Offshore Rift Research Articles

From widespread faulting to localised rifting: Evidence from K‐Ar fault gouge dates from the Norwegian North Sea rift shoulder

AbstractAlthough seismic and stratigraphic well information put tight constraints on rift basin evolution, eroded rift shoulders commonly expose polydeformed prerift basement whose deformation history may be difficult to constrain. In this work, we apply K‐Ar dating of fault gouge samples from 18 faults to explore the brittle deformation of the well‐exposed eastern rift margin to the northern North Sea rift. We find evidence of clay gouge formation since the Late Devonian, with distinct Permian and Jurassic fault activity peaks that closely match early stages of the two well‐established North Sea rift phases. A marked decay in fault density away from the rift margin confirms a close relationship between rifting and onshore faulting. The results show that initial rift‐related extension affected a much wider area than the resulting offshore rift. Hence our data support a rift model where strain is initially distributed over a several 100 km wide region, as a prelude to the development of the ~150–200 km wide Permo‐Triassic northern North Sea rift as defined by large marginal faults. Towards the end of the second rift phase, strain localises even more strongly to the 25–50 km wide Viking Graben. Interestingly, a period of early widespread extension is seen for both phases of North Sea rifting and may be a general characteristic of continental rifting. The documented prerift faulting and fracturing of the basement since the Devonian weakened the basement and probably facilitated the widespread initial extension that subsequently localised to form the northern North Sea rift, with further localisation to its relatively narrow central part (Viking Graben).

Hydrocarbon Enrichment and Main Controlling Factors in Offshore Rift Basins of China: A Case Study in the Beibuwan Basin

AbstractBeibuwan Basin is an important offshore oil and gas bearing basin of China. However, the distribution of oil and gas in different sags is uneven, and the rule of hydrocarbon accumulation and main controlling factors have not been clearly understood in this basin. Based on abundant seismic, well and analytical data, the differences and main controlling of tectonic evolution, sedimentary filling and source rock quality in sags are analyzed, and combined with the study of geothermal filed, the main controlling factors of differential hydrocarbon generation in sags of the Beibuwan Basin are clarified. On the basis of the research above, the hydrocarbon enrichment of the basin is clarified through the systematic analysis of reservoir‐caprock assemblage and accumulation characteristics of different structural units. The results show that there are four NE‐trending sag zones in the basin, and under the influence of fault activity, the scale of sags is regularly distributed. The scale of rifts in different sag zones decreases gradually from northwest to southeast and the scale of rifts in the middle of a single sag zone is normally larger than that at two ends. Under the control of the sag‐controlling faults activity, paleoproductivity and organic matter preservation conditions, the quality of source rocks in sags of the Beibuwan Basin decreases from northwest to southeast. The hydrocarbon‐rich sags in the Beibuwan Basin were formed under the control of source rocks and geothermal field, and the distribution of large‐medium scale fields is controlled by the favorable structural traps in the rift strata of inner gentle slope zone. The understanding of the hydrocarbon enrichment and main controlling factors in the Beibuwan Basin also provides a significant guidance for the oil and gas exploration in other rift basins.

Cenozoic tectonic evolution of southeastern Thailand derived from low-temperature thermochronology

Low-temperature thermochronological techniques, specifically apatite (U–Th)/He and apatite fission-track dating, were used to reconstruct the thermal history of southeastern Thailand. This area is intersected by vast and complex fault networks related to the Cenozoic Mae Ping and Three Pagodas Faults. These were identified from satellite imagery and confirmed by field observations. New apatite fission-track and apatite (U–Th)/He data were collected from crystalline basement blocks within these fault networks. Ages obtained range from 48 to 24 Ma, with most of the samples clustering between 36 and 24 Ma. Thermal history modelling indicates late Eocene–Oligocene exhumation of the exposed granitic and metamorphic basement rocks in southeastern Thailand. Exhumation was regional and was contemporaneous with sinistral fault activity during the late Eocene–early Oligocene along the Mae Ping Fault and Three Pagodas Fault. Moreover, this exhumation occurred coevally with a synrift phase of intracontinental offshore rift basin and half-graben basin development in the eastern Gulf of Thailand. The phase of exhumation ended in the early Miocene, as a result of the changing plate-tectonic forces along the complex plate boundaries of Sundaland. Supplementary material: AFT data, radial plots and the detailed procedure of thermal history modelling are available at https://doi.org/10.6084/m9.figshare.c.4633064

Quantitative analysis of the tectonic subsidence in the Potiguar Basin (NE Brazil)

The Potiguar Basin, located in the Brazilian Equatorial Margin, evolved from a complex rifting process implemented during the Atlantic Ocean opening in the Jurassic/Cretaceous. Different driving mechanisms were responsible for the onset of an aborted onshore rift and an offshore rift that initiated crustal rupture and the formation of a continental transform margin. Therefore, we applied the backstripping method to quantify the tectonic subsidence during the rift and post-rift phases of Potiguar Basin formation and to analyze the spatial variation of subsidence during the two successive and distinct tectonic events responsible for the basin evolution. The parameters required to apply this methodology were extracted from 2D seismic lines and exploratory well data. The tectonic subsidence curves present periods with moderate subsidence rates (up to 300 m/My), which correspond to the evolution of the onshore Potiguar Rift (∼141 to 128 Ma). From 128–118 Ma, the tectonic subsidence curves show no subsidence in the onshore Potiguar Basin, whereas subsidence occurred at high rates (over 300 m/My) in the offshore rift. The post-rift phase began ca. 118 Ma (Aptian), when the tectonic subsidence drastically slowed to less than 35 m/My, probably related to thermal relaxation. The tectonic subsidence rates in the various sectors of the Potiguar Rift, during the different rift phases, indicate that more intense faulting occurred in the southern portion of the onshore rift, along the main border faults, and in the southeastern portion of the offshore rift. During the post-rift phase, the tectonic subsidence rates increased from the onshore portion towards the offshore portion until the continental slope. The highest rates of post-rift subsidence (up to 35 m/My) are concentrated in the central region of the offshore portion and may be related to lithospheric processes related to the continental crust rupture and oceanic seafloor spreading. The variation in subsidence rates and the pattern of tectonic subsidence curves allowed us to interpret the tectonic signature recorded by the sedimentary sequences of the Potiguar Basin during its evolution. In the onshore rift area, the tectonic subsidence curves presented subsidence rates up to 300 m/My during a long-term rift phase (13 Ma), which confirmed that this portion had an extensional tectonic regime. In the offshore rift, the curves presented high subsidence rates of over 300 m/My in a shorter period (5–10 My), typical of basins formed in a transtensional tectonic regime.

Basement structure and its influence on the structural configuration of the northern North Sea rift

AbstractThe northern North Sea rift basin developed on a heterogeneous crust comprising structures inherited from the Caledonian orogeny and Devonian postorogenic extension. Integrating two‐dimensional regional seismic reflection data and information from basement wells, we investigate the prerift structural configuration in the northern North Sea rift. Three seismic facies have been defined below the base rift surface: (1) relatively low‐amplitude and low‐frequency reflections, interpreted as pre‐Caledonian metasediments, Caledonian nappes, and/or Devonian clastic sediments; (2) packages of high‐amplitude dipping reflections (>500 ms thick), interpreted as basement shear zones; and (3) medium‐amplitude and high‐frequency reflections interpreted as less sheared crystalline basement of Proterozoic and Paleozoic (Caledonian) origin. Some zones of Seismic Facies 2 can be linked to onshore Devonian shear zones, whereas others are restricted to the offshore rift area. Interpreted offshore shear zones dip S, ESE, and WNW in contrast to W to NW dipping shear zones onshore West Norway. Our results indicate that Devonian strain and ductile deformation was distributed throughout the Caledonian orogenic belt from central South Norway to the Shetland Platform. Most of the Devonian basins related to this extension are, however, removed by erosion during subsequent exhumation. Basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters, e.g., in the Stord and East Shetland basins. Prerift structures with present‐day dips >15° were reactivated, although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction.

Seismic characteristics and distribution of hydrothermal vent complexes in the Cretaceous offshore rift section of the Campos Basin, offshore Brazil

Hydrothermal vents are structures that occur close to volcanically active areas. Their occurrence can have considerable impacts on the evaluation of the oil reserves of a basin. In the Campos Basin, eastern Brazilian continental margin, no hydrothermal vents have been previously recognized and related with magmatic events on seismic sections. This work aims to map the hydrothemal vents, ducts and associated intrusive bodies observed in the rifte section. With the use of 2D seismic data and a well with lithologic information, the occurrence of ten hydrothermal vents and seven ducts were mapped and the interval with volcanic intrusive rocks was characterized. The identification of intrusive rocks in seismic data is given by reflectors with high amplitude and disk or half-moon geometry with abrupt endings. The vent geometries shows dome and eye-shaped types and the ducts are fault-related, downward-tapering cone type. This study highlights new geological elements that can influence the petroleum systems of the Campos Basin, such as the identification of hydrothermal vents in seismic sections and the correlation with regional tectono-magmatic events.

Active faulting at the western tip of the Gulf of Corinth, Greece, from high-resolution seismic data

Abstract The Gulf of Corinth is one of the fastest-spreading intra-continental rifts on Earth. GPS data indicate that the rift is currently opening in a NNE-SSW direction, with a rate of extension reaching up to 16 mm yr− 1 in its westernmost part. Although the rest of the offshore rift has been well studied, the western tip of the rift is still poorly explored. We present an accurate map of submarine faults in this area based on two high-resolution seismic reflection surveys (single-channel sparker). In the eastern part of the studied area, the sedimentary infill is affected by the known North Eratini, South Eratini, and West Channel faults. Further to the west, the seafloor is mostly flat and is bounded to the north by the normal, south-dipping, Trizonia fault. To the north, the shallower part of the Gulf shows to the east a diffuse pattern of normal and strike-slip deformation, which is replaced to the west by a 7.5 km long SE striking strike-slip fault zone, called the Managouli fault zone. To the westernmost tip of the Gulf, in the Nafpaktos Basin, two fault sets with different strikes are encountered; the one with a NE-SW strike exhibits a clear strike-slip component. The western tip of the Gulf of Corinth is the only part of the Corinth Rift where convincing evidence for strike-slip movement has been found. This fault pattern is likely related to the complex deformation occurring at the diffuse junction at the western tip of the Rift between three crustal blocks: Continental Greece, Peloponnese, and the Ionian Island-Akarnania block.

Modal decomposition of magnetic maps: the case of Cape Roberts aeromagnetic survey, Antarctica

This paper proposes a digital enhancement tool for magnetic anomaly maps. The magnetic anomaly map is decomposed by means of Eigenvalue Decomposition into a number of orthogonal bases. The dataset is then filtered accordingly to a specific variance pattern. The dataset is decomposed and the eigenvalues population is inspected so that the variance is evaluated yielding the definition of two thresholds. Subsequently, the dataset is reconstructed into three subsets which hold different features respectively. The proposed filtering procedure is first tested with a synthetic signal and then applied to the data of Cape Roberts (Antarctica) aeromagnetic survey, flown over an off-shore rift basin. The proposed method appears to be efficient in noise removal and acts as a digital enhancement tool which provides TMI anomaly maps revealing hidden lineaments, otherwise not visible. The methodology effectiveness as a hidden lineaments detection tool has been checked against independent data.

Fault architecture, basin structure and evolution of the Gulf of Corinth Rift, central Greece

ABSTRACTThe style of extension and strain distribution during the early stages of intra‐continental rifting is important for understanding rift‐margin development and can provide constraints for lithospheric deformation mechanisms. The Corinth rift in central Greece is one of the few rifts to have experienced a short extensional history without subsequent overprinting. We synthesise existing seismic reflection data throughout the active offshore Gulf of Corinth Basin to investigate fault activity history and the spatio‐temporal evolution of the basin, producing for the first time basement depth and syn‐rift sediment isopachs throughout the offshore rift. A major basin‐wide unconformity surface with an age estimated from sea‐level cycles at ca. 0.4 Ma separates distinct seismic stratigraphic units. Assuming that sedimentation rates are on average consistent, the present rift formed at 1–2 Ma, with no clear evidence for along‐strike propagation of the rift axis. The rift has undergone major changes in relative fault activity and basin geometry during its short history. The basement depth is greatest in the central rift (maximum ∼3 km) and decreases to the east and west. In detail however, two separated depocentres 20–50 km long were created controlled by N‐ and S‐dipping faults before 0.4 Ma, while since ca. 0.4 Ma a single depocentre (80 km long) has been controlled by several connected N‐dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca. 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N‐dipping faults; however, in the western Gulf strain is distributed across several active N‐ and S‐dipping faults throughout rift history, producing a more complex basin geometry.

Gas accumulation models and characteristics of gas fields in China

Due to different accumulation conditions between natural gas and oil, as well as the complexity of geological settings of the main gas bearing basins in China, accumulation of natural gas in Chinese gas fields is a complex process. In terms of timing of gas charging and entrapment, gas fields in China can be grouped into four types: fields with super late (Neogene–Quaternary) gas generation, migration and entrapment; fields with late (Paleogene–Neogene) gas generation, migration and entrapment; fields characterized by late (Tertiary–Quaternary) secondary accumulation of gas generated and accumulated earlier (primarily Mesozoic); and fields with early (primarily Mesozoic) gas generation, migration and entrapment. The first three types are all finalized during the Himalayan period. The Himalayan tectonic movements have, therefore, significant effects on the formation and distribution characteristics of natural gas pools in China. Currently, we are focusing our gas exploration in three major areas: the foreland basins in west–central China, three cratonic superimposed basins (Sichuan, Ordos and Tarim) and offshore rift basins. In these gas-rich basins, the final gas charging and entrapment took place in late and super late stages.

Deep seismic structure of the continental margin in the Gulf of Guinea: a summary report

Abstract Post-stack reprocessing of the PROBE seismic data set has resulted in a refined tectonic model of the continental margin between Cameroon and southern Gabon. The margin is divided into the North Douala and Gabon Basins by the Kribi Fracture Zone, a 75 km wide transform fault zone that trends northeast-southwest and intersects the coastline between 2 and 3°N. North of the Kribi Fracture Zone, oceanic crust extends essentially to the coastline and floors the narrow margin off Cameroon. Little or no offshore rift margin exists in the northern Douala Basin. Reflection Moho beneath oceanic crust is relatively weak but continuous throughout the northern Douala Basin. Oceanic crustal thickness is extremely uniform in the North Douala Basin, averaging c. 1.75 s in two-way travel time. A distinct northeast-southwest trending structural grain at both basement and reflection Moho levels occurs within the Kribi Fracture Zone. Much of the margin off Equatorial Guinea is influenced or controlled by fracture zone tectonics. South of the Kribi Fracture Zone, oceanic crust is offset c. 350 km to the southwest, resulting in a broad rift margin off Gabon that is unaffected by major-offset fracture zones. This margin seems to have evolved by a form of imperfect pure shear, involving brittle, upper continental crustal sheets overlying a series of ductile lower sheets that generally dip easterly to southeasterly. The ductile sheets may unroof at outer highs, which mark the boundary between oceanic and continental lithosphere. Dominant reflection events are often associated with the ductile shear zones that define the lower sheets, but a regionally coherent event representing reflection Moho cannot be delineated. The concept of reflection Moho beneath rifted continental crust may be invalid. The tectonic and rheological models presented here can be used to frame hydrocarbon exploration tactics and strategies along this passive margin. In a broader context, elucidation of margin architecture may be a cost-effective way to approach frontier passive margins in the next century.

Cenozoic transtension along the Transantarctic Mountains‐West Antarctic rift boundary, southern Victoria Land, Antarctica

Brittle fault arrays mapped along the structural boundary between the Transantarctic Mountains and the West Antarctic rift system are oriented obliquely to the axis of the mountains and offshore rift basins. The north to northwest trending regional rift boundary is thus not controlled by continuous rift border faults. Instead, the rift margin trend must be imposed by inherited lithospheric weaknesses along the ancestral East Antarctic craton margin. Fault kinematic solutions indicate that a dextral transtensional regime characterized the rift boundary in the Cenozoic and that dominantly transcurrent motion occurred during the most recent faulting episode. The Transantarctic Mountains are considered to be a rift‐flank uplift, yet no substantial isostatic uplift is expected in a transtensional setting, and the mechanism of large‐magnitude Cenozoic uplift of the mountains remains problematical. Regional deformation patterns in Victoria Land and the Ross Sea can be explained by a transtensional model and are not compatible with large‐magnitude crustal stretching within the West Antarctic rift system in the Cenozoic. The crustal thinning across the rift system more likely took place in the Mesozoic, when major West Antarctic crustal block motions occurred. The Cenozoic intracontinental deformation can be related to plate interaction resulting from the global Eocene plate reorganization, prior to the final separation between Antarctica and a narrow salient of the southeastern Australian margin. Displacement magnitude was probably minor, and thus early Tertiary east–west Antarctic motion is unlikely to account for discrepancies in global plate motion circuits.