Early Stages Of Rifting Research Articles

Rifting in the Paleoproterozoic Onega Basin: geochemistry of volcano-sedimentary rocks of the Zaonega Formation

The study of the volcanogenic-sedimentary sequence in the lower part of the Zaonega Formation in the Paleoproterozoic Onega structure (Karelian craton, Fennoscandian Shield) has shown that tuffs and high-silica rocks predominate in its composition. High-silica rocks (SiO2 up to 94 wt. %) are depleted of all elements and probably representing chemogenic siliceous silt. Tuff rocks are close to N-MORB basalts in terms of major element content and rare element distribution character. This association is common to the early stages of continental rifting in the Phanerozoic and may indicate the formation of volcanogenic-sedimentary complexes of the Zaonega Formation in the environment of continental rifting. The mafic rocks in the lower part of the Zaonega Formation are geochemically identical to dolerite dikes and N-MORB-type basalts of 2.10–2.14 Ga age. Their formation was probably related to the same episode of large-scale stretching and thinning of the continental lithosphere of the Karelian craton in the mid–Paleoproterozoic. In this case, the age limit of the Zaonega and underlying Tulomozero Formations should be somewhat older than the 2.06–2.10 Ga interval accepted in modern regional stratigraphic schemes of the Paleoproterozoic of Fennoscandian shield.

Quantitative Elimination of Seismic Pseudofaults and Fine Analysis of True Faults Underlying Igneous Rocks of No-Well Areas: A Case Study of Shuntuoguole Uplift in Tarim Basin

Abstract After multistage tectonic movement and evolution, large superimposed oil and gas basins generally developed many igneous rocks in the early rifting stages. The lithology and lithofacies of igneous rocks are complex, which is easy to lead to the distortion of the underlying migration velocity field and thus the response of seismic pseudofaults. Also, because of the obvious shielding and absorption effect of igneous rocks on seismic waves, the waveform quality of underlying strata is poor and the seismic response characteristics of faults are fuzzy. Currently, relevant studies have shown that the influence of igneous rock can be eliminated by the prestack depth migration with an accurate igneous rock velocity model. However, improving the accuracy of the velocity model needs to be corrected by well-logging data, resulting in poor applicability of the existing velocity modeling technology underlying igneous rocks without well, which is an obvious technical bottleneck. In this paper, the secondary strike-slip fault in Shuntuoguole low uplift of Tarim Basin, which has great oil and gas exploration potential but a very low degree of drilling, is selected as the research object. Aiming at difficult fault detection underlying igneous rocks caused by lack of drilling, the accuracy of fault seismic identification is improved by “interpretative fault preprocessing” and “fault sensitive attribute optimization.” In addition, through the “extreme hypothesis method” to maximize the complex migration velocity and simulate the underlying target layer distortion maximization, we realize the quantitative elimination of seismic pseudofaults. The practical application shows that this technology can determine the true and fake underlying faults quantitatively without establishing an accurate igneous rock velocity model. It is crucial not only for exploring oil and gas in the Tarim Basin’s secondary strike-slip faults but also for offering a method and technical guide for identifying faults in other basins affected by igneous rocks.

U‐Pb carbonate dating reveals long‐lived activity of proximal margin extensional faults during the Alpine Tethys rifting

AbstractSyn‐rift extensional faults play a significant role during the early stage of rifting. Constraining the age of faulting, and how and when deformation shifts from the proximal to the distal margin areas, is crucial for the reconstruction of the rifting process. Previous assessments of the structural and temporal evolution of rift‐related faults of the Adria proximal margin have primarily relied on indirect biostratigraphic evidence. Additionally, the majority of rift‐related faults underwent tectonic inversion during the Alpine orogeny. In this study, in‐situ U‐Pb geochronology was applied on syn‐kinematic calcites to unravel the activity of the Amora Fault, a remarkable example of a Jurassic growth fault unaffected by the Alpine orogeny. The obtained ages, spanning from Hettangian to Callovian, extend the AF activity beyond the previously established Early Jurassic time. This chrono‐structural model has significant implications on the role of major extensional faults in focusing deformation throughout the rift system's evolution.

Spatial evolution and temporal stability of hydrothermal processes at sediment-covered spreading centers: Constraints from Guaymas Basin, Gulf of California

The Guaymas Basin, Gulf of California is a hydrothermally active sediment-covered oceanic spreading center in the early stages of rifting. Hot-spring fluids were collected from its southern trough in 1998, 2003, and 2008 and analyzed for the concentration and isotopic composition of organic and inorganic aqueous species to assess subseafloor geochemical processes. Fluids discharged from the seafloor through hydrothermal edifices with measured temperatures of 91 to 317 °C. In general, fluids exiting larger structures were characterized by higher temperatures than fluids venting from smaller structures. Measured pH (25 °C) ranged from 5.7 to 7.1. Endmember fluids were characterized by near-zero concentrations of Mg and SO4, depletions in Na, and enrichments in K, Ca, and Cl relative to seawater, reflecting extensive interaction with sediment during circulation. Thermal maturation of sedimentary organic matter resulted in the addition of abundant ΣNH4+, ΣCO2, CH4, short-chain n-alkanes, and carboxylic acids to solution. Organic compounds added to fluids during recharge pass through high temperature deep-seated reaction zones and reach high levels of thermal maturity while others may be added by entrainment of lower temperature sedimentary pore fluids in hydrothermal upflow zones.The relative abundance of many organic compounds mobilized during hydrothermal alteration is consistent with metastable states of thermodynamic equilibrium that likely record subseafloor temperature conditions. The isotope composition of low molecular weight hydrocarbons and elevated concentrations of dissolved Cl suggest that temperatures may have exceeded the two-phase boundary for seawater and subsequently cooled before discharging at the seafloor. Periods of high temperature fluid-sediment or fluid-rock interaction followed by cooling may provide a mechanism for the aqueous mobilization and concentration of sulfide-forming metals in subseafloor mineral deposits. Fluid compositions have remained constant at the southern trough of Guaymas Basin for at least 26 years, indicating decadal stability in the physical and chemical conditions in subseafloor environments as well as the km-scale regional recharge zones containing unaltered sediment.

Hydrocarbon accumulation controlled by early stage rifting and late stage orthogonal subduction, the Madura Sub-Basin, Indonesia as example

Petroleum are mostly found in the sandstone reservoris whithin the Middle to Late Miocene Ngrayong and Wonocolo Formations in the Madura Sub-Basin, Indonesia. Oligo-Miocene reef carbonates have recently attracted the attention of industry. These reef carbonate buildups have the best petro-physical properties, but are the least well understood formations. Combined 2D seismic data interpretation, 3D seismic data attribute extraction, and drilling results, core data and logging data analysis shows that there are two tectonic systems co-existed in the study area and the rifting system trending NE, controls the development of the Kujung reef carbonate, while the subduction system trending EW, provides additional heat for the source rock and tilts the Kujung reef carbonate. The tilting of the carbonate reservoirs results the gas leaking, so gas reservoirs can only be found in the upper level reservoirs and south of the Rembang-Madura-Kangean (RMK) line.

Assessing the potential of syn-rift sediments for geochronological dating and its implications for the development of Makanjira-Shire basin in south Malawi Rift

The Upper Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa, presents an early-stage rift setting where rapid denudation processes take place and have profound influence on the geomorphological evolution of the region. The basin is key to the understanding of mechanisms involved in propagation and growthin young rifts. Although the tectonics in the region are well studied, lack of age constraints due to well-dated strata poses challenges in the understanding of the timing and mechanisms of rift evolution in this section of the south Malawi Rift.We used syn-rift sediments deposited from the rift shoulders to test the applicability of OSL and radiocarbon dating techniques in a poorly dated data-constrained region of the Malawi Rift.Our results suggest that proper sampling strategy is paramount in using the OSL technique for dating in areas of high dosage such as the one under consideration. However, the technique offers potential for use in these areas. Furthermore, we conclude from these findings that the evolution of the Upper Shire basin was triggered by tectonic movements along the Makongwa scrap that were responsible for redirecting the Likwenu River into the in the Zomba Graben through the Upper Shire at least during the Upper Pleistocene.

AGE AND STRATIGRAPHIC POSITION OF THE SUPRACRUSTAL COMPLEX (THE KASKAMA BLOCK, INARI TERRANE, NORTH-EAST KOLA-NORWEGIAN REGION OF THE FENNOSCANDIAN SHIELD)

New data of U-Th-Pb age (SIMS) of magmatic (T = 730–744°C) zircon from intermediate and acid metavolcanics (1923–1926 Ma) of the supracrustal complex in the Kaskama block of the Inari terrane (north-west of the Kola-Norwegian region of the Fennoscandian shield) were obtained, which makes it possible to attribute them to the Kalevian suprahorizon. The source of primary melts of metariodacites and metabasaltes in the Kaskama block was the paleoproterozoic continental lithosphere not younger than 2390–2384 Ma. It is shown that the age of the tonalites of the Kuroaivi massif (1936 ± 7 Ma, εNd(T) = +0.1) as well as the granitoid massifs of the Southern Pechenga zone (1950–1940 Ma), are older, within the errors of U-Th-Pb age determinations, than the volcano-sedimentary rocks of the Kaskama unit and the Vepsian of the Southern Pechenga zone. This period of granite formation separates the early geodynamic stages of the continental rifting at the Yatulian-Ludikovian time from regimes, similar to the suprasubductional formation of the continental lithosphere, of the Inari terrane (1926–1850 Ma).

Eocene sediments and a fresh to brackish water biota from the early rifting stage of the Upper Rhine Graben (west of oil field Landau, southwest Germany): implications for biostratigraphy, palaeoecology and source rock potential

Eocene sediments and a fresh to brackish water biota from the early rifting stage of the Upper Rhine Graben (west of oil field Landau, southwest Germany): implications for biostratigraphy, palaeoecology and source rock potential

The Magmatic Rifting of Santos Basin: Aeromagnetic Mapping of Dykes, Terranes and Marginal Structures and the Interplay Between Tectonism and Volcanism

AbstractWe present a new magnetic map integrating continental and oceanic features at the Santos Basin, in order to investigate the connection between onshore and offshore tectonics and magmatism at the early stages of rifting. The magnetic and seismic data evidence the offshore continuity of the Florianópolis and Serra do Mar dyke swarms, while the Ponta Grossa Dyke Swarm is interrupted by the marginal structures. This structural relationship brings to evidence a different timing of formation of the Ponta Grossa Dyke Swarm relative to the early opening of the margin. A very high amplitude magnetic lineament ‐ the Santos Marginal Anomaly (SAMA) reaches up to 1,500 nT and trends NE‐SW, with local E‐W inflections forming the Cananéia and Guaratiba Arcuate anomalies. Our modeling suggests that the SAMA can be associated with dykes and syn‐rift extrusive rocks linked at depth to larger magmatic bodies at the proximal and necking domains. Based on the increasing width and amplitude of the magnetic anomalies oceanwards, we propose a mechanism whereby the igneous accretion (i.e., the amount of magmatism) augmented as rifting evolved from continent to ocean, creating a volcanic rift axis. This rift was segmented by crustal discontinuities that probably nucleated on preexisting rheological contrasts, forming tectonomagmatic segments during the early extensional stages. The onshore‐offshore relationships highlight that where there was an agreement between basement fabric and dyke orientation, inheritance may have influenced their emplacement, although it was not a limiting factor for their formation. The regional geodynamics of the Gondwana opening was the first order control on the emplacements of the dyke swarms, with deviations of the magmatically influenced rift by tectonic inheritance.

Carboniferous–Triassic tectonic and thermal evolution of the middle crust section of the Dervio–Olgiasca Zone (Southern Alps)

AbstractA well‐preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio–Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre‐Alpine (rift‐related) history. We reconstruct the P–T–t–d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U–Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono‐thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite‐facies Carboniferous metamorphic basement (7.6–10 kbar and 610–660°C at 318–312 Ma) was affected by pervasive extensional deformation (5.1–7.6 kbar and 580–660°C) in the Middle‐ to Late‐Permian (257.5 ± 3.8 Ma). Pegmatite intruded at 249.8 ± 1.1 Ma in an extensional phase that re‐equilibrated rocks of the basement at 3.5–4.5 kbar and 560–600°C. During the Middle‐ to Late‐Triassic (241–235 Ma), the basement experienced static thermal recrystallization (T = 689 ± 41°C and ~5.0 kbar). This Late‐Anisian to Early‐Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large‐scale extensional detachments developed in the upper crust (e.g., the Lugano‐Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post‐Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting.

Skarn and peperite formation within the frame of rifting dynamics, sedimentation, and magmatic activities, Hammam Faraun, Gulf of Suez, Egypt

The Gulf of Suez Rift is a classic example of a rift zone worldwide for studying sedimentation, tectonics, hydrocarbon potentiality, and magmatism. Detailed field studies of the Gabal Hammam Faraun sub-block on the eastern flank of the Gulf of Suez revealed the interaction of sedimentation and rift-related magmatism. These interactive processes develop skarns along the basaltic sill-pre rift Late Cretaceous chalk and peperites along the Early Miocene carbonate-sill contact. generations of both rocks influence hydrocarbon potentiality by modifying reservoir rock porosity and permeability. Skarn rocks were recorded along the contacts of the basaltic sill and the pre-rift Late Cretaceous chalks. Peperites were recorded along the Early Miocene carbonates-sill contacts. The formational settings and associated processes responsible for developing skarn rocks and peperites during the early rifting stage have been postulated in an explanatory model based on mineralogical and textural criteria. The skarn rocks under study underwent four distinct evolutionary stages: an isochemical metamorphic, a prograde anhydrous, a retrograde metasomatic, and a meteoric event. The intermingling of syn-sedimentary basaltic sill emplaced under subaqueous conditions with the Early Miocene unconsolidated calcareous sediments of various rheologies produced fluidal and blocky peperites. The age of the rift initiation issue could be settled by discovering peperites in the Early syn-rift sediments in the study area and adjacent parts of the Gulf of Suez. The existence of magmatic bodies and the distribution of skarn and peperites across carbonate reservoirs are two key features that must be considered when developing a reservoir architectural model.

A study on geological structure prediction based on random forest method

The Xingmeng orogenic belt is located in the eastern section of the Central Asian orogenic belt, which is one of the key areas to study the formation and evolution of the Central Asian orogenic belt. At present, there is a huge controversy over the closure time of the Paleo-Asian Ocean in the Xingmeng orogenic belt. One of the reasons is that the genetic tectonic setting of the Carboniferous volcanic rocks is not clear. Due to the diversity of volcanic rock geochemical characteristics and its related interpretations, there are two different views on the tectonic setting of Carboniferous volcanic rocks in the Xingmeng orogenic belt: island arc and continental rift. In recent years, it is one of the important development directions in the application of geological big data technology to analyze geochemical data based on machine learning methods and further infer the tectonic background of basalt. This paper systematically collects Carboniferous basic rock data from Dongwuqi area of Inner Mongolia, Keyouzhongqi area of Inner Mongolia and Beishan area in the southern section of the Central Asian Orogenic Belt. Random forest algorithm is used for training sets of major elements and trace elements in global island arc basalt and rift basalt, and then the trained model is used to predict the tectonic setting of the Carboniferous magmatic rock samples in the Xingmeng orogenic belt. The prediction results shows that the island arc probability of most of the research samples is between 0.65 and 1, which indicates that the island arc tectonic setting is more credible. In this paper, it is concluded that magmatism in the Beishan area of the southern part of the Central Asian Orogenic belt in the Early Carboniferous may have formed in the heyday of subduction, while the Xingmeng orogenic belt in the Late Carboniferous may have been in the late subduction stage to the collision or even the early rifting stage. This temporal and spatial evolution shows that the subduction of the Paleo-Asian Ocean is different from west to east. Therefore, the research results of this paper show that the subduction of the Xingmeng orogenic belt in the Carboniferous has not ended yet.

Thermal evolution of a Variscan syn-orogenic intracontinental basin (Servoz basin, Western Alps): RSCM geothermometry and geochronology

The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.

Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and “simpler” electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF’s maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580–837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

Evolution of the Continental Margin of South to Central Vietnam and Its Relationship to Opening of the South China Sea (East Vietnam Sea)

AbstractThe continental margin of south to central Vietnam is notable for its high elevation plateaus many of which are covered by late Cenozoic basalt flows. It forms the westernmost margin of a wide continental rift of the South China Sea (East Vietnam Sea), and uplift has been considered a result of either rifting or younger intraplate basalt magmatism. To investigate margin development apatite thermochronometry was applied to a dense array of samples collected from across and along the margin of south to central Vietnam. Results, including thermal history models, identified a distinct regional episode of fast cooling between c. 37 and 30 Ma after which cooling rates remained low. The fast cooling coincides with a period of fast extension across the South China Sea (East Sea) region that preceded continental break‐up recorded by Oligocene grabens onshore. A thermal model is used test different processes that might influence the inferred cooling including a distinct pulse of exhumation; a decrease in exhumation followed by an associated transient decrease in geothermal gradients and, underplating coincident with rifting. Thermal relaxation following Mesozoic arc magmatism is ruled out as geotherms returned to background rates within 20 Myrs of emplacement, well before the onset of fast cooling. Models support fast cooling attributed to accelerated erosion during early stages of rifting. Some additional heating from either underplating and/or hot mantle upwellings is also possible. No evidence was found to support regional uplift associated with the intraplate magmatism, enhanced monsoon‐driven erosion, or seafloor spreading dynamics.

Stratigraphic reconstruction of the lower–middle Miocene Goto Group, Nagasaki Prefecture, Japan

AbstractThe Goto Islands are located at the westernmost tip of the Japan archipelago, and preserve a lower–middle Miocene sedimentary sequence deposited during rifting of the continental margin and opening of the Sea of Japan. The stratigraphy of the Goto Group and new K–Ar, fission‐track, and U–Pb age data were used to determine the initial conditions of rifting in southwest Japan. The thickness of the Goto Group is 2000–3000 m. The lower unit (ca. 22–17.6 Ma) consists of thick, greenish, volcaniclastic rocks with basaltic volcanic material, representing the initial stages of continental rifting. The middle unit (ca. −17.6 Ma) consists of alternating sandstones and shales deposited in lacustrine and meandering fluvial environments in a syn‐rift sedimentary basin during a period of volcanic activity. The upper unit (ca. 17.6–16.8 Ma) consists of thick sandstones of fluvial–deltaic facies that were deposited during rapid subsidence at the continental margin. This unit was deposited by a large fluvial system that flowed into the Sea of Japan. These sequences contain relatively cooler to warmer flora (Daijima‐type) and record the warm period of the Miocene Climatic Optimum. The Goto felsic volcanic rocks (16.8 ~ 15.4 Ma) unconformably overlie the Goto Group, and granitic magmatism (ca. 16–14.5 Ma) occurred after sedimentation of the Goto Group. The widespread lacustrine, meandering–braided fluvial, and vast deltaic systems of the Goto Group, and felsic volcanism, were formed due to rapid subsidence that produced a horst‐and‐graben basin during the early stages of rifting of a volcanic arc along the eastern margin of Eurasia. These events occurred from 22.0 to 16.8 Ma before and during the formation of the Sea of Japan.

The Limpopo Magma‐Rich Transform Margin, South Mozambique: 1. Insights From Deep‐Structure Seismic Imaging

AbstractA variety of structures results from the interplay of evolving far‐field forces, plate kinematics, and magmatic activity during continental break‐up. The east Limpopo transform margin, offshore northern Mozambique, formed as Africa and Antarctica separated during the mid‐Jurassic period break‐up of the Gondwana supercontinent. The nature of the crust onshore has been discussed for decades in an effort to resolve issues with plate kinematic models. Two seismic refraction profiles with coincident multichannel seismic reflection profiles allow us to interpret the seismic velocity structures across the margin, both onshore and offshore. These seismic profiles allow us to (a) delineate the major regional crustal domains; (b) identify widespread indications of magmatic activity; and (c) map crustal structure and geometry of this magma‐rich transform margin. Careful examination of the profiles allows us to make the following observations and interpretations: (a) on land, continental crust is overlain by a >10‐km thick volcano‐sedimentary wedge related to an early rifting stage, (b) offshore, thick oceanic crust formed due to intense magmatic activity, and between the two (c) a 50–60‐km wide transform zone where the crustal structures are affected by intense magmatic activity and faulting. The prominent presence of intrusive and extrusive igneous units may be attributed to the combination of a deep‐seated melting anomaly and a trans‐tensional fault zone running through thinned lithosphere that allowed melt to reach the surface. A comparison of the crustal thinning along other transform margins shows a probable dependence with the thermal and/or tectonic history of the lithosphere.

The stratigraphic evolution of the Lake Tanganyika Rift, East Africa: Facies distributions and paleo-environmental implications

Active continental rifts are ideal sites for understanding the break-up of continents, and long-lived rift lake environments are known as important reservoirs for endemic communities and biodiversity. The sedimentary fill of the Lake Tanganyika Rift records a long history of continental extension and variable tropical climate, that is unparalleled in its duration and fidelity. Recently acquired, state-of-the-art 2D seismic reflection data, together with reprocessed legacy data, are used to evaluate the evolution and distribution of sedimentary facies over the rift lake. Using seismic stratigraphic analysis, we reconstruct past depositional environments and the paleogeography of the lake and assess how tectonic-driven subsidence and hydroclimate variability modified the lake basin. We identify six syn-rift seismic units that overly the acoustic basement and identify depositional units beneath the syn-rift sequence that suggest episodes of pre-rift sedimentation. Based upon the seismic facies analysis, the earliest seismic stratigraphic unit is interpreted as deposited in an early-stage rift system of low-relief, that was dominated by alluvial, fluvial and shallow lacustrine conditions. Subsequent units exhibit attributes of a lacustrine environment of much greater water depth, enhanced catchment relief and accommodation, consistent with a more mature rift. In Seismic units 2–5, we observe extensive deltaic deposits and deep-water fans, and locally canyons, channels, channel-levee complexes, turbidites, slumps and other mass flow deposits. In the latter part of its history, erosional surfaces and abundant lowstand delta facies are observed, indicating the rift experienced dramatic hydroclimate cycles. We assess the relative timing of key features of the rift, including the emergence of major structures and rift segment boundaries, development of major drainages and linkages to upstream rift lakes. The evolving sedimentary facies of the rift illustrate a shallow- to-deep progression of rift valley environments and the more limited littoral habitats that influenced the evolution of its unique endemic organisms.

Transformation of sediment delivery and dispersal patterns controlled by relay-ramp evolution along the boundary fault of a lacustrine rift: The Eocene Shahejie formation, Dongying Sag, Bohai Bay Basin, NE China

The boundary fault of the Dongying Sag is divided into isolated and scattered segments and small-scale relay zones, which are important geomorphic features controlling sedimentation in the early stage of rift evolution. Previous studies have shown changes in facies in deposits of thousands of meters in thickness on fault-bounded margins, where deep-water fan systems are vertically stacked with delta deposits in the same way as in many basins around the world. However, changes in sedimentation processes and sedimentary systems and their relationship with the evolution of the main boundary fault remain a matter of debate. We focus on the transition from shallow-water to deep-water systems and their responses to relay-ramp evolution in the Yanjia Subsag of the Eocene Dongying Sag in the Bohai Basin. A relatively simple assemblage of local structures provides an ideal opportunity to investigate the impact of relay-ramp evolution on the early synrift deposits. An integrated dataset of 3D seismic volumes, cores and wireline logs is used to analyze the evolution of the Chennan Fault, to delineate thirteen lithofacies, and to recognize two depositional styles (including fan deltas and nearshore subaqueous fans) and their distribution during different stages of the relay-zone evolution. In the early stage of rifting, the depositional style changes, from the development of axial and transverse systems in the early to middle period to the development of transverse systems in the late period; this is mainly controlled by the evolution of the relay ramp. The change is accompanied by a rapid decrease in the rate of sediment supply, architectural changes in transport conduit from the relay ramp to a canyon, and local geomorphic changes that control transport routes, sediment delivery, and depositional locations. Floods ultimately become dominant transport mechanism with a new system, such that hyperpycnal flows form, leading to another transformation of the sedimentary systems from a fan-delta system to a nearshore subaqueous fan system. On the basis of these transformations in sedimentary systems we are able to propose new recommendations for reservoir exploration in continental rift basins, which include exploration potential, source reservoir cap assemblage, and spatiotemporal evolution of plays.

3D seismic classification of fluid escape pipes in the western Exmouth Plateau, North West Shelf of Australia

Fluid escape pipes are vertical pathways of focused flow venting from a variety of deep overpressure sources. These geological features are typical of many sedimentary basins, including proven petroliferous provinces worldwide, such as the North Sea and the Exmouth Plateau in the Northern Carnarvon Basin, NW Australia. High-quality three-dimensional (3D) seismic reflection data from the western Exmouth Plateau revealed the occurrence of exceptionally well-imaged fluid escape pipes affecting the Jurassic strata and the Triassic Mungaroo Formation, a key reservoir unit in the basin. A total of 171 fluid escape pipes, including blowout, seepage and hydrothermal pipes, were mapped, and their geomorphological characteristics were analysed. In the study area, these features form prominent vertical columns up to 4.5 km long disrupting continuous reflections of the Triassic to Jurassic section. Numerous fluid escape pipes terminate with palaeo-pockmarks affecting the Upper Jurassic syn-extension strata, providing evidence for pipe genesis during the early stages of the Late Jurassic rifting in the Exmouth Plateau. Fluid escape pipes were found rooting from different stratigraphic levels, suggesting multiple fluid sources within the Triassic sediments. Several fluid flow structures nucleated along or near rift-related fault planes within the Mungaroo Formation, providing further evidence of rifting as a main triggering factor of important fluid flow in the basin. In the study area, the presence of fluid escape pipes represents a significant risk for the preservation of potential hydrocarbon accumulations, as, when these features form, vertical fluid venting breaches through stratigraphy, compromising the integrity of seal units. This seems supported by the lack of significant discoveries within the area covered by seismic survey analysed in this study.