Reactivation Of Basement Faults Research Articles

Strontium isotope proxy of sedimentological records reveals uplift and erosion in the Southeastern Neo-Tethys ocean during the late Cretaceous

The mutual interplays between plate tectonic processes, orogenesis and continental uplift, high-flux magmatism, and high sedimentation rates can affect the geochemical signatures and composition of marine sediments. Here, we examine two major disconformities, the Cenomanian–Turonian boundary (CT-ES) and the middle Turonian (mT-ES) exposure surfaces, from the Upper Cretaceous sedimentary successions of the southeastern Neo-Tethys Ocean in the Zagros Basin (Iran). The disconformities are expressed as distinct positive peaks in rubidium (Rb) contents and 87Sr/86Sr isotopic ratios of carbonates. The 87Sr/86Sr values of samples bracketing the disconformities in seven well cores give average age ranges of 94.4–93.6 Ma for the CT-ES and 91–86 Ma for the mT-ES. These ages fall in the timespan of forearc/ophiolite formation through subduction initiation in the Neo-Tethys realm (southern margin of Eurasia), high convergence velocities between Arabia and Eurasia, and blueschist exhumation. All these processes involved buckling of the Neo-Tethyan lithosphere, initiating the Neo-Tethys closure and a high erosion rate on the Eurasian margin. The first two mechanisms exerted considerable compressional forces on the adjacent carbonate platforms, reactivated basement faults, and led to the uplift and erosion of the Arabian Plate, which provided the high contents of Rb and the high 87Sr/86Sr ratios in the carbonates.

Unravelling the tectonic phases: The impact of the South Caspian Block on Late Cenozoic deformation in the Central Alborz, Iran

Paleostress reconstruction through polyphase fault-slip data of a multi-deformed region suffered collisional tectonics can lead to detecting the stress phases. Based on earthquakes’ focal mechanisms and morphotectonic features, multiple deformation phases model has been proposed for the Alborz Mountains located in the collision zone between the Arabian (Central Iran) and Eurasian (South Caspian block) plates during the Late Cenozoic. In this study, paleostress analysis has been carried out in an area bounded by two (Kandovan and Taleghan) regional faults in the Central Alborz Mountains using fault-slip data. This analysis resulted in the identification of three main tectonic phases. The first, compressional phase is proposed to cause the inversion of the Alborz Mountains’ major initial normal faults to reverse faults during the convergence of the Arabian and Eurasian plates. The second, transpressional phase is offered as a factor for the reactivation of the hidden Alborz basement faults to form the NE-striking left-lateral strike-slip faults on the sedimentary cover. The third, transtensional phase is suggested to be responsible for the development of the NNE–SSW left-lateral transtensional faults. It is proposed that the second and third paleostress phases are affected mainly by the indentation of the South Caspian Block into the Alborz Mountains during the Late Cenozoic.

Induced seismicity traffic light protocol at the Alberta No. 1 geothermal project site

The Alberta No. 1 Geothermal Project (ABNo1) is a renewable energy project in Canada; it will use deep geothermal energy for electrical power generation and direct-use thermal power. However, concerns regarding induced seismicity must be addressed. This study proposes implementing the Traffic Light Protocol (TLP) as a risk management tool for conventional geothermal projects in Alberta. We focus on determining an appropriate TLP red-light threshold, employing a risk-informed approach for the Leduc Formation - one of the geothermal reservoirs proposed for development by ABNo.1. The recommended red-light threshold of ML 3.5 signifies the level at which operations must be halted (i.e., the red light) to mitigate nuisance impact, damage, and personal risk. The recommended yellow light threshold of ML 1.5 signifies the level at which assessment of injection operations must be re-assessed, the regulator must be informed, and a risk mitigation analysis must be undertaken to re-evaluate operations to keep induced seismicity at a low magnitude. Comparison to induced seismicity near the project site highlights key factors requiring consideration during the project's design phase: injection volume, chances of basement fault reactivation, and delayed seismic activity onset. Effective mitigation entails controlling injection rate, continuous fluid balance monitoring, allowing gradual and appropriate pressure dissipation rates, and potentially the use of tracer tests to deconvolve pathways. This study provides insights for stakeholders involved in injection projects, aiding risk mitigation and ensuring safe, sustainable energy extraction while also prioritizing the local community's well-being.

Coupling of thermal convection and basin-basement fluid mixing is critical for the formation of unconformity-related uranium deposits: Insights from reactive transport modeling

Thermal convection and fluid mixing are involved in the formation of many mineral deposits, but their relationships and roles in mineralization remain poorly understood because they are generally investigated separately in different studies. Here we present a case study combining thermal convection and fluid mixing to examine the critical factors that controlled the formation of high-grade, world-class unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada). Reactive transport modeling of various scenarios with different degrees of involvement of thermal convection and fluid mixing shows that significant URU mineralization occurs at the intersection of a basement fault with the basin-basement unconformity only if thermal convection and basin-basement fluid mixing take place concurrently. If there is no thermal convection in the basin, only sparse U mineralization occurs along the unconformity. If insufficient amount of reducing fluid is provided from the basement along the fault, no significant U mineralization occurs either. Furthermore, no significant U mineralization occurs if the U concentration in the basinal fluid is low. We conclude that the exceptionally rich U endowment in the Athabasca Basin is the result of coupling of three critical factors: high-permeability sandstone favoring thermal convection in the basin, ample supply of reducing agents by basement fluids along reactivated basement faults, and abundant U-rich basinal fluid available in the basin sequences. Similar coupling of fluid processes may be essential for the formation of other types of mineral deposits.

Structural evolution, segmentation and activity of the onshore-offshore Waimea-Flaxmore Fault System in south-eastern Tasman Bay, South Island, New Zealand

ABSTRACT The Waimea-Flaxmore Fault System (W-FFS) from the Nelson-Richmond urban area to D’Urville Island is analysed through six regional transects that join depth-converted seismic lines in Tasman Bay to onshore cross sections. Reverse reactivation of basement faults of the W-FFS by mechanisms of fault-propagation folding at c. 19-10 Ma occurred along the entire length of the fault system. However, the W-FFS remained Quaternary active only from the vicinity of Cape Soucis southwards into onshore Nelson, in contrast with cessation of activity along eastern Tasman Bay since 7 Ma. The inactive western faults of the W-FFS are buried below Plio-Quaternary marine sediments and segmented by the newly identified, c. E-W, Croisilles Fault Zone, Cross Point and D’Urville faults, with cumulative dextral offset of c. 27 km. The c. E-W faults extend from the onshore into Tasman Bay and are interpreted as the northernmost Late Miocene strands of the Marlborough Fault System that have accommodated incipient distributed shear in the crustal block north of the Queen Charlotte Fault Zone. Segmentation and along-strike changes of the W-FFS occur at the transition from the contractional domain of onshore Nelson to the northern Marlborough strike-slip domain, driven by large-scale kinematics of the Australia-Pacific plate boundary.

A synergistic approach to morphotectonic evolution for watershed management in the Bearma River Basin, Central India

Abstract Environmental catastrophes on a global scale have prompted a thorough evaluation of river morphology for sustainable basin development methods. Geomorphological investigations of river basins can provide significant information regarding Quaternary tectonic deformations. The present investigation intends to reveal tectonic imprints in the Bearma River Basin (BRB). Bearma is a significant river in central India that flows through Vindhyan Supergroup, Lameta, and Deccan Trap and contributes to developing the architecture of the marginal Gangetic plain. The digital elevation data has been utilized to obtain the morphotectonic indices, tectonic activity classes, and topographic characteristics. Bearma is an elongated basin with uplifted topography, continuously migrating channels, high hypsometric integral, and several stream length-gradient anomalies, indicating tectonically controlled. According to the tectonic activity index, 15.33%, 38.99%, and 46.55% areas of the BRB have high, moderate, or low tectonic activity, respectively. In conjunction with field investigations, the topographic and lineament study of the BRB has revealed significant relief variations and the importance of tectonic activity over erosion and depositional processes in determining the landscape. Reactivation of basement faults and subsurface lineaments caused by Himalayan tectonics and the Narmada Son North Fault have resulted in the recent deformation and development of the hydrographic network.

Recent basement fault reactivation and fluvial drainage modification in an intraplate setting, eastern Bundelkhand Craton, Madhya Pradesh, India

North-central India is underlain by three northeast-trending paleo-topographic ridges of Precambrian Indian basement, bounded by lithospheric-scale basement faults. The Pokhara Fault defines the eastern edge of the Faizabad paleo-topographic ridge, which exposes the Archean Bundelkhand Craton. Near Panna, Madhya Pradesh, the Pokhara Fault coincides with a monocline developed in the Proterozoic Vindhyan Supergroup rocks along the Bundelkhand cratonic margin. Fluvial morphology of basins along the monocline, as well as a deeply incised abandoned meander preserved where the Ken River flows through the monocline, are investigated using longitudinal stream profiles and other geomorphic data along the length of the monocline to assess potential linkages with active basement fault reactivation.Normalized steepness indices in analyzed channel profiles along the monocline increase significantly downstream, with some channels exhibiting convex, over-steepened segments. Normalized steepness indices, knickpoint elevations, and incision depths are on average much higher in streams along the southwest segment of the monocline compared to streams along the northeast segment. Most channel profiles exhibit a knickpoint with slope-break morphology. The obtained longitudinal channel profiles exhibit characteristics of a fluvial system in a transient state subjected to an increase in rock uplift that intensifies to the southwest over a reactivated basement fault. Using previously published relationships between erosion rates and channel steepness, we conservatively estimate the onset age of monocline growth to be Plio-Quaternary. Fluvial geomorphic features preserved near Panna also suggest that the Ken River is an antecedent river. The incised meander feature near the crest of the monocline appears to be the abandoned river valley of a former Ken River course that was orphaned during the evolution of the landscape by what is now the present-day Ken River.Our study supports growing evidence that Indian basement faults such as the Pokhara Fault are long-lived and have recorded multiple periods of reactivation. Recent fluvial modification in the Panna region demonstrates that Indian basement faults are also active on the south side of the Ganga Basin, far south of the Himalayan deformation front, and may be linked to intraplate seismicity and active tectonic modification of the landscape.

Structural styles of the Tierra del Fuego fold-thrust belt foothills, Argentina

Regional structural cross-sections based on seismic, borehole and surface data portray the foothills geometry of the Tierra del Fuego fold-thrust belt. Andean contraction was accommodated by two main deformation mechanisms: 1) A lower duplex thrust system that involves mainly Cretaceous deposits, with floor and roof thrusts at the base of the Cretaceous and close to the base of the Cenozoic, respectively; and 2) A series of detachment and faulted detachment anticlines in the Cenozoic, detached in mudstones above the roof thrust of the underlying duplex thrust system. Less common structures include fault bend and fault propagation folds. Basement fault reactivation is only locally important, with most Jurassic grabens and half-grabens preserved without inversion. Shortening along the foothills is modest, with values ranging from ∼8 to 4.6 kilometers. This region of the fold-thrust belt has hydrocarbon exploration interest; however, it is largely underexplored. Several large anticlines preserving Cenozoic clastic rocks have not been tested to date. The key risk for these structures is the charge, as the thick mudstone package forming their detachment isolates potential Cenozoic reservoirs from the Lower Cretaceous source rocks. Duplex thrust systems at depth may constitute a potential exploration play, provided brittle intervals were naturally fractured.

Multiphase faults activation in the southwest Huizhou Sag, Pearl River Mouth basin: Insights from 3D seismic data

The southwest Huizhou Sag is located in the Zhu I depression in the northern part of Pearl River Mouth Basin, which is influenced by the pre-existing basement fabrics and multiphase rifting, and has complex fault patterns in the Cenozoic. Using 3D seismic data of the southwest Huizhou Sag, we used seismic interpretation, fault pattern analysis from isochron maps, detailed fault throw analysis and structural evolution section analysis to constrain the initial formation timing, active periods and genesis of different striking normal faults, and the influence of basement faults on Cenozoic faults and the evolution progress of the sag was discussed, which significantly improve our understanding on the tectonic evolution of the northern Pearl River Mouth Basin. There are four groups of faults developed in the southwest Huizhou Sag, consisting of NE-SW, ENE-WSW, E-W and WNW-ESE striking, and the boundary faults of the subsag are mostly curved. Selective reactivation of ENE-WSW and WNW-ESW striking basement faults controlled the evolution of fault system during the Cenozoic. The southwest Huizhou Sag experienced multiple phases of extension, and the extension direction rotated clockwise from NW-SE to NNE-SSW. During the Early Eocene (Rift Ia), a large number of NE-SW striking faults were formed, accompanied by reactivation of ENE-WSW and WNW-ESW striking basement faults. These group of faults continued to be active during the Middle Eocene (Rift Ib) and some ENE-WSW striking faults were newly formed. The Late Eocene to Early Oligocene (Rift II) was dominated by E-W striking faults, and the depocenter shifted to the northwestern part of the study area. The Middle-Late Oligocene to Quaternary (Post-rift) was dominated by WNW-ESE striking faults, and the depocenter was not obvious.

Detailed Imaging of a Seismogenic Fault That Potentially Induced the Two 2019 Weiyuan Moderate Earthquakes in the Sichuan Basin, China

Abstract Induced earthquakes are becoming a significant issue. Several destructive induced earthquakes (Ms ≥5.0) have occurred in shale gas fields located at the southwestern Sichuan basin, China, in the recent years. Specifically, two moderate earthquakes with magnitudes of Ms 5.4 (occurred on 8 September 2019) and 5.2 (occurred on 18 December 2019) exceeded the largest earthquake ever recorded in the Weiyuan. It remains unclear which seismogenic faults were responsible for the two moderate events due to insufficient knowledge of the subsurface structure and contradictory hypocenter depths obtained from different seismological methods. In this study, mainshock locations, focal mechanisms, and precise relocations of microseismicity were used to map the 3D geometry of the seismogenic fault. The result shows that the two moderate earthquakes occurred on the same fault. High-resolution seismic reflection profiles indicate several preexisting faults at various depths and a potential seismogenic fault. No large-scale faults are identified in the sedimentary cover above the Cambrian at the shallow centroid depth of these earthquakes. However, the seismic reflectors show phase discontinuous feature, indicating the existence of preexisting faults in the basement. A preexisting fault in the basement is consistent with a seismogenic fault that may have induced the two moderate earthquakes. Our study suggests that poroelastic stress propagation is likely responsible for the basement fault reactivation. The rupture on a stressed basement fault was triggered and produced two moderate earthquakes in a short period during hydraulic fracturing. The previous strategy of avoiding direct injection of fluids into preexisting faults to reduce seismic risk in operation region is not enough to avoid the possibility of the coupled poroelastic stress triggering deep fault rupture. We need to reevaluate the potential seismic risk of shale gas development in Weiyuan, the southern Sichuan basin shale gas field considering the preexisting faults in sedimentary and in basement and the effect of the fully coupled poroelastic stress field.

Oligocene/Early Miocene E/W‐Shortening in the Oman Mountains Related to Oblique Arabia‐India Convergence

AbstractField survey, literature review, geological map interpretation, GPlates reconstruction and LA‐ICP‐MS U‐Pb dating of synkinematic calcite demonstrate that ∼E/W‐shortening in eastern Oman was significant and related to oblique convergence of Arabia and India from 32.5 to 20 Ma. Approximately N/S‐striking contractional structures, WNW to NNW‐striking sinistral faults and ∼E/W‐striking normal faults characterize a ∼250 km × ∼50 km wrench corridor in the eastern Oman Mountains (Hajar Wrench Corridor, HWC). Numerous faults/folds indicate that deformation of the HWC is widely distributed but concentrated along WNW to NNW‐striking major faults at the SW margin of the Saih Hatat Dome, forming the Hajar Shear Zone, which reactivated basement faults. GPlates reconstructions reveal that N‐drifting India rotated 8° counter‐clockwise with respect to fixed Arabia from 32.5 to 20 Ma, leading to a minimum of 100 km E/W convergence between both plates. This convergence created the sinistral HWC with a displacement of a few to several tens of kilometers. Independently from the GPlates time constraints, two U‐Pb ages of synkinematic calcites, crystallized along faults during HWC movement, yield compatible ages of 30.08 ± 0.47 and 22.31 ± 2.15 Ma (2 standard error). E/W‐shortening also affected the northern Oman Mountains, creating the ∼N/S‐striking Hagab Thrust in the Musandam Peninsula and the Jabal Hafit Anticline.

Characterization of Structural Geology of Faihaa Oilfield, Southern Iraq

Faihaa Oilfield is a new exploration Iraqi Oilfield located in southern Iraq and within Mesopotamian Plain, including the Block 9 exploration area, along the Iraqi-Iranian border. The study area included Faihaa Oilfield (in Iraq) and Yadavaran Oilfield (in Iran). They belong to one anticline (Dome) structure separated by the Iraqi-Iranian border, without a geological boundary between the fields. The current study aims to achieve structural geology analysis to the study area (Faihaa/Yadavaran structure). The structural analysis included geometric and genetic analyses of the study area. According to geometric and genetic analyses results, the Faihaa/Yadavaran structure is classified as an anticline, gentle, upright, non-plunge, and asymmetrical. According to thickness variation, there are two types of formations’ folds are recognized, Thickened and Supratenuous fold generated by two folding mechanisms bending and buckle mechanisms. Bending form Supratenuous fold, perhaps due to the vertical uplift of salt structure and/or basement faults, while buckle produces Thickened fold because of the parallel tectonic movement causes collision between Arabian Plate and Eurasian Plate. Consequently, a special strain pattern was formed and the reservoir quality in the crest of Mishrif and Yamama Formations was the best. The fold axis of the Faihaa/Yadavaran structure has a Boomerang shape, whereas it is almost straight from south to the center of the structure, then tends to the NW with different deviations over the study area formations. This direction may be due to the anticlockwise rotation of the Arabian Plate motion. The results of geometric and genetic analyses revealed that may be Faihaa/Yadavaran structure is one structural trap formed by tectonic activities; Hormuz salt structures, reactivated Basement faults, and Collision between Arabian and Eurasian Plates. The intensity of the tectonic activities of the study area maybe be less than surrounding Oilfields, therefore, the Faihaa Oilfield formation's depth was deeper than adjacent Oilfields.

Continental-scale structural heritage from rift extension to postrift inversion: Implications for the central Brazilian Equatorial Margin evolution

The link between major basement anisotropies and basin evolution is crucial for understanding the role of tectonic inheritance on continental margins. However, it is still a matter of debate how and why it happens, especially in the Equatorial Atlantic, where the interplay between Precambrian fabric and the shearing of major oceanic fractures has been studied. Here, we investigate the offshore Ceará Basin and its basement in the central part of the Brazilian Equatorial Margin to (1) map basement anisotropies underneath marginal basins and show how Precambrian crustal domains influenced basin evolution; (2) determine the deformation style of reactivated basement faults; and (3) assess the interplay between reactivated faults and the oceanic Romanche Fracture Zone. The combined geological and geophysical data show that reactivation of NE-SW to ENE-WSW continental-scale ductile shear zones that acted as crustal weakness zones started in the Early Cretaceous under a normal stress field. Stress field inversion occurred in the Barremian during the continental breakup when the South American plate started to became an independent intraplate area under a strike-slip stress regimes. The shear zones arrest at the E-W-striking Romanche Fracture Zone, which marks the continental-oceanic boundary. The major fault offsets occurred along the Transbrasiliano Shear Zone, which represented a collisional boundary between paleocontinents in the Neoproterozoic. Fault reactivation also occurred in the eastern boundary of the Ceará Terrace marginal ridge along the Transbrasiliano Shear Zone, uplifted during tectonic inversion. We conclude that fault reactivation mainly occurred between contrasting Precambrian terrains separated by ductile shear zones. Additionally, the folds associated with the Romanche Fracture Zone were mainly generated along reactivated faults that affected syn- to postrift units on the Equatorial Margin.

A wave to tidal influenced deltaic coastline in a carbonate environment: The Late Jurassic to Early Cretaceous La Casita Formation, northeastern Mexico

In this study, we track the evolution of a clastic shoreline before carbonate deposition takes over. The Late Jurassic to Early Cretaceous La Casita Formation in northeastern Mexico serves as an excellent example for such a shoreline. The work includes a re-interpretation of the tectono-sedimentary environment. The La Casita Formation includes nine facies associations related to river, wave, storm, and tidal processes along a deltaic coastline. Systems influenced by rivers develop more muddy successions with sedimentary structures characteristic of these systems (e.g., normal gradation, parallel lamination, and ripples). In comparison, the systems influenced by waves/storms and tides develop more sandy successions with the development of structures generated by oscillatory flows (ripples and hummocky cross-stratification) and bidirectional ripples and dunes with mud-drapes on their stoss side, respectively. The distribution of sub-environments (e.g., interdistributary bay and shoreface) was controlled by waves and tides. Paleocurrent directions indicate that the coastline was oriented NNW-SSE. The wave- and storm-dominated sedimentary environments contain a large diversity of trace fossils (BI ∼4) related to colonizing organisms in well-oxygenized environments with abundant food, normal salinity, and high temperature. The river- and tide-influenced settings are governed by a low trace fossil diversity (BI ∼2), indicating highly stressful conditions generated by the incursions of hypopycnal plumes.Allocyclic processes were associated with the reactivation of basement faults that controlled the exposure and distribution area of the basement rocks (e.g., the nearby Coahuila block and Tamaulipas Arch). In conclusion, the revision of the environmental evolution of the La Casita Formation has implications for the marine incursion of the Gulf of Mexico and its relationship with the pacific margin and the strike-slip basin development.

Evidence for Post‐Assembly Modification of Western Laurentia by Back‐Arc Extension

The crystalline crust that underlies the Western Canada Sedimentary Basin in northern Alberta is composed of tectonic domains that accreted to the margin of the Archean Rae province of western Laurentia, ca. 2.1-1.8 Ga. Previous tectonic models in this region invoke complex plate dynamics, including oppositely verging subduction zones, to explain the assembly of various geologic domains. In this paper, we introduce a new tectonic model that invokes back-arc extension within the Chinchaga Domain (CD). The basement crust in the CD hosts a vast, mid-crustal reflection sequence (Winagami Reflection Sequence), previously interpreted as a mafic sill complex that intruded into an atypically wide domain of Paleoproterozoic arc magmatism. The latter has been interpreted to have formed during Paleoproterozoic tectonic assembly through near-synchronous closure of multiple small oceanic basins and/or magmatism in a plate interior setting. Based on reinterpretation of regional geophysical data and post-compressional fabrics observed by re-examination of drill cores, we propose that observed temporal relationships and present-day configuration of Paleoproterozoic arcs is better explained by post-assembly extension of the Chinchaga Domain. The proposed post-assembly modification is analogous to recent tectonic evolution of the Basin and Range and can be explained through back-arc extension linked to slab rollback and/or a proximal coeval transform plate boundary. Our model implies a back-arc setting for the Winagami sill complex; it also provides a tectonic framework for explaining the origin of an enigmatic low δ18O anomaly (Kimiwan anomaly) and reactivated basement faults associated with recent induced seismicity.

Control of climate, sea-level fluctuations and tectonics on the pervasive dolomitization and porosity evolution of the Oligo-Miocene Asmari Formation (Dezful Embayment, SW Iran)

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, <20 μm), (ii) fine to medium crystalline (D2, 20–150 μm) and (iii) coarse crystalline dolomites (D3, >150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and intraparticles porosity. This study shows that the reservoir quality improves from finely crystalline dolomite towards samples dominated by coarsely crystalline dolomites.

Geochemistry of Late Permian basalts from boreholes in the Sichuan Basin, SW China: Implications for an extension of the Emeishan large igneous province

The emplacement of the Late Permian Emeishan large igneous province (ELIP) in the Upper Yangtze craton, SW China, is considered to be a crucial factor that triggered the end-Guadalupian biodiversity crisis. However, the contribution of ELIP remains debated because of its relatively modest known size. The restricted assessment of its areal extent is, in part, due to the lack of study of the potential volcanic rocks concealed beneath younger strata. Here we present the first geochemical and isotopic variations and detailed spatial distribution of Late Permian basaltic rocks penetrated by deep exploratory boreholes in the Sichuan Basin. The Late Permian basaltic rocks are mainly distributed in the western and northeastern parts of the Sichuan Basin and are positioned at the same stratigraphic level as the Emeishan flood basalts. Petrography, whole-rock trace elements and Sr-Nd-Pb isotopic compositions demonstrate that these rocks are alkaline basalts and have oceanic island basalt - like characteristics, indicating a similar origin as the Emeishan flood basalts. The trace element and isotope ratios suggest that the basaltic magmas in the western Sichuan Basin were derived by low degrees of partial melting of a garnet-bearing source. In contrast, the basalts in the northeastern part of the basin were generated by higher degrees of melting of a shallower mantle source in the spinel-garnet transition zone. Varying amounts of contamination by lower continental crustal melts are suggested by rough correlations between isotope and immobile trace element ratios. The spatial changes in composition and origin of the basaltic rocks in the Sichuan Basin can be interpreted as the result of varying extents of Emeishan mantle plume-lithosphere interaction under different lithospheric stress backgrounds, such that the reactivation of pre-existing basement faults controlled the emplacement of basaltic magma in the periphery of the ELIP. Our work reveals a substantially broader extent of the ELIP in buried sedimentary strata, which might have contributed to the abrupt end-Guadalupian climatic change.

Fluid evolution along the Patterson Lake corridor in the southwestern Athabasca Basin: constraints from fluid inclusions and implications for unconformity-related uranium mineralization

The Patterson Lake corridor (PLC) in the southwestern margin of the Athabasca Basin hosts several high-grade uranium deposits. These deposits are located in the basement up to 900 m below the unconformity surface, raising questions about their affiliation with typical unconformity-related uranium (URU) deposits elsewhere in the basin. Based on cross-cutting relationships four pre- and three syn- to post-mineralization quartz generations were identified. Fluid inclusion analyses indicate that pre-mineralization fluids have salinities ranging from 0.2 to 27.2 wt% NaCl equiv. (avg. 9.0 wt%), whereas syn-mineralization fluids have salinities ranging from 8.8 to 33.8 wt% NaCl + CaCl2 (avg. 25.4 wt%), with NaCl- and CaCl2-rich varieties. The homogenization temperatures (Th) of fluid inclusions from pre-mineralization quartz range from 80 to 244°C (avg. 147°C), and from syn-mineralization quartz range from 64 to 248°C (avg. 128°C). Fluid boiling is indicated by the co-development of liquid-dominated and vapour-dominated fluid inclusions within individual fluid inclusion assemblages from the syn-mineralization quartz and is related to episodic fluid pressure drops caused by reactivation of basement faults. Our results indicate that composition and P–T conditions of the ore fluids in the PLC are comparable to those of typical URU deposits in the Athabasca Basin, indicating that the uranium deposits in the PLC formed under similar hydrothermal conditions. Episodic reactivation of basement faults was an important driving force to draw uraniferous fluids from the basin and reducing fluids from the basement to the mineralization sites, forming deep basement-hosted deposits.Supplementary material: Table 1, microthermometric results of type-2 and -5 fluid inclusion assembladges and isolated inclusions from the Patterson Lake corridor, and table 2, microthermometric results of type-6 fluid inclusions from the Patterson Lake corridor are available at https://doi.org/10.6084/m9.figshare.c.5510179Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Buried thrust belt front of the western Central Andes of northern Chile: Style, age, and relationship with basement heterogeneities

We present a regional-scale study combining field, seismic, and limited borehole information to analyze the structural characteristics of the thrust belt front of the western Central Andes at 21°S. This region, located in northern Chile, consists of the easternmost part of the Coastal Cordillera, the Pampa del Tamarugal, and the western Domeyko Cordillera (or Chilean Precordillera). On the surface, a series of N–S-oriented folds alternating between monoclines and narrow anticline and syncline folds containing Mesozoic, Upper Cretaceous, and Cenozoic rocks form part of the main structural styles, which are frequently bound by west-verging reverse faults that form the basement of the deeper structural levels of the upper crust. The interpretation of a regional seismic grid composed of N–S, NE–SW, and WNW–ESE-oriented 2-D seismic profiles revealed the presence of several blind west-verging inversion anticlines that define the orogenic front of the Central Andes at this latitude. These anticline folds are related to the partial and/or total positive reactivation of ancient east-dipping Jurassic normal basement faults bounding half-graben structures and sub-basins developed within the Tarapacá Basin. Our results suggest that the thrust belt front in this region is preferably located under the Pampa del Tamarugal, and not along the topographic culmination of the western Precordillera as traditionally proposed. New tectonic restorations of 2-D structural models suggest that, during tectonic inversion, between 3 and 4 km of crustal shortening were accommodated by vertical motion along the inverted normal faults. The presence of Upper Cretaceous synorogenic deposits covering the Jurassic syn-rift deposits on the inversion anticlines indicates that both basin inversion and formation of the western thrust belt front began during the Late Cretaceous, and not during the Eocene “Incaic” tectonic phase as previously suggested by regional studies developed in the area.

Use of high-resolution stratigraphy and derived lithoclasts to document structural inversion: a case study from the Paleogene, Isle of Wight, UK

The effects of structural inversion generated by the Pyrenean Orogeny on the southerly bounding faults of the Hampshire Basin (the Needles and Sandown faults) and on Eocene sedimentation in the adjacent regions were studied in outcrop by sedimentary logging, dip records and the identification of lithoclasts reworked from the crests of anticlines generated during inversion. The duration and precise age of the hiatuses associated with inversion were identified using bio- and magnetostratigraphy and compared with the Geologic Time Scale 2020. The succession on the northern limb of the Sandown Anticline (Whitecliff Bay) includes five hiatuses of varying durations, which together form a progressive unconformity developed during the Lutetian–Priabonian interval (47–35 Ma). Syn-inversion deposits thicken southwards towards the southern margin of the Hampshire Basin and are erosionally truncated by unconformities. The effects of each pulse of inversion are recorded by successively shallower dips and the age and nature of clasts reworked from the crest of the Sandown Anticline. Most individual hiatuses are interpreted as minor unconformities developed subsequent to inversion, rather than eustatically generated sequence boundaries or transgressive surfaces. By contrast, the succession north of the Needles Fault (Alum Bay) does not contain hiatuses of magnitude or internal unconformities. Subsidiary anticlinal and synclinal structures developed in the NW of the island as a response to Eocene inversion events by the reactivation of minor basement faults. The new dates of the Eocene inversion events correspond closely with radiometric ages (48–36 Ma) derived from fracture vein-fill calcites to the west in Dorset.