High-pressure Belt Research Articles

Pore pressure distribution of a mega-splay fault system in the Nankai Trough subduction zone: Insight into up-dip extent of the seismogenic zone

We use the pore pressure distribution predicted from a waveform tomography (WT) velocity model to interpret the evolution of the mega-splay fault system in the Nankai Trough off Kumano, Japan. To map pore pressure around the mega-splay fault and plate boundary décollement, we integrate the high-resolution WT velocities with laboratory data and borehole well log data using rock physics theory. The predicted pore pressure distribution shows that high pore pressures (close to lithostatic pressure) along the footwall of the mega-splay fault extend seaward to the trough region, and the normalized pore pressure ratio is nearly constant over that extent. This continuity of the overpressured zone indicates that a coseismic rupture can potentially propagate nearly to the trough axis. We interpret a high-pressure belt within an accretionary wedge on the landward side of the present mega-splay fault as evidence of the ancient mega-splay fault. Because the ancient mega-splay fault soles into the active mega-splay fault, the active mega-splay fault may function as a basal detachment fault and is directly connected to the seaward plate boundary décollement.

Application Analysis of Comprehensive Advanced Geological Prediction in Karst Tunnel

Based on the DaDu mountain tunnel of new construction ShangHai-KunMing high-speed railway, combine with field geology investigation of the tunnel address area comparative analysis TRT advanced geological prediction, infrared probe water, ground penetrating radar advance geological forecasting and tunnel design data, aimed at fault fracture zone, high-pressure water-rich belt, the karst fissures developmental situation may have in construction period of DaDu mountain tunnel, carried out the comprehensive advanced geological prediction and then combined with the actual excavation situation contrast and verify the feasibility and practicality of the comprehensive geological advanced prediction technology, and it will have important significance of reference for the similar tunnel project in design and construction period.

A new petro-structural map of the Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps)

The Mt. Mucrone metagranitoid is an extensively investigated intrusive Permian body, located in the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone, within the high-pressure metamorphic belt of the Western Alps formed during the Alpine plate convergence. The structure of the northwestern sector of Mt. Mucrone, including both the metagranitoids and country rocks, has been mapped, although an integrated structural and petrological analysis is still lacking in its southwestern sector, a topic investigated in this contribution. During the field structural analysis, six groups of ductile structures were recognized to have evolved as follows: the D1 and D2 fabrics took place under eclogite-facies, D3 under blueschist-facies and D4 to D6 under greenschist-facies conditions, respectively. Foliation trajectories revealing the chronology of the superposed structures are represented in the analytical (drift and solid) and interpretative (solid) maps, both at a 1:10,000 scale, and a panel of structural cross-sections allows the 3D representation of the poly-deformed lithostratigraphy. The related metamorphism indicates the changes within the subduction-collision tectonic frame.

Tracing of the Deep Fluids in Western Tianshan by the Electron Microprobe Analyses of Omphacite Form High-Pressure Veins and Host-Rocks

Western Tianshan High-pressure (HP)-metamorphic belt is characterized by developed High-pressure (HP) veins, which are composed by HP-metamorphic minerals. The host rocks of the HP-veins are mainly composed by eclogites and blueschists. As the direct record of the deep fluids in the paleo-subduction zones, the HP-veins can provide us deep samples for probing into the deep fluids in the subduction zones. Fluids in the deep subduction zones play an important role in crust-mantle exchange related to plate subduction process. The electron microprobe analyses of HP-metamorphic minerals omphacite inside the veins and host rocks in western Tianshan high-pressure metamorphic belt is mostly paid attention. The result shows that the composition of the omphacite from HP-veins have the same composition of the omphacite from the host rocks, which indicates that the fluids from which the HP-vein precipitated originated from the host rock.

Expansion of the northern hemisphere subtropical high pressure belt: trends and linkages to precipitation and drought

A critical issue in research concerning long-term climate change is the relationship between circulation features and global temperature variations. We establish that the annual areal size of the northern hemisphere subtropical high pressure belt (SHPB), as defined by seven 500-hPa height isohypses, shares over 70% of the variability with global annual near-surface air temperature since 1948. The area enclosed by the 5850-m isohypse of the 500-hPa surface in the northern hemisphere has more than doubled since the 1950s, with greatest increases over northern Africa, the Middle East and India. A long-term historical run of a coupled global climate model shows rapidly increasing SHPB annual sizes, since the mid-1970s. Since the SHPB’s descending air produces increased aridity, SHPB expansion may transition humid regions to more arid lands. To examine this aspect, first, variations in recorded precipitation using a gridded database for the region experiencing expansion of the SHPB show a decrease in precipitation (though significant only at the 88% confidence level) over the last 60 years. Second, variations in a 0.5° spatial resolution monthly drought index, the Standardized Precipitation Evapotranspiration Index, are highly correlated (+0.78) with annual variations in the area enclosed by the 500-hPa height isohypses. These results support those of previous investigations that suggest further northward expansion of the northern hemisphere subtropical dry zones with continued global climate change.

Reconstructing Valanginian (Early Cretaceous) mid-latitude vegetation and climate dynamics based on spore–pollen assemblages

Changes in terrestrial vegetation patterns during the Valanginian (Early Cretaceous) and their link to major climatic and environmental alterations are poorly studied. In this study, the spatial and temporal changes in plant community structure are reconstructed based on spore–pollen records from two mid-latitude sites located in the Mid-Polish Trough (MPT, central Poland), and the Vocontian Basin (VB, southeast France). Stratigraphic control is provided by δ13Ccarb chemostratigraphy and calcareous nannofossil biostratigraphy. Reconstruction of hinterland vegetation is based on palynological investigations of 83 samples from hemipelagic (VB) and marginal marine (MPT) sediments rich in terrestrial palynomorphs. A total of 45 palynomorph taxa were identified at generic level (30 spores, 15 pollen). Vegetation around the MPT was dominated by araucarian/cupressacean conifers while that surrounding the VB was dominated by drought-resistant cheirolepidiacean conifers. At both sites the understorey and/or vegetation of open areas was dominated by pteridophytes. An early Valanginian gradual trend towards humid conditions at the MPT, well expressed by a distinct increase in the spore–pollen ratio, culminates in a short-lived spore-maximum stratigraphically located at the lower/upper Valanginian boundary. It is characterized by low conifer abundances and high abundances of the fern spore taxa Cyathidites, Leiotriletes and Gleicheniidites accompanied by enhanced abundances of the pteridosperm pollen Vitreisporites pallidus, whose parent plants are assumed to be indicative of swamp habitats. The spore-maximum is coeval to a similar peak observed in the VB, characterized by essentially the same taxa. Here, the spore-maximum is preceded by a protracted phase of arid conditions, characterized by low spore abundances and exceptionally high numbers of the cheirolepidiacean conifer pollen Classopollis. Changes in moisture, identified as the key climatic factor determining trends and turnovers in vegetation, were probably controlled by a monsoonal circulation. The supra-regional humid phase expressed by the coeval spore maxima was probably induced by an intensified monsoonal climate. The temporal influence of a northern hemisphere arid belt at the VB, under the influence of the subtropical high-pressure belt, may have caused the temporal drying not affecting the MPT site, located further north.

Mafic forearc cumulates and associated rocks in the central high-pressure belt of the Acatlán Complex of southern México: geochemical constraints

The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO2 (38–52 wt.%) and high Mg# (∼48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ϵ Nd(t) = 5.3–8.5 and 147Sm/144Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.

Stretching lineations in high-pressure belts: the fingerprint of subduction and subsequent events (Malpica–Tui complex, NW Iberia)

The evolution of tectonic flow has been studied in an exposed continental subduction system of the Variscan belt, the Malpica–Tui Complex of NW Iberia, where structural data are used to establish the kinematics and flow geometry during the whole deformation history. Structural analysis reveals reorientation of successive lineations by subsequent deformation events, and especially by late strike-slip tectonics. Analysis of the deflection patterns in the lineation map permits us to establish the original trend of lineations. The data expose the role of thrust and nappe tectonics and non-coaxial deformation in the hinterland of a collisional belt showing a dominant orogen-parallel lineation pattern. Following oblique subduction, contractional ductile thrusts transported and emplaced a part of the subduction system onto the adjacent mainland following vectors normal to the orogenic trend. However, the finite stretching lineation in associated recumbent folds is oblique to the orogenic trend and suggests overprinting by dextral shearing, probably owing to lateral components of the Gondwana and Laurussia convergence. Subsequently, extensional structures dismembered the tectonic pile, moving the pieces obliquely with respect to the trend of the collisional belt. Orogen-parallel strike-slip shear zones later produced major reorientation of the pre-existing tectonic fabrics towards their shear planes.

Drastic shrinking of the Hadley circulation during the mid-Cretaceous Supergreenhouse

Abstract. Understanding the behavior of the global climate system during extremely warm periods is one of the major themes of paleoclimatology. Proxy data demonstrate that the equator-to-pole temperature gradient was much lower during the mid-Cretaceous "supergreenhouse" period than at present, implying larger meridional heat transport by atmospheric and/or oceanic circulation. However, reconstructions of atmospheric circulation during the Cretaceous have been hampered by a lack of appropriate datasets based on reliable proxies. Desert distribution directly reflects the position of the subtropical high-pressure belt, and the prevailing surface-wind pattern preserved in desert deposits reveals the exact position of its divergence axis, which marks the poleward margin of the Hadley circulation. We reconstructed temporal changes in the latitude of the subtropical high-pressure belt and its divergence axis during the Cretaceous based on spatio-temporal changes in the latitudinal distribution of deserts and prevailing surface-wind patterns in the Asian interior. We found a poleward shift in the subtropical high-pressure belt during the early and late Cretaceous, suggesting a poleward expansion of the Hadley circulation. In contrast, an equatorward shift of the belt was found during the mid-Cretaceous "supergreenhouse" period, suggesting drastic shrinking of the Hadley circulation. These results, in conjunction with recent observations, suggest the existence of a threshold in atmospheric CO2 level and/or global temperature, beyond which the Hadley circulation shrinks drastically.

High pressure rocks of the Acatlán Complex, southern Mexico: Large-scale subducted Ordovician rifted passive margin extruded into the upper plate during the Devonian–Carboniferous

High pressure (HP) rocks in the Paleozoic Acatlán Complex of southern Mexico occur in a median belt extruded into the complex, which is the root zone of a klippe to the west. These HP rocks are predominantly clastic metasedimentary rocks and include a bimodal suite of amphibolites and granitoids. U–Pb LA-ICPMS zircon dating indicates that one of the amphibolites was intruded at or before 461±2Ma, and a megacrystic granitoid yielded an intrusive age of 488 +8/−7Ma, which is within the 490–450Ma range of other dated HP megacrystic granitoids. The amphibolites have a tholeiitic (transitional to alkalic), within-plate chemistry derived from a heterogeneous mantle by various degrees of partial melting. The host metasedimentary rocks contain detrital zircons derived both from the Acatlán granitoids and the ca. 1.0–1.3Ga Oaxacan Complex, and are inferred to represent rifted passive margin sediments. An age of 348 +4/−1Ma from zircons in a migmatized amphibolite is interpreted as the time of migmatization and dates decompression melting following peak pressure at depths of 45–55km. Retrogression from 530°C to 370°C occurred between 343 and 335Ma. The predominance of reasonably coherent, rifted passive margin rocks over a surface area of 10×90km suggests removal of a large slice of the upper plate during flat slab subduction. This slice was subducted to depths of ca. 45–55km where it was underplated and transferred to the overriding plate. The absence of Oaxacan Complex rocks in the HP belt suggests the Oaxacan basement was subducted to greater depths. Underplating was followed by extrusion facilitated by extension in the upper plate that produced Mississippian rift basins east of the median HP belt. Westward overthrusting of the HP rocks produced a klippe that overrode Mississippian clastic wedge sediments.

A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone, Acatlán Complex, southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481±16Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482±3Ma) and mica schist (youngest concordant detrital zircon: 433±3Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~50km (~16kbar and 750°C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329Ma (Ordovician-Mississippian), and was followed by F3 synformal folding. Cooling through ca. 385°C is indicated by 329±1 and 316–317±2Ma, 40Ar/39Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750Ma Goiás arc (Amazonia), 1–1.3Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.

Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic–Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet–clinopyroxene–phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~700°C and ~2.1–2.4GPa. SHRIMP-RG U–Pb dating of eclogite metamorphic zircon yielded a 75.5±2Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm–Nd clinopyroxene-two garnet–whole rock isochron from an eclogite band yielded a less precise but consistent age of 77±13Ma. The above features imply subduction to >60km depth of at least a portion of the North American sialic basement during Late Cretaceous collision.The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole+plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5±3.5Ma SHRIMP-RG U–Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results.Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacús gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua mélange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U–Pb, Rb–Sr, and K–Ar ages of the pegmatite range ~76–66Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary–present strike-slip faulting between the North-American and Caribbean plates.

An alternative Pangea reconstruction for Middle America with the Chortis Block in the Gulf of Mexico: tectonic implications

Understanding Pangea breakup requires a robust reconstruction, and this article focuses on the Middle America sector of the supercontinent. Although most Pangean reconstructions locate the Yucatan Block along the southern USA, the Chortis Block is generally placed off southern Mexico (Pacific model), undergoing sinistral relative motion during the Mesozoic and Cenozoic. However, the Pacific model is inconsistent with the absence of a Cenozoic fault linking the Cayman transforms and the Middle America Trench. We present an alternative Pangean reconstruction, where both the Yucatan and Chortis Blocks are placed in the future Gulf of Mexico, moving Mexico westwards along the Mojave–Sonora megashear to accommodate overlap with South America. Subsequent Mesozoic and Cenozoic evolution is inferred to have occurred in two stages: (i) Jurassic clockwise rotation along the Mojave–Sonora and West Florida megashears, followed by (ii) Cenozoic anticlockwise rotation along the Sierra Madre Oriental and East Yucatan megashears. The first stage is linked to the breakup of Pangea where the Gulf of Mexico formed as a pull-apart basin. The second stage is related to the evolution of the Caribbean where the Chortis and Yucatan Blocks rotated into the trailing side of the Caribbean Plate (pirate model). The new reconstruction is consistent with major parameters, such as (i) gravity, magnetic, and palaeomagnetic data; (ii) the westward continuation of the Cayman transform faults through the Chiapas foldbelt and along the N–S front of the Sierra Madre Oriental foldbelt; (iii) the 27–19 Ma removal of the southern Mexican forearc; (iv) offset of the Cretaceous volcanic arc (Guerrero-Suina); (v) the deflection of the Laramide orogen (Sierra Madre Oriental–Zongolica–Colon); and (vi) the continuity of Cretaceous platformal carbonates containing Caribbean fauna across Middle America. In this latter context, the Motagua high-pressure belt is interpreted as a Cretaceous extrusion zone into the upper plate above a subduction zone rather than as an oceanic suture.

A Carboniferous high-pressure klippe in the western Acatlán Complex of southern México: implications for the tectonothermal development and palaeogeography of Pangea

High-pressure (HP) rocks are critical for palinspastic restorations because they mark inferred subducted/extruded oceanic crust; knowledge of their geometric, geodynamic, and age relationships provide essential constraints on palaeogeographic reconstructions. The westernmost HP belt (Ixcamilpa) in the Acatlán Complex of southern Mexico has been inferred to be a mid-Late Ordovician backarc basin on the southern Iapetan margin that was subducted beneath eastern Laurentia and extruded up the subduction zone during the Early Silurian. Re-examination of Ixcamilpa HP rocks has revealed that they comprise lower Palaeozoic rift-passive margin protoliths and occur in a W-vergent klippe (not a suture) formed during polyphase deformation. Peak metamorphic mineral assemblages of blueschist-amphibole eclogite facies underwent retrogression through epidote amphibolite to greenschist facies. 40Ar/39Ar dating of various rocks yielded plateau ages of 344–339 Ma for calcic amphibole, 318 ± 4 Ma for glaucophane, and 329–325 ± 2 Ma for muscovite (excess argon), which clearly indicate a Carboniferous tectonothermal event. We interpret the 20 million years range in amphibole ages as reflecting progressive unroofing. The terminal stage of progressive thrusting placed the HP rocks above the middle Mississippian Zumpango Unit, during which a single penetrative sub-greenschist fabric was produced. Subsequent Permian or Laramide deformation refolded all the rocks about NE-trending upright folds. We postulate that the root zone of the HP nappe lies to the east in the median HP belt, which has a structure consistent with an extrusion zone. Inasmuch as similar units of the Acatlán Complex bound this HP root zone on either side, it is inferred to have been extruded into the upper plate above the subduction zone, and thus is not an oceanic suture. Our new data provide constraints for a Carboniferous palaeogeographic reconstruction, whereby subduction erosion of passive margin rocks took place along the western margin of Pangea and were subsequently extruded into the upper (Acatlán) plate.

Syn-collisional channel flow and exhumation of Paleoproterozoic high pressure rocks in the Trans-North China Orogen: The critical role of partial-melting and orogenic bending

Within the paleoproterozoic Trans-North China Orogen, the High-Pressure Belt (HPB) is made of high-pressure (~ 15 kbar) mafic granulites hosted in migmatitic gneisses. In this contribution, we document a set of structural analyses acquired over the whole HPB. We also proposed a morphological subdivision of the partially molten rocks that compose the HPB according to changes in melt fraction. A compilation of the P–T and radiochronological data carried out over the last 15 years is presented. The results highlight the concurrent effect of oroclinal bending and partial-melting in controlling the exhumation of the deeply buried continental crust. During ongoing compression of the thickening orogenic root, onset of partial-melting at peak metamorphism is responsible for a first strength drop that enhanced an eastward lateral flow. Radiometric ages show that the deep crust was partially molten over a 50 Ma lasting period during which it evolved in a diatexite core mantled by metatexites. This was responsible for a second strength drop with strain concentrated along the diatexite/metatexite boundaries, as exemplified by the newly documented Datong–Chengde Shear Zone, a ~ 400 km-long normal shear zone with a sinistral strike–slip component that accommodated the final uprise of the high-pressure rocks.

Prograde and retrograde metamorphic fabrics - a key for understanding burial and exhumation in orogens (Bohemian Massif)

In the Orlica–Śnieżnik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary–metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE–SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the mid-crustal metasedimentary rocks involved successive crystallization of chlorite–muscovite–ilmenite–plagioclase–garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P–T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and XFe are compatible with a P–T path from 3.5–5 kbar/490–520 °C to peak conditions of 6–7 kbar/∼630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ∼4 kbar/∼630 °C and to andalusite–cordierite stability at 2–3 kbar indicating exhumation from 25 km to around 9–12 km. Subsequent exhumation to ∼6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary–metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P–T–D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary–metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the ∼15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.

U–Pb Neoproterozoic–Ordovician protolith age constraints for high- to medium-pressure rocks thrust over low-grade metamorphic rocks in theIxcamilpa area, Acatlán Complex, southern Mexico

High-grade and high-pressure rocks in Acatlán Complex (southern Mexico) are inferred to have been emplaced either during the convergence and collision between Laurentia and Gondwana or during subduction on the western margin of Pangea. In the Ixcamilpa area, such rocks occur in a synformal nappe and are subdivided into (1) the Neoproterozoic–Ordovician Piaxtla Suite (metapsammite, meta-pelite, and amphibolite) that passes structurally upwards from blueschist through eclogite to amphibolite facies; intruded by (2) Cambro-Ordovician megacrystic granitoids; both of which were thrust westwards over (3) the Carboniferous Zumpango Unit consisting of clastic and meta-volcanic rocks. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) U–Pb zircon geochronology yielded age population peaks at (i) 435–490 Ma, probably derived from Acatlán granitoids; (ii) 500–700 Ma, likely derived from the Yucatan Peninsula and Brasiliano orogens; (iii) 800–900 Ma, with provenance in the Goiás arc of eastern Amazonia; and (iv) 950–1300 Ma, sourced either from Oaxaquia, Amazonia, or Laurentia: the younger ca. 310–360 Ma ages are limited to the Zumpango Unit and likely have a local provenance. The overall similarity of the Piaxtla rocks in the Ixcamilpa area and those in the Piaxtla-Mimilulco median belt suggests that Ixcamilpa nappe roots in the median belt, which is interpreted as an extrusion zone within the Acatlán Complex. Since neither high-pressure belt represents a closed ocean, deposition of the Neoproterozoic–Ordovician rocks is inferred to have taken place on the southern margin of Rheic Ocean adjacent to Oaxaquia–Amazonia, whereas the Carboniferous rocks were deposited on the western margin of Pangea synchronous with extrusion of the high-pressure rocks.

Late Quaternary fire regimes of Australasia

We have compiled 223 sedimentary charcoal records from Australasia in order to examine the temporal and spatial variability of fire regimes during the Late Quaternary. While some of these records cover more than a full glacial cycle, here we focus on the last 70,000 years when the number of individual records in the compilation allows more robust conclusions. On orbital time scales, fire in Australasia predominantly reflects climate, with colder periods characterized by less and warmer intervals by more biomass burning. The composite record for the region also shows considerable millennial-scale variability during the last glacial interval (73.5–14.7 ka). Within the limits of the dating uncertainties of individual records, the variability shown by the composite charcoal record is more similar to the form, number and timing of Dansgaard–Oeschger cycles as observed in Greenland ice cores than to the variability expressed in the Antarctic ice-core record. The composite charcoal record suggests increased biomass burning in the Australasian region during Greenland Interstadials and reduced burning during Greenland Stadials. Millennial-scale variability is characteristic of the composite record of the sub-tropical high pressure belt during the past 21 ka, but the tropics show a somewhat simpler pattern of variability with major peaks in biomass burning around 15 ka and 8 ka. There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 ± 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 ka and the history of biomass burning. However, changes in biomass burning in the last 200 years may have been exacerbated or influenced by humans.

Palaeosol stratigraphy across the Permian–Triassic boundary, Bogda Mountains, NW China: Implications for palaeoenvironmental transition through earth's largest mass extinction

Upper Permian and Lower Triassic palaeosols from northeastern Tethyan localities exposed within the Bogda Mountains, NW China, provide a wealth of information regarding long-term palaeoclimatic and palaeoenvironmental variations. Wuchiapingian palaeosols are characterized by intense redoximorphy, accumulation of vascular plant matter, accumulation of clay minerals and Fe-oxides, slickensides, and clastic dikes, suggesting a soil moisture regime that ranged from perennially wet to distinctly seasonal in soil moisture budget. Changsinghian to early Induan palaeosols include subsurface accumulations of clay and carbonate as well as surficial accumulations of organic matter, indicative of sub-humid to sub-arid soil moisture and variable soil moisture regimes. Induan to Olenekian palaeosols contain pedogenic CaCO 3 accumulations and gypsum pseudomorphs, indicating a drier environment characterized by net soil moisture deficiency. Elemental composition of palaeosol matrix was used to estimate palaeoprecipitation through the chemical index of alteration minus Potassium (CIA-K) proxy. Estimates from various Wuchiapingian strata indicate relatively stable palaeoprecipitation. During the late Changsinghian and early Induan, palaeoprecipitation appears to have decreased from 1100 to 230 (± 180) mm/year over less than 100 m of vertical stratigraphic section. In the Induan and Olenekian, palaeoprecipitation appears much less stable than in Wuchiapingian, with values vacillating from 290 to 1014 mm/year. The transition to a relatively unstable precipitation state coincides generally with the Permian–Triassic boundary, and may reflect climatic disturbances associated with the end-Permian extinction event in addition to altered atmospheric circulation patterns resulting from regional tectonics, moisture availability, and expansion of the subtropical high pressure belt.

New SHRIMP U-Pb zircon ages from the Heilongjiang High-Pressure Belt: Constraints on the Mesozoic evolution of NE China

The Heilongjiang Complex is a high-pressure accretionary belt composed mainly of serpentinite, glaucophane-bearing metabasalt, epidote-glaucophane-albite schist, greenschist, marble, two-mica schist, muscovite-albite schist, quartz schist, and quartzite (chert), located along the suture zone separating the Jiamusi and Songliao blocks in NE China. SHRIMP U-Pb zircon analyses of two samples of granitic gneiss from Yilan that are interleaved with the Heilongjiang Complex record diverse protolith ages that are consistent with derivation from regional Permian granitoids (263 ± 7 Ma) and the adjacent Mashan Complex (492 ± 6 Ma), indicating tectonic incorporation of older granitic components into the accretionary complex. The age of the basaltic protolith of two samples of glaucophane-albite schist from Yilan is ∼245 Ma and is slightly younger than the protolith age of two samples of amphibole schist from Mudanjiang, located about 200 km south of Yilan, that define weighted mean ^206^Pb/^238^U ages of 257 ± 4 and 257 ± 5 Ma. All four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at ∼200 to 190 Ma, most likely related to the subsequent high-pressure metamorphism, the timing of which has been determined in previous studies. The basaltic rocks record rifting and ocean development from ∼260 to 210 Ma, based on previous studies in which both E-MORB and OIB protolith signatures were identified. Detrital zircons in two samples of mica schist from Mudanjiang show distinct populations at ∼700 Ma, 500 to 450 Ma, 350 to 213 Ma (peak at 255 Ma), 290 to 200 Ma and ∼200 Ma. All except the youngest ages can be matched with rocks from the older components of the Jiamusi block and with other areas in the Central Asia Orogenic Belt (CAOB), supporting the view that the Jiamusi block was most likely the eastern segment of the CAOB prior to rifting. The youngest concordant magmatic detrital grain with an age of 207 ± 3 Ma defines the maximum age of sedimentation. This also limits the timing of high-pressure metamorphism in the Heilongjiang Complex to post-Late Triassic, consistent with argon data obtained in previous studies. Final closure between the Jiamusi and Songliao blocks took place in the latest-Triassic-Early Jurassic, with the two blocks accreted as a result of Pacific Ocean subduction. This suggests that the Heilongjiang Complex records the time when north-south closure of the CAOB in China gave way to westward movement related to the Pacific Ocean subduction, which has dominated the tectonics of NE China and Far East Russia since that time.