, and mooring measurements in 2011–2014 in the Romanche fracture zone (22°27′–22°32′W). The spillway exists quasi‐permanently in the regime of hydraulic control overflow and consists of two flows in the layer θ &lt; 1.20 °C (deeper than 4,150 m). The first flow enters the fracture from the south through a gap in the Southern Wall. The second flow is directed to the east along the narrow valley of the fracture. The sill depths of these passages are 4,570 and 4,430 m, respectively. The depths over the surrounding ridges are shallower than 4,100 m. Both flows descend to the local depression of the fracture with 5,000‐m depth. Bottom potential temperatures over both sills were 0.51 °C, and the measured velocities were 20–40 cm/s. When the flows descend down the slope, they accelerate in supercritical regime. Maximal measured velocities were up to 65 cm/s. Analysis of the 189‐day time series of velocities on the mooring deployed in the flow descending from the gap showed a very high level of fluctuation energy: 10 times greater than the energy of the barotropic tide at this point. The only energy source of these fluctuations is the available potential energy of the deep thermocline in the West Atlantic transforming to the kinetic energy of the deep spillway. The total transport of the bottom water below θ = 1.2 °C based on the lowered acoustic Doppler current profiler measurements is estimated at 0.29–0.35 Sv, which agrees with the predicted values.

Nonlinear waves in marginally stable flows of Antarctic Bottom Water

In this paper, we study internal waves and stratified flows in the Romanche Fracture Zone of the Mid Atlantic Ridge. Theoretical analysis involves mathematical formulation of stratified flows which uses LADCP- and CTD-data obtained from field observation in the cruises of RV Akademik Sergey Vavilov. We discuss the 2.5-layer model of the fluid with exponential stratification of both layers. The long-wave approximate model describing travelling waves is constructed by means of scaling procedure with a small Boussinesq parameter. It is demonstrated that solitary wave regimes can be affected by the Kelvin — Helmholtz instability arising due to interfacial velocity shear in upstream flow.

Morphotectonic development of the Ceará Terrace: a marginal ridge on the western side of the Romanche Fracture Zone in the Brazilian Equatorial Margin

The evolution of a rifted and sheared continental margin may control the formation of marginal ridges, which are prominent morphostructural features located at the extremities of fracture zones. We investigate the Ceara Terrace, a marginal ridge located at the western limit of the Romanche Fracture Zone in the Brazilian Equatorial Margin. Our data consist of 2000 km of 2D seismic lines parallel and orthogonal to the continental equatorial margin, as well as four exploratory wells. The Ceara Terrace has an evolution similar to that of the Ivory Coast-Ghana Ridge, the corresponding conjugate end of the Romanche Fracture Zone in the western African continental margin. The Ceara Terrace surface morphology displays an asymmetric ridge; the north slope is aligned to the Romanche Fracture Zone, related to a bathymetric step at ~ 850 m. The paleo relief represents a ridge consisting of a rift sequence (Lower Cretaceous) that is similar to the Ivory Coast-Ghana Ridge. This ridge is bounded by two half-grabens to the southeast associated with the reactivation of preexisting tectonic weakness zones related to the Precambrian Transbrasiliano Lineament. This structure was buried by a drift sequence, which comprises three sedimentary units: Unit 1, shale; unit 2, limestone that interfingers with shale and sandstone; and unit 3, limestone and shale. The fossil ridge is located near a seamount associated with important Oligocene volcanic units. Both conjugate marginal ridges were formed by a Late Albian to Cenomanian transpressional event (marked by folds and tectonic uplift) and flexural uplift due to erosion and thermal exchanges with oceanic spreading centers. Differences in the amplitude of uplift are related to the thermal influences of the distinct oceanic spreading centers. Furthermore, the results indicate that the studied area is still tectonically active.

Measurements of bottom currents in the western part of the Romanche Fracture Zone on cruises 37 and 38 of the R/V Akademik Sergey Vavilov

Measurements of bottom currents in the western part of the Romanche Fracture Zone on cruises 37 and 38 of the R/V Akademik Sergey Vavilov

Transpressional structures and hydrocarbon potential along the Romanche Fracture Zone: a review

Abstract The Romanche Fracture Zone was originally a corridor of Aptian-age dextral transtensional rifting along the Equatorial Atlantic margins. Late Albian plate tectonic compression occurred due to a change in plate vectors, when the African and South American continents were still in contact across a 500 km-long section of the Romanche Fracture Zone. This dextral compression produced reactivation of the rift faults to produce asymmetric landward-vergent anticlines and thrusts that trend ENE to NE. Fold-axial planes dip seaward, parallel to the rift faults. Minor asymmetric anticlines were developed on the long seaward-dipping fold limbs and these have subvertical axial planes. The asymmetry of the minor folds is due to the southward stratal dip having been oblique to the horizontal maximum principal stress during the Albian inversion. The folds on the African margin were subsequently tightened by compression in Santonian and Oligo-Miocene times. Aptian-age ENE strike-slip faults were reactivated during the compression phases to produce broad positive flower structures up to 30 km wide that formed topographical ridges along the original strike-slip faults. The intervening and broader flat-bottomed synclines do not appear to be associated with rift faults. The folding and thrust faulting created seabed relief of 1–2 km at the end of the Albian; evidenced by the amount of subsequent erosion that removed the better-quality reservoirs in the upper Albian sequence from the major fold crests. Consequently, there has been a significant number of failed oil exploration wells drilled along the fold crests. The fold ridges would have diverted turbidite channels in the onlapping Cenomanian–Campanian sequence and these will be preferentially located on the landward side of the anticlinal crests. Late Cretaceous stratigraphic and structural traps located between the major anticlines have not yet been explored for hydrocarbons along the Romanche Fracture Zone margins.