Continental Basement Research Articles

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

New U–Pb ages and lithochemical attributes of the Ediacaran Rio Doce magmatic arc, Araçuaí confined orogen, southeastern Brazil

The Araçuaí orogen of southeastern Brazil, together with its counterpart located in Africa, the West Congo belt, formed through closure of a gulf connected to the Adamastor Ocean by the end of the Ediacaran and beginning of the Cambrian. Convergence of the margins of the gulf led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma on a continental basement mostly composed of Rhyacian orthogneisses. The Rio Doce arc mainly consists of tonalite-granodiorite batholiths, generally crowded with mafic to dioritic enclaves, and minor gabbronorite-enderbite-charnockite plutons, suggesting mixing processes involving crustal and mantle sources. We investigate the basement, magma sources and emplacement ages of the Rio Doce arc. Our data suggest the arc comprises three main granitic rock groups: i) Opx-bearing rocks mostly of enderbite to charnockite composition; ii) enclave-rich tonalite-granodiorite (ETG); and iii) enclave-poor granite-granodiorite with minor tonalite (GT). The Opx-bearing rocks are magnesian, calc-alkalic to alkali-calcic and metaluminous. Together, the ETG and GT rock groups range in composition from tonalite to granite, are metaluminous to slightly peraluminous, show a predominantly medium- to high-K, expanded calc-alkaline signature, and other geochemical and isotopic attributes typical of a pre-collisional volcanic arc formed on a continental margin setting. Mineralogical, chemical, and geochronological data suggest the involvement of HT-melting of granulitic (H2O-depleted) sources of Rhyacian age for the generation of Opx-bearing granitic rocks, additionally to magma mixing and fractional crystallization processes. In conclusion, the studied rock groups of the Rio Doce arc were likely formed by interactions of mantle and crustal processes, in an active continental margin setting. These processes involved ascent of mantle magmas that induced partial melting on the continental basement represented by the Rhyacian gneisses.

Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

AbstractWe study the erosive convergent margin of north‐central Chile (at ~31°S) by using high‐resolution bathymetric, wide‐angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2‐D velocity‐depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic‐continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large‐scale crustal thinning and long‐term subsidence of the outermost fore arc.

Heat flow in the southern Chile forearc controlled by large-scale tectonic processes

Between 33°S and 47°S, the southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, several major oceanic fracture zones on the subducting Nazca Plate, the active Chile Ridge spreading centre, and the underthrusting Antarctic Plate. The heat flow through the forearc was estimated using the depth of the bottom simulating reflector obtained from a comprehensive database of reflection seismic profiles. On the upper and middle continental slope along the whole forearc, heat flow is about 30–60 mW m–2, a range of values common for the continental basement and overlying slope sediments. The actively deforming accretionary wedge on the lower slope, however, in places shows heat flow reaching about 90 mW m–2. This indicates that advecting pore fluids from deeper in the subduction zone may transport a substantial part of the heat there. The large size of the anomalies suggests that fluid advection and outflow at the seafloor is overall diffuse, rather than being restricted to individual fault structures or mud volcanoes and mud mounds. One large area with very high heat flow is associated with a major tectonic feature. Thus, above the subducting Chile Ridge at 46°S, values of up to 280 mW m–2 indicate that the overriding South American Plate is effectively heated by subjacent zero-age oceanic plate material.

Beneath Bass Strait: Linking Tasmania and Mainland Australia using Ambient Seismic Noise

One of the most hotly debated topics in Australian geology pertains to the tectonic relationship between Tasmania and mainland Australia. Among the main difficulties in reconciling mainland Australian and Tasmanian tectonics is the lack of Precambrian exposure in the Lachlan Orogen (Victoria), which contrast with the West Tasmania Terrane that exhibits numerous outcrops of Proterozoic rocks, apparently excluding any tectonic affinity between them. Furthermore, the West Tasmania Terrane differs significantly from the East Tasmania Terrane in that the latter does not contain any evidence of Precambrian rocks and no evidence of a Proterozoic continental basement has been reported, either in outcrop nor inferred from geophysical surveys. Furthermore, the presence of Bass Strait and the Mesozoic and Cainozoic sedimentary and volcanic sequences that mask the older terranes, makes the link between Tasmania and southeast mainland Australia even harder to decipher. This has significantly impeded the ability of conventional surface mapping to unravel the tectonic history of this area, which remains one of the great challenges of Australian Earth sciences. The focus of this study is ambient seismic noise data from 24 broadband stations, which span northern Tasmania, several islands in Bass Strait and southern Victoria, thus allowing a dense coverage of surface wave paths that can be exploited to image the 3-D structure of the crust joining Tasmania and Victoria in high detail. To produce the highest quality Greenâ??s functions, careful processing of the data has been performed, after which Group and Phase velocity dispersion measurements have been carried out using a frequency-time analysis method on the symmetric component (average of the casual and acasual signal) of the empirical Green functions. The next step is to invert the dispersion curves for maps of Group and Phase velocity, which is expected to shed new light on the structure beneath Bass Strait.

Structural and Coal-Controlled Characteristics of Boli Coal Basin

Boli coal basin has abundant coal resources and its structure shows southward protruding arcuate structure. There is difference about basement in western and eastern, so it can be further divided into continental craton basement in west and continental craton edge active zone basement in east. For this reason, the coal-bearing strata at the west is stable and its structure is relatively simple. On the contrary the eastern coal-bearing strata is less, poor-quality and thickness is unstable. There are two groups of faults and four folds that mainly control coal-bearing strata distribution. It can be divided into six coal-controlled structural styles in Boli coal basin based on system analysis and summarized about geological exploration data.

Continental subduction channel processes: Plate interface interaction during continental collision

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of peridotite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and mantle-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.

Reply to comment by P. Olivier on “Preorogenic exhumation of the North Pyrenean Agly Massif (Eastern Pyrenees, France)”

We make this reply to Philippe Oliver's comments on the deformation of the Mesozoic cover of the northern Agly massif.

Characterizing the continental basement of the Central Andes: Constraints from Bolivian crustal xenoliths

A critical factor in understanding the development of active continental margins is knowledge of the crustal basement on which magmatic arcs are built. This study reports results from a whole-rock geochemical and zircon U-Pb geochronological study of a suite of crustal xenoliths from the Bolivian Altiplano, Central Andes, that provide new insight into the evolution and composition of the continental basement beneath the region. The xenoliths are hosted in Pliocene–Pleistocene trachyandesitic to dacitic lavas that erupted from monogenetic volcanic centers in the Andean backarc region and comprise both igneous and metamorphic lithologies, including diorites, microgranites, gneisses, garnet–mica schists, granulites, quartzites, and dacites. The xenolith suite exhibits significant Sr-isotopic heterogeneity, with values extending from 0.7105 to 0.7368. Pb isotopic signatures reflect the crustal domains previously constrained from scattered exposures of basement rocks throughout the region. Ion microprobe U-Pb dating of cores and rims from zircon separates from two of the sampled xenoliths reveals predominant early Phanerozoic age peaks (ca. 500 Ma; population 1), late Mesoproterozoic age peaks (1.0–1.2 Ga; population 2), and Paleoproterozoic age peaks (1.7–1.9 Ga; population 3). Populations 1 and 2 are well documented throughout the Andes and correspond to periods of supercontinent formation (e.g., Rodinia at ca. 1.0 Ga) and breakup. Population 3, which is poorly represented in the zircon record of the Andes as a whole, may record geological events during the construction of the Paleoproterozoic Amazonian Craton. The presence of the three age peaks in the detrital zircon population record of a single crustal xenolith demonstrates the important role of crustal recycling in the construction of the modern-day Andean margin. The lithological character of the xenoliths and their detrital zircon ages are inconsistent with current understanding of the eastern extent of the Arequipa-Antofalla basement block beneath the Bolivian Altiplano and instead indicate that it terminates further to the west than previously assumed.

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite–Granite Complex by a 100–250m wide shear zone. Fine-grained syn-kinematic andesine+Mg-hornblende assemblages in meta-tonalites of the Diorite–Granite Complex indicate shearing at T=710±40°C at P<0.5GPa, followed by deformation at greenschist facies conditions. 40Ar/39Ar step-heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the Triassic–Jurassic boundary and persisted until t<188Ma, with the Mafic Complex cooling rapidly at the footwall of the Diorite–Granite Complex at ca. 165.4±1.7Ma.Final exhumation to the basin floor was accommodated by low-angle detachment faulting, responsible for the 1–10m thick damage zone locally capping the Mafic Complex. The top basement surface is onlapped at a low angle by undeformed Mesozoic sandstone, locally containing clasts of footwall rocks. Existing constraints from the neighboring Corsica ophiolites suggest an age of ca. 165–160Ma for these final stages of exhumation of the Santa Lucia basement.These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5Myr through the consecutive activity of extensional shear zones and detachment faults. High thermal gradients may delay the switch from ductile shear zone- to detachment-dominated crustal thinning, thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting.

Late Cretaceous subduction of the continental basement of the Maya block (Rabinal Granite, central Guatemala): Tectonic implications for the geodynamic evolution of Central America

The Rabinal Granite is a peraluminous S-type composite pluton formed upon partial melting of a metasedimentary source region that fringes the southernmost North America plate in central Guatemala. It is therefore considered, together with the intruded metasedimentary sequences, to be part of the continental basement of the Maya block. This leucocratic K-feldspar–plagioclase–quartz–muscovite ± biotite granite shows increasing deformation along its southern margin, where it is cut across by the dextral, Late Cretaceous, top-to-the-NE Baja Verapaz shear zone. Although it has been recently dated at 562–453 Ma (isotope dilution–thermal ionization mass spectrometry), the new data presented here, including laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb and 40 Ar- 39 Ar geochronology and electron-probe mineral chemistry, allow us to more precisely establish the timing of intrusion and metamorphic overprinting of the Rabinal Granite. The zircons dated by LA-ICP-MS indicate a crystallization age of 471 +3 / −5 Ma (Early Ordovician), as well as abundant inherited cores with Pan-African and Mesoproterozoic dates. Laser total fusion Ar-Ar analyses of magmatic low-silica muscovite (Si = 6.2–6.4 atoms per formula unit) indicate cooling following magmatic crystallization during the mid-late Paleozoic and variable extents of resetting of Ordovician micas during Cretaceous metamorphism and deformation. The pressure-temperature ( P - T ) conditions of the inferred Ordovician metamorphism that produced partial melting of the metasedimentary source of the Rabinal Granite and the ascent and crystallization of the granitic melt are uncertain, but a clockwise P - T -time path with maximum P and T of P and T of ∼8.5 kbar and ∼300 °C, respectively. This second event, dated at 70.1 ± 0.6 Ma by means of laser total fusion 40 Ar- 39 Ar analyses on high-Si muscovite grains, is interpreted to be the result of subduction and accretion of the basement of the Maya block during the latest Cretaceous, likely in a transpressional tectonic regime related to the lateral collision of the Maya block with the Pacific (Farallon)–derived Caribbean arc. This finding represents the first direct evidence for latest Cretaceous subduction of the metamorphic Paleozoic basement of the Maya block, north of the Baja Verapaz shear zone.

Geochronology and petrology of the Early Carboniferous Misho Mafic Complex (NW Iran), and implications for the melt evolution of Paleo-Tethyan rifting in Western Cimmeria

We report new petrological, geochemical and geochronological data from the Misho Mafic Complex (NW Iran), which represents a significant component of the West Cimmerian domain in Paleo-Tethys. The Misho Mafic Complex (MMC) mainly consists of gabbros crosscut by abundant basaltic dykes and the overlying basaltic sheeted dyke complex. Gabbros are intrusive into the Precambrian continental basement representing the northern margin of Gondwana. The U–Pb zircon age of a leucogabbro dyke reveals that the igneous emplacement age of the MMC is 356.7±3.4Ma (Early Carboniferous). The gabbros and basaltic dykes are represented by (1) a subgroup of rocks showing normal mid-ocean ridge basalt (N-MORB) affinity, and (2) another subgroup of rocks displaying plume-type MORB (P-MORB) affinity. These subgroups of rocks are coeval. The N-MORB rocks have almost flat N-MORB normalized incompatible element patterns, low Th/Yb, Ta/Yb, Zr/Y ratios, and high Zr/Nb ratios. The P-MORB rocks show significant OIB-type trace element signatures, such as enrichments in Th, Ta, Nb and light rare earth elements (LREE) with respect to N-MORB composition, high Th/Yb, Ta/Yb, Zr/Y ratios, and low Zr/Nb ratios. Petrogenetic modeling suggests that N-MORB rocks were generated by ~13% partial melting of a depleted MORB mantle (DMM) source, whereas P-MORB rocks were generated by ~4–6% partial melting of a DMM source metasomatized by variable proportions of OIB-type (plume-type) enriched components. The mantle melting for both N-MORBs and P-MORBs appears to have started initially deep in the garnet-facies mantle, and then shifted to shallow levels in the spinel-facies mantle where it experienced higher degrees of melting. The MMC collectively formed as a product of interaction between a depleted MORB-type asthenosphere and plume-type material. Its mafic–ultramafic rocks represent an early Carboniferous magmatic event developed during the continental break-up of the northern edge of Gondwana that led to the opening of Paleo-Tethys, that was originally triggered by a mantle plume. This model is consistent with well-documented late Devonian–early Carboniferous mantle plume activity to the east, along the Paleo-Tethys margins in central-eastern Asia, and suggests that the initial rift-drift tectonics of Paleo-Tethys was strongly affected by plume-related magmatism and associated lithospheric weakening at a regional scale.

Geochemistry of komatiites from the Tipasjärvi, Kuhmo, Suomussalmi, Ilomantsi and Tulppio greenstone belts, Finland: Implications for tectonic setting and Ni sulphide prospectivity

Komatiites and komatiitic basalts within the Neo-Mesoarchaean, ca. 2.8Ga greenstone belts in the western part of the Karelian craton in Finland formed in diverse palaeoenvironments. The lavas in the Tipasjärvi and Kuhmo greenstone belts formed in an oceanic plateau environment, whereas the komatiites of the Suomussalmi belt formed in a continental rift setting, consistent with Nd and Pb isotope data of Suomussalmi felsic volcanics indicating the presence of older (>3Ga) crust in the belt. The komatiites and komatiitic basalts of the Ilomantsi greenstone belt show trace element systematics similar to those of modern continental arc volcanics, implying interaction of the lavas with felsic arc volcanics. PGE contents are low (2–4ppb Pt and Pd each) relative to most other Al-undepleted komatiites from elsewhere, interpreted to result from equilibration with sulphides during emplacement. Xenocrystic zircon grains, up to 3.2Ga in age, in some felsic volcanics and granitoids that are coeval with the komatiites indicate the presence of older continental basement in the Ilomantsi area. The Ilomantsi komatiites are interpreted to have formed at an active continental margin where basement rocks were extensively intruded and assimilated by felsic plutonic massifs. The komatiites of the Tulppio belt in northern Lapland resemble those of the Tipasjärvi, Kuhmo and Suomussalmi belts, but due to limited data, their emplacement environment remains uncertain.The Ni sulphide prospectivity of the Finnish komatiite belts appears to be relatively low, despite the identification of geochemical signatures of PGE depletion and crustal contamination in the Ilomantsi belt and, to a lesser degree, Suomussalmi and Tulppio belts, the presence of locally sulphide-bearing felsic volcanics and sedimentary rocks in the footwall of some of the komatiites, and the discovery of several small magmatic sulphide deposits in the Kuhmo, Suomussalmi, Ilomantsi and Tulppio belts. The main reasons are that the Finnish komatiites are relatively differentiated compared to those in mineralised belts elsewhere, and the Finnish belts contain a relatively large proportion of compound sheet flows relative to adcumulates, suggesting that dynamic lava channel environments where significant amounts of sulphide-bearing substrate could be assimilated and deposited are uncommon. Furthermore, the samples analysed to date lack the signature of scattered, but widespread PGE enrichment that is characteristic of mineralized komatiite belts and that has been interpreted to reflect wide-scale saturation of the magmas in Ni-sulphide melt.

Rehabilitation Idea Begins at Sumers

The Demirci migmatitic quartzofeldspathic gneisses of the Sunnice Massif, exposed in the ‹stanbul Zone north of Bolu, have compositions that show them to be paragneisses, associated with subordinate para- amphibolites. They consist principally of metamorphosed, rather calcareous greywackes, with sediment derived from a continental arc source. They occur in the para-autochthon below a nappe pile (Cele meta-ophiolite), deformed and subjected to amphibolite facies metamorphism before the emplacement of late Neoproterozoic metavolcanic rocks later subjected to greenschist-facies metamorphism (Yellice volcanics). As they are overlain in sequence by nappes comprising sections of the Cele meta-ophiolite with ocean floor basalt and island arc tholeiite compositions respectively, the Demirci paragneisses may represent a section of the ancient continental basement on to which the meta-ophiolite was obducted during the Proterozoic.

Petrogenesis and U–Pb zircon chronology of adakitic porphyries within the Kop ultramafic massif (Eastern Pontides Orogenic Belt, NE Turkey)

High-silica adakites intrude the Kop ultramafic massif in the eastern Pontide orogenic belt of NE Turkey. These dominantly dacitic to rhyodacitic plagioclase porphyries form clusters of small intrusions. U–Pb dating of zircons from two intrusions yields ages between 52 and 59Ma. The best-constrained crystallization age is 55.83±0.04Ma, indicating that these are the oldest adakitic rocks of the eastern Pontides. The rocks show geochemical signatures typical of adakites. The absence of systematic geochemical variation within the adakite suite indicates a subordinate role for fractional crystallization and binary mixing, and suggests that individual intrusions may represent separate, small batches of magma. Modeling of partial melting suggests that the degree of melting was 5–20%, that the residues after melting likely contained garnet, clinopyroxene, and rutile, and that plagioclase was either absent or totally consumed during partial melting. The most likely source compositions had large-ion lithophile elements >10 times, and heavy rare earth elements <10 times, chondritic concentrations. Modeled bulk distribution coefficients indicate that the adakites were generated as silicic magmas, and are not derived by fractional crystallization from basaltic or andesitic parental melts. Sr, Nd, and Pb isotopic data indicate at least three isotopically distinct sources, one of which is depleted mantle. The other components include upper crust (possibly a mixture of subducted sediment and Hercynian continental basement rocks) and a component that may include both lower crust and sublithospheric mantle. These adakites are unique among the late Cretaceous and Cenozoic igneous rocks of the eastern Pontides in having a large proportion of the depleted mantle component. The prominent role of the depleted mantle in their petrogenesis suggests that the adakites may be related to a slab window in a south-directed subduction zone.

Formation age and tectonic environment of the Gantaohe Group, North China Craton: Geology, geochemistry, SHRIMP zircon geochronology and Hf-Nd isotopic systematics

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen, North China Craton, and is mainly composed of greenschist-facies metabasalt, meta-sandstone and dolomitic marble. We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group. SHRIMP dating yielded a weighted mean 207Pb/206Pb age of 2087±16 Ma (MSWD=1.3) for magmatic zircons, but there are also abundant ca. 2.5 Ga inherited zircon xenocrysts. The magmatic zircons shows a large ɛ Hf(t) variation in ɛ Hf(t) from −7.17 to +0.45, suggesting an isotopically highly heterogeneous source for the metabasalt. Chemically all samples show no distinct Zr or Hf anomalies, and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram, and their whole-rock ɛ Nd(t) values range from 4.0 to −0.8. We suggest that the basalt is formed by partial melting of a depleted mantle source, followed by significant crustal contamination. Field observations, the presence of abundant inherited zircon, as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement. These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean.

A crust-scale 3D structural model of the Beaufort-Mackenzie Basin (Arctic Canada)

The Beaufort-Mackenzie Basin was initiated in the Early Jurassic as part of an Arctic rifted passive continental margin which soon after became overprinted by Cordilleran foreland tectonics. Decades of industrial exploration and scientific research in this petroliferous region have produced a wide spectrum of geological and geophysical data as well as geoscientific knowledge. We have integrated available grids of sedimentary horizons, well data, seismic reflection and refraction data, and the observed regional gravity field into the first crust-scale 3D structural model of the Beaufort-Mackenzie Basin. Many characteristics of this model reflect the complex geodynamic and tectonostratigraphic history of the basin.The Mesozoic–Cenozoic sedimentary part of the model comprises seven clastic units (predominantly sandy shales) of which the modelled thickness distributions allow to retrace the well-established history of the basin comprising a gradual north(east)ward shift of the main depocentres as well as diverse phases of localised erosion. As a result of this development, the present-day configuration of the basin reveals that the sedimentary units tend to be younger, more porous, and thus less dense towards the north at a constant depth level.By integrating three refraction seismic profiles and performing combined isostatic and 3D gravity modelling, we have modelled the sub-sedimentary basement of the Beaufort-Mackenzie Basin. The continental basement spans from unstretched domains (as thick as about 42km) in the south to extremely thinned domains (of less than 5km thickness) in the north where it probably represents transitional crust attached to the oceanic crust of the Canada Basin. The uppermost parts of the continental crust are less dense (ρ=2710kg/m3) and most probably made up by pre-Mesozoic meta-sediments overlying a heavier igneous and metamorphic crust (ρ=2850kg/m3).The presented crust-scale 3D structural model shows that the greatest thicknesses of Mesozoic–Cenozoic sediments (almost 17km) are not found in the north where the sub-sedimentary crust is thinnest, but farther south where the crust is thicker and the Moho quite deep. We causally relate the huge amounts of foredeep deposits overlying a Moho depression to a flexural response of the lithosphere to orogenic loading induced by the Brooks Range orogen in the south. Our 3D model provides an ideal base and reference for future numerical studies including reconstructions of the development from a passive margin to a foreland basin and simulations of the present-day thermal field of the basin.

Water deep inside the mountains: Unique water samples from the Gotthard rail base tunnel, Switzerland

The construction of the new 56km long Gotthard railway base tunnel in Switzerland opened a large number water conducting fractures and 122 water samples were collected along the Amsteg section of the tunnel and then chemically analyzed. The Amsteg section of the tunnel cuts across granites, gneisses and metarhyolites of the Alpine basement. The overburden of basement rocks above the tunnel axis is up to 2200m thick with rock temperature reaching 45°C. Sodium is the prime cation of most waters; calcium, potassium and magnesium are low to very low in concentration. The associated anions are carbonate/bicarbonate, sulfate and chloride in widely varying concentrations. High to ultra-high fluoride concentrations of up to 29mg/L F and high pH values of up to 10.4 are significant properties of the waters. Six chemical types of water can be distinguished reflecting the different types of fractured rock present in the section. Most of the waters can be viewed as ternary mixtures of dissolved natrite (Na2CO3), thenardite (Na2SO4) and halite (NaCl).The geological and hydrogeological context suggests a meteoric origin of the waters. The unique water compositions and characteristics result from interaction with continental basement rocks along the flow path. The assemblage of major minerals of the different rocks is generally very similar, but the rocks differ with respect to minor minerals and accessories. Derived rock data suggest that dissolution and precipitation of calcite and fluorite, albite dissolution, leaching of salty inclusions, sulfide oxidation by CO2, alteration of biotite to chlorite, precipitation of stilbite and hematite are the important processes that create the observed water compositions. Although fracture water acquires its composition exclusively by chemical interaction with crystalline basement rocks with very similar mineral assemblages, the resulting waters are of great chemical variability.

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.

Two-stage growth of the Calabrian accretionary wedge in the Ionian Sea (Central Mediterranean): Constraints from depth‐migrated multichannel seismic data

New structural images of the Calabrian accretionary wedge are presented from depth‐migrated multichannel seismic data. A combined interpretation swath‐mapping bathymetry allows us to identify five morphological domains on the basis of their tectonic style. (1) Beneath the undeformed Abyssal Plain, a set of deep, NW vergent reverse faults cuts the Ionian oceanic crust and thick pre-Messinian sediments. (2) Towards the NW, the low-taper post-Messinian wedge overlying a shallow NW dipping décollement, at the base of the Messinian evaporites and exhibiting strong tectonic thickening. (3) Beneath the flat Central Transition Zone, a backthrust marks the contact between the post- and pre-Messinian wedges. Here, the décollement dip increases (>3°) cutting through deeper sediments to reach 10km depth. (4) Beneath the pre-Messinian Calabrian wedge, some steep landward dipping features are imaged where underplating may occur. (5) Beneath the inner plateau, a fore‐arc basin lies above the top of the Calabrian continental basement imaged at 8km depth. The architecture of the Calabrian accretionary complex is very similar to the Mediterranean Ridge. Both systems consist of (a) an external low-taper post-Messinian wedge overlying a thick undeformed section of underthrust Mesozoic sediments and (b) an internal pre-Messinian wedge where the décollement steps down and where the underthrust section is presumably underplated. We perform area balancing and show that since the Messinian, the Calabrian accretionary wedge has undergone extremely rapid outward growth at an average rate of 30km/Ma, which makes it the fastest growing accretionary wedge over the past 5Ma.