Mesozoic Structures Research Articles

Cominco American well: Implications for the reconstruction of the Sevier orogen and basin and range extension in west-central Utah

Re-evaluation of acoustic, gamma ray and dip meter logs from the Cominco American Federal No. 2 well in the Sevier Desert basin of west-central Utah sheds new light on the interpretation of Neoproterozoic and Cambrian stratigraphy and Mesozoic structure in a region that has been influential in the development of ideas about crustal shortening and extension. The most prominent of several major thrust faults (the Canyon Range and Pavant thrusts) have been interpreted by DeCelles and Coogan (2006) [Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Geological Society of America Bulletin, v. 118, n. 7-8, p. 841-864, http://dx.doi.org/10.1130/B25759.1] as having been cut and in part re-activated between late Oligocene and Holocene time by as much as 47 km of displacement on the gently west-dipping Sevier Desert detachment. This interpretation, which is based upon a combination of outcrop, seismic reflection and well data, depends critically on the Canyon Range thrust intersecting the Cominco well at a depth of 2,551 to 2,557 m (8,370-8,389 ft.), and terminating downwards against a re-activated Pavant thrust. Our work suggests that the fault at 2,551 m (8,370 ft.) is a strand of the Pavant thrust, and that the Canyon Range thrust cuts the well at a depth of 1,222 m (4,010 ft.). This alternative interpretation depends in turn on identification of the section between the two faults as terminal Neoproterozoic to middle Cambrian Prospect Mountain Quartzite through Chisholm Formation rather than Neoproterozoic “Pocatello Formation,” “Blackrock Canyon Limestone” and lower Caddy Canyon Quartzite. To support this interpretation we present evidence for stratigraphic repetition and for deformation at the 1,222 m (4,010 ft.) level. Use of the lithostratigraphic terms “Pocatello” and “Blackrock Canyon” in west-central Utah is shown to be inappropriate, and among the reasons that the critical interval in the Cominco well has been misinterpreted by some authors. If the Canyon Range and Pavant thrusts are both found in the Cominco well, as we suggest, then they cannot be used as a piercing point for the estimation of displacement on the Sevier Desert detachment or as justification for the existence of the detachment. Published estimates of extension across the Sevier Desert basin therefore need to be reduced, potentially to as little as ∼10 km.

Three potential exploration areas of Southern Junggar Basin, NW China: Arguments from structural modeling

Three potential exploration fields of the Southern Junggar Basin are proposed and discussed through seismic structural interpretation and three-dimensional geological modeling based on quantitative structural analysis techniques. The first field comprises Tertiary imbricate wedges in the core of the second anticlinal belt. These small-scale wedges contribute to different properties of oil, gas and water in different parts of the same anticline. The second field consists of deep Mesozoic structures of the Jurassic-Cretaceous fault-bend folds, which are different in sequences and levels from the overlying Tertiary wedges. The third field is composed of the lateral transfer structures mainly in the second anticlinal belt, which are formed by transfer action between adjacent anticlines. Identification of the above three structural types must be based on pertinent seismic deployment and imagery processing. For the exploration of deep structures, a three-dimensional medium model, in which shear module, Lame's constants and density are specified to truly represent mechanical properties of the rock units, must be constructed first to calculate the restoration strains within the model by using the volumetric restoration technique. The strain data can help predict the fracture azimuth, density and intensity of deep reservoirs.

Types, features, and prospecting potential for Mesozoic metal ore deposits in Zhejiang Province, southeast China

The geotectonic units of Zhejiang Province include the Yangtze Plate in the northwest juxtaposed against the South China fold system in the southeast along the Jiangshan–Shaoxing fault. The South China fold system is further divided into the Chencai–Suichang uplift belt and the Wenzhou–Linhai geotectogene belt, whose boundary is the Yuyao–Lishui fault. The corresponding metallogenic belts are the Mo–Au(–Pb–Zn–Cu) metallogenic belt in northwest Zhejiang, the Chencai–Suichang Au–Ag–Pb–Zn–Mo metallogenic belt, and the coastal Ag–Pb–Zn–Mo–Au metallogenic belt. The main Mesozoic metal ore deposits include epithermal Au–Ag(Ag), hydrothermal vein-type Ag–Pb–Zn(Cu), and porphyry–skarn-type Mo and vein-type Mo deposits. These ore bodies are related to the Mesozoic volcanic-intrusive structure: the epithermal Au–Ag(Ag) deposits are represented by the Zhilingtou Au–Ag deposit and Houan Ag deposit and their veins are controlled by volcanic structure; the hydrothermal vein-type Ag–Pb–Zn deposits are represented by the Dalingkou Ag–Pb–Zn deposit and also controlled by volcanic structure; and the porphyry–skarn-type Mo deposits are represented by the Tongcun Mo deposit and the vein-type Mo deposits are represented by the Shipingchuan Mo deposit, all of which are related to granite porphyries. These metal ore deposits have close spatio-temporal relationships with each other; both the epithermal Au–Ag(Ag) deposits and the hydrothermal vein-type Ag–Pb–Zn deposits exhibit vertical zonations of the metallic elements and form a Mo–Pb–Zn–Au–Ag metallogenetic system. These Jurassic–Cretaceous deposits may be products of tectonic-volcanic-intrusive magmatic activities during the westward subduction of the Pacific Plate. Favourable metallogenetic conditions and breakthroughs in the recent prospecting show that there is great resource potential for porphyry-type deposits (Mo, Cu) in Zhejiang Province.

Estructura herciniana y alpina de las colinas de Barcelona:

Line 9 of the underground railway is currently being constructed in Barcelona. This undertaking necessitates tunnelling through a number of hills that are mainly made up of Paleozoic rocks, which exhibit a complex structure due to the superposition of Variscan, Mesozoic, Paleogene and Neogene structures. We present a 1:5000 scale map of the hills of Barcelona. Unpublished field notes from surveys carried out in the 1940s and in the early 1970s were crucial for drawing up this detailed map, which together with subsurface data from public works and our study of the few remaining outcrops, enabled us to provide fresh insights into the structure of this area. We also discuss the age of the structures on the basis of cross-cutting relationships and regional considerations. Our conclusions highlight the ongoing need for a geological survey of cities given that our understanding of their geology depends on impermanent outcrops and on the recovery of lost subsurface data. These considerations call for a suitable management of the geological information in urban areas with a complex geology for planning and developing safe infrastructures.

Lineamentos estruturais na borda leste da Bacia do Paraná em Santa Catarina: análise multiescala com base em imagens LANDSAT e SRTM

Este trabalho apresenta o estudo de lineamentos estruturais de uma porção da borda leste da Bacia do Paraná em Santa Catarina e do Escudo Catarinense. A área engloba rochas da Formação Serra Geral, da Sequência Gondwânica e pré-cambrianas do referido escudo. Os lineamentos estruturais foram interpretados a partir da análise digital de imagens do satélite LANDSAT e de imagens de relevo sombreado, geradas a partir do Projeto SRTM, com o uso de técnicas de geoprocessamento. Os dados obtidos a partir das imagens LANDSAT e do projeto SRTM mostraram-se eficazes na identificação de lineamentos rúpteis nas escalas 1:100.000 e 1:500.000. A aplicação desta metodologia mostra que as direções dos lineamentos estruturais, separados por domínios litológicos, são dependentes das imagens utilizadas de cada sensor, e que os diagramas de roseta obtidos a partir das imagens do Projeto SRTM não seguem o mesmo padrão de acordo com a escala adotada. A integração dos resultados obtidos (LANDSAT e SRTM) mostra três padrões principais de orientações: NW-SE, NS ± 5° e NE-SW. Os compartimentos estudados não apresentam diferença significativa entre as escalas analisadas, com exceção da Formação Serra Geral que, na escala 1:100.000, destaca a direção NE-SW. Os lineamentos do Escudo Catarinense, na escala 1:500.000, exibem direções preferenciais NS ± 5°. No Paleozoico (Sequência Gondwânica) e Mesozoico (Fm. Serra Geral), as estruturas estão orientadas preferencialmente NW-SE e, secundariamente, NS ± 5°. Na região do Domo de Lages, onde ocorrem os grupos Itararé e Guatá, foram identificados poucos lineamentos. Na escala 1:100.000, os compartimentos investigados apresentam, em geral, lineamentos com intensidade e densidade mais homogêneas, sugerindo ser esta escala a mais adequada para identificação de estruturas rasas.

On the Geologic Structure at the Epicenter of the 1886 Charleston, South Carolina, Earthquake

The study focuses on evidence of Cenozoic faulting in the epicentral area of the 1886 Charleston, South Carolina, earthquake and its connection with Mesozoic structure. The seismic data consist of several reflection profiles collected near Summerville, South Carolina, in the period 1975–1983. Reprocessing of the data reveals an extensive early Mesozoic extensional basin, approximately 20 km in width, between Summerville and Charleston. The basin is delineated by the geometry of reflections that image early Mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies. Cenozoic compressional reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary of the basin is marked by a sharp gradient in the magnetic field. Folded Cretaceous and Tertiary Atlantic Coastal Plain sediments in association with diffractions and truncated reflections from the early Mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted. Instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin. Modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km. We propose that the Charleston earthquake occurred due to compressional reactivation of a Mesozoic fault in a localized zone of intense early Mesozoic continental rifting.

Multi-phase tectonic structures in the collision zone of the Kolyma-Omolon microcontinent and the eastern margin of the North Asian craton, Northeastern Russia

Abstract. The sequence of formation of structures is established in the zone of junction of the eastern margin of the North Asian craton and the northeastern flank of the Kolyma-Omolon microcontinent, in the area of bend of the Kolyma structural loop. Detailed structural studies revealed two phases in the formation of Mesozoic structures – an early thrust phase and a late strike-slip phase. Structures formed during each of the phases are described. Thrust structures are represented by the Setakchan nappe on which the minimum amount of horizontal displacement is estimated at 13–15 km. Later superposed left-lateral strike-slip faults have a north strike. Formation of these latter structures occurred during the second phase of collision between the Kolyma-Omolon microcontinent and the eastern margin of the North Asian craton.

Upper mantle flow beneath and around the Hangay dome, Central Mongolia

Mongolia represents the northernmost area affected by the India–Asia collision, and it is actively deformed along transpressive belts closely associated with large-scale strike-slip faults. The active and past mantle flow beneath this region is, however, poorly known. In order to investigate deep mantle deformation beneath central Mongolia and its relation with the surrounding major structures such as the Siberian craton, the Gobi–Altay belt and the Baikal rift, a NS-trending profile of broadband seismic stations has been deployed in the summer 2003 from the southern Siberian craton to the Gobi–Altay range, crossing the entire Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases. In eastern Mongolia, the permanent station ULN in Ulaanbaatar reveals the presence of two anisotropic layers, the upper one being oriented NE–SW, close to the trend of lithospheric structures and the lower one NW–SE, close to the trend of Eurasia absolute plate motion. Along the NS profile in central Mongolia, seismic anisotropy deduced from SKS splitting reveals a homogeneous NW–SE trending structure, fully consistent with the observations made in the Altay–Sayan in western Mongolia. The observed delay times of 1.5 to more than 2.0 s favor consistent mantle flow over large mantle thicknesses. Since the lithosphere is less than 100 km thick beneath central Mongolia and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the absolute plate motion, we propose that both the lithosphere and the asthenosphere may join their anisotropic effects beneath central Mongolia to explain the large delay times. Although GPS vectors represent the instantaneous displacement of the Earth's surface and SKS splitting the time and vertical integration of finite strain at depth, we use the opportunity of the dense geodetic measurements available in this region to discuss the anisotropy pattern in term of present-day deformation. In the Eurasia-fixed reference frame, GPS and SKS both depict a similar trend beneath central Mongolia, suggesting a lithospheric block “escaping” toward the east that could orient olivine a-axes in the upper mantle, within a transpressive tectonic regime. A different behaviour is observed in western Mongolia: the GPS vectors trend NS, close to the regional compression direction, whereas the fast SKS directions trend EW, suggesting a tectonic regime close to a mode of axial shortening, generating the development of an EW-trending foliation at depth. We therefore propose that Mongolia is a place where active and frozen lithospheric deformation may add their effects, together with the sublithospheric flow. In the three sources of anisotropy inferred, a fundamental role is played by the Siberian craton that acted as an undeformable core of the continent through time: the frozen Paleozoic and Mesozoic structures wrap around the craton, building up the fast anisotropic direction pattern; the present-day sublithospheric flow induced by the plate motion is likely deflecting around its deep roots; finally, the present-day tectonic regime appears to be controlled by the presence of the craton to the north.

Age of the Berezitovoe gold-base metal deposit in the western Selenga-Stanovoi Superterrane and its relation to magmatism

The Selenga‐Stanovoi Superterrane [1] in the southeastern margin of the North Asian Craton is one of the key structures of eastern Asia. Its geological structure is mainly composed of conditionally defined Early and Later Precambrian rock complexes, as well as numerous Paleozoic‐Mesozoic intrusive and volcanoplutonic associations. The intricate and multistage evolution of tectonic structures was characterized by the functioning of magmatic and ore-forming systems of various ages and types related to different geodynamic settings, resulting in the formation of no less complicated metallogenic specialization of the region. Numerous data obtained recently make it possible to specify the ages of igneous and metamorphic rock complexes and, correspondingly, to revise the existing concepts of the regional geological structure. However, isotopic‐geochronological information on most ore objects remains insufficient. Therefore, it is rather difficult to correlate tectonic, magmatic, and ore-forming processes. According to metallogenic models, the majority of ore objects are Mesozoic structures [2 and others]. However, these models are based largely on the macroscopic association of mineralization with different magmatic processes. Systematic geochronological studies of ore deposits with application of advanced methods were introduced only during the past few years [3‐5 and others]. In this communication, we

The deep seismic reflection MARCONI-3 profile: Role of extensional Mesozoic structure during the Pyrenean contractional deformation at the eastern part of the Bay of Biscay

The new MARCONI-3 deep seismic profile allows recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. It denotes that the easternmost part of the Bay of Biscay consists of a thick wedge of uppermost Cretaceous to Cenozoic synorogenic sediments lying unconformably on the top of a thinned continental crust with the Mesozoic Parentis Basin to the north and the coeval Landes High to the south. The Parentis Basin appears as a major half-graben bounded southwards by a north-dipping planar fault. It is filled by a thick sequence of Jurassic–Upper Cretaceous carbonates affected by salt domes and squeezed diapirs made up of Triassic evaporites and mudstones. These salt tectonic structures also affect the overlying uppermost Cretaceous to Lower Miocene synorogenic deposits which are folded upon these structures. The Landes High includes a thin pre-Upper Cretaceous cover tilted to the south. In the Basque shelf, it is deformed by a basement-involving thrust wedge emplaced during the Late Eocene–Miocene that constitutes the North-Pyrenean contractional front. Geometric relationships and thickness variations depict that this overall structure results from the following. • Mesozoic extensional stage which includes a Late Jurassic (?)–Late Aptian syn-rift stage in which the Parentis Basin formed; and an Albian–early Late Cretaceous post-rift stage in which diapirs of Triassic evaporites grew close to this major fault. • Compressive deformational stage coeval to the Pyrenean orogeny which led to (1) the development of a latest Cretaceous up to Middle Miocene foreland basin; and (2) from Late Eocene, the formation of a basement-involving thrust wedge in the innermost foreland basin, the squeezing of the diapiric structures formed previously in the Parentis Basin and, later (Middle Miocene), the partial inversion of some pre-existent faults located in the southern Parentis Basin margin. This geodynamic evolution together with the structure of the area evidences that the extensional structure resulting from the opening of the Bay of Biscay played an important role both in the location of the North-Pyrenean front and in the North-Pyrenean foreland contractional deformation features. Specially, the lack of significant inversion structures in the Parentis Basin, despite that it belongs to a severely thinned crustal area before the Alpine compression, denotes that the Mesozoic Landes High acted as an important buffer for the propagation of the Pyrenean contractional deformation to the north. This deformational buffer was active until Early Miocene and probably vanished afterwards during the last stages of Pyrenean orogen development when some basement faults reactivated in the Parentis Basin.

Position of the Blyb Complex in the pre-Mesozoic crustal structure of the Front Range Zone in the Northern Caucasus

A number of Variscan nappe complexes were recognized in the Late Mesozoic structure of the Front Range Zone of the Greater Caucasus in the 1970s. They consist predominantly of greenstone units and override one another in a consecutive order. The only exception is the upper, Atsgara Nappe, which is composed of crystalline schists, amphibolites, and microgneisses. Crystalline schists, gneisses, amphibolites, and other rocks of the so-called Blyb Complex occur at the base of the nappe packet. The affinity of crystalline rocks of the Blyb Complex to one of the upper Variscan nappes is substantiated in this paper. The Middle Paleozoic rocks, which originally were located below the Blyb Complex in the Front Range structure, overrode its rocks along the surface of the Blyb Thrust Fault in the Early Triassic. Since that time, the crystalline rocks of the Blyb Complex have occupied the lowermost position in the structure of the Front Range. The absence of Upper Paleozoic rocks in the footwall of the thrust fault is due to the fact that, in the Late Paleozoic, the area underlain by the Blyb Complex was an inlier and a source of clastic material. The hanging wall of the Blyb Thrust Fault may be traced farther southward into the Main Range Zone, where it most likely consists of the Laba Group and other rocks. As has been established previously, the lower portion of the Laba Group consists of analogues of the Middle Paleozoic successions of the Front Range Zone, while its upper portion consists of crystalline schists of the Lashtrak Nappe, which occupy a position similar to that of the Atsgara Nappe metamorphic rocks. These relationships suggest that the rock complexes of the Front Range Zone could have undergone repeated displacements due to post-Variscan (Indosinian) tectonic events and overrode crystalline rocks in the Main Range Zone and more easterly areas. Owing to the uplift of the Central Caucasus, they are now eroding. The difference in the metamorphic grade of the Blyb Complex and the rocks of the Atsgara and Marukha nappes is due to the fact the Blyb Complex lies close to the root zones of nappes or belongs to different nappe sheets. The Blyb Thrust Fault pertains to the Indosinian faults that played the main role in the formation of the Front Range structure.

Fault reactivation and rift localization: Northeastern Gulf of Aden margin

The Gulf of Aden and the Sheba spreading ridge (Gulf of Aden) forms the southern boundary of the Arabian Plate. Its orientation (075°E) and its kinematics (about 030°E divergence) are interpreted as the result of an oblique rifting. In this contribution, a field study in the northeastern Gulf of Aden allows us to confirm the Oligo‐Miocene synrift directions of extension and to precise the normal fault network geometry. The synrift extensions are 020°E and 160°E (possibly in this chronological order); the normal faults strike 070°E, 090°E, and 110°E. The results show that some characteristics are consistent with oblique rifting analogue models, while some others are not. Especially, fault reactivation of Mesozoic structures is shown to have occurred significantly at the beginning and during rifting. These data are therefore compared to analogue models of oblique reactivation, and this comparison demonstrates that fault reactivation played a key role during the early stage of the Gulf of Aden rifting. Finally, scenarios of the lithospheric evolution during the eastern Gulf of Aden opening (preexisting weaknesses in the lithosphere or not) are discussed to better constrain the deformation history of the northern margin. Especially, we show that rift localization processes may imply stress rotations through time.

MesozoicndashCenozoic evolution of North Atlantic continental-slope basins: The Peniche basin, western Iberian margin

New regional (two-dimensional) seismic reflection data, published Deep-Sea Drilling Project–Ocean Drilling Program reports, and unpublished shallow-offshore well information characterize the Mesozoic–Cenozoic evolution of the western Iberian continental slope north of 3845N. Two distinct sectors bounded by first-order transfer faults exist between the Galicia Bank and the Nazar fault. The northernmost sector 1 is filled by Triassic–Aptian (prebreakup) sequences, reaching more than 3.5 s two-way traveltime (TWTT) in thickness in distinct half grabens. Salt pillows, salt ridges, minibasins, and salt-detached overburden faults were generated during the Mesozoic and reactivated in the Cenozoic. Sector 2 shows Triassic–Jurassic units more than 2.0 s TWTT thick, underlying east-tilting half grabens of Early Cretaceous age. Salt structures in this sector evolved into mature salt diapirs. Postbreakup units are up to 2.0 s TWTT thick in both sectors. The evolution of the study area replicates evolutionary settings that have previously been proposed for nonvolcanic passive margins. However, some distinct features are noted: (1) widespread Triassic–Berriasian units deposited over rotated tilt blocks represent the early rifting stage; (2) Early Cretaceous subbasins showing rift-climax units, most likely formed during the advanced rifting stage, are spatially constrained to an approximately 100-km (62-mi)-wide region stretched along the continental slope; and (3) listric blocks and their associated low-angle (deep) detachment faults, formed on the distal margin during the advanced rifting and transition to sea-floor spreading stages, show no developed rift-climax units above them. From the early rifting stage onward, Mesozoic faults and halokinetic structures induced local differences in the thickness and character of seismic facies. Cenozoic (Alpine) tectonism promoted the reactivation of older Mesozoic structures.

Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros–Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches

The Zagros Simple Fold Belt is characterized by elongated, curved, or sigmoidal folds. The trend of these structures together with the structural style, change suddenly across the Zagros–Makran syntaxis which separates the continental collision domain of Zagros from the oceanic subduction one in Makran. This work focuses on the Minab anticline, outcropping in the easternmost part of Zagros. In order to understand the kinematics of a sigmoidal fold and underscore possible vertical axis rotations in the eastern side of the syntaxis, we performed a joint study of magnetic fabric, microtectonics and paleomagnetism of the northern termination of this fold. The two limbs have been sampled (7 sites, 134 samples) along three cross-sections corresponding to three different orientations of the fold axis. The rocks are weakly deformed fine-grained Mio-Pliocene reddish siltstones. The shortening directions deduced from both magnetic fabric analysis and microtectonic observations are consistent with each other, they are horizontal and roughly perpendicular to the local fold axis, following the torsion of the fold hinge line, and indicating a tectonic origin of the magnetic fabric. Rockmagnetic analyses (thermomagnetic curves, hysteresis loops) point to the presence of magnetite in the PSD and MD ranges as the main magnetic carriers, together with a minor contribution from hematite. Apart from a post-tilting sub-actual VRM and/or CRM (component A), paleomagnetic analyses yield mainly two pre-tilting magnetization components: Component B is carried by magnetite, spanning the intermediate to high unblocking temperature range (300 °C ≤ T ubs ≤ 580 °C). Component C has unblocking temperatures characteristic of hematite (580 °C ≤ T ubs ≤ 680 °C). Both are ante-folding, based on positive reversal and fold tests, inside each of the cross-section but also for the three sections together. However, because component C is biased by some inclination flattening, only component B is taken into account afterwards. After full tectonic correction, the site mean direction of component B calculated for 6 out of the 7 sites is not statistically different from the individual site mean directions. The intersection direction determined by small circle analysis in these six sites is: D = 18.0° I = 33.7° k = 482 alpha 95 = 2.6°. Compared to the Mio-Pliocene direction expected in this area for Africa, it corresponds to an overall clockwise rotation of the whole structure of some 18.3° ± 5.1°, consistent with the regional geodynamic context. The absence of differential paleomagnetic rotation from one cross-section to the other, and the fact that the magnetic lineation together with the shortening direction deduced from microtectonic analysis follows the fold curvature demonstrate that the torsion of this fold is not secondary but rather coeval with the fold amplification. The torsion of the fold would stem from underlying inherited Mesozoic structures and the rheological response, during the deformation, of two incompetent décollement layers undergoing differential velocities and a change of thickness, in a constant stress field direction.

Mesozoic episodic magmatism in South Korea and its tectonic implication

NE trending granitoid batholiths in South Korea are part of the huge Phanerozoic granitoid belt in east Asia. The crystallization ages of granitoid plutons show three episodes of Mesozoic magmatism in South Korea: Triassic (248–210 Ma), Jurassic (197–158 Ma), and Late Cretaceous–early Tertiary (110–50 Ma), with a Late Jurassic–Early Cretaceous (∼50 m.y.) hiatus. The granitoid ages and kinematic analyses of the Mesozoic structures in South Korea suggest that the Middle Jurassic (180–170 Ma) dextral strike‐slip ductile shear zones formed by the northwest (orthogonal) subduction of the Izanagi plate, whereas the Cretaceous sinistral faults formed by the northward (oblique) subduction of the same plate, which generally agree with proposed reconstruction of plate motions, albeit uncertain, around east Asia. Among the models proposed to explain magmatic lull in a continental arc, shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in South Korea and Japan. The huge width (∼500 km) of the east Asian granitoid belt may be explained by oceanward retreat of subduction zone in the Cretaceous in association with change in the movement direction of the subducting plate.

Hydrocarbon habitat of the offshore Bohai Basin, China

The Bohai Basin is a major continental petroliferous basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total petroleum resources in the basin. Although the basin has been explored for nearly 40 years, the offshore part of the basin is just reaching a moderately mature stage of exploration, and its petroleum geological features are gradually becoming known. The subduction of the Pacific Plate to the Eurasia Plate resulted in the convective upwelling of the mantle in the Bohai area and triggered the development of NNE and NE striking extensional faults along the Mesozoic structures in the Late Paleocene, causing the formation of the rift basin. During the Oligocene, the depocenters and subsidence centers of the greater Bohai Basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin. Since the Oligocene, especially during the Neogene, due to the indentation of the India Block to the Eurasia Plate, the right-lateral strike-slip movement of the Tan-Lu fault was relatively intense in the offshore part of the basin. These characteristics led to the differences between the offshore and onshore sectors in sedimentation, structural features, and petroleum geology. During the deposition of the Dongying Formation, the offshore sector became the depocenter and subsidence center of the basin, resulting in the deposition of the lower Dongying Formation mudstone source rocks; while these mudstones are immature in the onshore part of the basin. In the offshore Bohai, most major discoveries have been made in the drape anticlines on the rises in the Upper Tertiary reservoirs; while the Lower Tertiary reservoirs within the half-grabens only serve as secondary reservoirs. As the Neogene depocenter of the basin, the offshore part accommodated some fine-grained shallow lacustrine clastics of the lower Minghuazhen Formation, serving as important regional seals for the Neogene reservoirs. A set of shallow faults and a central faulted anticlinal belt in the shallow Upper Tertiary sequences were formed during the Neogene in the offshore sector under the influence of right-lateral strike-slip faulting of the Tan-Lu fault. These shallow faults networked with deep extensional faults, serving as hydrocarbon conduits. The central faulted anticline belt, along the Tan-Lu fault system, also provides significant traps.

Hydrocarbon habitat of the offshore Bohai Basin, China

The Bohai Basin is a major continental petroliferous basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total petroleum resources in the basin. Although the basin has been explored for nearly 40 years, the offshore part of the basin is just reaching a moderately mature stage of exploration, and its petroleum geological features are gradually becoming known. The subduction of the Pacific Plate to the Eurasia Plate resulted in the convective upwelling of the mantle in the Bohai area and triggered the development of NNE and NE striking extensional faults along the Mesozoic structures in the Late Paleocene, causing the formation of the rift basin. During the Oligocene, the depocenters and subsidence centers of the greater Bohai Basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin. Since the Oligocene, especially during the Neogene, due to the indentation of the India Block to the Eurasia Plate, the right-lateral strike-slip movement of the Tan-Lu fault was relatively intense in the offshore part of the basin. These characteristics led to the differences between the offshore and onshore sectors in sedimentation, structural features, and petroleum geology. During the deposition of the Dongying Formation, the offshore sector became the depocenter and subsidence center of the basin, resulting in the deposition of the lower Dongying Formation mudstone source rocks; while these mudstones are immature in the onshore part of the basin. In the offshore Bohai, most major discoveries have been made in the drape anticlines on the rises in the Upper Tertiary reservoirs; while the Lower Tertiary reservoirs within the half-grabens only serve as secondary reservoirs. As the Neogene depocenter of the basin, the offshore part accommodated some fine-grained shallow lacustrine clastics of the lower Minghuazhen Formation, serving as important regional seals for the Neogene reservoirs. A set of shallow faults and a central faulted anticlinal belt in the shallow Upper Tertiary sequences were formed during the Neogene in the offshore sector under the influence of right-lateral strike-slip faulting of the Tan-Lu fault. These shallow faults networked with deep extensional faults, serving as hydrocarbon conduits. The central faulted anticline belt, along the Tan-Lu fault system, also provides significant traps.

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential. Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift. During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature. There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality. In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Recôncavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge. Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.

The MacCulloch Field, Block 15/24b, UK North Sea

Abstract The MacCulloch Field lies within Block 15/24b in the UK Central North Sea and is located on the northern flank of the Witch Ground Graben. It was discovered by Conoco well 15/24b-3 in 1990. MacCulloch Field is a four-way dip closure at Top Paleocene over a deeper Mesozoic structure. The reservoir consists of Upper Balmoral Sandstones containing 32-37° API oils derived from Kimmeridge Clay Formation shales and sealed by shales belonging to the Sele Formation. The field contains recoverable reserves of 60-90 MMBOE. Reservoir quality is generally very good, with an average porosity of 28% and core permeabilities (Kh) between 200 mD and 2D. AVO anomalies and a seismic flat spot are associated with oil filled reservoir and the oil-water contact in certain areas of the field.

Geological setting of the Argentine Frontal Cordillera in the flat-slab segment (30°00′–31°30′S latitude)

The Argentine Frontal Cordillera, between latitudes 30°20′ and 31°20′S, presents an eastward-verging polycyclic structure. Two large groups of rocks with markedly different structural nature can be distinguished: a Paleozoic basement with characteristic thin-skinned structure and the Andean cover showing remarkable extensional Mesozoic structures inverted by Tertiary compressional tectonic events. These groups are divided by a large tectonic unconformity separating not only rocks with different style and degree of deformation but also different paleogeographic settings. The Cenozoic tectonic inversion is linked with the subhorizontal subduction of the Nazca Plate in the Pampean flat-slab segment and subsequent uplift of the Andean Cordillera. The Paleozoic basement is composed of Devonian and Permo-Carboniferous marine sedimentary rocks, separated by an angular unconformity. These rocks were deposited in siliciclastic and mixed sedimentary platforms, respectively, and intruded by Permo-Triassic granitoid rocks. The Devonian rocks were deformed by the Famatinian orogeny in Late Devonian times. The most important Paleozoic structures are related to the Gondwanic orogeny, of Permian age. These include east-vergent thrusts and related folds that record a large shortening component. The Andean cover, lying unconformably over the Paleozoic basement, is primarily volcanic and volcanoclastic, with some interbedded continental sedimentary rocks. Two main stratigraphic groups can also be defined here: an older sequence linked to an extensional tectonic event and a synorogenic sequence connected to a compressional tectonic event that produced an inversion of previous extensional features. The lower unit (Choiyoi Group: Permian and Triassic) is affected by normal faults with downthrown western blocks and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic basement in a typical thick-skinned tectonic style and are grouped in bands with N–S strike. The uppermost units (Vizcachas, Melchor, and Olivares groups: Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplift of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. Displacement on reverse faults, measured in the Tertiary synorogenic rocks, is usually less than 1 km. The Tertiary crustal shortening calculated from cross-sections is about 10%, in contrast with the estimated 50% shortening of the Precordillera unit. All this shows that most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplifted block in which the extensional structures have been preserved, and that the Rodeo–Upsallata basin is a piggy-back type.