Fold Zone Research Articles

Tomographic Determination of the Upper Crustal Structure in Jiashi Strong Earthquake Swarm Region‐Joint Inversion of Explosion and Earthquake Data

AbstractThe Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units, i.e. the Tianshan fold zone, Pamir tectonic arc and Tarim block. Many strong earthquakes occurred recently in this area. In order to obtain the fine crustal velocity structures of this area, the data from a 3‐D seismic transmission experiment and a portable seismograph network acquired in the Jiashi strong earthquake swarm region in 1997 are processed. Joint inversion of explosion and earthquake data is carried out to reconstruct the 3‐D images of velocity perturbations of the upper crust under the seismic array. The temporary seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths, and the additional 20 seismic stations were deployed in this area to record local earthquakes. The results show that the structure of the upper crust in the study area is characterized by obvious inhomogeneities both laterally and vertically. The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocity with the increase of depth. There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area. It extends downwards to the depth of 16 km at least, and is surrounded by relatively low velocities. VP/VS ratios are also characterized by high values in this region. This kind of structural differences may be related to the occurrence of the Jiashi strong earthquake swarm. It can be found from the P wave velocity images at the depth of 16 km that the Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations. The existence of the high P wave velocity anomaly in the crust may play an important role in the seismogenic process.

Paleomagnetism of Cretaceous rocks in the Jiaodong Peninsula, eastern China: Insight into block rotations and neotectonic deformation in eastern Asia

Although first‐order agreement of paleomagnetic directions between south China and Siberia shows that terranes between them were sutured by Cretaceous times, deformation by ongoing tectonism has continued and been especially focused near the continental margin of eastern Asia. To help evaluate differential rotations and relate them to regional strike‐slip and extensional tectonic regimes, we have conducted paleomagnetic study on Cretaceous sediments and volcanic rocks from the Jiaodong Peninsula south of the Bohai Sea in eastern China bordering the Tan‐Lu Fold Zone (TLFZ). Paleomagnetic results from 73 sites in the (Early Cretaceous) Laiyang and Qingshan groups and (Late Cretaceous) Wangshi Group yield magnetizations of primary or synfolding (mid‐Cretaceous) origins. Large differential block rotations of Early Cretaceous age are identified near the Muping‐Jimo Fracture Zone (MJFZ) dissecting the peninsula although Late Cretaceous declinations are regionally systematic and show post‐Late Cretaceous counterclockwise rotation of ∼14° in the Jiaolai Basin relative to the reference North and South China blocks. Block boundaries are identified as the TLFZ to the west and the MJFZ in the southeast. The eastern Jiaodong Peninsula is rotated clockwise relative to the North and South China blocks by a comparable amount (∼20°) to the southern Korean Peninsula to the east and to the Liaodong Peninsula and Yanji Basin to the north. Although these blocks are unlikely to be tightly coupled, their collective clockwise rotation mirrors counterclockwise rotations north of the Japan Sea and probably resulted from regional extension focused above uprise of asthenosphere beneath eastern Asia during Cretaceous times.

Velocity structure of the lithosphere on the 2003 Mongolian-Baikal transect from SV waves

The S wave velocity distribution in the Earth’s crust and the first two hundred kilometers of the upper mantle is inferred from data of a seismological linear network including 18 broadband stations installed in the framework of the international teleseismic experiment carried out in 2003 in the south of Siberia and in Mongolia. Models were constructed by using P-to-S received function inversion beneath each station. Vertical cross sections of S wave velocities from the surface to depths of 65 and 270 km covering the entire 1000-km profile are constructed by the linear spline interpolation of individual velocity models. The vertical sections are also represented as anomalies relative to the standard velocity model. The most intense low velocity anomalies (from −3 to −6%) in the crust and upper mantle are identified beneath the Sayan, Khamar-Daban, and Khangai highlands and the Djida fold zone and agree both with other geophysical data and with the distribution of Late Cenozoic volcanic fields. The results of this work suggest that the activation of Mongolian-Siberian highlands is largely connected with uplift of the asthenosphere to the base of the crust.

A preliminary study on the lithospheric thermal-rheological structure of the East Qinling orogenic belt

This paper considers the lithospheric rheological structure of the East Qinling orogenic belt to explore its geodynamics. The lithospheric rheological structure was calculated by the constraints of the lithospheric temperature structure. The thermal-rheological stratification structures of the lithosphere in the East Qinling orogenic belt present different features from each other within different tectonic units. The hinterland fault-bounded fold zone (HLZ) and the North Qinling thick-skinned imbricated thrust zone (NQL) in the northern half part of the Qinling orogen, with a temperature of 305°C for the Moho boundary, are characterized by “cold” geotherm, thickened lithosphere and the model C for rheological stratification structure. The South Qinling tectonic zone (SQL), with a mean temperature of 642°C and a high temperature of 826°C for the Moho boundary, has obvious features with the model H of “hot” geotherm, thinned lithosphere and intensive rheological behavior within moderate-lower crust and top of the upper mantle. During post-orogenesis, the NQL, being the convergent frontal region of continental subduction beneath the Qinling orogen by both the North China craton (NC) and Yangtze craton (YZ), is in a coexistence period of a dominantly thickened lithosphere and an initial delamination, and the SQL, probably under pluming, has been developing new delamination and underplating and partial melting within the crust in its axel area and recycling for mass and energy (in the forms of heat transfer and convection) between the crust and mantle.

Measurement of terrace deformation and crustal shortening of some renascent fold zones within Kalpin nappe structure

The Kalpin nappe structure is a strongest thrust and fold deformation belt in front of the Tianshan Mountains since the Cenozoic time. The tectonic deformation occurred in 5―6 striking Meso-zoic-Cenozoic fold zones, and some renascent folds formed on the recent alluvial-proluvial fans in front of the folded mountains. We used the total station to measure gully terraces along the longitudinal to-pographic profile in the renascent fold zones and collected samples from terrace deposits for age de-termination. Using the obtained formation time and shortening amount of the deformed terraces, we calculated the shortening rate of 4 renascent folds to be 0.1±0.03 mm/a, 0.12±0.04 mm/a, 0.59±0.18 mm/a, and 0.26±0.08 mm/a, respectively. The formation time of the renascent folds is some later than the major tectonic uplift event of the Qinghai-Tibet Plateau 0.14 Ma ago. It may be the long-distance effect of this tectonic event on the Tianshan piedmont fold belt.

Soils and landscapes of the Kzyladyr karst field in the Southern Urals

The specificity of soil and landscape conditions in the area of the Kzyladyr karst field in the western folding zone of the Southern Urals is discussed. Unique karst landscapes and soils developing from hard gypsum rocks, gypsiferous weathered mantles, and gypsiferous saline rocks are characterized. These soils strongly differ from the background soils on silicate rocks in their morphology and physicochemical properties. At present, the territory under study is subjected to considerable anthropogenic loads and requires special protection measures. It is suggested that it should be included in the system of specially protected territories. The relic, rare, and unique soils of the karst field should be included in the Red Data soil book.

Atomistic Failure Mechanism of Single Wall Carbon Nanotubeswith Small Diameters

Single wall carbon nanotubes with small diameters (<5.0 Å)subjected to bending deformation are simulated by orthogonaltight-binding molecular dynamics approach. Based on the calculations ofC–C bond stretching and breaking in the bending nanotubes, weelucidate the atomistic failure mechanisms of nanotube with smalldiameters. In the folding zone of bending nanotube, a large elongationof C–C bonds occurs, accounting for the superelastic behaviour. TheC–C bonds parallel to the axis direction of nanotube are brokenfirstly due to the sustained longitudinal stretching strain, givingrising to forming one-notch or two-notch bond-breaking mode dependingon nanotube chiralities. The direct bond-breaking mechanism isresponsible for the brittle fracture behaviour of nanotubes with smalldiameters.

Kraishtides (Kraištiden) 70 years later: Myth or Reality?

The notion of Kraishtides underwent considerable transformations during 70 years since its introduction in the tectonic nomenclature of the Balkan Peninsula. Initially it was introduced for the allegedly youngest (Early Miocene) fold and thrust (orogenic) zone (Krajštiden) situated obliquely to Balkanides and South Carpathians. This notion evolved into a fault belt composed of normal faults that were transformed into reverse faults (Krajštiden-Lineament), and in a lineament-geosynclinal zone (Kraishtid lineament-geosyncline) of repeated opening (in latest Jurassic, Palaeogene and Neogene times) of basins of rift character, and following (“Austrian”, “Pyrenean” and “Savian”) compression and closures. All these meanings of the term may be regarded as redundant now. They are inconsistent with the modern knowledge about the geological structure and evolution of the region, and do not fit the modern tectonic ideas. Only the term “Kraishtid (Strouma) Lineament” could be preserved as a synonym to “Strouma (Kraishtid) fault belt”. However, the word “Kraishtid (Krajštiden)” refers to an orogenic belt (zone), and cannot be preserved as a toponym of fault belt. “Kraishtides” are neither a structural zone with a definite characteristics. The structure of SW Bulgaria is a peculiar mosaic of fold zones and blocks. In the Late Jurassic – Early Cretaceous palaeogeodynamic setting, a part of this area was the westernmost part of the Nish-Troyan flysch trough whereas South of it, the largest area belonged to an uplifted “plateau”. In mid-Cretaceous times (Aptian – Albian), the “Austrian” orogenesis led to the closure of the relics of the flysch trough, and the formation of the complex Morava-Rhodope (Macedonian-Rhodope) zone (in its westernmost part composed of the Morava, Strouma, and Ograzhden units). In Late Cretaceous times, the whole Morava-Rhodope zone represented an area of thickened continental crust (elevated frontal arc, “plateau”) whereas North of it, the Srednogorie volcanic arc and see was opened. In Palaeogene times, the whole Morava-Rhodope zone was again a volcanic island arc with a WNW-ESE trend. Only in Palaeogene and Neogene times, the Strouma (Kraishtid) lineament played the role of a fault belt with considerable dextral strike-slip movements, and repeating rifting in transtensional conditions.

Experimental characterisation in sheet forming processes by using Vickers micro-hardness technique

In this paper, an experimental micro-hardness procedure is proposed to evaluate the evolution of HSLA steel behaviour during each sequence of sheet forming process. As micro-hardness technique offers a reliable inspection, it was retained here to follow the mechanical characteristic changes, which may happen during manufacturing progress. This contribution consists in characterisation of sheet material at different steps: virgin sheet, unreeled sheet, straightened sheet and bent sheet. Measurement performed on virgin HSLA steel showed that material is highly heterogeneous within the sheet thickness. The micro-hardness profiles examined after the bobbing-off step showed a high sensitivity of sheet behaviour to straightening operation that is widely adopted in steel working in order to make sheet sufficiently flat for forming. A level of hardening ratio between virgin material and straightened material has been clearly observed. Moreover, micro-hardness is investigated on bent parts at the fold zone for displaying the mechanical properties modifications under a large gradient deformation. In this way, hardening phenomenon and damage phenomenon, which are generally activated simultaneously for elastoplastic steel, are quantified accurately. Results compared into them gave a good idea about the interaction of process–material during manufacturing.

Fluorine-beryllium deposits of the Vitim Highland, western Transbaikal region: Mineral types, localization conditions, magmatism, and age

The fluorine-beryllium deposits of the Vitim Highland are represented by two mineral types: feld-spar-fluorite-phenakite-bertrandite and thorite-fluorite-phenakite. Their localization is controlled by fault zones of various orders, folds, and stocks and dikes of the Early Mesozoic subalkali quartz syenite and syenite porphyry. The ore is represented by mineralized crush zones and metasomatic veins hosted in intercalated carbonate and aluminosilicate rocks. The deposits were formed under hydrothermal conditions at 360–90°C; their age is estimated at (243 ± 3)-(260 ± 2) Ma.

Multiple phases of Tertiary extension and synextensional deposition of the Miocene-Pliocene Salt Lake Formation in an evolving supradetachment basin, Malad Range, southeast Idaho, U.S.A.

The extensional history of the Malad and Bannock ranges in southeast Idaho and northern Utah involves multiple phases of Tertiary normal faulting and synextensional deposition. Detailed geologic mapping, structural and stratigraphic analyses, and geochronologic data from this study elucidate previously defined deformational events in the region, and define two new extensional episodes. The largest-magnitude extension took place along low-angle normal faults of the ∼10–4-Ma Bannock detachment system, with concurrent sedimentation of the Miocene–Pliocene Salt Lake Formation in a regionally continuous supradetachment basin. This basin developed by ∼10.2 Ma, and was preceded by two phases of smaller-magnitude, aerially restricted normal faulting and sedimentation. In the Henderson Creek quadrangle of the southern Malad Range, the earliest event involved ∼8% north–south extension during deposition of the Paleocene–Eocene Wasatch (?) Formation. This conglomerate unit was deposited in an asymmetric, south-tilted half-graben bounded on the south by the syntectonic Willow Spring normal fault. The next pre-detachment extensional event produced a north-striking, east-tilted half-graben in which the Middle to Late Miocene Skyline Member of the Salt Lake Formation (∼11.9–10.2 Ma) was deposited as an ash-rich alluvial fan. This half-graben is bounded on the east and south by the syntectonic Red Knoll and Spring Trail normal faults. Detrital zircon age data from a tuffaceous sandstone bed in the Skyline Member suggest incorporation of reworked zircons from the ∼12.5–15-Ma Owhyee-Humboldt volcanic field in southwest Idaho with 10.3-Ma glass. The inception of the regional Bannock detachment system is recorded by the breakup and ∼16% west-southwest extension of its hanging wall by a set of north- to north–northwest-striking Late Miocene normal faults. These faults were associated with syntectonic deposition of the Cache Valley Member of the Salt Lake Formation (∼10.2− <9.2 Ma) in a regional-scale lake system. During deposition, the northeast-dipping Steel Canyon normal fault accommodated uplift of an intrabasinal horst that shed a wedge of Third Creek Member conglomerate eastward into the lake system between ∼10.0 and <9.2 Ma. The Third Creek Member interfingers with a Cache Valley Member lake-margin tufa-bearing facies, which changes eastward into a deeper-water, micritic limestone-bearing facies. The most recent extensional event in the Henderson Creek quadrangle involved ∼9% east–west extension on the north-striking Pliocene–Quaternary Wasatch fault. A segment boundary of the Wasatch fault, consisting of a 2.5–3.5-km-wide relay ramp that formed between two right-stepping en echelon segments, lies just north of the Idaho-Utah border. In addition, a broad, north- to north–northwest-trending antiformal zone of extensional folds is present in the south half of the study area and is interpreted as a double-rollover anticline that formed progressively in Late Miocene and Pliocene–Quaternary time above two oppositely dipping listric normal faults.

Surface deformations associated with the October 2004 Mid-Niigata earthquake: Description and discussion

AbstractThe Mid-Niigata earthquakes (mainshock: JMA M6.8, the largest aftershock: JMA M6.4) occurred on 23 October 2004 in the Shinano River fold and thrust zone, western margin of Northeast Japan. Two major Quaternary reverse fault systems, fault related to Higashiyama—Tamguiyama fold in western side and Muikamachi fault in eastern side, had already been mapped in aftershock area from the geological structure and tectonic morphostructure. These active fault and fold strike almost north-northeast—south-southwest. We investigated destructions of man-made structures and surface deformation using the releveling data by Geography Survey Institute. In turn, we examined about the origin of observed surface destructions from the compare with aftershock distribution. Our leveling and G.S.I leveling results indicate the deformation concordant with the Higashiyama-Tamugiyama fold and the offset along the a part of the northern half of Muikamachi fault. The vertical displacements are estimated to be ca. 70 cm and less than 20 cm, respectively. The surface destructions were identified almost only along the Suwatoge flexure (correlated to the eastern limb of the Higashiyama-Tamugiyama fold), Tagawa anticline, Obiro fault and Muikamachi fault. The shortening amounts were estimated to be ca. 16 cm at the Suwatoge flexure, less than 2 cm at the Tagawa anticline and ca. 8 cm at the Muikamachi fault from the compressional destruction of man-made structure. The maximum vertical displacement and shortening was appeared at the bottom of Suwatoge flexure. The Suwatoge flexure is located onto the shallower extension of the row of aftershocks which was correlated to main shock earthquake. The Muikamachi fault is located near the shallower extension of the source fault of the largest aftershock. The stress regime inferred from the character of the observed surface destructions is concordant with the trend of geological structure and focal mechanism solutions. The small compressional destructions of man-made structures are byproducts created by buried faulting and folding. Thus, the Higashiyama-Tamugiyama folding structure grew in association with the buried faulting in the 2004 earthquake.

Seismological and field observations from the 1990 November 6 Furg (Hormozgan) earthquake: a rare case of surface rupture in the Zagros mountains of Iran

We document a unique example of a co-seismic surface rupture in the Zagros mountains of SE Iran. Using P- and SH-waveform inversion, aftershock distributions and field observations we show that the Mw 6.4 1990 November 6 Furg (Hormozgan) earthquake was associated with ∼15 km of south-facing surface ruptures with an average vertical displacement of ∼1 m. Earthquakes of Mw 6–7 are common in the Zagros mountains of Iran. However, no co-seismic surface ruptures associated with thrust faulting have been reported before now, and blind thrusting appears to be more common. Source parameters determined from seismology and measurements from the surface ruptures indicate slip on a fault dipping ∼30◦N. Although there is likely to be considerable uncertainty in at least some locations, the aftershock activity measured in the month following the earthquake appears to be broadly confined to a diffuse area around the northward-dipping fault. The geomorphology of the epicentral region contains indications of both long-term Quaternary uplift and cumulative fault scarps from repeated earthquakes. The Furg earthquake probably occurred on a reactivated section of the High Zagros fault, a geological suture from the early history of the Iran–Eurasia collision. The 1990 earthquake is likely to have ruptured to the surface because of its unusual location in the High Zagros zone, away from the present-day focus of seismic activity in the Simply Folded zone of the Zagros. It is possible that other sections of the High Zagros sutures still have the potential to generate earthquakes.

Influence of mechanical stratigraphy and initial stress state on the formation of two fault propagation folds

Kinematic and mechanical modeling of the Rip Van Winkle (SE New York, USA) and La Zeta (SW Mendoza, Argentina) anticlines illustrate the influence of mechanical stratigraphy and initial stress state on the kinematics of fault propagation folding. In both anticlines, faults nucleating at distinct stratigraphic levels open upward into triangular zones of folding. Folding intensity and finite strain attenuates with distance from the fault tip. Trishear reproduces the bulk geometry and finite strain of the relatively homogeneous limestone sequence of the Rip Van Winkle anticline and predicts an initial location of the fault tip consistent with the field observations. Folding of the heterogeneous sedimentary section of La Zeta anticline, however, cannot be simulated by the trishear model alone. An additional mode of deformation involving transport of material from the backlimb into the hinge area and extension parallel to the direction of fault propagation is necessary to reproduce the geometry and finite strain of the anticline. Mechanical, distinct element modeling (DEM) of the anticlines indicates that their contrasting kinematics could have resulted from differences in mechanical stratigraphy and initial stress state. Folding of a homogenous, normally consolidated assemblage (initial horizontal to vertical stress ratio, K o=1) is trishear like and resembles the Rip Van Winkle anticline. Folding of a heterogenous (layered such as La Zeta), over-consolidated assemblage ( K o>1) departs from the trishear model and resembles La Zeta anticline. Based on the DEM simulations, we postulate that the Rip Van Winkle anticline formed at high depths (high overburden loads and lithostatic stress conditions), and that La Zeta anticline formed at shallow depths, after substantial uplift and erosion of the Andean mountain front (which induced over-consolidation and high K o).

Structures that accommodated differential vertical axis rotation of the western Transverse Ranges, California

New paleomagnetic and structural data from the northwestern Transverse Ranges of California define the location, geometry, and kinematics of structures that facilitated differential vertical axis rotation. Paleomagnetic declinations in the southern Coast Ranges indicate negligible amounts of vertical axis rotation, which contrasts sharply with data from rocks of the same age in the western Transverse Ranges that record approximately 90° of clockwise rotation since early Miocene time. This change in paleomagnetic declinations occurs across an east‐west trending zone of reverse faults and folds that includes the western Big Pine–Pine Mountain fault, the Santa Ynez fault, and the structures in between. The structures in this zone exhibit increased amounts of shortening to the west such that the zone closed in a fan‐like fashion, resulting in rotation of the southern edge with respect to the northern edge. The new data and interpretations refine previous models of vertical axis rotation in southern California by describing the deformation present at rotation boundaries that allowed distinct areas to rotate relative to the surrounding crust.

Structural and lithological continuity and discontinuity in the Otago Schist, Central Otago, New Zealand

The Otago Schist in Central Otago has undergone complex late metamorphic and post‐metamorphic deformation during uplift and exhumation. Two late metamorphic structural generations can be recognised and these may be genetically related. The earlier Manorburn Generation has been widely recognised and described previously. This generation has fold axes subparallel to a prominent syn‐metamorphic quartz rodding lineation. A later generation, herein named Poolburn Generation, has folds which superficially resemble Manorburn Generation, but has fold axes that are at a high angle to the quartz rodding lineation. Both generations occur in mappable fold zones (kilometre‐scale) that are generally not vergence boundaries, and some minor relative displacement may occur across fold zones. Fold zones occur within structurally and lithologically uniform schist domains. Abrupt changes in lithological sequences and orientations of structural elements such as Manorburn and Poolburn Generation fold axes and quartz rodding lineations occur at post‐metamorphic faults which separate different schist domains. Central Otago schist can be subdivided on the regional scale (tens of kilometres) into at least nine schist domains whose structural and lithologic continuity is disrupted by fault discontinuities. The domains and bounding discontinuities developed during Late Jurassic and Early Cretaceous uplift. Syn‐metamorphic compressive ductile deformation evolved to localised fold zones in the early stages of this uplift. Subsequently, regional extension caused juxtaposition of domains with different textural zones, and schists from slightly different structural levels. The Caples/Torlesse Terrane boundary is a composite feature, and different segments formed at different stages through the transition from ductile compression to brittle extensional deformation.

Kinematics of large scale asymmetric folds and associated smaller scale brittle — Ductile structures in the proterozoic somnur formation, pranhita — Godavari Valley, South India

The development of structural elements and finite strain data are analysed to constrain kinematics of folds and faults at various scales within a Proterozoic fold-and-thrust belt in Pranhita-Godavari basin, south India. The first order structures in this belt are interpreted as large scale buckle folds above a subsurface decollement emphasizing the importance of detachment folding in thin skinned deformation of a sedimentary prism lying above a gneissic basement. That the folds have developed through fixed-hinge buckling is constrained by the nature of variation of mesoscopic fabric over large folds and finite strain data. Relatively low, irrotational flattening strain (X:Z-3.1-4.8, k<1) are associated with zones of near upright early mesoscopic folds and cleavage, whereas large flattening strain (X:Z-3.9-7.3, k<1) involving noncoaxiality are linked to domains of asymmetric, later inclined folds, faults and intense cleavage on the hanging wall of thrusts on the flanks of large folds. In the latter case, the bulk strain can be factorized to components of pure shear and simple shear with a maximum shearing strain of 3. The present work reiterates the importance of analysis of minor structures in conjunction with strain data to unravel the kinematic history of fold-and-thrust belts developed at shallow crustal level.

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi: Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-are-naceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [high Th/Sc (0.57−3.59), La/Sc (1.46−12.4), La/Yb (5.84−19.0)] and variable Th/U ratios, with ΣREE=129−296µg/g, δEu=0.51−0.86, and (La/Yb)N=3.95−12.9. The Nd isotopic model agest DM of these rocks vary from 1597 to 2124 Ma. Their143Nd/144Nd values are low [ɛNd(0)=−11.4 to −15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and K-rich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199±26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model aget DM (1597−2124 Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic (1100−1600 Ma).

Cenozoic lithospheric extension induced magmatism in Southwest Japan

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.