Basement Faults Research Articles

Basement Controls on Structural Evolution in Thin‐Skinned Thrust Belts—Implications for Migration of Deformation Into Orogenic Forelands

AbstractFold and thrust belts on the outer fringes of contractional mountain belts are classically interpreted as forming in sedimentary rocks that have detached from the underlying continental crust that acts as a rigid, undeformed basement. This is “thin‐skinned tectonics” and the Jura fold and thrust belt of northern Switzerland is an archetype. We analyze published cross‐sections and interpret new high resolution seismic imagery, to show that it is not simply thin‐skinned. Faults in the underlying basement coincide with folds and thrusts in the overlying cover. We argue that, by off‐setting the basal evaporite detachment, the basement faults form steps that nucleate structures in the otherwise thin‐skinned thrust belt. However, the basement faults are not simply inherited structures, as previously thought, but were locally activated during thrusting. Impacting the evolutionary sequence of these thrusts in the cover. In 3D, the folds that nucleate on basement steps can propagate laterally into areas where basement faulting is only weakly developed. It is the availability of exceptional seismic imagery that permits these deductions. Most other fold and thrust belts worldwide do not have this image quality and interpretations can rely on single profiles. Therefore, the role of basement faults may have gone un‐noticed.

Reactivation of critically-stressed basement faults and related induced seismicity in the southeastern Sichuan basin

Reactivation of critically-stressed basement faults and related induced seismicity in the southeastern Sichuan basin

Quaternary Activity and Paleoearthquakes of the Fushan Fault, Shanxi, China

The AD 1209 M6.5 Fushan earthquake caused significant casualties and damage. The Fushan Fault, forming the boundary between the Linfen Faulted Basin and uplifted Taihang Mountains, may have been the seismogenic fault, but research is lacking. Based on UAV and field surveys, we found that the Fushan Fault has a surface exposure length of 24 km and displaces Holocene strata. Samples from offset layers within a trench showed that the most recent event occurred within the last 7 ka (i.e., Holocene activity) and that the fault has the potential to generate earthquakes exceeding magnitude 7. Since 17 ka (late Quaternary), two significant paleoearthquakes have been identified: (1) between 17 and 7 ka (displacement: 2.04 m, average slip: 0.2 mm/yr) and (2) within the last 7 ka (displacement: 3.93 m, average slip: 0.56 mm/yr). Since the Late Pleistocene, the displacement rate has increased, indicating an increasing potential seismic hazard. These results were confirmed by terrestrial LiDAR; the bedrock fault surface fractal dimensions are consistent with two paleoearthquake events since the late Quaternary (coseismic displacements of 2.51 and 3.18 m). This article uses an empirical formula to evaluate the potential maximum magnitude of the Fushan Fault based on the relationship between the distribution range of the fault surface and the magnitude. Therefore, the maximum assessed earthquake magnitudes of the Fushan Fault are Ms = 7.07, 6.94, and 7.31. This assessment result basically matches the strength of the 6.5 magnitude Fushan earthquake in 1209 AD. By comparing with historical records, our results confirm that the Fushan Fault was the seismogenic structure responsible for the AD 1209 M6.5 Fushan earthquake.

Unsupervised machine learning models applied to basement faults: An example from the Dibrugarh region, NE India

Unsupervised machine learning models applied to basement faults: An example from the Dibrugarh region, NE India

Intra‐Craton Faults Characteristics, Development and Its Hydrocarbon Accumulation Significance in Mesozoic Strata at Southwestern Ordos Basin, China

ABSTRACTThe Ordos Basin is among China's largest petroliferous basins, with its southwestern part being a key exploration area. Prior studies indicated that the basin's internal structure was relatively simple, featuring minor developmental faults and primarily stratigraphic–lithologic reservoirs. However, recent research has identified numerous strike‐slip faults in the basin, and their relationship to oil and gas accumulations remains unclear. This study, using integrated interpretations of field outcrops, imaging logging and 3D seismic reflection data, clarifies the characteristics, morphology and formation mechanisms of multi‐period faults in the southwestern Ordos basin. Additionally, the study investigates the relationship between these faults and oil and gas accumulations. Results show that Mesozoic fractures in the southwestern basin are primarily NE‐ and NW‐trending. Seismic profiles reveal that these faults exhibit complex geometries, including upright structures in the Middle to Upper Triassic and floral structures in the Cretaceous. Coherence slices show that Lower Jurassic faults have linear structures NE‐ and NW‐trending, while Cretaceous faults exhibit parallel linear structures ENE‐trending. The study of fault displacement and morphology suggests two evolutionary patterns for Mesozoic faults in the basin: layered development and basement‐activated faulting. The widespread ENE‐ and NW‐trending faults represent a specific mode of tectonic stress transfer in stable cratonic areas with minimal basement fault influence. Conversely, some ENE‐trending faults are significantly influenced by basement activation during various geological periods, penetrating deeply into strata and exhibiting distinct segmentation on a planar scale. This differential fault development results in an uneven distribution of Jurassic oil and gas reservoirs. Significant accumulations of Jurassic oil and gas are found in the ENE‐trending tension–torsional strike‐slip sections, whereas many NW‐trending faults may negatively impact oil and gas reservoirs. This study elucidates the characteristics of Mesozoic faults in the southwestern Ordos Basin, offering valuable guidance for oil and gas exploration and development in the region.

Inter-basin groundwater flow in the Ordos Basin: Evidence of environmental isotope and hydrological investigations

The Ordos Basin, located in arid and semi-arid region of China, is famous for its abundant groundwater resources and artesian features. The source of groundwater recharge, whether from local precipitation or external sources, has been debated. This study aims to elucidate the groundwater circulation mechanism in the Ordos Basin through scientific expedition, environmental isotope method, and hydrological drilling exploration, providing valuable insights for other artesian basins. Comprehensive analysis indicates that groundwater in the Ordos Basin is recharged by modern precipitation, primarily from high-elevation areas outside the basin. Deep groundwater from these external sources ascends to the aquifer through basement fault zones. Evidence from hydrogen and oxygen isotopes, hydraulic gradients, and water quantities suggests that the Tibetan Plateau is the most potential recharge source. Based on the distribution of Cenozoic basalt and data from seismic observation wells, we propose that leakage water from the Tibetan Plateau rift valley is transported to the Ordos Basin through fast channels, possibly lava tubes, and then upwelling through basement fault zones. This work provides a new perspective on the mechanism of inter-basin groundwater circulation.

An alternative formation mechanism for strike-slip fault in stable intracratonic basin

The cratonic strike-slip fault is a kind of fault developed by strike-slip motion in relatively stable tectonic settings. Compared to strike-slip faults developed in an active tectonic settings, cratonic strike-slip faults have small-scale characteristics, weak activity and are difficult to identify without high-quality 3D seismic data. To date, there have been few in-depth investigations of this kind of strike-slip fault, leading to a poor understanding of its characteristics and genetic mechanisms. In this study, we extensively investigated the cratonic strike-slip faults discovered in recent years in the Ordos Basins of China via 3D seismic data, and conducted comparative analyses with similar features observed in other intracratonic basins and active tectonic settings. This study suggests that cratonic strike-slip faults have some special characteristics, such as small displacement and low maturity even when they have been active during several orogenic processes, vertical stratified structural style, and decoupling with the basin periphery structures in terms of trending direction and kinematics. This paper proposed an alternative formation mechanism for cratonic strike-slip faults. The cratonic strike-slip faults nucleate from fractures induced by regional compressive stress instead of being directly inherited from preexisting faults or basement faults. These fractures experience brittle shearing movement under various structural processes, such as plane-differential compression, oblique compression, block rotation, and finally evolve into long strike-slip faults. The development of cratonic strike-slip faults probably record the special stress transmission within the stable craton, the weak in-plane tectonic stresses in the intracratonic basin, which mainly operate at shallow depths. This formation mechanism can provide a new perspective for the understanding of brittle deformation on the earth's surface, and help to elucidate the occurrence of earthquake activity in stable blocks.

Topographic ridges express late Quaternary faulting peripheral to the New Madrid seismic zone, intraplate USA: Their tectonic implications

Northeast-trending linear topographic ridges in Pliocene and Quaternary sediments adjacent to the New Madrid seismic zone (NMSZ), intraplate North America, have been long speculated to be neotectonic landforms related to reactivation of basement faults of the eastern Mississippi Valley Rift margin. Earthquake epicenters and paleoseismological studies show that eastern rift margin faults (ERMF) were active during late Quaternary south of a restraining bend in the Mississippi Valley Rift fault complex, but the ERMF zone north of the restraining bend (ERM-N) and underlying the linear ridges is seismically quiescent. A previous P-wave seismic reflection survey across the most prominent linear ridge (Rives lineament) revealed that it overlies a horst in Eocene sediment. To ascertain whether the Rives lineament could have been produced by surface faulting/folding, we collected electrical resistivity tomography (ERT) and ground penetrating radar (GPR) profiles to image the ridge's shallow subsurface features. We interpret shallow folding and faulting of late Pleistocene sediment in our ERT and GPR images. Additionally, convexity of multiple topographic profiles of scarps along margins of the Rives lineament was quantified and compared to the convexity of profiles of known neotectonic scarps and of fluvial terrace riser profiles within the region. Comparison of these profiles reveals a high degree of similarity between the scarps along the Rives lineament margins and the Holocene Reelfoot thrust scarp. When combined, our results strongly suggest late Pleistocene or Holocene folding and faulting created this ridge despite its current seismic quiescence, and thus they provide useful insight for assessing the seismic hazard potential of quiescent faults in strike-slip systems in general. We conclude that the late Quaternary series of movements documented on the Reelfoot thrust was initiated to accommodate crustal shortening after transpressional fault movements on the ERM-N became inactive in late Pleistocene or early Holocene and that movement along the ERM-N may resume if the Reelfoot thrust has an interval of inactivity in the future. Our results suggest how fault interactions within a strike-slip system can change, making faults with significant amounts of seismic potential appear as though they are inactive.

Effect of hydrogen on the basal dissociation of 〈a〉-type screw dislocation core in titanium

The existence of basal dissociated screw dislocation core structure of titanium with different H concentrations is investigated by first-principles calculation. The calculation of stacking fault energy of the basal plane with H in different layers suggests that a high H concentration would be beneficial for basal dissociation. The direct calculation based on the dislocation core instead proved that a small amount of H cannot stabilize the basal dissociated core structure. When H concentration increases, the basal dissociated core configuration and some mixed cores with basal faults could be more stable than the pyramidal dissociated structure.

Influence of reactivated basement structures on evolving orogens: Along-strike diachronous Himalayan metamorphism in far west Nepal

Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.

The geometry of fault reactivation and uplift along the central part of the Maacama Fault Zone, Northern California Coast Ranges (USA)

Abstract Fault reactivation of bedrock structures in active fault zones influences stress state and earthquake rupture phenomena through the introduction of weak slip surfaces that impact fault zone geometry and width. Yet, geometric relationships between modern faults and older reactivated faults are difficult to quantify in rocks that have experienced multiple deformation episodes. We used new geologic mapping, geomorphic tools, and structural modeling to quantify rock uplift and subsurface fault geometry of the central part of the Maacama Fault Zone near Ukiah, California, USA, and the surrounding area. Results suggest that the northern Mayacamas Mountains are in a tectonically driven disequilibrium, with differential rock uplift focused on the western side of the range. Steeply east-dipping fault surfaces and splays characterize the geometry of the Maacama Fault Zone. We mapped two newly identified faults to the east of the main Maacama Fault, the Cow Mountain–Mill Creek Fault, and Willow Creek Fault, which align with a moderately east-dipping cluster of microseismicity between 4–10 km depth beneath the Mayacamas Mountains. Static stress modeling on the Maacama Fault Zone and newly identified faults to the east quantify slip tendency values of 0.5–0.4, which suggests that the faults are moderately to poorly suited for slip in the modern stress field and may be weak. We infer that modern uplift is driven by oblique reverse, up-to-the-east, dip-slip motion on the reactivated Cenozoic Cow Mountain–Mill Creek and Willow Creek Faults as material is advected through a restraining bend on the Maacama Fault. This study shows that reactivated bedrock faults increase the fault zone width and introduce fault surfaces that contribute a component of vertical deformation and uplift in major strike-slip fault zones. Deformation is accommodated on an interconnected network of new and reactivated faults that delineate a complex seismic hazard.

Preliminary ESR dating results of fault barite shed new insights into the retrospective history of bedrock fault activities in basaltic regions

The chronology of fault activity in bedrock is critical to constraining and understanding periods of active faulting, assessing seismic hazards, and mitigating the effects of earthquakes. However, because of the lack of suitable materials for dating, the temporal reconstruction of faulting in bedrock remains highly challenging for geologists. In the present study, we determine for the first time the electron spin resonance (ESR) ages of fault barite (BaSO4), which is produced by episodes of intense faulting on basalt bedrock fault surfaces. Three barite samples were obtained from a basalt fault section (27°5′23″N, 100°25′45″E, 1.8 km above sea level) of the Lijiang–Xiaojinhe Fault (LXF), southeastern Tibetan Plateau, for ESR measurements. Similar to marine barite, the ESR spectrum of fault barite shows an electron-type center with g = 2.0037, 2.0034, and 2.0028 attributed to SO3−. The signal intensity systematically increased with increasing gamma-ray dose. Dose rates were calculated using a model based on the location and burial depth of the barite samples, as well as their surrounding bedrock. The three barite samples yield ESR ages of 131 ± 26, 503 ± 61, and 1416 ± 246 ka, respectively, which indicate that the LXF was active during the Early and Middle Pleistocene. The three ESR ages for fault barite from basalt extend the time range of activity of the LXF compared with previous carbonate ESR and radiocarbon dating results. Consequently, we propose that ESR dating of barite is valuable for reconstructing the history of bedrock fault activity. However, given that this investigation represents a preliminary application of the fault-barite ESR method, further study is needed to confirm its usefulness and the accuracy and precision of dating results.

Development of arcuate faults and controls on hydrocarbon accumulation in the Tiantai slope, Xihu Depression, East China Sea basin

The strike-slip movement of NW-trending transfer zone in the Tiantai slope of the Xihu Depression governed the development of the Cenozoic arcuate faults. These faults significantly influenced trap formation, sand-bodies distribution, and hydrocarbon migration. Based on seismic, drilling data and fault activity analysis methods, this paper describes the origin and evolution of the arcuate faults and their impact on hydrocarbon accumulation. These arcuate faults exhibit a change from NE to NW strike orientation in plan view, and are distributed in the hanging wall of the NW-trending basement faults. The main arcuate faults penetrate the bottom of the Cenozoic strata and extend upward through the top of the lower member of the Huagang Formation, markedly affecting the depositional dynamics of the Pinghu Formation. The segments of the arcuate faults began to grow during the Middle Eocene, and experienced hard linkages in the Late Eocene. The activity of these arcuate faults diminished significantly and gradually stopped during the Middle and Late Oligocene. The segmentation and linkage growth processes of the arcuate faults controlled the formation and evolution of traps. Additionally, the activities of these faults influenced the distribution of sandbodies, with the segmentation points of the arcuate faults serving as important conduits for sediment input. Moreover, the arcuate faults served as effective pathways for vertical hydrocarbon migration. The area with the arcuate faults present favorable conditions for hydrocarbon accumulation in the Tiantai slope of the Xihu Depression.

Hydrothermal activity near the Permian–Triassic transition in the south‐western Ordos Basin, China: Evidence from carbonate cementation in Upper Permian sandstones

ABSTRACTCarbonate cementation in the Upper Permian sandstones informs the timing and temperature of hydrothermal activity in the south‐western Ordos Basin. This study presents a detailed examination of these hydrothermally influenced carbonate cements, constraining their age, carbonate diagenesis and relationship with regional geodynamic evolution. Sedimentological analyses demonstrate the development of deltaic plain and front sand bodies in the study area, which resulted in interbeds of volcanic matrix‐rich sandstones with matrix‐free sandstones. Petrography and electron microprobe analysis reveal four carbonate mineral growth phases of matrix‐free sandstones in the following sequence; scarce pure siderite, scarce Mg‐rich siderite, abundant blocky calcite and moderately abundant grain‐replacing calcite. The fluid inclusion data show anomalies of homogenization temperature of blocky carbonate cements during early diagenesis, over a wide range of ca 148 to 228°C. In addition, blocky carbonate cements show low δ13C (−5.9 to −13.1‰ Vienna PeeDee Belemnite) and δ18O (clustered tightly from −12.4 to −14.6‰ Vienna PeeDee Belemnite) values, interpreted to result from elevated temperatures during cementation, associated with activation of basement faults and concomitant hydrothermal fluid intrusion triggered by oceanic crust subduction in the south‐west margin of the Ordos Basin. Using in situ calcite U–Pb geochronology, the timing of hydrothermal activity was constrained to ca 247.0 ± 11 to 248.2 ± 4.7 Ma. This work provides a case study for applying intergranular calcite U–Pb dating to determine the absolute timing of fluid flow in sedimentary basins, offering tremendous potential to capture snapshots of various diagenetic evolution stages in sediments. The proposed diagenetic model can also provide new insights and understanding regarding hydrothermally influenced sediments. More importantly, hydrothermal activity may have commenced earlier than previously thought. The North Qinling Orogen uplift and associated Mianlue oceanic crust subduction may have begun at the Permian–Triassic transition with a protracted hydrothermal event in the south‐western Ordos Basin.

Surface Rupture of the 2008 Mw 6.6 Nura Earthquake: Triggered Flexural‐Slip Faulting in the Pamir‐Tien Shan Collision Zone

AbstractThis study investigates the intricate relationship between earthquake sources and seismogenic surface ruptures in a complex tectonic setting with active faults in the continental collision zone between the southern Tien Shan and the northern Pamir Mountains in Central Asia. The study focuses on the 2008 Mw 6.6 Nura earthquake along the Pamir Frontal Thrust, where the seismogenic surface rupture occurred unexpectedly within the footwall and 10 km away from the source thrust fault. This discrepancy raises questions about the interactions and potential trigger mechanisms between tectonic structures during earthquake rupture. Using unmanned aerial vehicle photography and field inspection, our investigation integrates detailed fault‐zone mapping with tectono‐geomorphic observations to unravel potential interactions between subsurface structures and surface‐deformation phenomena. Our findings suggest that a combination of slip along deep‐seated basement faults and remotely triggered flexural slip within folded Paleogene strata led to surface rupture of overlying Quaternary glacial deposits. Geomorphological and geochronological analyses coupled with systematic displacement measurements furthermore reveal evidence of similar past ruptures within the regional fault system, suggesting a recurrence interval of 1.7 kyr and a Holocene vertical offset rate of 0.4 mm/yr. The analysis of the Nura rupture zone contributes significantly to evaluate linkages between surface and subsurface structures regarding fault‐zone behavior and seismic hazard assessments. Importantly, our results highlight the critical role of on‐site investigations in regions with poorly defined surface ruptures, where misinterpretation may lead to the underestimation of the impact of seismic events and limitations in assessing earthquake history and strain accumulation.

Kinematics of rift linkage between the Eastern and Ethiopian rifts in the Turkana Depression, Africa

AbstractRift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.

Regional variance in alluvial sedimentation and revised stratigraphy of the Klipheuwel Group and Franschhoek Formation

Abstract The siliciclastic sediments of the Klipheuwel Group mark the transition from Pan-African tectonism and amalgamation of Southwest Gondwana to the shallow marine deposits of the Cape Supergroup in the Western Cape. The rocks are preserved in a number of seemingly isolated depositories and previous studies have mainly focused on selected occurrences. The lack of any integration of regional sedimentological or structural characteristics has, to date, resulted in only tentative stratigraphic correlations and subdivisions of the Klipheuwel Group. Lithological and structural data from the Klipheuwel Group indicate at least two separate depocentres that accommodated the post Pan-African peneplanation of the Saldania Belt and clastic detritus in distinctly different palaeoenvironments. A northern depocentre can be distinguished from a southern depocentre, each characterised by distinct architectural elements and facies associations that reflect topographic and structural controls. The northern depocentre, including Klipheuwel Group occurrences at Eendekuil, Redelinghuys and Elands Bay, show laterally persistent braided fluvial facies associations that developed multi-storey superimposed ‘braided sheet’ deposits. Here, the rocks show gentle dips and thicknesses range from 300 to 450 m. The southern depocentre, including the Klipheuwel and Klapmutskop localities, are characterised by much larger thicknesses (up to >2 000 m), steep dips of the rocks and laterally discontinuous braided fluvial facies associations that developed staggered channelised braided deposits. The lateral continuity of fluvial facies in the northern depocentre reflect sedimentation on a peneplained basement. The larger thickness, steep dips and strongly channelised deposits in the southern facies, in contrast, indicate deposition in actively subsiding half-graben structures that reactivated basement faults. The spatially closely associated Franschhoek Formation shares numerous characteristics with the Klipheuwel Group but preserves Pan-African strains similar to that of the underlying Malmesbury Group, that may indicate its formation as compressional piggyback basins with synorogenic sedimentation.

Riedel shear structures reactivation may induce earthquakes through long-term steam injection: A case study of a heavy oil production field in northwestern China

Conventional heavy oil exploitation methods involve steam stimulation and flooding. An oil field in northwestern China has been producing heavy oil via steam injection for several decades. The production area has been seismically quiet until an increase in seismicity occurred several years ago. An array of 40 seismographs has been deployed between July and October 2021 to monitor seismicity and resolve the possible causes of the recent seismicity. Using an end-to-end machine-learning-based high-precision earthquake location workflow, we analyze a microseismic sequence comprised of 178 events that occurred in the study area. Numerical simulations incorporating Coulomb failure stress suggest that prolonged steam injection can reactivate faults and induce seismic events. Similarly, fluid diffusion through conduits may achieve the same effect. Analysis of the focal mechanism solutions of 21 strike-slip and thrust events with [Formula: see text] in conjunction with the background stress regime ([Formula: see text] azimuth = N15°W) reveal that the stress distribution is compatible with a left-lateral Riedel shear structure (RSS) model. Therefore, we can speculate that steam injection may induce earthquakes by reactivating preexisting RSS fault structures. To conclude, the recent seismic events could have been induced by two possible mechanisms: (1) long-term steam injection may cause the static stress level on the faults beneath the reservoir to build up to critical levels, following which a slight stress disturbance can trigger an earthquake and (2) fluid conduits may transport condensed water to basement faults, weakening the faults through fluid diffusion.

A Comprehensive Analysis of Basement Faults in Miocene Deposits: Fars and Bandar Abbas Perspectives

A Comprehensive Analysis of Basement Faults in Miocene Deposits: Fars and Bandar Abbas Perspectives

Source and U-Pb Chronology of Diagenetic Fluids in the Permian Maokou Formation Dolomite Reservoir, Eastern Sichuan Basin, China

The hydrothermal dolomitization, facilitated by basement fault activities, had an important impact on the Permian Maokou Formation dolomite in the Sichuan Basin, which experienced complex diagenesis and presented strong reservoir heterogeneity. The source and age of diagenetic fluids in this succession remain controversial. In this study, various analyses were implemented on samples collected from outcrops and wells near the No. 15 fault in the eastern Sichuan Basin to reconstruct the multi-stage fluid activity and analyze the impact on reservoir development, including petrology, micro-domain isotopes, rare earth elements, homogenization temperature of fluid inclusions, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating. The homogenization temperature of primary brine inclusions in fine-grained matrix dolomite and saddle dolomite is concentrated between 100 and 150 °C, which indicates that the impacts of abnormally high temperatures of other geological bodies. The δ13C and δ18O value and low 87Sr/86Sr value indicate that the diagenetic fluid of fine-grained matrix dolomite is mainly Permian seawater. The U-Pb ages of fine-grained matrix dolomite are ~260 Ma, which coincides with the age of the main magmatism of Emeishan Large Igneous Province (ELIP), and hydrothermal fluid provided a favorable high-temperature environment in the penecontemporaneous stage. While highly radiogenic 87Sr/86Sr compositions suggests those of saddle dolomite, the high-temperature Sr-rich formation fluid. The U-Pb ages of saddle dolomite are 245–250 Ma, which coincides with the age of the 255~251 Ma magmatism of ELIP. This indicates that those should be the diagenetic products of the ELIP hydrothermal fluid in the shallow burial stage. The U-Pb age of coarse-grained calcite is 190–220 Ma, and it should be the diagenetic product of the deep burial stage. Brine inclusions associated with primary methane inclusions were developed in coarse-grained calcite, with a homogenization temperature range of 140.8–199.8 °C, which indicates that the formation fluid activities were related to hydrocarbon charging. The Permian Maokou Formation dolomite was firstly formed in the penecontemporaneous shallow burial stage, and then it was subjected to further hydrothermal dolomitization due to the basement faulting and the abnormally high heat flow during the active period of ELIP. Hydrothermal dolomitization contributed to the formation and maintenance of intercrystalline and dissolution pores, whereas it also formed saddle dolomite to fill the pores, and reduce the pore space. The influence of deep fluid activities on reservoir evolution is further distinguished.