Differential Subsidence Research Articles

Tectono‐stratigraphic evolution of Shillong Plateau, North East India through the Permian‐Eocene window

The Shillong Plateau in the north‐eastern Indian Plate signifies a cut‐off patch of the Indian Peninsular Shield, bordered by fault and thrust sheets, with thick alluvium cover along its periphery. The basement rock in the plateau accomplished about 2 km relief difference with the adjoining Sylhet Trough of Bangladesh in the south. The plateau witnessed Permo‐Carboniferous Gondwana sedimentation at its western margin, with Early Cretaceous basaltic traps and continuous fluvio‐marine sedimentation along its southern edge since the late Cretaceous. Although it is largely believed that strike‐slip movement along the Dauki Fault shifted the plateau about 250 km eastward, however, the driving mechanism for Shillong Plateau detachment and conversion of rift‐intercratonic basin to platformal configuration has drawn more attraction. This article focuses on the tectono‐stratigraphic evolution of Shillong Plateau and its adjoining regions on the basis of previous works, to try to understand the distinct tectonic and environmental factors. Early Permian continental rifting to the south‐west of the plateau and south‐east and south‐west of the Malda High presumably separated the Shillong Plateau from Chotanagpur Granite Gneiss Complex (CGGC). Extrusion of Rajmahal Traps west of the Malda High and Bengal‐Sylhet‐Mikir volcanism along an Early Cretaceous geofracture probably accelerated the east‐northeast migration of Shillong Plateau with respect to the CGGC. Since the Late Cretaceous, sedimentation at the southern fringe of Shillong Plateau commenced with an intracratonic basin which gradually transformed into a passive margin configuration during the Late Palaeocene‐Eocene Epoch during growth of the Indian Ocean. Differential subsidence in southern Shillong Plateau caused clastic sedimentation without considerable transportation. At the beginning, the Late Cretaceous intracratonic basin had received volcanoclastic, fluvio‐lacustrine sediments in a series of graben/half‐grabens associated with extensional faulting and volcanism, as represented by the Lower Mahadek and Jadukata formations in the plateau. The upper Mahadek and later Palaeogene sequences along the southern margin of the plateau record marine sedimentation, initiated with an estuarine‐marshy environment during the Late Cretaceous to Palaeocene, followed by a platformal marine succession since the Mid‐Eocene. The Calcutta‐Mymensingh Eocene Hinge Zone that separates a stable shelf from the deep basinal part might be traced through the south‐eastern fringe of Shillong Plateau and presumably continues further north‐east between the Naga and Disang thrusts in the eastern part of Dhansiri Valley.

Assessment of Soil Collapse and Subsurface Features Using Integrated Geophysical Methods in Bismayah Pumping Station, Southeast Baghdad, Iraq

The Bismayah pumping station was exposed to differential subsidence in the near surface soil due to the dewatering process. The geophysical surveys via ground penetrating radar (GPR) and electrical resistivity imaging (ERI) were used to assess the subsidence in subsurface soils. Four profiles of ERI using Wenner-Schlumberger array and four lines of GPR using antenna 250 MHz were implemented around the pumping station. The results show many underground features that are interpreted as concrete slabs of the storm and sewage tanks. These features reflect high resistivity values on the ERT profiles and high amplitude reflections on the GPR lines. Moreover, the results revealed the existence of pipe leakage on the northern side of the station, which in turn showed a very distinctive shape and low resistivity values. However, the results do not present any evidence of the existence of soil subsidence in the area around the station area. Keywords: Bismayah pumping station; ERI; GPR; Integrated geophysical methods.

Mapping of Genetic Sequences of the Cambrian Series in the Jbel Saghro Massif, Eastern Anti-Atlas, Morocco: Implications for Eustatic and Tectonic Controls

In this paper, a sedimentological and sequence stratigraphy analysis was performed on Lower and Middle Cambrian deposits of Jbel Saghro, Eastern Anti-Atlas. The field data analysis and the application of sequence stratigraphy concepts were used to classify sedimentary processes and depositional environment, and to define the Lower to Middle Cambrian basin’s detailed geometry. The Cambrian sedimentation of northeastern Saghro indicates a deltaic environment, which is composed of two depositional sequences. These sequences are made of a transgressive system-tract with retrograding sediments and a highstand system tract with prograding sediments. In response to sea-level change, these system-tracts were formed by several genetic units, and limited by various stratigraphic surfaces. The genetic unit stacking-patterns combined with the study of synsedimentary tectonics enabled to follow the sedimentary record’s Spatio-temporal evolution and its three-dimensional geometry. The study area deposits display significant dissimilarities in thickness. The western part shows a Lower Cambrian hiatus and an important reduction of the thickness in the Middle Cambrian deposits. However, the marine trend (progradation/retrogradation) remains similar in the study area. This suggests the same eustatic origin of all genetic sequences and variations in their preservation rate. This configuration is the result of differential subsidence that affected the Anti-Atlas during the Cambrian.

Understanding along-strike variability in controlling mechanisms of paleoenvironmental conditions and stratigraphic architecture: Ordovician successions in the Alborz Mountains of Iran at the northern Gondwana margin

Changes in stacking patterns of facies, stratal architecture and shoreline trajectory along depositional strike are mainly dependent on along-strike differences in factors such as physical processes (rivers, waves, tides), tectonics, eustatic changes, sediment supply, and local physiography. The lower-upper Ordovician successions (Lashkerak Formation) deposited in a Peri-Gondwana shallow continental shelf have been interpreted using the sequence stratigraphic concepts. This approach allowed to highlight main differences along-strike in the co-existing depositional systems (coastal plains, estuaries, deltas, and shoreline to shelf systems) in the Alborz basin along the northern edge of Gondwana. On the basis of sedimentary and ichnological characteristics, the siliciclastic successions are divided into four facies associations: (i) Wave-dominated estuarine, (ii) Wave-dominated shoreface-offshore complex, (iii) Mixed fluvial (F)- and wave (W)-influenced delta (Fw and Wf), and (iv) Tide-dominated, fluvial-influenced delta (Tf). Six third-order depositional sequences (DS1-DS6) were recognized, each of them are bounded by unconformities and pronounced shifts in the facies. Along depositional strike (NE/SW- a distance of c. 150 km), significant intrabasinal changes in sediment supply produced evident lateral changes in the shoreline trajectory and stratal architecture, of the DS1-DS3 (Tremadocian-Floian), may be attributed to differences in physical processes (rivers, waves, tides activity) and topographic irregularities derive probably by different rates of subsidence in the basin. During the deposition of the DS1-DS3, a significant along-strike differences in the composition of ichnological suites and the lifestyle of benthonic organisms, dependent on the relative supremacy of wave, storm, river, and tidal processes. Sedimentary supply and processes are the main controlling factors for the stratigraphic architecture rather than the tectonic activity between Tremadocian and Floian. While during the Darriwilian-Katian interval (DS4-DS6), the tectonic activity predominates with the beginning of extensional phases related to the rifting in the Alborz basin causing the activation of syn-depositional normal faults and the development of block-faulted craton-ward margin. Typically, differential subsidence due to extensional tectonic activity causes the uplift of tectonic blocks west of the Alborz Basin into the emerging areas, on one hand, and simultaneous deepening-upward stacking patterns (i.e., increased subsidence rates) in the hanging-wall depocenters of east of the Alborz Basin, on the other hand. This outcrop-based study suggests that the identification of genetically related depositional units in a larger paleogeographic context is fundamental for understanding along-strike variability in controlling mechanisms of the spatial distribution and stratigraphic architecture of coexisting and linked depositional systems.

Monitoring of Land Subsidence and Ground Fissure Activity within the Su-Xi-Chang Area Based on Time-Series InSAR

Serious land subsidence and ground fissure (GF) disasters have brought huge economic losses to the Su-Xi-Chang area (China) and threatened the safety of its residents. To better understand the development of these disasters, it is urgent to carry out long-term and large-scale deformation monitoring in this region. In this study, based on time-series interferometric synthetic aperture radar (InSAR) technology, ground deformation characteristics were obtained at different periods. Meanwhile, Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) version 5.00 was used to study the stress, seepage field, and displacement changes in the soil layers caused by pumping activities at the bedrock bulge. The results showed that three subsidence centers were located in Suzhou, Wuxi, and Changzhou from 2007 to 2010. The ground fissures in Guangming village had obvious differential settlements and intense activities. The land subsidence in the Su-Xi-Chang area was under control from 2018 to 2021, while there was a relative rebound in most areas. Combined with numerical simulation and geological data, we demonstrated that pumping activities would accelerate and intensify the land subsidence process, and differential subsidence was prone to occur at the buried hill, which in turn led to the formation of ground fissures. By comparing the characteristics of ground deformation in different periods, it was proven that banning groundwater exploitation is an effective measure for preventing and controlling such disasters.

Palaeogeographic reconstruction and sequence architecture of the middle‐upper Jurassic successions of Hawraman Basin (NW Iran): Implications for tectono‐depositional processes of the northeastern passive margin of the Arabian Plate

The middle‐upper Jurassic successions (Sargelu, Najmah, and Gotnia formations) of the NW sector of the Zagros Mountain belt are studied to generate a sequence stratigraphic model that helps to understand the Jurassic tectono‐depositional processes and palaeogeographic reconstructions of the northeastern passive margin of the Arabian Plate. The major geological structures observed in northwestern Iran indicate the predominance of an extensional deformation during the Toarcian to Aalenian owing to the eastern Mediterranean rifting. This extensional regime reached its maximum from the Early to Middle Jurassic, causing the fragmentation/drowning of the long‐lived Triassic to Early Jurassic carbonate platform and the development of the Hawraman intrashelf basin (HIB). Associated changes in marine current dynamics, trophic conditions, and degree of substrate oxygenation led to the bloom of oligotypical biota (Bositra limestones) creating unfavourable conditions for the development of other benthic communities (Sargelu Formation). Later, from the Callovian to Oxfordian, changes in carbonate production types (switch from the Bositra filament facies to the stromatolitic‐thrombolitic boundstones) remarkably decreased the dip angle of the depositional profile (Najmah Formation). A second‐order tectono‐eustatic cycle (a major long‐term transgressive/regressive [T/R] cycle) including six minor short‐term T/R facies cycles (TR1–TR6) is identified in the study area. The development of the T/R facies cycles during the Middle Jurassic (TR1–TR3; give‐up sequences; Sargelu Formation) as a result of combined global eustatic transgression and linear subsidence resulted in drowning and backstepping of the platform. In contrast, the short‐term cycles that developed in the upper Jurassic (TR4–TR6; keep‐up sequences; Najmah and Gotnia formations), following a sea‐level fall, resulted in progressive shallowing of the carbonate platform. This study suggests that extensional deformation of the area reached its peak in the transition of the Early to Middle Jurassic, leading to the creation of HIB, but either decreased or played a subordinate role to eustatic sea‐level variations during the deposition of the TR1–TR3 facies cycles (Bajocian‐Callovian). Changes in tectonic regime from an extensional deformation (Early Jurassic) to cooling subsidence (Middle Jurassic), accompanied by a long‐term transgressive sea‐level trend during the Middle Jurassic, were important factors in the development of anoxic trough environments with the deposition of organic‐rich intervals, represented by the TR1–TR3 facies cycles. Tectonic instability (tectonic uplift plus considerable differential subsidence due to the reactivation of faults) in the eastern margin of the Arabian platform during the Late Jurassic led to the development of the TR4–TR6 facies cycles.

Deformation and Mechanical Characteristics of Existing Foundation Pit and Tunnel Itself Caused by Shield Tunnel Undercrossing

This paper establishes a three-dimensional symmetrical shield model to investigate the influence of a double-line shield tunnel undercrossing an existing foundation pit and of changed grouting pressure on the deformation and mechanical characteristics of both the foundation pit and the tunnel itself, and it proposes a method of symmetrical segmented pressure, in which different grouting pressure is applied in different sections of the tunnel. The monitoring data are used to verify the reliability of the model, and the maximum relative error is 5.44%. The numerical results show that the maximum subsidence of the retaining pile and anchor are 3.76 mm and 10.33 mm, respectively, and the maximum tensile stress of the anchor is increased by 32.4%. The subsidence shape of the foundation pit raft is an arch with four corners warping upward and the maximum subsidence difference is 3.17 mm. Uneven subsidence of the tunnel occurs along the longitudinal direction, and large and small subsidences are located at the outside and underpart of the foundation pit, respectively, and the maximum and minimum values are 11.15 mm and 2.13 mm, respectively, and the maximum subsidence difference is 9.02 mm. The deformation and mechanical characteristics of both the foundation pit and the tunnel are significantly decreased by appropriately increasing the grouting pressure, and it is recommended that the grouting pressure should not exceed 300 kPa. The proposed method of segmented pressure can reduce the differential subsidence by 47.2% and the maximum tensile stress by 27.2%, so it can significantly reduce the uneven subsidence of the tunnel and improve the tunnel stress condition. The research results can provide a theoretical basis for the safe construction of shield tunnels under the existing foundation pit.

Types of Beixi slopes and the process of tectonic evolution

The phenomenon of oil and gas accumulation on the Beixi slope is obvious, which is one of the important directions of oil and gas pre-exploration and increasing reserves and production in the future. Taking the Beixi slope as the target area, according to the geological characteristics, the Beixi slope is divided into the differential subsidence type of the same fault depression, the reverse normal fault tilting and lifting slope and the compound slope. The evolution profile of Beixi slope is restored by using 2Dmove application software. based on the joint action of structure and deposition of Beixi slope, the fault evolution process of Beixi slope can be divided into four stages: rift stage 1 (Tongbamiao formation-N14 reflection layer), rift stage 2 (N14 reflection layer-T22 reflection layer), fault depression conversion stage (T22 reflection layer-T04 reflection layer) and depression inversion stage (above T04 reflection layer).

Urban growth and land subsidence: Multi-decadal investigation using human settlement data and satellite InSAR in Morelia, Mexico

Limited attention is typically paid to the cause-effect relationship between land subsidence due to aquifers overexploitation in expanding metropolises and urban growth models and patterns. This paper implements an integrated urban and satellite Interferometric Synthetic Aperture Radar (InSAR) approach to investigate subsidence, multi-decadal urban growth and peopling trends in the Metropolitan Area of Morelia (ZMM) in the Mexican state of Michoacán. Stacking of JRC's Global Human Settlement Layer, DLR's World Settlement Footprint and INEGI's National Geostatistical Framework datasets reveals a predominant edge-expansion growth model, with urban densification in 1975–2020 and some sprawling in 1990–2000. Population of the ZMM doubled in the last 30 years, reaching over 1 million inhabitants. The ENVISAT and Sentinel-1 InSAR analysis confirms that subsidence is structurally-controlled by the main normal faults within the Cuitzeo half-graben. Differential sinking and ground discontinuities are aligned with buried tectonic faults and contrasting compressible sediment thickness. Non-linearly deforming subsidence bowls develop at extraction wells in both old and newly urbanized sectors of the ZMM. Maximum vertical displacement velocities increased from −2.5 cm/year in 2003−2010 to −9.0 cm/year in 2014–2021, with subsidence migrating towards recently urbanized zones. More than 250 new groundwater wells were added to the public registry since 2000, many of which within new urban sectors. Time-lapse InSAR reveals a 4 km2 rapidly subsiding bowl that formed at the largest social housing neighbourhood of Villas del Pedregal, as building lots were progressively completed and sold, and new wells registered. With angular distortions due to the differential subsidence reaching 0.12% in 2014–2021, new buildings and roads are exposed to fracturing and surface faulting risk of comparable level as the city historic building blocks located along the main faults. By providing useful insights into the relationship between urban growth and land subsidence in the ZMM, the approach proves valuable for application to other metropolises worldwide.

The Limpopo Magma‐Rich Transform Margin, South Mozambique – 2: Implications for the Gondwana Breakup

AbstractThe rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high‐resolution seismic reflection data paired with data from industry‐drilled wells and proposes a structural model of the Limpopo transform margin in a magma‐rich context. Results indicate that the Limpopo transform margin is characterized by an ocean‐continent transition that links the Beira‐High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano‐sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo‐Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady‐state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE‐ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma‐rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn‐transfer and ‐transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large‐scale differential subsidence along the transform margin.

Elongated Giant Seabed Polygons and Underlying Polygonal Faults as Indicators of the Creep Deformation of Pliocene to Recent Sediments in the Grenada Basin, Caribbean Sea

AbstractBased on 2D seismic profiles, multibeam and seabed grab cores acquired during the Garanti cruise in 2017, 1–5 km wide seabed giant polygons were identified in the Grenada basin, covering a total area of ∼55,000 km2, which is the largest area of outcropping polygonal faults (PF) ever found on Earth so far. They represent the top part of an active 700–1,200 m thick underlying polygonal fault system (PFS) formed due to the volumetric contraction of clay‐ and smectite‐rich sediments, initiated in the sub‐surface at the transition between the Early to Middle Pliocene. The short axes of the best‐fit ellipses obtained from a graphical center‐to‐center method were interpreted as the local orientation of a preferential contraction perpendicular to the creep deformation of slope sediments. In the North Grenada Basin, the polygons are relatively regular, but their short axes seem to be parallel to a N40°E extension recently evidenced in the forearc, possibly extending in the backarc, but not shown in the study area. They are most probably related to a progressive burial due to a homogeneous subsidence. In the South Grenada Basin, the polygons are more elongated and their axes are progressively rotating southeastward toward the depocenter, indicating a creep deformation toward the center of the basin created by a differential subsidence. Seabed polygons and underlying PF could thus be indicative of the deformation regime of shallow sediments related to main slopes controlled by two different basin architectures.

Characterization of Irreversible Land Subsidence in the Yazd‐Ardakan Plain, Iran From 2003 to 2020 InSAR Time Series

AbstractGroundwater extraction rates exceeding recharge are occurring throughout Iran to sustain industrial and agricultural activities, resulting in land deformation in many areas, particularly in the Yazd‐Ardakan Plain (YAP) in central Iran's dry and desert regions. In this study, Interferometric Synthetic Aperture Radar (InSAR) time series analysis and statistical models are applied to characterize the controls on land subsidence in the YAP from 2003 to 2020. Our results reveal the existence of a northwest‐southeast elongated area of 234.45 km2 experiencing subsidence at rates up to 15 cm/yr. In the YAP, the international Airport, railway, transit road, and several industrial and historical sites are threatened by the differential subsidence. Well data confirm that groundwater levels have decreased by 18 meters between 1974 and 2018, driving the compaction of sediments within the underlying aquifer system. Our statistical analysis shows that the thickness of a shallow, clay‐rich aquitard layer controls the extent of the observed subsidence and the Independent Component Analysis of the InSAR time series shows that inelastic compaction is dominated. This work reveals that current groundwater extraction practices in central Iran are not sustainable and result in permanent subsidence, ground fractures with impact on infrastructures, and a permanent decrease in water storage capacity.

Mechanical Characteristics of Structures and Ground Deformation Caused by Shield Tunneling Under-Passing Highways in Complex Geological Conditions Based on the MJS Method

This study defined the height ratio of soft-rock strata and established a numerical model for analyzing shield construction in upper-soft, lower-hard composite strata together with field monitoring data. In this way, the influence of shield tunneling while passing under the pile foundation of the culvert at a short distance (the shortest distance is 1.4 m) in the typical upper-soft, lower-hard composite strata in Guangzhou can be examined. Moreover, the reinforcement effects of the ground, the bridges, and the culverts, using the strata-reinforcing plan dominated by the metro jet system (MJS) in a narrow space, are evaluated. Based on the results, (i) the maximum ground subsidence is found at the position in which the height ratio of the soft rock is 1.0. (ii) However, differential subsidence might be found in the subsequent shield construction when the soft-rock height ratio of the adjacent excavated surface ranges from 0 to 0.2 and from 0.5 to 1. (iii) The concentrated release of stress has a greater impact on the structure than the geological conditions of the shield tunneling face. (iv) Reinforcing with the MJS method contains the concentrated release of stress. This study can provide a reference for controlling the deformation of the under-crossing structure in the shield construction of the upper-soft, lower-hard composite strata.

Formation mechanism of pumping-induced earth fissures associated with a pre-existing normal fault, Beijing, China

The Shunyi (SY) earth fissures originated due to groundwater pumping and have caused severe damage to the Beijing Capital International Airport (BCIA) and other infrastructure in Beijing, China. They are distributed along the pre-existing NE-SW trending normal Shunyi-Liangxiang (SL) fault zone, with 1–30 cm vertical offsets. Previous studies of this long-term ground displacement have concluded that a subsidence bowl has formed on the footwall (northwestern side) of the SL fault in the BCIA. However, based on our observations, the relative subsidence occurred exclusively on the southeastern side, behaving like the activation of the SL fault. This complex deformation pattern has led to the formation mechanism of the SY earth fissures remaining unclear. In this study, two groups of numerical simulations based on the geological and hydrogeological settings of the SY earth fissures were performed. In Scenario I, the pumping load was on the footwall, whereas in Scenario II, the pumping load was on the hanging wall. Our results show that there are always two areas in the faulted model that have undergone pumping-induced subsidence. The first subsidence area indicates that the actual regional subsidence bowl occurred extensively around the pumping well. This subsidence was blocked by the pre-existing normal fault zone. The second subsidence area is localized and occurred abruptly in the hanging wall adjacent to the fault zone regardless of the effect of the position of the pumping load. This localized differential subsidence, representing the secondary surface faulting, was accompanied by earth fissure formation due to concentration of the surface shear strain. These simulation results agree with the actual land deformation across the SY earth fissures. Furthermore, our results suggest that both the formation location and deformation pattern of the SY earth fissures were dependent upon the characteristics of the pre-existing SL fault zone. This is mainly attributed to the fact that the fault zone has a lower resistance than the surrounding materials. Our findings reveal the formation mechanism of the SY earth fissures and provide a new understanding of pumping-induced earth fissures in other faulted areas.

Tectono‐stratigraphic evolution of the offshore western Niger Delta from the Cretaceous to present: Implications of delta dynamics and paleo‐topography on gravity‐driven deformation

AbstractThe interaction between sedimentary wedge dynamics and paleo‐fracture zones is investigated offshore western Niger Delta lobe (WNDL) to reconstruct the evolution of the delta from the Cretaceous to present. This was achieved through detailed regional seismic interpretation, calibrated with well data. Our results suggest that high sedimentation rates in the WNDL since the Serravallian–Tortonian triggered the migration of the ‘Oligocene‐Tortonian extensional zone’ and gravity spreading seawards (from a present‐day onshore to a present‐day offshore position), with extensional, translational and contractional deformation. An additional increase in sedimentation rate since the early Pliocene, further accelerated gravity spreading and the development of the present‐day contractional front. A five‐stage tectono‐stratigraphic evolution of the offshore WNDL from the late Cretaceous to present is proposed. Paleo‐topographies formed by the Charcot and Chain Fracture Zones exerted depositional control on the stratigraphic architecture of the offshore WNDL from the Cretaceous to Serravallian. Differential subsidence on both sides of the relict Charcot and Chain transform faults is responsible for the segmentation of gravity‐driven deformation of the eastern and western Niger Delta lobes. In addition, a comparison of the stratigraphic architecture of the eastern Niger Delta lobe (ENDL) and WNDL demonstrates a similar overall progradation and sediment bypass to the deep basin during the Pliocene. During the Pleistocene, the two lobes show a distinct evolution and architecture: the ENDL shows an overall retrogradation and sediment sequestration on the shelf, whereas the WNDL displays an overall progradation and sediment bypass. This study documents long‐term and large‐scale control of delta dynamics and paleo‐topography on gravity‐driven deformation of the offshore eastern and western Niger Delta lobes, and similar analysis could be applied in the reconstruction of other passive margin basins.

Tectonic Activity Near the Rio Grande Rise Increases Fluid Flux in Old Oceanic Crust

AbstractOceanic plateau crust is thicker and hotter than the surrounding “normal” oceanic crust, causing differential subsidence and subsequent strain between the two domains. Here, we show that recently active sedimentary faults that have been imaged adjacent to the Rio Grande Rise, an oceanic plateau in the western South Atlantic, have extensions that can penetrate at least ∼1.5 km into the upper oceanic crust. Our high‐resolution seismic velocity model indicates that these fault zones significantly reduce seismic velocities and increase inferred porosity and permeability. We also present physical property‐constrained fluid flow models that predict substantially increased fluxes along these pathways compared to crust not affected by faulting. As these phenomena are effectively enabled by the proximity of an oceanic plateau, we suggest that similar geologic settings globally may also exhibit elevated levels of tectonic and hydrothermal activity, with implications for the hydration of mature oceanic crust and the oceanic crustal biosphere.

Channel and inter-channel morphology resulting from the long-term interplay of alongslope and downslope processes, NE Rockall Trough, NE Atlantic

With this contribution we use a pair of overlapping 3D seismic surveys and two exploration wells to document the response of long-lived slope channels to the onset of bottom currents sweeping the lower slope in the NE Rockall Basin, offshore Ireland. Downslope gravity current activity, linked to a phase of uplift, prevailed throughout the Eocene and led to the formation of multiple channels, most notably a large-scale sinuous channel complex (Channel 4 Complex) tied to a persistent sediment entry point on the margin. Channels fed lobes on the floor of the basin, with increased axial tilting forcing gravity currents to flow parallel to the base of slope. A phase of margin-wide differential subsidence and basin deepening in the Late Eocene then activated bottom current circulation across the basin. Northward-flowing bottom currents first erosionally refashioned the lower slope creating a prominent unconformity with contourites that then initially forming infill drifts. Bottom currents swept along and obliquely upslope, building plastered drifts that straddled the lower sections of the still active channels. The drifts modified and amplified the spurs separating the active channels, with interaction between along and downslope processes accreting sediment that built and maintained channel relief and allowed the channel mouths to extend further basinward over and across the earlier lobes. Contourite-forced channel extension on account of lower slope depositional re-profiling represents another manifestation of the interplay between slope channels and bottom currents. In the case of NE Rockall, the accreted base of slope wedge and mounded geometry resembles similar features elsewhere along the eastern Rockall margin. Those have previously been ascribed to mass-transport and base-up channel initiation, however, the similarities are so striking that we propose that these also are the result of lower slope re-profiling by bottom currents.

Geometrical Breakdown Approach to interpretation of depositional sequences

AbstractSeismic and sequence stratigraphic analyses are important methodologies for interpreting coastal and shallow-marine deposits. Though both methods are based on objective criteria, terminology for reflection/stratal stacking is widely linked to eustatic cycles, which does not adequately incorporate factors such as differential subsidence, sediment supply, and autogenic effects. To reduce reliance on model-driven interpretations, we developed a Geometrical Breakdown Approach (GBA) that facilitates interpretation of horizon-bound reflection packages by systematically identifying upward-downward and landward-seaward trajectories of clinoform inflection points and stratal terminations, respectively. This approach enables a rigorous characterization of stratal surfaces and depositional units. The results are captured in three-letter acronyms that provide an efficient way of recognizing repetitive stacking patterns through discriminating reflection packages objectively to the maximum level of resolution provided by the data. Comparison of GBA with selected sequence stratigraphic models that include three and four systems tracts and the accommodation succession approach shows that the GBA allows a greater level of detail to be extracted, identifying key surfaces with more precision and utilizing more effectively the fine-scale resolution provided by the input seismic data. We tested this approach using a synthetic analogue model and field data from the New Jersey margin. The results demonstrate that the geometric criteria constitute a reliable tool for identifying systems tracts and provide an objective and straightforward method for practitioners at all levels of experience.

Evolution from syn‐rift carbonates to early post‐rift deep‐marine intraslope lobes: The role of rift basin physiography on sedimentation patterns

ABSTRACTThe stratigraphic architecture of Early Jurassic strata exposed along a >10 km long transect in the Chachil Graben, an exhumed marine rift depocentre in the Neuquén Basin (Argentina), provides insights into the sedimentological and stratigraphic expression of the syn‐rift to post‐rift transition. A change from syn‐rift intrabasinal carbonate to post‐rift extrabasinal siliciclastic sedimentation is recorded, as well as variations in sediment supply and dispersal patterns across rift‐related topography. The late syn‐rift was marked by a transgression and development of a shallow‐marine carbonate system, including carbonate platform deposits perched on fault‐block highs and periplatform deposits accumulated in fault‐block lows, which overlies continental volcano‐sedimentary syn‐rift deposits. Differential subsidence and basin deepening induced retrogradation of the carbonate system, which was progressively drowned and overlain by organic‐rich calcareous mudstone that draped across rift structures at the onset of the early post‐rift. The first extrabasinal siliciclastic influx led to progradation of an early post‐rift intraslope lobe complex into the graben, which is associated with kilometre‐scale clastic injectites. The depositional architecture, facies distribution and pinch‐out style of intraslope lobes record the effects of an inherited compaction hinge, which acted as an oblique counterslope to sediment gravity flows. The occurrence of combined‐flow bedforms, widespread erosion, and limited facies segregation across lobes bearing different hybrid event bed types, is in sharp contrast to sedimentological characteristics of existing intraslope lobe models. Documentation of the syn‐rift to post‐rift transition stratigraphy permitted identification of changes in thickness and facies resulting from the passive infill of inherited topography with early post‐rift differential compaction. This architecture contrasts markedly with those developed during syn‐rift normal faulting. Furthermore, the influence of local inherited topography on the development of early post‐rift lobes is key to improve subsurface prediction of sandstone distribution and quality during assessment of hydrocarbon reservoirs and carbon storage sites.

Plio–Quaternary Structural Evolution of the Outer Sector of the Marche Apennines South of the Conero Promontory, Italy

Some new results and preliminary remarks about the Plio–Quaternary structural and evolutionary characteristics of the outer Marche Apennines south in the Conero promontory are presented in this study. The present analysis is based on several subsurface seismic reflection profiles and well data, kindly provided by ENI S.p.A. and available on the VIDEPI list, together with surface geologic–stratigraphic knowledge of Plio–Quaternary evolution from the literature. Examples of negative vs. positive reactivation of inherited structures in fold and thrust belts are highlighted. Here, we present an example from the external domain of the Marche Apennines, which displays interesting reactivation examples from the subsurface geology explored. The study area shows significant evolutionary differences with respect to the northern sector of the Marche region previously investigated by the same research group. The areal distribution of the main structures changes north and south of the ENE–WSW oriented discontinuity close to the Conero promontory. Based on the old tripartite classification of the Pliocene, the results of this work suggest a strong differential subsidence with extension occurring during the Early Pliocene and principal compressive deformation starting from the Middle Pliocene and decreasing or ceasing during the Quaternary. The main structure in this area is the NNW–SSE Coastal Structure, which is composed of E-vergent shallow thrusts and high-angle deep-seated normal faults underneath. An important right-lateral strike–slip component along this feature is also suggested, which is compatible with the principal NNE–SSW shortening direction. As mentioned, the area is largely characterized by tectonic inversion. Starting from Middle Pliocene, most of the Early Pliocene normal faults became E-vergent thrusts.