East Margin Research Articles

Fault characteristics in exhumed basement rocks; implications for understanding subsurface basement faults

We studied fault core geometry and mechanical properties of exhumed basement rocks at two localities (Storskora and Lislaskora) in Sotra Island, western Norway. We combine outcrop studies with in-situ measurements of the rock stiffness (Young's modulus) to characterize the faults. Faults were investigated both along and across strike using multiple 1D scanlines on the outcrop. Our results show that both fault core thickness and stiffness values vary along the faults. Thicker fault cores (up to 0.8 m and 1.9 m in Storskora and Lislaskora loaclities, respectively) show higher values of stiffness (Young's modulus) up to 70 GPa. The stiffness values of the fault core are generally higher than those measured on the damage zone of the faults in this area. The presence of epidote and compacted fault gouge in the fault core can cause the increase in estimated fault core stiffness. In contrast, fractures are dominant in the damage zones causing local reductions in the stiffness. A map of recent seismic events in this area shows potential seismic activities along some of the major exposed faults in the Sotra Island (e.g. Rustefjorden Fault). Based on the evidence from outcrop, inferred displacements, and interpretation of an available reflection seismic section, we found that the exposed faults could be secondary and part of the damage zone of the Øygarden Fault Complex in the east margin of the rift system in the North Sea. The results of this study could be utilized to predict the architecture and changes in rock stiffness of basement-involved faults in the subsurface.

New Magnetotelluric Data Reveal Deep Fault Boundaries and Contrasting Late Cenozoic Fault Kinematics Between the Qilian Shan Thrust Wedge and Beishan‐Alxa Block, Western China

AbstractThe structural connectivity and kinematic relationship between the Altyn Tagh sinistral strike‐slip fault (ATF) and Qilian Shan fold‐and‐thrust belt along the north Tibetan margin east of 96°E is an important question for tectonicists interested in the evolving active deformation field of Central Asia and associated earthquake hazards of China's Hexi Corridor region. New results from a detailed 130‐km‐long N‐S magnetotelluric (MT) survey from the Qilian Shan to Beishan elucidates the locations and down‐dip orientations of major faults. Importantly, the results indicate that the Heishan‐Jinta’Nanshan fault system roots steeply into the lower crust, is unconnected to the Qilian Shan thrust wedge, and has reactivated the margin of the North China Craton and an older, regional ductile shear belt. The structurally linked ATF‐Heishan‐Jinta’Nanshan system defines a fundamental kinematic boundary in central Asia between the NE directed Qilian Shan thrust belt to the south and the eastwardly extruding Beishan‐Alxa Block to the north.

Deep Fault‐Controlled Fluid Flow Driving Shallow Stratigraphically Constrained Gas Hydrate Formation: Urutī Basin, Hikurangi Margin, New Zealand

AbstractThe Hikurangi Margin east of New Zealand's North Island hosts an extensive gas hydrate province with numerous gas hydrate accumulations related to the faulted structure of the accretionary wedge. One such hydrate feature occurs in a small perched upper‐slope basin known as Urutī Basin. We investigated this hydrate accumulation by combining a long‐offset seismic line (10‐km‐long receiver array) with a grid of high‐resolution seismic lines acquired with a 600‐m‐long hydrophone streamer. The long‐offset data enable quantitative velocity analysis, while the high‐resolution data constrain the three‐dimensional geometry of the hydrate accumulation. The sediments in Urutī Basin dip landward due to ongoing deformation of the accretionary wedge. These strata are clearly imaged in seismic data where they cross a distinct bottom simulating reflection (BSR) that dips counterintuitively in the opposite direction to the regional dip of the seafloor. BSR‐derived heat flow estimates reveal a distinct heat flow anomaly that coincides spatially with the upper extent of a landward‐verging thrust fault. We present a conceptual model of this gas hydrate system that highlights the roles of fault‐controlled fluid flow at depth merging into strata‐controlled fluid flow into the hydrate stability zone. The result is a layer‐constrained accumulation of concentrated gas hydrate in the dipping strata. Our study provides new insight into the interplay between deep faulting, fluid flow and gas hydrate formation within an active accretionary margin.

Development genetic and stability classification of seasonal glacial lakes in a tectonically active area—A case study in Niangmuco, east margin of the Eastern Himalayan Syntaxis

The Niangmuco region on the east margin of the Eastern Himalayan Syntaxis features a large number of glacial lakes. The development process and stability classification of glacial lakes is of great significance to the study of seasonal glaciers in the eastern Himalayan margin, with implications for economic development and disaster prevention. Based on Landsat remote sensing image data from 2000 to 2021, this study analyzed the development and change characteristics of glacial lakes in the Niangmuco region during the past 21 years, and classified the stability of lakes with areas >0.02 km2 using the fuzzy consistent matrix method. In this area, 126 glacial lakes were identified within an elevation range of 3044–4156 m with a total area of 10.94 km2. These lakes primarily included glacial erosion lakes, valley lakes, tectonic lakes, and landslide dam lakes. Specifically, glacial erosion lakes accounted for 88.9% of the total number of lakes and 60.3% of the total lake area, followed by valley lakes with 6.3% and 23.7%, respectively. From 2000 to 2010, the total area of glacial lakes decreased from 10.53 km2 to 10.09 km2, which may be attributable to climate fluctuations. Subsequently, the area of lakes increased significantly to 10.94 km2 in 2021, an increase of 0.41 km2. Compared with 2000, among the lakes with a growth rate of 0.019 km2/a in 21 years, glacial erosion lakes exhibited the largest change. Among the classified glacial lakes in the study area, 95.7% were stable and relatively stable, mostly comprising glacial erosion lakes at high altitudes between 3468 and 4156 m. Only 4 unstable and extremely unstable glacial lakes were identified, and they were located near a fault zone. The findings show that the development and the change of glacial lakes in the area are primarily controlled by temperature and precipitation, and the topography and fault activity have important influences on the stability of glacial lakes.

Strontium isotopes as tracers for water-rocks interactions of groundwater to delineate iodine enrichment in aquifer of Datong Basin, northern China

Groundwater iodine has direct importance for human dietary iodine intake in areas where drinking water is of groundwater origin. Iodine affected aquifer system in Datong Basin of North China was studied with a focus on strontium isotopes and major ion chemistry, in order to estimate the controlling hydrological and geochemical processes. The data revealed a rather complicated mixing pattern of various groundwater source end-members that were subject to different water-rock interactions, such as silicate weathering, evaporite and carbonate dissolution. Groundwater from the recharge area of east margin was characteristic by 87Sr/86Sr ratios higher than 0.71643, which implied the silicate weathering process. Groundwater from the discharge area and west margin might be originated from evaporite and carbonate dissolution processes with low 87Sr/86Sr ratios. Mixing models of three end-members based on Sr isotope and contents reflected that the type of water-rock interactions shifted from silicate weathering towards evaporite dissolution along the groundwater flow path from east margin to basin center. Abundant iodine and natural organic matters (NOMs) were discovered in groundwater located at the discharge area with Sr isotopic values between 0.710 and 0.717, implying evaporite dissolution governing iodine and NOMs enrichment in groundwater. Silicate weathering process had negligible influence on iodine and NOMs enrichment in groundwater. While carbonate dissolution made negative contribution for iodine releasing to aqueous phases. The PHREEQC inverse modeling results illustrated that dissolution of halite, calcite, gypsum and kaolinite and cation exchange significantly changed chemical composition of groundwater along the groundwater paths from the two basin margins to central area.

Detailed View of the Seismogenic Structures and Processes of the 2022 Bayan Har Intraplate Earthquake Swarm on the East Margin of the Qinghai–Tibet Plateau

Abstract At 16:03 on 9 June 2022 (UTC), an Mw 5.5 earthquake followed by several Mw>4 events, including the largest event of Mw 5.8 within a few hours, occurred in the Maerkang area near the Caodeng Hot Spring Town, located in the south-central part of the Bayan Har plate on the eastern margin of the Qinghai–Tibet plateau. The earthquake swarm allows understanding the tectonic stress environment of the Bayan Har plate and is an example of a typical moderate-to-strong intraplate earthquake swarm. This article comprehensively analyzes the detailed seismogenic fault structure of the swarm by means of precise hypocenter relocation, focal mechanism inversion for Mw>4 earthquakes, inversion of the tectonic stress field in different regions of the Bayan Har plate, tidal strain calculation, and seismicity statistics. The results show that the swarm was not directly related to the nearby mapped Songgang fault, but rather resulted from the successive activation of a series of unknown faults. The precise hypocenter distribution, together with focal mechanism solutions of major earthquakes, illuminates five major seismogenic faults with conjugate relationships and stepover. Spatial and temporal migration of hypocenters, stress transfer, and tidal correlations demonstrate that cascade triggering, afterslip, and overpressured fluid might have jointly played a role in causing the earthquake swarm. As an output of this research, a set of verifiable datasets are provided as a basis for further in-depth research.

Proterozoic fold and thrust belt imaged on reflection seismic in Arabian plate basement

Major structures are imaged on 2D reflection seismic data within the Precambrian basement of the western Rub’ Al-Khali of Saudi Arabia for the first time. In this area the base-Cambrian Angudan Unconformity that defines top basement is at a depth of 1–2 km with a gentle easterly dip. Large antiformal structures are clear on 2D reflection seismic within the basement. The antiforms are separated by zones that are interpreted as low-angle faults. No basal detachment is seen and the faults pass below the base of reflection seismic at a depth of approximately 12 km. Individual folds have dimensions in the order of 20 km wavelength and 2 km amplitude and occur between 3 and 10 km depth. The structural style is interpreted as compressional by comparison with thrust systems and accretionary prisms elsewhere. Regional studies indicate that basement is comprised of a series of north-south trending oceanic Proterozoic terranes that were brought together in the Ediacaran East African Orogeny. Based on current regional tectonic maps the newly-imaged fold belt is on the east margin of the Khida-Abas terrane. Depth conversion shows the thrusts dip at approximately 10° north in the line of section. A novel combination of cross-section to structural trend obliquity estimates combined with matched filtering of potential field data indicate the fold and thrust belt trends 015°-195° with ESE vergence. This compressional belt may be some internal structure of the Khida-Abas terrane, or may be a non-outcropping accretionary prism on the margin of that terrane, or may even represent a set of amalgamated microterranes that do not outcrop along trend. Either way, these observations represent a major, previously undescribed element of this sector of the East African Orogen.

Geology of amethyst quartz deposits from Montezuma and surrounding areas of Minas Gerais and Bahia States

Amethyst quartz deposits occur in some regions of Minas Gerais state, Brazil, related to different geological environments. The most important are associated with hydrothermal veins crossing the Espinhaço mountain range, mainly in small and eroded mountains near its east margin, related to the Macaúbas Group. In the border region between the states of Minas Gerais (MG) and Bahia (BA), some of these deposits produce amethyst crystals that become green under heat treatments and are known in the gemmological market as “prasiolite.” The aim of the present study was to provide details of the clear structural control over the mineralized veins. The quartz veins are hosted in quartzites, which have long been the subject of controversy regardingtheir stratigraphic position. In this study, they are inserted into the Santo Onofre (MG) and Serra de Inhaúma (BA) sequences of unknown ages. These quartzites show a folded subvertical foliation, indicating a posterior deformational phase that had not been observed in rocks from the Macaúbas Group in other regions of MG state. At the Montezuma mine (MG), the main veins are around 0.70-1.10 m thick with steep dips to the NE, perpendicular to the NE-SW foliation; bedding orientation is approximately N-S, dipping to the E. In the Coruja mine area (BA), host quartzites have foliation attitudes varying between N35-45°E/ subvertical, and mineralized veins are around 1 m thick, nearly concordant with the foliation. At Tibério’s mine (BA), the mineralization is related to narrow fractures, less than 30 cm wide, and perpendicular to the host quartzite foliation, with directions/dips around N20°W/80°SW. In described deposits, amethyst veins occur structurally related to axial surfaces in foliation folds.

Seismically-induced down-sagging structures in tephra layers (tephra-seismites) preserved in lakes since 17.5 cal ka, Hamilton lowlands, New Zealand

We analysed numerous soft-sediment deformation structures (SSDSs) identified in seven unconsolidated, up to 8-cm thick, siliceous tephra layers that had been deposited in ~35 riverine-phytogenic lakes within the Hamilton lowlands, northern North Island, New Zealand, since 17.5 calendar (cal) ka BP. Based on sediment/tephra descriptions and X-ray computed tomography scanning of cores taken from ten lakes, we classified these SSDSs into elongated load structures (i.e., down-sagging structures) of different dimensions, ranging from millimetre- to decimetre-scale, and centimetre-long dykes. Down-sagging structures were commonly manifested as intrusions of internal tephra beds of very fine to medium sand into underlying organic lake sediments. The tephra layers commonly exhibited an upper silt bed, which was not directly affected by deformation. Dry bulk density and grain size distribution analyses of both the organic lake sediment and the internal tephra beds provided evidence for the deformation mechanism of down-sagging structures and their driving force: the organic lake sediment and the upper silt bed are less liquefiable, whereas the very fine to medium sand internal tephra beds are liquefiable. The tephra layers and encapsulating organic lake sediments formed three-layer (a–b–a) density systems, where ‘a’ denotes the sediment unit of lower density. We infer that downward-directed deformation was favoured by the a–b–a density system with the upper, less-liquefiable, silt bed within the tephra layer preventing upward intrusion during the liquefaction process. The spatial distribution and ages of SSDSs within the lakes provided some evidence that liquefaction of the older tephra layers, i.e., Rerewhakaaitu, Rotorua, and Waiohau tephras, deposited 17.5, 15.6, and 14 cal ka BP, respectively, was triggered by a seismic source to the northeast of the Hamilton lowlands (i.e., Kerepehi and/or Te Puninga faults). In contrast, the liquefaction of the younger tephra layers, i.e., Opepe, Mamaku, and Tuhua tephras, deposited 10.0, 8.0, and 7.6 cal ka BP, respectively, may have been triggered by movement on local faults within the Hamilton lowlands, namely the Hamilton Basin faults, or by distant faulting at the Hikurangi subduction margin east of North Island.

Transtensional tectonics during the Gondwana breakup in northeastern Brazil: Early Cretaceous paleostress inversion in the Araripe Basin

Intraplate deformation during the Mesozoic breakup of West Gondwana predominantly occurred along major ancient lithospheric discontinuities. The Precambrian Patos Shear Zone (PASZ) is an inherited lithospheric discontinuity in the interior of northeastern Brazil which is closely linked to the evolution of the Araripe Basin. In this study, unlike previous interpretations of purely extensional tectonics, we conducted stratigraphic and structural field mapping as well as paleostress reconstruction to demonstrate the role of intraplate transtensional tectonics during the Araripe Basin evolution. Limited to the north by the PASZ and associated with sinistral reactivation, a set of normal to oblique NE–SW faults constituting a transtensional horsetail structure generated the initial basin geometry. Stratigraphic records of this rift phase comprise the Jurassic to Early Cretaceous Brejo Santo, Missão Velha, and Abaiara formations. Paleostress reconstruction showed a NE–SW/subhorizontal σ1 axis, a vertical σ2 axis, and a NW–SE/subhorizontal σ3 axis, resulting in a sinistral transtensional tectonic regime, which was mainly governed by intraplate processes that reflected the opening of the Brazilian East Margin. The subsequent Aptian post-rift early phase was characterized by NE–SW reverse faults and a succession of normal to oblique normal faults striking NW–SE to WNW–ESE. This fault arrangement generated a new NW–SE transtensional basin, which controlled the Barbalha Formation and modified and partially preserved the previous NE-SW transtensional basin. Paleostress reconstruction showed a NW–SE/subhorizontal σ1 axis and NE–SW/subhorizontal σ3 axis for strike-slip faults but NE–SW extension for normal faults. This is related to a second dextral reactivation of the PASZ following propagation of intraplate stress during the opening of the Brazilian Equatorial Margin. In summary, tectonic evolution of the Araripe Basin was strongly influenced by stress propagation from the Gondwana breakup into intraplate settings, with two distinct reactivations of the PASZ pre-existing basement structures which results in a paleostress inversion during basin evolution.

Subduction of the Palaeo‐Pacific Plate beneath the Jiamusi Block: Constraints from the Late Triassic–Late Jurassic sedimentary records in the eastern Jiamusi Block

The Jiamusi Block is a crucial geological unit joining multiple tectonic regimes, including the Late Palaeozoic Palaeo‐Asian Ocean and the superposition of the Early Mesozoic Palaeo‐Pacific tectonic domain. However, the duration of the Palaeo‐Pacific Plate subduction beneath the Jiamusi Block has long been controversial. In this paper, we focus on clastic rocks from the Nanshuangyashan and Yunshan formations on the east margin of the Jiamusi Block and use the modal analysis of sandstones and detrital zircon U–Pb dating to trace the subduction history of the Palaeo‐Pacific Plate. Our analysis results suggest that the two groups of samples are characterized by a high content of monocrystalline quartz and a low content of lithic and feldspar, indicating that the sedimentary provenances were stable continental blocks and magmatic arcs. The detrital zircon ages of the Nanshuangyashan Formation have a major peak at ca. 245 Ma and two secondary peaks at ca. 470 and 228 Ma, respectively. The Yunshan Formation has a major peak at ca. 494 Ma and a secondary peak at ca. 265 Ma. The provenances of the Nanshuangyashan Formation were attributed to the Jiamusi Block, Khanka Block, and a Late Triassic magmatic arc. Similarly, the provenances of the Yunshan Formation were attributed to the Jiamusi and Khanka blocks. Detrital zircon analysis results show that the eastern margin of the Jiamusi Block was in the same back‐arc basin settings in the Late Triassic and the Late Jurassic. Thus, we propose that the eastern margin of the Jiamusi Block had been affected by the Palaeo‐Pacific Plate subduction in the Late Triassic and had continued to the Late Jurassic.

Structure and thermochronology of basement/cover relations along the Defiance uplift (AZ and NM), and implications regarding Laramide tectonic evolution of the Colorado Plateau

Interpretation of the late Cretaceous to Eocene Laramide tectonic evolution of the Colorado Plateau is hampered by the difficulty of placing precise temporal constraints on fault-induced basement uplift, for there is a paucity of exposed basement from which thermochronologic ages might be obtained. The Defiance and Zuni uplifts, located on the southeasternmost Colorado Plateau, offer rare basement exposure that provide additional temporal insights. The Zuni uplift exposes abundant Precambrian basement rock, which has been the subject of previous apatite thermochronologic study to interpret low-temperature tectonic/exhumation history. In the Defiance uplift region, which is the main focus of this study, there are two outcrop areas of Precambrian-basement rock along the trace of the East Defiance monocline. Both sites are quarries. Uplift/erosion consequences of Pennsylvanian-Permian Ancestral Rocky Mountains (ARM) deformation are explicit in the geology of these quarries, for the contact between the Supai Group (Permian) and underlying basement (1,703 ± 1.3 Ma, zircon U-Pb) is a nonconformity. Here, we apply a combination of structural analysis of Laramide fold/fault relations and multi-method thermochronology to the exposed granitic basement of both the Defiance and Zuni uplifts. Zircon U-Pb, zircon (U-Th)/He, apatite fission-track, apatite (U-Th-Sm)/He, and hematite (U-Th)/He reveal a poly-phase thermo-tectonic history. Initially, the Paleo-Proterozoic 1.7 Ga basement cooled to ∼400 °C by at least 1.4 Ga, followed by 1.4–1.0 Ga unroofing to depths of ∼8 km. Following cooling, the Defiance-Zuni granitic basement experienced protracted residence at temperatures ≤200 °C (∼8 km) between <i>ca.</i> 900 and 600 Ma. Sedimentary evidence and hematite (U-Th)/He dates bracket the ARM event (400–200 Ma), which may suggest fluid-rock interaction or near-surface exposure associated with the ARM. Following the ARM, the Defiance uplift experienced heating (&gt;120 °C) associated with burial consistent with the stratigraphic overburden, until approximately 80 Ma. Finally, the Defiance-Zuni region experienced initial cooling at &lt;70 Ma, with the main phase of exhumation to the upper crust (&lt;2 km) at <i>ca.</i> 60–40 Ma. Detailed structural modeling along 15 normal-profile cross-sections across the east margin of the Defiance uplift reveals that Laramide trishear monoclinal folding was generated by an oblique-slip master fault that partitioned ∼8 km of strike-slip and ∼1.5 km of reverse-slip displacement. Inferred strike-slip compartmental faulting in the Zuni uplift appears to fit coherently within this overall kinematic model in relation to Laramide loading direction. In the context of geodynamic models for flat-slab subduction during the Laramide tectonic event, consideration of new data from the Defiance uplift (along with other recently reported thermochronology data) provides support for models that bring the flat slab beneath North America along an east-northeastward trajectory, rather than along a more northward trajectory.

Assessment of eutrophication through ecological indicators at the entrance of a tropical urbanized estuary

Dissolved nutrients, phytoplankton biomass and other ancillary variables, were obtained at two sub-embayments at the entrance of Guanabara Bay (Rio de Janeiro, Brazil) and a composite trophic status index (TRIX) was used to assess the water quality of the study area. The role of bottom sediments in nutrient dynamics was also investigated through the evaluation nutrients, phytoplankton biomass and other sedimentary variables, since this compartment acts as a geological record of anthropogenic input. Jurujuba Sound, at the east margin, and Flamengo-Botafogo Sounds, at the western margin, were sampled during neap and spring tides in the dry season. Signs of eutrophication were detected through the extreme variations of dissolved oxygen concentrations at both margins, being more accentuated at Jurujuba Sound (2.20-14.07 mg.l-1). Dissolved inorganic nitrogen was elevated at both margins and mostly composed of ammonium, surpassing 87% at Flamengo Botafogo Sounds, which suggests a continued input of raw sewage at the western margin of the bay. TRIX revealed poor water quality for most stations at the study area, varying from 4.53 to 7.29 at Jurujuba Sound and from 5.67 to 7.87 at Flamengo-Botafogo Sounds. The increase of TRIX from neap to spring tide was registered at both margins, revealing decrease of water quality. The differences in grain size between both margins played a key role in nutrient dynamics, with predominance of fine sediments at Jurujuba Sound and coarser particles at the opposite margin. Accumulation of high concentrations of TOC (0.87-6.57%) and inorganic phosphorus (154.34-1516.82 µg.g-1) were favored by the predominance of fine sediments at Jurujuba Sound. The assessment of eutrophication in water column and bottom sediments revealed the maintenance of this process at the entrance of Guanabara Bay sustained by the recurrent anthropogenic input, what demands urgent action from public policies to mitigate this situation.

The Pore Structure of Marine to Continental Transitional Shales in the Permian Shanxi Formation on the East Margin of the Ordos Basin, China

The pore structure is an important factor in determining the storage capacity for shale gas development. Twelve samples were selected from the marine to continental transitional shales in the Shanxi Formations on the eastern margin of the Ordos Basin, and their nanoscale pore structure was investigated by a variety of low-temperature N2 adsorption (LT-N2GA) and low-pressure CO2 adsorption (LP-CO2GA) and high-pressure mercury intrusion (HPMI) experiments. The shale pores are complex and are mainly mesopores, which range mainly from 10 nm to 40 nm and greatly contribute to the pore volume (PV), whereas pores with a diameter less than 0.8 nm greatly contribute to the specific surface area (SSA) of the shale. The pore structure is affected by the TOC content, organic matter maturity, and illite/smectite (I/S) mixed layer content. The total PV increases with the increase in TOC content, organic matter maturity, and I/S mixed layer content. The effects of pores on the occurrence shale gas were determined by high-pressure methane adsorption experiments. The maximum adsorption amount of methane was positively correlated with the SSA of the micropores, indicating that the micropores have a large SSA, which controlled the adsorbed gas content of the shale. The mesopores provide the majority of the PV, which mainly corresponds to the volume of free gas in the shale, and the macropores are mainly micron-sized pores, which can form the main migration channels for shale gas.

Tectonic evolution of the northern Verkhoyansk Fold-and-Thrust Belt: insights from palaeostress analysis and U–Pb calcite dating

AbstractA combined structural and geochronological study was carried out to identify the tectonic evolution of the northern Verkhoyansk Fold-and-Thrust Belt, formed on the east margin of the Siberian Craton during late Mesozoic collision. Fault and fold geometries and kinematics were used for palaeostress reconstruction along the Danil and Neleger rivers cross-cutting the central and western parts of the Kharaulakh segment of the northern Verkhoyansk. Three different stress fields (thrust, normal and strike-slip faulting) were identified after separation from heterogeneous fault-slip data. Thrust and normal faulting stress fields were found in both areas, whereas a strike-slip faulting stress field was only found in Neoproterozoic rocks of the Neleger River area. U–Pb laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) dating of calcite slickenside samples reveals the following succession of major deformation events across the northern Verkhoyansk: (i) The oldest tectonic event corresponding to the strike-slip faulting stress field with NE–SW-trending compression axis is Early Permian (Cisuralian, 284 ± 7 Ma) and likely represents a far-field response to the late Palaeozoic collision of the Kara terrane with the northern margin of the Siberian Craton. (ii) A slickenfibrous calcite age of 125 ± 4 Ma is attributed to the Early Cretaceous compression event, when the fold-and-thrust structure was formed. (iii) U–Pb slickenfibre calcite ages of 76–60 Ma estimate the age of a Late Cretaceous – Palaeocene compression event, when thrusts were reactivated. Slickensides related to both (ii) and (iii) compressional tectonic events formed by similar stress fields with W–E-trending compression axes. (iv) From the Palaeocene onwards, extensional tectonics with approximately W–E extension predominated.

New understandings on gas accumulation and major exploration breakthroughs in subsalt Ma 4 Member of Ordovician Majiagou Formation, Ordos Basin, NW China

Geological conditions and main controlling factors of gas accumulation in subsalt Ma 4 Member of Ordovician Majiagou Formation are examined based on large amounts of drilling, logging and seismic data. The new understandings on the control of paleo-uplift over facies, reservoirs and accumulations are reached: (1) During the sedimentary period of Majiagou Formation, the central paleo-uplift divided the North China Sea in central-eastern of the basin from the Qinqi Sea at southwest margin of the basin, and controlled the deposition of the thick hummocky grain beach facies dolomite on platform margin of Ma 4 Member. Under the influence of the evolution of the central paleo-uplift, the frame of two uplifts alternate with two sags was formed in the central-eastern part of the basin, dolomite of inner-platform beach facies developed in the underwater low-uplift zones, and marl developed in the low-lying areas between uplifts. (2) From the central paleo-uplift to the east margin of the basin, the dolomite in the Ma 4 Member gradually becomes thinner and turns into limestone. The lateral sealing of the limestone sedimentary facies transition zone gives rise to a large dolomite lithological trap. (3) During the late Caledonian, the basin was uplifted as a whole, and the central paleo-uplift was exposed and denuded to various degrees; high-quality Upper Paleozoic Carboniferous—Permian coal measures source rocks deposited on the paleo-uplift in an area of 60 000 km2 providing large-scale hydrocarbon for the dolomite lithological traps in the underlying Ma 4 Member. (4) During the Indosinian—Yanshanian stage, the basin tilted westwards, and central paleo-uplift depressed into an efficient hydrocarbon supply window. The gas from the Upper Paleozoic source rock migrated through the high porosity and permeability dolomite in the central paleo-uplift to and accumulated in the updip high part; meanwhile, the subsalt marine source rock supplied gas through the Caledonian faults and micro-fractures as a significant supplementary. Under the guidance of the above new understandings, two favorable exploration areas in the Ma 4 Member in the central-eastern basin were sorted out. Two risk exploration wells were deployed, both revealed thick gas-bearing layer in Ma 4 Member, and one of them tapped high production gas flow. The study has brought a historic breakthrough in the gas exploration of subsalt Ma 4 Member of Ordovician, and opened up a new frontier of gas exploration in the Ordos Basin.

Southeast Solomon Islands in regional perspective: settlement history, interaction spheres, Polynesian outliers and eastward dispersals

This paper reviews the prehistory of the greater southeast Solomons region in the light of the 46 years of research which has been conducted since Green and Yen published the preliminary results of their Southeast Solomons Culture History Project in 1976. Green saw the region as key to investigating some of the major questions relating to Oceanic culture history, and as subsequent archaeological, linguistic and genetic research has shown, this has proven to be the case. The evidence is reviewed for initial Lapita and subsequent settlement, the development of a Marginal East Melanesia–Central Micronesia Interaction Zone, early proto-Polynesian settlement of the Polynesian Outliers and the probable role of the region in the settlement of East Polynesia.

The genesis of the granitic rocks associated with the Mo-mineralization at the Hongling deposit, eastern Tianshan, NW China: Constraints from geology, geochronology, geochemistry, and Sr-Nd-Hf isotopes

The eastern Tianshan is an important molybdenum metallogenic belt in the southern margin of the Central Asian Orogenic Belt (CAOB). The newly discovered Hongling Mo deposit is located in the north–east margin of the Kanggur–Huangshan shear zone in eastern Tianshan, NW China. However, the genesis of the granitic rocks associated with the Mo-mineralization is unclear. The monzogranite, monzogranite porphyry and porphyritic granite from the Hongling deposit are high-K calc-alkaline series and characterized by high SiO2 and Al2O3 and low MgO, TiO2, and P2O5 concentrations. They are enriched in K, Rb, U and Pb, and depleted in Nb, Ba, P, and Ti, with negative Eu anomalies. Zircon LA–ICP–MS U–Pb dating yielded mean ages for the monzogranite of 238 ± 5 Ma and 234 ± 3 Ma, monzogranite porphyry of 234 ± 2 Ma, and porphyritic granite of 232 ± 3 Ma, respectively, which coincide with molybdenite Re–Os model ages ranging from 227 ± 4 to 228 ± 4 Ma. The zircons from the monzogranite, monzogranite porphyry and porphyritic granite yield εHf (t) of 8.3–15.4, 8.4–16.1, and 7.8–14.7, respectively, and TDMc (Hf) ages of 0.23–0.76 Ga. The Hongling granites show a centralized distribution toward lower (87Sr/86Sr)i values of 0.70387–0.70450, higher εNd(t) values of 2.6 to 3.7, and TDM2(Nd) ages of 0.71–0.80 Ga. The geochemical characteristics of granitoids from the Hongling deposit are practically the same as the fertile granitoids from the Donggebi and Baishan deposits. The geochemical and isotopic data indicate that the Hongling granitoids were likely derived from the partial melting of the juvenile lower crustal materials generated during Precambrian to Paleozoic, with involving minor mantle components and related to intraplate extensional setting.

Morphostratigraphy of barrier spits and beach ridges at the east margin of Salgada Lagoon (Southeast Brazil)

At wave-dominated coasts, it is common the presence of coastal barriers that can be of different typologies and whose dynamics can influence the formation of lagoon systems. Along southeastern and south Brazil the dynamics involving barrier-lagoon evolution is related to wave and aeolian process associated with sea-level behavior. For a better understanding about the relationship regarding to coastal process, barriers, and lagoons the application of geophysical techniques enables to identify morphostratigraphic characteristics improving the acknowledgment of the main processes forming these morphologies. In this study, a 1.4 km long common-offset GPR profile, in cross-shore direction, was surveyed in order to obtain the internal depositional architecture of the sandy coastal barrier to the east of Salgada lagoon, located in the wave-dominated Paraiba do Sul river delta, in southeastern Brazil. The GPR allows to describe morphostratigraphic characteristics imprinted by sediment dynamics and how it is related to the lagoon system formation. Eight radarfacies were identified, grouped according to the coastal barrier pattern, divided in barrier spit and beach ridge, and sediment transport characteristics. Five radarfacies comprise the barrier spit environment and three the beach ridge environment. The barrier spit environment radarfacies showed different characteristics of the portions of the spit sequence (initial, medium or distal), and along with its subsurface imaging angle (parallel, oblique or transversal to the spit growth direction) providing highly detailed data regarding those barrier types. This recognition allowed the interpretation of a previous spit, approximately 3 m deep, located under another one that overlapped it. Also, spit accretion records were found, which occur due to the seaward exposure to high-angle wave action. Meanwhile, the beach ridge environment radarfacies revealed the typical pattern of progradation due to swash processes, and also the occurrence of berm ridge accretion and overtopping/overwashing processes. The integrated analyses using the subsurface data indicated that the evolution of the targeted coastal barriers and the lagoon can be divided into four stages. The first stage is the most landward barrier spits growth, where Salgada lagoon was beginning its closing process. This is followed by the second stage with the continuity of barrier spits growth but with changes in spits orientation. The third stage represents an erosive phase, with a major erosive truncation that probably eroded a significant area of the spits. At the last stage suggested there was a subsequent accumulation phase, marked by the formation beach ridges. Besides that, it is also important to recognize that the development of coastal barriers of different types at the site showed an alignment variability of paleocoastlines. While barrier spits revealed variation in their orientation, beach ridges have kept their alignment more constant. This fact occurred independently of erosive episodes. Furthermore, the GPR profile acquisition strategy made it possible to investigate the spit from different perspectives, which helped with the interpretation of deeper radarfacies.

Depositional environment, age determination and source diagnose of oils from the western Chepaizi Uplift (Junggar Basin) constrained by molecular biomarkers

The western Chepaizi Uplift of petroleum-enriched Junggar Basin hosted multiple oil-bearing intervals, i.e., Neogene Shawan Formation (N1s), Lower Cretaceous Tugulu Group (K1tg), and Upper Carboniferous Xibeikulasi Formation (C2x). However, their origins had always been debated, due to the occurrence of Jurassic, Cretaceous and Paleogene source rocks in the neighboring Sikeshu Sag. In this paper, two oil groups, i.e., group A (N1s and K1tg oils from the west margin of western Chepazi Uplift), and group B (oils from the east margin of the western Chepaizi Uplift). The latter can further subdivided into family B1(Upper Jurassic Qigu Formation (J3q) oil and N1s oils from east margin of western Chepaizi Uplift close to the Sikeshu Sag, and oil sands from the Sikeshu Sag) and family B2 (N1s and C2x oils from eastern margin of the western Chepaizi Uplift away from the Sikeshu Sag) can be defined. Age-specified biomarker ratios (24-norcholestane, Oleanane index and extended tricyclic terpane ratio) implied Jurassic and Paleogene source ages with minor Cretaceous contributions. Depositional environment-diagnosed biomarkers (Pr/Ph, Pr/nC17, Ph/nC18, methyldibenzothiophene/methylphenanthrene, C22/C21 tricyclic terpanes, C24/C23 tricyclic terpanes, C29αβ/C30αβ hopane, C31 hopane (22R)/C30αβ hopane, and gammacerane/C30 hopane) showed lacustrine source input with high hypersalinity for family B1 but freshwater lacustrine facies for group A and family B2. C27–C28–C29 regular steranes and triaromatic steroids distributions indicated ascending order of high plant contribution from family B1 to family B2 to group A. Combined with stable carbon isotope, the uplift-scale petroleum contribution to multiple oil-bearing intervals by multiple source intervals were lined out. Group A oils implied origins from source rocks deposited in freshwater lacustrine facies under weak oxidation conditions with predominant organic input of high plants, and genetically matched with the Lower Jurassic Badaowang Formation (J1b) mudstone source rock. Family B1 oils showed provenance from source rocks occurred in reducing source facies with high hypersalinity and great contribution of lower organisms, and well corelated with both J1b and Paleogene Anjihahe Formation (E2-3a) source rocks. However, where away from the generating Sikeshu Sag, especially for family B2 oils, the hydrocarbon contribution of E2-3a source rocks was decreased, mainly controlled by their variable distribution and thermal evolution.