Extensional Sedimentary Basins Research Articles

Lithospheric Structure of the Circum‐Pannonian Region Imaged by S‐To‐P Receiver Functions

AbstractThe lithosphere‐asthenosphere boundary and mid‐lithospheric discontinuities are primary attributes of the upper mantle. The Pannonian region is an extensional sedimentary basin enclosed by collisional orogens. Here, we estimate the negative phase depth of S‐to‐P receiver functions to image the lithospheric thickness and other discontinuities with high resolution, based on the recent dense seismological broadband networks. The lithosphere‐asthenosphere boundary is relatively shallow (<90 km) in the Pannonian Basin system, and deeper (∼90–140 km) in the surrounding orogens, where average surface heat flow values are higher (120 mW/m2) and lower (50–70 mW/m2), respectively. The 1D and 2D common conversion point migration with 3D velocity model provide comparable but different resolution images beneath the wider region of the Pannonian Basin. We obtained deeper values in the Western (∼120 km) and Southern‐Carpathians orogens (∼135 km). Furthermore, we provide new information on the lithospheric thickness and its seismic properties in the eastern part of the study region (e.g., Apuseni Mountains (∼95 km), Eastern‐Carpathians (∼120 km), Moesian Platform (∼90 km) and Transylvanian Basin (∼85 km). The shallower negative phase depth can be interpreted as the lithosphere‐asthenosphere boundary beneath the Pannonian Basin system in agreement with its high heat flow values. In contrast, the deeper negative phase depth estimates in the colder surroundings can be interpreted as intra‐ or mid‐lithospheric discontinuities, when compared with local seismic tomography models. In this region, the correlation with heat flow implies that the observed negative phase depth is of thermo‐chemical or rheological nature.

A Bayesian framework for subsidence modeling in sedimentary basins: A case study of the Tonian Akademikerbreen Group of Svalbard, Norway

Quantitative subsidence analysis techniques have been widely utilized in ancient extensional basins to evaluate age relationships and unravel regional sedimentation patterns; however, uncertainties associated with various model inputs, such as lithological parameters, water depth, and relative or direct age uncertainties are often neglected. Here, we modify existing decompaction, backstripping and age-depth modeling procedures for post-rift thermally subsided basins through the introduction and propagation of uncertainties using both Monte Carlo and Markov chain Monte Carlo methods in a new program called SubsidenceChron.jl. As a case study for its potential utility in ancient extensional basins, we applied this technique to a globally relevant fossiliferous early Neoproterozoic (Tonian) sedimentary succession in northeastern Svalbard, Norway. Using two new geochronological constraints (Re-Os age from the Akademikerbreen Group and detrital zircon U-Pb maximum depositional age from the Veteranen Group) along with a published Re-Os age from the Polarisbreen Group and previously established age constraints for the onset of the Sturtian snowball Earth glaciation, our model generates a posterior stretching factor (β) of 1.29+0.08/−0.06 and a posterior thermal subsidence initiation time (t0) of 840.40+18.64/−23.61 Ma. These results, along with the calculated age estimations for different stratigraphically important horizons throughout this succession, generally agree with those suggested by previous studies based on global chemostratigraphic correlations. The fewer assumptions made in our case study, as well as the incorporation and propagation of uncertainties on model inputs in SubsidenceChron.jl more broadly, contribute to important and quantifiable uncertainties in our age-depth model results. We suggest this approach will be relevant to future subsidence and age-depth models for Precambrian and Phanerozoic extensional sedimentary basins, in addition to providing a simple test of age models built solely on chemostratigraphic correlations.

Crustal and uppermost mantle structures imaged by teleseismicP-wave traveltime tomography beneath the Southeastern Korean Peninsula: implications for a hydrothermal system controlled by the thermally modified lithosphere

SUMMARYThe southeastern Korean Peninsula (SeKP) has experienced intense deformation owing to subduction and backarc extension at the eastern continental margin of the Eurasian Plate, leading to the formation of complex tectonic structures. Abnormally high surface heat flux, Cenozoic volcanism, signatures of mantle degassing and hydrothermal alteration, and several active fault systems with extensional sedimentary basins have been identified; however, the major driving forces that promote local seismic events and hydrothermal activities remain enigmatic. Here, we constructed 3-D P-wave velocity of the crust and upper mantle in the SeKP for the first time using a teleseismic traveltime tomography method and an extensive data set obtained from a dense seismic network. Our model revealed three distinct velocity patterns at different depths: (1) in the upper crust (depth ∼0–10 km), a low-velocity anomaly beneath the Cenozoic sedimentary basin exhibiting a prominent lateral velocity contrasts with higher velocities in the Cretaceous sedimentary and plutonic rocks; (2) a N–S trending low-velocity anomaly extending from the lower crust to the uppermost mantle (depth ∼20–35 km) beneath the major active fault systems interpreted as a thermally or mechanically weakened structure that could transfer high surface heat flux and transport mantle-driven gases and (3) a low-velocity anomaly adjacent to the Cenozoic basin in the upper mantle at depths of 35–55 km interpreted as the higher temperature upper mantle. Via a series of geodynamic simulations, we demonstrated that the extensional deformation at the eastern continental margin during the Early to Middle Miocene locally enhanced the temperature of the crust and upper mantle beneath the SeKP. We propose that a hydrothermal system, resulting from the thermally modified lithosphere of the continental margin, has contributed to the enhanced local seismicity and geothermal activities observed in the SeKP region.

Structural and geochronological constraints for the Mesozoic nappe structures of the Taqian–Fuchun thrust zone, eastern Jiangnan orogenic belt, South China

The NE‐trending Jiangnan orogenic belt has experienced multi‐tectonic episodes since the Mesozoic separates the Yangtze Block in the north‐west from the Cathaysia Block in the south‐east. The Taqian–Fuchun thrust zone in north‐east Jiangxi Province is located in the eastern Jiangnan orogenic belt. It is considered as an essential unit to understand the geometry, kinematics and dynamics of the Mesozoic nappe structures in the eastern Jiangnan orogenic belt and an ideal region to reveal the Mesozoic tectonic evolution in South China. Here, we present new detailed structural data coupled with zircon U–Pb and apatite fission‐track dating studies, establishing a four‐stage deformation history and describing the geometry, kinematics, ages and dynamic background. The Middle–Late Triassic NW–SE compression event (D1, 243–226 Ma) is mainly characterized by regional angular unconformity between the Lower and Upper Triassic and asymmetric folding of the underlying strata. The associated regional NW–SE shortening was related to the intercontinental collision. The Middle–Late Jurassic NW–SE compression event (D2, 175–160 Ma) is characterized by the angular unconformity between the Middle Jurassic and Cretaceous and the SE‐vergent imbricate nappe structures. The shortening has been associated with the NW‐ward subduction of the Paleo‐Pacific Plate. The Late Jurassic–Cretaceous NW–SE extension event (D3, 160–67 Ma) includes the transition stage from compression to extension (160–145 Ma) and the intensive extension stage (145–67 Ma). The former is characterized by the development of the extensional detachment structures and intermediate‐acid magmatism, while the latter is characterized by the development of Cretaceous extensional sedimentary basins and A‐type granites. This extension was correlated to the slab roll‐back of the Paleo‐Pacific Plate. The Paleocene–Eocene compression event (D4, 67–35 Ma) is manifested by the thrusting of metamorphic rocks in the Shuangqiaoshan Group to the Cretaceous red beds. This shortening possibly originated from the collision between the India and Eurasian Plates. This study provides new constraints on the Mesozoic structural style and establish a reliable geochronological framework of the Mesozoic tectonic evolutions of the eastern Jiangnan orogenic belt.

Geoscience Poster G3: Application of a probability model to detect unrecognised igneous intrusions in sedimentary basins

Poster G3 Mafic igneous intrusions are a common feature in extensional sedimentary basins, particularly those located at volcanic rifted margins, and are important in both exploration and development contexts due to their range of interactions with the petroleum system and their role as potential drilling hazards. Experience from a range of basins containing mafic igneous intrusions suggests that seismically resolvable intrusions are typically accompanied by a large number of intrusions that are too thin to be confidently identified and interpreted from seismic reflection surveys. The increased vertical resolution of wireline log data affords an opportunity to identify such sub-seismic-scale intrusions, though in many wells with full wireline suites igneous intrusions are often misidentified as sedimentary units, including felsic intrusions whose physical properties are more similar to sedimentary rocks. Here we apply a wireline-log-based probability model to well data from a number of basins. In previous applications, the model has proven highly effective in predicting the occurrence of carbonate cementation zones in sandstones in comparison to neural network approaches. We demonstrate its ability to predict the presence of igneous intrusions that were not previously identified by either seismic interpretation, or through the analysis of well-derived datasets. The broader application of this model to large suites of legacy data could lead to improved knowledge of the occurrence of intrusions in basins with implications for basin modelling and well planning. To access the poster click the link on the right. To read the full paper click here

Application of a probability model to detect unrecognised igneous intrusions in sedimentary basins

Mafic igneous intrusions are a common feature in extensional sedimentary basins, particularly those located at volcanic rifted margins, and are important in both exploration and development contexts due to their range of interactions with the petroleum system and their role as potential drilling hazards. Experience from a range of basins containing mafic igneous intrusions suggests that seismically resolvable intrusions are typically accompanied by a large number of intrusions that are too thin to be confidently identified and interpreted from seismic reflection surveys. The increased vertical resolution of wireline log data affords an opportunity to identify such sub-seismic-scale intrusions, though in many wells with full wireline suites igneous intrusions are often misidentified as sedimentary units, including felsic intrusions whose physical properties are more similar to sedimentary rocks. Here we apply a wireline-log-based probability model to well data from a number of basins. In previous applications, the model has proven highly effective in predicting the occurrence of carbonate cementation zones in sandstones in comparison to neural network approaches. We demonstrate its ability to predict the presence of igneous intrusions that were not previously identified by either seismic interpretation, or through the analysis of well-derived datasets. The broader application of this model to large suites of legacy data could lead to improved knowledge of the occurrence of intrusions in basins with implications for basin modelling and well planning.

Compression‐Induced Anomalous Subsidence in the Extensional Sedimentary Basin: A Numerical Study From the Pearl River Mouth Basin, Northern South China Sea Margin

AbstractCompressional uplift has long been considered as the basic response to basin inversion. However, significant discrepancies are recently observed in the Pearl River Mouth Basin (PRMB), where anomalous subsidence occurs during the Miocene compression phase. Whether the subsidence is another scenario that is associated with the tectonic inversion under certain circumstances remains unknown. Here we conduct systematic numerical modeling to explore the patterns of rift basin evolution from extension to compression. Our results show that two distinct inversion types develop depending on the stretching degree and tectonic quiescence. A less stretched continental crust would facilitate the uplift of basin center accompanied by erosion during shortening; otherwise, the basin center exhibits anomalous subsidence with abundant sediment accumulation. We further demonstrate that these two contrasting types, consistent with the Miocene evolutionary pathways of the eastern and western PRMB, respectively, are strongly conditioned by the along‐strike variation of stretching degree during extension.

Fluid migration recorded by fluid inclusions in crack-sealed quartz veins and sandstone host rock; Cameros Basin, Spain

The role of deep hydrothermal fluid circulation through fractures and their impact on the sandstone host rock is studied in an extensional sedimentary basin (Cameros Basin, Spain) affected by a post-extensional hydrothermal metamorphism. The quartzarenites of the Urbión Group constituted a hydrocarbon carrier affected by very low to low-grade hydrothermal metamorphism during Late Albian to Coniacian. This process generated abundant quartz veins and transformed the quartzarenites into quartzites. This study compares the microthermometry of the fluid inclusion asemblages (FIAs) in the veins and in the quartz grain overgrowths in the quartzites, in order to understand the behavior of the hydrothermal fluids through fractures and their effects in the host rock. Fluid inclusions in the quartz grain overgrowths contain liquid and vapor at room temperature and homogenize to the liquid (Th: 124–265 °C, H2O + NaCl system). Those of quartz veins present both liquid and vapor CO2 and an aqueous liquid phase (room temperature). Final homogenization is to the liquid (Th: 109–282 °C, H2O + NaCl + CO2, mean values of amount-of-substance fractions: 0.92, 0.01, 0.07). Large Th variation within each FIA is common, due to crack and sealing processes and to reequilibration by successive thermal pulses. In contrast, the narrow Th range in each FIA towards the top of the record indicates that these inclusions are probably not reequilibrated. Two growing stages are recognized under SEM-CL in the quartz grain overgrowths, one diagenetic and another hydrothermal, the later with FIAs showing Th similar than the veins. The results can help in the evaluation of the geo-energy resources in sedimentary basins.

Early post-rift confined turbidite systems in a supra-detachment basin: Implications for the early to middle Miocene basin evolution and hydrocarbon exploration of the Pannonian Basin

Understanding the interplay of sedimentation and deformation style is essential to develop an adequate model of the evolutionary history of an extensional sedimentary basin. Due to their volume and temporal extent, late post-rift deposits are often in the focus of research; however, early post-rift deposits are less studied. The aim of the study is to reconstruct the tectono-sedimentary evolution of a syn -rift to early post-rift succession in the hanging wall of a low-angle detachment zone and to recognize the effects of this deformation style on turbidity current pathways and depocenters. The small Őri sub-basin of the western part of the Miocene Pannonian basin is used as a study area where the oldest early post-rift turbidites of ~15–12.8 Ma occurred. These sediments were deposited in a supradetachment setting, where inherited and actively forming structures controlled the sediment pathways and accommodation of both the distally and locally sourced marine turbidites. Seismic data, well logs and core information were integrated to identify and analyze unconformities, structural elements, and turbidite deposits. Syn-rift, early post-rift and late post-rift system tracts were identified. The syn -rift/post-rift unconformity is a result of combined eustatic sea level drop and the change in deformation intensity. The main low-angle detachment fault, orthogonal hanging wall transfer faults and related folds together controlled the complex synformal shape of the basin, which was related to major corrugations of the footwall detachment. Four turbidite phases were distinguished based on upward increasing sand content, areal extent, reservoir quality, and degree of confinement as the turbidity currents reached the basin margins. (1) Small fans deflected by transfer faults developed, followed by (2) an axial turbidite system that reached an anticline and was deflected by it. (3) Lateral confinement by the anticline increased significantly and thick sandstone bodies with abrupt pinch-outs formed, while the influence of smaller faults diminished. (4) The axial turbidite systems covered the basin floor and their interaction with basin margins created continuous sand bodies of confined character. The sand fairways and types of confinement of a supra-detachment basin differ from high-angle normal fault systems. The exhumed metamorphic rocks provided the sediment, and the extension-parallel antiformal and synformal corrugations acted as lateral confinement. The gentle back-tilt of the detachment hanging wall block exerted a frontal confinement for the extensive turbidite system at the distal part of the basin, but the gentle NE-ward dip over the detachment still permitted long run-out distances. • Combination of eustasy and tectonics leads to syn-rift-early post-rift unconformity. • Corrugation fold of a detachment creates an intricate topography for sediment flows. • Deformation activity decreases during deposition of early post-rift turbidites. • Lateral, frontal confinement of turbidity currents controls reservoirs quality. • Differences between sand fairways in high- and low-angle extensional systems.

The meeting place of backarc and foreland rifting: The example of the offshore western Sicily (Central Mediterranean)

Coeval rifting processes affecting both backarc, thrust belt and foreland occurred in offshore western Sicily (Central Mediterranean) as indicated by the analysis of a large seismic data set calibrated with well data, onshore data and dredges in a dedicated GIS software environment. Structural pattern of the investigated area, 3-D geological models and a Pliocene-Lower Pleistocene evolution of extensional sedimentary basins have been reconstructed. In detail, two zones characterized by basins with different directions of extension have been recognized: one trending NE-SW, located south of the Egadi Islands; another trending E-W, positioned north of the Egadi Islands. The first group of basins represents the continuation of those of the Africa foreland basin (Pelagian Block), as the second one characterizes the opening of the backarc (southern Tyrrhenian margin). Both basins affect the collisional chain (Maghrebian thrust belt). An eastward progression of extension, from lower Pliocene to lower Pleistocene, based on different onset of the basin fill, has been documented in the Southern Tyrrhenian margin. The study area represents the meeting place of backarc and foreland rifting that affected the late Miocene thrust belt. The recognized pattern of extensional deformation of the upper plate adds some constraint on the geodynamic evolution of the African and Eurasia plates and supports a model of westward dipping slab in the Central Mediterranean. It is hypothesized that the along-trench variation of the nature of the plates, due to paleo-margin configuration, has a profound impact on the upper crustal deformation during the convergence and thus the meeting place of backarc and foreland rifting could represent the surficial signature of the slab detachment since Pliocene times. • 3D models are constructed from geological and subsurface data • The study area represents the meeting place of backarc and foreland rifting affecting the late Miocene thrust belt. • A lower Pliocene-lower Pleistocene eastward progression of extension has been documented in the Southern Tyrrhenian margin. • The geological signature of slab detachment during Pliocene times is where backarc and foreland rifting meets.

Inversion of two-phase extensional basin systems during subduction of the Paleo-Pacific Plate in the SW Korean Peninsula: Implication for the Mesozoic “Laramide-style” orogeny along East Asian continental margin

During subduction, continental margins experience shortening along with inversion of extensional sedimentary basins. Here we explore a tectonic scenario for the inversion of two-phase extensional basin systems, where the Early−Middle Jurassic intra-arc volcano-sedimentary Oseosan Volcanic Complex was developed on top of the Late Triassic−Early Jurassic post-collisional sequences, namely the Chungnam Basin. The basin shortening was accommodated mostly by contractional faults and related folds. In the basement, regional high-angle reverse faults as well as low-angle thrusts accommodate the overall shortening, and are compatible with those preserved in the cover. This suggests that their spatial and temporal development is strongly dependent on the initial basin geometry and inherited structures. Changes in transport direction observed along the basement-sedimentary cover interface is a characteristic structural feature, reflecting sequential kinematic evolution during basin inversion. Propagation of basement faults also enhanced shortening of the overlying sedimentary cover sequences. We constrain timing of the Late Jurassic−Early Cretaceous (ca. 158−110 Ma) inversion from altered K-feldspar 40Ar/39Ar ages in stacked thrust sheets and K-Ar illite ages of fault gouges, along with previously reported geochronological data from the area. This “non-magmatic phase” of the Daebo Orogeny is contemporaneous with the timing of magmatic quiescence across the Korean Peninsula. We propose the role of flat/low-angle subduction of the Paleo-Pacific Plate for the development of the “Laramide-style” basement-involved orogenic event along East Asian continental margin.

Interrelation between surface and basement heat flow in sedimentary basins

Petroleum system models (PSM) result critically depend on the computed evolution of the temperature field. As PSM typically only resolve the sedimentary basin and not the entire lithosphere, it is necessary to apply a basement heat flow boundary condition inferred from well data, surface heat flow measurements, and an assumed tectonic scenario. The purpose of this paper is to assess the use of surface heat flow measurements to calibrate basin models. We show that a simple relationship between surface and basement heat flow only exists in thermal steady-state and that transient processes such as rifting and sediment deposition will lead to a decoupling. We study this relationship in extensional sedimentary basins with a 1D lithosphere-scale finite element model. The numerical model was built to capture the large-scale dynamic evolution of the lithosphere and simultaneously solve for transient thermal processes in basin evolution, such as sedimentation, compaction-driven fluid flow, and seafloor temperature variations. Our analysis shows that several corrections need to be applied when using surface heat flow information for the calibration of basement heat flow in PSM. Not doing so can lead to significant errors of up to 30-50 °C at typical petroleum reservoir and source rock depths. We further show that resolving sediment blanketing effects in basin modeling is crucial, with the thermal impact of sediment deposition being at least as important as rifting-induced basement heat flow variations.

Tectonic evolution and extension at the Møre Margin – Offshore mid-Norway

Lithospheric stretching is the key process in forming extensional sedimentary basins at passive rifted margins. This study explores the stretching factors, resulting extension, and structural evolution of the Møre segment on the Mid-Norwegian continental margin. Based on the interpretation of new and reprocessed high-quality seismic, we present updated structural maps of the Møre margin that show very thick post-rift sediments in the central Møre Basin and extensive sill intrusion into the Cretaceous sediments. A major shift in subsidence and deposition occurred during mid-Cretaceous. One transect across the Møre continental margin from the Slørebotn Subbasin to the continent-ocean boundary is reconstructed using the basin modelling software TecMod. We test different initial crustal configurations and rifting events and compare our structural reconstruction results to stretching factors derived both from crustal thinning and the classical backstripping/decompaction approach. Seismic interpretation in combination with structural reconstruction modelling does not support the lower crustal bodies as exhumed and serpentinised mantle. Our extension estimate along this transect is ~188±28km for initial crustal thickness varying between 30 and 40km.

The characteristics of Precambrian sedimentary basin and the distribution of deep source rock: A case study of Tarim Basin in Neoproterozoic and source rocks in Early Cambrian, Western China

Abstract Based on outcrops of the whole basin, drilling and seismic data, this paper makes a systematic analysis of Neoproterozoic basin stratigraphic distribution and tectonic evolution characteristics in Tarim. Under the control of the aggregation and breakup of Rodinia supercontinent, the extensional sedimentary basin formed with North-South differences in the Neoproterozoic, separated by the middle ancient land. There is a sedimentary basin in the north, east-west banded, with two thickness centers, Aksu region in northwest and Manjiaer-Kuruketage region in northeast and maximum thickness about 2 500−3 500 m. There are two sedimentary basins in the south, north-east banded, with two thickness centers, Yecheng and Hetian with maximum thickness about 1 500−2 000 m. The evolution of the basin can be divided into two stages, rift in Nanhuan (Cryogenian) and depression in Sinian (Ediacaran), the latter stage deposits larger range but with thinner thickness of strata. Its main rock types contain volcanic rock, clastic rock, moraine rock and carbonate rock. The distribution of source rock during Early Cambrian period is mainly controlled by the development characteristics of the Neoproterozoic sedimentary basin.

Miocene progressive forearc extension in the Central Mediterranean

Miocene extensional sedimentary basins are being increasingly recognized in Central Mediterranean. However the paleogeographic reconstruction of the Paleo-Tyrrhenian during this time span results a difficult task because the subsequent Pliocene-Quaternary backarc opening moved away these older sedimentary basins. Our study outlines principal deformation events recorded by sediments that were deposited within minor basins of the Tyrrhenian Sea region and provides constraints on the duration of these events.The interpretation of seismic profiles, wells, and outcrops data together with a review of the middle-upper Miocene successions permitted us to recognize Miocene third order depositional sequences, reconstruct the fault pattern, and furnish a detailed evolution of the sedimentary basins that preceded the Pliocene-Quaternary Tyrrhenian high stretching. Based on the stratigraphic and tectonic constraints together with the age migration of the depocenters, we reconstructed a detailed paleogeographic evolution of central Mediterranean. Sedimentary basins architecture and fault pattern indicate a system of approximately N-S oriented normal faults and approximately E-W transform faults that were active since the Late Oligocene. Two period of extensional/transtensional tectonics (late Oligocene-Lower Burdigalian and Upper Langhian-Tortonian) were interrupted by a compressional event (late Burdigalian-Lower Langhian).Tacking in account the position of the Miocene volcanic arc (Sardinia), we propose that the coeval sedimentary basins developed in the central Mediterranean as forearc extensional/transtensional basins during the progressive collision with the African plate and before the Tyrrhenian backarc opening. Unlike published late Oligocene-Tortonian reconstructions of the western-central Mediterranean realm that report a linear migration of backarc extension, our results support a geodynamic model characterized by a progressive deformation of the forearc extension in the overriding plate.

Mesozoic and Early Cenozoic sediment influx and morphology of the Mozambique Basin

The Mozambique Basin is one of the oldest extensional sedimentary basins developed along the eastern African margin. The basin hosts a continuous record of sediments since the Jurassic separation of Antarctica from Africa. The objectives of this study were to extend the regional stratigraphic framework north of the Zambezi Delta into the deep abyssal plains and review the early evolution of the Mozambique Basin using nine multi-channel seismic reflection profiles.We identify six major stratigraphic units that were deposited in Jurassic, Early Cretaceous, Late Cretaceous, Paleogene, Neogene and Quaternary times. Mesozoic sedimentation rates of 5–10 cm/kyr and 1–3 cm/kyr during the Paleogene are calculated in the deeper basin. The presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until Early Cretaceous times. The Mesozoic sediments have a high seismic velocity that exceeds 4.5 km/s, except in a distinct Early Cretaceous low-velocity (3.7 km/s) zone that may indicate the presence of undercompacted, overpressured shales. In spite of the fact that the Zambezi catchment was much smaller in pre-Miocene times, the high Late Cretaceous sedimentation rates can be attributed to rapid denudation of the African continent after a major tectonic uplift episode at approximately 90 Ma. Increased sediment influx into the basin from the Zambezi in Late Cretaceous times resulted in the formation of an elongated submarine fan lobe into the Mozambique Channel north of Beira High. Strong north-south bottom currents commenced within the channel in Late Cretaceous times, forcing the aggradation of sediments on the southern flank of the lobe. In addition, we observe several current-controlled sediment deposits in the deeper basin that are influenced by north-south bottom currents. Low Paleogene sedimentation rates are attributed to a sediment-starved basin during a relative quiet tectonic phase onshore.

Late Cretaceous-Oligocene magmatic record in southern Mexico: The case for a temporal slab window along the evolving Caribbean-North America-Farallon triple boundary

Continental magmatism in southern Mexico is expected to record the eastward displacement of the Farallon-North America-Caribbean triple junction. However, a trench-transversal belt of magmatism in the central western Guerrero State does not fit into a regular pattern of arc migration and reorientation following the formation of the WSW trending Acapulco trench in the Cenozoic. We revised the magmatic pattern of southern Mexico using an updated database and new laser ablation inductively coupled plasma–mass spectrometry sensitive high-resolution ion microprobe, Ar-Ar ages, and geochemical and geologic data for the coastal part of the anomalous Guerrero belt. Our data reveal a persistent magmatic activity between ~75 and 35 Ma, with a changing character at the Paleocene-Eocene boundary (circa 56 Ma). Late Cretaceous-Paleocene granitoids have an adakitic signature imprinted by stable garnet in the source and show no plagioclase fractionation, indicative of wet and oxidized magmas. Eocene rocks consist of an almost bimodal suite of plutonic bodies covered by a succession of mafic lavas. Granitic plutons show plagioclase fractionation and flat middle rare earth element-heavy rare earth element; gabbros have a tholeiitic character, indicative of dryer and more reduced magmas. They appear later and were emplaced at shallow depth in an extensional sedimentary basin. We interpret the magmatic record of the Guerrero belt as the response to two concurrent processes: (1) a temporal window in the Farallon slab induced by the concurrent subduction along the two noncollinear trench segments of southern Mexico (WNW trending) and of the Chortís block (NNW trending) and (2) a scissor-like transtensional rifting associated to counterclockwise rotation and eastward motion of the Chortís block.

A multidisciplinary study of the emplacement mechanism of the Qingyang–Jiuhua massif in Southeast China and its tectonic bearings. Part II: Amphibole geobarometry and gravity modeling

The Late Mesozoic geology of the Lower Yangtze area is characterized by extensional sedimentary basins, numerous granitic plutons and several world-class ore deposits. Regionaly, the coeval tectonic and geodynamic framework remains inadequately investigated. In order to provide a more comprehensive understanding of the regional tectonic context, we performed a multidisciplinary study of the emplacement mechanism of the Qingyang–Jiuhua massif. It consists of the granodioritic Qingyang and the monzogranitic Jiuhua plutons, U–Pb dated at 142±1.0Ma and 131±2.6Ma, respectively. Biotite and amphibole cooling ages are 5–8myr younger. Results deduced from field structural observation, petrographic and magnetic fabrics, paleomagnetism show that this massif was probably vertically emplaced by permissive intrusion coeval with weak regional extension. However, detailed information on the characteristics of this tectonic event and its emplacement depth was not documented. Therefore, we carried out an investigation of amphibole geobarometry and gravity modeling in order to address the emplacement mode of the plutons within their regional tectonic framework. Amphibole data show that the Qingyang–Jiuhua massif was emplaced in the upper crust at a depth between 5 and 12km. Gravity modeling indicates that the massif is laccolithic. It is characterized by several NE–SW-striking linear thickening zones that we interpret as the feeder roots of the massif. These results led us to conclude that (1) the Qingyang–Jiuhua massif was fed by vertical, NE–SW striking tension gashes; (2) consequently the late Early Cretaceous tectonics in the study area was characterized by NW–SE extension.

Was there an Ediacaran Clymene Ocean in central South America?

Previous studies have proposed that a major suture resulted from the collision between the Amazonian and Sao Francisco-Congo cratons during the Cambrian, following the closure of a supposed Clymene Ocean. The proposal tentatively located this ocean along the Araguaia and Paraguay belts at the eastern margin of the Amazonian Craton, and its southern extension reached the Pampean belt in Argentina. In the present study we will argue that the existence of Ediacaran-Cambrian oceanic lithosphere in central South America is highly unlikely. West Gondwana was assembled during the convergence between the Amazonian, West African, Sao Francisco-Congo and Rio de La Plata cratons as well as the Saharan Metacraton, leading to the closure of the Goias-Pharusian Ocean during the Neoproterozoic. Final closure and continental collision resulted in the development of the Transbrasiliano-Kandi mega-shear zone that cuts through several mobile belts, but leaves the cratonic areas totally untouched. Consistent results of radiometric dating along the Transbrasiliano (TB) mega-shear in South America and of metamorphic rocks of the Brasilia Belt have indicated that the Neoproterozoic collision finished at ca. 620 Ma. After isostatic uplift, cooling, and denudation, between 590 and 500 Ma, emplacement of undeformed K-rich postorogenic granites represented the main tectonic event. At this time or afterwards, a series of small extensional sedimentary basins formed in graben troughs, most of which are within the TB tectonic corridor. They all were of extensional character, contrasting clearly with the convergent tectonics occurring within the coeval Pampean Orogen in Argentina. The main arguments showing that an Ediacaran to Cambrian oceanic closure in central Brazil is untenable include: (i) the assembly of West Gondwana was completed by ca. 600 Ma, when the convergence between the Amazonian, Sao Francisco and Rio de La Plata cratons had already ended. After this, there is no geological evidence of an oceanic lithosphere (for example, ophiolites, magmatic arcs, et cetera), ruling out the possible existence of an Ediacaran or Cambrian Clymene Ocean in Central Brazil; (ii) the Gurupi and Araguaia belts in Brazil, as well as the Bassaride and Rokelide belts in West Africa, are regarded as aulacogenic-type systems formed within an intraplate tectonic setting. Their tectonic history precedes the collision between the Amazonian and Sao Francisco-Congo cratons, as demonstrated by the linear structures of the Transbrasiliano megashear which truncate the N-S structural trends of the Araguaia Belt; (iii) there is a close correlation between the Corumba Group of the Paraguay Belt in Brazil and the Arroyo del Soldado Group in Uruguay. These sedimentary sequences belonged to the same Ediacaran continental shelf and this is a powerful indicator for an Ediacaran connection between the Amazonian and Rio de La Plata cratons, which precludes the existence of a wide ocean (for example, the Clymene) between them. On the other hand, the tentative correlation between the Sierras de Cordoba and the Paraguay Belt cannot be accepted, because these are far apart and there is no similarity in lithology, metamorphism, or structural trends; (iv) the Puga paleopole is the most important evidence for the hypothesis of the Cambrian Clymene Ocean, however the age of about 600 Ma for this paleopole, taken on the basis of Sr and C isotopes, is loosely constrained. In addition this is located at low latitude, not far from the present pole, and therefore could be related to a younger remagnetization; (v) the Pampean Orogen is made up of medium- to high-grade metamorphic rocks constrained between 560 Ma and 520 Ma and therefore was tectonically active during most of the Cambrian. However, at this time, an oceanic lithosphere is not evident in the vicinity of the Paraguay belt, and in central Brazil extensional rather than convergent tectonic processes have been observed.

Geological history of Britain and Ireland (second edition), edited by Nigel Woodcock and Rob Strachan. Wiley-Blackwell, Chichester, 2012. No. of pages: 442. Price: UK£37.50. ISBN 978-1-4051-9381-8 (paperback).

The theme of Geological History of Britain and Ireland is clear from its title, but it is pleasing that it covers some aspects of the offshore geology as well. The preface correctly identifies it to be aimed primarily at an undergraduate audience. It will, however, also provide a useful summary text for those with more experience. The editors, who contribute more to the writing than anyone else, were assisted by a team of 14 other authors. The book falls into two clear parts of approximately equal length. The first half covers Precambrian evolution through to the Variscan orogeny (ending in the Carboniferous) and is dominated by a description of areas affected by contractional tectonics. The second half covers the Carboniferous to present-day history and is dominated by a description of areas that lie within or adjacent to extensional sedimentary basins. The first half of the book largely succeeds in its objectives, but the second half sadly comes up short in a modern context. The Precambrian to Variscan part of the book is written mainly by the editors and close collaborators. They are writing about geographical areas and periods of geological history in which they are acknowledged experts, and this comes through in the text which is authoritative and up to date. My favourite chapter in the book covers the Variscan orogeny, written by Lawrence Warr. This chapter is a succinct and lively summary, and the author clearly understood his brief. Honourable mention also goes to Nigel Woodcock's chapter on the Acadian orogeny, which probably more than any other introduces new concepts to describe the geological evolution of parts of the UK which have previously been intensively studied and documented. This first part of the book does take until page 49 before the description of the oldest rocks in Britain and Ireland (the Lewisian Complex) begins; and in the context of the book's title, I felt that much of the preceding 48 pages could be omitted or condensed. Descriptions of tectonically-influenced areas are also heavily reliant on the ‘terrane’ concept. I have always felt, however, that, at the scale of island Britain, the terrane concept is not really appropriate, originating as it does from the entire length of the American–Canadian Pacific coastline. This, then, brings us to the second half of the book, describing the Carboniferous-and-younger history. Overall, this left me very disappointed, because I do not feel that the text brings this half of the book to life. It is dominated by rather uninspiring descriptions of stratigraphic sequences from particular areas, in a style which seems little different from any of the traditional stratigraphy texts from the past 35 years or more. Our current understanding of the dynamics of sedimentary basins can achieve better results and communication than this. Perhaps the main reason that the text is rather turgid and descriptive is because it is not helped by the supporting illustrations. We are working at a time when seismic sections are routinely available for publication, and are the primary means of illustrating both the structural and large-scale depositional geometries of sedimentary basins. Yet, in describing the Carboniferous-and-younger evolution, only two seismic sections are included and one of these appears to be from northern France! The absence of seismic sections is compounded by a corresponding absence of interpretive cross-sections. Clearly, the authors did not attend my own undergraduate stratigraphy classes which always concluded with a requirement to draw a cross-section through the area of study. What we get instead of seismic sections and cross-sections are many pages devoted to generalized stratigraphic logs. These really do not assist the description of an area's geological history to any great extent. Perhaps even more astonishingly, it seems that the modern stratigrapher feels it more important to be able to characterize and illustrate the isotope content of the rocks they are working on rather than understand their regional-scale depositional setting in any form of dynamic context. The focus of the second half of the book on describing the basic rock successions, rather than the regional geological history, also leads to some major omissions such as Arthurs Seat volcano in Edinburgh, perhaps the most iconic and historically-significant piece of Carboniferous geology in the UK. In conclusion, what we have here is a book of two halves, in which the weak second half lets down the strong first half. If there is to be a third edition I would strongly urge the editors to reappraise the approach to the second half of the book, and perhaps draft in more authors with an industry background who are used to looking at the evolution of sedimentary basins in a dynamic, rather than simply stratigraphic, context.