Rift Sequence Research Articles

From oblique arc-continent collision to orthogonal plate subduction in the southeastern central Asia Orogenic Belt during Paleozoic: Evidence from superimposed folds at the northern margin of the north China Craton

• Two stages of folds are identified on the northern margin of the North China Craton. • The D 1 folds were crossly superimposed by the D 2 folds and both formed in Paleozoic. • A dynamic transition occurred in middle Paleozoic in the northernNorth China Craton. Accretionary kinematic history of the Central Asia Orogenic Belt (CAOB) remains unclear in previous geological studies. Based on detailed structural analysis of newly identified folds on the northern margin of the North China Craton (NCC), we propose a tectonic model suggesting that Paleozoic accretion of the orogenic belt includes two phases of deformation. The first phase of deformation is oblique collision of the Bainaimiao Arc and the NCC, which was followed by orthogonal subduction of the Paleo-Asian Ocean in the second phase of deformation. The change of plate kinematics is supported by two generations of folding, namely D 1 (D B ) and D 2 (D A ). Both D 1 (D B ) and D 2 (D A ) folds developed in the Bayan Obo Group (BOG), a Meso- and Neoproterozoic rift sequence deposited on the passive continental margin of the NCC. D 1 (D B ) folds were superimposed by the east-west trending D 2 (D A ) folds. The D 1 (D B ) folds developed as a result of oblique collision of the Bainaimiao arc and the NCC. The D 2 (D A ) folds developed in response to the southward orthogonal subduction of the Paleo-Asian Ocean Plate. They formed between latest Silurian and Early Permian, coeval with the two accretion events at the northern edge of the NCC. Our study suggests that multiphase deformation of sedimentary layers on the subducting plates provide important constraints for kinematic evolution of plate tectonics.

The importance of recognizing multiples in legacy data: A case study from the Brazilian equatorial margin

Summary The Ceará Basin is a deepwater exploration frontier basin that comprises part of the Brazilian equatorial margin. This basin has been receiving renewed attention from the petroleum industry since the discovery of important deepwater oil fields in its African counterpart. However, detailed seismic stratigraphic, depositional, and structural frameworks for the Ceará Basin are still lacking in the literature. We have analyzed a series of 2D seismic data sets and stumbled into the pitfalls of migration artifacts (i.e., multiples) ultimately realizing that reprocessing was the best option to avoid the mistake of interpreting these artifacts as geologic features. Multiples can be difficult to identify in seismic data in which they mimic the true geology of the region, and they often present a pitfall for less experienced interpreters. Indeed, the identification and removal of multiples is crucial because they do not reflect the true geology in the subsurface and may otherwise lead to incorrect business decisions. Geological feature: Stratigraphy of the Ceará Basin, offshore Brazil Seismic appearance: Strong seismic horizons mimicking geological layering Alternative interpretations: Multiples arising from poor seismic migration processing Features with similar appearance: Strong seismic horizons reflecting basement and carbonates Formation: Rift sequence of the Ceará Basin Age: Cretaceous Location: Ceará Basin, offshore Brazil Seismic data: Obtained by the Brazilian National Petroleum Agency and reprocessed by the authors Analysis tool: Reprocessing

Geochronology of the Volyn volcanic complex at the western slope of the East European Craton – Relevance to the Neoproterozoic rifting and the break-up of Rodinia/Pannotia

Thick Neoproterozoic rift sequences along the western margin of the East European Craton (EEC) are classically attributed to rifting leading to break-up of super-continent Rodinia/Pannotia. Rifting was associated with intense, mainly mafic, volcanism, known as the Volyn Series, in the Lublin-Podlasie Basin (LPB) and in the Orsha-Volyn Aulacogen. Legacy deep boreholes in Poland provide continuous section through the volcanic series. Zircon from nine samples of tuff layers interbedded with basalt, from four boreholes, were dated with the U–Pb method with SHRIMP. The samples contain syn-eruptive zircons, together with zircon recycled from previous ash falls and zircon from the surrounding Proterozoic basement. The youngest volcanic zircon cluster from each sample of the studied tuff samples yielded concordia ages stratigraphically consistent, which document eruption events that took place between 567.1 ± 4.7 Ma and 551.0 ± 4 Ma. The most intense development of lavas and pyroclastics took place at ca. 557–555 Ma and ca. 567–560 Ma. Older volcanogenic zircon grains, inherited from previous phases of volcanic activity, preceding actual eruptions, were are dated at 588.0 ± 8 Ma to 580 ± 8.1 Ma. Age spectrum of xenocrysts zircon grains, inherited from the crystalline basement rocks of the LPB, ranges from ca. 1830 Ma to ca. 1480 Ma, coherent with the known late Paleoproterozoic and Mesoproterozoic age of crust-forming events in the western part of the EEC. The late Ediacaran syn-rift igneous activity in the LPB continued over a time span of approx. 37 My. It developed along both the SW margin of emerging Baltica and in the western the Orsha-Volyn Aulacogen. The study area was located at the intersection of these regional structures, being thus a site of development of an incipient triple junction, related to Rodinia/Pannotia break-up. Further extension along the SW margin of Baltica led to its separation from the plate adjacent to the west, and to the subsequent development of continental passive margin. Final emergence of Baltica, as an individual palaeocontinent, took place at ca. 550 Ma.

Unraveling the origin of the Parnaíba Basin: Testing the rift to sag hypothesis using a multi-proxy provenance analysis

Syneclises are long-lived sedimentary basins characterized by complex subsidence and erosion histories. The premise that these geotectonic units evolve from initial rifting processes following thermal (or flexural) subsidence is widespread in the geologic sciences and, to this day, remains a controversial issue. Seeking to test this hypothesis, we proceeded a novel multi-proxy provenance study aiming to identify differences (and/or similarities) in the sedimentary signal and source areas of the Jaibaras (rift) and Parnaíba (sag) basins. We conducted a detailed analysis of trace elements geochemistry of detrital rutile grains, macroscopic gravel composition and paleocurrents from the sedimentary deposits of the Aprazível Formation (Ediacaran - Cambrian, top of Jaibaras Basin) and the Ipu Formation (Ordovician, basal unit of Parnaíba Basin). Our data reveal that important changes in source areas occurred between the end of the rifting and the beginning of the sag phase, reinforcing the hypothesis that the evolution of the Jaibaras and Parnaíba basins were not genetically related. Our results demonstrate that conglomerates of the rift sequence are predominantly composed of volcanic, sedimentary, and metamorphic angular to sub-angular clasts, pointing to diverse, nearby source areas. Contrastingly, conglomerates of the initial sag sequence have greater sedimentary maturity, with dominant rounded vein quartz clasts and other minor source contributions, which suggest distant source areas, showing a consistent paleocurrent direction towards NW. Indeed, the detrital rutile trace elements geochemistry demonstrates that the source areas of these two units were distinct, revealing an important decrease in the input of granulite facies and metamafic grains in the sag basin comparing with the rift succession. In conclusion, as well as paleomagnetic and geochronological studies, the provenance methods using a multi-proxy approach proved to be an effective and powerful technique for distinguishing modifications in the sedimentary signal between rift-to-sag sequences.

Thermal Evolution of Asymmetric Hyperextended Magma‐Poor Rift Systems: Results From Numerical Modeling and Pyrenean Field Observations

AbstractWe investigate the thermal and structural evolution of asymmetric rifted margin using numerical modeling and geological observations derived from the Western Pyrenees. Our numerical model provides a self‐consistent physical evolution of the top basement heat flow during asymmetric rifting. The model shows a pronounced thermal asymmetry that is caused by migration of the rift center toward the upper plate. The same process creates a diachronism for the record of maximum heat flow and maximum temperatures (Tmax) in basal rift sequences. The Mauléon‐Arzacq basin (W‐Pyrenees) corresponds to a former mid‐Cretaceous asymmetric hyperextended rift basin. New vitrinite reflectance data in addition to existing data sets from this basin reveal an asymmetry in the distribution of peak heat (Tmax) with respect to the rift shoulders, where highest values are located at the former upper‐ to lower‐plate transition. This data set from the Arzacq‐Mauléon field study confirms for the first time the thermal asymmetry predicted by numerical models. Numerical modeling results also suggest that complexities in synrift thermal architecture could arise when hanging‐wall‐derived extensional allochthons and related Tmax become part of the lower plate and are transported away from the upper‐ to lower‐plate transition. This study emphasizes the limitations of the common approach to integrate punctual thermal data from pre‐rift to synrift sedimentary sequences in order to describe the rift‐related thermal evolution and paleothermal gradients at the scale of a rift basin or a rifted margin.

Tectonic and stratigraphic evolution of southern Jacuípe Basin (Brazil) based on seismic sequence stratigraphy

Jacuípe Basin, located at the northeastern Brazilian passive margin, has great hydrocarbon potential, but is considered a new frontier basin due to the lack of integrated studies. The present paper aims to comprehend the history of the basin based on the interpretation of several 2D seismic lines and well data. No pre-rift strata were detected. Within the rift supersequence, which overlies the crystalline basement of the São Francisco craton, four unconformity-bounded third-order sequences (labeled Rift 1 to 4) were recognized. Rift sequences 1 and 2 have scattered depocenters, reflecting the process of synthetic and antithetic fault installation and linkage. Rift sequence 3 has a widespread record throughout the basin. Rift sequence 4 records the ending of mechanical subsidence with minor faulting, and is followed by an uplift which caused the initial continental shelf to be exposed during most of the subsequent depositional events. The drift supersequence was split in two phases or stages. The first drift stage has sedimentation confined to the continental slope due to the uplift of the shelf region, whereas during the second drift stage sedimentation surpasses the border fault and the sedimentary succession overlies directly the rift supersequence. A structural fault map was built, depicting main faulting controls and trends, basin hinge line, main depocenters, the Jacuípe External High and a volcanic plug. The paper also clarified the still controversial southern geologic limit of the basin, and closes with a suggestion of an updated chronostratigraphic chart.

Late Neoproterozoic–early Paleozoic seismic structure–stratigraphy of the SW Tarim Block (China), its passive margin evolution and the Tarim–Rodinia breakup

A complete structural and stratigraphic record of extensional tectonics and passive margin evolution is displayed by the late Neoproterozoic–early Paleozoic syn- and post-rift sequences exposed in the SW Tarim Block. The syn-rift strata, represented by the upper Neoproterozoic Nanhua and Sinian Systems, occur in NE–SW– and WNW–ESE–trending graben and half–graben structures and comprise clastic terrestrial and bimodal volcanic rocks that are interspersed with diamictites. Nearly 100- to 150-km-long, sub-elliptical basins with ~400-m-thick upper Neoproterozoic Nanhua System deposits are nested within these grabens. Stratigraphically higher, upper Sinian units include littoral to shallow marine clastic rocks, grading upwards into massive dolomites and marking an important transition from terrestrial to a shallow marine depositional environment in the passive margin evolution of SW Tarim. Unconformably overlying these Neoproterozoic syn–rift rock units are the lower Cambrian sandstone, mudstone and dolomite, and the middle Cambrian dolomitic limestone and stromatolites collectively making up the post–rift sequence. These rocks represent a shallow marine transgressive sequence, deposited on a submerged carbonate platform of the rifted margin of SW Tarim. The regional unconformity between the syn– and post–rift sequences marks a sharp breakup unconformity, pointing to a major tectonic transition from continental rifting to drifting in the history of the Tarim–Rodinia breakup. This extensional tectonics and passive margin evolution during the late Neoproterozoic–early Cambrian indicate that the collisional Pan–African orogenic events, which affected several other East Gondwana continents, did not involve the Tarim Block.

Rift activity and sequence stratigraphy of the Oligocene Dongying Formation in the Nanpu Sag, eastern China: Implications for rift sequence stratigraphy in lacustrine basins

Rifting plays an important role in accommodation variations in rift basins. However, fluvial discharge as another important factor affecting lacustrine rift sequence stratigraphy is not well explored. In this work, seismic and well data are used to investigate the stratigraphic features, rift activity, and effect of fluvial discharge in the Nanpu Sag, Bohai Bay Basin. The syn‐rift deposition of the Dongying Formation (28.5–23.8 Ma) is divided into four sequences (SQ1, SQ2, SQ3, and SQ4), and the rift‐related subsidence rate along two parallel boundary faults is calculated. The results indicate that the rift activity weakens from SQ1 to SQ4 and that the two parallel boundary faults consist of a total of four fault segments. As the fault segmented, three propagation folds formed intrabasin highs on which the Beipu, Laoyemiao, and Gaoliu outlets developed. Well logs and 3D seismic data show that from SQ1 to SQ3, retrogradational stacking patterns occur in Beipu, Laoyemiao, and the hanging‐wall ramp, while progradational stacking patterns occur in the Gaoliu outlet. In SQ4, the sequence stacking patterns are progradation‐dominated. By evaluating channel erosion and sediment supply, the fluvial discharge can be estimated, and the result indicates that the Gaoliu outlet has the strongest fluvial discharge from SQ1 to SQ3 with strong channel erosion, and in SQ4, the fluvial discharge of each outlet is comparable and relatively low. The high fluvial discharge of the Gaoliu outlet from SQ1 to SQ3 is considered to be the primary reason for the lake level rise in the intervals between the pulses of rift activities. Sequence stacking patterns and depositional trends in our study intervals are not in line with the rift subsidence rate; this indicates that rifting is not the only factor controlling the rift sequence stratigraphy and variations in the accommodation of lacustrine rift basins. While the Gaoliu outlet has predominant fluvial discharge from SQ1 to SQ3, progradational parasequence sets deposit in the Gaoliu outlet. In contrast, the retrogradational parasequence sets occurred in the Beipu outlet, Laoyemiao outlet, and the hanging‐wall ramp. As the fluvial discharge of the Gaoliu outlet decreased in SQ4, the fluvial discharge in each outlet became low, and the stacking patterns were dominated by progradation. Consequently, this research suggests that fluvial discharge is a significant factor controlling the accommodation creation in lacustrine rift basins, and a sequence model which gives a full consideration of fluvial discharge is constructed for lacustrine rift basins.

Modelling Rifting Sequence Stratigraphy Coupled with Surface Process and Thermo-Mechanical Modelling

Rift settings preserve high-fidelity records of their depositional history in response to multiple processes, such as climate change that significantly influences the sediment input, and tectonic deformation which contributes to accommodation generation or consuming. Integrated studies of geomorphology, thermochronology, analog experiments and numerical modelling improved our understanding of the rifting processes and associated structural evolution. However, the interplay between climate change, sediment transport from eroding highland to rift basins and rift-related deformation is poorly understood.We present a forward numerical scheme that couples surface process with thermo-mechanical modelling on a rift setting. In the coupling numerical framework, a 2D (potentially 3D) lithospheric scale model is set up. The erosion, sediment transport and deposition are controlled by surface processes with the boundary conditions of climate force (precipitation) and erosion coefficient. The resulting sediment volumes are transferred to the thermo-mechanical system, which has significant effect of crustal deformation. The produced tectonic uplift or subsidence then contributes to the change of surface topography and thus the sediment routing. We focus on investigating the climatic controls on source dynamics, sediment transport, and the deposition in both marine and nonmarine environments. We then quantify the influence of sediment accumulation on crustal deformation and rift evolution. The resulting stratigraphic architecture will be analyzed through evolving stratal stacking patterns and shoreline trajectories to explore the feedbacks between erosion/deposition patterns and the rift structural. We will then apply our modelling to typical rifting examples such as East Africa rift system.

Neoproterozoic igneous activity and Permo-Triassic metamorphism inthe Gapyeong area within the Gyeonggi Massif, South Korea, and theirimplication for the tectonics of northeastern Asia

Although Neoproterozoic rocks were recently found in the western margin of the Gyeonggi Massif, no Neoproterozoic rocks were believed to be present within the interior of this massif. However, in this study, Neoproterozoic rocks are recognized for the first time within the interior of the Gyeonggi Massif through the discovery of the Neoproterozoic volcano-sedimentary rocks of the Gapyeong Formation (GF). The GF is located in the Gapyeong area within the northern Gyeonggi Massif to the north of the Permo-TriassicHongseong-Odesan continental collision boundary in South Korea. The GF mainly consists of mafic granulite interlayered with metacalcsilicate, marble, biotite gneiss, and metatrachyandesite and contains a serpentinized ultramafic block and hornblende biotite gneiss slab. The ages and whole-rock geochemistry indicate that the interlayered metatrachyandesite and mafic granulite in the GF were trachyandesite and basalt that formed through bimodal volcanism in a continental rift tectonic setting at ca. 854 Ma, which matches the timing of the Rodinia supercontinent breakup. The GF can be correlated to the 900–850 Ma Neoproterozoic rifting sequences in the southern Nangrim and northern Gyeonggi Massifs in the Korean Peninsula, which can be correlated to the 930–830 Ma Neoproterozoic rifting sequence along the southern margin of the North China Craton. The GF is difficult to correlate with the ca. 890–790 Ma subduction-related Neoproterozoic rocks in the Hongseong area in the southwestern Gyeonggi Massif, which are considered equivalent to rocks in the northern margin of the South China Craton. The GF underwent upper granulite-facies metamorphism (816–882 °C, 9.0–10.9 kbar) at ca. 250 Ma during the collision between the North and South China Cratons. The serpentinite block in the GF formed in a suprasubduction zone, and the hornblende biotite gneiss slab in the GF was metamorphosed under lower granulite-facies conditions (725–767 °C, 7.0–8.4 kbar) at ca. 414 Ma, indicating that the block and slab were tectonically incorporated into the GF. A strong mylonitic texture is observed in the GF but not in the basement Paleoproterozoic gneiss except at its boundary contact with the GF. These data indicate that the GF, together with the serpentinite block and hornblende biotite gneiss slab, was tectonically transported to the present area after the Triassic metamorphism.

The fluid evolution of the Nimbus Ag-Zn-(Au) deposit: An interplay between mantle plume and microbial activity

The Nimbus Ag-Zn-(Au) volcanic-hosted massive sulfide (VHMS) deposit represents an exceptional orebody in the Yilgarn Craton of Western Australia. It is located in a zone of juvenile crust running N-S through the Eastern Goldfields Superterrane. However, unlike most other VHMS occurrences associated with the 2690–2680 Ma rift sequence (Teutonic Bore, Jaguar, Bentley, Erayinia), it is hosted by the ca. 2705 Ma plume-related stratigraphy, which is more typically associated with komatiite-hosted nickel-sulfide mineralization. The Nimbus deposit displays hybrid VHMS-epithermal characteristics resulting from low temperature and shallow water conditions, which developed a quartz-carbonate-sericite dominated alteration assemblage in the dacite host rocks. Sulfide mineralization comprises pyrite, sphalerite, galena, arsenopyrite, Ag-Sb-Pb-Bi sulfosalts, and rare chalcopyrite. In this study, we applied a combination of in-situ analytical techniques to monitor the evolving sulfur isotope signature of the mineralizing fluids, and propose that the Nimbus VHMS deposit developed as a result of a bimodal fluid history. In the first stage, the hydrothermal system – powered by plume-related magmatism – deposited a series of barren pyrite lenses with colloform textures. Their variable but consistently negative mass-independent sulfur isotope fractionation signature (Δ33S = −0.81%) reflects the interaction between mantle-derived magmatic fluids and Archean seawater. The ubiquitous presence of carbon-rich porous textures and the remnants of carbonaceous “nests” in colloform pyrite also indicate a significant contribution of microbial sulfate reducers during the incipient stages of sulfide precipitation. The gradual formation of colloform lenses eventually sealed the hydrothermal system establishing a physical barrier that limited the interaction with seawater. This process promoted the onset of higher temperature and pressure conditions required to form the high-grade Zn-Ag mineralization, sourcing sulfur and fluids largely from a magmatic reservoir (Δ33S = +0.09%). Whereas the identification of a predominantly magmatic sulfur source is supported by sulfur isotope signatures, the recognition of a magmatic origin of the fluid may only be inferred indirectly. It is consistent with the observed quartz-sericite alteration style and the proposed mechanism of sulfide deposition via decompression, which does not require any interaction with seawater. The trace element distribution of the ore-related sulfides also supports a closed hydrothermal system developed through multiple fluid pulses. Indeed, this ore-forming scenario is commonly favored by high confining pressures, which at Nimbus were established following the deposition of the colloform pyrite lenses that progressively sealed the hydrothermal system creating the favorable conditions for mineralization.

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.

Seismic interpretation of the diapiric structures and gas chimneys in the Liwan gas field, Pearl River Mouth Basin, northern South China Sea

Voluminous diapiric intrusions are present in 3D seismic data from the Liwan gas field area in the Baiyun sag of the Pearl River Mouth Basin, South China Sea. These diapirs are subdivided into magmatic diapirs and mud diapirs according to their seismic signatures and formation mechanisms. Saucer- and mound-shaped reflectors with anomalously high amplitudes distributed throughout the Early Miocene strata are interpreted as sills and laccoliths and are typified as magmatic diapirs. Mud diapirs in the study area can be divided into two types: wavy reflectors occurring at the basement of the sag caused by the gravitational instability of plastically deformed mudstones and columnar upheaval anomalies rooted in Paleogene rift sequences. Gas chimneys are characterized by vertical weak/blank zones accompanied by pull-down effects. Vertical gas chimneys and mud diapirs pierce the Middle Miocene strata. The vast majority of them are triggered by normal faulting; however, no deep-seated faults can be detected around the peripheries of igneous intrusions. Migration forces and pathways are represented by vertical mud diapirs and gas chimneys and thus control the distribution of shallow gases with negative-amplitude anomalies as well as gas hydrates.

Field and geochronological evidence for origin of the Contendas-Mirante supracrustal Belt, São Francisco Craton, Brazil, as a Paleoproterozoic foreland basin

The low-grade Contendas-Mirante meta-volcano-sedimentary belt (CMB) was previously interpreted as an Archean greenstone belt with the uppermost formation deposited in the Paleoproterozoic. New zircon U-Pb LA-ICP-MS ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development. Phyllites of the Lower Group and meta-arenites of the Upper Group yielded similar young zircon populations at ca. 2080Ma, which are interpreted as the maximum deposition age for the entire sequence. The unique exception is the 3273–3338Ma zircon-bearing quartzite that occurs in spatial association with the 3300Ma rhyolite. A granite sheet intrusive into metagraywacke of the Lower Group constrains the minimum deposition age of 2045±26Ma for the CMB. Altogether four main age clusters of detrital zircon grains were recognized, namely 2075–2200Ma, 2200–2440Ma, 2500–2770Ma, 3270–3380Ma. These age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the São Francisco craton. Maximum deposition age for both groups indicates that sedimentation occurred immediately prior the high-grade metamorphism in the adjacent Archean-Paleoproterozoic Jequié Block and Itabuna-Salvador-Curaçá Orogen. These observations, along with coarsening upwards in the sediments, indicate that the CMB represents a foreland basin developed along the eastern margin of the Gavião Block during plate convergence. Paleoarchean basement rocks were reactivated and emplaced into the CMB during basin inversion and is timely correlated with the granitic intrusion. Recognition of older supracrustal fragments is ambiguous, one main example is the 3273–3338Ma zircon-bearing quartzite, which correlates with the northern Jacobina basin along the Contendas-Mirante lineament (e.g., Bahia gold belt) and may represent part of an Archean intracontinental rift sequence.

Architecture of the intracontinental Jaibaras Rift, Brazil, based on geophysical data

Qualitative and quantitative integration and interpretation of magnetic, gravity and magnetotelluric data help to determine the internal architecture of the Jaibaras rift, and allow assessing the evolution of the Jaibaras Rift within the Precambrian crystalline basement of Borborema Province, NE Brazil. This was achieved by 2D joint modeling of magnetic and gravity data in five sections across the main axis of the Jaibaras Rift. Surface data, rock density measurements, depth constraints from 2D Euler deconvolution and geophysical information from previous work in the area were integrated to constrain the modeling. The magnetic and gravity profiles of the Jaibaras Rift indicate estimated source bodies at depths up to 2.5 km, showing complex configuration for the structural framework, with a set of asymmetric grabens and horsts. The 2D magnetotelluric inversion shows that the Jaibaras Rift is marked by low resistivity values, and maximum thickness of the sedimentary package up to approximately 3 km. Shallow dipping conductive material may represent either a suture zone between the Ceará Central and Médio Coreaú domains or a set of fractures due to horizontal σ1 stress in the Ceará Central Domain. The Jaibaras Rift displays a very complex internal structure, with discontinuous sequences of grabens and horsts, and a significant volume of surface and subsurface volcanic rocks. The sedimentary packages with volcanic rift sequences have variable thicknesses, from 1 to 3 km. These rock units are controlled by normal faults that developed from older discontinuities, such as the Transbrasiliano lineament.

Exploration for Late Cretaceous turbidites in the Equatorial African and northeast South American margins

AbstractThe Suriname–Guyana and Ghana–Ivorian Basins exhibit strong geological similarities which are of interest from a petroleum exploration point of view. These include (1) a well-developed system of Late Cretaceous erosional canyons allowing coarse-grained shallow-water clastics to enter the deep marine basin to form attractive turbidite exploration targets; (2) a broad shelf with strong longshore currents which sort and transport coarse clastics into the canyon heads; (3) an organic-rich Cenomanian–Turonian hydrocarbon source rock which is thermally mature in the centre of the basin; and (4) a series of extensional fault-networks along the shelf margin that extend upwards from the Rift Sequence into the overlying Drift Sequence and which, along with the canyon geometries, enable migrating oil and gas to accumulate in combination structural and stratigraphic traps. In June 2007, Tullow and its partners made an important discovery in offshore Ghana, at Mahogany, which subsequently became the giant Jubilee field. Tullow is currently applying the same geoscientific technologies in offshore Suriname in the search for analogous subtle combination traps.

Hyper-extended rift systems in the Xisha Trough, northwestern South China Sea: Implications for extreme crustal thinning ahead of a propagating ocean

From the apex of the NW Sub-basin of the South China Sea, we present new long-cable deep-penetration seismic profiles and deepwater drillings together with corrected seismic interpretations from previous studies. Structural mapping demonstrates that the Xisha Trough is characterized by a hyper-extended rift system that shows evidence of extremely thinned crust and rotated fault blocks. A series of NE, NEE and W-E striking listric normal faults along the Xisha Trough, which merge at depth into Moho reflection, record strongly extensional deformation. Based on the absence of an acoustic basement beneath a wedge-shaped rifted sequence, we document three diapiric magma structures exiting the Moho and penetrating into the extremely thinned continental crust, which is covered by a Cenozoic sequence. For the Xisha Trough we can demonstrate, based on seismic data and calculated fault slip rates, the faults bounding the Xisha Trough were initially formed by distributed rifting, and were reactivated and focused on faulting along the axis of the Xisha Trough. However, the peak faulting migrated along the margin strike from east to west and terminated at the apex of the Xisha Trough. One of the key processes, controlling the evolution of the Xisha Trough, is depth-dependent extension, which may arise from differential lithospheric stretching by regional dynamic lower crustal/mantle flow.

Sequential patterns in Cenozoic marginal basins of the Northwest Pacific

AbstractBy using magnetic anomaly identifications, onshore and offshore exploratory wells, ODP drillings, isotopic dating and orthochronology results, this paper summarizes the tectonic event sequence (including extension, back‐arc spreading, subsidence, convergence/compressive deformation and thrusting) of the marginal basins of the NW Pacific and extracted sequence patterns of the marginal basins according to the tectonic event sequence from an array of oil company regional seismic lines and other geological data. Three important evolutionary stages of the Northwestern Pacific (NW Pacific) marginal sea basins are distinguished: from the Palaeocene to Eocene, from Oligocene to Middle Miocene and from the later Middle Miocene to the present day. Each stage corresponds to a rifting sequence (Palaeocene to Eocene), a drifting sequence (Oligocene to Middle Miocene) and a subsidence/convergence sequence (later Middle Miocene to the present day). The rifting sequence and drifting sequence developed sequentially in the South China Sea Basin, from Palaeocene to Eocene and from Oligocene to Middle Miocene, while in the Japan and Okhotsk Sea basins, an early stage rifting sequence was not developed in the Palaeocene to Eocene interval. However, the drifting and rifting sequence developed synchronously from the Oligocene to Middle Miocene. From the Later Miocene time, all the marginal basins of the Northwest Pacific basically stopped drifting and began to enter the subsidence (post‐drifting) stage. An arc‐continent collision from the south caused the formation of a convergence sequence in the southern margin of each marginal basin. The NW Pacific Plate collision with the Kuril–Kamchatka Arc led to the deformation of the subsidence sequence in the southern margin of the Kuril Sea Basin. The NW Pacific Plate convergence with the Japan Arc led to the formation of a convergence sequence in the southern margin of the Japan Sea Basin. The Borneo–Palawan Arc convergence with the Dangerous Grounds–Reed Bank Block in the South China Sea Basin from 15 Ma led to the formation of the convergence sequence in the southern margin of the South China Sea Basin. The degree of thrusting deformation of the convergent sequence increased gradually from north to south in the marginal basins of the Northwest Pacific. Copyright © 2016 John Wiley & Sons, Ltd.

Geology and U/PB geochronology of the Gamtoos Complex and lower Paleozoic Table Mountain Group, Cape Fold Belt, Eastern Cape, South Africa

The Cape Fold Belt (CFB) along the southern coast of South Africa contains several tectonic windows that expose low-grade “basement” rocks as inliers that are inferred to be late Neoproterozoic in age subjected to tectono-metamorphism during the Pan African Saldanian Orogeny (ca. 650 to 550 Ma). Whilst a Saldanian tectonic history along the northwest to southeast trending western branch of the CFB has been well documented, such Neoproterozoic deformation along the east-west trending southern branch of the CFB is not established. This work presents new field observations and SHRIMP U/Pb geochronologic data from the Kleinrivier Sequence within the Gamtoos Inlier exposed along the eastern-most known tectonic window, near Port Elizabeth, and which is here renamed the Gamtoos Complex. The study area includes 3 tectonic sequences from the Gamtoos Complex (“pre-Cape”) and 2 sequences from the overlying Table Mountain Group of the Cape Supergroup. The lower sequences of the Gamtoos Complex comprise north-east verging thrust packages within a regional antiform, all of which have a common oriented planar tectonic fabric (S 2 ) coincident with those in the lower Paleozoic rocks. Phyllites from the Kleinrivier and upper Sardinia Bay sequences (spanning the pre-Cape and Cape boundary) are similarly affected by a slaty cleavage (S 1 ), crenulation cleavage (S 2 ) and multiple generations of quartz veins. U/Pb data are from zircons of metasediments and igneous rocks from the Gamtoos Complex and from the overlying rocks of the Table Mountain Group. Meta-greywackes of the Kleinrivier Sequence (youngest zircon: 523 ± 6 Ma) are probably Cambrian in age. However, some units from the Kleinrivier Sequence are intruded by felsic sills with a Concordia age: 530.2 ± 4.4 Ma and therefore require further U/Pb detrital zircon analyses. Mafic sills and dykes also intrude the Kleinrivier Sequence and were subsequently deformed and contain S 2 . Boulder conglomerates in the Sardinia Bay Sequence (youngest matrix zircon: 521 ± 6 Ma) are separated from the Kleinrivier Sequence by an angular unconformity. It also contains granite-boulders (Concordia age: 530.2 ± 4.7 Ma), and is overlain by the feldspathic psammites and quartzites of the Sardinia Bay Sequence with zircons as young as 510 ± 7 Ma (e.g. middle Cambrian). In total, 75% of the detrital zircons from all sequences date between early-Mesoproterozoic and early-Neoproterozoic (ca. 1455 to 852 Ma) and are likely sourced from the gneisses of the Namaqua-Natal province that are known to underlie the CFB, whilst the Neoproterozoic to mid-Cambrian zircons (ca. 828 to 431 Ma) have been sourced from Pan African mobile belts, possibly the Mozambican and/or Saldanian Belts to the east and west, respectively. Such “Pan African” detrital zircons are more prevalent in the Sardinia Bay Sequence and form the dominant component in the Lower Table Mountain Group, suggesting a lesser source influence from the Namaqua Natal Mobile Belt. The conglomerates at Sardinia Bay possibly represent mid-Cambrian rift sequences, similar to the Klipheuwel Group that overlie the Malmesbury Sequence and Cape granites along the western branch of the CFB. The Sardinia Bay Sequence has zircons equivalent to those of the Peninsula Formation (youngest zircon: 516 ± 5 Ma), and therefore should be linked to represent the lower Table Mountain Group.

Climate‐modulated sequence development in a tropical rift basin during the Late Palaeocene to Early Eocene super greenhouse Earth

AbstractSequence developments in rift basins are considered to be influenced largely by tectonics and to a lesser extent by eustatic sea‐level and climate. Studies indicate that in passive margin basins, climate can mask the effects of tectonics and eustasy by modulating the sediment supply. It is, however, less understood how the sedimentary sequence in rift basins might respond to strong climatic fluctuations where tectonic pulses generate rapid accommodation space. Here a case study has been provided to assess the effect of climate vis à vis sea‐level and tectonics on sequence development in the Cambay rift basin, western India, during the Early Palaeogene (Late Palaeocene to Early Eocene) super greenhouse globe. Facies analysis of this shale–lignite sequence suggests deposition in a lagoon/bay, developed over the Deccan Trap basement. Detailed sequence stratigraphic analysis using basin‐wide representative composite sections, marker lignite seam, event bed and high‐resolution carbon isotope (δ13C) chemostratigraphy suggest an overall transgressive motif. Among the three prominent Early Eocene eustatic highstands, only the one at ca 53·7 Ma is expressed by the thickest coal accumulation throughout the basin. Expression of the other sequence stratigraphic surfaces is subdued and can be due to the overall finer grain size of the sediment or local variation in the subsidence rate at different fault‐controlled mini‐basins. Enigmatic presence of a maximum flooding surface coinciding with the 53·7 Ma climate event (Eocene Thermal Maximum 2), manifested by negative carbon isotope excursion, indicates possible influence of climate over and above tectonics in developing the rift sequence. Qualitative rainfall variation assessed using the magnitude of the carbon isotope excursion and pollen abundance show that a relatively dry/low precipitation climatic phase during the Eocene Thermal Maximum 2 hindered the siliclastic supply to the basin. Thus, it has been inferred that climate‐induced high siliciclastic supply possibly enhanced the autocyclic reorganization and hindered the development of the key sequence stratigraphic surfaces across the basin during climate extremes.