Present-day Orientation Research Articles

Precise Pb-Pb baddeleyite geochronology, geochemistry, and Sr-Nd isotopic constraints on the 2.36 & 1.88 Ga mafic dykes from the Bastar craton, India: Implications for their petrogenesis in conjunction with the Dharwar mafic dykes

This research focuses on the geochronology and geochemistry of WNW-NW-NNW trending Paleoproterozoic mafic dykes from the Bastar craton in India. The weighted mean 207Pb/206Pb ages of baddeleyite grains obtained from the Tuskal, Pakhanjore, and Gariaband regions were 2365.25 ± 0.54 Ma, 2365.70 ± 0.48 Ma, and 2363.85 ± 0.79 Ma, respectively. Baddeleyite grains from a mafic dyke near Narayanpur-Kondagaon yielded a weighted mean 207Pb/206Pb age of 1886.5 ± 1.8 Ma. In the TAS (Total Alkali Silica) diagram, the 2.36 Ga and 1.88 Ga mafic dykes have basalt to basaltic andesite composition. Dykes from both age groups are represented by the tholeiitic magma series in the AFM diagram. In the spidergrams, the dykes show LREE and LILE enrichment, with the 2.36 Ga dykes showing flat HREE and the 1.88 Ga dykes showing slightly inclined HREE patterns. The REE non-modal batch melting model indicates derivation of the 2.36 Ga mafic dykes from a shallower spinel lherzolitic source, whereas the 1.88 Ga mafic dykes were derived from a deeper spinel-garnet lherzolitic mantle source. The 2.36 Ga dykes have near chondritic to slight super chondritic initial Nd isotopic values (εNd(2.36 Ga) = −0.95 to 2.36), whereas the 1.88 Ga dykes have sub chondritic initial Nd isotopic values (εNd(1.88 Ga) = −2.37 to −9.67). Present-day orientations and similar precise ages of the mafic dykes reported from the Bastar and Dharwar cratons indicate the counter-clockwise rotation of the Bastar craton during the Paleoproterozoic.

Location of the Lhasa terrane in the Late Cretaceous and its implications for crustal deformation

The Cenozoic crustal deformation of the Asian continent is constrained by the location of its southern margin in the Late Cretaceous, which is, however, affected by potential remagnetization and inclination shallowing of redbeds in the Linzhou Basin. Our paleomagnetic results from the Shexing Formation redbeds in the Late Cretaceous yield a mean paleo-inclination of ~21.5°, consistent with the recalculation of previous data. Rock-magnetic, paleomagnetic and petrographic analyses indicate that the redbeds have recorded the original paleomagnetic field and have not suffered from significant inclination shallowing. The paleopole from 70 redbeds sites is at 71.8°N, 281.5°E with A 95 = 2.6°, yielding a paleolatitude of ~12.0 ± 2.6°N at ~70–91 Ma for the Linzhou Basin. This result indicates a latitudinal convergence of 1264 ± 477 km north of the sampling area within Asia since the Late Cretaceous. Combined with previous data, our results show that the southern margin of Asia was at ~11–15°N in the Late Cretaceous, aligning in a WNW-ESE direction, which is similar to its present-day orientation. A review of available data shows that the sharp bending of the southern margin of Asia around the eastern Himalayan syntaxis (EHS) is probably due to different crustal movement patterns around the EHS. The shape of the southern margin of Asia determines the arc-shaped orientation of Himalayan orogen. • The Shexing Formation redbeds have recorded the prefold magnetizatioin. • The southern margin of Asia was oriented in a WNW-ESE direction at ~12°N. • Different movement around the eastern Himalayan syntaxis make a sharp turn. • The southern margin of Asian plays a primary role in shaping the Himalayan orogen.

Improved full waveform moment tensor inversion of Cratonic intraplate earthquakes in southwest Australia

SUMMARYIn contrast to global observations in stable continental crust, the present-day orientation of the maximum horizontal stress in Western Australia is at a high angle to plate motion, suggesting that in addition to large-scale plate driving forces, local factors also play an important role in stress repartitioning. As a reliable stress indicator, full waveform moment tensor solutions are calculated for earthquakes that occurred between 2010 and 2018 in the southern Yilgarn Craton and the adjacent Albany-Fraser Orogen in southwestern Australia. Due to regional velocity heterogeneities in the crust, we produced two geographically distinct shear wave velocity models by combining published crustal velocity models with new ambient noise tomography results. We applied a full waveform inversion technique to 15 local earthquakes and obtained 10 robust results. Three of these events occurred near Lake Muir in the extreme south of the study area within the Albany-Fraser Orogen. The focal mechanism of the 16th September 2018 Lake Muir event is thrust; two ML≥ 4.0 aftershocks are normal and strike-slip. Our results are consistent with field observations, fault orientations inferred from aeromagnetic data and surface displacements mapped by Interferometric Synthetic Aperture Radar which are all consistent with reactivation of existing faults. The other seven solutions are in the southeastern Yilgarn Craton. These solutions show that the faulting mechanisms are predominantly thrust and strike-slip. This kinematic framework is consistent with previous studies that linked active seismicity in the Yilgarn Craton to the reactivation of the NNW–SSE oriented Neoarchean structures by an approximately E–W oriented regional stress field. Our results suggest that the kind of faulting that occurs in southwest Australia is critically dependent on the local geological structure. Thrust faulting is the dominant rupture mechanism, with some strike-slip faulting occurring on favourably oriented structures.

Kinematics and U-Pb zircon ages of the sole metamorphics of the Marmaris Ophiolite, Lycian Nappes, Southwest Turkey

ABSTRACTIn the eastern Mediterranean, the Lycian Nappes are found in the structurally uppermost position in the Anatolide-Tauride belt related to the closure of the Neotethys. In Western Turkey, the Marmaris Ophiolite with the metamorphic sole occupies the uppermost tectonic position in the Lycian belt. The metamorphic sole is represented by discontinuous tectonic slices composed of amphibolites, phyllites, micashists and quartzo-feldspathic micaschists. Zircons from the micashists and quartzo-feldspathic micaschists display dark cores and rims. The cores yield ages between 229 and 175 Ma, inner rims yield ages between 153 and 143 Ma and the outer rims show a concordia age of 96.7 ± 0.79 Ma. In terms of their Th/U ratios, the cores and inner rims indicate igneous origin, whereas the outer rims indicate accretion during metamorphism. By dating of these zircons, the deposition time for the protolith of micaschists and quartzo-feldspathic micaschists could be constrained as the Early Cretaceous. Present-day orientation of the kinematic data from the sole metamorphics and the uppermost part of the Karabörtlen formation clearly suggest a top-to-the NE sense of shear. By taking into account the 25º–30º anticlockwise post-emplacement rotation of Southwest Turkey, it follows that the Lycian Nappes were emplaced eastward onto the Menderes Nappes. This tectonic model disagrees with the previous tectonic models suggesting northward or southward movement of the Lycian Nappes onto Menderes Nappes.

Triassic to recent tectonic evolution of a crestal collapse graben above a salt-cored anticline in the Glückstadt Graben/North German Basin

In this study we investigate faulting above a salt wall in the Glückstadt Graben/North German Basin. Two supra-salt faults are mapped from coast to coast over a distance of 6–9km based on offshore and onshore seismic data. These faults form a ca. 2km wide crestal collapse graben and pierce the seafloor. Salt wall evolution started in the early Late Triassic to Early Jurassic due to regional extension and resulting sub-salt faulting. The salt wall was eroded following exposure to costal and sub-aerial erosion by the regional Mid-Late Jurassic to Early Cretaceous uplift. Late Cretaceous to Early Paleogene compressional tectonics reactivated the vertical salt movement and shortened the salt wall, creating a salt-cored anticline with the crestal collapse graben above. The supra-salt faults were reactivated between the Late Eocene and Middle Miocene when the principal horizontal stress orientation changed from a NE–SW to a NW–SE, the present-day orientation. Stratigraphic data indicate that these faults moved mainly in the Cenozoic. Several observations strongly suggest that the faults continued developing during the Pleistocene until today: (i) the Pleistocene Unconformity is concave upwards and cut by faults; (ii) growth strata within the marine Holocene deposits above the graben imply recent tectonic movements; (iii) onshore high-resolution P-wave vibroseis data of the south-eastern Eckernförde Bay suggest about 10m of faulted Holocene strata; and (iv) marine seismic data show the faults piercing the seafloor. We suggest that the recent salt tectonics and upward propagation of supra-salt faults resulted from differential ice-sheet loading. That effect on the salt wall stopped once ice grew over the whole structure, at which time the wall subsided because of ice loading. The salt wall and faults were reactivated again once the ice front retreated so that the ice loaded only one side of the structure.

Asymmetrical magnetic fabrics in the Egersund doleritic dike swarm (SW Norway) reveal sinistral oblique rifting before the opening of the Iapetus

The 616 ± 3 Ma (Ediacaran) Egersund doleritic dike swarm cuts across the Rogaland anorthosite province and its granulitic country rocks, in SW Norway. The structure of eight out of eleven main dikes of the swarm was investigated using the anisotropy of magnetic susceptibility (AMS) technique. Thermomagnetic data and values of the bulk magnetic susceptibility reveal a magnetic mineralogy dominated by Ti-poor titanomagnetite. Magnetic fabric and global petrofabric are coaxial, except in sites strongly affected by hydrothermal alteration, as demonstrated through image analysis. Asymmetrical dispositions of the magnetic foliation and lineation support the existence of a syn-emplacement, sinistral strike-slip shearing resolved on dike walls. Such asymmetrical fabrics are attributed to a transtension tectonic regime, in a context of oblique extension during the continental rifting phase which preceded the opening of the Iapetus Ocean along the SW margin (present-day orientation) of Baltica.

Integration of palaeomagnetic data, basement-cover relationships and theoretical calculations to characterize the obliquity of the Altomira–Loranca structures (central Spain)

Abstract The main objective of this work is to characterize the structures belonging to the Altomira Range and Loranca Basin (SW Iberian Chain, Central Spain) in terms of understanding their present-day orientation, highly oblique with respect to the NW–SE orientation of adjacent structures of the Iberian Chain. The Altomira and Loranca fold and thrust belts present a slightly curved geometry with a general north–south orientation at their central sector, and structures oriented NNE–SSW and north–south to NNW–SSE at their northern and southern sectors, respectively. Palaeomagnetic data from Middle Eocene–Lower Miocene rocks (including clays, fine sandstones and limestones) reveal the absence of vertical-axis rotations in the central sector of the studied area, where structures are oriented north–south, and up to 17° of clockwise and 21° of anticlockwise vertical-axis rotations in the northern and southern sectors, respectively. These data are supported by calculations of the theoretical vertical-axis rotations from shortening estimates and basement-cover structural relationships. This approach highlights the importance of integrating different datasets to characterize the obliquity of fold and thrust belts. Supplementary material: Complete information about the paleomagnetic sites and directions is available at http://www.geolsoc.org.uk/SUP18841

Upper Katian (Ordovician) bentonites in the East Baltic, Scandinavia and Scotland: geochemical correlation and volcanic source interpretation

AbstractAltered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob's Linn bentonites from southern Scotland suggest a common volcanic source in Katian times.

The Beni Bousera Peridotite (Rif Belt, Morocco): an Oblique-slip Low-angle Shear Zone Thinning the Subcontinental Mantle Lithosphere

Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW-SE stretching lineation that progressively rotates towards a NNE-SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet-spinel mylonites, Ariegite subfacies fine-grained porphyroclastic spinel peridotites, Ariegite-Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2*0 GPa in garnet-spinel mylonites to 1150°C at 1*8 GPa in the Seiland domain. A pervasive diffuse dunitic-websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for <200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km−1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ∼90 to ∼60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.

Late Ordovician cooling event: Evidence from the Siberian Craton

The Upper Ordovician of the Tungus Basin on the Siberian Platform is represented by cool-water carbonates. Onset of cool-water conditions starts at the Darriwilian, and it is associated with widespread phosphatization, especially in the Volginian, Kirensko-Kudrinian and Chertovskian regional stages (mid-Darriwilian–lower Sandbian). The shift from tropical-type to temperate-type carbonates was preceded by destruction of the warm-water “carbonate factory” and a large input of siliciclastic material (Baykit Sandstone) during the Vikhorevian and Mukteian regional stages (lower to mid-Darriwilian). Wide distribution of temperate-type carbonates across the Siberian Platform, located in the low latitudes in the Ordovician time, can be explained by upwelling of cold oceanic waters and their penetration into epicontinental seas. The same situation was recorded for the Upper Ordovician of the North American Platform. Newly discovered K-bentonite beds in the uppermost Sandbian–Katian of the Tungus Basin are also associated with cool-water carbonates. The K-bentonite beds point to extensive volcanism on or near the western (in present-day orientation) margin of the Siberian Craton in the Late Ordovician time. Timing of the volcanism is surprisingly close to the period of volcanic activity of the Taconic Arc near the margin of Laurentia. Both upwelling and volcanism seem to be activated by the same plate-tectonic reorganization, and both these processes seem to play an important role in a global, later Ordovician cooling terminated by the Hirnantian glaciation.

'A Giant with Clay Feet': Quebec Students and their Historical Consciousness of the Nation

The aim of this paper is to explore French Canadian (Québec) students' historical consciousness of the nation through the lens of Social Identity Theory (SIT). Relying on a sample of 142 Quebéc's historical narratives written by Francophone Québécois students, the paper revisits findings from a previous study on the historical consciousness of young Québécois. Informed by SIT principles, our narrative analysis shows how most Franco-Québécois categorize the past in homogenous categories (e.g., the imperialist Anglophone; the surviving Francophone) and frame their stories into particular modes of present-day orientations. Implications of this studyfor history education are also discussed.

Petrogenesis and tectonics of late Permian felsic volcanic rocks, eastern Qiangtang block, north-central Tibet: Sr and Nd isotopic evidence

The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO2 concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (87Sr/86Sr) i of 0.70319–0.70547, and ϵNd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean.

Amalgamation of the North China Craton: Key issues and discussion

Geological and geophysical data indicate that the Precambrian basement of the North China Craton (NCC) formed by amalgamation of a number of micro-continental blocks. The number of blocks, when they existed and how they came together are controversial, and in particular the following issues are disputed: (1) the timing of collisional event(s) leading to the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen (TNCO); (2) the polarity of the subduction between the Eastern and Western blocks; (3) the validity of an old continental block (Fuping Block) that collided with the Eastern Block at ∼2.1Ga; (4) the tectonic setting of the northern margin of the NCC in the Paleoproterozoic; (5) the tectonic nature of high-pressure (HP) and ultrahigh temperature (UHT) granulite-facies events in the Khondalite Belt of the Western Block; and (6) the tectonic setting of the Paleoproterozoic Jiao-Liao-Ji Belt in the Eastern Block.Analysis and integration of available stratigraphic, structural, geochemical, metamorphic and geochronologic data enable the development of an internally consistent and coherent model for assembly and stabilization of the various Archean blocks of the NCC in the Paleoproterozoic. All metamorphic ages obtained for the TNCO are around 1.85Ga, which establishes that the final amalgamation of the Western and Eastern blocks of the craton occurred at ∼1.85Ga. The TNCO is characterized by a fan-shaped pattern of structural features, with the top-to-the-NW and top-to-the-SE thrusting in the northwest and southeast, respectively. This pattern does not constrain subduction polarity for the collisional assembly of the Eastern and Western blocks. Structures in lithospheric mantle and asthenosphere in the TNCO have been significantly modified/replaced in the Mesozoic and Cenozoic, and hence the present-day orientation of these structures, even if they relate to Paleoproterozoic assembly of the craton cannot be used to infer associated subduction polarity. There are no unique structural data or available metamorphic data to supporting the existence of an old continental block that intervened between the Eastern and Western Blocks, which collided with the Eastern Block at ∼2.1Ga. Available data are also inconsistent with the existence of the Paleoproterozoic Inner Mongolia-North Hebei Orogen along the northern margin of the NCC that formed through accretion of an exotic arc at ∼2.3Ga and incorporated into the Paleoproterozoic Columbia (Nuna) Supercontinent at 1.92–1.85Ga. We interpret the north Hebei portion of this inferred orogen as part of the TNCO, and the Inner Mongolian portion as an independent continental block (Yinshan Block). This block is separated from the Ordos Block by the Paleoproterozoic Khondalite Belt. The high-/medium-pressure granulite facies metamorphic event in the Khondalite Belt is considered to have resulted from collision between the Yinshan and Ordos blocks to form the Western Block at ∼1.95Ga, whereas the ∼1.92Ga UHT metamorphism within the belt was related to the underplating or intrusion of mantle-derived magmas during the post-collisional extension. The Jiao-Liao-Ji Belt in the Eastern Block likely formed through Paleoproterozoic rifting to form the Longgang and Langrim blocks, and subsequent basin closure and collision in the period 2.2–1.9Ga.

Testing the transpression hypothesis in the western part of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming

A system of subvertical, northeast-striking shear zones collectively called the Cheyenne belt comprises the fundamental boundary between the Archean Wyoming province and the Paleoproterozoic Colorado province in the Medicine Bow Mountains of southeastern Wyoming. These shear zones are generally interpreted as a mid-crustal thrust system rotated to its present-day subvertical orientation during late-stage, orogen-scale folding. However, the map geometry, presence of large domains of S tectonites, and vertical mineral lineations closely match numerical simulations of transpressional shear zones. This transpression hypothesis is tested using two detailed case studies that integrate detailed geologic mapping, kinematic analyses of S and L-S tectonites, quartz crystallographic fabric analyses, and deformation mechanism analyses of shear zones in the western part of the Medicine Bow Mountains. These data point toward general shear deformation that accommodated two coeval deformation components: (1) southeast-side-up, dip-slip or southeast-side-up/dextral, oblique-slip motion and (2) foliation-normal shortening. According to the broadest definition, these are transpressional shear zones. However, the strike-slip component is small, and these data do not require any significant oblique plate motion during the formation of the Cheyenne belt shear zones. The lack of evidence for overprinting of initial southeast-side-up fabrics by horizontal shortening, the near parallelism of the subvertical shear zones throughout the upper and middle crust, and the presence of subvertical mafic dike swarms in Archean basement rocks immediately adjacent to the Cheyenne belt all indicate that the Cheyenne belt shear zones were not rotated into their present-day subvertical orientations during late-stage deformation. Instead, we interpret the shear zones as a ∼1,750-Ma steeply dipping stretching fault system between the penetratively deformed young, hot, rheologically weak rocks of the Colorado province and the old, cold, rheologically strong Archean rocks north of the belt.

Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia*

In easternmost Australia, the New England Orogen contains a geological record dominated by subduction-related rocks, with plate convergence during the Late Devonian to Triassic being related to a west-dipping subduction system, assuming present-day orientation, at the boundary of eastern Gondwanaland and the Panthalassan Ocean. A well-preserved Late Paleozoic accretionary wedge contains deep-marine turbidites deposited as trench fill, plus infaulted slices of oceanic crust. The turbidites are mostly first-cycle, immature, quartz-poor, volcanic-derived sedimentary rocks, some of which contain detrital hornblende, along with less-common quartz-rich sandstones to the east. In this study, detrital zircons from sandstones in various tectonic blocks of the New England Orogen are dated by the U–Pb SHRIMP and LA-ICPMS techniques and detrital hornblendes by the Ar–Ar technique to constrain the age and provenance of sedimentary rocks in the accretionary wedge. All samples, except two quartz-rich sandstones from the northern Shoalwater Formation, have maximum depositional ages of 355–316 Ma, indicating that the accretionary wedge evolved over a period of at least 40 Ma, with principal sources from a contemporaneous active continental margin volcanic arc. Quartz-rich sandstones from the easternmost part of the accretionary wedge (Shoalwater Formation and eastern Beenleigh Block) contain a greater range of individual detrital zircon ages from Late Paleozoic to Archean (several individual grains >3000 Ma). These ages indicate that, although detritus from Carboniferous volcanic arc sources was involved, quartz-rich detritus mostly derived from the continental interior dominated the depocentres. We suggest that these quartz-rich sandstones accumulated from longitudinal transport along the trench, like the modern-day Barbados Ridge accretionary wedge, along with breaching of the marginal arc by streams draining the continental interior.

LA-ICP-MS U–Pb zircon geochronology of early Neoproterozoic mafic-intermediat intrusions from NW margin of the Yangtze Block, South China: Implication for tectonic evolution

Proterozoic magmatism along the margin of the Yangtze Block is extensive and provides important evidence in the plate tectonic reconstructions for the Proterozoic. The Bikou Terrane on the northwestern margin of the Yangtze Block consists of complex Archean to Paleozoic metamorphosed sedimentary and volcanic sequences, intruded by several mafic-intermediate bodies. Three major intrusions are dated using laser ablation ICP–MS U–Pb of zircons. This dating yielded ages 884 ± 5.5 Ma, 884 ± 14 Ma and 877 ± 13 Ma for the Pingtoushan diorite, Guankouya diorite and Liujiping gabbro, respectively. These are the oldest intrusions found along the margin of the Yangtze Block. Geochemical analyses of these mafic-intermediate intrusions show they have negative Nb–Ta anomalies that are best interpreted as having been formed in an arc setting. The older magmatism and unique rock associations of the Bikou Terrane suggest that it was an isolated terrane that existed in a marginal sea to the Yangtze Block. The eastward subduction (present-day orientation) resulted in early arc magmatism (∼880 Ma) within the Bikou Terrane and lasted to ca. 750 Ma. This study supports the model that the Yangtze Block was surrounded by ocean and developed arc magmatism along its western margins in Neoproteroic time, with continent-directed subduction that commenced in the early Neoproterozoic. The Bikou Terrane was subsequently accreted onto the northwest margin of the Yangtze Block at about 750 Ma.

The Yanbian Terrane (Southern Sichuan Province, SW China): A Neoproterozoic arc assemblage in the western margin of the Yangtze Block

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important elements in the reconstruction of Neoproterozoic plate tectonics. The Yanbian Terrane in the western margin of the Yangtze Block is a typical arc assemblage composed of a flysch-type sedimentary sequence intruded by gabbroic and granodioritic plutons. The sedimentary sequence consists chiefly of tuffaceous material with interlayered chert, sandstone, and pillow basalts. Laser ablation ICP-MS U–Pb dating of detrital zircons from the sandstones yield ages as young as 840 Ma. The Gaojiacun and Lengshuiqing mafic intrusions are dated at 812 ± 3 Ma and 806 ± 4 Ma, respectively, using the SHRIMP zircon U–Pb technique. Geochemical data show that both the Gaojiacun and Lengshuiqing intrusions have arc signatures, with ɛNd( t) values of +1.5 to +6.0, initial 87Sr/ 86Sr ratios of 0.705–0.706 and pronounced negative Nb–Ta and Zr–Hf anomalies. Their geochemical variations are best explained by fractional crystallization without major crustal contamination. The Yanbian Terrane represents a typical arc assemblage formed on the western edge of the Yangtze Block during Neoproterozoic time. The sedimentary sequence was deposited in an oceanic setting, probably in a back arc basin environment. The depleted, subduction-modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block.

The geodynamic setting of the Phanerozoic basins of Africa

The Pan-African event assembled the supercontinent of Gondwana during the Late Proterozoic, and is manifest within the African plate by an anastomosing pattern of tectonic sutures and mobile belts. The development of African basins during the Phanerozoic can be thought of as relating to the polyphase break-up of Gondwana, which was accomplished, in general, by reactivation along the Pan-African sutures. The present African plate occupied a relatively central position in Gondwana, with only the north-westernmost (Rif-Tell/Atlas) and southernmost (Cape Fold Belt) tips of the plate (present-day orientation) exposed directly to convergent plate margin activity. Although the African plate can be generally considered to have undergone extension throughout the Phanerozoic, compressive tectonism associated with the closure of the Paleotethys (Hercynian Rif-Tell) and the Neotethys Oceans (Alpine Atlas) and subduction of the Paleopacific plate (Hercynian Cape Fold Belt) are exceptions. Rifting associated with the break-up of Gondwana was dominant from the Late Carboniferous onwards, and basins developed as a result of this extension both at the margins and within the continental African plate. Rifting began in earnest during the Early-Middle Jurassic, when East and West Gondwana separated, and the central Atlantic Ocean began to open. From the Late Jurassic to Early Cretaceous, the South Atlantic opened progressively northwards, and intraplate extension led to the development of the West, Central and East African rift basins. The final episode of rifting took place between the Late Eocene and Early Miocene, and opened the Dead Sea–Red Sea–Gulf of Aden basins, with rifting in the East African Rift continuing to the present day. During the Phanerozoic, Africa (and Gondwana) migrated across the South Pole northwards to its present latitude, and the fill of preserved basins reflects a concomitant change in paleoclimate. Rises in sea level, in part related to the demise of Gondwanan glaciers during the Paleozoic, locally flooded the African plate. The rich fossil record of life on the African plate during the Mesozoic and Cenozoic can also be coupled with a more benevolent climate at lower latitudes as Gondwana passed away from south polar regions. The presence of plumes beneath the African plate during the Mesozoic and Cenozoic (the Karoo and Afar plumes, respectively), coupled with an absence of slab pull at the margins of the young post-Gondwana oceanic crust, has rendered Africa in a relatively stationary position since the Mesozoic, with only a slow rotation of the plate as the Central and South Atlantic, Indian Ocean and Red Sea have opened.

Neotectonics of The Netherlands: a review

Earthquakes and vertical land movements inferred from geodetic levelling results demonstrate that the Netherlands is situated on a tectonically active part of the Earth's surface. Tectonic subsidence analyses of the sedimentary records in the basins and the history of tectonic uplift inferred from Meuse river terraces show that the current tectonic activity is part of a deformation phase which began in the late Early Miocene, which we take as the start of the neotectonic period. The neotectonic faulting mode is normal-slip. This is in accordance with the present-day orientation of the maximum horizontal stress, and the vertical orientation of the maximum stress. However, the neotectonic fault zones are reactivated Variscan or older wrench faults. These faults have been reactivated repeatedly during the Mesozoic and Cenozoic in normal and wrenching modes, and therefore represent fundamental crustal weakness zones. As a result, the surficial neotectonic fault pattern resembles the inherited wrenching fault pattern, although the faulting mode is normal-slip. This is illustrated for the area where normal faults have displaced the Meuse fluvial terrace system in the southeastern part of the Netherlands. The neotectonic vertical motions in and around the Roer Valley Rift System have affected the courses of the Rhine and Meuse rivers from Pliocene times until the present. In addition, a fluvial terrace staircase developed along the Meuse river in response to the uplift of the Ardennes-Rhenish Massif and its foreland, for example near Maastricht. Furthermore, small fault scarps are present along segments of the bounding fault zones of the Roer Valley Graben: the Peel Boundary Fault Zone and the Feldbiss Fault Zone. In other parts of the Netherlands the neotectonic vertical motions have no direct expression in the morphology. Examples of possible indirect control on the morphology in these areas occur in glacial landscapes, the Zuiderzee Basin and the coastal zone.

SHRIMP U–Pb zircon geochronological and geochemical evidence for Neoproterozoic arc-magmatism along the western margin of the Yangtze Block, South China

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important in the reconstruction of Neoproterozoic plate tectonics. In the Panxi Belt, adjacent to the eastern margin of the Tibetan Plateau, there are many metamorphic complexes associated with Neoproterozoic granites. These are granitic gneisses of upper greenschist to amphibolite metamorphic facies, which have traditionally been considered the Archean basement of the Yangtze Block, although their origin and age of formation were poorly understood. This study provides the first reliable, SHRIMP U–Pb zircon dating results for the gneissic complexes and the Neoproterozoic granites. Three samples of the Kangding gneissic complex yielded identical ages of 797±10, 795±13 and 796±14 Ma. The Gongcai gneissic complex has zircons dated to be 824±14 Ma with metamorphic rims of 177±3 Ma, whereas the Gezong granite has an older age of 864±8 Ma. Other gneissic complexes include the Miyi complex that has a younger age of 764±9 Ma. Geochemical data show that the Kangding gneissic complex has arc signatures, representing metamorphic products of Neoproterozoic, arc-related acidic plutons. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block. There is a well-defined arc assemblage with an identical Neoproterozoic age along the eastern margin of the Yangtze Block. Thus, during Neoproterozoic time, both the western and eastern margins of the block were active arcs separated by the Trans-Yangtze basin. The Yangtze Block must, therefore, have been an isolated continent, although it was presumably located near the Rodinian supercontinent.