Present Erosion Surface Research Articles

Structural and stratigraphic history of the southern domain of the Indian Mountain Deformed Zone, southeastern New Brunswick, Canada: Tournaisian (Lower Carboniferous) tectonic ‘pop-up’ and collapse

The southern domain of the Indian Mountain Deformed Zone (IMDZ) in southeastern New Brunswick marks a major right-lateral strike-slip fault belt active during late Tournaisian (Lower Carboniferous) sedimentation. The rocks of the Sussex Group, representing a depositional cycle from subsidence to basin inversion, occupy this zone and lie unconformable on crystalline basement, the latter representing a partially exhumed portion of the adjacent (to the south) buried Westmorland uplift. Deformation is related to early reverse faults/thrusts, later strike-slip faults. and latest normal faults. The Gorge Fault zone in the southern domain of the IMDZ demonstrates many essential features of the entire zone. The offset of The Gorge Fault zone increases to the east. In the west it forms a blind thrust and asymmetric anticline whereas in the east it expands into a reverse fault/thrust complex. A progressive evolution from reverse faults/thrusts to strike-slip fault movement resulted in a tectonic pop-up, culminating in gravitational collapse along normal faults with listric profiles that flatten out within 100–200 metres of the present erosion surface. Megabreccias formed during deposition of the Sussex Group are contemporary with reverse fault/thrusts. The geometry of the various faults is best explained by progressive deformation within an overall right-lateral strike-slip regime under general shear, with early formed features rotating both congruently and incongruently to the main IMDZ boundaries. Further complexity is a consequence of many reverse faults/thrusts and normal faults occurring close to a free surface and the latter a response to gravitational instability of the pop-up structure controlled by topography. A revised stratigraphy for the Sussex Group in the Indian Mountain Deformed Zone and its interpretation is integral to constructing the structural history. Two units, Stilesville Formation and Briggs Cross Formation, are formally defined here.

Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering

The timescale of the modification stage of basin-sized impact structures is not well understood. Owing to ca. 10 km of erosion since its formation, the Vredefort impact structure, South Africa, is an ideal testing ground for deciphering post-impact modification. Here, we present geophysical and geochemical evidence from the Vredefort Granophyre Dikes, which were derived from the - now eroded - Vredefort impact melt sheet. The dikes have been studied mostly in terms of their composition, while the timing and duration of their emplacement remain controversial. We examined the modern depth extent of five dikes, with three from the inner crystalline core of the central uplift, and two from the boundary between the core and the supracrustal collar of the central uplift, using two-dimensional electrical resistivity tomography. We found that the core dikes terminate near the present erosion surface (i.e., <5 m depth). In contrast, the dikes at the core-collar boundary extend to a depth ≥ 9 m. These observations suggest that the core dikes are exposed near their lowermost terminus. In addition, we obtained bulk geochemical composition of the dikes, finding that the andesitic composition phase is present in the core-collar dikes that is not found in the core dikes. The presence of this phase indicates the episodic emplacement of impact melt into subvertical crater floor fractures.We conclude that the dike formation was protracted and occurred over a time span of at least 104 years. The sequential formation of the Vredefort Granophyre Dikes points to horizontal extension of crust below the impact melt sheet above a kinematic velocity discontinuity, a crustal instability resulting from the dynamic collapse of the transient cavity.

Dismembered Ophiolite of the Olkhon Composite Terrane (Baikal, Russia): Petrology and Emplacement

Dismembered ophiolites in the Early Paleozoic Olkhon terrane, a part of the Baikal collisional belt in the southern periphery of the Siberian craton, occur as fault-bounded blocks of ultramafic and mafic rocks from a few meters to hundreds of meters in size. The ultramafic rocks are mainly dunite–harzburgite peridotites with gradual transitions between the lithologies, as well as moderate amounts of enstatitite, wehrlite, and clinopyroxenite, but no lherzolite. Most peridotites have strongly depleted chemistry and a mineralogy corresponding to the harzburgite type usual for ophiolites of suprasubduction zones (SSZ). The mafic rocks are leuco- to melanocratic gabbros with different relative percentages of clinopyroxene, olivine, and plagioclase, which enclose thin layers and lenses of clinopyroxenite and anorthosite. They bear back-arc basin geochemical signatures, a setting inferred for the Neoproterozoic southern Siberian craton. The gabbroic rocks are of two geochemical groups; most of their trace-element patterns show Ta-Nb minimums and Sr maximums common to suprasubduction zone ophiolites. Judging by the Ol + Opx + Chl + Chr mineral assemblages, the Olkhon peridotites underwent low amphibolite and amphibolite regional metamorphism at 500–650 °C. The occurrence of the ultramafic and mafic bodies is consistent with formation in an accretionary wedge metamorphosed during a collisional orogeny. The mantle and crustal parts of the Olkhon ophiolite suite apparently were incorporated into the terrane during the frontal collision of perio-oceanic structures with the Siberian craton. Then, in a later oblique collision event, they became dismembered by strike-slip faulting into relatively small bodies and fault blocks exposed in the present erosional surface.

Mineral footprints of the Paleoproterozoic sediment-hosted Abra Pb-Zn-Cu-Au deposit Capricorn Orogen, Western Australia

The minerals exploration undercover relies on the understanding of the ore-forming processes and the associated alteration characteristics that are used in vectoring towards a deposit. Previously, the proposed process for forming the 1590 Ma Abra base-metal deposit, hosted at 250 m depth in the Paleoproterozoic Edmund Basin of the Western Australian Capricorn Orogen, was through exhalation to the seafloor (SEDEX-style). Lacking any visible signs of alteration, sediments overlying the Abra deposit were perceived as younger, unaltered, and cutting the mineralisation and associated alteration comprised of siderite, chlorite and minor sericite. However, a recent study delineated a phengitic (i.e. Al-poor, Mg-, Fe-, Si-rich) white mica footprint in regolith above the deposit from hyperspectral field and remote sensing data. This study links the surficial mineral footprint to the deposit below through detailed analysis of hyperspectral data from drill core. The hyperspectral data processing results were validated using optical microscopy, micro-XRF and SEM-EDS analysis, and compared with geochemical, lithological and structural drill core data. Hyperspectral short-wave infrared (SWIR) data revealed a vertically zoned alteration footprint from the deposit to the surface, where very fine-crystalline phengitic white mica (i.e. sericite) and chlorite are most abundant a few tens of metres above the deposit. The abundance of chlorite gradually decreases, and the distal phengitic alteration is cut by the present erosional surface, but only moderately affected by weathering, confirming the surficial footprint detected in the previous study. Similarly, a vertically zoned carbonate alteration was delineated from thermal infrared (TIR) reflectance spectra from drill cores up to 2 km away from the deposit. Carbonate alteration from proximal to distal comprised of Mn-rich siderite, ankerite and ferroan dolomite, which replace the dolomite matrix in coarse-grained stratigraphic sequences. The replacement of dolomite matrix suggests that these permeable horizons acted as fluid conduits, which channelled hydrothermal fluids into sedimentary strata from a feeder structure. The presence of a vertically zoned carbonate and chlorite-phengite footprint in the overlying sediments advocates for their deposition before the ore formation at Abra, and a sub-seafloor replacement within a permeable sedimentary sequence as the ore-forming process. Comparable alteration footprints have been described around some world-class sediment-hosted base-metal deposits, including the Sullivan in the British Columbia, which is considered as a classic SEDEX-style by some and an exhalative-replacement hybrid by others. Also, identical footprints are common around the volcanic-hosted massive sulphide and are associated with the carbonate replacement-style deposits – an overlap which is an interesting future research space where hyperspectral technologies could provide answers. Validated hyperspectral data combined with geochemical, lithological and structural information enabled the identification of a subtle mineral footprint above the Abra deposit, and the established method can be used for vectoring towards other buried mineralisations – even through weathering.

Targeting Iron Ore in Banded Iron Formations Using ASTER Data: Weld Range Greenstone Belt, Yilgarn Craton, Western Australia

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image analysis is a proven method for mapping mineral and geochemical zonation associated with a variety of ore types, including orogenic Au, porphyry Cu-(Mo), porphyry-skarn, Pb-Zn-Au, and Mn systems. Only recently has this technique been applied, in a general sense, to mineral alteration mapping and exploration for Fe ore deposits hosted by banded iron formations (BIFs). For this reason, the Archean Weld Range greenstone belt that hosts the Beebyn and Madoonga Fe ore deposits has been chosen as a case study area to test the effectiveness of ASTER imaging techniques for the identification of Fe orebodies. Banded iron formations in the Weld Range district crop out as a series of parallel, 10- to 500-m-wide, 55 wt % Fe) iron ore deposits host Archean hypogene magnetite and specular hematite orebodies that are locally replaced by more recently formed, supergene goethite-hematite ore within several hundred meters of the present erosion surface. A common feature of all ore types hosted by BIFs is a high Fe content relative to SiO2. Consequently, all types of Fe ore in the Weld Range district are best identified by the ferric iron to silica index and the opaques to silica index, for the reason that these ASTER image products detect surfaces that are rich in (opaque) Fe oxide minerals and have a low silica abundance. Gabbro, dolerite, and basalt country rocks located within 20 m of high-grade Fe ore zones in BIFs are altered to hypogene Fe-rich chlorite and, more rarely, are altered by Fe-rich talc. These hypogene alteration zones are best detected by the ferrous iron content in MgOH minerals and carbonates and the FeOH group abundance products, which identify hypogene Fe chlorite and Fe talc. This study demonstrates that integrated remote spectral sensing techniques (ASTER, airborne hyperspectral, and radiometric) used in conjunction with geophysical surveys (aeromagnetic and gravity) are useful for district-scale exploration for Fe orebodies hosted by BIFs. The spectral sensing techniques are a rapid, cost-effective, and efficient means for generating and ranking exploration targets that are located in areas with restricted physical access.

Uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the Hoting area, Central Sweden: a palaeomagnetic and40Ar/39Ar study

Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked contact test for two Subjotnian dykes demonstrates the primary nature of the dyke magnetization. From the magnetic properties and the palaeomagnetic data, it can be concluded that the overprinting in the hybrid zone of one of the basic dykes is a partial thermoremanent magnetization. Reliable results were also obtained from a palaeointensity study of samples from the hybrid zone in the baked host rock. The study was performed in the laboratories at Luleå and at Scripps and a mean intensity of the Earth's magnetic field of 5.8 ± 1.9 μT was determined. The maximum temperature due to the dyke intrusion in the hybrid zone has been defined and from that an ambient temperature of 375 °C in the host rock at the time of intrusion has been calculated. This calculated temperature is not contradicted by the 40Ar/39Ar data. A palaeothermal gradient in the crust at ca. 1.6 Ga is calculated at ca. 34 °C km−1 and yields a depth of burial of the present erosion surface at ca. 10.4 km. This implies an uplift rate of 0.65 km (100 Ma)−1. A slow cooling of the gabbroic host rock (ca. 3.5 °C Ma−1) has been calculated from the difference in the U-Pb age of zircon (1.786 ± 0.010 Ga) and the 40Ar/39Ar biotite ages (1.648 ± 0.012 Ga; 1614 ± 0.024 Ga) of this study. This slow cooling resulted in a palaeomagnetic age of ca. 1.7 Ga for the gabbro, which is also the age of the determined palaeointensity. The calculated ca. 1.614 Ga palaeomagnetic pole from the basic dykes fulfils most of the criteria for a reliable pole and may be regarded as a new key-pole for Fennoscandia.

Condições de gênese do depósito de Au-Cu (Bi) Pombo (Terra Nova do Norte - MT) estimadas com microtermometria de inclusões fluidas e termometria de clorita

The Pombo deposit contains hydrothermal quartz-sericite veins with gold and copper along a shear zone in which mineralization developed under decreasing pressure, consequence of the transition from a transpressional to a distensional state. Mineralizing fluids derived from an earliest, high salinity solution, by successive fractionation and immiscibility. Primary, contemporaneous, fluid inclusions from quartz of hydrothermalized rocks and ores displayed the initial participation of fluids modeled by CO 2 , H 2 O-NaCl-KCl-(F) and H 2 O-CO 2 -NaCl-KCl-(F) systems and, at the end, of fluids from H 2 O-NaCl-CaCl 2 system. Late-magmatic and hydrothermal sodic alterations developed at temperatures greater than 320oC and pressures from 2.6 to 1.3 kb. Afterwards, fluids from the same systems, with salinities lower than 15% weight NaCl eq. and temperatures higher than 370oC, gave rise to hydrothermal potassic alteration at host rocks, followed by propylitic and phyllic alterations at lower temperatures and pressures, from 1.9 to 1.4 kb. In the shear zone’s axial regions, at more than 40m below the present erosion surface, fluids with 30 to 40% weight NaCl eq. precipitate minerals of the “deep” ore at temperatures greater than 470oC and pressure from 1.3 to 1.1 kb. That fluid boiled at depth lesser than 40m, at temperatures greater than 470oC and pressure smaller than 1.1kb. Mineralization ceased when the hydrothermal system was dominated by mixing of hydrothermal fluid and meteoric water, which generated an acidic and oxidizing fluid, modeled by H 2 O-NaCl-CaCl 2 system that caused precipitation of great volume of sericite and gold and transformed ore bodies to quartz-microgreisen Au-Cu veins.

Condições de gênese do depósito de Au-Cu (Bi) Pombo (Terra Nova do Norte – MT) estimadas com microtermometria de inclusões fluidas e termometria de clorita

The Pombo deposit contains hydrothermal quartz-sericite veins with gold and copper along a shear zone in which mineralization developed under decreasing pressure, consequence of the transition from a transpressional to a distensional state. Mineralizing fluids derived from an earliest, high salinity solution, by successive fractionation and immiscibility. Primary, contemporaneous, fluid inclusions from quartz of hydrothermalized rocks and ores displayed the initial participation of fluids modeled by CO2, H2O-NaCl-KCl-(F) and H2O-CO2-NaCl-KCl-(F) systems and, at the end, of fluids from H2O-NaCl-CaCl2 system. Late-magmatic and hydrothermal sodic alterations developed at temperatures greater than 320ºC and pressures from 2.6 to 1.3 kb. Afterwards, fluids from the same systems, with salinities lower than 15% weight NaCl eq. and temperatures higher than 370ºC, gave rise to hydrothermal potassic alteration at host rocks, followed by propylitic and phyllic alterations at lower temperatures and pressures, from 1.9 to 1.4 kb. In the shear zone’s axial regions, at more than 40m below the present erosion surface, fluids with 30 to 40% weight NaCl eq. precipitate minerals of the “deep” ore at temperatures greater than 470ºC and pressure from 1.3 to 1.1 kb. That fluid boiled at depth lesser than 40m, at temperatures greater than 470ºC and pressure smaller than 1.1kb. Mineralization ceased when the hydrothermal system was dominated by mixing of hydrothermal fluid and meteoric water, which generated an acidic and oxidizing fluid, modeled by H2O-NaCl-CaCl2 system that caused precipitation of great volume of sericite and gold and transformed ore bodies to quartz-microgreisen Au-Cu veins.

Biotite and muscovite 40Ar–39Ar geochronological constraints on the post-Svecofennian tectonothermal evolution, Forsmark site, central Sweden

In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar–39Ar biotite and some 40Ar–39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1,000 m long borehole samples at Forsmark, central Sweden. The 13 surface samples were collected across 3 branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar–39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below c. 300°C at 1.73–1.66 Ga, and that the rocks could have accommodated strain in a brittle manner between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to fault along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from 40Ar–39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.67 Ga, when the area cooled from c. 500 to 300°C. Cooling rates of 1.9–4°C/m.y. have been attained. Between 1.68 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole 40Ar–39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple cooling after the Svecofennian orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activities further to the west or to a combination of these possibilities.

Structure of Ahmic domain and its vicinity, southwestern Central Gneiss Belt, Grenville Province of Ontario (Canada)

In the Central Gneiss Belt of Ontario (CGB), the parautochthon and a stack of four thrust sheets have been transformed into a complex system of inclined and upright folds. One large upright structure, the 50 km long Wahwashkesh-Ahmic Lakes antiform (WALA), folds the parautochthon and three thrust sheets represented by rock assemblages in the Ahmic, Parry Sound or Shawanaga domains. Despite its tight map pattern west of Magnetawan village, WALA is a gentle to open, disharmonic, noncylindrical, buckle fold that deforms the main foliation and other ductile-style mesoscopic structures in the Grenville gneisses. The main foliation is subhorizontal or moderately inclined, at most localities, but deforms steeply dipping quartz–feldspar veins, possible vestiges of gneissic layers, in the core region of the Ahmic domain. Such planar features could be due to a subhorizontal shortening strain associated with the development of 1080 Ma ductile thrusts in the CGB. The main foliation in the Ahmic domain, on the other hand, is attributable to 1020 Ma vertical thinning of the thrust stack. Transverse gradients of total strain, documented by others over distances of hundreds of meters, characterize the walls of major ductile thrusts. Our results of detailed strain-fabric mapping in metaplutonic rocks presently situated at the Ahmic-Parry Sound domain border, apparent sole-thrust segment of a synformal klippe, provide no evidence for such transverse gradients. We hypothesize that the original walls of the thrust were removed from the level of the present erosion surface, presumably by dip-slip extension faulting.

The Spi Lake Formation of the central Hearne domain, western Churchill Province, Canada: an axial intracratonic continental tholeiite trough above the cogenetic Kaminak dyke swarmGeological Survey of Canada Contribution 20070462.

The Spi Group is a package of plagioclase porphyritic, amygdaloidal basalt flows and sandstone (Spi Lake Formation) conformably overlain by coarse conglomerate and sandstone (Old Boot Formation). These unconformably overlie the Kaminak Lake segment of the Central Hearne supracrustal belt, western Churchill Province. The Spi Group developed in a small, isolated basin (8 km2) and is older than the Paleoproterozoic Hurwitz Group. Our data demonstrate that the basalts of the Spi Lake Formation are genetically related to the ca. 2450 Ma Kaminak dyke swarm, a speculation previously promoted by some workers based on field characteristics alone. They are continental tholeiites with large ion lithophile and light rare-earth element enriched multielement profiles with prominent high field-strength element troughs. The 143Nd/144Nd compositions overlap the contemporaneous chondritic bulk-earth data. All of these are features we infer to have resulted from mixing of melts from dominantly depleted mid-ocean ridge basalt (MORB) mantle with low-degree partial melts of metasomatically enriched sub-continental lithospheric mantle. These composite magmas then underwent clinopyroxene + plagioclase ± olivine fractionation with minor assimilation of local tonalitic middle crust. The Spi Lake basin is &lt;1 km wide, trends 020° parallel to Kaminak dykes but is discordant to the more common 045° to 070° trends of fold keels of the Hurwitz Group. The basin is a graben filled first with basalt and then molassoid sediments, representing the expression of crustal extension associated with Kaminak dyke emplacement. This indicates that the present erosional surface in the region is broadly at the same level as at the time of both Hurwitz and Spi Group deposition.

Structural Controls on Massive Sulfide Deposition and Hydrothermal Alteration in the South Sturgeon Lake Caldera, Northwestern Ontario

Synvolcanic structures played a fundamental role in the genesis, morphology, and siting of volcanogenic massive sulfide ores and associated hydrothermal alteration in the Archean South Sturgeon Lake caldera complex. The most voluminous and persistent hydrothermal venting and massive sulfide deposition occurred along synvolcanic rifts and grabens associated with faults and tectonic fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reservoir. The type of fracturing is highly variable and changes with the composition, competency, degree of consolidation, and alteration of host rocks. Synvolcanic structures and fracture styles also vary according to the amount and type of tectonic movement, including extension-related collapse, shearing and faulting perpendicular to the principal direction of extension, and orthogonal faulting and shearing. Permeable conduits were created by tension fracturing along fault zones, brittle deformation at the intersections of orthogonal faults, and by extensional fractures in stockworks. In texturally uniform footwall rocks, the distribution of alteration zones was controlled by the morphology of the structural conduit. In rocks with vertical and/or lateral facies, permeability, and competency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Some crosscutting synvolcanic structures, alteration zones, and intrusions appear as stratiform units at the present erosion surface due to regional deformation and the present attitude of the volcanic stratigraphy. Hydrothermal mineral assemblages (e.g., quartz, carbonates, chlorite, pyrite, chalcopyrite) infilling structurally induced fractures provide good evidence of fluid migration pathways. However, mineralogy can vary significantly according to the fluid characteristics, host rock geochemistry, and subsequent metamorphic history of the area. Clearly, one of the best methods for locating volcanogenic massive sulfide deposits is to delineate the attitudes of synvolcanic structures, and explore those that show evidence of associated high-temperature hydrothermal mineral assemblages. Excellent exploration targets occur where synvolcanic structures with hydrothermal alteration intersect paleo-seafloor horizons.

Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the Mianlue suture

The geologic framework of the Phanerozoic Qinling–Dabie orogen was built up through two major suturing events of three blocks. From north to south these include the North China craton (including the north Qinling block), the Qinling–Dabie microblock, and the South China craton (including the Bikou block), separated by the Shangdan and Mianlue sutures. The Mianlue suture zone contains evidence for Mesozoic extrusion tectonics in the form of major strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200 Ma thrust belt that reformed by 200–150 Ma thrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian to Early Triassic metasandstones and metapelites, forming a regional-scale block-in-matrix mélange fabric. Five distinct tectonic units have been recognized in the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices including Early Paleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing rift margin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentary rocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separate basin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinct deformation episodes (D 1, D 2 and D 3 during 240–200 Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the D 1 episode. Thrust slices 20 km south of the Mianlue suture are related to this vertical extrusion due to the same rock assemblages, ages and kinematics. The D 2 and D 3 episodes folded all the units in a thick-skinned style about east–west (D 2) and west–northwest (D 3) axes in the Mianlue suture zone. An early foreland propagating sequence of accretion of Late Paleozoic rocks deposited above the Yangtze craton is not involved in D 1 deformation but is temporally equivalent to the D 2 and D 3 deformation in the Mianlue suture. Two stages of strike–slip faulting mainly occurred at the end of D 2 and D 3, respectively. During D 2 deformation, the Bikou block was obliquely indented to the ESE into the Mianlue suture, rather than being thrust over the Mianlue suture from the north as a part of the Qinling–Dabie microblock. During D 3 deformation, however, the Bikou block was bounded by the south boundary fault of the Mianlue suture, and the Yangpingguan fault on the south. These faults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganze orogen. The other basement blocks north of the Mianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of the Wudang dome, during the D 3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D 3 emplacement of granite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D 3 deformation. Therefore, based on insights from the evolution of the Mianlue suture, the D 2 and D 3 episodes in the Mianlue suture and its neighbors are not responsible for and associated with the two-stage extrusion of the Dabie UHP-HP terranes from the Foping dome to the present erosional surface (more than 350 km).

Diagenetic history of the Podhale-Orava Basin and the underlying Tatra sedimentary structural units (Western Carpathians): evidence from XRD and K-Ar of illite-smectite

AbstractDiagenesis in the Podhale and Orava Paleogene flysch basins and in the underlying Mesozoic structural units was studied by XRD measurement of the percent smectite in the mixedlayer illite-smectite from shales and K-Ar dating of the illite-smectite from bentonites, supported by XRD quantitative mineral analysis, grain density, and porosity measurements of the bulk shales. The diagenetic mineral reactions identified in the flysch shales include illitization of smectite (&gt;60 to &lt;5%S), dissolution of K-feldspar and kaolinite, crystallization of quartz, albite and chlorite. An unusually large amount of basin history information was obtained by combining the illite-smectite data from wells and from the present erosional surface of the basin.The rocks underwent burial diagenesis at a stable geothermal gradient similar to the present-day value of 21±2°C/km. The maximum burial temperatures were reached very quickly (high sedimentation rate) close to the basin inversion time, at ∼17 Ma in the western part and 18 Ma in the eastern part.The basin floor, which included the present-day Tatra Mts., was inclined towards the East. The thickness of the sedimentary filling of the basin ranged from 3.5–4.5 km in the western Tatra (removed entirely), to 5–6 km in the western Podhale (&lt;3–4 km removed), to 6.5–7.5 km in the eastern Podhale (&gt;4–5 km removed), and even more in the eastern Tatra and Spisská Magura close to the Ružbachy Fault. These data imply a major subsidence followed by uplift of the Podhale plus Tatra block along the Ružbachy Fault and the deposition of a thick sequence of Lower Miocene sediments over the entire area (latter removed by erosion).The Mesozoic rocks of all the structural units underlying the flysch basin underwent advanced diagenesis (maximum palaeotemperatures of 160–270°C) during an Upper Cretaceous tectonic burial event at ∼80–90 Ma. The tectonic overload was removed before the Eocene transgression (49–42 Ma).

The Acadian fold belt in the Meguma Terrane, Nova Scotia: Cross sections, fold mechanisms, and tectonic implications

Structural data and cross sections from the eastern part of the Devonian‐Carboniferous fold belt of the Meguma Terrane give insight into its structural development and crustal tectonics. The sandy Goldenville Formation, the active competent member during buckling, formed a multilayer several kilometers thick that lay beneath the denser, softer muds and silts of the Halifax Formation. Cross sections reveal 2 orders of folds: 11–18 km wavelength folds are interpreted as resulting from buckling under the influence of gravity; 4–6 km wavelength folds reflect the thickness of the multilayer with buckle shortening in the range 32–44%. Depth to detachment derived from the sections (11–13.5 km below present erosion surface) is such that given the depth of the detachment below the paleosurface during the main phase (pre‐Devonian granite) of folding, it would likely have been a relatively soft and thick shear zone that would have influenced profile geometry of the main phase folds at this stage of fold belt development. Granite production that may have been triggered by mantle delamination during continuing convergence accompanying terminal Pangean assembly and ocean basin closure was followed by rapid exhumation. Later localized deformation (Late Devonian–Carboniferous) in the fold belt took place as the Meguma Terrane was displaced along the terrane‐bounding Cobequid‐Chedabucto fault system. The characteristic asymmetric cross sections of structures of this phase are attributed to cooling and strengthening of the detachment that was a consequence of the postgranite uplift and exhumation.

Anisotropie de susceptibilité magnétique (ASM) et chimie des traces : une nouvelle méthodologie pour démêler processus hydrothermaux et supergènes. Application au gisement à Ba–Fe–F de Chaillac (Indre, France)

Abstract Anisotropy of magnetic susceptibility (AMS) determinations and trace-element analyses have been firstly carried out, at Chaillac, on two ferruginous sandstone sections of hydrothermal and lateritic origins, respectively. The goethite has been identified as the main AMS carrier. Distinct AMS patterns and characteristic Ti–As anomalies have been observed, which allows to quantify the importance of hydrothermal and lateritic processes when both are superimposed. These preliminary results show that the ‘Fe–Ba complex’ of hydrothermal origin at Chaillac has been lateritized for a few meters from the present erosion surface.

(U+Th)–He evidence of Jurassic continuous hydrothermal activity in the Schwarzwald basement, Germany

(U+Th)–He model and isochron ages of 21 botryoidal hematites collected from the Hohberg base-metal vein system in the Triberg granite complex, Mittelschwarzwald (Germany), cover most of the Jurassic period. The possibility of quantitative 4He loss triggered by successive hydrothermal events, although conceivable for individual hematites — based on a 4He retentivity study and calculated opening temperatures of hematite for 4He of >(90–180)°C — can largely be ruled out. Thus the range of model and isochron ages indicates that the majority of the hematites are not cogenetic. This is supported by a broad range of Pb total concentrations in the Hohberg botryoidal hematites (150–2700 ppm), by comparison with virtually identical Pb total concentrations found in cogenetic hematites from four different vein systems in the Schwarzwald. The Th/U ratios of the hematites vary by a factor of ∼700 (0.01–7.25) mainly as a result of the range of Th concentrations (0.1–74 ppm). The incorporation of substantial amounts of Th in hematite is thought to result from increased Th transport mobility, possibly caused by association with colloids and/or complexing in the ore solution. δ 18O analyses of a ∼160 Ma old cogenetic hematite–quartz pair indicate that precipitation occurred from a fluid of magmatic origin at a temperature of ∼180°C. The regional temperature peak in the basement at the present erosional surface during the Jurassic (∼130°C) provides a lower temperature limit of hematite formation. This implies that hematite emplacement occurred at temperatures roughly ranging between ∼130 and ∼180°C. The Jurassic continuous hydrothermal activity suggests that the pre-rifting European craton is characterized by local thermal fluxes during a long time span.

The late Archaean and early Proterozoic: what happened next? (extended abstract)

One of the most interesting processes of the Archaean and early Proterozoic is the development of continental crust, from the generation of greenstone belts to the gradual amalgamation, rifting and regrouping of continental fragments. The geological history of Western Australia is an excellent illustration of this process. This abstract gives a brief and simplified overview of the main events. Western Australia consists of two Archaean blocks, the Pilbara and Yilgarn Cratons, which have completely different histories (Figure 1). The Pilbara Craton is discussed in detail by White et al., Nijman and Zegers et al. in this issue and consists of half a dozen oval to round granitoid batholiths surrounded by greenstone belts and small sedimentary basins. This apparently simple structure was built up during three main cycles of greenstone formation and sedimentation, each associated with intrusion of granitoid rocks within the batholiths (Figure 2). The relatively simple-looking round and oval shape of the batholiths is therefore misleading, since each batholith is composed of several intrusions of different age; their present shape is largely due to the development of shear zones between the surrounding rocks and the batholiths. Although younger sequences occur dominantly in the west of the Pilbara, and older ones in the east, there are no indications that the Pilbara is made up of terranes of different age. Cratonisation of the Pilbara took place around 2.77 Ga, followed by deposition of a platform sequence of the Mt. Bruce supergroup (Fortescue and Hamersley Basins) between 2.77 and 2.4 Ga (Figures 1, 2). The Yilgarn Craton has a completely different history. This craton has a structural ‘grain’ of elongate greenstone belts and granitoid batholiths trending NNW-SSE. Granitoid rocks occupy up to 70% of the present erosion surface. Contrary to the Pilbara, the Yilgarn Craton consists of up to a dozen crustal fragFigure 1. Schematic map of Western Australia showing the main tectonic units and basins, as discussed in the text and shown in Figure 2. E-Gf. – eastern Goldfield terrane, LG – Lake Grace terrane, Bo. – Boddington terrane, Ba. – Balingup terrane, Bi. – Biranup Complex, No. – Nornalup Complex. Area shown measures 1600 km from north to south.

Orogenic evolution of the New England Appalachians of southwestern Connecticut

New U-Pb ages are presented that place constraints on the Paleozoic orogenic evolution of the New England Appalachians in southwestern Connecticut. Beardsley and Pumpkin Ground orthogneisses yield concordant U-Pb zircon ages of 446 ± 2 Ma and 428 ± 2 Ma, respectively. These orthogneisses were considered conformable metavolcanic members of the Harrison Gneiss but are now recognized as structurally juxtaposed metaplutonic units. Pumpkin Ground orthogneiss contains young inherited zircons and has K-feldspars with elevated 207 Pb/ 204 Pb, a characteristic of Avalonian crust. The Ansonia leucogranite (406 ± 13 Ma; discordant zircon and monazite) marks the inception of abundant leucogranite magmatism that heralds the onset of the Acadian orogeny. The Shelton muscovite granite, previously interpreted as a metamorphosed siliceous tuff (Shelton white gneiss), yields a concordant U-Pb garnet age of 380 ± 3 Ma, in agreement with U-Pb monazite ages. Middle Devonian granites have K-feldspars with elevated 207 Pb/ 204 Pb and zircons with possible Avalonian inheritance. Morphologically distinct monazites from staurolite-kyanite-grade pelitic schists of the Trap Falls Formation display complicated U-Pb systematics that indicate Acadian metamorphism was a prolonged event, lasting from ca. 395 to 376 Ma. A U-Pb sphene age of 362 ± 1.5 Ma from the Pumpkin Ground orthogneiss suggests that cooling through the ∼550 °C isotherm was attained by 362 Ma. Based on an assumed temperature of 700 °C at 375 Ma, the cooling rate immediately following the Acadian orogeny was ∼12 °C/Ma. Constraints on deformation are provided by foliated syn-metamorphic granites and cross-cutting post-tectonic intrusions. Deformed intrusions include the Ansonia leucogranite (406 ± 13 Ma), a transposed dike of muscovite granite (391 ± 3 Ma; concordant garnets), and the Shelton muscovite granite (380 ± 3 Ma). Deformation accompanied or post-dated emplacement. Late-kinematic pegmatite and post-kinematic granite yield U-Pb monazite ages of 375 ± 1 Ma and 376 ± 2 Ma, respectively, and show that kinematic deformation pre-dated 375 Ma. The Alleghenian orogeny in southwestern Connecticut was associated with the growth of morphologically distinct metamorphic monazite in the Ansonia leucogranite at 296 ± 2 Ma and emplacement of the Pinewood Adamellite at 291 ± 4 Ma. During this orogeny, temperatures approached but did not exceed ∼550 °C at the present erosional surface. Combined field and laboratory studies provide a geologic link to the U-Pb geochronology and highlight necessary revisions to current tectonic models for the New England Appalachians of southwestern Connecticut. Plutonic rocks constitute a large proportion of exposed units and record nearly continuous magmatism from 446 Ma to 375 Ma. This constrasts sharply with models advocating little to no magmatism in southwestern Connecticut. Crustal-scale imbrication of basement rocks with Grenvillian and Avalonian Pb isotopic signatures occurred by Early Silurian time. The Acadian was a prolonged (ca. 395 to 376 Ma) middle- crustal-level orogenic event that generated considerable volumes of leucogranite.

The significance of Rb-Sr ages of biotite and phlogopite for the thermal history of the Central and Southern Marginal Zones of the Limpopo Belt of southern Africa and the adjacent portions of the Kaapvaal Craton

Generally, Rb-Sr biotite and phlogopite ages range from ∼ 2050 Ma to ∼ 1950 Ma between the Murchison-Thabazimbi and Triangle-Tuli-Sabi shear zones of southern Africa in the rocks comprising the Central and Southern Marginal Zones of the Limpopo Belt and adjacent portions of the Kaapvaal Craton. In some shear zones, however, similar ages ocurr as young as ∼ 800 Ma. In contrast, within the Kaapvaal Craton south of the Murchison-Thabazimbi Shear Zone, Rb-Sr biotite ages range between ∼ 3150 Ma and ∼ 2100 Ma and show no regular distribution. In all cases, the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature, approximately 320°C. The regular pattern of 2050 Ma to 1950 Ma Rb-Sr biotite ages north of the Murchison-Thabazimbi Shear Zone suggests that the present erosional surface approximates a paleo-isothermal surface. This surface was exposed by uplift and erosion which was completed by ∼ 1900 Ma to 1800 Ma ago when the sedimentary and volcanic rocks of the Soutpansberg Group were deposited. The regional uplift was accommodated by motion on the Murchison-Thabazimbi and Triangle-Tuli-Sabi Shear Zones and probably was related to tectonism associated with the emplacement of the anorogenic Bushveld and Phalaborwa complexes. Other anorogenic granitic magmatism accompanied uplift. The depth represented by the paleo-isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely. It is suggested, however, that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift.