Late Neoproterozoic to Holocene thermal history of the Precambrian Georgetown Inlier,northeast Australia

Carboniferous‐Permian volcanic complexes and isolated patches of Upper Jurassic — Lower Cretaceous sedimentary units provide a means to qualitatively assess the exhumation history of the Georgetown Inlier since ca 350 Ma. However, it is difficult to quantify its exhumation and tectonic history for earlier times. Thermochronological methods provide a means for assessing this problem. Biotite and alkali feldspar ⁴⁰Ar/³⁹Ar and apatite fission track data from the inlier record a protracted and non‐linear cooling history since ca 750 Ma. ⁴⁰Ar/³⁹Ar ages vary from 380 to 735 Ma, apatite fission track ages vary between 132 and 258 Ma and mean track lengths vary between 10.89 and 13.11 μm. These results record up to four periods of localised accelerated cooling within the temperature range of ～320–60°C and up to ～14 km of crustal exhumation in parts of the inlier since the Neoproterozoic, depending on how the geotherm varied with time. Accelerated cooling and exhumation rates (0.19–0.05 km/10⁶ years) are observed to have occurred during the Devonian, late Carboniferous‐Permian and mid‐Cretaceous — Holocene periods. A more poorly defined Neoproterozoic cooling event was possibly a response to the separation of Laurentia and Gondwana. The inlier may also have been reactivated in response to Delamerian‐age orogenesis. The Late Palaeozoic events were associated with tectonic accretion of terranes east of the Proterozoic basement. Post mid‐Cretaceous exhumation may be a far‐field response to extensional tectonism at the southern and eastern margins of the Australian plate. The spatial variation in data from the present‐day erosion surface suggests small‐scale fault‐bounded blocks experienced variable cooling histories. This is attributed to vertical displacement of up to ～2 km on faults, including sections of the Delaney Fault, during Late Palaeozoic and mid‐Cretaceous times.     

U–Pb Dating of Hydrothermal Zircons from the Neoproterozoic Liushanyan VMS Cu–Zn Deposit,Central China: Evidence for a Triassic Deformation Event

The Liushanyan deposit is an important volcanic‐host massive sulfide (VMS) Cu–Zn deposit in the Qinling‐Tongbai‐Dabie orogenic belt, central China, with reserve of 2.38 Mt Cu and 16.11 Mt Zn. Orebodies occur in the meta‐quartz keratophyre of the Liushanyan formation. In this paper, we present textural features and laser ablation ICP‐MS U–Pb dating results of zircons from the ore‐bearing mylonitized meta‐quartz keratophyre. The hydrothermal zircons are distinct from metamorphic zircons in this rock, showing low cathodoluminescence (CL) response and hydrothermal rims (black in CL images). They have relatively flat light rare earth element patterns and high La content and low (Sm/La)_N and Ce/Ce* values. These features are typical of hydrothermal zircons. The cores of metamorphic zircons yield a weighted mean ²⁰⁶Pb/²³⁸U age of 900 ± 26 Ma, interpreted as the volcanic and related VMS mineralizing age. Two much younger events are also recorded by zircons in this rock: (i) the Early Silurian amphibolites–greenschist facies metamorphism at 435 ± 26 Ma; and (ii) the growth of hydrothermal zircons at ca. 241 ± 1 Ma, associated with the ductile shear deformation. The Silurian metamorphic event is probably associated with the arc–continent collision, while the Triassic ductile deformation event formed in the final continent–continent collision setting.     

Grenvillian-aged orogenesis in the Palaeoproterozoic Gascoyne Complex, Western Australia: 1030–950 Ma reworking of the Proterozoic Capricorn Orogen

In situ SHRIMP U–Pb geochronology of monazite and xenotime in pelitic schists from the central Gascoyne Complex, Western Australia, shows that greenschist to amphibolite facies metamorphism occurred between c. 1030 and c. 990 Ma. Monazite from an undeformed rare‐element pegmatite from the same belt gives a ²⁰⁷Pb/²⁰⁶Pb age of c. 950 Ma, suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism. Metamorphism in the central Gascoyne Complex was previously interpreted as Barrovian, largely based on the identification of kyanite in peak metamorphic assemblages, and has been attributed to intense crustal shortening and substantial tectonic thickening during Palaeoproterozoic continent–continent collision. However, the stable Al₂SiO₅ polymorph has been identified in this study as andalusite rather than kyanite, and the prograde assemblages of staurolite–garnet–andalusite–biotite–muscovite–quartz indicate temperatures of 500–550 °C and pressures of 3–4 kbar. These data show that the Palaeoproterozoic Gascoyne Complex underwent an episode of Grenvillian‐aged intracontinental reworking concentrated in a NW–SE striking corridor, during the Edmundian Orogeny. Until now, the Edmundian Orogeny was thought to have involved only reactivation of structures in the Gascoyne Complex, along with deformation and very low‐ to low‐grade metamorphism of Mesoproterozoic cover rocks some time between 1070 and 755 Ma. However, we suggest that it involved regional amphibolite facies metamorphism and deformation, granite magmatism and pegmatite intrusion between c. 1030 and c. 950 Ma. Therefore, the Capricorn Orogen experienced a major phase of tectonic reworking c. 600 Myr later than previously recognized. Our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events.     

Quantitative Expression of Heat Flow versus Tectonic Deformation in the China Continent： The Effects of Plastic-Flow Network and Stable Block

1 Introduction It has been understood in the study of terrestrial heat flow that the distribution of heat flow in the interior of continent is influenced by a large number of factors, involving heat sources (e.g. mantle heat flow, heat production of radioactive elements in the crust, magmatic activity, and heat production of tectonic deformation), heat transfercondition (e.g. thermal conductivity and thickness of media), groundwater circulation, etc. On the background of these factors it is pa…     

Recent,very rapid retreat of a temperate glacier in SE Iceland

Iceland's glaciers are particularly sensitive to climate change, and their margins respond to trends in air temperature. Most Icelandic glaciers have been in retreat since c. 1990, and almost all since 1995. Using ice‐front measurements, photographic and geomorphological evidence, we examined the record of ice‐front fluctuations of Virkisjökull–Falljökull, a steep high‐mass‐turnover outlet glacier in maritime SE Iceland, in order to place recent changes in a longer‐term (80‐year) context. Detailed geomorphological mapping identifies two suites of annual push moraines: one suite formed between c. 1935 and 1945, supported by lichenometric dating; the other between 1990 and 2004. Using moraine spacing as a proxy for ice‐front retreat rates, we show that average retreat rates during the 1930s and 1940s (28 m a^?1) were twice as high as during the period from 1990 to 2004 (14 m a^?1). Furthermore, we show that both suites of annual moraines are associated with above‐average summer temperatures. Since 2005, however, retreat rates have increased considerably – averaging 35 m a^?1 – with the last 5 years representing the greatest amount of ice‐front retreat (～190 m) in any 5‐year period since measurements began in 1932. We propose that this recent, rapid, ice‐front retreat and thinning in a decade of unusually warm summers has resulted in a glaciological threshold being breached, with subsequent large‐scale stagnation of the glacier terminus (i.e. no forward movement) and the cessation of annual push‐moraine formation. Breaching this threshold has, we suggest, caused further very rapid non‐uniform retreat and downwasting since 2005 via a system feedback between surface melting, glacier thinning, decreased driving stress and decreased forward motion.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Reworking of the Gangdese magmatic arc,southeastern Tibet: post‐collisional metamorphism and anatexis

The Gangdese magmatic arc, southeastern Tibet, was built by mantle‐derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo‐Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post‐collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma respectively. Meta‐granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post‐collisional amphibolite facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km ^{? 1}, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high‐P metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.     

Determination of the Neoproterozoic Shicaogou Syn-collisional Granite in the Eastern Qinling Mountains and Its Geological Implications

The Shicaogou granite has been identified as a magnesian (Fe-number=0.71-0.76), calcic to calc-alkalic (MALI=3.84-5.76) and peraluminous (ASI=1.06-1.13) granite of the syn-collisional S-type, with high SiO2(>71%), A12O3 (>13%) and Na2O+K2O (6.28%-7.33%, equal for NaO2 and K2O). Trace element and REE analyses show that the granite is rich in LILE such as of Rb, Sr, Ba and Th, and poor in HFSE like Yb, Y, Zr and Hf. Its Rb/Sr ratio is greater than 1; the contents of Nb and Ta, and the ratio of Nb/Ta as well as the REE geochemical features (e.g. REE abundance, visible fractionation of LREE and HREE and medium to pronounced negative Eu anomalies) are all similar to those of crust-origin, continent-continent syn-collisional granite. Moreover, the granite exhibits almost the same pattern as that of the typical continent-continent syn-collisional granite on the spider diagram and all samples fall within the syn-collisional granite field.The cathodoluminescence (CL) investigations have revealed that the zircon f     

Ice‐distal landscape and sediment signatures evidencing damming and drainage of large pro‐glacial lakes,northwest Russia

Lyså, A., Jensen, M. A., Larsen, E., Fredin, O. & Demidov, I. N.^* 2010: Ice‐distal landscape and sediment signatures evidencing damming and drainage of large pro‐glacial lakes, northwest Russia. Boreas, Vol. 40, pp. 481–497. 10.1111/j.1502‐3885.2010.00197.x. ISSN 0300‐9483. Sediments from river sections and the morphology of the upper reaches of Severnaya Dvina and Vychegda in northwest Russia show evidence of the existence of large ice‐dammed lakes in the area twice during the Weichselian. During the Late Weichselian, three separate ice‐dammed lakes (LGM lake(s)) existed, the largest one at about 135 m a.s.l. having a volume of about 1510 km³. Stepwise and rapid lake drainage is suggested to have taken place within less than 1000 years. The locations of various passpoints controlled the drainage, and when the lake was at its maximum level water spilled southeastwards into the Volga basin. Later, but before the lake water finally drained into the White Sea, water was routed northeastwards into the southeastern part of the Barents Sea. The oldest lake, the White Sea lake, existed around 67–57 ka ago, slightly in conflict with earlier palaeogeographic reconstructions regarding the chronology. The extent of the lake was constrained by, in addition to the Barents Sea ice‐sheet margin in the north, thresholds in the drainage basin. Later, one threshold was eroded and lowered during the LGM lake drainage. Given a lake level of about 115 m a.s.l., a lake area of about 2.5 × 10⁴ km³ and a water volume of about 4800 km³, the lake drainage northwards and into the ocean probably impacted the ocean circulation.     

Pressure-induced incipient amorphization of α-quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm^?1, and additional vibrations at 480–485, 520–523 and 608 cm^?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ～2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm^?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm^?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.     

Isotopic constraints on the genesis and age of autochthonous glaucony in the Oligo-Miocene Torquay Group, south-eastern Australia

The Oligo‐Miocene Torquay Group at Bird Rock in south‐eastern Australia comprises a sequence of fine‐grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool‐water, mid‐shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb–Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution–crystallization reactions, which occurred during very early diagenesis in a closed or isochemical system, isolated from the ambient marine environment. The constituent ions of the glaucony were derived primarily from terrigenous clay minerals, but considerable potassium may have been sourced indirectly from sea water, through potassium enrichment of clay precursors. The pore fluids associated with glauconitization were marine derived, but progressively modified by the dissolution–crystallization of detrital clay minerals and autochthonous glaucony. Rb–Sr data for the BW horizon indicate that dating glauconies may be somewhat problematic, as co‐genetic glauconitic minerals can show a range of initial strontium compositions, which reflect the incorporation of strontium derived from mineralogical precursors and/or contemporaneous sea water. Rb–Sr isochrons indicate that the glaucony of the BW horizon formed at 23 ± 3 Ma. This age is in good agreement with both the established biostratigraphy and a ⁸⁷Sr/⁸⁶Sr age for the horizon (23 ± 1 Ma), but could only be determined using the independent age constraint and the estimate of the ⁸⁷Sr/⁸⁶Sr ratio of contemporaneous sea water provided by analysis of associated biogenic carbonate.     

Mafic plutonic rocks in a continental‐arc setting: geochemistry of 1.87–1.78 Ga rocks from south‐central Sweden and models of their palaeotectonic setting

Mafic intrusive rocks (1.79–1.78 Ga) of the Transscandinavian Igneous Belt (TIB) and the c. 1.87 Ga Hedesunda Igneous Complex in the Fennoscandian Shield of south‐central Sweden were studied using whole‐rock and isotope geochemistry. Rock types vary from gabbros/norites (and leucogabbros) to quartz diorites, with Mg# between 76 and 49, and wt% SiO₂ between 43.6 and 59.7, indicating some variation in evolutionary levels and variable cumulus components. Geochemical signatures are calc‐alkaline to shoshonitic, large ion lithophile elements and light rare earth elements enriched and high‐field strength elements depleted of continental‐arc type. ε_Nd(t) ranges between +1.0 and +2.7, and ⁸⁷Sr/⁸⁶Sr(t) between 0.7020 and 0.7038. There is no systematic correlation between chemical parameters and isotope ratios. These isotopic data overlap with other mafic plutonic TIB rocks; samples from the Dala Province (DP) tend to overlap with the c. 1.7 Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios. With two exceptions, the ε_Nd(t) of +1 to +2 conform to an isotopically ‘mildly depleted’ source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above ε_Nd(t) +2.0 are scarce in the TIB. Mantle sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (T_DM c. 2.0 Ga), that is during subduction in the preceding Svecofennian (2.0–1.87 Ga) and/or during the TIB‐0&1 event (1.85–1.78 Ga). The palaeotectonic settings inferred are active continental margins; N–S‐directed convergence at 1.87 Ga and E–W‐directed at 1.79–1.78 Ga. Copyright © 2008 John Wiley & Sons, Ltd.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

High‐P tectono‐metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines,Italy

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly‐deformed basements, the loss of information on pre‐kinematic phases of mylonites hinders large‐scale correlations based on tectono‐metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)–temperature (T)–deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine‐grained K‐white mica and chlorite enveloping mica‐fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg‐rich cores and Mg‐poor rims. The amphibolite contains green amphibole+plagioclase+garnet+quartz+ilmenite defining S₁ with a superposed mylonitic fabric localized in decimetre‐ to centimetre‐scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P–T constraints that are linked to the reconstructed metamorphic‐deformational stages. For the first time an early high‐P stage at >11 kbar and 510°C was constrained, followed by a temperature peak at 550–590°C and 9–10 kbar and a retrograde stage (<475°C, <7 kbar), during which ductile shear zones developed. The inferred clockwise P–T–D path was most likely related to crustal thickening by continent‐continent collision during the Variscan orogeny. A comparison of this P–T–D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono‐metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.     

Rock nail reinforcement of a free surface

Rock nailing of the surface instability test specimen is considered as a means to reinforce the rock and increase its free surface strength (FSS). This technique is modeled using Cox's original shear‐lag method. In the continuum sense, nail reinforcement is modeled as a body force that acts as a confinement. Results are presented for a single nail in a surface instability test specimen and for a nail‐reinforced test specimen with a given nail density. The results show that nail length and the nail–rock contact modulus are important parameters. The effect of nail reinforcement increases with the second power of nail length. Rock dilation is also important since the action of the nails is mobilized from the difference in displacement between the rock and the nail. The results show the potential for a significant increase in the FSS of the rock. Copyright © 2011 John Wiley & Sons, Ltd.     

Quick Evaluation of Present-Day Low-Total Organic Carbon Carbonate Source Rocks from Rock-Eval Data: Middle–Upper Ordovician in the Tabei Uplift, Tarim Basin

Previous studies have postulated the contribution of present-day low-total organic carbon(TOC) marine carbonate source rocks to oil accumulations in the Tabei Uplift, Tarim Basin, China. However, not all present-day low-TOC carbonates have generated and expelled hydrocarbons; therefore, to distinguish the source rocks that have already expelled sufficient hydrocarbons from those not expelled hydrocarbons, is crucial in source rock evaluation and resource assessment in the Tabei Uplift. Mass balance can be used to identify modern low-TOC carbonates resulting from hydrocarbon expulsion. However, the process is quite complicated, requiring many parameters and coefficients and thus also a massive data source. In this paper, we provide a quick and cost effective method for identifying carbonate source rock with present-day low TOC, using widely available Rock-Eval data. First, we identify present-day low-TOC carbonate source rocks in typical wells according to the mass balance approach. Second, we build an optimal model to evaluate source rocks from the analysis of the rocks' characteristics and their influencing factors, reported as positive or negative values of a dimensionless index of Rock-Eval data(IR). Positive IR corresponds to those samples which have expelled hydrocarbons. The optimal model optimizes complicated calculations and simulation processes; thus it could be widely applicable and competitive in the evaluation of present-day low TOC carbonates. By applying the model to the Rock-Eval dataset of the Tabei Uplift, we identify present-day low-TOC carbonate source rocks and primarily evaluate the contribution equivalent of 11.87×10~9 t oil.     

Climate changes during the last c. 7500 years as recorded by the degree of peat humification in the Lofoten region,Norway

Vorren, K.‐D., Jensen, C. E. & Nilssen, E. 2012 (January): Climate changes during the last c. 7500 years as recorded by the degree of peat humification in the Lofoten region, Norway. Boreas, Vol. 41, pp. 13–30. 10.1111/j.1502‐3885.2011.00220.x. ISSN 0300‐9483. Two peat cores from two neighbouring bogs in Lofoten, northern Norway were densely AMS dated and analysed for humification. The two cores have been influenced by human agricultural impact, especially c. 1600 cal. a BP, which may have affected the local hydrology of the bogs. From 7400 cal. a BP onwards, 19 distinct wet‐shifts are recorded in the two cores. Eight or nine of these correspond chronologically to periods of low solar activity. This correlation is most convincing during the last 2000 years. Some wet‐shifts are connected with a solar low‐activity period during the Subboreal/Subatlantic transition, which in central Europe is dated at 2750–2565 cal. a BP. For Lofoten, the corresponding Subboreal/Subatlantic transition – or the wet‐shift marking this transition – is dated at c. 2600 cal. a BP. Some wet‐shifts occur just before or just after solar low‐activity periods, but only four of the nineteen wet‐shifts are clearly not temporally connected with periods of low solar activity. Compared with the wet‐shifts in NW European (mainly British Isles) bogs, there are more frequent wet‐shifts in northern Norway. Compared with other peat cores in northern Norway, especially for the interval 6500–5000 cal. a BP, Lofoten deviates by its lack of wet‐shifts. As in England, Scotland and Ireland, there is regional variability in the temporal formation of wet‐shifts in northern Norway.     

Xiba Granitic Pluton in the Qinling Orogenic Belt,Central China: Its Petrogenesis and Tectonic Implications

Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U–Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite (218±1 Ma). The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with (La/Yb)N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 (mean= 0.97). Most of these samples from Xiba granitic pluton exhibit εNd(t) values of ?8.79 to ?5.38, depleted mantle Nd model ages (TDM) between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios (87Sr/86Sr)i from 0.7061 to 0.7082, indicating a possible Meso- to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher (87Sr/86Sr)i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.     

Surface wetness and mire development during the late Holocene in central Sweden

Andersson, S. & Schoning, K. 2010: Surface wetness and mire development during the late Holocene in central Sweden. Boreas, Vol. 39, pp. 749–760. 10.1111/j.1502‐3885.2010.00157.x. ISSN 0300‐9483. Late Holocene mire development and surface wetness changes have been studied in a small mixed mire located in central Sweden. Today the mire is characterized by a mainly ombrotrophic centre dominated by Sphagnum mosses, with Carex content increasing towards the more minerotrophic mire margins. Two peat sequences extracted from the central ombrotrophic part were investigated for stratigraphy, humification, testate amoebae analysis, C/N ratio and δ¹³C and δ¹⁵N stable isotopes. Three main stages of mire development are identified, with the first stage, between c. 4200 and 2600 cal. yr BP, characterized by water‐logged conditions, suggesting a minerotrophic fen stage. The second stage, between c. 2600 and 1000 cal. yr BP, is characterized by more ombrotrophic conditions and Sphagnum‐dominated vegetation. The onset of the prominent change at c. 2600 cal. yr BP could have been initiated by climate change coincident with a change in solar activity. The last stage, between c. 1000 and 50 cal. yr BP, is dominated by more ombrotrophic conditions, suggesting increased precipitation. This study shows that the response of hydrological proxies in a mixed mire during its development towards more ombrotrophic conditions might result in conflicting results, a finding that needs to be considered in palaeoenvironmental reconstructions from mires that change between ombrotrophic and minerotrophic settings.     

Retrogression by deep infiltration of meteoric fluids into thrust zones during late-orogenic rapid unroofing

Hypersaline (Na–Ca–Cl) fluids are associated with late‐stage quartz veining and retrogression of garnet, kyanite and other high P–T phases in the vicinity of thrusts and major lithological boundaries in the Scandian nappes of the Troms‐Ofoten region, northern Norway. They record early Devonian fluid infiltration during rapid exhumation in the final stages of Caledonian orogenesis. The δ¹⁸O and δD characteristics of these late fluids provide compelling evidence for deep circulation of meteoric fluids. The sub‐greenschist to low greenschist facies retrogression (P=2±1 kbar; T =300–350 °C) suggests infiltration to depths of 7–9 km in a regime of supra‐hydrostatic fluid pressure. Peak metamorphic quartz veins and associated fluids have δD and δ¹⁸O characteristics consistent with a metamorphic origin (δD ?47 to ?75; δ¹⁸O+8.6 to +17.4). However, late quartz veins and associated fluids show a broad spread of δD from ?42 to ?148, interpreted in terms of meteoric fluid infiltration. Such negative δD values are only recorded in present‐day high‐latitude or high‐altitude settings, and since north Norway was in an equatorial setting (10° S) in the early Devonian, a high‐altitude origin is deduced. By calculation, and by comparison with modern examples, the early Devonian mountains of the north Norwegian Caledonides are interpreted to have had a topography in excess of 5 km. The deep circulation of surface waters is interpreted in terms of topographically driven flow, linked with a hydrothermal system induced by elevated geothermal gradients due to rapid uplift. Whilst the case for deep penetration of surface‐derived fluids has been promoted for Mesozoic and younger mountain belts, this study represents one of the first documented examples for a Palaeozoic orogenic belt. It suggests that many of the fundamental processes operating during the exhumation of mountain belts are similar irrespective of age.