Ungava Orogen Research Articles

Roberts Lake Syncline mafic lavas (NE Superior craton): A proposed extension of the Cape Smith belt

The Superior craton is bordered on its northern and eastern sides by the approximately coeval Cape Smith fold belt and the Labrador Trough, two major Paleoproterozoic volcano-sedimentary belts that record broadly similar tectonic evolution but which show some distinctive characteristics and have notably different mineral resource endowments. The Roberts Lake Syncline, situated directly between the Cape Smith and Labrador Trough, hosts two Paleoproterozoic mafic volcanic sequences and has uncertain tectonic affinities to its neighboring fold belts. It was previously linked to Labrador Trough magmatism but re-interpretation of the regional geology suggests a closer resemblance to the successions of the Cape Smith belt. We present new lithogeochemistry that shows that the Roberts Lake Syncline comprises two distinct magmatic events rather than the single event previously interpreted. The lower and more voluminous suite of the Roberts Lake Syncline lavas comprises multiply-saturated basalts characterized by normalized trace element patterns similar to E-type mid-ocean ridge basalts (E-MORB). The upper suite of the Roberts Lake Syncline stratigraphy is composed of volcanic rocks with depleted, N-MORB-like trace element signatures rocks spanning a range from komatiites and picrites with near-primary compositions to high-Mg basalts with flat normalized trace element patterns. Both suites were apparently undersaturated with sulfide liquid in their mantle source. The E-MORB magmas were affected by a sulphide-saturation event, leading to widespread and variable degrees of depletion of chalcophile elements, despite showing no significant crustal contamination. The N-MORB magmas also underwent saturation with sulfide melt, generating minor occurrences of magmatic sulfide mineralization hosted by ultramafic intrusions within the older volcanic suite and small but significant depletions in chalcophile elements in the erupted picritic basalts. The geochemistry of the lower E-MORB suite is similar to that of the 2170 Ma Biscotasing dike swarm that occurs in and near the Labrador Trough and the 1998 Ma Minto-Povungnituk large igneous province (LIP) of the Cape Smith belt, whereas the upper N-MORB-like suite in the Roberts Lake Syncline has compositions resembling those of the 1880 Ma Circum-Superior LIP. Our detailed geochemical comparison suggests a close resemblance to the chemical stratigraphy observed within the Cape Smith belt, leading us to consider the Roberts Lake Syncline as the southeasternmost portion of the Ungava Orogen rather than the northernmost part of the Labrador Trough. These insights contribute to a revised LIP and plume centre map for this NE portion of the Superior craton and help to constrain the large-scale tectonic architecture of the margins of the Superior Province.

Petrological and geochemical characteristics of a Paleoproterozoic magmatic arc (Narsajuaq terrane, Ungava Orogen, Canada) and comparisons to Superior Province granitoids

The Narsajuaq terrane is a major component of the Paleoproterozoic Ungava (Trans-Hudson) Orogen of northern Quebec and is a well-exposed mid-crustal block interpreted to be the plutonic roots of a magmatic arc. The Narsajuaq terrane is comprised of a series of hornblende ± pyroxene-bearing plutons and supracrustal rocks which were accreted to the Archean Superior Province basement during the Paleoproterozoic. On the basis of geological field relationships and geochronological data the plutonic rocks of the Narsajuaq terrane are divided into four main groups: (1) the Cape Smith suite (1898-1839 Ma) consists of various deformed and undeformed plutons that are found intrusive into the Cape Smith Belt lithologies; (2) the Gneissic suite, which ranges in age from 1863-1844 Ma and consists of a well-layered sequence of diorite and tonalite, intruded by and interlayered with variably deformed and metamorphosed granitic veins; (3) the Younger suite, consisting of discrete, kilometre-size bodies of metamorphosed plutons (diorite-monzodiorite-tonalite-granite) intrusive into the Gneissic suite, ranging in age from 1836-1821 Ma; and (4) granitic sheets of the Anatectic suite which intruded the Narsajuaq terrane ca 1800 Ma. A series of metamorphosed sedimentary rocks (predominantly semi-pelites) of the Sugluk Group are also found in the Narsajuaq terrane, and contain zircons of both Archean and Paleoproterozoic age. Plutons of the Gneissic and Younger suites are predominantly calc-alkaline and metaluminous to mildly peraluminous and contain variable but generally moderate to high concentrations of the large-ion lithophile elements and light rare earth elements (REEs), pronounced negative Nb and Ti anomalies, and fractionated REEs, comparable to modern arc granites. A range in isotopic compositions is noted for the suites, with the Gneissic and Younger suites having initial εNd and 87Sr/86Sr(i) values ranging from juvenile (+4.0 and 0.7020, respectively) to enriched (−10.7 and 0.7048). The Anatectic suite granites range to extremely enriched isotopic compositions with initial εNd up to −18.5 and 87Sr/86Sr(i) = 0.7064. The Cape Smith suite has a much more restricted isotopic composition which is predominantly juvenile (initial εNd = +3.2to+1.5 and 87Sr/86Sr(i) = 0.7020 to 0.7024), and suggests a minor role for crustal contamination, primarily via sediment subduction. The isotopic signature of the Narsajuaq terrane plutons indicate that at least three sources were involved in their petrogenesis: depleted mantle, average 2.8 Ga Superior Province crust and older (3.1 Ga) Superior Province crust. When compared to plutonic suites from the NE and SE Superior Province (Minto block, Abitibi, Pontiac and Opatica belts), the Narsajuaq terrane suites define a similar chronology and evolution, indicating a commonality and dominance of arc-accretionary processes for crustal formation in the Precambrian Shield of Canada. Higher trace element abundances in the Narsajuaq terrane plutons compared to the Archean samples may be related to different subduction zone fluxes which may reflect different bulk compositions of subducted oceanic sediments during the Proterozoic.

Geology, U – Pb, and Pb – Pb geochronology of the Lake Harbour area, southern Baffin Island: implications for the Paleoproterozoic tectonic evolution of northeastern Laurentia

Geological and geochronological results of an investigation of the Paleoproterozoic siliciclastic and carbonate supracrustal rocks of the Lake Harbour Group (LHG) and surrounding tonalitic gneisses on southern Baffin Island are presented. Conventional U – Pb geochronology of monazite from rocks of the LHG suggest that penetrative deformation of these rocks occurred prior to, or during, peak metamorphic conditions at ca. 1845 –1840 Ma. Conventional U – Pb zircon results indicate that much of the tonalitic gneiss ranges in age from [Formula: see text] to 1827[Formula: see text]. The tonalitic gneisses and Lake Harbour Group units were tectonically imbricated by ca. 1805 Ma, and are part of a southwest-verging thrust belt interpreted from regional considerations to represent the northern continuation of the Ungava Orogen. The present results indicate that current tectonic models for the evolution of northeastern Laurentia that involve a dominantly Archean southeastern Rae province require revision. It is proposed that much of the metaigneous material that lies between the Archean Superior and Nain cratons represents a composite subduction-related domain.

Mineral Corona Formation During High-P Retrogression of Granulitic Rocks, Ungava Orogen, Canada

Crystalline basement exposed in tectonic windows within the Ungava Orogen records a polycyclic Archean granulite-facies to Paleoproterozoic amphibolite-facies history. Amphibolite-facies assemblages comprise garnet coronas around plagioclase, clinopyroxene or cummingtonite coronas on orthopyroxene, hornblende coronas on clinopyroxene±orthopyroxene, sodic rims on calcic plagioclase, and/or titanite coronas on ilmenite. Petrographic observations and model reactions suggest that growth of coronitic garnet is closely associated with amphibolitization of twopyroxene gneisses. Calcic plagioclase constitutes a key reactant in all garnet-producing reactions and possibly acted as a rate-controlling phase. Multi-equilibrium thermobarometric calculations show good convergence of possible equilibria in the amphibolite-facies rocks, indicating that coronitic textures need not imply complete chemical disequilibrium. P—T determinations for the amphibolite-facies gneisses beneath the thrust belt of Ungava Orogen are in the range 7.7–9.8 kbar and 585–723°C. These values are consistent with prograde determinations from pelitic schists within the thrust belt. Estimates of water activity cluster into two populations. High aH2O values are obtained for highly strained basement rocks adjacent to the thrust belt whereas low aH2O values are derived for orthogneiss samples which show no thrust-related fabrics and are distal to the thrust belt.

Stitching together the Ungava Orogen, northern Quebec: geochronological (TIMS and ICP–MS) and geochemical constraints on late magmatic events

Late magmatic activity in the Ungava Orogen of northern Quebec is manifest as granitic dykes and small, rare plutons that crosscut all tectono-stratigraphic elements of the orogen. Conventional U–Pb geochronology (thermal ionization mass spectrometry (TIMS)) on one particularly important pluton that cuts all these domains (the Lac Duquet monzogranite) indicates its age of emplacement at 1742.2 ± 1.3 Ma. This undeformed and nonmetamorphosed pluton postdates the youngest structures in the orogen (D4 folds), thereby constraining the timing of the latest deformation to >1742 Ma. Laser ablation inductively coupled plasma–mass spectrometry (ICP–MS) on zircons from the same sample identified a large range in 207Pb/206Pb ages of inherited grains from 1.7 to 3.2 Ga, corresponding to the ages of the host rocks for the pluton. This high-K peraluminous monzogranite pluton contains moderate to high concentrations of large ion lithophile elements and fractionated and enriched light rare earth elements, similar in composition to the surrounding continental crust and to other crustally derived granites. Initial 87Sr/86Sr values of 0.7040–0.7051 and εNd ranging from −4.4 to −9.7 indicate incorporation of a significant amount of older material in the petrogenesis of the pluton. It is proposed that anatexis of the surrounding continental crust due to structural thickening during the waning stages of the Ungava orogeny resulted in the generation of the Lac Duquet pluton and was the source for its inherited zircons.

Large‐scale fluid infiltration, metasomatism and re‐equilibration of Archaean basement granulites during Palaeoproterozoic thrust belt construction, Ungava Orogen, Canada

Abstract The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite‐granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses. The older suite comprises c. 2.7‐Ga granulite facies assemblages (orthopyroxene‐clinopyroxene‐hornblende‐plagioclase‐ilmenite ± biotite ± quartz) that record moderate pressures (±5 kbar) and high temperatures (±800° C). A younger, c. 1.8‐Ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal décollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re‐equilibration of the Archaean granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet‐hornblende‐quartz ° Cummingtonite coronas between plagioclase and orthopyroxene‐clinopyroxene, as well as titanite coronas on ilmenite. Multi‐equilibrium thermobarometry on the coronitic assemblages documents re‐equilibration of the granulitic gneiss to 7.7 kbar at 644° C in the south and 9.8 kbar at 700° C in the north. The variably deformed, amphibolite facies domain sandwiched between the coronitic garnet zone and the basal décollement is marked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium flow of an aqueous‐chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt, with fluid flow probably focused along the basal décollement.

Syn-tectonic magmatism and the development of compositional layering, Ungava Orogen (northern Quebec, Canada)

Layer- and foliation-parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now-exhumed middle crustal sections of both Archean and Paleoproterozoic age in the northern Ungava peninsula, Quebec (Canada). In the Archean Superior Province, diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite-facies conditions coeval with voluminous granitic magmatism. The Paleoproterozoic Narsajuaq arc contains evidence for contemporaneous magmatism, transpressional deformation and granulite-facies metamorphism prior to its collision with the Superior Province basement. In both plutonic domains, regional compositional layering is defined by (1) metre to kilometre-scale alternation of generally well foliated tonalite and quartz diorite bodies; and (2) centimetre to kilometre-scale, variably deformed granitic veins and sheets that lie parallel to layering/tectonic foliation in the host rocks. Syn-tectonic intrusion of a substantial portion of the veins along extension fractures sub-parallel to layering/foliation (i.e. at high angle to the regional shortening direction) is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures. Compositional layering generated and/or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust.

Geochemistry of mafic magmas from the Ungava orogen, Québec, Canada, and implications for mantle reservoir compositions at 2.0 Ga

The Ungava orogen of northern Québec is one of the best preserved Proterozoic mobile belts of the world, recording > 200 Ma of plate divergence and convergence. Voluminous magmatism associated with rifting of the Superior Province basement ≈2.04 Ga resulted in the development of a volcanic rift margin sequence and an ocean basin. Four distinct mafic magma suites were erupted: (1) continental basalts (Eskimo Formation, western and central Povungnituk Group) with moderate to high Zr/Nb and negative Nb anomalies which have interacted with the continental crust (ϵ Nd(2.0 Ga)) from −7.4); (2) mafic lavas from the Flaherty Formation, eastern Povungnituk Group and some Watts Group lavas associated with passive margin rifting, having slightly enriched isotopic signatures ( ϵ Nd(2.0 Ga) = +2.7 to +4.4) compared to the contemporaneous depleted mantle, high (Nb/Y) n and low Zr/Nb ratios (≈4.4 and ≈8.9, respectively); (3) a highly alkaline OIB-like suite ( ϵ Nd(2.0 Ga) = +2.3 to +3.2, (Nb/Y) n> 12) within the Povungnituk Group composed of nephelinites, basanites and phonolites; and (4) depleted Mg-rich basalts and komatiitic basalts ( ϵ Nd(2.0 Ga) ≈ + 4.5 to + 5.5) with trace-element characteristics of N-MORB, but with higher Fe and lower Al than primitive MORB (Chukotat Group, Ottawa Islands and some Watts Group samples). The ocean basin into which these lavas were erupted was subsequently destroyed during subduction between ≈1.90 and ≈1.83 Ga, resulting in the development a magmatic arc (Narsajuaq terrane and Parent Group). The Ungava magmas provide a unique window into the mantle at 2.0 Ga. The chemical and isotopic similarity of these Proterozoic magmas to modern-day magmas provides strong evidence that the interplay between depleted mantle, OIB mantle and sub-continental mantle during the Proterozoic was comparable to that of the modern Earth.

Constraints on Pb closure temperature in titanite based on rocks from the Ungava orogen, Canada: Implications for U-Pb geochronology and P-T-t path determinations

On the basis of multiequilibrium metamorphic pressure-temperature ( P-T ) estimates, we have determined that 660–700 °;C is a reasonable minimum estimate of the closure T of Pb diffusion in titanite, a temperature significantly higher than previously suggested. Activity-corrected reactions that produce titanite define narrow T stability fields (≈40 °;C) that correspond closely to metamorphic P-T determinations derived for coronitic garnet-clinopyroxene-hornblende samples. Growth of titanite, occurring between 1789 and 1814 Ma in the underthrust Archean basement of the Paleoproterozoic Ungava orogen (northern Quebec), is associated with retrograde metamorphism during underthrusting of the Archean basement (Approx.)1 b.y. after formation of the original granulitic assemblages.

Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations

Recent detailed geological mapping and UPb geochronological studies in the Torngat, Ungava and Baffin orogens of Northeast Laurentia have provided new information regarding the tectonic assembly of this region between ∼1.92 and 1.74 Ga. In contrast with an earlier hypothesis whereby the geometry of Archean continental fragments was thought to have resulted entirely from northerly indentation of the Superior Province and consequent extrusion tectonics, interpretation of recently acquired data shows that Superior Province indentation was the last of multiple collisional events during the formation of Northeast Laurentia. In the present model, we propose that Southeast Rae and North Rae provinces were separate prior to ∼1.92 Ga, when regional tectonism changed from extension to contraction. Westerly subduction and westward translation of the Archean Disko terrane and Nain Province between the North Rae and Southeast Rae provinces resulted in the development of a continental magmatic arc in the Cumberland batholithic complex and in the Torngat orogen, and deposition of the upper sequences of the Penrhyn-Piling-Karrat sequences in the Foxe and Rinkian belts of the Baffin orogen (1880–1895 Ma). Continental collision between the Disko terrane and North Rae Province produced shortening across the Rinkian and Foxe belts at, or before, 1876 Ma. Meanwhile, early arc magmatism in the Ungava orogen (1861–1898 Ma) accompanied foredeep or transtensional basin formation in the New Quebec orogen (∼1870–1883 Ma) and the onset of deformation in the southeastern Nagssugtoqidian orogen (∼1883 Ma). Nain-Southeast Rae provinces collision at ∼1860 Ma across the Torngat orogen caused renewed granitic magmatism and metamorphism in the Cumberland batholithic complex between 1857 and 1853 Ma. Initiation of felsic volcanism in the Makkovik-Ketilidian orogen at this time indicates a shift in the locus of plate subduction from the Southeast Rae Province to the southern margin of the Nain Province. In the Ungava orogen, northerly subduction changed to southerly subduction resulting in the generation of a second arc suite at between 1844 and 1826 Ma. Formation of the de Pas batholith in the hinterland of the New Quebec orogen at ∼1840 Ma accompanied arc-continent and Superior-Southeast Rae collision from 1845 to 1829 Ma. At this time, accretion of arc material in the Makkovik-Ketilidian orogen was contemporaneous with high-grade sinistral shearing along the reactivated Torngat orogen and the northwestern Nagssugtoqidian orogen. Southeast-directed thrusting along the southeastern margin of the Cumberland batholithic complex, and reactivation of the Rinkian belt is predicted to have also occurred at this time. At ⩽1826 Ma, the onset of arc-continent collision across the Ungava orogen initiated closure of the North Rae-Southeast Rae provinces juncture and generation of the Ford Lake plutonic suite. Subsequent Superior-Southeast Rae continental collision (∼1810 Ma) resulted in dextral “escape tectonics” in the Foxe and Rinkian belts, and thick-skinned folding of basement and cover sequences in the Ungava orogen. High-grade metamorphism and granitoid magmatism in the Makkovik-Ketilidian orogen also occurred. Reactivation of the Nain-Southeast Rae (east-directed thrusting) and Superior-Southeast Rae (west-directed thrusting) margins at ∼1790 Ma resulted in “pop-up” of the Southeast Rae Province between its more rigid neighbors. Final cooling and the emplacement of post-tectonic granitoid rocks took place at ∼ 1740–1790 Ma.

UPb geochronology of the western Cape Smith Belt, Canada: new insights on the age of initial rifting and arc magmatism

Abstract The Cape Smith Belt of northern Quebec is the foreland to hinterland transition of the Ungava Orogen, a component of the Paleoproterozoic Trans-Hudson Orogen of North America. UPb ages for rift and volcanic-arc related units from the Cape Smith Belt are presented and used to evaluate models of its tectonic evolution. A gabbroic sill intrusive into the rift-related sediments near the base of the Povungnituk Group is 2038 −2 +4 Ma old, indicating that rifting of the Superior Province was initiated prior to that time, ∼80 m.y. earlier than previously reported. A granodiorite intrusive into the volcanic rocks of the Povungnituk Group is dated at 1991±2 Ma indicating that magmatism associated with the rifting event was active at that time, ∼30 m.y. earlier than formerly thought. The age of Parent Group arc-related magmatism ranges from 1874 −3 4+ Ma, the age of a diorite intrusion, to 1860±2 Ma, the age of another diorite intrusion and of a rhyolite. A granodiorite intrusive in the Parent Group yields an age of 1845±6 Ma. Magmatism of the Parent Group is thought to be related to northward-dipping subduction of the Superior Province margin below the Watts Group. A gabbroic intrusion in the 2000 Ma old Watts Group is dated at 1858±2 Ma. These ages are consistent with a tectonic model in which the rocks of the Cape Smith Belt formed in a single, continental rift to oceanic progression, followed by subduction which led to an arc-continent collision. A comparison of UPb ages for similar events in different sectors of the Trans-Hudson Orogen in Canada, reveals that early events are diachronous and range from 2169 Ma to 2038 Ma. The main collisional events are coeval in the Ungava and New Quebec Orogens and in some of the western sectors at 1830–1860 Ma. Younger ages of 1800–1820 Ma are obtained for the late collisional stages throughout the Trans-Hudson Orogen.

Terrane accretion in the internal zone of the Ungava orogen, northern Quebec. Part 2: Structural and metamorphic history

The tectonic history of the early Proterozoic Ungava orogen is marked by structural–metamorphic episodes that both predate and postdate a collision between a magmatic arc terrane and the northern continental margin of the Superior Province. Distinct precollisional tectonic histories are documented for the rocks forming the lower plate of the Ungava orogen (the Archean Superior Province basement and an Early Proterozoic rift-to-drift margin sequence) and the orogenic upper plate (Early Proterozoic ophiolitic and magmatic arc units). The lower-plate units preserved in the external part of the orogen (Cape Smith Thrust Belt) record the development of a foreland thrust belt characterized by south-verging faults ramping up from a basal décollement located at the basement–cover contact. The plutonic core of the magmatic arc contains structures and metamorphic assemblages indicative of an episode of dextral transcurrent deformation contemporaneous with granulite-facies metamorphism and arc plutonism. The "tectonically suspect" ophiolitic and arc units were accreted to the thrust belt along south-verging faults, which reimbricated the foreland thrust belt and which resulted in at least 100 km of displacement of upper-plate units with respect to the autochthonous basement. Collisional thickening and consequent exhumation resulted in relatively high-pressure, greenschist- to amphibolite-facies metamorphism of lower-plate cover units, and in the retrogression of high-grade assemblages in the arc rocks. Postaccretion shortening resulted in folding of both the allochthonous rocks and the footwall basement.

Terrane accretion in the internal zone of the Ungava orogen, northern Quebec. Part 1: Tectonostratigraphic assemblages and their tectonic implications

The tectonostratigraphic record of the Ungava orogen contains evidence for the interaction of divergent, transform, and convergent plate boundaries over a > 0.2 Ga period in the Early Proterozoic. Three principal tectonic domains are recognized: (1) autochthonous basement plutonic and supracrustal rocks of Archean age; (2) autochthonous and allochthonous sedimentary and volcanic units associated with a ca. 1.99–1.92 Ga rift-to-drift margin; and (3) "suspect' ' crustal components of a ca. 2.00 Ga ophiolite and ca. 1.90–1.83 Ga island-arc terrane. Domain 1 is the stratigraphic or structural basement to domains 2 and 3. Field relationships indicate that domain 2 units were accumulated on or near domain 1, whereas no tectonostratigraphic linkages are known between domain 3 and the others. Accumulation of continent-derived sedimentary rocks, continental tholeiites, komatiitic basalts, and mafic lavas equivalent to modern normal mid-oceanic ridge basalts (n-MORB's) in domain 2 is interpreted as recording continental rifting leading to the possible formation of oceanic crust. Pillowed basalts, sheeted dykes, and cumulate rocks suggest that the crustal portion of an ophiolite is preserved within domain 3. The ophiolite is interpreted as a "suspect" fragment of oceanic crust, which may represent an earlier rifted portion of domain 2, possibly juxtaposed by transform plate boundary motions. Within domain 3, calc-alkaline volcanic and plutonic units and sequences of volcanogenic and siliciclastic sedimentary rocks are thought to indicate the development of a magmatic arc and fore-arc basin and thus the establishment of a convergent plate boundary. The most internal lithotectonic packages in domain 3 are predominantly crystalline thrust sheets containing (i) an older intrusive suite, interpreted as the plutonic foundation of the magmatic arc, and (ii) younger plutons, interpreted as the result of a reversal in subduction polarity. The reversal may have followed attempted subduction of domains 1 and 2 beneath the upper-plate arc assemblage of domain 3.

Footwall involvement during arc-continent collision, Ungava orogen, northern Canada

Geological evidence and theoretical models are presented to illustrate deformation paths followed by continental crust during underthrusting, subduction and collision. Fieldwork in an early Proterozoic collisional orogen indicates that a thrust belt and its autochthonous footwall basement are obliquely exposed in a crustal cross-section with significant structural relief. The collisional orogen records an evolution in the style and extent of involvement of the Archaean footwall from: (1) underthrusting without evidence of internal deformation; (2) limited thrusting and ductile deformation of footwall rocks; to (3) folding of the entire thrust belt and its footwall into cuspate-lobate, long wavelength structures. The structural evolution occurred during hydration of basement rocks coeval with collision-related metamorphism. Lithospheric strength profiles, constrained by simple numerical heat conduction models, suggest that the observed deformation path for the footwall crust was controlled by weakening due to heating of basement rocks, and by changes in the relative strengths of different crustal layers. The profiles predict a very weak lower crust and upper mantle at the time of footwall-involved folding, suggesting that the lower crust and upper mantle deformed by ductile flow concomitant with buckling of the stronger upper crust.

Long-lived continent-ocean interaction in the Early Proterozoic Ungava orogen, northern Quebec, Canada

The Early Proterozoic Ungava orogen contains evidence for the development of a longlived (>80 m.y.) and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc-continent collision. A magmatic arc was built on 2.00 Ga oceanic crust above a north-dipping subduction zone between about 1.90 and 1.86 Ga. Development of a south-verging continental thrust belt containing units associated with prior rifting of the Archean Superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca. 1.87 Ga. Subduction of the Superior province continental crust is inferred to have led to a reversal in subduction polarity. A younger, south-dipping subduction zone is suggested by a suite of 1.85-1.83 Ga quartz diorite to granite plutons that intrude the upper- plate oceanic crust and the older arc. The arc-continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement.