Relay Ramps Research Articles

Tectonic Control of Groundwater Recharge and Flow in Faulted Volcanic Aquifers

AbstractGroundwater dynamics in continental rift zone settings remain poorly understood because of the spatial heterogeneity in flow, storage, and recharge dynamics. The Central Kenya Rift is an excellent example where, though groundwater is important for domestic, irrigation, and geothermal energy exploitation, its hydrogeological properties remain largely unknown. Existing conceptual groundwater models assume flow from the high‐elevation, humid rift flanks to the low‐elevation, semiarid rift floor, but the role of the faults that fracture the aquifers is commonly unaccounted for. We applied geochemical, isotopic (δ18O, δD, 87Sr/86Sr, 3H‐3He), multivariate statistical methods, and knowledge of geological structures to determine recharge sources, flow, and residence times to revise the conceptual flow model. Results show that groundwater is primarily recharged by meteoric waters, river input, and Lake recharge. The faults impart a control on the groundwater flow within four sub‐compartments. Major differences in flow patterns exist between the eastern and western rift flanks: surface and groundwater transfer from the eastern flanks to the rift floor occurs via relay ramp structures, and flow on the west side takes place laterally from the high escarpment. Although 3H‐3He dating shows that the age of groundwater ranges from a few to >>50 years, most of the groundwater in the rift‐floor area is free of 3H and was recharged before the 1960s. Hence, we propose that these areas receive episodic recharge and represent the most sensitive groundwater resources in the rift. To inform sustainable groundwater development, a robust monitoring network is required to capture the heterogeneous groundwater dynamics.

The development of the eastern Orpheus rift basin, offshore eastern Canada: A case study of the interplay between rift-related faulting and salt deposition and flow

The salt-rich Orpheus rift basin, part of the eastern North American (ENAM) rift system, formed during the Late Triassic to Early Jurassic prior to opening of the Atlantic Ocean. Using a dense grid of 2D seismic-reflection lines, data from nearby wells, and information from adjacent ENAM rift basins, we have established a tectonostratigraphic framework, identified key structural elements, and reconstructed the deformation history for the eastern part of the basin. Our work shows that a series of E-striking, S-dipping faults with normal separation bound the basin on the north. Deformation within the basin is complex with forced folds above deep-seated intrabasin faults, detachment folds, detached thrust faults, and salt diapirs. The synrift evaporite sequence consists of an older massive salt unit that underlies a younger unit with two distinct interfingering facies. Facies A, consisting of salt and interbedded sedimentary rocks (likely shales), developed near the border-fault system and its relay ramps. Facies B, consisting of massive salt with few interbedded sedimentary rocks, developed toward the basin center. The youngest synrift unit accumulated exclusively within minibasins near the northern border-fault system. Based on location, this youngest synrift unit likely consists of coarse-grained and poorly sorted alluvial-fan or talus-slope deposits shed from the footwall. All synrift units are intruded by igneous sheets likely associated with the Central Atlantic Magmatic Province and, thus, are mostly Late Triassic in age (or possibly older). The border-fault system profoundly affected deposition within the eastern Orpheus rift basin by providing pathways for clastic sediment input into the salt-rich basin. These depositional patterns subsequently influenced deformation associated with lateral salt flow during minibasin formation. In regions with interbedded salt, detachment folds and thrust faults developed, whereas salt walls and stocks developed in regions with more massive salt.

Modes of Oblique Inversion: A Case Study From the Cretaceous Fold and Thrust Belt of the Western Transdanubian Range (TR), West Hungary

AbstractInversion of pre‐existing normal faults is a frequent phenomenon in orogenic belts. However, their impact on the evolution of the Cretaceous Eoalpine orogeny is poorly studied, despite the widespread presence of Triassic and Jurassic normal faults. Our study area is the SW part of the Transdanubian Range (TR), which represents the uppermost thick‐skinned nappe of the Eoalpine orogeny in the Alpine realm. The variable trend of Cretaceous contractional structures within the Transdanubian Range has been traditionally explained by poly‐phase shortening. However, based on the combined analyses of field observations and 3D seismic data, we found that the complex geometry of Cretaceous contractional structures can be explained rather by the oblique inversion of segmented Late Triassic grabens. In general, the SW part of the TR can be characterized by N‐S trending contractional structures. NW‐SE striking oblique ramps are nucleated above Late Triassic normal faults, whereas NE‐SW trending folds and thrust with limited strike‐parallel length evolved due to the inversion of pre‐existing breached relay ramps, which connected the segments of Late Triassic normal faults. We determined different types of oblique ramps, incuding normal faults, which suffered transpressional reactivation, oblique hanging wall breakthrough thrusts, oblique footwall short‐cut and hanging wall by‐pass thrusts. Only very steep normal faults, or those that strike at low angle to the regional E‐W shortening direction, were not reactivated.

The effect of breached relay ramp structures on deep‐lacustrine sedimentary systems

AbstractFault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between en‐echelon fault segments, which become modified through progressive deformation, and may develop through‐going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep‐marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high‐quality subsurface data from the Early Cretaceous deep‐lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay‐ramp, an ideal example, formed during the syn‐rift and was later covered by a thick transitional and early post‐rift succession. Major transitional and early post‐rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo‐bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure‐bounding fault to the north. Reactivation of basin‐bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin‐located relay ramps on sedimentary systems from syn‐rift, transitional and into the early post‐rift phase. It suggests that these structures can become reactivated during post‐rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small‐scale lobes, with small, often line‐sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs.

Joint‐Rupture Pattern and Newly Generated Structure of Fault Intersections on the Northern Margin of the Linhe Basin, Northwestern Ordos Block, China

AbstractThe activity of major active faults around the Ordos Block is of great interest to seismologists, as at least fourM ≥ 8 earthquakes have been recorded. However, the Linhe Basin, which has the thickest Cenozoic sediments, has no record of large earthquakes. Does this basin have the structural conditions required for large earthquakes? The northern boundary fault of the Linhe Basin is composed of the NE‐striking Langshan piedmont fault (LPF), E‐W‐striking western section of the Seertengshan piedmont fault (WSPF), and NW‐striking eastern section of the Seertengshan piedmont fault (ESPF). Based on large‐scale active fault mapping, this article analyzes data from 23 trenches, using an unmanned aerial vehicle to measure the faulted landform, and combines these data with Quaternary dating methods to acquire the paleoearthquake sequences of the LPF, WSPF, and ESPF. Furthermore, this article explores their rupture modes and discusses the structural evolution of two intersection points. Through paleoseismic comparison, seven trenches revealed historical earthquakes from 7 BC, with a common magnitude ofM8.1. The trenches also revealed seven paleoseismic events since the Holocene, which conformed to the periodic model with a period of 1.37 ± 0.11 ka. The elapsed time since the latest event (2.0 ka) has exceeded the earthquake recurrence period; thus, the area is currently at risk of anM7.4–8.0 earthquake. The NE‐striking LPF and E‐W‐striking WSPF are connected by two left‐stepping small fault segments. The E‐W‐striking WSPF and NW‐striking ESPF are connected by a large triangular relay ramp.

Influence of variable decoupling between vertically separated fault populations on structural inheritance – The Laminaria High, NW Shelf of Australia

AbstractWhen extension events are greatly separated in time, older faults may be buried and stratigraphically separated from newly developing faults at shallower depths. During rifting, the buried structures may reactivate and propagate upwards to be expressed within the shallow system. The degree of linkage between structural levels determines the influence that the deeper structures can exert over the geometry and evolution of the incipient fault system. In this study we use 3D seismic reflection data to examine how a deep fault population across the Laminaria High, NW shelf of Australia influences the development of a younger fault system at shallow depths. These fault populations are non‐parallel and decoupled across a mechanically weak interval. The majority of shallow faults are not linked to those at depth. However, the reactivation and upward propagation of some of the deeper faults produce anomalously oriented structures at shallow depths, hard‐linked to the deeper structures. One fault in particular shows along‐strike variability, with the deep segment reactivated and present at shallow depths in the west. To the east, the shallow and deep fault segments become decoupled across the mechanically weak interval, although some soft‐linkage and strain transfer still occurs. We suggest that this switch in the degree of coupling along the fault is due to the geometry of the deeper structure, with the transition corresponding to a prominent relay ramp. We show how the geometry of a deeper fault may affect its propensity to reactivate during subsequent extensional events, ultimately affecting the degree of structural inheritance that is expressed within younger, shallower fault populations.

Fault-related fracture modeling in a pre-salt lacustrine carbonate reservoir from Santos Basin, offshore Brazil: Predicting preferential fluid flow paths using 3D geological and flow simulation models

The authors propose an integrated workflow from seismic to flow simulation to predict sub seismic and inter-well fracture distribution. This methodology provides insights in order to fulfill a knowledge gap in reservoir 3D modeling of naturally fractured carbonates applied in the industry. The workflow includes interpretation of borehole image logs, 3D seismic, geophysical and geomechanical attributes, and calibration with dynamic data. The main natural fracture parameters analyzed for the case study are direction, dip, and fracture intensity. The studied area is a naturally fractured Aptian lacustrine carbonate reservoir within the Santos Basin, offshore Brazil. The isolated platform developed over a series of elongated NE-SW horsts structures. The main seismic scale structures in the reservoir are NE-SW trending faults that show normal displacement, defining en échelon relay ramps that contain NW-SE sub seismic faults and diffuse fracture swarms. The damage zones that surround these main faults define continuous fracture corridors intercepted by oblique sub-seismic faults in relay-ramp zones. Structural analysis showed that fracture intensities are higher at the north domain in the Barra Velha Formation. The fractures were simulated stochastically using a discrete fracture network (DFN), generating a dual porosity and dual permeability model (DPDK) for the flow simulation resulting in a suitable model configuration for the history match.

Architecture and depositional processes of sublacustrine fan systems in structurally active settings: An example from Weixinan Depression, northern South China Sea

Sublacustrine fans developed in structurally active settings are commonly characterized by complicated depositional characteristics, which create great challenges to hydrocarbon exploration. To advance understanding their sedimentary architecture and depositional processes, a large-scale sublacustrine fan system in the Eocene Weixinan Depression, northern South China Sea has been identified and investigated through the comprehensive analysis of 3-D seismic, well-logging, and core data. This fan was deposited during a lake-level falling stage, covering an area of 55 Km2. It can be divided into two sections in the vertical direction by the sedimentary characteristics. The lower section mainly consists of thin layers of well-sorted fine sandstones with parallel bedding, which are interpreted as deposits of turbidity currents. By contrast, the upper section comprises thick layers of gravelly coarse sandstones with various deformation structures and widespread floating clasts, which should be the deposits of cohesive debris flows. Provenance analysis suggests that the development of complicated sedimentary characteristics has a close relationship with the occurrence of multiple sediment supply directions. The lower section was sourced by the western delta with long-distance transported well-sorted sediments, while the northern fan delta sourced the upper section with short-distance transported coarse-grained sediments. Its depositional process was mainly controlled by paleo-geomorphology, sediment supply, and relative lake level changes. The lower section, controlled by the flexure slope belt, is a typical sand-rich sublacustrine fan characterized by the low gradient (<1°–1.5°), while the upper section, controlled by the steep slope fault belt, is a gravel-rich sublacustrine fan dominated by intense faulting and high gradient (3.2°–4.5°). In addition, during the depositional period of the upper section, the relay ramps between en-echelon faults provided major entry points for coarse-grained sedimentary flows into the depression. Therefore, a deep-lacustrine model is proposed for the sublacustrine fan system, showing the detailed “triggering-transportation-deposition” process. This study outlines the complexity of the sedimentary architecture of sublacustrine fans in structurally active settings, and the proposed model may be applicable in other lacustrine basins.

Faults resembling channel-like features

We analyzed a synthetic transfer zone and its associated fault planes and relay ramp in Penobscot, a potential offshore field in the Scotian Basin. Transfer zones are structural areas where one fault dies out and another fault begins, forming a relay ramp in the middle. They can be categorized as divergent, convergent, and synthetic transfer zones depending on the relative location and dipping directions of the faults. These zones not only play an important role in fluid migration but also help interpreters delineate secondary features such as fractures, splay shears, and Riedel faults. Commonly, those faults would branch into smaller splays and the relay ramp can get “breached” with connecting faults with the increase of slip. The study area in the Scotian Basin is characterized by two major listric normal faults dipping in the same direction giving rise to a synthetic transfer zone. These faults are clearly visible on seismic attributes, including curvature and coherence slices extracted along the top of the Cretaceous Petrel Formation. However, when analyzing the seismic attributes along the overlying Wyandot Formation’s top, we observe channel-like features, which run parallel as well as at an angle to these faults. However, when we performed further analysis using seismic amplitude’s vertical slices, interpreted horizons, and seismic attributes, we found that these features are not channels. We divided the features into two types: The first type is parallel to the main faults and can be associated with the grabens formed by synthetic and antithetic secondary faults (northeast–southwest); the second type is related to the polygonal faulting associated with differential compaction and gravitational loading of the Wyandot Chalk Formation. Apart from the two lineations, there are north-northeast–south-southwest-oriented lineations, which are an impression of basement faulting, and north-northwest–south-southeast-oriented lineations representing the acquisition footprint. Geological feature: Synthetic and antithetic faults in a synthetic transfer zone Seismic appearance: Channel-like feature on seismic attribute horizon slices Alternative interpretations: Channels Features with similar appearance: Steep slope channels Formation: Wyandot Age: Late Cretaceous Location: Penobscot, Nova Scotia, Canada Seismic data: Open-source SEG Analysis tools: Coherence and curvature attributes

Influence of extensional faults and relay ramp on palaeo‐topography and lacustrine carbonate facies: Purbeck Limestone Group (Upper Jurassic – Lower Cretaceous), Wessex Basin, UK

AbstractLacustrine carbonate facies distribution is controlled by multiple environmental parameters including climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, integrated with outcrop logging, correlations, and facies models, are used to create a new tectono‐sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half‐graben sub‐basins south of two extensional east‐west faults, with widespread subaerial emergence of footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in‐situ microbial mounds within bedded inter‐mound packstones‐grainstones. Mounds occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish‐water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate‐rich waters sourced from the northern limestone footwall blocks that fed into extensive shallow‐water areas on the low‐angle relay ramp slope.

Relay ramps and rhombochasms in the northern Appalachian Basin: Extensional and strike-slip tectonics in the Marcellus Formation and Utica Group

Relay ramps and rhombochasms in the northern Appalachian Basin: Extensional and strike-slip tectonics in the Marcellus Formation and Utica Group

Spatiotemporal evolution of transfer structures and linked fault systems in an extensional setting: Southwest Gebel Akheider, Cairo-Suez District, Egypt

Detailed geological field mapping of the area located to the southwest of Gebel Akheider, south-central part of the Cairo-Suez District (Egypt), was carried out to identify the geometry, structural style, and evolution of the normal fault systems in the study area. The present structural geometry of the study area is of Miocene age and is the result of the propagation, kinematic interaction, and eventual linkage of extensional fault segments. Eight mapped relay ramps formed between synthetic non-overlapping and overlapping normal faults. Fault displacement profiles showing variation in displacement along strike revealed steep displacement gradients and local displacement minima at the locations of these relay ramps. Conjugate normal faults in the area also showed signs of kinematic interaction. Fault displacement profiles revealed complimentary displacement distribution between interacting overlapping conjugate faults. In competent rock units (Middle Eocene limestone), this interaction resulted in the formation of horsts and grabens relay zones, while in incompetent rock units (Middle Miocene soft marl and sandy limestone), this interaction resulted in a series of en echelon plunging anticlines and synclines (accommodation folds). Several transfer faults marked by points of local displacement minima were also identified, forming a distinctive zigzag pattern in map view that is characteristic of the Cairo-Suez District. Curved fault traces represent another form of fault linkage in the study area. Fault displacement profiles indicated two different growth mechanisms, either as the result of bilateral growth of two along-strike faults, or the intersection of previously isolated and differently oriented fault segments.

Shortcut Faults and Lateral Spreading Activated in a Pull-Apart Basin by the 2018 Palu Earthquake, Central Sulawesi, Indonesia

Our understanding of pull-apart basins and their fault systems has been enhanced by analog experiments and simulations. However, there has been scarce interest to compare the faults that bound pull-apart basins with surface ruptures during earthquakes. In this study, we investigated the effects of a 2018 earthquake (Mw 7.5) on a pull-apart basin in the Palu–Koro fault system, Sulawesi Island, Indonesia, using geomorphic observations on digital elevation models and optical correlation with pre- and post-earthquake satellite images. A comparison of active fault traces determined by geomorphology with the locations of surface ruptures from the 2018 earthquake shows that some of the boundary faults of the basin are inactive and that active faulting has shifted to basin-shortcut faults and relay ramps. We also report evidence of lateral spreading, in which alluvial fan materials moved around the end of the alluvial fan. These phenomena may provide insights for anticipating the location of future surface ruptures in pull-apart basins.

Impact of growth faults on mixed siliciclastic‐carbonate‐evaporite deposits during rift climax and reorganisation—Billefjorden Trough, Svalbard, Norway

AbstractFault‐controlled mixed siliciclastic‐carbonate‐evaporite depositional systems exhibit distinct sensitivity to tectonic and eustatic controls that are expressed in the sedimentary architecture. In the Upper Carboniferous Billefjorden Trough (Svalbard, Norway), up to 2,000 m of a warm and arid climate syn‐rift basin fill comprises such depositional systems, documented in this study with traditional field techniques supported by helicopter‐ and ground‐based LIDAR models. The basin evolved from siliciclastics‐dominated red beds and paralic units that filled a symmetrical basin, to a rift climax half‐graben with alluvial fans entering the basin along relay ramps of the master fault zone (Billefjorden fault zone). Faults located in the hanging wall dip‐slope prevented the progradation of coarser material to the eastern part of the basin. Later, structural reorganisation in the dipslope led to the cessation of easternmost faults with deformation focusing along one major lineament (Løvehovden fault zone) antithetic to the master fault zone. The basin subsidence became more symmetrical, with main central depocentre and shallower platforms near the basin flanks. Footwall anticlines from faults displacement gradients were sensitive to periodical exposure and recorded dissolution breccias and footwall synclines preserved evaporites coupled with shallow marine siliciclastic deposits. Concurrently, thick gypsum/anhydrite deposits in the basin centre reflect glacio‐eustatic lowstands, whereas evenly thick carbonate deposition characterises highstands. While most analysis of syn‐rift basin fill is based on siliciclastics deposits, we here demonstrate the complexity of tectonism versus eustatic sea level changes in a mixed carbonate‐evaporite syn‐rift deposits. Tectonic influence is ascribed to the deposition of alluvial fans that prograded from the master fault towards the basin centre. On the dipslope glacio‐eustatic signals outperformed tectonic influence on deposition. Sea level lowstands promoted deposition of red sabkha mudstones and gypsum/anhydrite, salinas evaporites or dissolution breccias, interbedded with highstand carbonate beds.

Mesozoic structural inheritance in the Cenozoic evolution of the central Catalan Coastal Ranges (western Mediterranean): Structural and magnetotelluric analysis in the Gaià-Montmell High

The control exerted by the Mesozoic basin configuration on the Cenozoic tectonic evolution of the Catalan Coastal Ranges has been frequently recognized as a key factor to explain its present-day structure. However, details of this structural inheritance and its evolution through geological time is still under discussion. In this work we present two structural cross-sections based on fieldwork, well and magnetotelluric data in order to illustrate the structural styles and tectonic evolution of the Gaià-Montmell High. Here, the Montmell Fault not only constitutes the SW segment of one of the major Neogene faults in the Catalan Coastal Ranges (the Montmell-Vallès Fault System), but also the NW limit of a Late Jurassic-Early Cretaceous extensional basin(the Montmell-Garraf Basin), facts that denote a major role of this fault in the tectonic evolution of the area. The present-day structure of the Gaià-Montmell High resulted, therefore, from two successive episodes of inversion during the Cenozoic. The first one reactivated the Montmell Fault as compressional during the Paleogene. As a result, and among other inversion-related structures, the Gaià-El Camp Thrust developed sa major NW-directed basement footwall shortcut. Later on, the previously formed compressional structure during the Paleogene became reactivated as extensional during the Neogene. During this phase, the reactivation of the Montmell Fault looks limited and, hence, the extension is transmitted to the Baix Penedès Fault. The reactivation of the Gaià-El Camp Thrust is also manifest in the development of an array of extensional faults in the backlimb of the Carme-Cabra Anticline that corresponds to the NE-end of El Camp Fault. This episode of negative inversion developed accommodation zones between the four major faults present in the area ( Vallès-Penedès, Montmell, El Camp and Baix Penedès faults) that are characterized by the presence of relay ramps with breaching faults.

Synthetic transfer zone characterization using seismic attributes: An example from the Parihaka fault system in the Taranaki Basin, New Zealand

Synthetic transfer zones develop between fault segments that dip in the same direction, with relay ramps connecting the fault blocks separated by the different fault segments. The characteristics of the transfer zones are controlled by the lithology, deformation conditions, and strain magnitude. The Parihaka fault is a northeast–southwest-trending set of three major en echelon faults connected by relay ramps in the Taranaki Basin, New Zealand. The structure in the basin is defined by extension during two episodes of deformation between the late Cretaceous and Paleocene and between the late Miocene and recent. To better understand the evolution of a synthetic transfer zone, we studied the geometry and secondary faulting between the individual fault segments in the Parihaka fault system using structural interpretation of 3D seismic data and seismic attributes. This interpretation allows for a unique application of seismic attributes to better study transfer zones. Seismic attributes, such as coherence, dip, and curvature, are effective tools used to understand the detailed geometry and variation in displacement on the individual faults, the nature of secondary faulting along the transfer zones, and the relationship between the faults and drape folds. The seismic characterization of the fault system of Miocene to Pliocene age horizons highlights variations in the degree of faulting, deformation, and growth mechanism associated with different stages of transfer zone development. The coherence, dip, and curvature attributes indicate a direct correlation with structural parameters such as deformation, folding, and breaching of relay ramps. All three attributes enhance the visualization of the major and associated secondary faults and better constrain their tectonic history. The observed correlation between the seismic attributes and structural characteristics of transfer zones can significantly improve the structural interpretation and exploration workflow.

Significance of fault seal in assessing CO 2 storage capacity and containment risks – an example from the Horda Platform, northern North Sea

An understanding of fault seal is crucial for assessing the storage capacity and containment risks of CO 2 storage sites, as it can significantly affect the projects on across-fault and along-fault migration/leakage risk, as well as reservoir pressure predictions. We present a study from the Smeaheia area in the northern Horda Platform offshore Norway, focusing on two fault-bounded structural closures, namely the Alpha and Beta structures. We aim to use this study to improve the geological understanding of the northern Horda Platform for CO 2 storage scale-up potentials and illustrate the importance of fault seal analysis in containment risk assessment and storage capacity evaluation of a CO 2 storage project. Our containment risk assessment shows that the Alpha structure has low fault-related containment risks; thus, it has a potential value to be an additional storage target. The Beta structure shows larger fault-related containment risks due to juxtaposition of the prospective storage aquifer with the basement across the Øygarden Fault System. The storage capacity of Smeaheia will be determined by the long-term dynamic interplay between pressure depletion and recharging. Our study shows that across-fault pressure communication between Smeaheia and the depleting Troll reservoir is likely to be through several relay ramps of the Vette Fault System. However, Smeaheia also shows pressure-recharging potentials, such as through the subcropping areas at the Base Nordland Unconformity. The depletion observed in the newly drilled well 32/4-3S gives a good validation point for our fault seal predictions and provides valuable insights for future dynamic simulations. Thematic collection: This article is part of the Geoscience for CO 2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

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.

Present day strike-slip deformation within the southern part of the İzmir-Balıkesir Transfer Zone based on GNSS data and implications for seismic hazard assessment in western Anatolia

Herein, a combined analysis of Global Navigation Satellite System-derived strain rate maps, in accordance with recent seismicity, was presented to reveal that the N-S extension is accommodated primarily by strike-slip faulting of the İzmir-Balıkesir Transfer Zone (İBTZ), where a counter clockwise rotation (~25-100°/Myr) along the vertical axis is dominant. The results indicated that strike-slip segments within the İBTZ show variable transport sense and amount of slip along them, and they connect by hard linkage relay ramps with the dip to oblique slip normal faults. According to the strain map, the Karaburun Peninsula has the largest strain rates, at 137 nano strain (nstrain)/yr extension (NE-SW) and 126 nstrain/yr (NW-SE) compression. To the south, the largest strain areas begin to shrink where the NW-trending sinistral Riedel Fault is located. The smallest strains in the region were measured on the NE-trending Tuzla Fault, compatible with the right lateral component. Based on this, the northern part of the Karaburun Peninsula has the shortest recurrence period in the region. The geodetic earthquake recurrence periods throughout the region comprise 800 yr for magnitudes 7 and above and 70 year for magnitudes between 6 and 7. The period was calculated as 30 years for M > 5.5 (with 99% probability) and 100 years for M > 6 (with 95% probability). These were consistent with the geodetic earthquake recurrence periods (25-30 years for M > 5.5 and 80-100 years for M > 6). This result showed that the seismic hazard sources in the region have increased the earthquake risk, which may cause loss of life and property in the near future.

Axial and transverse depositional systems of a syn-rift basin fill (Bohai Bay Basin, China)

For improved prediction of reservoir distribution and quality in the infill of rift basins, it is paramount to characterize the extent and relative dominance of simultaneously active depositional systems fed by axial and transverse drainages, and to understand how these vary in response to geological controls. This study demonstrates how integration of seismic, wireline-log, core, petrographic and heavy-mineral datasets, combined with existing palaeotopographic reconstructions, can be utilized to characterize the spatiotemporal distribution and reservoir potential of syn-rift depositional systems associated with axial and transverse sediment routes to a lacustrine basin. In the Eocene of the Dongpu Depocentre (Bohai Bay Basin, China), three distinct delta types are identified: footwall-derived coarse-grained fan deltas and sand-rich deltas, and axial deltas that are dominated by siltstone and fine sandstone. The spatiotemporal distribution of these systems in the basin fill arose from the interaction of tectonic, lake-level, and climatic controls. Sedimentological and provenance analyses indicate the predominance (>85%) of axial systems over transverse systems in the infill of this elongate, asymmetrical graben. Locally, the interaction and amalgamation of transverse and axial systems was controlled by the presence of intra-basinal highs, through their influence on sediment-delivery pathways, and by high rates of sediment supply that caused overfill of isolated depocentres. Texturally mature, well-sorted fine sandstones of axial deltaic systems, which were sourced from comparatively large catchments, are attractive reservoir targets. By contrast, conglomerates of transverse fan deltas are poorly sorted and have more limited reservoir potential. However, footwall-derived fans may serve as viable reservoir targets locally, where fed by major hinterland drainages associated with long sediment-transport distances. Identification of the position of relay ramps in seismic data and recognition of bedrock-incised drainage outlets on the footwall basement of the basin margin may allow prediction of the presence and location of reservoir units associated with oversized deltas.