Pull-apart Basin Research Articles

Oblique rifting along transfer zones: The structural evolution model revealed by physical modeling

Oblique rifting is a common process that acts during the rupture of continental masses and can evolve to the passive margin stage. The inter-rift (or intra-rift) interaction can occur by transfer zones (TZs). These zones can be formed at the most diverse scales and, on a large scale, can result in the development of oblique rifts. To understand how TZs nucleate and evolve at the plate scale, we simulated, through physical modeling, the development of oblique transfer zones that connect two parallel rifts (with no lateral overlap) developed orthogonally to the extensional direction. Our results showed that obliquity and rheology could play an important role in the structural architecture of rifts that originated in such a structural environment. When the obliquity ranges from low to intermediate (i.e., 20°≤ α ≤ 45°), these rifts can be compartmentalized into two main structural domains - extremity, and central regions - with diachronic evolution and distinct kinematics. Extremity regions are characterized by oblique-slip deformation with intense rotation/tilting of the substrate. The central region is dominated by transcurrent (and oblique-slip) deformation with the presence of marginal plateaus, pull-apart, and oblique basins. Small variations in the obliquity (around 5°) are enough to affect parameters like faults' dip and geometry, and depocenters’ width and depth, thereby promoting changes in the morphology, geometry, architecture, and configuration of TZs. In a more mature stage, and on a continental scale, the TZs evolve into oblique rifts whose margins are composed of oblique and transcurrent fault segments (transfer/transcurrent) that together define a typical zig-zag geometry. The sizes and numbers of these segments depend on the obliquity and the rheology. Transcurrent fault segments are more common when the obliquity is 20° and 25°, in the model with quartz sand only. The modeling data presented here can be compared with the equatorial Atlantic margin of South America, assuming that it may represent an important plate-scale oblique transfer zone (20°) developed between the South and Central Atlantic.

Evolution of pull-apart basins with overlapping NE-trending strike-slip fault systems in the northern South China Sea margin: Insight from numerical modeling

A pull-apart basin is a significant type of oil-bearing basin. Its formation mechanism is a hot and challenging issue in geodynamic research that involves the interaction of three-dimensional faults and stress states. In particular, the systematic and quantitative interpretation of the three-dimensional mechanism is not clear. Therefore, we use ASPECT simulation software to analyze how the main dynamic parameters affect the evolution of the basins. The following features were recognised: (1) The greater the degree of overlapping between parallel strike-slip faults, the longer the time of rapid subsidence at the initial stage, but the larger the range of subsidence and the shallower the depth of subsidence at the final stage. (2) All models generate two small basins at the beginning, but they connected together at different stages. The antithetic end-member models of different spacing faults subsided rapidly at the initial stage. However, the two independently developed small basins were connected with time. (3) Based on the comprehensive comparison of numerical modeling results with the evolution of strike-slip faulting in the Pearl River Mouth Basin (PRMB) in the northern South China Sea margin, this paper proposes that the 120° overlapping releasing stepover pattern is likely dominant at the early stage. Furthermore, this study finds a linear relationship between the strike-slip displacements and the depths of basin depocenter. Based on this relationship, the displacements of master strike-slip faults on both sides of the Baiyun Sag of the PRMB are less than 3.5 km. It can further help to understand the formation mechanism of the PRMB.

Thermal evolution of a Variscan syn-orogenic intracontinental basin (Servoz basin, Western Alps): RSCM geothermometry and geochronology

The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.

A quick comparison of Pliocene and Upper Miocene shale resources in northern, central and southern parts of Song Hong basin with reference to their gas potential

This research is a follow-up of a more comprehensive PhD study on assessment of shale gas resources in the northern Song Hong basin that was conducted at the Asian Institute of Technology (AIT). The Song Hong basin, a typical pull-apart Cenozoic basin, had experienced a post-extensional stage accompanied by seafloor spreading from Upper Oligocene to Lower Miocene with its stratigraphy characterised by a fault-controlled syn-rift continental sequence followed by a post-rift marine sequence. In recent years, a number of gas fields have been discovered in the Song Hong basin with the Oligocene-Eocene and the Lower-Middle Miocene shales as the major and minor source rocks, respectively. On the other hand, the Pliocene and Upper Miocene shales, present in the stratigraphy from the north to the south of the Song Hong basin, have generally been considered as the seals, but not the source rocks in some previous studies. In July 2020, an exploration well (Ken Bau-2X) was drilled in Block 114 by ENI, reaching a total depth of 3,658 m and encountering a pay of about 110 m in several intervals of Upper Miocene sandstones interbedded with shales, confirming a considerable gas accumulations discovered in Vietnam so far. The interesting thing is that this well only encounter the Pliocene and Upper Miocene shales, the Oligocene-Eocene or Middle-Lower Miocene sediments underlying is absent or very thin. Therefore, potential source rock of these shales should be considered in the area, in particular with reference to petroleum system of the central Song Hong basin.
 
 In this study, a preliminary comparison of the Pliocene and Upper Miocene shale resources in the northern, central and southern blocks in the Song Hong basin was conducted based on the analysis results of XRD, Rock-Eval pyrolysis, vitrinite reflectance, respectively. While the Pliocene and Upper Miocene shales in many areas of Song Hong basin, show a very low or no hydrocarbon generation potential, the very deep and thick Pliocene and Upper Miocene shales in the center and adjacent areas, deposited in a marine environment under the special conditions of abnormal pressure and high geothermal gradient, can be potential source rocks that have possibly generated and released a large amount of hydrocarbons. Further geochemical analyses and petroleum system modelling of the Pliocene and Upper Miocene shales in particular and for the whole central Song Hong basin are recommended.

Palaeo-thermal history of the Blanice Graben (the Bohemian Massif, Czech Republic): The origin of anthracite in a late-Variscan strike-slip basin

Coal petrology data and vitrinite reflectance modelling were combined with micro-geochemical and fluid inclusion analyses of coal mineralization and apatite fission track analysis (AFTA) of coal-bearing sediments to constrain the thermal history of the Blanice Graben, a narrow, nearly 150-km long, late-Variscan strike-slip basin filled with Permo-Carboniferous continental coal-bearing deposits. The vitrinite reflectance modelling and the ranking of coals, which increased along the meridional axis of the graben from bituminous coals (Rr ∼ 0.70%) to high-rank anthracites (Rr = 2.85–4.77%), provide evidence of maximum palaeo-temperatures ranging from about 120–145 °C to 310–390 °C. This corresponds to the diagenetic realm in the north of the graben that graded to high-grade anchizonal to low-epizonal metamorphic conditions in the south. The present level of coalification resulted from the Permian burial, 2500–5000 m thick, which aggregated under a high geothermal gradient (∼ 90 °C/km). In central and southern relics of the graben, however, specific microscopic features of coals and fluid inclusion and chlorite thermometry suggest that the anthracite rank was attained by the combined effect of a stratigraphic burial, a regional discharge of ∼300 °C hot, mineralized fluids that ascended along faults into buried coal-bearing strata and an intense Permian faulting and shearing of coal seams, which collectively contributed to the anthracitization process. The heat source, promoting high geothermal gradient and circulation of hot fluids through the coal-bearing sediments, was probably associated with a mantle magma chamber hidden deeply below the graben and manifested at current errosional level by ∼303 and ∼ 270 Ma old microdiorite dykes. Similar dramatic geothermal settings, characterized by elevated geothermal gradients, localized fault-controlled incursions of hot hydrothermal solutions, and rapid coalification of peat layers at relatively shallow depths of burial, dominated in many late Variscan coal-bearing basins during a period of post-collisional extension at ∼315–280 Ma. The AFTA method, applied to sedimentary apatites, provides evidence that following the climax of the Variscan orogeny, the Blanice Graben coal-bearing deposits were rapidly uplifted and, throughout the Mesozoic and Cenozoic times, they experienced only prolonged slow cooling (0.46–0.54 °C/My) and moderate burial temperatures of 50–70 °C. Its final ascent towards the present-day erosional surface occurred during a period of accelerated uplift that has affected the Bohemian Massif since the Oligocene.

Airborne Magnetic Survey and Remote Sensing Applied to Structural Study in Vohilava Area Madagascar

Mananjary district is known for its gold production. To better understand the gold mineralization distribution, a regional structural study was undertaken using airborne geophysics and remote sensing (RS). Magnetometry data, Landsat Operational Land Imager (OLI) 8 satellite-image and DEM (Digital Elevation Model) have been processed and interpreted. Signals associated with deep structures are weak and dominated by strong signals from surficial magnetic sources. To overcome this problem, local-phase based filters were used. A deep knowledge of each filter allows to use it separately or in combination with other filters. RS is a powerful tool for a regional study. Color composite technique and DEM analysis were helpful to bring out faults and structures. The combination of magnetometry and RS allows geologists to efficiently delineate prospective zones. The study has unveiled a dextral strike-slip fault (SSF) over 240km and associated faults, folds, flower structure and pull-apart basins within the Mananjary gold district. Moreover, the SSF structurally controlled the spatial distribution of Vohilava-Nosivolo and Maha groups. R Riedel shear faults appear as North North East – South South West faults. R’ Riedel shear faults occur in the north of the study area with East North East -West South West orientation. Tectonic block rotation occurred between Ampasinambo and Soavina. P’ shear faults striking West North West – East South East occur in Vohilava. Some folds are associated with the SSF as well. Vohilava positive flower structure which used to be a syncline and its antiform equivalent are SSF footprints. The latter was active since Mesoarchean.

The 2021 Mw6.7 Lake Hovsgol (Mongolia) Earthquake: Irregular Normal Faulting with Slip Partitioning Controlled by an Adjacent Strike-Slip Fault

In transtensional regions, structures striking obliquely to the extension direction generally exhibit oblique or partitioned slips. However, their on-fault partitioning patterns and controlling factors are less known, hindering our understanding of the evolution of the rifting process. Here, we study the slip distribution of the 2021 Mw6.7 Lake Hovsgol (Mongolia) earthquake occurred in a pull-apart basin using InSAR observations. Our preferred slip model shows a remarkable feature, with three zones exhibiting distinct slip directions at different depths. The Coulomb stress change analysis reveals that this pattern is likely controlled by the left-lateral motion of the Mondy Fault to the north, which also inhibits the growth of a boundary fault to the east of the lake, shaping the asymmetric graben structure in this region. Our results imply the important role of major strike-slip faults bounding the pull-apart basin in the formation and evolution of the oblique rift.

Structure of the Northern Altar Pull-Apart Basin Revealed by a 2D Reflection Seismic Survey: Evolution of the Gulf of California Shear Zone in Northwest Mexico

Structure of the Northern Altar Pull-Apart Basin Revealed by a 2D Reflection Seismic Survey: Evolution of the Gulf of California Shear Zone in Northwest Mexico

Role of an advanced gravity data analysis in improving the geologic understanding of the northern Tebessa region, Northeastern Algeria

The Tebessa region in the extreme northeastern part of the Saharan Atlas of Algeria is characterized by a succession of anticline and syncline structures, diapirs, grabens, and a thick sedimentary cover underlying the Paleozoic basement. The region constitutes a complex structural system. The extensional structures (i.e., grabens and basins) in the Tebessa region belong to the foreland grabens of the North Africa Alpine Belt and their evolution is related to the tectonic history of the western Mediterranean region. The Tebessa region has been the subject of several geologic studies in the last decades. However, the subsurface structural configuration remains unknown, resulting in uncertainty about the exact origin of the grabens present in the Tebessa region.The current research involves gravity data analysis in order to understand the geologic subsurface system. Accordingly, gravity data are used to delineate the fault systems and the tectonic framework by using the 2-D Source Edge Detection (SED) technique, Centre for Exploration Targeting (CET) grid analysis, Euler Deconvolution, multiscale edge detection method (Worming), and 3-D modeling.Results indicate that the main tectonic trends within the study area are NE-SW, E-W, ENE-WNW, and NW-SE directions, making the northern Tebessa a complex structural region. The eastern part of the Tebessa region is characterized by a basement uplift which contributes to the positive gravity anomalies of the diapirs. In addition, the grabens have a sedimentary deposit thickness of about 2–8 km and underlain by Triassic evaporite layers that facilitated their opening by providing rheological weakness.The grabens in the Tebessa region have different mechanisms that have been responsible for their formation. However, the Early Mesozoic extensional tectonic regime initiated some grabens by creating the bounding normal faults, followed by Cenozoic strike-slip motions, when grabens opened as pull-apart basins.

Relative tectonic activity assessment of the Northern Sumatran Fault using geomorphic indices

Geomorphic indices have widely been applied to assess relative tectonic activity. The relative tectonic activity is mainly associated with relative uplift and long-term tectonic deformation. By considering this, the strike-slip basins along the Northern Sumatran Fault become suitable locations for the assessment. This study aims to evaluate the relative tectonic activity of the Northern Sumatran Fault with the focus on its strike-slip basins using geomorphic indices. This study utilized the Indonesian national digital elevation model (DEMNAS) with spatial resolution at 8.25-m. Here are the six geomorphic indices applied in this study along with the average values of the results: mountain front sinuosity (1.14–1.94), valley width-to-height ratio (0.58–1.93), stream length-gradient index (93.81–599.70), index of basin shape (1.66–3.16), asymmetric factor (7.45–19.93), and hypsometric integral (0.41–0.59). These results were used to generate the index of relative tectonic activity. The Northern Sumatran Fault exhibits, generally, alternating moderate and high tectonic activities and there is a northward decrease from very high to moderate tectonic activity in its northern part. The variation of relative tectonic activity of the fault coincides with the distribution of seismicity. This may suggest that the long-term tectonic deformation of the Northern Sumatran Fault has persisted to recent time.

Development of the Whitehorse trough as a strike-slip basin during Early to Middle Jurassic arc-continent collision in the Canadian Cordillera

Abstract The Whitehorse trough is a synorogenic basin in the northern Cordillera that resulted from arc-collision processes along the northwestern margin of North America, but its filling history and tectonic significance remain uncertain. New detrital zircon U-Pb-Hf isotope analyses of 12 rock samples, including six basal sandstones that sit unconformably on Triassic rocks of Stikinia, were combined with published detrital zircon and fossil data to establish the depositional ages of synorogenic Laberge Group strata in Yukon and test proposed links between Intermontane terrane exhumation and basin-filling events. Laberge Group strata yielded 205–170 Ma and 390–252 Ma detrital zircon populations that indicate derivation from local Late Triassic to Middle Jurassic arc and syncollisional plutons and metamorphosed Paleozoic basement rocks of the Stikinia and Yukon-Tanana terranes. Basal sandstone units have Early Jurassic depositional ages that show the Whitehorse trough filled during early Sinemurian, late Sinemurian to Pliensbachian, and Toarcian subsidence events. Late Triassic to Early Jurassic detrital zircon grains confirm that syncollisional plutons near the northern trough were exhumed at 0.5–7.5 mm/yr and replicate their excursion to subchondritic Hf isotope compositions as a result of increasing crustal contributions from Rhaetian to Sinemurian time. The new detrital zircon data, combined with recent constraints for Triassic–Jurassic metamorphism and magmatism in Yukon, require modification of published forearc to syncollisional basin models for the Whitehorse trough. We reinterpret Jurassic subsidence patterns and architecture of the Whitehorse trough to reflect sinistral transtension within a transform fault system that resulted from the reorganization of subduction after end-on arc collision.

Geology, Geochemistry, and Geochronology of the Giant Rio Tinto VMS Deposit, Iberian Pyrite Belt, Spain

Abstract The Rio Tinto deposit is a giant volcanogenic massive sulfide deposit (VMS) that contains more than 500 Mt of pyrite-rich massive sulfides and more than 2 Gt of mineralized stockwork. Three broad lithostratigraphic groups occur in the regional stratigraphy: the phyllite-quartzite group, the volcano-sedimentary complex, and the Baixo Alentejo Flysch Group. These three major packages reflect the evolution of a depositional environment from a stable platform to deposition in pull-apart continental basins during oblique subduction and collision and coeval synorogenic flysch sequence. The volcano-sedimentary complex, which hosts massive sulfide mineralization at Rio Tinto, can be divided into four major units: (1) the Mafic Siliciclastic Unit, (2) the Lower Sedimentary Unit, (3) the Felsic Unit, and (4) the Upper Sedimentary Unit. The Felsic Unit is further subdivided based on new U-Pb zircon geochronology into three distinct subunits. Felsic Unit I (ca. 356 Ma) includes dome complexes dominated by rhyodacite and reflects the onset of felsic magmatism in the region. Felsic Unit II (ca. 352–348 Ma) represents the main interval of volcanic activity, also dominated by rhyodacite domes and related aprons, and is associated with widespread VMS mineralization. Felsic Unit III (ca. 340 Ma) reflects a late pulse of rhyolitic volcanism. Massive sulfides occur as two different styles of mineralization: (1) replacive ores as discordant pipes hosted by glass-rich felsic rocks and enclosed by a large zone of stockwork-like mineralization and (2) overlying shale-hosted exhalative mineralization in small anoxic basins, probably formed during the collapse of the volcanic domes of Felsic Unit II in the Middle-Late Tournaisian. New lithogeochemical data illustrate two types of mafic rocks in the Mafic Siliciclastic Unit: a basaltic andesite and a high–Ti-Zr basalt, both of tholeiitic affinity. Using immobile element ratios (heavy rare earth elements [HREEs], Al, Y, Zr, and Ti) of the Felsic Unit, fundamental differences have been recognized between the subunits. The unmineralized Felsic Unit I is characterized by high Zr content (225–300 ppm) and a pronounced Eu negative anomaly, and probably represents the most fractionated rocks. Felsic Unit II is characterized by Zr values between 50 and 200 ppm. The low Zr values of the mineralized unit contrast with the typically high Zr values of the felsic rocks related to volcanogenic massive sulfides elsewhere and, at a regional scale, can help to discriminate potentially fertile domes from barren volcanism.

The Neoproterozoic basement of the Parnaíba Basin (NE Brazil) from combined geophysical-geological analysis: A missing piece of the western Gondwana puzzle

A regional Pre-Silurian unconformity marks the erosive planar base of the Phanerozoic Parnaíba Basin (PB), and indistinctly truncates key tectonic units for the understanding of the Neoproterozoic evolution of western Gondwana in northeastern Brazil. We present a comparative analysis of geophysical and geological datasets in the PB and propose a new tectonic configuration for its pre-Silurian basement, composed of different terranes amalgamated during the Brasiliano orogeny. Using integrated seismic interpretation and gravity modelling, constrained by an updated crustal thickness map, well data and a compilation of recent geophysical studies, we have identified two main blocks in the center of the basin. They represent pre-Brasiliano inliers, surrounded by Brasiliano mobile belts, and assigned to two major crustal building blocks of western Gondwana: the Amazonian-West Africa (AWB; to the west/north) and the Central African blocks (CAB; to the east/northeast). The Grajaú block belongs to the AWB and is characterized by low gravity anomaly, thicker crust (41–45 km), transparent seismic character of the basement and a high velocity lower crust. The Teresina block belongs to the CAB and is characterized by slightly thinner crust (39–41 km), higher values of gravity and magnetic anomalies and by the presence of mid-crustal reflectors (MCR), observed in seven seismic lines and here interpreted as a remnant of a paleosuture zone between the two blocks. Along a NE-SW 500-km seismic and gravity profile, we interpreted the MCR as composed of crustal-scale thrust faults verging westwards and defining the Barra do Corda belt, which was formed by the closure of the Goiás-Pharusian ocean along the Transbrasiliano-Kandi corridor and subsequent collision of the Grajaú and Teresina blocks. This belt deforms the eastern margin of the Ediacaran Riachão foreland basin (RB), observed in seismic and well data beneath the SW portion of PB and overlying the southern Grajaú block. The western margin of RB is bounded by eastwards verging thrust faults, interpreted as a zone of back-thrusts and thinned crust (∼36 km) in the eastern prolongation of the Araguaia belt beneath PB. To the east, the limit between the Teresina block and the Borborema Province is marked by the NE-SW Transbrasiliano Shear Zone, along which Neoproterozoic mylonites were recovered from wellbores, and a system of narrow pull-apart basins is interpreted in the seismic data, triggered by Early Paleozoic transtensional reactivations of the TBSZ, prior to the deposition of the basal Ordovician-Silurian sequence of PB. This complex collisional tectonic setting of PB basement contradicts the idea of a stable cratonic block beneath it, inciting new formation models that account for the observed crustal and lithospheric heterogeneities.

Swarm seismicity illuminates stress transfer prior to the 2021 Fagradalsfjall eruption in Iceland

The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was followed by effusive lava outflow lasting six months. It was preceded by an intensive earthquake swarm lasting one month with the largest earthquake exceeding ML 5. We analyze seismic data recorded by the Reykjanet local seismic network to trace the processes leading to the eruption to understand the relation between seismic activity and magma accumulation.Precise relocations show two hypocenter clusters of the 2021 swarm in the depth range of 1-6 km; a NE-SW trending cluster that maps the dyke propagation, and a WSW-ENE trending cluster that follows the axis of the oblique plate boundary. Additionally, we relocated the preceding earthquake swarms of 2017, 2019 and 2020 and found that they form two branches along the oblique plate boundary, which coincide with the WSW-ENE trending cluster of the 2021 swarm. These branches form a stepover of ∼1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the place where the eruption occurred, suggesting that magma erupted at the place of crustal weakening.The strong seismic activity started with a ML 5.3 earthquake of 24 February 2021, which triggered the aftershocks on the oblique plate boundary and in the area of magmatic dyke, both in an area of elevated Coulomb stress. The seismicity shows a complex propagation of the dyke, which started at its northern end, migrated southwestward and then jumped back to the central part where the effusive eruption took place. The observed N-S striking focal mechanisms are interpreted as right-lateral antithetic Riedel shears that accommodate the left lateral slip along the oblique plate boundary. The co-existence of seismic and magmatic activity suggests that the past seismic activity weakened the crust in the eruption site area, where magma accumulated. The following ML 5.3 earthquake of 24 February 2021 triggered the seismic swarm and likely perturbed the magma pocket which led to the six-months lasting eruption that started on 19 March.

The near-surface structure in the area of the Børglum fault, Sorgenfrei-Tornquist Zone, northern Denmark: Implications for fault kinematics, timing of fault activity and fault control on tunnel valley formation

Previous studies showed evidence that the Sorgenfrei-Tornquist Zone (STZ) in northern Denmark is seismically active and fuelled the demand for further analyses of the fault structure and its seismic hazard potential. We focus on the area of the Børglum fault, which represents the northern boundary fault of the STZ and extends at least 250 km from the western Baltic Sea across Jutland into the southeastern North Sea. The Børglum fault has previously been interpreted to represent a glacially-triggered fault. Two shear-wave reflection seismic sections were acquired to image the near-surface structure in the vicinity of the Børglum fault. Detailed insights into the acquisition, processing and interpretation of the seismic sections will be provided. The seismic sections indicate that the Børglum fault in the study area is not an isolated fault, but most likely represents a complex fault system with a strike-slip component. The fault interpretation is based on systematic vertical reflector offsets, the observation of steeply dipping thin transparent zones that separate the offset reflectors, abrupt lateral changes in the reflector pattern and in some cases, the presence of a fault shadow (a diffuse zone on a seismic section below a fault). This interpretation is mainly based on the identification of positive flower structures on the seismic sections, and supported by the presence of elongated sedimentary mini-basins and topographic depressions in the study area, which are partly interpreted to represent strike-slip basins. A WNW-ESE striking tunnel-valley system is imaged on the seismic sections. The spatial relationship between the faults and the tunnel valley suggests a fault-control on tunnel-valley system formation in this area. The seismic sections indicate that faults in the study area were probably active at the end of the Saalian glaciation (MIS 6). In addition, soft-sediment deformation structures and shear-deformation bands developed in Late Pleniglacial to Lateglacial sediments point to repeated phases of fault activity. The reflector pattern on one of the seismic sections indicates that a major fault nearly reaches the Earth's surface. A step in the topography above the fault probably represents a fault-scarp. Because the ongoing glacial-isostatic adjustment will lead to a change in the stress magnitude in this area, future movements along the faults are possible.

A forearc pull-apart basin under oblique arc-continent collision: Insights from the North Luzon Trough

The North Luzon Trough, on the East Asian continental margin, is under transpression due to the oblique convergence between the Eurasian and Philippine Sea plates. Based on a seismic profile across the Manila subduction system and other geological data, we propose a strike-slip pull-apart basin model to account for the formation of the North Luzon Forearc Basin. The subduction system experienced four tectonic evolutionary stages: (1) initial subduction (~22–20 Ma): the Eurasian Plate began to subduct beneath the Philippine Sea Plate, and the incipient Manila Trench-North Luzon Volcanic Arc system started to form; (2) upper crustal deformation (~20–15 Ma): the overriding Philippine Sea Plate was strongly deformed by continuous subduction, and thrust faults, volcanic activity, and possibly strike-slip faulting weakened the forearc areas; (3) initiation of strike-slip faults (~15–6 Ma): continuous rotation of the Philippine Sea Plate initiated strike-slip faults in the weak zones of the forearc areas; (4) pull-apart basin formation (~6–0 Ma): the pull-apart basin formed in the forearc due to sinistral movement of two sets of strike-slip faults. We propose an indentation-escape process involving coupling between the sinistral strike-slip faults in the Taiwan-Luzon area and dextral strike-slip faults in the East Asia continental margin, for the evolution of the eastern South China Sea Basin margin.

Origin of Deep Methane from Active Faults along the Itoigawa–Shizuoka Tectonic Line between the Eurasian and North American Plates: 13C/12C and 14C/12C Methane Profiles from a Pull-Apart Basin at Lake Suwa

Stable and radiocarbon isotope analyses provide valuable insights into carbon dynamics in hydrosphere ecosystems. A lake containing seeps of deep methane (CH4) may provide an ideal environment for elucidating such dynamics, including the involvement of deep carbon in the surface ecosystem and its food web. In this study, we performed carbon isotope analysis of seep gas, lake water, and a cyanobacterial bloom sample collected at Lake Suwa, a pull-apart basin on the Itoigawa–Shizuoka tectonic line, Japan. CH4 collected from two seep sites 100 m from the lakeshore was 14C-depleted (Δ14C = −972.1 to −989.8‰), indicating that the origins of seep CH4 and benthic bubble CH4 (Δ14C = +21.7‰) collected near the lakeshore differed. The results suggested that benthic bubble CH4 was produced by methanogenic archaea in benthic sediments and that seep CH4 was produced by methanogenic archaea using relic carbon. The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14C = −103.1 to −630.6‰) and near the lakeshore (Δ14C = −100.9‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. The Δ14C values of the cyanobacterial bloom sample also showed values similar to those of DIC (Δ14C = −110.3‰), suggesting that 10–63% of DIC in surface water and 10% in primary producers originated from relic carbon, including CH4 and CO2, and was incorporated into the surface ecosystem of Lake Suwa.

Analogue modeling of the role of salt in the structuration of thin-skinned pull-apart basins: The case study of El Hamma basin, Central Tunisia

Pull-apart basins are structurally complex features that typically form in between two laterally offset strike-slip zones. The vast majority of analogue experiments on the formation of this type of basins have been conducted using basal discontinuities that transmitted from the bottom up strike-slip motion into the overlying sedimentary cover. Here, we present a series of models whose aim is to demonstrate that pull-apart basins above a viscous layer can be initiated not from the base up, but by the sole effect of lateral forces that allow two blocks to move laterally one past the other. This occurs independently from the basement, from which the brittle cover is fully decoupled by the viscous layer. This work was inspired by a natural example, the El Hamma Basin, located in Central Tunisia West of the city of Gabes and is bounded by two N100-110 trending major fault corridors, the Oum Ali- Fejij corridor in the North, and the Oglet Merteba-Matmata in the South. The area underwent multiple tectonic episodes of both extension and compression from Late Liassic to the Plio-Quaternary times. The region was subjected to regional NE-SW extension during the upper-cretaceous and major NW-SE compression during upper Miocene (Tortonian) and early quaternary (Early Pleistocene). The latter stage resulted in right-lateral strike slip along both corridors with faults forming a pull-apart basin that comprises two lateral mega-grabens separated by a central horst. Although one sub-circular diapir in the area (the El Melah diapir) crops out along the northern wrench zone, whether or not the mobile Triassic series extend farther South of it, beneath the pull-apart basin, is still debated.Through a series of analogue experiments using an innovative technique that generates two narrow, parallel strike-slip zones located within the sedimentary cover, rather than in the basement. Experimental results show remarkably similar geometry and evolution with those of the El Hamma basin. Moreover, from a regional perspective, our results suggest that the pinch out of the Triassic evaporites extends further South than previously thought and underlies at least part of the El Hamma basin.

Extreme continental weathering in the northwestern Tethys during the end-Triassic mass extinction

The end-Triassic mass extinction (ETE) is thought to have been triggered by widespread eruption and emissions of volcanic and contact metamorphic carbon and sulfur gases from the Central Atlantic Magmatic Province (CAMP) flood basalts. Although palynological studies of shallow marine deposits in the basins of Denmark and Germany suggest that deforestation and catastrophic soil loss occurred during the CAMP volcanism and ETE interval, the intensity and timing of continental weathering in the region of northwestern Tethys Ocean remain unclear. Here we present Sr, C, and O isotope data, as well as multivariate statistical analyses of major element contents, for carbonate–clastic deposits in the Kardolína section, Slovakia, to develop a continental weathering record in the NW Tethys during the ETE event. This section consists of a shallow marine carbonate sequence in the Rhaetian Fatra Formation and the overlying Hettangian Kopieniec Formation. The Fatra Formation represents ramp facies deposits that formed in a restricted pull-apart basin of the Fatric Zone along the southern margin of the Bohemian Massif in the region of northwestern Tethys. Carbon isotope analysis of the limestones revealed two negative carbon isotope excursions (NCIEs) in the uppermost Fatra Formation. These two NCIEs occurred in the latest Rhaetian and can be compared to the “precursor” and “initial” NCIEs reported for the northwestern Tethys. Strontium isotope analysis of the limestones revealed an abrupt increase in Sr isotope ratios between the precursor and initial NCIEs, which indicates that continental weathering of the Bohemian Massif increased rapidly in the latest Rhaetian. Multivariate analysis of major element contents in the carbonate rocks also shows that intense chemical weathering of the hinterland was accelerated after the precursor NCIE, supporting the inferences drawn from Sr isotopes. Furthermore, our study reveals that the carbonate depositional environments of the Fatra Formation changed to the formation of specific Fe-rich oolites with increased continental weathering after the precursor NCIE. The concentrations of redox-sensitive major elements (e.g., Mn and Fe) and multivariate analysis of the major element data suggest that the Fe-rich oolites were formed by an influx of O-depleted water into the shallow water depositional environment of the Fatra Formation. A possible origin for this O-depleted water is the oxygen minimum zone (OMZ) that formed at intermediate water depths in European basins (e.g., the German and Eiberg basins) during the latest Rhaetian. Our results suggest that the marine environment in the European basins may have developed an OMZ due to the increase in continental weathering during the latest Rhaetian, and these environmental changes may have had an important role in the marine ETE event. • Sr, C, and O isotope data of the Rhaetian succession in Slovakia were determined. • Two negative carbon isotope excursions (NCIEs) were detected in latest Rhaetian. • Abrupt increase in Sr isotope ratios recognized between these two NCIEs. • Continental weathering was temporarily enhanced during the end-Triassic extinction. • Intense weathering led to development of oxygen minimum zone in NW Tethys.

The Role of Public Health and Earthquake Occurrences in Myanmar: A Case Study of the 2017 Taikkyi Earthquake

Most of the historical earthquakes and recent seismicity in Myanmar indicate that earthquake occurrences are closely associated with major fault zones of two different structural trends in N-S direction and NW-SE direction. Series of pull-apart basins along the Burma plate both in Myanmar and in the Andaman Sea are bounded by north- striking faults and controlled by ENE-WSW trending normal faults with dipping either way. The Burma plate has been rifting and drifting towards north, with relative to SE Asia by the back-arc spreading process in Central Andaman Basin since Neogene. Northward movement of the Burma plate accommodates the crustal displacement which in turn, cause continental intraplate earthquakes along the basin bounding faults in the Central Myanmar Basin. Myanmar region is tectonically and seismically active as indicated by occurrences of 15 major earthquakes with M ≥ 7.0 in the past 100 years. Departments of Disaster management are formed all over Myanmar in every township, consisting of sub-committees: information sub-committee, search and rescue sub-committee, public health and first aid sub-committee, training and fund raising sub-committee. Disaster responses mainly involve search and rescue, helping the injured with medical aid, then providing emergency shelter, food and water.