Strike-slip Fault Zone Research Articles

Dynamic Simulations of Earthquake Cycles in Strike-Slip Fault Zones

Dynamic Simulations of Earthquake Cycles in Strike-Slip Fault Zones

Seismic sedimentology analysis of fluvial-deltaic systems in a complex strike-slip fault zone, Bohai Bay Basin, China: Implications for reservoir prediction

The Miocene fluvial-deltaic system in the Bohai Bay Basin exhibits difficulties in favourable reservoir prediction in a complex strike-slip fault zone in the Tanlu fault system. On the basis of 3D seismic data and 12 wells, we utilize two different RGB colour-blending methods from various datasets. The first method is derived from the 3D stacked, −90° phase seismic volume, and the second method is generated from the root mean square (RMS) volume attribute, which is extracted from −90° phase seismic data. Combined with the linear combination analysis of user control, we attempt to better quantify reservoir thickness and evolution of the fluvial-deltaic system in the Guantao Formation. The results show that stratal slices of the second RGB colour-blending method have a much greater ability to characterize sedimentary facies evolution and to analyse sand-thickness variations, which can effectively decrease the disadvantageous influence of multiple fault blocks on stratal slice views. In the vertical section, the linear combination analysis and the first RGB colour-blending method, however, have greater utility and resolution, which can be used to characterize the sand body by the width of seismic events or display the various sand thicknesses in different colours. The high-resolution anatomy of the fluvial-deltaic system in the Bohai Bay Basin not only explains current exploration effects but also effectively predicts thick sandstones with the most production potential in this complex region. This study may also have great significance for other complex areas or strata.

Seismicity and landform development of the dextral Kalabagh Fault Zone, Pakistan: Implications from morphotectonics and paleoseismology

The Kalabagh Fault Zone in the western Sub-Himalayas is a dextral strike slip fault zone that exhibits a push-up block in the stepover zone of the two fault segments. The inception of the push-up block occurred along the restraining faults that form two morphotectonic bends along the mountain front at Zaluch and in the South of Thathi. The landform was further developed by en-echelon fold and thrust sequence up to Khairabad in the North and Ghundi village in the South. The aseismically moving fault zone has experienced earthquakes during late Quaternary forming surface expressions in the West of the push-up block at Khairabad, Larkakki and Ghundi. The Khairabad and Ghundi sections are the active tectonic fronts of the stepover zone. Displacements caused by these seismic activities has deformed Quaternary sediments creating stream deflections, surface ruptures, pressure ridges, uplifted sediments at the mountain front, foreland ridges and uplifted alluvial fan. Offsets in the late Quaternary sediments indicate the Kalabagh Fault is active and seismic events of Mw ≥ 6 has formed these ground effects. Furthermore, stratigraphic correlations of fluvial sediments indicate the Indus River has been flowing in the NS direction along the northern segment of the Kalabagh Fault. Uplift of the push-up block in the Kalabagh Fault Zone and subsequent deposition of the alluvial fans has caused westward diversion of the Indus River.

Morphotectonic Structures along the Southwestern Margin of Lesvos Island, and Their Interrelation with the Southern Strand of the North Anatolian Fault, Aegean Sea, Greece

A hydrographic survey of the southwestern coastal margin of Lesvos Island (Greece) was conducted by the Naftilos vessel of the Hellenic Hydrographic Service. The results have been included in a bathymetric map and morphological slope map of the area. Based on the neotectonic and seismotectonic data of the broader area, a morphotectonic map of Lesvos Island has been compiled. The main feature is the basin sub-parallel to the coast elongated Lesvos Basin, 45 km long, 10–35 km wide, and 700 m deep. The northern margin of the basin is abrupt, with morphological slopes towards the south between 35° and 45° corresponding to a WNW-ESE normal fault, in contrast with the southern margin that shows a gradual slope increase from 1° to 5° towards the north. Thus, the main Lesvos Basin represents a half-graben structure. The geometry of the main basin is interrupted at its eastern segment by an oblique NW-SE narrow channel of 650 m depth and 8 km length. East of the channel, the main basin continues as a shallow Eastern Basin. At the western part of the Lesvos margin, the shallow Western Basin forms an asymmetric tectonic graben. Thus, the Lesvos southern margin is segmented in three basins with different morphotectonic characteristics. At the northwestern margin of Lesvos, three shallow basins of 300–400 m depth are observed with WNW-ESE trending high slope margins, probably controlled by normal faults. Shallow water marine terraces representing the last low stands of the glacial periods are observed at 140 m and 200 m depth at the two edges of the Lesvos margin. A secondary E-W fault disrupts the two terraces at the eastern part of the southern Lesvos margin. The NE-SW strike-slip fault zone of Kalloni-Aghia Paraskevi, activated in 1867, borders the west of the Lesvos Basin from the shallow Western Basin. The Lesvos bathymetric data were combined with those of the eastern Skyros Basin, representing the southern strand of the North Anatolian Fault in the North Aegean Sea, and the resulted tectonic map indicates that the three Lesvos western basins are pull-aparts of the strike-slip fault zone between the Skyros Fault and the Adramytion (Edremit) Fault. The seismic activity since 2017 has shown the co-existence of normal faulting and strike-slip faulting throughout the 90 km long Lesvos southern margin.

Investigating the Deformation and Failure Mechanism of a Submarine Tunnel with Flexible Joints Subjected to Strike-Slip Faults

Knowledge from historical earthquake events indicates that a submarine tunnel crossing active strike-slip faults is prone to be damaged in an earthquake. Previous studies have demonstrated that the flexible joints are an effective measure for a submarine tunnel crossing a strike-slip fault. The background project of this paper is the second submarine tunnel of Jiaozhou bay. In this work, model tests and numerical simulations are conducted to investigate the deformation and failure mechanism of a submarine tunnel with flexible joints under a strike-slip fault dislocation. The influence of strike-slip faults on a tunnel with flexible joints has been investigated by examining the deformation of rock mass surface, analyzing lining stains, and crack propagation from model tests. Numerical simulations are conducted to study the effects of the design parameters of a tunnel with flexible joints on the mechanical response of the lining. The results showed that the ‘articulated design’ measure can improve the ability of the tunnel to resist the strike-slip faults. In terms of the mechanism of design parameters of a tunnel with flexible joints, this paper finds that increasing the lining thickness, decreasing the lining segment length, and decreasing the tunnel diameter to a reasonable extent could effectively improve the performance of this faulting resistance measure for a tunnel under the strike-slip fault zone dislocation. Compared with the horseshoe tunnel cross-section, the circular tunnel cross-section can improve the ability of the faulting resistance of a tunnel with flexible joints, while the optimal angle of the tunnel crossing the fault zone is 90º. It is concluded that the wider fault zone, smaller flexible joint width, and less stiffness of the flexible joint could make lining safer under a strike-slip fault dislocation. The above research results can serve as a necessary theoretical reference and technical support for the design of reinforcement measures for a submarine tunnel with flexible joints under strike-slip fault dislocation.

Sigmoidal normal faults and evidence for vertical-axis block rotation in an oblique convergent margin: A 3D seismic example from offshore Colombia

Sigmoidal fold and fault geometries are typical kinematic indicators of strike-slip fault zones. We document kilometer-scale, normal faults with sigmoidal plan-view geometries within the dextral pull-apart Bahia Basin, at the rear of the obliquely convergent South Caribbean Deformed Belt, offshore Colombia. Using 3D seismic reflection data calibrated to wells, closely spaced, low-displacement, planar normal faults are mapped within the Miocene strata. A series of seismic horizontal (time) slices and computed seismic attributes are used to interpret the 3D configuration of these faults. The closely spaced faults display an east–west trend with a progressive rotation into a northwest–southeast trend. In map view, the fault traces curve toward their tips, describing a sigmoidal-Z geometry that terminates at discrete northeast–southwest-trending fault zones. The structures observed may correspond to either tension fractures, which form theoretically at 45°, or antithetic shear fractures with normal displacement formed at 50°–70° to the boundaries of a dextral shear zone. These scenarios lead to a clockwise block rotation of between 20° and 40° within the shear zone. This study shows the first example of vertical-axis block rotations observed offshore in the western end of the South Caribbean margin and is an important example of the use of 3D seismic data to identify rotations where paleomagnetic studies are not available.

Geodynamics and Neotectonics of the Cordillera Septentrional and Samaná Peninsula, Dominican Republic: implications for seismotectonics and seismic hazard assessment

The Dominican Republic is exposed to a high seismic hazard due to its geodynamic location between two active margins of oblique convergence. New geodynamic, neotectonic and seismotectonic data of the Cordillera Septentrional and Samaná Peninsula, and also the onshore area of the northern microplate, have allowed the location of the main seismotectonic fault zones. Structural, stratigraphic and morphotectonic information obtained from the fault zones at several scales has allowed us to interpret their kinematics and consistency with the obtained regional stress tensor deduced from dynamic inverse methods. This information complemented by the record of historical seismicity, displacement velocity and seismic parameters of the fault zones has allowed us to evaluate the seismic hazard of the studied area.The method used for the evaluation of the seismic hazard is probabilistic and considers three types of seismic sources: the subduction zone, the main strike-slip fault zones and the areas located between them. This method allows us to calculate the iso-lines of peak ground acceleration (PGA) with a resolution of 0.05º for an exceedance probability of 10% (50 years) and a return period of 475 years. Resulting PGA bands are arranged sub-parallel to the North Hispaniola, Septentrional and Camú Fault Zones, as well as the main ONO-ESE-trending faults systems, detecting a maximum of PGA towards the SE of the modelled area. Consequently, the cities with the lowest seismic hazard are those located towards the NW of the maximum PGA zone.

The uppermost Messinian–Quaternary evolution of the Anamur–Kormakiti zone: The transition between the Outer Cilicia and Antalya basins, northeastern Mediterranean

A dense grid of high-resolution multichannel seismic reflection profiles are used to delineate the stratigraphic and structural architecture of the Anamur–Kormakiti zone which separates the Outer Cilicia Basin from the eastern Antalya Basin. The data showed that the uppermost Messinian–Quaternary structural framework of the region is characterized by two arcuate south-convex sinistral strike-slip fault zones in the northeast and east which converge to form a ∼70–80 km wide NW–SE trending zone, consisting of two internally-parallel dextral strike-slip fault zones in the northwest and west. A narrow NNW–SSE trending zone occupied by similarly trending positive flower structures across the Anamur–Kormakiti zone suggests that this region is probably a rotational stepover between two oppositely moving strike-slip fault zones. The regionally extensive strike-slip fault zones are largely developed during the Pliocene–Quaternary along the southern fringes of the Taurus Mountains in response to the westward escape of the Aegean–Anatolian Microplate following the collision and suturing of the Arabian Plate to the Eurasian Plate during the end of Miocene–Early Pliocene.

Surface mapping, structural modelling and kinematics along the sinistral strike-slip fault zone, NE Potwar, Pakistan

AbstractThis research was carried out to understand the nature of strike-slip Jhelum Fault zone and to propose a model for the surface to subsurface deformation pattern. Field data along with satellite images are used to construct the geological map. Moreover, the subsurface model has been proposed using the mechanism of dip-isogons in computer application which connects points of equal inclination or dip on the outer and inner bounding surfaces of a folded layers. The proposed geological map and subsurface model shows that the Jhelum Fault when propagated in the south from Hazara-Kashmir Syntaxis forms a continuous shear zone on surface with some discontinuous exposure of splay faults rather than exposed as continuous discrete break. Likewise, the subsurface cross sections show that deformation along the fault zone is accumulated by splay faults from the main Jhelum Fault, which forms a positive flower structure with steep north-eastward dips, which is characteristics of strike-slip movement along Jhelum Fault Zone. The vertical stratigraphic throw along these faults shows small offsets and little east–west shortening, indicating that the major slip along the fault is strike slip.

A natural example of brittle-to-viscous strain localization under constant-stress conditions: a case study of the Kellyland fault zone, Maine, USA

AbstractThis article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a ∼50 m wide band of pulverized foliated granite, (3) a ∼2.8 m wide breccia zone hosting coeval shear zones, and (4) a >100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96–104 MPa at temperatures of 400–450 °C and is marked by coeval pulverization and formation of breccia. Interseismic viscous creep at similar flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted shear zones, brittle fractures and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the >100 m wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive creep at 112–124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle–viscous transition and uppermost viscous zone are the strongest part of the crust.

Study on the law of drillability of stratum in ShunBei #1 block

ShunBei 1 block is located in the northwest of Shuntuoguole low uplift in the north of Tarim Basin, with a block area of 4453km2. The reservoirs are mainly distributed in the Middle Ordovician carbonate rocks, and the burial depth is generally more than 7500m. A large number of strike-slip fault zones are developed in ShunBei #1 block, which results in low drilling rate, difficult drilling and poor formation drillability. In this paper, through rock drillability experiment and acoustic test experiment, the experimental analysis of the core of the study block is carried out, and the fitting relation formula of drillability level value is determined by comparing a variety of fitting methods. Based on the calculation model of drillability class value, this paper calculates the drillability class value of Well X in No. V fault zone of ShunBei #1 block, and establishes the drillability profile of Well X. Through the analysis of the drillability profile and drilling conditions, it is believed that with the increase of formation depth, the drillability class value of formation increases, and the drillability of formation becomes worse. Because the fluctuation of the strata drillability class value in the fault zone is small, but the strata drillability class value is large. The drillability class value of induced fracture zone strata changes dramatically, showing obvious sawtooth shape, but in general, the value of drillability class value is consistent with that of conventional strata. The strata drillability level of sliding crushing zone increases obviously, but its fluctuation is small. In addition, the drillability of the strata under different fault types is compared. It is concluded that the drillability class value of the strata controlled by the compression fault zone and the strata controlled by the translation and pull-apart fault zone fluctuates very little, but the drillability class value of the strata controlled by the compression fault zone is larger, and the formation drilling difficulty is greater, and the drillability is worse.

The likely Quaternary active El Arrayán fault, Santiago, Chile

Understanding the location and nature of Quaternary active crustal faults is critical to the reduction of both fault rupture and strong ground motions hazards in built environments. Recent work along the San Ramón Fault (SRF) demonstrates that crustal seismic sources are important hazards within the Santiago Metropolitan region of Chile. In this paper, we present the evidence of a second likely Quaternary active fault (the El Arrayán Fault, EAF) that runs through the northeastern part of Santiago, in the Lo Barnechea area. The EAF is a strike-slip fault zone at least 13 km-long, has a steep dip (mean dip 77º NNE), and a WNW-ESE general trend (~110º) that cuts across folded rocks of the Abanico Formation and Quaternary deposits. Sequences of meter-wide fault rocks in El Arrayán area are coincident with the tectonic geomorphology of the EAF, with fault scarps, deflected streams, saddles, “island hills”, sag ponds and linear valleys in the La Dehesa area and coincident with fault strike from the bedrock exposures. Although direct dating is not yet available, fault exposures tied to fault-related tectonic and geomorphic features that crosscut Late-Quaternary morphologies and deposits (including hillslopes, ridges, landslides and alluvial fans), provides strong evidence that the EAF is a Quaternary active and seismogenic crustal fault. Sinistral slickensides and left-lateral deflected streams indicates a left-lateral kinematics (with slight up to the north reverse motion) for the EAF. Due to the location, geometry, kinematics and likely Quaternary activity of the EAF, this structure is interpreted as an arc-oblique transfer zone of the West Andean Thrust (WATS). If the EAF has stick-slip behaviour, it is potentially an important source of fault rupture and strong ground motions and could be responsible for earthquakes up to Mw 6.4 based on earthquake scaling laws and microseismicity observations in and around Santiago. The lack of fault zone avoidance criteria (i.e., do not build in and around active faults) in Chilean law, requires immediate enhanced fault mapping, legislation and the implementation of active fault rupture avoidance areas to reduce the risk associated with active crustal structures in the built environment.

Cenozoic tectonic evolution of the eastern Liaodong Bay sub-basin, Bohai Bay basin, eastern China — Constraints from seismic data

The eastern Liaodong Bay sub-basin is located in the northeastern Bohai Bay basin, where Cenozoic fault patterns are thought to be largely affected by the movement of the Tan-Lu strike-slip fault zone. Based on three-dimensional seismic data, we identify the initial timing of faulting on normal faults with various orientations in the eastern Liaodong Bay sub-basin, document the structural features and evolution of the fault system and compare the fault patterns with analog models. The basin experienced phases of extensional, strike-slip and inversion-type deformation from the Paleogene syn-rift stage to the Neogene-Quaternary post-rift stage. During the Eocene, the eastern Liaodong Bay sub-basin was in a nearly NNW-SSE oriented extensional setting, with active faults predominantly trending NE-SW to ENE-WSW. During the Oligocene, the eastern Liaodong Bay sub-basin was in a predominantly dextral strike-slip setting. In the study area, the NE-SW trending boundary faults were reactivated, and the reactivated pre-existing internal faults rotated to a more E-W trend. Some of the newly formed extensional faults were orthogonal to the eastern boundary fault F1 and were oriented NW-SE, while others were predominantly oriented E-W. The renewed minor phase of N–S extension during the Miocene caused the reactivation of some pre-existing faults striking NE-SW (boundary faults) and E-W (internal faults); the faults exhibit a bidirectional character in the post-rift section, and the later inversion caused regional compressional deformation. These results suggest that the reactivation of pre-existing faults under phases of extension, strike-slip and inversion provides a more comprehensive explanation for the fault patterns and anticlines observed in the eastern Liaodong Bay sub-basin.

Hydrothermal activity along a strike-slip fault zone and host units in the São Francisco Craton, Brazil – Implications for fluid flow in sedimentary basins

This study combines multiscale analyses of geological, fault, fracture, and stable isotope data to investigate strike-slip deformation and channeling of hydrothermal fluids along the Cafarnaum fault and calcite veins at different distances from the fault, which is a structure in the São Francisco Craton, northeastern Brazil. Meteoric fluids with δD values near −45‰ and δ18O values near −6.5‰ and temperatures at 40–70 °C precipitated as calcite veins in the host carbonate units. The Cafarnaum fault, a N-S-striking vertical, ~170 km long fault zone, juxtaposes Neoproterozoic carbonate rocks in the western block and Mesoproterozoic siliciclastic rocks in the eastern block. A zone of restraining bends occurs at the central part of the fault, whereas termination zones of horsetail geometry occur at both ends of the Cafarnaum fault. These zones are marked by NW-SE-striking extensional faults that are oblique to the main N-S-striking fault zone, where hydrothermal deposits occur. The zone of influence of the Cafarnaum fault is ~ 20 km wide around the main fault. The fault formed during the Brasiliano orogeny (740–560 Ma) after Neoproterozoic carbonate platform deposition. In contrast with the host units, fluids along the fault zone originated in deeper levels of the crust and show much lower δ18O values, indicating higher crystallization temperatures. These fluids caused brecciation in the Neoproterozoic carbonate host rocks, whereas a subsequent decrease in fluid pressure and cooling near the surface resulted in the precipitation of a hydrothermal paragenesis in veins, also affecting the host rock.

Geothermal exploration and reservoir modelling of the United Downs deep geothermal project, Cornwall (UK)

Two deep geothermal wells, one of which is the deepest onshore UK well, have been successfully drilled to intersect the Porthtowan Fault zone in the early Permian high heat flow Carnmenellis Granite, part of the major post-Variscan Cornubian batholith which underlies SW England. The drilling target was prognosed at 4 to 5 km depth from existing surface data and limited geophysics. It is a late Variscan complex strike-slip fault zone with an episodic reactivation history, possibly as late as the Tertiary, and is associated with fluid flow on a geological timescale including present day geothermal springs in mines.Logging and initial hydraulic testing have been carried out to help define the reservoir volume, where temperatures at 5 km depth are around 180 °C. Initial reservoir modelling based on the fracture data and the induced micro-seismicity during drilling and injection testing afterwards indicates that open fractures aligned close to the maximum horizontal stress at >4 km depth are in critical extension or shear and a subset of them are hydraulically active. These early results are encouraging for this project, which aims to produce power and heat, as well as for other similar geothermal ventures in the region.

Geological controls on the natural CO2 accumulation in the Surennuoer Oilfield of the Hailar Basin, China

Naturally occurring CO2 often coexists with hydrocarbon accumulations on continental margins around the world, the presence of which may affect hydrocarbon production as a greenhouse gas. In this paper we focus on the natural CO2 in the Surennuoer Oilfield of the Hailar Basin, and investigate CO2 origin, distribution, related tectonic controls and accumulation mechanisms using geochemical, petrophysical and seismic data. Results show that the CO2 is of inorganic origin due to the degassing of magma from the deep crust or mantle, and is mainly stored in structural traps of N1 Member. There are close spatial and temporal relationships among CO2 distribution, major fault distribution and underneath igneous bodies, indicating a genetic link. The magmatic events and major faults are proposed to be the key controlling factors of CO2 distribution, which provided CO2 supply and migration pathways for both magma and degassed CO2. Additionally these faults also play a role in forming structural traps to store CO2. The sandstone reservoirs of low porosity and low permeability may play a limited role due to their poor reservoir quality. Two CO2 accumulation models are outlined in the Surennuoer strike-slip fault zone and the S3 fault zone. The former is characterized by shallower magmatic igneous body and a small amount of CO2 in the shallower N2 Member in addition to N1 Member. The Surennuoer strike-slip faults may have further channelled magma upwards which initially intruded along the basin-controlling fault, while the magma under the S3 fault zone did not migrate further upward along the S3 faults. Generally this study provides a detailed understanding of CO2 distribution, accumulation models and geological controls which may guide future hydrocarbon production in the study area.

Early Neoproterozoic Gold Deposits of the Alto Guaporé Province, Southwestern Amazon Craton, Western Brazil

Abstract The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D1 to D2) to transpressional (D3) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D2 strike-slip and reverse fault zones, respectively. However, the veins formed during D3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ18O values of quartz (12 ± 1‰) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δD for hydrous minerals (–116 to –55‰) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Discovery and exploration technology of fault-controlled large oil and gas fieldsof ultra-deep formation in strike slip fault zone in Tarim Basin

Discovery and exploration technology of fault-controlled large oil and gas fieldsof ultra-deep formation in strike slip fault zone in Tarim Basin

Seismic attributes for characterization and prediction of carbonate faulted karst reservoirs in the Tarim Basin, China

An ultradeep, faulted karst reservoir of Ordovician carbonate was discovered in the Shunbei area of the Tarim Basin. Fractured-cavity reservoirs buried beneath the large thickness of upper Ordovician mudstone were formed along the fault-karst belts. The hydrocarbon accumulation in these reservoirs is controlled by the fault system, and the oil-gas accumulation was affected by karstification and hydrothermal reformation. Previous studies and 2D modeling revealed that the reservoirs had “bright spot” amplitude responses like “string beads,” and they have developed along the strike-slip faults. However, describing such a complex fault-controlled karst system is still a difficult problem that has not been well addressed. We have sought to instruct the attribute expression of faulted karst reservoirs in the northern part of the Tarim Basin. We applied coherence and fault likelihood (FL) seismic attributes to image faults and fractures zones. We then used a trend analysis method to calculate the residual impedance from the impedance of the acoustic inversion, using the fact that residual impedance has higher lateral resolution in reservoir predictions. Finally, we integrated the coherence, FL, and residual impedance attributes into a new seismic attribute, the “fault-vuggy body,” with a certain fusion coefficient. The fault-vuggy body attribute establishes a connection between faults and karst cavities. This method could help in the characterization and prediction of carbonate faulted karst reservoirs. Available drilling data were used to validate that the fused fault-vuggy body attribute was an effective reservoir prediction method. As the seismic sections and slices along the layer help delineate, the distribution of bright spots and strike-slip faults indicates that the main strike-slip fault zones are the most favorable reservoirs in the Shunbei Oil and Gas Field.

Permian rifting processes in the NW Junggar Basin, China: Implications for the post-accretionary successor basins

The Junggar Basin, bounded by the West Junggar along late Carboniferous strike-slip faults in the west, is a post- orogenic successor basin within the Neoproterozoic to Paleozoic Central Asian Orogenic Belt (CAOB). The Junggar Basin is a key region for understanding the Permian rifting processes since it is located in an extensional step-over zone between the Irtysh and North Tianshan strike-slip fault zones that were most active in the Permian. Based on new seismic reflection profiles, detrital petrology and zircon U-Pb ages, together with regional geological data and a refraction seismic profile, this study investigates the tectonic evolution and the geodynamic mechanics of the basin. The early Permian phase of syn-rift mechanical subsidence in the western Junggar Basin formed half grabens. Volcanic detritus is abundant, and detrital zircon U-Pb ages show sediment supply was entirely local. During the middle Permian post-rift phase with thermal subsidence, sediment supply exceeded subsidence, and there are increasingly distant sources of sediment. The Permian regional extensional stress field between major post- accretionary strike-slip fault zones triggered the reactivation of the pre-existing late Carboniferous strike-slip faults in West Junggar and produced thinning of the crust beneath the Junggar Basin, with a β crustal stretching factor of 1.74. Seismic refraction data reveals asthenospheric upwelling resulted in underplated mafic lower crust (6–10 km). The location of initial rift axis in the Junggar Basin was controlled by the juxtaposition of thin and weak crust beneath the basin and relatively strengthened thick crust in the West Junggar attributed to the cooling of A-type granites, resulting from localized strain. Similar location of post-orogenic rifting in southwestern CAOB, in areas of high heat flow following collision, suggests that Junggar Basin is an example of a more general process.