Post-rift Phase Research Articles

Coupled thermo-mechanical 3D subsidence analysis along the SW African passive continental margin

Sedimentary basins along the SW African margin deliver information about processes, which occurred in the past and serve as a good starting point for reconstructing the margin’s evolution. By integrating detailed information on the present-day configuration of the SW African margin into a 3D thermo-mechanical forward modeling framework, we attempt a margin-wide reconstruction of its subsidence history since its onset during breakup. The 3D forward modeling approach as applied on the SW African margin area makes use of the coupling between thermal relaxation of the lithosphere and flexural isostatic balance in response to sediment deposition and build-up of thermal stresses during lithosphere cooling. On the one hand, our results provide useful information about the behavior of the lithosphere during breakup and the subsequent post-rift phase. On the other hand, restored paleobathymetries for specific time intervals during the post-rift evolution give evidence for possible uplift events superposed on the long-term subsidence history of the margin. Restored paleobathymetries are in agreement with conclusions derived from former studies and provided strong indications for the presence of a rather heterogeneous crustal configuration of the margin marked by a mechanical decoupling of the upper and lower crustal domains during most of the rifting phase.

The chronology and tectonic style of landscape evolution along the elevated Atlantic continental margin of South Africa resolved by joint apatite fission track and (U‐Th‐Sm)/He thermochronology

AbstractAtlantic‐type continental margins have long been considered “passive” tectonic settings throughout the entire postrift phase. Recent studies question the long‐term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U‐Th‐Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 µm, and mean (U‐Th‐Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130 Ma and 110–90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1–3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle‐driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km.

Early evolution of the southern margin of the Neuquén Basin, Argentina: Tectono-stratigraphic implications for rift evolution and exploration of hydrocarbon plays

Long-lived rift basins are characterized by a complex structural and tectonic evolution. They present significant lateral and vertical stratigraphic variations that determine diverse basin-patterns at different timing, scale and location. These issues cause difficulties to establish facies models, correlations and stratal stacking patterns of the fault-related stratigraphy, specially when exploration of hydrocarbon plays proceeds on the subsurface of a basin. The present case study corresponds to the rift-successions of the Neuquén Basin. This basin formed in response to continental extension that took place at the western margin of Gondwana during the Late Triassic–Early Jurassic. A tectono-stratigraphic analysis of the initial successions of the southern part of the Neuquén Basin was carried out. Three syn-rift sequences were determined. These syn-rift sequences were located in different extensional depocentres during the rifting phases. The specific periods of rifting show distinctly different structural and stratigraphic styles: from non-volcanic to volcanic successions and/or from continental to marine sedimentation. The results were compared with surface and subsurface interpretations performed for other depocentres of the basin, devising an integrated rifting scheme for the whole basin. The more accepted tectono-stratigraphic scheme that assumes the deposits of the first marine transgression (Cuyo Cycle) as indicative of the onset of a post-rift phase is reconsidered. In the southern part of the basin, the marine deposits (lower Cuyo Cycle) were integrated into the syn-rift phase, implying the existence of different tectonic signatures for Cuyo Cycle along the basin. The rift climax becomes younger from north to south along the basin. The post-rift initiation followed the diachronic ending of the main syn-rift phase throughout the Neuquén Basin. Thus, initiation of the post-rift stage started in the north and proceeded towards the south, constituting a diachronous post–rift event. This arrangement implies that the lower part of Cuyo Cycle, traditionally related to regional thermal subsidence, may be deposited during either mechanical subsidence or thermal subsidence according to its position within the basin.

Regional Depositional Model Of South Makassar Basin Depocenter, Makassar Strait, Based On Seismic Facies

South Makassar Basin Depocenter (SMBD) is located in Southern Makassar Strait which has petroleum potential by the presence of oil and gas indications within the area based on Airborne Laser Fluorescence survey. However, detail studies within this area are not developed well. One of the studies which can be utilized for further discoveries of oil and gas field in SMBD is a study of depositional model using seismic facies method to maximize limited seismic and well data. Interpretation of depositional model in Eocene (syn-rift phase) was varied from alluvial plain and alluvial fan complex, continued gradually to platform. In Oligocene time (post-rift phase), massive transgression caused a major deepening in entire SMBD where the depositional environment changed to basinal plain. In early Miocene – recent interval (syn-orogenic phase), SMBD was relatively in stable condition at basinal plain environment. Lower Tanjung Sequence will be the most prolific petroleum play in SMBD due to its adequate source rock, reservoir rock and seal rock as well. Keywords: South Makassar basin, depositional model, seismic facies

Neogene strontium isotope stratigraphy, foraminifer biostratigraphy, and lithostratigraphy from offshore wells, Queen Charlotte Basin, British Columbia, Canada

Strontium isotope ages of foraminifers from Early Miocene to Late Pliocene (Neogene) sequences (21.2–3.4 Ma) are reported for the first time from the Queen Charlotte Basin (QCB) in Queen Charlotte Sound, offshore British Columbia. These ages, together with a revised foraminifer biostratigraphy and log data from two offshore wells, provide a high-resolution chronostratigraphy for the southern part of the QCB. The data show thick 1717–2636 m Miocene sequences overlain by much thinner Pliocene and younger units (<240 m). Assessments of foraminifer biofacies indicate common transported neritic and shelf faunas into slope or bathyal environments where changes in basin water depths indicate significant deformation and erosion. Additional stratigraphic information for six offshore wells in Hecate Strait show the common occurrence of amorphous carbonates in the upper sections of the penetrated basin successions. Amorphous carbonates and coals are less common in the Harlequin D-86 and Osprey D-36 wells of Queen Charlotte Sound. Coals are especially common to abundant in the sedimentary sections penetrated by the Hecate Strait wells and are potential sources of seep fluids and gases. The occurrences of glassy-textured coals, zeolite-like minerals, recrystallized foraminifers, and inverted stratigraphic units in the basal parts of Miocene sub-basins indicate heating, deformation or slumping, and upward mobilization of fluids or gases after the Early Miocene. Results support syn-rift and post-rift depositional and deformational phases. Comparisons between the different geological processes and events in the Queen Charlotte and Tofino basins reveal details of the complex evolution of these Cenozoic basins, plate margins, and ridge junctions.

North America’s Midcontinent Rift: When rift met LIP

Rifts are segmented linear depressions that are filled with sedimentary and igneous rocks; they form by extension and often evolve into plate boundaries. Flood basalts, a class of large igneous provinces (LIPs), are broad regions of extensive volcanism formed by sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift (MCR) is an unusual combination, because its 3000-km length formed during a continental breakup event 1.1 Ga, but it contains an enormous volume of igneous rocks that are mostly flood basalt. We show that MCR volcanic rocks are significantly thicker than other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR a rift’s geometry but a LIP’s magma volume. Structural modeling of seismic-reflection data shows that LIP volcanics were deposited during two phases—an initial rift phase where flood basalts filled a fault-controlled extending basin and a postrift phase where LIP volcanics and sediments were deposited in a thermally subsiding sag basin without associated faulting. The crust thinned during the initial rifting phase and then rethickened during the postrift phase and later compression, yielding the present thicker crust observed seismologically. The restriction of extension to a single normal fault in each rift segment, steeply inward-dipping rift shoulders with sharp hinges, and persistence of volcanism after rifting ended gave rise to a deep flood basalt–filled rift geometry not observed in other presently active or ancient rifts. The unusual coincidence of a rift and LIP arose when a new rift associated with continental breakup interacted with a mantle plume or overrode anomalously hot or fertile upper mantle.

Role of rift transection and punctuated subsidence in the development of the North Falkland Basin

The results of well-constrained seismic interpretation and new mapping of three-dimensional (3D) seismic data volumes demonstrates that the North Falkland Basin consists of two superimposed failed rift basins: a Late Jurassic NW–SE-striking Southern Rift Basin (SRB); and an Early Cretaceous north–south-striking Northern Rift Basin (NRB). The SRB is best developed in coastal waters of the Falkland Islands, where it comprises a series of extensional sub-basins that are transected by faults belonging to the more substantive NRB. Regional interpretation demonstrates that the NRB consists of a southward-tapering, asymmetric extensional basin containing a thick (in excess of 10 km) sequence of sediments. Its syn-rift subsidence history was controlled by a major west-dipping normal fault array comprising several fault segment precursors, which, together with corresponding antithetic faults, effectively subdivides the hanging wall into a series of sub-basins throughout its length. The NRB initially developed in a fluvial and later lacustrine environment before becoming predominantly marine in the Tertiary. A prograding delta system filled the basin from the north during the early post-rift phase. Contemporaneously, sediment was shed off the segmented basin-bounding fault via long-established feeder drainage systems through breached relay ramps into the depocentre. The resultant sediment dispersal led to deposition of numerous lacustrine turbidites that created the Sea Lion fans and its affiliates, the location of which mimics, and is thus interpreted to have been controlled by, the underlying syn-rift sub-basins. Post-rift subsidence was punctuated by an important, but short-lived, phase of basin inversion during the Aptian that created a large, broad and gentle north–south-striking anticline that runs along the central basin axis. Whilst the episode of basin inversion arrested subsidence, it did not inhibit petroleum prospectivity. The syn-rift lacustrine source intervals did subsequently pass through the critical moment in the Cretaceous leading to hydrocarbon maturation and the migration of waxy oil, a process that continues to the present day.

Reconstruction of the southwestern African continental margin by backward modeling

As the evolution of the southwestern African passive continental margin and in particular, the vertical movements in this area are still a matter of debate, this study aims to quantify the past vertical movements by reconstructing the paleobathymetric evolution of the southwestern African margin during the post-rift phase. To quantify the vertical movements, inferred from paleobathymetries, different subsidence components that have affected the passive continental margin are calculated and interpreted with respect to the generating processes.Our investigation takes advantage of information on the present-day sedimentary thicknesses and the crustal configuration along the SW African margin. By using this detailed information and modeling backward in time we attempt to separate different subsidence components such as the load induced subsidence and the thermal subsidence for different time intervals during the post-rift phase. Calculating the amounts of each subsidence component and subtracting them from the present-day configuration yields paleobathymetries which are interpreted with respect to the evolution of the southwestern African passive margin.These restored paleobathymetries for different time steps during the post-rift phase indicate spatial and temporal variations of vertical movements including phases of seafloor uplift. As a result, our investigation allows to give minimum estimates of the vertical movements that have occurred during the post-rift phase. In addition to predominant subsidence, about 1000 m of seafloor uplift are deduced for the northern and southern parts of the SW African margin. Quantifying the amounts of vertical movements cannot finally clarify the generating processes behind these movements but the timing and magnitude of seafloor uplift indicate a deep mantle mechanism.

Hydrocarbon source rock generative potential of the Sudanese Red Sea basin

To date, the Sudanese Red Sea like the Red Sea as a whole, represents a grossly under-explored basin. Recently, the basin has seen increased exploration interest, with more than 15 wells being drilled, some of which have been petroleum discoveries (gas and condensate with oil shows). A similar exploration trend has been seen in the Saudi, Eriteria and Egyptian sectors of the Red Sea. The Red Sea rift marks the break-up of the Afro-Arabian plate in Eocene–Oligocene time. The tectonostratigraphic evolution can be subdivided into three major phases: Pre-rift, Syn-rift and Post-rift phase. Generally, terrestrials and marginal to deep marine sediments were deposited with both good reservoir quality and potential source rocks being present. The pre-Salt Rudeis/Kareem shales are expected to be good petroleum source rocks for the basin. Moreover, shales of base Zeit (post-Salt) are considered the main source of the known accumulated hydrocarbon (gas and condensate) in the shallow targets with the condensate charge also expected from intra-evaporites shales and pre-salt sources along the salt windows. The measured amount of TOC, HI, and hydrocarbon generation from pyrolysis of kerogen of the examined samples indicate source rock generative potential of these intervals. The TOC contents for base Zeit, and interbeded shales of Dungnab Formations, range from 0.60& to 5.40 wt. %, while the hydrogen index (HI) values are in the range of 50-600mg HC/gTOC suggesting dominantly type III-II kerogen. The high TOC (>1.5) and HI (>300) values of base Zeit and Dungnab Formations are limited to some intervals within these formations. Consequently, the base Zeit source rocks are considered very good for condensate and gas generation. In comparison to their equivalents Burqan and Maqna Group sources (HI = 350–600 mg HC/gTOC) on the Saudi Red Sea sector, Rudeis/Kareem shales could perhaps possess good quality type-II kerogen. The Red Sea’s high pressure and temperature regimes have a significant impact on hydrocarbon maturation, generation and the quality of the preserved accumlations. Presently, the Zeit source varies in maturity across the basin and within the oil window in the Suakin and Bashayer areas and at gas window elsewhere to the east towards offshore. On the other hand, the pre-salt Rudies/Kareem Formations are currently in the late oil to wet gas zone and probably out of gas phase distally offshore. However, the salt tectonics makes for challenging exploration. The salt movement is considered recent as it is only the recent sediments that appear to be controlled by salt movement.

Analyses on the tectonic thermal evolution and influence factors in the deep-water Qiongdongnan Basin

To reveal the tectonic thermal evolution and influence factors on the present heat flow distribution, based on 154 heat flow data, the present heat flow distribution features of the main tectonic units are first analyzed in detail, then the tectonic thermal evolution histories of 20 profiles are reestablished crossing the main deep-water sags with a structural, thermal and sedimentary coupled numerical model. On the basis of the present geothermal features, the Qiongdongnan Basin could be divided into three regions: the northern shelf and upper slope region with a heat flow of 50–70 mW/m2, most of the central depression zone of 70–85 mW/m2, and a NE trending high heat flow zone of 85–105 mW/m2 lying in the eastern basin. Numerical modeling shows that during the syn-rift phase, the heat flow increases generally with time, and is higher in basement high area than in its adjacent sags. At the end of the syn-rift phase, the heat flow in the deepwater sags was in a range of 60–85 mW/m2, while in the basement high area, it was in a range of 75–100 mW/m2. During the post-rift phase, the heat flow decreased gradually, and tended to be more uniform in the basement highs and sags. However, an extensive magmatism, which equivalently happened at around 5 Ma, has greatly increased the heat flow values, and the relict heat still contributes about 10–25 mW/m2 to the present surface heat flow in the central depression zone and the southern uplift zone. Further analyses suggested that the present high heat flow in the deep-water Qiongdongnan Basin is a combined result of the thermal anomaly in the upper mantle, highly thinning of the lithosphere, and the recent extensive magmatism. Other secondary factors might have affected the heat flow distribution features in some local regions. These factors include basement and seafloor topography, sediment heat generation, thermal blanketing, local magmatic injecting and hydrothermal activities related to faulting and overpressure.

Geologia do Grupo Paranoá na porção externa da Faixa Brasília

The Paranoa Group represents a psamo-pelitic-carbonated succession deposited in shelf conditions. The stratigraphy of this important lithostratigraphic sequence of the Brasilia Fold Belt was originally proposed by letter-code designation that includes 11 units with the following stratigraphic stacking: SM, R1, Q1, R2, Q2, S, A, R3, Q3, R4 and PC. The present paper formalizes these units and proposes the following denomination to the formations, named from base to top: Ribeirao Sao Miguel, Corrego Cordovil, Serra da Boa Vista, Serra Almecegas, Serra do Parana, Ribeirao Picarrao, Ribeirao do Torto, Serra da Meia Noite, Ribeirao Contagem, Corrego do Sansao and Corrego do Barreiro. The deposition of the Paranoa Group is considered to have been processed in the Mesoproterozoic (1,542 to 1,042 Ma). This age interpretation is supported by the stratigraphic position (as it occurs over the post-rift phase sediments of the Arai Group and under slates and carbonates of the Bambui Group), by the presence of conical stromatolite (conophyton) and by isotopic data. The sedimentation was controlled by transgressive-regressive cycles, including grading and traction processes, debris flows, tides, waves, storms and basin bottom paleogeography. The sediments were affected by low-grade metamorphism and the deformation resulted in the formation of monoclines, different styles of folds (chevrons, in box and cylindrical ones) and folding interference resulting in domes and structural basins. The regional deformation of the Paranoa Group is mainly controlled by the Parana and Rio Maranhao thrust systems and by the Ribeirao Sao Miguel shear belt.

Upper mantle viscosity and dynamic subsidence of curved continental margins

Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s.

Permian–Holocene tectonostratigraphic evolution of the Mandal High, Central Graben, North Sea

A three-dimensional seismic data set and published data from exploration wells were used to reconstruct the tectonostratigraphic evolution of the Mandal High area, southern North Sea, Norway. The Mandal High is an elongated southeast-northwest–trending horst. Three fault families in the Lower Permian sequence, inherited from the basement structural grain of Caledonian origin, are interpreted: (1) a north-northwest–south-southeast–striking fault family, (2) a northeast-southwest–striking fault family, and (3) a near east-west–striking fault family. In addition, an east-southeast–west-northwest–striking fault family (4) that formed during Late Jurassic rifting and was reverse reactivated in the Late Cretaceous is interpreted. We suggest that inversion occurred because of small dextral motion along fault family 1. A final fault family (5) displays various strike orientations and is associated with salt movements.Seven chronostratigraphic sequences defined by well data and recognized on three-dimensional seismic data are interpreted and mapped: Early Permian rifting in a continental environment; Late Permian deposition of the Zechstein salt and flooding; Triassic continental rifting; uplift and erosion in the Middle Jurassic with deposition of shallow-marine and deltaic sediments; rifting and transgression in a deep-marine environment during the Late Jurassic; a post-rift phase in a marine environment during the Early Cretaceous; and flooding and deposition of the Chalk Group in the Late Cretaceous. An eighth sequence was interpreted—Paleogene–Neogene—but has not been studied in detail. This sequence is dominated by progradation from the east and basin subsidence. Well and seismic data over the Mandal High reveal that large parts of the high were subaerially exposed from Late Permian to Late Jurassic or Early Cretaceous, providing a local source of sediments for adjacent basins.Similar to the Utsira High, where several large hydrocarbon discoveries have been recently seen, the Mandal High might consist of a set of petroleum plays, including fractured crystalline basement and shallow-marine systems along the flanks of the high, thereby opening up future exploration opportunities.

Continental Flood Basalts and Rifting: Geochemistry of Cenozoic Yemen Volcanic Province

Rift formation is a crucial topic in global tectonics. The Yemen rift-related area is one of the most important provinces, being connected to the rifting processes of the Gulf of Aden, the Red Sea and Afar Triangle. In this paper, a review of the Yemen volcanic province and its relations with the Red Sea rifting are presented. Tertiary continental extension in Yemen resulted in the extrusion of large volumes of effusive rocks. This magmatism is divided in the Oligo-Miocene Yemen Trap Series (YTS) separated by an unconformity from the Miocene-Recent Yemen Volcanic Series (YVS). Magmas of the YTS were erupted during the synrift phase and correlate with the first stage of sea-floor spreading of the Red Sea and the Gulf of Aden (30 - 15 Ma), whereas the magmas of the YVS were emplaced during the post rift phase (10 - 0 Ma). A continental within plate character is recognized for both the YTS and YVS basalts. The YTS volcanic rocks are contemporaneous with, and geochemically similar to, the Ethiopian rift volcanism, just as the volcanic fields of the YVS are geochemically alike to most of the Saudi Arabian volcanics. YTS and YVS have analogous SiO2 ranges, but YVS tend to have, on average, higher alkalis and MgO contents than YTS. Fractional crystallization processes dominate geochemical variations of both series. Primitive magmas (MgO > 7.0%) are enriched in incompatible elements and LREEs with respect to primitive mantle, but YVS are more enriched than YTS. To first order, the different geochemical patterns agree with different degrees of partial melting of an astenospheric mantle source: 25% - 30% of partial melting for YTS and 10% - 3% for YVS. Secondly, the higher degree of enrichment in incompatible elements of YVS reflects also greater contribution of a lithospheric mantle component in their source region.

The long-term topographic response of a continent adjacent to a hyperextended margin: A case study from Scandinavia

We present data that link Scandinavia’s passive-margin domains under a unified system invoking isostatically driven, postextension phase vertical adjustments to severe crustal thinning. Topographic and geological data indicate that the relative location of the first landward occurrence of total crustal embrittlement or deformation coupling—the Taper Break—controlled and continues to control Scandinavia’s post-thinning geomorphic evolution. Formed during Late Jurassic or Early Cretaceous thinning, yet marked today by seismicity, the Taper Break closely approximates the boundary between (1) less-stretched lithosphere that increases in rigidity both toward land and through postrift time, and (2) the highly attenuated, pervasively faulted, permanently weakened lithosphere of the distal margin. Following the stretching, thinning, and exhumation phases proposed by other workers, an accommodation phase is warranted. Commencing during “sag” basin time and continuing today, it is probably driven by thermal cooling and mass transfer from the escarpment to the basins offshore. The accommodation phase does not entirely coincide with the traditional postrift phase as the former may contain the latter. During accommodation, the original synrift escarpments can be eroded to very low base levels. Sharply tapered margin segments can undergo subsequent rejuvenation by out-of-sequence normal faulting and footwall uplift, probably in response to tensile bending stresses engendered by lithospheric-scale flexure. Accommodation-phase uplift at passive margins is the inexorable and penultimate phase of hyperextension, and may perhaps be followed by the onset of subduction localized by the weakened lithosphere of the distal margin and the ocean-continent transition.

Structural evolution of the northern East China Sea Shelf Basin interpreted from cross-section restoration

The northern East China Sea Shelf Basin consists of three depressions (the Domi, Jeju, and Socotra Depressions), separated by basement highs or rises. Reconstruction of depth-converted seismic reflection profiles from these depressions reveals that the northern East China Sea Shelf Basin experienced two phases of rifting, followed by regional subsidence. Initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the plate margin. Major extension (~15 km) took place during the early phase of basin formation. The initial rifting was terminated by regional uplift in the Late Eocene-Early Oligocene, which was probably due to reorganization of plate boundaries. Rifting resumed in the Early Oligocene; the magnitude of extension was mild (<1 km) during this period. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 m of sediments and basement rock were removed by erosion during and after the second phase of uplift. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Subsequent erosion removed about 900 m of sediments. The regional subsidence has dominated the area since the Late Miocene.

Seismic stratigraphy and structural analysis of the northern East China Sea Shelf Basin interpreted from multi-channel seismic reflection data and cross-section restoration

Analysis of multi-channel seismic data from the northern East China Sea Shelf Basin (ECSSB) reveals three sub-basins (Socotra, Domi, and Jeju basins), separated by structural highs (Hupijiao Rise) and faulted basement blocks. These sub-basins show a typical rift-basin development: faulted basement and syn-rift and post-rift sedimentation separated by unconformities. Four regional unconformities, including the top of acoustic basement, have been identified and mapped from multi-channel seismic data. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area. The depths of the acoustic basement range from less than 1000 m in the northwestern part of the Domi Basin to more than 4500 m in the Socotra Basin and 5500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1500 m in the northwest where the acoustic basement is shallow and reach about more than 5500 m in the south. Interpretation of seismic reflection data and reconstruction of three depth-converted seismic profiles reveal that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian Plate, caused by the subduction of the Pacific Plate beneath the Eurasian Plate. Extension was the greatest during the early phase of basin formation; estimated rates of extension during the initial rifting are 2%, 6.5%, and 3.5% in the Domi, Jeju, and Socotra basins, respectively. A regional uplift terminated the rifting in the Late Eocene-Early Oligocene. Rifting and extension, although mild, resumed in the Early Oligocene; while fluvio-lacustrine deposition continued to prevail. The estimated rates of extension during the second phase of rifting are 0.7%, 0.8%, and 0.5% in the Domi, Jeju, and Socotra basins, respectively. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the post-rift phase of regional subsidence. Regional subsidence dominated the study area between the Early Miocene and the Late Miocene. An inversion in the Late Miocene interrupted the post-rift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Uplift and subsequent erosion were followed by regional subsidence.

Interplay of phosphogenesis and hydrothermalism in the latest Ediacaran rift of the High Atlas, Morocco

Two dissolution surfaces, capped by phosphatic crusts that contain up to 40 wt.% of P 2O 5, are well exposed in the latest Ediacaran Tabia Member (Adoudou Formation) of the Ouneïn region, High Atlas. Both the Tabia dolostones and capping phosphorites are crosscut by several generations of synsedimentary faults, fissures, and hydrothermal dikes hosting Cu ore bodies, in a scenario of down-faulted blocks. Stratigraphic and facies relationships suggest the preservation of a major pulse of intra-cratonic rifting, responsible for the development of an unstable carbonate platform subjected to sharp uplift and tilting perturbations and dissolution processes, associated with hydrothermal dike-swarm injection. The whole framework is sealed with the Tamjout Bed (Tifnout Member), which marks the end of abundant regional tilting (syn-rift phase) and the beginning of a thermal subsidence-dominated regime (post-rift phase) in the Moroccan margin of West Gondwana.

Sedimentação e deformação tectônica cenozoicas na porção central da Bacia Potiguar

Neogene and Quaternary sedimentary covers occur along the Brazilian coast and have been frequently described together as a single unit. The study of Brazilian sedimentary basins concentrates on their rift phase, whereas the post-rift phase has been considered a tectonic quiescent period. In the Potiguar basin, although post-rift Cretaceous units are well investigated, the Neogene and Quaternary sedimentary covers, as well as their identification and differentiation, are still poorly known. A few previous studies have demonstrated that post-rift sedimentary units with no apparent deformation have a complex deformation pattern in all scales of observation. The study of this deformation, however, did not include Neogene and Quaternary units. The main aim of the present study is the characterization of Neogene and Quaternary sedimentary units that outcrop in the central part of the Potiguar Basin, State of Rio Grande do Norte, Brazil, and related tectonics. The study has concentrated on the description of the Barreiras Formation and overlying Quaternary alluvial, marine, and aeolian deposits at 1:100,000 scale. Facies analyses, grain size studies, and luminescence dating were carried out. Ten informal and formal lithostratigraphic sedimentary units were described, in addition to the Precambrian crystalline basement. The main results indicate that several Quaternary alluvial deposits were previously mapped as the Miocene Barreiras Formation. It was possible to locate the new boundaries of the Quaternary sedimentary deposits and their stratigraphic relationships with older units. In addition, it was possible to identify the major fault systems in the basin that show NW- and NE-trending directions, which coincide with macro landforms. It follows that these major fault systems, mainly the NW-trending system, control the deposition of Neogene and Quaternary sedimentary units.

Cenozoic deformation in the George V Land continental margin (East Antarctica)

This investigation is based on the analysis of multichannel seismic data collected in the continental shelf of the George V Land, between 140°E and 155°E on the East Antarctic margin. Most of the East Antarctic continental shelf is covered by permanent, thick sea and terrestrial ice and it is therefore still unexplored. This is the reason why the tectonic deformation affecting the Antarctic margin during the Mesozoic rift from the Australian plate and the Cenozoic post-rift phase is poorly known. The few coastal polynyas (such as the Mertz polynya, in the George V Land continental shelf) are the only places where the oldest sedimentary section can be studied with the existing technology. The data presented were not collected to address tectonic questions, however the relevance of this study is to document for the first time the occurrence of rift and post-rift tectonic structures in the sedimentary section near the coast, where the oldest sediment section is shallowest. These considerations are particularly relevant as Mertz-Ninnis trough, also known as George V Basin is located near the area of transition (around the Spencer Fracture Zone) between the extensionally-dominated Wilkes Land–Great Australian Bight Basin conjugate segment of the Australian–Antarctic Rift and the transtensional, strike-slip kinematics of the Otway Basin–South Tasman Rise–Oates Land segment. The tectonic structure in the George V Land sector presented in this study is two-fold with two rift phases: one being connected to the breakup process and the later one associated to a change in plate rotation. A former extensional phase opened structural grabens, with axis oriented WNW–ESE and possibly NE–SW. A latter transpressional phase reactivated previous structures and tilted, faulted and folded sedimentary strata, located in the inner continental shelf. The first tectonic phase is likely related with the Cretaceous rifting between the Antarctic and Australian plates. The second tectonic phase might be related to the onset of the fast spreading phase of Pacific–Indian Ocean, that caused uplift, inversion and folding of post-rift strata in a narrow east–west oriented region, near coastal basement outcrop, in Paleocene–Eocene times.