Search

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordoyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Haroya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordoyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordoyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Scapolite, a volatile-bearing framework silicate, is found in pegmatites and leucosomes spatially associated with (ultra)-high pressure ((U)HP) eclogites in the Nordoyane domain of the Western Gneiss Region (WGR), Norway. The field relationships, petrology, and stable isotope geochemistry of scapolite-bearing rocks offer insights into the role of fluids in partial melting of WGR gneisses during the Scandian orogeny. Three types of scapolite pegmatite are distinguished based on mineralogy, texture, and scapolite chemistry. The most abundant Type 1 pegmatites are associated with dioritic to granodioritic migmatites, including local diatexite, developed in dioritic to granitoid orthogneisses. They are zoned from scapolite+quartz cores to hornblende+plagioclase rims, and concentrated in low-strain areas including boudin necks between disaggregated eclogite bodies. They are interpreted as late-stage differentiates of the (grano)dioritic migmatites, which also contain accessory scapolite in leucosomes. Type 2 and 3 pegmatites also contain coarse-grained scapolite but lack hornblende-rich selvages; Type 2 scapolites are texturally and chemically distinct from other varities. Type 1 and 3 pegmatites and host rock migmatites contain meionitic scapolite with high S- and C– contents, whereas Type 2 pegmatites contain marialitic scapolite with high Cl-contents. The pegmatites crystallised at amphibolite-facies conditions, ca. 750–800 °C and 1 GPa. Scapolites in Type 1, 2, and 3 pegmatites and host rock leucosomes share similar δ13C (−7.9 to −2.2‰) and δ34S (−1.1 to 5.7‰) compositions, interpreted to indicate a common source of volatile components. Comparison with literature data suggests a mantle source for the S and C in the scapolite-forming fluids. We propose that infiltration of mantle-derived fluids into the Baltican continental slab during subduction or the early stages of exhumation drove partial melting in subducted Baltican crust.

Subduction and exhumation of ultrahigh-pressure (UHP) metamorphic terranes are typically envisaged as short-lived processes associated with the transition from oceanic subduction to continent-continent collision. Norway’s Western Gneiss Region, by comparison, is a giant, late-orogenic UHP terrane that underwent protracted residence at UHP conditions during the Scandian phase of the Caledonian orogeny followed by relatively slow exhumation. Here, we use two-dimensional numerical thermal-mechanical models to explore the tectonics of orogens of this type and the associated controls on the size of their UHP terranes and the duration of UHP metamorphism. The models have four tectonic phases designed to capture the main stages of the Caledonian evolution: oceanic subduction and microcontinent accretion; continental margin subduction; plate quiescence; and postorogenic extension during plate divergence. Contrasting styles of exhumation are explored by varying the strength of the margin crust and investigating melt-induced weakening. The tectonic and metamorphic evolution of the Western Gneiss Region is consistent with a model in which continental margin crust was subducted beneath a thick orogenic wedge where it underwent metamorphism at (U)HP conditions for at least 15 million years (Myr) as subduction ended. The buoyant Baltican crust must have been especially strong in order to have stayed coupled to the underlying lithosphere during this phase, perhaps reflecting its refractory composition and/or a lack of fluids. Subsequent exhumation of the Western Gneiss Region can be explained by orogen-scale extension resulting from minor (∼100 km) plate divergence, with removal of the orogenic wedge by combined top-to-the-hinterland transport, normal faulting, and erosion. We conclude that large, long-duration UHP terranes are fundamentally different from transient smaller ones. The latter are often explained by the paradigm of buoyant exhumation. This paradigm is incomplete, but both types can be explained by control of the system by the exhumation number (ratio of buoyancy force to basal traction). By implication, the existence of this type of large UHP terrane is a consequence of the high strength of the subducted crust.

Building on our previous results, we use 2-D upper mantle-scale thermomechanical numerical models to explore key controls on the evolution of Alpine-type orogens and the Alps per se, focusing on (ultra)high-pressure ((U)HP) metamorphic rocks. The models show that UHP rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit. Here we test the sensitivity of the models to surface erosion rate, crustal heat production, plate convergence/divergence rates, geometry of the subducting continental margin, and strength of the retrocontinent. Surface erosion affects crustal exhumation but not early buoyant exhumation. Metamorphic temperatures increase with crustal radioactive heat production. Maximum burial depth prior to exhumation increases with plate convergence rates, but exhumation rates are only weakly dependent on subduction rates. Onset of absolute plate divergence does not trigger exhumation in these models. We conclude that contrasting peak pressures, exhumation rates, and volumes of (U)HP crust exhumed in the Alps orogen primarily reflect along-strike contrasts in the geometry, thermal structure, and/or strength of the subducting microcontinent (Brianconnais) and continental (European margin) crust. The experiments also support the interpretation that the Western Alps (U)HP Internal Crystalline Massifs exhumed as composite, stacked plumes and that these plumes drove local crustal extension during orogen-scale shortening. For weak upper plate retrocrusts, postexhumation retrothrusting forms a retrowedge. Overall, these results are consistent with predictions using the exhumation number (ratio of buoyancy to side traction forces in the conduit), which expresses the combined parameter control of the depth/volume of crustal subduction and the transition to buoyant exhumation.

In order to understand how orogens “work,” a quantitative approach demonstrating proof of concept is essential. Our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of “working” numerical models that are consistent with both the underlying physics and first-order geological constraints. We present a simple conceptual temperature-magnitude (T-M) framework for orogenesis in terms of the progression from small-cold to large-hot orogens, and we use forward numerical models to test hypotheses corresponding to specific stages along the T-M spectrum. Small-cold orogens are analyzed using crustal-scale singularity ( S ) point models, in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics. The transition from oceanic subduction to continental collision, and the subsequent evolution of large-hot orogens, has been investigated using both crustal- and upper-mantle–scale models, the latter including dynamic subduction of suborogenic mantle lithosphere. Large-hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot, weak, lower-crustal infrastructure. Beneath plateaus, tectonic processes are dominated by ductile flow of weak crust in response to differential pressure, while plateau flanks form external thrust-sense wedges. We discuss four topical issues in orogenic tectonics, including the response of the suborogenic mantle lithosphere to convergence, the interaction of climate and tectonics, the current debate concerning wedge versus channel-flow models to explain the Himalayan-Tibetan system, and the interpretation of metamorphic architecture in terms of orogenic processes. We conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins, accompanied by shortening, thickening, and heating of deformed crust.

Eocene (ultra)high-pressure ((U)HP) rocks exposed in the Western Alps are generally interpreted to result from subduction of European continental crust beneath Adria and its subsequent exhumation. However, the roles of extension (either from plate divergence or internal to the orogen) and erosion during exhumation remain controversial. Here we use 2D numerical geodynamic models to explain the formation and exhumation of (U)HP rocks in an Alpine-type orogen and interpret the results in the conceptual Prowedge–Uplifted Plug–Retrowedge–Conduit (PURC) framework. (U)HP metamorphism of oceanic and microcontinent crust in the models results from burial and accretion to a subduction channel/conduit formed beneath an advancing retrocontinent. Rapid exhumation from (U)HP conditions is achieved by the buoyancy-driven transport of a composite plume of stacked (U)HP oceanic and microcontinent crust from the subduction conduit to the overlying orogenic prowedge, accommodated by coeval thrusting and normal-sense shearing. Subsequent ‘trans-crustal’ exhumation is achieved by a combination of doming/internal extension and later retrotransport of the (U)HP plume through the uplifted plug, during underthrusting of the thick continental margin crust, coupled with increased erosion. Our proposed mechanism implies that exhumation-related normal-sense shearing in the Western Alps, per se , was driven from below by the buoyancy of the ascending plume, and that extension owing to plate divergence is not required to explain (U)HP rock exhumation. The efficiency of the exhumation mechanism depends strongly on the buoyancy and strength of the (U)HP plume, suggesting that in order to exhume rapidly, it must achieve a critical size. By implication, the multiple small Eocene (U)HP complexes within the Internal Crystalline Massifs may have been exhumed as part of a single composite body comprising diverse units aggregated from different levels of the subduction conduit during burial or ascent, rather than as individual small bodies exhumed in separate pulses.

The contact aureole at the eastern margin of the South Mountain Batholith (Halifax Pluton) underlies most of the city of Halifax, Nova Scotia. The Halifax Group in the study area includes two lithological units, the Cunard and Bluestone formations. Before intrusion, both had been affected by greenschist facies regional metamorphism and deformed into northeast–southwest-trending, regional upright folds associated with a strong slaty cleavage. Contact metamorphic isograds trend obliquely across the Halifax peninsula, at a high angle to regional structural trends. At 2.5–3 km from the intrusive contact, sparse cordierite spots mark the outer limit of the contact aureole. The biotite-in isograd is marked by the development of biotite within chlorite + muscovite stacks inherited from regional metamorphism. Pyrrhotite is the dominant sulphide mineral throughout the contact aureole. With increasing metamorphic grade, assemblages in both units are marked by increasing modal abundance of cordierite and biotite, with K-feldspar variably developed within ca. 600 m of the contact. However, there is a marked difference in the distribution and appearance of andalusite between the two units. In aluminous pelites of the Cunard formation, idioblastic chiastolite appears before biotite more than 1500 m from the contact. In the less aluminous Bluestone formation, andalusite is present only within ca. 500 m of the contact, where it forms xenoblastic, spongy crystals. In both units, the assemblage andalusite + biotite + K-feldspar ± cordierite is developed near the contact, with local fibrolite and evidence of incipient partial melting. Petrographic constraints suggest pressure–temperature conditions at the contact of ca. 2.5–3.0 kbar (1 kbar = 100 MPa) at ca. 650 °C.

Ultrahigh-pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Haroya in the Nordoyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P–T history of the host eclogite. The coesite–eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr38–41Alm35–37Grs23–26Spss1) and omphacite (Na0.35–0.40Ca0.57–0.60Fe2+0.08–0.10Mg0.53Fe3+0.01AlVI0.40–0.42)2(AlIV0.03–0.06Si1.94–1.97)2O6, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr38–40Alm40–44Grs16–21Spss1), diopside rims on omphacite (Na0.04–0.06Ca0.88–0.91Fe2+0.09–0.13Mg0.81–83Fe3+0.08AlVI0.03)2(AlIV0.07–0.08Si1.92–1.93)2O6, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P–T conditions estimated using thermocalc are ∼3 GPa and 760 °C for M1, consistent with the presence of coesite, and ∼1 GPa and 813 °C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P–T conditions, confirming suggestions that large portions of the WGR were buried to depths of ∼100 km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top-northwest shearing, followed by subhorizontal sinistral shearing along northwest-dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P–T conditions, and exhumation paths of WGR coesite–eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision.

There is a growing need for decentralized wastewater treatment systems that support small abattoirs. Constructed wetlands (CW) are a potential solution; however, research on this subject has been limited to warm climates. Therefore, a research/demonstration site was established at a small-scale abattoir in Great Village, Nova Scotia, Canada. At the site, a 58.5-m2 two-celled surface flow CW with Typha latifolia was constructed. On average, the abattoir produced 0.75 m3of wastewater per animal slaughtered (a mixture of cattle, pigs, and lambs) with an annual inflow of 124 m3 and high monthly variability. The residence time of the CW (during the growing season) was determined by a tracer test to be 111 d, and the active volume of the system was 89%. Overall, mass removals during two years of continuous monitoring were BOD5 (95%), TSS (72%), TDS (81%), TP (88%), SRP (97%), TKN (87%), and TAN (87%). The average E.coli level in wetland effluent (88 CFU 100 mL-1) was below the regulatory limit of 200 CFU 100 mL-1, although 13% of samples exceeded this value. Treatment efficiencies were generally higher during the growing season than the non-growing season.

(Ultra)high-pressure [(U)HP] rocks form and exhume from deep within subduction channels, but subsequent horizontal transport in the shallower orogenic crust makes it difficult to reconstruct their tectonic histories. We use a conceptual framework and numerical models to show that buoyant exhumation from within a subduction conduit formed during one-sided subduction may lead to emplacement of (U)HP rocks into either the lower plate (prowedge) or upper plate (retrowedge) of an orogen, depending on whether the upper plate crust deforms or acts as a backstop during exhumation. Both modes may operate at different positions or different times within an orogen, leading to emplacement of (U)HP rocks into both plates without changing subduction geometry. We propose that retrotransport during exhumation may explain some (U)HP rocks (e.g., Liverpool Land) situated in the upper plate of the Greenland-Norwegian Caledonides.

The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps 19–74 ) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps 22–71 ) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions ( e.g. , sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps 21–52 ), whereas type-3B crystals have a Mn-rich rim (Sps 15–21 ). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions ( e.g. , apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps 4–11 ) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps 8–16 ) was partially resorbed and overgrown by a Mn-rich rim (Sps 11–26 ). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.

Numerical models of tectonic processes in large hot orogens offer insight into the styles, controls and consequences of ductile flow of middle and lower orogenic crust. We present results from crustal-scale thermal-mechanical models illustrating contrasting styles of syn- and post-convergent ductile flow. Crustal viscosity in the models is controlled by temperature-dependent flow laws, scaled to represent lithological strength variations. An additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting. Two end-member tectonic styles, in which lower crustal flow is driven either by gravity or tectonics, develop in response to the initial properties and evolving strength of the model crust. Homogeneous channel flow, driven by the gravitational potential energy difference between a plateau and its foreland, is characteristic of Himalayan–Tibetan (HT-series) models with relatively weak, laterally homogeneous crust and moderately high heat production. In contrast, Grenville Orogen (GO-series) models with relatively strong, laterally heterogeneous crust do not reach the low effective viscosity (ηeff≤1019 Pa.s) required for gravity-driven channel flow. Instead, tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor. The dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland. When tectonic convergence is turned off, the model orogens undergo gravitational spreading, in which ductile middle and lower crust flow laterally from the plateau towards the foreland, driving thrusting on the orogenic flanks and ductile thinning in the orogenic core. The resulting geometry can resemble that produced by syn-convergent ductile flow, except for the development of regional-scale normal-sense shear zones in the post-convergent stage. Model feasibility is tested against data from the western Grenville Orogen in Ontario. The comparison suggests that high-grade metamorphic rocks in the orogenic core record the effects of protracted syn- and post-convergent ductile flow, whereas those on the flanks record mainly post-convergent thrusting and extension. The models suggest a number of ways to distinguish among different styles of syn- and post-convergent ductile flow; metamorphic P–T–t paths alone are not diagnostic of tectonic process without additional information on crustal-scale structure and timing. In combination with erosion, both types of flow may contribute to exhumation of regionally extensive high-grade gneiss terranes.

The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal-sense displacement on an orogen-parallel shear zone during lithospheric contraction. Here, in situ monazite U(-Th)-Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure-temperature-time ( P-T-t ) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite-bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite-bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal-sense (top-to-the-north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P-T-t paths are compatible with modelled P-T-t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid-crustal material.

Numerical models offer powerful insights into tectonic processes, especially when their validity can be tested against geological and geophysical observations from natural orogenic belts. Here we explain some of the criteria for success in integrating orogenic models with data, using examples from the Grenville Orogen. Model designs must be simplified by comparison with nature to illuminate the first-order processes that control orogenic evolution, which limits the extent to which model results can reproduce geological observations. For the western Grenville Orogen, observed variations in geological properties are represented by lower crustal blocks with strength decreasing from the exterior to the interior of the model. GO-series models with this design reproduce the first-order crustal architecture of the Georgian Bay and Montréal – Val d’Or Lithoprobe transects. Both constant-convergence and stop-convergence models produce similar geometries, but only stop-convergence models produce normal-sense shear zones like those observed. EGO-series models, incorporating an initial weak zone bounded by stronger lower crustal blocks, predict exhumation of high-pressure rocks as observed in the eastern Grenville Orogen, although other aspects of these model results are not as successful. The single most important test of a geodynamic model is its ability to integrate diverse and independent observations in a self-consistent manner. Other criteria include consistency with crustal-scale geometry and structural and metamorphic histories. By these criteria, the present models account reasonably well for the syn- and post-convergent evolution of the western Grenville Orogen, but further work is required to produce a fully satisfactory model for the eastern end of the system.

Sulfide minerals (Po + Py + Ccp >> Sp + Gn + Apy) are common constituents of the Meguma Supergroup (MSG) country rocks, and are also common in the marginal facies of the South Mountain Batholith (SMB), Nova Scotia, relative to the interior of the batholith. The distribution and relative modal proportions of sulfides in the granites near the contact indicate that they are probably contaminants from the MSG country rocks. We trace the textural and chemical changes that occur in Py, Po, and Ccp from the country rocks through their xenoliths to the granite during their assimilation in the granite. Pyrrhotite in the country rock ranges from large polycrystalline porphyroblasts with many inclusions to small individual grains free of inclusions, all with low concentrations of Co and Ni. Chalcopyrite in the country rocks occurs mainly as anhedral inclusions in pyrrhotite, and as small isolated grains in the silicate matrix. Pyrite in the country rocks occurs as euhedral cubes or as anhedral cores to pyrrhotite grains. Relative to their occurrences in the MSG country rocks, characteristics of sulfide grains in the granitic rocks include generally smaller grain-sizes, Po grains with fewer silicate and oxide inclusions, development of some Po grains with an inclusion-rich core and an inclusion-free rim, Po grains that have a lower Fe:S ratio and contain higher concentrations of Co (up to 0.17 wt%) and Ni (up to 0.41 wt%), local increases in the Ccp:Po ratio, better development of crystal faces on Ccp inclusions in Po, development of micro-dropper or silicate-melt-displacing textures, and a general absence of Py except for its association with Ccp in microgranitoid enclaves. The grain-size reduction is consistent with disintegration of polycrystalline MSG sulfide grains, either in the solid state or in a partially melted state, as well as dissolution in the SMB magma. The higher concentrations of Co and Ni in the SMB pyrrhotite are consistent with cation exchange with the granitic melt. Most of the remaining characteristics are consistent with partial melting of the multiphase MSG sulfide blebs related to their incorporation in the SMB magma. From known phase-relations in the system Cu–Fe–S, we deduce that temperatures of the SMB magma exceeded the temperature of beginning of melting of the composition-dependent multiphase MSG sulfide aggregates, and thus at least some of them underwent partial melting. Other more refractory grains, which have undergone only textural (annealing, ductile deformation) and chemical modification in the solid state, are simply modified xenocrysts.

The distribution of ultra-high-pressure (UHP) metamorphic rocks demonstrates that burial (to > 100 km) and rapid exhumation (> 1 cm a − 1 ) of continental crust is a normal part of early (∼ 10 Ma) continental collision. Currently, there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper-crustal structures resulting from early collisional UHP rock exhumation. Characteristic features requiring explanation include: structural domes that are cored by UHP nappes; associated medium- to high-pressure nappes displaying a distinct “pressure gap”; overlying lower-grade rocks, including suture zone ophiolites; and, coeval foreland-directed thrust-faults and syn-exhumation normal faults. We present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge. Competition between down-channel shear traction and up-channel buoyancy forces, expressed as the exhumation number, E , controls burial and exhumation, leading to rapid up-channel flow when E > 1. Exhuming UHP material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge, driving thrusting and extension. This solution is compelling because it explains both the geology and the petrology of the Tso Morari and other UHP complexes, and because it demonstrates that pulse-like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures. This places constraints on the exhumation mechanism and provides a test of alternative models. Other proposed mechanisms, such as continuous circulation in a lithospheric-scale wedge or overpressured subduction channel, predict different types of upper-crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of UHP terranes.

Electron-microprobe data were obtained from monazite along five transects within the Longstaff Bluff Formation, a thick package of Paleoproterozoic turbiditic rocks exposed on Baffin Island, northern Canada, and metamorphosed at conditions of the upper greenschist to granulite facies during the Trans-Hudson Orogeny. During prograde metamorphism, the LREE content of monazite progressively decreased, whereas the U, Th, Y and HREE content increased, with sharp changes at the transition from Sil–Kfs rocks to incipiently migmatitic rocks. In situ chemical dating of monazite yielded a range of ages from 1913 to 1741 Ma. The age data form five distinct clusters (1910, 1880, 1845, 1805, and 1765 Ma), of which at least four are interpreted to represent different episodes of monazite growth. The textural relationship and petrographic context of the grains were used to constrain the significance of the data. The two oldest grains (1910 Ma) could be detrital or metamorphic. The 1880 Ma age is interpreted to reflect monazite growth near the onset of regional deformation. The 1845 Ma age cluster is mostly found in the three southern (higher-grade) transects, the 1805 Ma event is widespread throughout the study area, and the 1765 Ma episode of monazite growth has been recognized mostly in the northern (lower-grade) part of the study area. The results suggest a northward progression of metamorphism from granulite- to lower-amphibolite-facies assemblages, with textural data suggesting that the widespread 1805 Ma event was responsible for migmatite formation in the southernmost part of the study area.

Evaluating the origin of garnet, cordierite, and biotite in granites provides important insight into closed- and open-system magma evolution. We present field, textural, major- and trace-element mineral chemical, and Sr^Nd whole-rock isotopic data on garnet-, cordierite-, and biotite-rich zones from the peraluminous South Mountain Batholith. We infer that: (1) garnet-rich zones of decimeter to meter size with � 30 vol. % large, subhedral garnet with abundant inclusions of detrital country-rock monazite represent partially assimilated metapelitic country rocks, where garnet is the incongruent product of biotite-dehydration melting; (2) cordieriterich zones tens of meters to kilometers in dimension, with � 5v ol. % large, subhedral to euhedral, zoned cordierite, formed by crystallization from relatively evolved magmas and subsequent crystal accumulation; (3) biotite-rich zones with large, subhedral to euhedral biotite with abundant euhedral apatite inclusions, making up � 80 vol. % (centimeter-scale) or � 25 vol. % (kilometer-scale) of the rocks, formed dominantly by fractional crystallization throughout the chemical evolution of the batholith. Our results suggest that for garnet and cordierite, a combination of textural and mineral chemical characterization is probably sufficient to determine their origin in granites. However, for biotite and other readily equilibrated minerals, evaluating both mineral and rock textures and majorelement, trace-element, and isotopic compositions is essential.

The tectonic styles of the transition from oceanic subduction to continental collision and their implications for exhumation of UHP material are tested in a series of numerical thermal-mechanical models that address subduction-collision dynamics at constant convergence rate. Reference models examine the sensitivity to continental mantle lithosphere density, pro-continental margin temperature and rheology, and retro-continental margin rheology. Three tectonic styles are observed: (1) continental subduction without significant deformation of the over-riding continent (favoured by relatively dense mantle lithosphere and a strong retro-continental margin), (2) deformation and subduction of the leading edge of the retro-continent (favoured by low-density mantle lithosphere and strong retro-continental crust), and (3) backthrusting of the retro-continental margin and subsequent double subduction (favoured by low-density mantle lithosphere and weak retro-continental crust). Continental (U)HP material is formed in all three tectonic styles, although it is only efficiently exhumed in the first case. Three mechanisms contribute to (U)HP exhumation, their importance depending on the evolving geometry of the subduction zone, the strength of the continental margin crust and the density of the subducting mantle lithosphere. Weaker/hotter continental margins decouple in the subduction channel and are subsequently exhumed by buoyancy, and/or driven circulation of material and/or forced expulsion in response to insertion of strong continent into the subduction channel (plunger effect). Stronger/colder continental material is subducted without detachment and is then transported laterally, underplating the retro-continental mantle lithosphere. Neither surface erosion nor slab break-off are required for UHP exhumation.

Numerical models for channel flow in the Himalayan–Tibetan system are compatible with many tectonic and metamorphic features of the orogen. Here we compare the provenance of crustal material in two channel flow models (HT1 and HT111) with observations from the Himalaya and southern Tibet. Thirty million years after the onset of channel flow, the entire model crust south of the India–Asia suture still consists only of ‘Indian’ material. The model Greater Himalayan Sequence (‘GHS’) is derived from Indian middle crust originating 1000 km south of the initial position of the suture, whereas the Lesser Himalayan Sequence (‘LHS’) is derived mainly from crust originating 1400 km south of the suture. Material tracking indicates little or no mixing of diverse crustal elements in the exhumed region of the model ‘GHS’, which is derived from originally contiguous materials that are transported together in the top of the channel flow zone. These results are compatible with provenance data indicating a clear distinction between GHS and LHS protoliths, with the GHS originating from a more distal position (relative to cratonic India) than the LHS. In model HT111, domes formed between the suture and the orogenic front are cored by ‘Indian’ middle crust similar to the ‘GHS’, consistent with data from the north Himalayan gneiss domes. Material tracking shows that plutons generated south of the suture should have ‘Indian’ crustal signatures, also compatible with observations. Model ‘GHS’ pressure–temperature–time (P-T-t) paths pass through the dehydration melting field between 30 and 15 Ma, consistent with observed leucogranite ages. Finally, exposure of midcrustal ‘GHS’ and ‘LHS’ material at the model erosion front is consistent with the observed appearance of sedimentary detritus in the Lesser Himalaya. We conclude that channel flow model results are compatible with provenance data from the Himalaya and southern Tibet. Numerical models for channel flow in the Himalayan–Tibetan system (e.g. Beaumont et al. 2001) predict that low-viscosity middle crust has flowed outward from beneath the Tibetan Plateau, and has been exhumed between the Main Central Thrust zone (MCT) and South Tibetan Detachment system (STD) in response to focused denudation at the erosion front. Model results are compatible with a number of first-order tectonic and metamorphic features of the orogen (Beaumont et al. 2001, 2004; Jamieson et al. 2004). However, previous work did not address in detail the distribution of material from different sources within the model orogen. Material tracking in the models can be used to predict the provenance of rocks now exposed in the Greater Himalayan (GHS) and Lesser Himalayan (LHS) sequences and in the north Himalayan gneiss (NHG) domes, as well as the likely sources of Miocene leucogranites throughout the region. Conversely, provenance data from the orogen provide an important test of the models and additional constraints on material flow patterns within the model orogen. Here we present results from two channel flow models (HT1 and HT111) that display contrasting tectonic styles in the model region corresponding to the Himalaya and southern Tibet and that are reasonably consistent with a range of first-order observations from the orogen. The purpose of this paper is to compare the distribution of crustal material at and below the model surfaces with provenance data from metamorphic, plutonic and sedimentary rocks in the orogen. In order to facilitate comparison between model results and data from the Himalayan–Tibetan orogen, the following nomenclature has been adopted in this paper. Model times are given in Ma, meaning ‘millions of years before the end of the model’, equivalent to ‘millions of years before present’ in nature. Model features appear in quotation marks; corresponding features of the real Himalayan–Tibetan system do not. The pro-side of the model is referred to as ‘India’ (south) and the retro-side of the model is referred to as ‘Asia’ (north). The upper crust above the model orogen is referred to as ‘Tethyan’ material. The protolith boundary between incoming pro-crust and the outflowing channel is referred to as the MCT (analogous to the Main Central Thrust zone), and the boundary between the extruded channel From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 165–182. 0305-8719/06/$15.00 # The Geological Society of London 2006. and the overlying upper crust is referred to as the STD (analogous to the South Tibetan Detachment system; Jamieson et al. 2004). The initial boundary between pro-side and retro-side crust, interpreted to be equivalent to the Indus–Yarlung Tsangpo suture zone, is referred to simply as the suture. The terms distal and proximal refer to relative distance from the pro-ward end of the model (‘cratonic India’); pro-side distal material originates closer to the model suture than proximal material.

Crustal-scale channel flow numerical models support recent interpretations of Himalayan—Tibetan tectonics proposing that gravitationally driven channel flows of low-viscosity, melt-weakened, middle crust can explain both outward growth of the Tibetan Plateau and ductile extrusion of the Greater Himalayan Sequence. We broaden the numerical model investigation to explore three flow modes: homogeneous channel flow (involving laterally homogeneous crust); heterogeneous channel flow (involving laterally heterogeneous lower crust that is expelled and incorporated into the mid-crustal channel flow); and the hot fold nappes style of flow (in which mid-/lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid-crust). The three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material. The hot fold nappe mode is driven tectonically by, for example, collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows. The heterogeneous mode combines tectonic and gravitationally driven flows. Preliminary results also demonstrate the existence and behaviour of mid-crustal channels during advancing and retreating dynamical mantle lithosphere subduction. An orogen temperature—magnitude (T-M) diagram is proposed and the positions of orogens in T-M space that may exhibit the flow modes are described, together with the characteristic positions of a range of other orogen types.

We present 40Ar/39Ar hornblende and muscovite data from a 200 km transect across the northern flank of the Paleoproterozoic Trans-Hudson Orogen (THO) exposed along Reindeer Lake in Saskatchewan, Canada. The transect crosses a series of distinct lithotectonic domains that experienced pressure–temperature (P–T) conditions of 600–730 °C at 4.5–6.0 kbar (1 kbar = 100 MPa) during D2 peak metamorphism at 1820–1790 Ma. The distribution of 40Ar/39Ar hornblende ages shows that most of the transect cooled through approximately 500 °C by ca. 1765 Ma, with two significant exceptions. At the north end of the transect, older ages in the Peter Lake Domain (up to 2200 Ma) suggest that this region did not experience high-grade metamorphism that affected the rest of the transect and may indicate a structural break between these Archean rocks and the Cree Lake Zone to the north. In the south-central part of the transect, U–Pb (monazite, titanite) and 40Ar/39Ar (hornblende, muscovite) ages indicate that rocks in the vicinity of the Duck Lake Shear Zone, a ductile thrust zone separating the Kisseynew and La Ronge domains, cooled at a rate of 6 °C/Ma and that cooling was delayed by ca. 20–25 million years relative to that of adjacent regions. Based on generic thermal–tectonic models for large hot orogens, we suggest that delayed cooling in the vicinity of the shear zone reflects post-orogenic thermal relaxation of lower crustal isotherms that were perturbed by the effects of convergence.

The links between an early phase of orogenesis, when orogens are commonly wedge shaped, and a later phase, with a plateau geometry, are investigated using coupled thermal-mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust, and simple frictional-plastic and viscous rheologies. Models in which rheological properties do not change with depth or temperature are characterized by growth of back-to-back wedges above the subduction zone. Wedge taper is inversely dependent on the Ramberg number (Rm; gravity stress/basal traction); increasing convergence velocity or crustal strength produces narrower and thicker wedges. Models that are characterized by a decrease in crustal viscosity from η c to η b with depth or temperature, leading to partial or full basal decoupling of the crust from the mantle, display more complex behaviour. For models with a moderate viscosity ratio, η b /η c ∼ 10 - 1 , the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust. A redaction in the viscosity ratio (η b /η c ∼ 10 - 2 ) is sufficient to cause a transition of the central wedge region to a plateau. This transition depends on the basal traction, therefore the thickness of the weak basal layer also affects the transition. Further reduction of the viscosity ratio (η b /η c ∼ 10 - 4 ) leads to full basal decoupling and the development of plateaux in all cases considered. In most models, the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust. Where the crust is fully decoupled, large-scale model geometries for both depth- and temperature-dependent rheologies are similar with gravity-driven flow concentrated in the low-viscosity region. However, strong lateral temperature gradients within these models, controlled by the interaction of horizontal and vertical thermal advection, diffusion and heterogeneous thickening of the radioactive crustal layer, lead to differences in the velocity and deformation fields between the two cases, particularly at the plateau margins. The results suggest that simple depth-dependent viscosity models may be reasonable approximations for describing the large-scale geometry of fully developed plateaux, but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry.

We present new field observations and petrologic and geochronological data from the Muskoka domain in the southwestern Grenville Province of Ontario in an attempt to constrain the relationship between amphibolite-facies and granulite-facies gneisses in areas of transitional metamorphic grade, and to examine their implication for tectonometamorphic models for the Grenville Province of Ontario. The predominant medium-grained amphibolite-facies migmatitic orthogneisses of the Muskoka domain contain several generations of leucosome, some of which are related to southeast-directed extensional structures. The amphibolite-facies granitoid gneisses contain numerous mafic enclaves with granulite-facies assemblages recrystallized from anhydrous precursors during Grenvillian metamorphism. Other associated granulites are characterized by their patchy occurrence and gradational contacts, similar to the charnockites in southern India. Patchy granulites, leucocratic vein networks in mafic enclaves, and crosscutting leucocratic granulite veins are interpreted to have formed as a result of local differences in reaction sequences and (or) fluid compositions. The U–Pb zircon lower intercept age of the patchy granulites overlaps with the previously determined range of 1080–1060 Ma for high-grade metamorphism in the Muskoka domain, while zircon and titanite from a crosscutting granulite vein crystallized at about 1065–1045 Ma, supporting a Grenvillian age for granulite formation. Peak metamorphic conditions of 750–850°C and 10–11.5 kbar (1 kbar = 100 MPa) were determined from the mafic enclaves, whereas the more felsic migmatites reequilibrated at somewhat lower temperatures. The high temperatures caused extensive migmatization and facilitated rheological weakening of the Muskoka domain 10–25 million years after the start of the Ottawan orogeny in the Central Gneiss Belt.

Coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr−1, and a slope-dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m−3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m−3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro-side of the system, and is driven by a depth-dependent pressure gradient that is facilitated by the development of a temperature-dependent low viscosity horizon in the mid-crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post-convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust-related exhumation on the flanks. Velocities in the pro-side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro-side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal-sense offset at its upper boundary, reverse-sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

Recent interpretations of Himalayan–Tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the Tibetan plateau1,2,3, and that ductile extrusion of high-grade metamorphic rocks between coeval normal- and thrust-sense shear zones can explain exhumation of the Greater Himalayan sequence4,5,6,7. Here we use coupled thermal–mechanical numerical models to show that these two processes—channel flow and ductile extrusion—may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low-viscosity material. Our models provide an internally self-consistent explanation for many observed features of the Himalayan–Tibetan system8,9,10.

High-P/low-T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560–550 Ma volcanic and sedimentary rocks and co-magmatic plutonic units of the Caledonian terrane. The HMS consists of albite- and garnet-porphyroblastic mica schist, with minor marble, calc-silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc-silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618–615 Ma, a minimum age for high-P/low-T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625–600 Ma subduction-generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic-Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now-adjacent Brookville terrane.

Revised cross sections of the western Grenville Province incorporate new geologic results and reprocessed seismic reflection data. The geology is presented in terms of three tectonic elements: (1) "pre-Grenvillian Laurentia and its margin" with ca. 1740 and 1450 Ma continental arc plutons and associated supracrustal rocks; (2) "Composite Arc Belt" of allochthonous ~1300-1250 Ma volcanic arcs and sedimentary rocks; and (3) "Frontenac-Adirondack Belt" characterized by supracrustal and granitoid rocks, and anorthosites, of uncertain affinity, that may represent a distinctive part of the Composite Arc Belt or an offshore (micro)continent. Rocks of the Composite Arc and Frontenac-Adirondack belts were amalgamated with each other by ca. 1160 Ma, were then thrust over Laurentia during ca. 1080-1035 Ma and ca. 1010-980 Ma phases of convergence, and were dissected and exhumed by

40Ar/39Ar dates from eight muscovite separates and two whole-rock samples of slate, siltstone, and sandstone provide detrital, metamorphic, and post-metamorphic overprinting ages for the low-grade metamorphic rocks of the Meguma Supergroup in the Mahone Bay region of southern Nova Scotia. Variably reset detrital muscovite ages of 600-550 Ma agree with previously obtained U-Pb dates from detrital zircon. Separates of foliation-parallel muscovite yield precise 40Ar/39Ar ages of 395-388 Ma. Petrographic and metamorphic data indicate that these are metamorphic mica growth ages. The range overlaps with the younger limit of previously reported whole-rock ages (410-380 Ma), but is more tightly constrained. An apparent inconsistency between previous 40Ar/39Ar whole-rock ages as old as 410 Ma and regional deformation that affected sedimentary rocks as young as 385 Ma has been resolved by the younger ages reported here and by recent adjustments to the early Paleozoic time scale which suggest that the Lochkovian to Emsian Torbrook Group was deposited between 419 and 394 Ma. A muscovite plateau age of 376 Ma from one sample may be related to late-stage flexural slip deformation in the Ovens gold district.

Continental collision generally occurs after a protracted history of rifting, sedimentation, and subduction. Inherited material heterogeneities and thermal effects from terrane accretion and amalgamation may control subsequent collisional deformation. Such strength contrasts may have played an important role in the Silurian evolution of the Newfoundland Appalachians. A cross-sectional plane-strain numerical model is used to investigate the effect of a weak zone embedded between stronger model crust (the "vise") on the deformation style of orogens. In contrast to collision of uniform strong crust in which deformation is directly related to the underlying subduction of mantle lithosphere, deformation in vise models can propagate out to the ends of the vise in a diffuse manner. This distributed tectonic style depends on the relative strength of the embedded weak zone, the degree of coupling of this zone to underlying layers, and the effect of gravity acting on thickened crust. For weak coupling at the base of the crust, results are insensitive to the behaviour of the underlying model mantle (e.g., subduction, subduction retreat, or pure-shear thickening). Vise model results are in first-order agreement with the following characteristics of Newfoundland Silurian tectonics: (i) a diffuse deformation style distributed over a weak core zone more than 100 km wide, (ii) lack of indirect evidence of mantle dynamics from crustal reflectivity fabric, and (iii) lack of clear evidence for significant crustal thickening during orogeny.

In the southwestern Grenville Province, the Central Gneiss Belt consists of a belt of parautochthonous rocks in the north and a collage of allochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario, the allochthon-parautochthon boundary is marked by the Shawanaga shear zone, a northwest-directed thrust zone between the Britt and Shawanaga domains that was reactivated during ductile, top-to-the-southeast extensional shearing. UPb ages of 1042+4/−2, 1019±4 and 988±2 Ma for granitic pegmatite dykes that are pre-kinematic, late syn-kinematic, and post-kinematic with respect to top-side-down displacement constrain major extensional transport on the Shawanaga shear zone to ca. 1020 Ma. However, nearly all the zircons in the dykes are inherited from a pre-Grenvillian, ca. 1460 Ma granitic host rock to the dykes. Recognition of the inherited nature of these grains comes from the multi-dyke dating approach employed here, and illustrates a potential pitfall in dating shear zone movement using only single dykes interpreted as syn-kinematic. UPb ages of metamorphic titanite from 14 samples collected in a transect across the Shawanaga shear zone span a 93 m.y. segment of concordia. These ages do not correlate with titanite grain size or vary systematically from domain to domain. However, when sample microtexture, strain state, structural position, titanite morphology, and pegmatite UPb data are considered together, the titanite ages can be reasonably inferred to date: 1. (1) regional metamorphism (1049-1045 Ma); 2. (2) cooling below the titanite isotopic closure temperature (∼600°C) during extensional unroofing (1028-1018 Ma); 3. (3) recrystallization-controlled titanite growth and (or) isotopic resetting in high-strain zones (1008-1000 Ma); 4. (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made up of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain but rather occurred within localized regimes, some which were clearly structurally controlled. The 93 m.y. spread of concordant titanite ages within a 20 × 20 km area demonstrates that cooling through isotopic closure is only one of several possibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes.

The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso-proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late-stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon-parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

New U–Pb zircon and titanite data from the Muskoka domain, Grenville Province, Ontario, provide protolith and metamorphic ages for the southwestern Central Gneiss Belt. Discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 ± 6 Ma and a lower intercept age of 1064 ±18 Ma. U–Pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207Pb/206Pb age of 1079 ± 3 Ma. A weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the Central Metasedimentary Belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 ± 13 Ma and a lower intercept age of 1066 ± 8 Ma. The charnockitic vein yielded concordant zircon ages of 1077 ± 2 Ma. The upper intercept ages are interpreted in terms of protolith crystallization, and the concordant and lower intercept ages as Grenviilian high-grade metamorphism and associated anatexis. We have found no evidence for a ca. 1190–1160 Ma metamorphic event in these rocks, as required by some regional tectonic interpretations. We conclude that emplacement of the Central Metasedimentary Belt over the Central Gneiss Belt, which caused high-grade metamorphism in the Muskoka domain, occurred at or shortly before ca. 1080 Ma, and that this marks the time of accretion of the Central Metasedimentary Belt to the southeast margin of Laurentia.

The Blair River Complex in northern Cape Breton Island, Nova Scotia, is interpreted to be an exposure of Grenvillian basement which formed the southeastern tip of a promontory on the proto-Atlantic continental margin of North America. U-Pb (zircon) data reported here confirm Grenvillian ages for the major components in the Blair River Complex. The locally granulite-facies Sailor Brook gneiss has a minimum protolith age of 1217 Ma and was metamorphosed at 1035 +12/−10 Ma. The Lowland Brook Syenite has an igneous crystallization age of 1080 +5/−3 Ma, and the Otter Brook granitic orthogneiss yielded an igneous crystallization age of 978 +6/−5 Ma. An igneous crystallization age could not be obtained for anorthositic rocks in the Blair River Complex, but the Red River Anorthosite Suite was metamorphosed at 996 +6/−5 Ma. Silurian thermal activity is recorded in the Blair River Complex by granitic magmatism dated at 435 +7/−3 Ma and a widespread amphibolite-facies metamorphic overprint of similar age. These events are corroborated by 206Pb/238U ages of ca. 423 Ma for titanite from seven widely distributed samples from both Silurian and Proterozoic units, interpreted to represent cooling from igneous or peak-metamorphic conditions through temperatures of ca. 550–600°C. Rutile in one sample gave a U-Pb age of 410 ± 2 Ma, which may represent postmetamorphic cooling through 405°C. The U-Pb cooling ages are supported by similar 40Ar/39Ar ages of ca. 420 Ma for phlogopite and muscovite from rare calc-silicate lenses in the Blair River Complex, and for hornblende from a Silurian diorite intrusion. The widespread Silurian metamorphism of this inlier is similar in age to that in western Newfoundland and indicates involvement of the Blair River Complex in Silurian orogenesis. Our data support the interpretation that the Blair River Complex represents Laurentian Grenvillian basement, analogous to basement exposures in the Humber Zone of western Newfoundland.

We report structural and metamorphic data from a c. 25-km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them. Moderately to steeply dipping foliations with strong down-dip lineations in all three divisions appear to be associated with NW-directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S > L) with shallow, SE-SSE-plunging lineations in which sparse, dominantly (but not exclusively) normal-sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi-abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust-related deformation, with P-T-t paths that indicate near-isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P-T-t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre-1003 Ma). The GFTZ contains steeply to moderately dipping, thrust-related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990-980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.

International audience; As the continental crust thickens during mountain building, it can become hot enough to start melting, leading to a profound reduction In its strength. Melt-weakened crust can flow outward or upward in response to the pressure gradients associated with mountain building, and may be transported hundreds of kilometres laterally as mid-crustal channels. In the Himalayan-Tibetan system, melting began about 30 million years ago, and widespread granite intrusion began at 20-23 Ma. Geophysical data indicate that melt is present beneath the Tibetan plateau today, and deeply eroded mountain belts preserve evidence for melt-enhanced ductile flow in the past. Flow of partially molten crust may limit the thickness and elevation of mountain belts and has influenced the deep structure of continents.

[1] Rocks metamorphosed to high temperatures and/or high pressures are rare across the Himalayan orogen, where peak metamorphic conditions recorded in the exposed metamorphic core, the Greater Himalayan Sequence (GHS), are generally at middle to upper amphibolite facies. However, mafic garnet-clinopyroxene assemblages exposed at the highest structural levels in Bhutan, eastern Himalaya, preserve patchy textural evidence for early eclogite-facies conditions, overprinted by granulite-facies conditions. Monazite hosted within the leucosome of neighboring granulite-facies orthopyroxene-bearing felsic gneiss yields LA-MC-ICP-MS U-Th-Pb ages of 13.9 ± 0.3 Ma. Monazite associated with sillimanite-grade metamorphism in granulite-hosting migmatitic gneisses yields U-Th-Pb rim ages between 15.4 ± 0.8 Ma and 13.4 ± 0.5 Ma. Monazite associated with sillimanite-grade metamorphism in gneiss at structurally lower levels yields U-Pb rim ages of 21–17 Ma. These data are consistent with Miocene exhumation of GHS material from a variety of crustal depths at different times along the Himalayan orogen. We propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming Indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh-pressure eclogites in the western Himalaya. Their formation and exhumation in the Miocene therefore do not require diachroneity in the timing of the initial India-Asia collision.

The Long Range Inlier, a steep-sided plateau underlain mainly by Grenvillian gneisses, is the most prominent topographic feature of western Newfoundland. Apatite fission-track analysis of 31 samples from the Long Range Inlier and its surroundings yielded measured apparent ages of 343–152 Ma. Age versus elevation plots, track-length distributions, and model thermal histories indicate that the region experienced slow cooling in the late Paleozoic, with apparent exhumation rates of 7–9 m∙Ma−1 and cooling rates of 0.08–0.28 °C∙Ma−1. Model thermal histories suggest that the present upper surface of the Long Range plateau cooled below ~120 °C in Ordovician times. The thermal histories are compatible with, but do not require, some exhumation of the Long Range Inlier along Acadian thrust faults. Results from Early Carboniferous sedimentary rocks of the Deer Lake Basin are similar to Long Range Inlier data from similar elevations, implying that at some time between ~350 and 300 Ma, the entire region was buried to depths sufficient to induce total annealing (T > 120 °C) in these samples. Closure ages determined from model thermal histories indicate that regional cooling to temperatures below ~120 °C began before 300 Ma. The Carboniferous sedimentary cover was largely removed by Jurassic time, perhaps in response to lowering of regional base level by rifting associated with the opening of the Atlantic Ocean.

Bacteria fate and transport within constructed wetlands must be understood if engineered wetlands are to become a reliable form of wastewater treatment. This study investigated the relative importance of microbial treatment mechanisms in constructed wetlands treating both domestic and agricultural wastewater. Escherichia coli (E. coli) inactivation, adsorption, and settling rates were measured in the lab within two types of wastewater (dairy wastewater lagoon effluent and domestic septic tank effluent). In situ E. coli inactivation was also measured within a domestic wastewater treatment wetland and the adsorption of E. coli was also measured within the wetland effluent. Inactivation of E. coli appears to be the most significant contributor to E. coli removal within the wastewaters and wetland environments examined in this study. E. coli survived longer within the dairy wastewater (DW) compared to the domestic wastewater treatment wetland water (WW). First order rate constants for E. coli inactivation within the WW in the lab ranged from 0.09 day(-1) (d(-1)) at 7.6 degrees C to 0.18d(-1) at 22.8 degrees C. The average in situ rate constant observed within the domestic wetland ranged from 0.02 d(-1) to 0.03 d(-1) at an average water temperature of 17 degrees C. First order rate constants for E. coli inactivation within the DW ranged from 0.01 d(-1) at 7.7 degrees C to 0.04 d(-1) at 24.6 degrees C. Calculated distribution coefficients (K(d)) were 19,000 mL g(-1), 324,000 mL g(-1), and 293 mL g(-1) for E. coli with domestic septic tank effluent (STE), treated wetland effluent (WLE), and DW, respectively. Approximately 50%, 20%, and 90% of E. coli were "free floating" or associated with particles5 microm in size within the STE, WLE, and DW respectively. Although 10-50% of E. coli were found to associate with particles5 microm within both the STE and DW, settling did not appear to contribute to E. coli removal within sedimentation experiments, indicating that the particles the bacteria were associated with had very small settling velocities. The results of this study highlight the importance of wastewater characterization when designing a treatment wetland system for bacterial removal. This study illustrated the level of variability in E. coli removal processes that can be observed within different wastewater, and wetland environments.

Interlayered mafic and felsic metavolcanic rocks and metasedimentary rocks of Ordovician to Silurian age are characteristic of the Aspy terrane of northwestern Cape Breton Island. These rocks were affected by medium- to high-grade metamorphism and were intruded by synkinematic granitoid orthogneisses during Late Silurian to Early Devonian times. They were intruded by posttectonic Devonian granitic plutons and experienced rapid Devonian decompression and cooling. The chemical characteristics of the mafic metavolcanic rocks indicate that they are tholeiites formed in a volcanic-arc setting. The volcanic rocks of the Aspy terrane differ from many other Silurian and Silurian–Devonian successions in Atlantic Canada, which have chemical and stratigraphic characteristics of volcanic rocks formed in extensional within-plate settings, and are somewhat younger than the Aspy terrane sequences. Aspy terrane units are most similar to Ordovician–Silurian volcanic and metamorphic units in southwestern Newfoundland, including the La Poile Group and the Port aux Basques gneiss. Together with other occurrences of Late Ordovician to Early Silurian volcanic-arc units, they indicate that subduction-related compressional tectonics continued into the Silurian in parts of the northern Appalachian Orogen. The complex Late Silurian – Devonian tectonic history of the Aspy terrane may reflect collision with the southeastern edge of a Grenvillian crustal promentory.

Albite porphyroblasts are widely distributed in pelitic and semi-pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P-T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite-bearing schists, here termed PMAQ (paragonite-muscovite-albite-quartz), MMAQ (microcline-muscovite-albite-quartz), and PMMQ (paragonite-muscovite-margarite-quartz), can be distinguished on the basis of pre-porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a(Na+/H+) versus log a(K+/H+)] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre-existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples.

Pressure-temperature-time (P-T-t) paths summarize the changes in pressure and temperature imposed on a metamorphic rock during orogenesis. They provide a convenient framework for the interpretation of complex metamorphic histories and also offer insight into the thermal and tectonic factors controlling metamorphism in collisional orogens. P-T-t data are acquired through a combination of textural observations, thermobarometry, and thermochronometry, and assembled into a P-T-t path using geological constraints. One-dimensional P-T-t models, assuming instantaneous deformation and thermal relaxation by conduction, are flexible and useful for testing tectonic models, particularly where geochronological constraints are available. Two-dimensional models allow more sophisticated deformation geometries and allow the effects of advection to be incorporated. Analysis of collisional orogens in terms of critical wedge theory can yield P-T-t paths that reflect coupling between thickening, uplift, exhumation, erosion, and convergence. Where rates of erosion approach rates of tectonic uplift, as is currently happening in the Southern Alps of New Zealand, high-grade metamorphic rocks can be exhumed rapidly from considerable depth. Alternatively, rapid exhumation may reflect gravity-driven extension in an over-steepened or thermally weakened orogen.

[1] The exhumation of crustal ultra-high-pressure (UHP) material depends on temporal and spatial variations in its detachment within the subduction channel. This dependence is investigated using numerical models with variable initial crustal strengths, representing a range of initial crustal compositions, and parameterized strain weakening, representing a range of processes that reduce effective crustal viscosity during deformation. Competition between down-channel shear traction, favoring subduction, and up-channel buoyancy, favoring exhumation, is expressed as the exhumation number, E, which can vary with time and position along the channel. Exhumed lower strength crust, which resists subduction owing to weak down-channel traction, records peak conditions 38 kbar. Given sufficient strain weakening, exhumation proceeds at >60 km Ma−1, indicating that buoyancy (E ≫ 1) drives exhumation in these models. In all models, exhuming UHP material forms a deforming ductile plume, with a range of possible structural relationships predicted between exhumed UHP and HP materials.

The loading of microbial contaminants was examined within the Thomas Brook watershed, a 784 ha mixed land-use catchment located in the headwaters of the Cornwallis River drainage basin (Nova Scotia, Canada). The objectives were to: (i) examine spatial and temporal characteristics of fecal bacteria loading during the growing season from five subwatersheds, and (ii) develop areal fecal indicator organism export coefficients for rural landscapes. Fecal coliform, Escherichia coli , total suspended solids (TSS) concentrations and stream flow were monitored at five locations in the watershed over six consecutive growing seasons (May–Oct, 2001–2006). A nested watershed monitoring approach was used to determine bacterial loading from distinct source types (residential vs. agricultural) during both baseflow and stormflow periods. Areal bacterial loading rates increased in each nested watershed moving downstream through the watershed and were highest in the three subcatchments dominated by agricultural activities. Upper watershed bacterial loading throughout the growing season from an agricultural subcatchment (Growing Season Avg 8.92 × 10 10 CFU ha −1 ) was consistently higher than a residential subcatchment (Growing Season Avg 8.43 × 10 9 CFU ha −1 ). As expected, annual average stormflow bacterial loads were higher than baseflow loads, however baseflow loads still comprised between 14 and 35% of the growing season bacterial loads in the five subwatersheds. Fecal bacteria loads were greater during years with higher annual precipitation. A positive linear relationship was observed between E. coli and TSS loading during the 2005 and 2006 growing seasons when both parameters were monitored, indicating that the processes of sediment transport and bacterial transport are linked. It is anticipated that computed areal microbial loading coefficients will be useful in developing watershed management plans. More intensive sampling during stormflow events is recommended for improving these coefficients.

[1] UHP rocks commonly form and exhume during the transition from oceanic subduction to continental collision. Their exhumation in subduction channels depends on the balance between down-channel shear traction and up-channel buoyancy. Thermal-mechanical upper-mantle-scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance. Changes in shear traction leading to crustal decoupling/detachment are investigated by varying the onset of strain weakening, thermal parameters, and convergence velocity. Variations in buoyancy force are investigated by modifying subducted material density and volume. The model results are interpreted in terms of the exhumation number E, which expresses the role of the pressure gradient, channel thickness, effective viscosity, and subduction velocity. Peak metamorphic conditions, exhumation velocity, and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of E. The models reproduce natural PTt constraints and indicate that neither slab breakoff nor surface erosion is required for UHP exhumation.

U–Pb dates from zircon, titanite, and monazite in plutons of the Bras d'Or and Mira terranes of southern Cape Breton Island, combined with 40Ar/39Ar and other radiometric age data, indicate that the Bras d'Or and Mira terranes had separate magmatic and metamorphic histories until at least the Middle Cambrian and possibly until the Devonian. The Bras d'Or Terrane is characterized by abundant late Precambrian (ca. 565–555 Ma) dioritic to granitic plutons, as exemplified by the Shunacadie granodiorite ([Formula: see text], U–Pb zircon). Early Ordovician granitic plutons, such as the Kellys Mountain leucogranite with a U–Pb (zircon) age of 498 ± 2 Ma, occur locally. Titanite and 40Ar/39Ar (hornblende) ages from plutons and metamorphic units suggest that widespread metamorphism may have occurred between these plutonic events and (or) during the Early Ordovician plutonism.Plutons in the Mira Terrane appear to have been emplaced at about 620 Ma or before, as exemplified by the Chisholm Brook granodiorite ([Formula: see text]), and to have been unaffected by younger thermal events, as indicated by close similarity of U–Pb (zircon and titanite) and 40Ar/39Ar (hornblende) ages. Plutons of Devonian age are localized in an easterly trending belt in the central part of the terrane. In contrast to the Bras d'Or Terrane, widespread latest Precambrian (ca. 565–555 Ma) to Early Ordovician plutonism and metamorphism appear to have been absent. It is unlikely that the Mira and Bras d'Or terranes were juxtaposed before the Early Ordovician.

Metamorphic mineral assemblages and textures from Early Palaeozoic continental margin rocks in north-western Newfoundland indicate that different structural levels have contrasting metamorphic histories. Rocks of the East Pond Metamorphic Suite, which represent the older, structurally lower level of the margin, experienced an early high-pressure–low-temperature stage of metamorphism (10–12 kbar minimum, 450–500°C) which produced eclogite in mafic dykes and phengite–garnet assemblages in pelites. This was overprinted by higher temperature–lower pressure amphibolite facies metamorphism (700–750°C, 7–9 kbar minimum) which produced complex symplectic textures in rocks of all compositions. Rocks of the Fleur de Lys Supergroup, which were deposited in the stratigraphically higher levels of the rifted margin, reached pressures of 7–8.5 kbar at about 450°C during the early stages of metamorphism, overprinted by assemblages which indicate maximum temperatures of 550–600°C at about 6.5 kbar. The metamorphic history of both units is interpreted to be the result of thermal relaxation following initial burial of a continental margin by overriding thrust sheets. Since there is no evidence that maximum pressures or temperatures within the Fleur de Lys Supergroup were ever as high as those reached in the East Pond Metamorphic Suite, these rocks may have followed parallel, ‘nested’P–T–t paths, with the more deeply buried East Pond Metamorphic Suite subjected to greater thermal relaxation effects. Quantitative modelling of P–T–t paths is not possible with the present data, owing to both large uncertainties in P–T estimates, and in the time of metamorphism.

New U-Pb ages from zircon, titanite, or monazite separates from 12 units in the Aspy and Bras d'Or terranes, Nova Scotia, range from late Precambrian to Devonian and generally support the interpretation that the 2 terranes had differing histories until juxtaposition in the Devonian. Five plutons of dioritic, tonalitic, and granodioritic compositions in the northern Bras d'Or terrane yielded zircon ages between 575 (maximum) and 556 ± 4 m.y. Younger titanite ages of 548 ± 2 and 521 ± 2 m.y. from two of these plutons may indicate younger metamorphism. Previously undocumented Early Ordovician plutonism (493 ± 2 Ma, zircon) and metamorphism (496 ± 5 Ma, titanite) also occurred in the Bras d'Or terrane. In the Aspy terrane, a zircon age of 433 +7/-4 m.y. was obtained for a rhyolite unit in the upper part of a widespread metavolcanic-metasedimentary sequence, which together with higher-grade, probably correlative units, is characteristic of the terrane. The age is interpreted to indicate that most metamorphic rocks in the terrane are Ordovician to Early Silurian in age, in contrast to Precambrian ages in the Bras d'Or terrane. Devonian plutonism was widespread in the Aspy terrane, whereas Devonian plutonism in the Bras d'Or terrane appears restricted to the vicinity of the boundary with the Aspy terrane. Zircon, titanite, or monazite ages from 4 plutonic and orthogneissic units in the Aspy terrane and in the Bras d'Or terrane adjacent to the terrane boundary range from 403 ± 3 to 375 +5/-4 m.y.