bluuebunny/crossref_metadata_2025 huggingface dataset:
Upper mantle structure of the northern Cascadia subduction zone
Authors: Bostock M. G. , Vandecar J. C.
Published: April 2011

Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.
Seismic reflection constraints from Lithoprobe line 29 on the upper crustal structure of the northern Abitibi greenstone belt
Authors: Bellefleur Gilles , Barnes Arthur , Calvert Andrew , Hubert Claude , Mareschal Marianne
Published: April 2011

Detailed reprocessing of east–west Lithoprobe seismic reflection line 29 includes cross-dip analysis to improve the continuity of the reflectors and median filtering to attenuate shear wave refractions. The interpretation provides additional constraints on the tectonic models of northern Abitibi, but cannot be used to invalidate either of the two models recently presented for the area. However, the seismic profile defines a large east-dipping faulted contact between the Brouillan tonalite and the volcanic rocks exposed west of the intrusion. The moderate dip of the reflectors and their extensive lateral continuity, combined with geological information, provide evidence for an east-dipping thrust sequence and suggest an allochthonous origin for the Brouillan tonalite. The maximum thickness of the volcanic sequence in the northern Abitibi greenstone belt is 8 km, but could be as low as 4 km if Opatica orthogneisses are considered to underthrust northern Abitibi. The mid-crustal reflections confirm the east–west continuity of south-vergent imbrications also observed on a north–south reflection line (28) through northern and central Abitibi. Breakage and displacement of some mid-crustal reflectors may define a west-to-east thrust sequence of sense opposite to that which thrust the Brouillan pluton over the volcanic rocks.
Sm–Nd isotopic geochemistry of Precambrian to Paleozoic granitoid suites and the deep-crustal structure of the southeast margin of the Newfoundland Appalachians
Authors: Kerr Andrew , Jenner George A. , Fryer Brian J.
Published: April 2011

In the Eastern Central Mobile Belt of the Newfoundland Appalachians, late Precambrian basement inliers have εNd from −3 to +2, but Cambro-Ordovician metasedimentary rocks have initial εNd below −7. This region is inferred to have an "inverted" crustal residence structure, which influenced subsequent Appalachian-cycle magmatism. Ordovician and Silurian granitoid suites have εNd of −8 to −2, bracketing both basement and cover, but peraluminous, "S-type" granites have the lowest εNd. Devonian granites have initial εNd values from −5 to +1, and low εNd is associated with peraluminous character. These Paleozoic granites show geographic trends, with lowest εNd values in areas where metasedimentary rocks are abundant. They are suggested to contain anatectic material from both Precambrian basement and metasedimentary cover, but some "I-type" suites probably also include a mantle-derived component. In the adjacent Avalon Zone, Precambrian plutonic suites mostly have εNd from +1 to +6, but there are negative εNd values (−8 to −4) in the westernmost Avalon Zone. Devonian plutonic suites mostly have εNd from +2 to +5. Thus, the Precambrian crust of the Avalon Zone is largely "juvenile," except at its westernmost edge. Contrasts across the Eastern Central Mobile Belt–Avalon Zone boundary, defined by the Dover–Hermitage Bay fault system, indicate a major, crustal-scale structure, and suggest an isotopically distinct "central block" beneath the central Appalachian Orogen, rather than a simple extension of "Avalonian" crust. Similar geographic–isotopic patterns have been reported in Nova Scotia and New Brunswick, suggesting that this pattern represents a first-order deep-crustal subdivision of the northern Appalachian Orogen.
Sm–Nd isotopic evidence for crustal contamination in the ca. 1750 Ma Wanapitei Complex, western Grenville Province, Ontario
Authors: Prevec S. A.
Published: April 2011

The Wanapitei Complex consists of a variably metamorphosed gabbronorite lying immediately adjacent to the Grenville Front Boundary Fault. U–Pb zircon geochronology indicates a crystallization age of [Formula: see text] for a noritic component, with both Grenville-aged (ca. 1000 Ma) metamorphism and minor older inheritance indicated. Geochemical evidence is consistent with plagioclase–pyroxene fractionation, but indicates additional open-system behaviour. [Formula: see text] data indicate contamination of a depleted mantle by light rare earth element-enriched material during the Penokean. This was followed during emplacement by extensive contamination of the then isotopically near-chondritic magma with variable amounts of evolved Archean crustal material, on the order of 40% in extreme cases, generating εNd(1.75) values between 0 and −7.5.
Mount Churchill, Alaska: source of the late Holocene White River Ash
Authors: Richter D. H. , Preece S. J. , Mcgimsey R. G. , Westgate J. A.
Published: April 2011

Although the White River Ash was first recognized by F. Schwatka in 1883, the source of this widespread bilobate Plinian deposit in northwestern Canada and adjoining eastern Alaska has not been definitively identified. Recent investigations disclose that Mount Churchill, in the St. Elias Mountains of south-central Alaska, contains a summit crater whose rim is chiefly composed of ash, coarse pumice, and exotic rock fragments. The pumice clasts decrease in size and the exotic rock fragments disappear with increasing distance from the summit area. Pumice from the Mount Churchill summit, and from the White River Ash deposits up to 25 km from the summit area, are mineralogically and chemically similar. Microprobe analyses of the summit pumice glass are virtually identical to those of more distant proximal White River Ash glass. Fe–Ti oxide compositions in the Mount Churchill proximal tephra indicate that all tephra sampled during this study is from the younger, eastern lobe of the White River Ash. These field and laboratory data demonstrate that Mount Churchill is a Holocene volcano and the source of the eastern lobe of the White River Ash. Glass compositions and distribution patterns, however, indicate both lobes of the ash have a common source.
U–Pb geochronology of the Nain craton on the eastern margin of the Torngat Orogen, Labrador
Authors: Scott David J.
Published: April 2011

New U–Pb geochronological results provide insight into the geological evolution of the northernmost Nain craton. A tonalitic orthogneiss, retrogressed from granulite to amphibolite facies, is interpreted to have crystallized at 2832 ± 3 Ma; zircon overgrowths, dated at 2769 ± 5 Ma, are interpreted as metamorphic in origin, whereas metamorphic monazite crystallized at 2692 ± 2 Ma. The crystallization age of this sample suggests it may be correlative with similar rocks of the Kanairiktok Plutonic Suite of the Hopedale block, whereas the age of the zircon overgrowths is synchronous with metamorphism in the Saglek block and may indicate a link at this time. A foliated tonalité crystallized at 2802 ± 2 Ma; titanite in this sample records deformation at 1745 ± 4 Ma; the emplacement age suggests a link with Hopedale block, whereas the titanite growth is clearly related to Torngat orogeny. Two foliated granites crystallized at 2587 ± 3 and 2564 ± 2 Ma, respectively, synchronous with late magmatic events recognized in the southern Nain craton. An Avayalik mafic dyke was emplaced at [Formula: see text] Ma, and subsequently metamorphosed at [Formula: see text] Ma. An intraboudin granitic pegmatite crystallized at 1780 ± 2 Ma, and titanite in a diorite formed at 1735 ± 3 Ma, recording the effects of deformation in the Komaktorvik shear zone during Torngat orogeny.
U–Pb geochronology of a Paleoproterozoic continental magmatic arc on the western margin of the Archean Nain craton, northern Labrador, Canada
Authors: Scott David J.
Published: April 2011

The Paleoproterozoic Torngat Orogen, in northernmost Labrador and northeastern Quebec, records the collision between the western margin of the Nain Province and the eastern margin of the Rae Province. Six samples from a suite of calc-alkaline plutonic rocks that intrude the Nain craton have been dated using the U–Pb method at 1895 ± 2, 1888 ± 2, [Formula: see text], 1885 ± 2 (two samples), and >1859 Ma, and support the interpretation that east-dipping subduction occurred below the Nain craton during this interval. A granitic dyke that truncates the sinistral shear fabric in the Abloviak shear zone is 1824 ± 2 Ma, placing a younger limit on the timing of this deformation in the area. A second granitic dyke, emplaced synchronously with dextral deformation in the bend in the Abloviak shear zone, is 1798 ± 3 Ma. A model is proposed in which subduction occurred beneath the Nain craton along its entire exposed length from 1.91 to 1.86 Ga, and collision occurred between 1.86 and 1.84 Ga, followed by oblique uplift of the southern part of the Rae craton from 1.80 to 1.71 Ga.
Petrogenesis and regional tectonic significance of Late Devonian mafic intrusions in the Meguma Zone, Nova Scotia
Authors: Tate Marcus C. , Clarke D. Barrie
Published: April 2011

Late Devonian (377–368 Ma, 40Ar/39Ar; 376 Ma, U–Pb) mafic intrusions in the Meguma Zone crop out as dykes, plugs, and synplutonic bodies of gabbro, diorite, or lamprophyre. All of the intrusions have similar lithologie characteristics and hydrous ferromagnesian mineral assemblages, and they appear to represent a genetically related series of mafic bodies with similar petrogenetic histories in the crust of the Meguma Zone. The intrusions show wide chemical variation of SiO2 (45.7–65.7 wt.%), Al2O3 (8.9–26.5 wt.%), MgO (2.8–26.5 wt.%), CaO (1.2–11.2 wt.%), and K2O (0.1–4.4 wt.%), and they have calc-alkaline, high-K calc-alkaline, and shoshonitic characteristics. Large-ion lithophile elements (LILE) are present at variable but high concentrations (e.g., Ba = 62–1920 ppm, Sr = 176–2567 ppm) relative to most high field strength element (HFSE) abundances (e.g., Y = 10–37 ppm, Zr = 8–421 ppm), and light rare-earth elements (LREE) have much higher concentrations than heavy rare-earth elements (HREE) (La/Lu = 24–330). Initial Sr isotopic ratios (0.7044–0.7079) and εNd values (−4.36 to 3.69) are highly variable. Scatter on major oxide variation diagrams probably results from the fractionation of all the major modal phases in the intrusions (olivine, augite, hornblende, and (or) plagioclase), and the cumulate characteristics of some bodies support this suggestion. Nevertheless, parallel patterns for the intrusions on mid-ocean ridge basalt (MORB) normalized spider diagrams support the notion of similar mafic parent melts, and Sr–Nd isotopic data identify contamination by continental crust in only one of the intrusions. The most primitive picrite contains approximately basaltic HFSE in conjunction with HREE at 5–11CN, perhaps suggesting that the magmas emanated from depleted peridotite or pyroxenite, but high alkalies, LILE (<60MN), and LREE (10–100CN), and elevated initial Sr ratios in all of the intrusions, also require the existence of an enriched source component. Troughs in the spider diagrams at Ta, Nb, and Ti, and Sr–Nd isotopic values comparable with modern island-arc basalts, suggest that fluids derived from subducted ocean lithosphère metasomatized the mantle. Tectono-magmatic discriminators imply a continental margin arc environment rather than an island arc, and the intrusions record either Early Devonian subduction of Iapetus Ocean lithosphère beneath the Avalon terrane, Middle Devonian subduction of Theic Ocean lithosphère beneath the Meguma terrane, or an inherited subduction signature formed during a much older event.
The intracratonic Paleoproterozoic Forward orogeny, and implications for regional correlations, Northwest Territories, Canada
Authors: Cook D. G. , MacLean B. C.
Published: April 2011

Reflection seismic data from the Colville Hills and Anderson Plain document an intracratonic compressional event, herein named the Forward orogeny, which affected strata considered equivalent to the >1663 Ma Hornby Bay Group outcropping on Coppermine Homocline. Forward orogeny structures were peneplaned and unconformably overlain by strata considered equivalent to the >1267 Ma Dismal Lakes Group. A comparable tectonic history is recorded in the exposed rocks of Coppermine Homocline to the east. Structural orientations indicate a general northwest–southeast direction of maximum compression during an early phase and a more west–east direction in a later phase. Regional sequence A of G.M. Young and co-workers is subdivided into preorogenic sequence A1 (Hornby Bay and equivalents), and postorogenic sequences A2 (Dismal Lakes and equivalents) and A3 (Coppermine River group and equivalents). The Forward orogeny, dated as approximately 1663 Ma (the age of the syntectonic Kaertok Formation on Coppermine Homocline), is not related to the 1.84–1.9 Ga Wopmay Orogen. Relationships between the Forward orogeny and the Racklan orogeny, in the Wernecke and Ogilvie mountains, remain unresolved because the age of the Racklan is uncertain.
Rock slope movements along the Mount Currie "fault scarp," southern Coast Mountains, British Columbia
Authors: Bovis Michael J. , Evans Stephen G.
Published: April 2011

Rock slope movements are reported from Mount Currie ridge, near Pemberton in the southern Coast Mountains of British Columbia. The site displays linear trenches, antislope scarps, and tension cracks typical of mountain ridges undergoing slow deformation; however, the most prominent linear scarp on the ridge has been previously interpreted as a fault scarp. Kinematic tests on rock discontinuities demonstrate the feasibility of both toppling and plane sliding at this site. Ground-motion vectors based on a 4 year, laser electronic distance measurement (EDM) record of slope movements are generally compatible with the results of the kinematic tests. These results cast doubt on a strictly seismic interpretation of the Mount Currie scarp.
Geochronology of the Little Nahanni pegmatite group, Selwyn Mountains, southwestern Northwest Territories
Authors: Mauthner M. H. F. , Mortensen J. K. , Groat L. A. , Ercit T. S.
Published: April 2011

Geochronological and Pb isotopic studies were carried out to constrain the petrogenesis, emplacement age(s), and relationships of the Little Nahanni pegmatite group to other granitoids in the region. A U–Pb age of 81.6 ± 0.5 Ma for columbite was obtained. K–Ar ages for micas were determined to be 65.4 ± 4.0 Ma for muscovite and 65.8 ± 3.4 and 65 ± 3 Ma for lepidolite. The U–Pb age is interpreted as the emplacement age for the pegmatites, and the K–Ar ages are interpreted to be the timing of a thermal event responsible for resetting the K–Ar mica system. The U–Pb columbite age is 8–13 Ma younger than the average age of the plutons in the Selwyn plutonic suite and indicates that these pegmatites represent a magmatic event that has not been previously recognized. Likewise, the K–Ar ages point to a later thermal event not previously recognized in this area. The results of our studies demonstrate the utility of the U–Pb columbite geochronometer in dating relatively young pegmatite systems.
Mesoproterozoic sedimentation, magmatism, and metamorphism in the southern part of the Grenville Province (western Quebec): U–Pb geochronological constraints
Authors: Friedman R. M. , Martignole J.
Published: April 2011

U–Pb data provide new constraints on the age of sedimentation, metamorphism, magmatism, and deformation in the Grenville Province of western Quebec. A metapelite, an alaskitic gneiss, and an amphibolite were sampled within an area of 1 km2 in the Mont-Laurier terrane. The metapelite yielded detrital-metamorphic zircons that gave 207Pb/206Pb ages of ca. 1205–2200 Ma. The youngest detrital components, between 1210 and 1300 Ma and possibly as old as [Formula: see text] Ma, provide a maximum age range for the deposition of this rock. Data for the alaskitic gneiss suggest that it is either derived from an igneous (volcanic) protolith with a minimum age of ca. 1250 Ma and a maximum age of [Formula: see text] Ma, or is a dyke emplaced at ca. 1140–1170 Ma. The amphibolite yielded zircon interpreted as metamorphic, with a minimum age of 1118 Ma, and a maximum age not likely older than ca. 1160 Ma. Zircons from charnockites and monzonites of the Morin plutonic complex gave zircon igneous ages between ca. 1157 and 1165 Ma. High-grade metapelites of the Réservoir Cabonga terrane yielded metamorphic zircon ages of 1140-1160 Ma. Metamorphic monazites from both the Réservoir Cabonga and the Mont-Laurier terranes yielded ages of 1138−1182 Ma, interpreted as the crystallization age or the time that significant Pb loss ceased. These ages indicate that the two terranes underwent the same long-lasting metamorphic event. The overlap between ages of metamorphic zircons and monazites on the one hand and the age of anorthosite–charnockite magmatism on the other hand suggests a long-lasting high-grade metamorphism with heat contribution from crystallizing plutons. A posttectonic aplite dyke from the interior of the Mont-Laurier terrane gives a zircon minimum age of 1054 Ma, considered a minimum age for penetrative deformation in this part of the Grenville Province. Rutile ages of 945–955 Ma record cooling through about 400 °C in both the Réservoir Cabonga and the Mont-Laurier terranes.
Late Quaternary history of the Mackenzie–Beaufort region, Arctic Canada, from modelling of permafrost temperatures. 1. The onshore–offshore transition
Authors: Taylor Alan E. , Dallimore Scott R. , Outcalt S. I.
Published: April 2011

Five offshore drill holes northeast of Richards Island reveal permafrost conditions, which are interpreted in terms of the sea-level and paleoenvironmental history of the Canadian Beaufort Shelf. The top of ice-bonded permafrost lies 76–88 m below the seabed within 12 km of the shore, 11 m below the seabed 20 km offshore, and at 60 m some 50 km offshore. The base of permafrost is 500 m to over 700 m deep. Temperature–depth gradients are negative or nearly isothermal, and permafrost is degrading. Geothermal modelling of the temperatures and permafrost observations indicates that the sites were subaerially exposed throughout most of the Wisconsinan, and during the early Holocene. Three sites 2–10 km from the present shoreline experienced inundation at 3.5–4.0 ka. These times support dates of marine transgression predicted by the sea-level curve, but the sites may have been occupied by thermokarst lakes prior to transgression. In contrast, the site 20 km offshore with shallow ice bonding was inundated about 0.4 ka; this area may have persisted as an island until recently. Shoals observed nearby may be analogues of this process, being outliers that are now being eroded below sea level. Marine transgression occurred about 6 ka some 50 km offshore.
The Ainslie Detachment: a regional flat-lying extensional fault in the Carboniferous evaporitic Maritimes Basin of Nova Scotia, Canada
Authors: Lynch Gregory , Giles Peter S.
Published: April 2011

The Ainslie Detachment occurs near the base of the Carboniferous Windsor Group, forming a regional flat-lying extensional fault distributed across 10 000 km2. New mapping has delineated the structure through southwestern Cape Breton Island and into central Nova Scotia. Shearing is concentrated at the top of the basal Macumber limestone along its contact with overlying evaporites and younger allochthonous units. The highly contrasting rheologies of the formations created an anisotropic zone of weakness which acted as an upper crustal stress guide, stratigraphically controlling the trajectory of the detachment through the basin. The detachment is characterized by an approximately 3–10 m thick calc-mylonite zone, with an intense planar fabric featuring alternating very fine grained shear planes and coarser annealed layers. Coarser layers are boudinaged into pinch and swell structures, locally producing segmented augen. Highly strained intraclasts, ooids, and peloids, recrystallized carbonate boudins, and carbonate vein segments are included in the calc-mylonite as semirigid inclusions and rotated porphyroclasts. Thick zones of fault breccia straddle portions of the detachment and overprint the mylonite, demonstrating an evolution to brittle conditions during progressive shear. Listric faults in the hanging wall of the detachment feature a ramp and flat geometry, with an upper detachment occurring along the upper contact of the Windsor Group with the overlying Namurian Mabou Group. Locally up to 2 km of the stratigraphic succession has been removed, with faults cutting downsection in a westerly direction producing rollover in the hanging wall.
Geochemistry and provenance of metasedimentary rocks from the Archean Golden Pond sequence (Casa Berardi mining district, Abitibi subprovince)
Authors: Flèche M. R. , Camiré G.
Published: April 2011

The Archean Golden Pond sequence is made up of deformed and metamorphosed conglomerates, greywackes, and mafic volcanic rocks, and is overlain by ferrugineous metasedimentary rocks of the North iron formation. The clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering. Their source area was dominated by the presence of 60–80% high-Al2O3 felsic volcanics having strongly fractionated [La/Sm]N (= 3.7 ± 0.3) and very low Ta/Th ratios (= 0.09 ± 0.02), with lesser proportions of basaltic (10–30%) and ultramafic volcanic rocks (1–10%). The ferrugineous metasedimentary rocks can be modelled by mixing 20–40% siliciclastic material, of the composition of the average Golden Pond greywacke, with an Fe- and Si-rich precipitate (molecular Fe/Si = 0.6 ± 0.2). The high-Al2O3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment, but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenospheric mantle source, possibly in an oceanic rift environment. Therefore, it is suggested that the ultramafic, mafic, and felsic volcanic rocks were brought to the same erosional level by dissection of the arc system and rapid exhumation of the felsic arc lithologies and the deeper ocean floor. Intrabasinal hydrothermal activity associated with contemporaneous mafic volcanism and (or) graben development may have also been responsible for the local production of the Fe-rich precipitates of the North iron formation.
A proposed nomenclature for stages and series for the Cambrian of Laurentia
Authors: Palmer A R
Published: April 2011

An internally consistent succession of six named regional stages (from youngest to oldest: Sunwaptan, Steptoean, Marjuman, Delamaran, Dyeran, Montezuman), with their boundaries defined by regional extinction events within the trilobite record, is proposed for the trilobite-bearing rocks of Laurentia. Sub-trilobite stage(s) are recommended but unnamed. The three youngest stages have been named previously, but the lower boundary of the Marjuman is shifted downward. The three older stages within the trilobite record are new. If deemed necessary, four regional series are also suggested (from youngest to oldest: Millardan, Lincolnian, Waucoban, and Begadean); each of the three youngest series accommodates two stages.
Geochemical and isotopic (Nd, O) data from Ordovician felsic plutonic and volcanic rocks of the Miramichi Highlands: petrogenetic and metallogenic implications for the Bathurst Mining Camp
Authors: Whalen Joseph B , Rogers Neil , van Staal Cees R , Longstaffe Frederick J , Jenner George A , Winchester John A
Published: April 2011

Middle Ordovician felsic magmatism contemporaneous with Bathurst Camp Pb-Zn volcanogenic massive sulphide(VMS) deposits consists of strongly altered volcanic to subvolcanic rocks, belonging to the Tetagouche Group, and relativelyunaltered granitoid plutons, which are divided into northern, central, and southern groups within the Miramichi Highlands.Calc-alkalic felsic volcanic rocks and northern plus central plutons have EpsilonNd(T) values ranging from -8.2 to -1.9 and -4.0 to +0.3, respectively. They exhibit within-plate-type volcanic and transitional I- to A-type granite geochemical characteristics.Granitoid rock Delta18O values range from +8.0 to +10.1. Published granitoid rock Pb isotopic compositions overlapunpublished galena data from Bathurst VMS deposits. Field, geochemical, and isotopic evidence indicate that these volcanicand granitoids rocks are consanguineous and mainly derived from Proterozoic orolder infracrustal sources. Alkalic felsic volcanic rocks, and associated alkaline basaltic rocks, are more juvenile (EpsilonNd(T) = +3.2 to +4.2) and were possibly derivedfrom slightly enriched mantle sources. Southern plutons exhibit continental arc-type features. The felsic magmatism and VMS deposits likely formed in an Okinawa-type back-arc basin developed from rifting the Early Ordovician Popelogan continentalarc, of which the southern plutons are remnants. Correlations between pluton groups and volcanic formations indicate that felsic magmatism was erupted through and onto the Miramichi Group. As most felsic volcanic formations lack plutonicequivalents, the Tetagouche Group probably does not represent disrupted slices of an originally conformable stratigraphic section. This supports a model in which thrust slices juxtapose remnants of volcanic centres erupted at different locationswithin a back-arc basin.
The Great Falls Tectonic Zone: suture or intracontinental shear zone?
Authors: Boerner D E , Craven J A , Kurtz R D , Ross G M , Jones F W
Published: April 2011

The Great Falls Tectonic Zone is generally considered to be the boundary between the Archean Hearne and Wyoming provinces. Although completely buried beneath the western Canadian sedimentary basin, the zone can be studied indirectly through variations in Phanerozoic sedimentation patterns, faulting, basement geochronology, and xenoliths, and with geophysical remote sensing methods. While tectonically active ca. 1.8 Ga and clearly truncating the potential field fabrics of Wyoming Province and Medicine Hat Block, the Great Falls Tectonic Zone lacks a colinear magmatic arc, suggesting that the Hearne-Wyoming juxtaposition did not involve subduction of oceanic lithosphere. Furthermore, electromagnetic studies fail to detect a response that can be interpreted as a plate-edge foreland basin, typical of exposed Proterozoic suture zones. The only conductivity anomaly associated with the zone is weak and appears at depths exceeding 20 km, well below the top of the Proterozoic basement. Taken together, these observations suggest the Great Falls Tectonic Zone may be better interpreted as a reactivated Archean(?) intracontinental shear zone rather than a Proterozoic age suture between Archean provinces.
The response (1958-1997) of permafrost and near-surface ground temperatures to forest fire, Takhini River valley, southern Yukon Territory
Authors: Burn C R
Published: April 2011

Forest fires in permafrost areas often modify ground surface conditions, causing deepening of the active layer and thawing of near-surface permafrost. Takhini River valley lies in the discontinuous permafrost zone of southern Yukon Territory. The valley floor is covered by glaciolacustrine deposits, which are locally ice rich. In 1958 extensive forest fires burned most of the vegetation and the soil organic horizon in the valley, but, 50 km west of Whitehorse, 1 km2 of spruce forest adjacent to the Alaska Highway escaped burning. Permafrost beneath this stand of trees is in equilibrium with surface conditions: the active layer is 1.4 m thick, the base of permafrost is at 18.5 m, the annual mean temperature at the top of permafrost (1.5 m) is -0.8°C, and the temperature gradient in permafrost is constant with depth. At burned sites nearby there has been little regeneration of forest vegetation since the fire, and long-term permafrost degradation has occurred. At one burned site, the permafrost table is more than 3.75 m below the ground surface, the mean annual ground temperature is -0.2°C or warmer throughout the profile, the annual mean temperature at 1.5 m is 0.1°C, and permafrost is thawing from top and bottom. A simplified analytical model for thawing of permafrost indicates that over a millennium will be required to degrade permafrost completely at this site, if thawing proceeds from the top down. The result demonstrates the persistence of ice-rich permafrost a few metres below the ground surface, even at sites near the southern margin of permafrost in Canada.
U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications
Authors: Keppie J Duncan , Davis D W , Krogh T E
Published: April 2011

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton Island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after ~977 Ma, extended through a period of volcanic activity between ~694 and 637 Ma, and ended before the ~629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between ~694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between ~612 and 607 ± 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at ~553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-arc basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic arc following cessation of subduction, respectively. The combination of ~700-550 Ma arc magmatism and ~1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico.
Structural and U-Pb geochronological evidence for 1.47 Ga rifting in the Belt basin, western Montana
Authors: Sears J W , Chamberlain K R , Buckley S N
Published: April 2011

Structural mapping and U-Pb dates from mafic sills in the Perma culmination of west-central Montana constrain the timing of a major rifting event in the early history of the Belt basin. Magmatic zircon from the Plains and Paradise mafic sills within the Prichard Formation yielded U-Pb ages of 1469 ± 2.5 and 1457 ± 2 Ma, respectively. The older sill invaded unconsolidated, wet sediments at a depth of less than 1.6 km, and its emplacement is coeval with widespread soft-sediment folding, mud diapirism, and bathymetric collapse of the basin between members E and F of the Prichard Formation. The youngest sill intruded near the base of the exposed section after significant accumulation of sediment had occurred in the basin. Correlation of these data with recent geochronologic results from the Moyie sills in British Columbia and Purcell lava in Montana suggests that the rifting and subsidence event was basin-wide, and that at least two thirds of the deposition of the Belt Supergroup occurred between ~1470 and 1440 Ma. These results support a continental rift model for the formation of the Belt basin.
Impact of a base metal smelter on the geochemistry of soils of the Flin Flon region, Manitoba and Saskatchewan
Authors: McMartin I , Henderson P J , Nielsen E
Published: April 2011

The regional dispersal patterns of six trace metals (As, Cd, Cu, Hg, Pb, Zn) emitted from the base metal smelter at Flin Flon were examined in surface soils and at depth, using an extensive regional geochemical database for humus and the underlying till. Humus is enriched in those elements emitted from the smelter, and regional dispersal patterns reflect the historical record of smelter contamination. The concentrations of the smelter metals decrease with increasing distance from the stack, until background values are reached, indicating atmospheric fallout from the smelter plume. Smelter contamination is generally restricted to the surface organic-rich horizons, and concentrations of smelter metals in till reflect the absence of significant contamination at depth in the upper C horizon of soils. The maximum radius of contamination varies among the major smelter metals, ranging from 70 km for Cd to 104 km for As. No direct relationship exists among emission, deposition, and sink concentrations, reflecting the complexity of factors influencing total metal concentrations in soils. Factors considered in this study include the natural geochemical signature of the underlying substrate, natural soil-forming processes such as biogeochemical enrichment in the surface organic layer and post-depositional mobilization of metals, and the variation in total metal concentrations among the different fractions and types of samples analysed. With increasing distance from the smelter, these factors become significant and the relative proportion of anthropogenic contamination in the surface terrestrial environment is more difficult to estimate.
Biostratigraphy of the Vendian-Cambrian Sukharikha River section, northwestern Siberian Platform
Authors: Rowland Stephen M , Luchinina Veronica A , Korovnikov Igor V , Sipin Dmitri P , Tarletskov Alexander I , Fedoseev Artem V
Published: April 2011

The Sukharikha River section contains more than 800 m of fossiliferous Vendian and Lower Cambrian carbonate rock deposited in ramp, shelf, and slope environments. A diverse fauna of small shelly fossils, calcibionts, brachiopods, trilobites, and archaeocyaths has allowed us to develop a multi-taxa biostratigraphic framework for this section. A dearth of distinctive fossils low in the Sukharikha Formation prevents us from determining the position of the Vendian-Cambrian boundary. Abundant small shelly fossils and archaeocyaths in the uppermost Sukharikha Formation and low in the Krasnoporog Formation provide ample biostratigraphic control near the base of the Tommotian Stage, but the Nemakit-Daldynian - Tommotian boundary, as defined at Ulakhan-Sulugur on the Aldan River, is temporally ambiguous. For this reason there is no precise definition of this boundary. In the Sukharikha River section we have provisionally placed the base of the Tommotian Stage at the first occurrence of Nochoroicyathus sunnaginicus Zone archaeocyaths, about 1.5 m below the top of the Sukharikha Formation. However, we suppose that this horizon actually predates the deposition of nominally basal Tommotian taxa in the Aldan region. A new global stratotype section for the Nemakit-Daldynian - Tommotian boundary should be selected, and this section may turn out to be a good candidate. The paleontological richness (especially small shelly fossils and archaeocyaths), the apparent absence of long depositional hiatuses, and the presence of well-preserved limestones suggest that the Sukharikha River section contains the combination of paleontological, sedimentological, and isotopic data to resolve some fundamental problems in Early Cambrian stratigraphy.
A three-dimensional gravity model of the southern contact of the Sebago pluton, Maine
Authors: Behn Mark D , Eusden, Jr. J Dykstra , Notte III John A
Published: April 2011

The Sebago pluton is a two-mica granite that intruded the metasedimentary rocks of the Central Maine Terrane around 292 Ma. In recent years, geologists have raised an increasing number of questions related to the overall thickness of the Sebago pluton and the position of its subsurface contact with the underlying metasedimentary rocks. Past studies have shown the Sebago pluton to be a thin, 1-2 km thick, subhorizontal sheet dipping 3° to the northeast. This study examines anomalies in the Earth's gravitational field related to the southern portion of the Sebago pluton, specifically to determine the thickness of the pluton and to locate the subsurface contact between the pluton and the underlying metasedimentary rocks. A three-dimensional model shows the thickest portions of the pluton (~1.8 km) to occur at the bottom of a bowl hape along the southwestern contact. Moreover, the model shows the pluton to thin toward the northern and eastern regions of the study area, where the average thickness is less than 0.5 km. The pluton appears to extend southward below the cover of the metasedimentary rocks along the southwestern contact. Thus, contrary to previous models, the Sebago pluton is not a northeasterly dipping sheet of uniform thickness, but rather an arched sheet with an irregular thickness extending beneath the metasedimentary rocks along both its northern and southern contacts. Based on this new geometry, either the relationship of the pluton to the surrounding metamorphic zones must be modified, or the possibility must be considered that the Sebago pluton is actually a composite batholith, composed of a younger (Permian) granite to the north and an older (Carboniferous) granite to the south.
Wide-angle seismic imaging of pristine Archean crust in the Nain Province, Labrador
Authors: Funck Thomas , Louden Keith E
Published: April 2011

A detailed refraction - wide-angle reflection seismic experiment was carried out in northern Labrador to determine the velocity structure of relatively unaltered Archean crust in the Nain Province. Six 3-component land seismometers were used to record an airgun source along a profile parallel to the coast. Forward modeling of traveltimes and amplitudes yields a P- and S-wave velocity model that shows two crustal blocks separated by a fault. Magnetic data suggest, but do not prove, that the fault is the offshore continuation of the Handy fault. A southwards thickening of the lower crust across the fault indicates that a transcurrent component might have been associated with the faulting. The total crustal thickness is 33 km to the north and 38 km to the south of the fault. The presence of PmS reflections imply a sharp transition at the Moho. Upper crustal velocities of 5.8-6.3 km/s and Poisson's ratios of 0.20 and 0.24, north and south of the fault respectively, are consistent with a gneissic composition, but suggest a higher quartz content in the northern block. Velocities in the middle crust increase to 6.5 km/s, where a discontinuity at a depth between 16 and 18 km marks the transition to the lower crust with velocities between 6.6 and 6.9 km/s. Poisson's ratios of 0.24 and 0.26 indicate, respectively, a felsic middle crust and an intermediate composition for the lower crust. The absence of a high-velocity basal layer is in accordance with other examples of Archean crust.
Anatomy of a terrane boundary: an integrated structural, geographic information system, and remote sensing study of the late Paleozoic Avalon-Meguma terrane boundary, mainland Nova Scotia, Canada
Authors: Webster T L , Murphy J B , Barr S M
Published: April 2011

Remote sensing and geographic information system analysis complimented by geological mapping have resulted in a new interpretation of the Late Carboniferous evolution of the Avalon-Meguma terrane boundary (known as the Minas Fault Zone) in the Canadian Appalachian Orogen. Various images, including optical, radar, and shaded-relief elevation, have been integrated with magnetic and gravity data to compliment mapping in the vicinity of the exposed terrane boundary in mainland Nova Scotia. Throughout much of the region, the style of deformation is typical of dextral motion along the east-west Chedabucto Fault, the most prominent structure in the Minas Fault Zone. Lineament analysis of the shaded-relief elevation and radar images has identified an important lineament trending east-northeast which corresponds to the axial trace of folds that rotate clockwise into parallelism with the Chedabucto Fault. However, in eastern mainland Nova Scotia, the shaded-relief and geophysical images, together with field data, suggest that the Chedabucto Fault was offset by sinistral motion along the north-northwest-trending Country Harbour Fault. Following this event, the region in the vicinity of this offset became a restraining bend during renewed dextral motion along the Chedabucto Fault, resulting in the formation of a positive flower structure represented by the exposure of Early Devonian volcanic and sedimentary rocks in the Guysborough block. The processes described are probably typical of recurrent motions along terrane boundaries.
Lithogeochemistry of volcano-plutonic assemblages of the southern Hanson Lake Block and southeastern Glennie Domain, Trans-Hudson Orogen: evidence for a single island arc complex
Authors: Maxeiner Ralf O , Sibbald Tom II , Slimmon William L , Heaman Larry M , Watters Brian R
Published: April 2011

This paper describes the geology, geochemistry, and age of two amphibolite facies volcano-plutonic assemblages in the southern Hanson Lake Block and southeastern Glennie Domain of the Paleoproterozoic Trans-Hudson Orogen of east-central Saskatchewan. The Hanson Lake assemblage comprises a mixed suite of subaqueous to subaerial dacitic to rhyolitic (ca. 1875 Ma) and intercalated minor mafic volcanic rocks, overlain by greywackes. Similarly with modern oceanic island arcs, the Hanson Lake assemblage shows evolution from primitive arc tholeiites to evolved calc-alkaline arc rocks. It is intruded by younger subvolcanic alkaline porphyries (ca. 1861 Ma), synvolcanic granitic plutons (ca. 1873 Ma), and the younger Hanson Lake Pluton (ca. 1844 Ma). Rocks of the Northern Lights assemblage are stratigraphically equivalent to the lower portion of the Hanson Lake assemblage and comprise tholeiitic arc pillowed mafic flows and felsic to intermediate volcaniclastic rocks and greywackes, which can be traced as far west as Wapawekka Lake in the south-central part of the Glennie Domain. The Hanson Lake volcanic belt, comprising the Northern Lights and Hanson Lake assemblages, shows strong lithological, geochemical, and geochronological similarities to lithotectonic assemblages of the Flin Flon Domain (Amisk Collage), suggesting that all of these areas may have been part of a more or less continuous island arc complex, extending from Snow Lake to Flin Flon, across the Sturgeon-Weir shear zone into the Hanson Lake Block and across the Tabbernor fault zone into the Glennie Domain.
Early Cretaceous kyanite-sillimanite metamorphism and Paleocene sillimanite overprint near Mount Cheadle, southeastern British Columbia: geometry, geochronology, and metamorphic implications
Authors: Digel Scott G , Ghent Edward D , Carr Sharon D , Simony Philip S
Published: April 2011

Mapping of isograds related to regional amphibolite-facies metamorphism constrains a three-dimensional model of isogradic surfaces near Mount Cheadle in the northern Shuswap metamorphic complex (lat. 52°20'N, long. 119°05'W). Kyanite and sillimanite coexist in a lens-shaped zone, bounded by the kyanite-out and sillimanite-in isogradic surfaces, that is 50 km long, up to 10 km thick, and up to 20 km wide. Textural equilibrium, simple regular geometry of isogradic surfaces, and simple mineral assemblages suggest that metamorphism occurred at P-T conditions near those of the kyanite-sillimanite equilibrium curve. Reconstruction of isotherms in the kyanite + sillimanite zone suggests that the metamorphic field gradient was about 14°C·km-1. A 5 km thick, staurolite-free kyanite zone adjacent to the sillimanite-in isograd suggests a pressure range of about 1.5 kbar (1 kbar = 100 MPa) for Bathozone 5 of D.M. Carmichael. Regional metamorphism was Early Cretaceous (monazite U-Pb geochronology) with quenching in the Late Cretaceous, possibly caused by motion on the basal thrust beneath the Malton complex. A younger generation of sillimanite grew in discrete outcrop-scale ductile shear zones, veins, and pods in a north-south-oriented belt (50 km by 20 km). U-Pb dates on zircon, monazite, and titanite indicate an age of the sillimanite overprint of 65-59 Ma. It may have resulted from the influx of hot fluids associated with widespread Late Cretaceous and Paleocene leucogranite emplacement concomitant with extensional faulting.
A comparison of the age and composition of the Shelburne dyke, Nova Scotia, and the Messejana dyke, Spain
Authors: Dunn Andrew M , Reynolds Peter H , Clarke D Barrie , Ugidos Jose M
Published: April 2011

New 40Ar/39Ar age determinations on biotite from the Shelburne dyke of Nova Scotia and from the Messejana dyke of Spain are identical at 203 ± 2 Ma. In addition, both dykes have similar chemical compositions and probably represent similar magma sources, petrogenetic processes, and tectonic settings related to an early phase of opening of the North Atlantic Ocean.
On the origin of the Oak Ridges Moraine
Authors: Barnett P J , Sharpe D R , Russell HAJ , Brennand T A , Gorrell G , Kenny F , Pugin A
Published: April 2011

Landscape analysis, mapping, sedimentology, shallow geophysics, and borehole data are integrated to better understand the complex landform-sediment geometries and event sequences of the Oak Ridges Moraine, southern Ontario. A model for the origin of the Oak Ridges Moraine is based on the recognition that the moraine is built on a high-relief, erosional surface (unconformity) consisting of drumlin uplands and a network of deep, steep-walled, interconnected valleys (tunnel channels). The development of the moraine is thought to have occurred in four stages: I, subglacial sedimentation; II, subaqueous fan sedimentation; III, fan to delta sedimentation; IV, ice-marginal sedimentation. The model traces the transition from subglacial to proglacial conditions during moraine formation and examines the order and timing of sedimentation. It is thought that the early stages of moraine construction are better exposed in the east; in the west, these stages are buried by later stages.