F2 Folds Research Articles

Polyphase folding at upper structural levels in the Borbera Valley (northern Apennines, Italy): implications for the tectonic evolution of the linkage area between Alps and Apennines

Abstract The Borbera Valley (northwestern Italy) is located in a complex geological area where the linkage between Alps and Apennines occurs. In this area the Antola Unit (Late Cretaceous–Palaeocene) is unconformably overlain by the Upper Eocene–Miocene succession of the Tertiary Piedmont Basin. The structural analysis indicates the occurrence of a folding phase of Late Oligocene–Early Miocene age, characterised by recumbent F2 folds. These folds are superposed onto D1 structures related to an early folding phase of Middle Eocene, affecting only the Antola Unit. The occurrence of map-scale D2 folding phase structures that affect the Tertiary Piedmont Basin succession suggests that the linkage area between Alps and Apennines was reactivated during the Late Oligocene–Early Miocene. To cite this article: M. Marroni et al., C. R. Geoscience 334 (2002) 565–572.

Late-orogenic backfolding and extension in the Brooks Range collisional orogen, northern Alaska

New structural observations across a crustal section spanning relatively unmetamorphosed fold–thrust belt rocks and epidote–amphibolite grade rocks document the late-orogenic kinematic history of the Brooks Range, Alaska, and provide insight into the character, relative timing, and geodynamic setting of late-orogenic backfolding and extension in collisional orogens. Greenschist-grade rocks on the north flank of a metamorphic culmination display mesoscopic, open-to-close, S-vergent folds (and associated cleavages) and small-displacement normal faults, suggesting a sequence of D2 backfolding and D3 extension following the protracted phase of N-directed D1. At deeper levels, epidote–amphibolite facies rocks display a pervasive gently N-dipping S2 crenulation cleavage that is axial-planar to kilometer- and smaller-scale tight to isoclinal recumbent F2 folds. Porphyroblast–matrix relationships suggest that folds at the deeper levels initiated during synmetamorphic D2 backfolding. Much of the strain recorded by S2 and the tightening of folds, however, probably accumulated during post-metamorphic D3 extension.40Ar/39Ar data suggest an early- to mid-Albian age for D2 backfolding, which may be related to formation of a regional-scale backfold. Backfolding, together with apparently synchronous thrusting near the range front may represent the final stages of Cretaceous contractional deformation in the orogen. Contraction was immediately followed by extension, which, aided by erosion, produced a short-lived period of cooling and exhumation beginning at ∼103Ma. The available data suggest that extension is post-contractional, and thus is not viewed as a driving force of thrusting in the foreland. Extension was not limited to range-bounding normal faults, but rather was distributed over much of the southern half of the Brooks Range, with character of extensional fabrics being dependent on the geometry of D2 fabrics.Inasmuch as backfolding and extension are common features within collisional orogenic belts, the conclusions drawn here have implications regarding fundamental processes associated with the late-stages of collisional orogenesis. In particular, the study provides an example of the roles that backfolding and extension have played in the geodynamic development of a collisional orogen.

A reappraisal of polymetamorphism in the Eastern Ghats belt — A view from north of the Godavari rift

Evidence collated from different parts of the Eastern Ghats belt north of the Godavari rift (barring the “Western Charnockite Zone” ) indicates that this sector evolved through a series of compressive structures (F1 to F3), with prolific migmatization in quartzofeldspathic and metapelitic gneisses synchronous with F1 shortening, as was the syn-F1 emplacement of profuse megacrystic K-feldspar-bearing granitoid bodies. Thereafter, melt productivity of the rocks (synchronous withF 2– F3 folding) sharply decreased. Mineral parageneses stable in the S1, S2 and S3 fabrics indicate persistence of granulite facies conditions. P-T estimates on orthopyroxene + garnet + plagioclase + quartz assemblages anchored to recrystallized mosaic that overgrow all penetrative fabric elements in mafic granulites, granitoids and quartzofeldspathic gneisses are in the range of 900‡-950‡C and P≅ 8–9 kbar. This estimate is comparable to those retrieved from sapphirine-bearing paragenesis in Mg-Al metapelites that appear to be diachronous in relation to the fabric elements, and arguably disrupt the granoblastic mosaic. These facets in the northern sector of the orogenic belt are compatible with either a single cycle of tectonic events (i.e., F1, F2 and F3 in continuum), or temporally-separate thermo-tectonic events, with the peak of earlier metamorphism (pre- to syn-F1) at lower temperature (in the granulite facies) in comparison to the record of high post-F3-Tmax values.It is suggested on the basis of the above evidence that the late Proterozoic/Pan-African granulites in the Eastern Ghats belt north of the Godavari rift, are unlikely to be reworked equivalents of any older granulitic crust, such as the ∼1.6 Ga granulites south of the rift. Instead, the temporally disparate sectors may represent different crustal segments with unconnected pre-amalgamation tectonic history. However, if the ∼ 1.6 Ga granulites of the Western Charnockite Zone continue northwards across the rift, as suggested by recent isotope data, there are serious doubts as to the validity of a north-south division within the Eastern Ghats belt.

Unequivocal assignment of the fullerene carbons of diethyl 1,2‐methano[60]fullerene 61,61‐dicarboxylate using 2D INADEQUATE NMR spectroscopy

AbstractAssignment of all carbons of the [60]fullerene sphere of diethyl 1,2‐methnano[60]fullerene 61,61‐dicarboxylate using 2D INADEQUATE NMR spectroscopy is described. Acquiring the INADEQUATE data with small spectral widths and optional F1 folding with States‐TPPI phase implementation provides a convenient route to high‐resolution data. A description of the topology of the [60]fullerene sphere and its comparison with x‐ray crystallographic analysis is also presented. Copyright © 2001 John Wiley & Sons, Ltd.

Structure of the Lac Nominingue – Mont-Laurier region, Central Metasedimentary Belt, Quebec Grenville Province

The Lac Nominingue – Mont-Laurier region of the Central Metasedimentary Belt, Grenville Province of Quebec, comprises the granulite-facies Bondy gneiss complex (core of the Bondy gneiss dome) and overlying Sourd group metasedimentary rocks. A metamorphic foliation – transposed compositional layering (S1; host to peak-pressure parageneses) has been folded by isoclinal folds (F2 and F3) crosscut by leucosomes that host peak-temperature assemblages. The orthopyroxene isograd cuts obliquely across F3 folds, indicating that 1.20–1.18 Ga granulite-facies metamorphism post-dated D3. D3 structures are cut by ductile shear zones and boudinaged in D4 and are folded by regional-scale, open, upright north–south folds (F5). Folds with shallowly dipping axial surfaces (F6) are subsequently developed in the Sourd group. F5 (and probably F6) developed prior to intrusion of the ca. 1165 Ma Chevreuil suite. In the Nominingue–Chénéville deformation zone (NCDZ) east of the Bondy gneiss dome, Chevreuil intrusions contain north-striking magmatic and tectonic foliations. These, along with host gneisses and metasedimentary rocks, are displaced by conjugate ductile shear zones (northeast dextral and south-southeast sinistral) and north-northeast-striking thrusts. Late open folds (F8) with east-northeast-striking axial surfaces produce dome and basin interference patterns. F2 to F5 folds may have formed during either subhorizontal, east–west contraction or east–west extension resulting from orogenic collapse or convective lithospheric thinning following crustal thickening during terrane assembly in the Elzevirian orogeny. Structures in the NCDZ imply ESE–WNW contraction and NNE–SSW (orogen-parallel) extension in D7 syn- to post-intrusion of the Chevreuil suite. F8 folds imply a late, Grenvillian SSE–NNW contraction.

Tectonic and kinematic evolution within mid-crustal orogenic root zones: a case study from the Caledonides of northwestern Ireland

The Slieve League Peninsula of northwest Ireland lies on the western limb of a major orogenic strike-swing in which regional foliation trends have deviated from the northeast–southwest trends typical of much of Scotland, to west–east orientations. Across-strike coastal exposures on the western tip of the peninsula through Neoproterozoic Dalradian metasediments enable a detailed examination and analysis of the structural evolution of a Caledonian orogenic root zone which has been previously correlated with the Loch Awe Syncline of southwest Scotland. Minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships. Over much of the area, a composite, steep northeast–southwest-trending S2–S3 foliation containing a gently southwest-plunging quartz mineral elongation lineation is the dominant fabric at outcrop, and is associated with MP2 almandine–amphibolite facies metamorphism. F2 folds are isoclinal with curvilinear hinges and similar geometry. They typically plunge steeply towards the southwest and display variable (dextral) or north-directed vergence, whilst minor F3 fold hinges plunge moderately towards the southwest and typically verge (sinistrally) towards the south. Major, composite D1–D3 tectonic slides are developed in the Argyll Group. Structural and stratigraphic relationships indicate that D1 induced large-scale reversals in younging across tectonic slides, resulting in reversals in subsequent F2 and F3 facing patterns. Tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding, together with extensional crenulations, sheared quartz pods and metre-scale asymmetric boudinage of metadolerites, all of which indicate dextral (D2) and sinistral (D3) shear. After unfolding subsequent folds (F4), this corresponds to top-to-the-north (D2) and top-to-the-south (D3) translations. D4 results in regionally northwest-verging structures, with minor crenulations and the S4 cleavage transecting fold hinges in an anticlockwise sense, suggesting a dextral component of deformation. The detailed kinematic data indicate that the overall geometry of this western, deep-level arm of the root zone is not a product of the classic mushrooming fountain of nappes model, but rather major interference between consistent northerly directed D2 thrusting and a later phase of southeast-directed (D3) retrocharriage (‘back-folding’) which intensifies towards the south.

Deformation history of the Naraku Batholith, Mt Isa Inlier, Australia: Implications for pluton ages and geometries from structural study of the Dipvale Granodiorite and Levian Granite

Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa Inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith, and both contain an intense tectonic foliation, S1, which is interpreted to have formed during the north‐south shortening associated with D1 of the Isan Orogeny. The geometry of S1 form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south‐southwest‐plunging antiform, which was produced by the regional D2 of the Isan Orogeny. S1 form surfaces in the Levian Granite define open F2 folds with wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre‐ or syn‐ the regional D1. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S3 of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre‐D1 and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.

Deformation history of the blueschist-facies sequences from the Villa de Cura unit (Northern Venezuela)

The Serrania del Interior terrane is located in Northern Venezuela, that represents the southern margin of the Caribbean plate. This terrane includes the Villa de Cura unit, which mainly consists of multiple sub-units of metamorphosed and deformed Cretaceous volcano-sedimentary sequences. These sequences are commonly interpreted as generated in a supra-subduction zone setting. The structural history of the Villa de Cura blueschists-facies units includes four main deformational phases, from D1 to D4. The first D1 phase is mainly represented by a relict S1 schistosity developed under HP/LT conditions. The relic S1 schistosity is deformed by isoclinal to subisoclinal F2 folds showing similar geometry. The F2 folds are characterized by a continuous S2 crenulation cleavage developed under greenschist-facies metamorphism. The parallelism between the A2 axes and the related L2 mineral lineations suggests an interpretation of the F2 folds as sheath folds developed during non-coaxial deformation. The kinematic indicators suggest a top-to-W sense of shear for the D2 phase. The D3 phase is distinguished by asymmetric, parallel F3 folds with a S vergence. Finally, the D4 phase consists of F4 open, gentle folds with high-angle to sub-vertical axial planes. The collected data suggest a complex deformation history, characterized by coupling of strike-slip tectonics and shortening during the retrograde evolution of the blueschist-facies sequences.

Deformation history of the Yolla Bolly terrane at Leech Lake Mountain, Eastern belt, Franciscan subduction complex, California Coast Ranges

The roles of volume loss, coaxial versus noncoaxial flow, and blueschist exhumation in subduction-related accretionary wedges are still poorly understood. In our study at Leech Lake Mountain in the Eastern belt of the Franciscan subduction complex, we focus on these subjects. In the specific example of the Franciscan, the tectonic significance of the boundary between the Eastern and Central belts remains controversial. The Leech Lake Mountain area in northern California is situated immediately above this boundary and, therefore, appears to be of crucial importance for understanding aspects of the tectonic evolution of the Franciscan. The structural development at Leech Lake Mountain is characterized by three deformational events. D2 produced the regional cleavage (s2) during blueschist-facies metamorphism. D2 is the only entirely ductile deformation event and was associated with accretion-related internal stacking within the Yolla Bolly terrane. The s2 cleavage is folded by F3 folds, which are overturned to the west. Spacing of a related s3 cleavage is generally at the decimeter scale, but locally is spaced at the centimeter scale. The s2 and s3 cleavages, and F3 folds, are overprinted by tight to open F4 folds at the decimeter to map scale. Map-scale F4 folds are largely upright, whereas small-scale F4 folds are either upright or overturned to the east. Our finite-strain analysis relates deformed lengths to original lengths in the rock and thus provides an absolute reference frame, which allows us to detect deformation-related volume changes. Absolute finite-strain data from 20 samples support earlier studies indicating that the development of the moderately dipping s2 cleavage was accompanied by pronounced volume loss, averaging 36%. The directions of the principal finite-strain axes are scattered; therefore, we calculated a tensor average of the data. The principal stretches of the tensor average are S X : S Y : S Z = 1.06:0.91:0.66, indicating that the accumulation of ductile strain was characterized by subvertical shortening, which was largely compensated for by volume loss and not by orthogonal extension. Evidence for noncoaxial deformation in our samples is limited, indicating that the accumulation of ductile strain was largely coaxial. Our preferred tectonic interpretation is that accretion and blueschist-facies metamorphism of the Yolla Bolly terrane at Leech Lake Mountain occurred during D2 under an almost coaxial deformation regime. During and after D2, the rocks were considerably exhumed and were telescoped by D3 top-to-the-west out-of-sequence thrusts at shallow-crustal levels. In the Leech Lake Mountain area, D3 thrusting juxtaposed the Eastern and Central belts along the Red Mountain fault and cut out the lower tectonic units of the Eastern belt. The difference in the degree of peak metamorphism between the Eastern and Central belts is modest, indicating that displacement at the Red Mountain fault was not large. We propose that D3 out-of-sequence thrusts attenuated the metamorphic and stratigraphic section across the entire Franciscan and overlying Great Valley forearc. The final D4 event produced large-scale folds. All three events resulted from horizontal shortening. There is no evidence for a major phase of horizontal extension, which might have aided exhumation of the Eastern belt blueschists. Modest tectonic exhumation was due to vertical ductile shortening during D2.

The Werner Lake Co-Cu-Au Deposit of the English River Subprovince, Ontario, Canada: Evidence for an Exhalative Origin and Effects of Granulite Facies Metamorphism

The Werner Lake Co-Cu-Au deposit is confined to a mixed unit of orthopyroxene- bearing amphibolites, ultramafic rocks, garnetiferous biotite schists, calc-silicate rocks, and garnet-rich quartzites. Mineralization includes disseminated sulfides in the garnetiferous biotite schists and semimassive-massive cobaltite-rich orebodies. Field relationships and geochemical data suggest that the garnetiferous biotite schists were most likely derived from amphibolites, whereas the calc-silicate rocks and garnet-rich quartzites probably represent metamorphosed exhalites. Mass changes for the formation of the garnetiferous biotite schists include gains in Fe, Mn, and K but loses in Ca and Na. The garnetiferous biotite schists are also characterized by depletion in whole-rock δ 18O values (4–6.5‰ V-SMOW) relative to the associated amphibolites (6.8–8.3‰) and ultramafic rocks (5.6–6.6‰), which is interpreted as an indication of high- temperature interactions with seawater. Near zero d34S values of cobaltite and sulfides indicate a juvenile source for S. These data and similarities to strata-bound Co-Cu and Cu-Co deposits suggest that the Werner Lake deposit most likely formed from syngenetic exhalative processes. The granulite facies metamorphism (680°–780°C, 4.5–6.6 kbars, and a H2O of 0.12–0.25) at the Werner Lake deposit was accompanied by penetrative deformation and was followed by retrogression and late hydrothermal alteration. However, high-grade metamorphism and deformation did not obliterate the primary metal and alteration zonation in the Werner Lake deposit. We attribute this preservation of primary features to the dry character of the granulite facies metamorphism. This is supported by the preservation of contrasting oxygen isotope signatures in the ores and associated lithologies. Localized, solid-state remobilization of cobaltite-rich ores is indicated by the presence of durchbewegung textures, the alignment of cobaltite porphyroblasts parallel to the S2 foliation and thickening of cobaltite-rich orebodies close to F2 fold hinges. Chalcopyrite-rich veins with biotite-rich selvages suggest a local, fluid- facilitated remobilization during retrogression. Granulite facies metamorphism, deformation, and subsequent retrogression might have locally upgraded the cobaltite-rich ores. Late hydrothermal alteration did not significantly affect the Co mineralization in the Werner Lake deposit.

Structural setting, textures, and timing of hydrothermal vein systems in the Val d'Or camp, Abitibi, Canada: implications for the evolution of transcrustal, second- and third-order fault zones and gold mineralization

In the Val d'Or camp, Archean Abitibi greenstone belt, Canada, numerous gold-mineralized second- and third-order fault zones are spatially associated with the transcrustal Cadillac Tectonic Zone (CTZ). This situation is used to test whether fluid systems in the CTZ have a similar structural timing to those in the gold-hosting structures, and hence the CTZ could represent the main fluid conduit in the camp. The transcrustal CTZ at Orenada No. 2 contains structurally complex vein systems, with mineralized quartz-tourmaline veins related to both D2 oblique-reverse faulting and F3 dextral asymmetric folding, both of which have been overprinted by unmineralized subhorizontal and subvertical quartz veins. Quartz ± tourmaline veins within second- and third-order shear zones at Paramaque and Rivière Héva also formed during D2 deformation and have been, at least at Rivière Héva, deformed by F3 asymmetric folding. In contrast, mineralized quartz vein systems at Cartier Malartic are controlled by F3 folding and overprinted by late-stage D3 faults which host late quartz-tourmaline veins. Quartz vein textures are consistent with these timing relations, because D2-controlled veins contain deformed quartz grains, whereas quartz in D3-controlled veins is unstrained. The D2 and D3 timing of mineralized quartz veins in the transcrustal CTZ and in second- and third-order structures is consistent with the notion that the CTZ represents the main fluid conduit and that mineralization occurred in linked second- and third-order structures. The different timing of quartz-tourmaline veins in different shear zones indicates that the veins were probably hydraulically linked to the CTZ during at least two different episodes. The location of Cartier Malartic structurally below the CTZ indicates that fluids travelled either downward from the main conduit or that the shear zone was part of the CTZ.

TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK, SUPERIOR PROVINCE, QUEBEC, CANADA

Previously considered one of Earth’s largest granulite terranes, the Minto block, in northern Quebec, consists dominantly of granitic rocks with sparse supracrustal remnants that record low-pressure, high-temperature metamorphism to the amphibolite and granulite facies at ca. 2.7 Ga. Much of the region is underlain by plutonic rocks, including igneous charnockitic bodies, that provide little evidence of metamorphism. In contrast, pelitic assemblages in low-grade greenstone belts and their higher-grade shoulders define four metamorphic zones: 1) sub-staurolite-grade slates, found only in the Duquet belt, 2) staurolite – andalusite – sillimanite – garnet-zone schists, present in all belts, 3) marginal garnet – sillimanite ± kyanite migmatites, and 4) garnet – sillimanite ± cordierite and garnet – orthopyroxene granulites, as enclaves within plutonic units. A prograde low-pressure, hightemperature regional facies-series is defined by andalusite – sillimanite assemblages, thermobarometric analyses that indicate peak conditions ranging from 3 kbar, 560°C to 8.4 kbar, 840°C, and short segments of anticlockwise P–T trajectories preserved in two of the lowest-grade areas. Kyanite occurs in migmatites in the vicinity of a ca. 2.81 Ga shear zone bounding two distinct supracrustal assemblages and marks an earlier Barrovian event. All belts are polydeformed, with a common, broadly synmetamorphic, NNW-striking, steep foliation and down-dip lineation related to regional-scale F2 folds. Distributed late-metamorphic dip-slip shear appears responsible in some areas for cross-strike juxtaposition of low- and high-grade metamorphic zones. Older, generally poorly preserved D1 deformation, recognized in several belts, may represent accretionary structures. Variably developed younger deformation includes NNW dextral transcurrent brittle–ductile shear zones (S3), variably developed east-trending F4 folds, and WNW-striking pseudotachylite zones. U–Pb monazite ages of 2702 Ma date high-grade metamorphism of a pelitic granulite, crystallization of two diatexites, and staurolite-zone metamorphism in one schist. Together with similar zircon ages of crust-derived granites, these data point to a major collisional orogeny at this time. Older titanite (2748–2765 Ma) indicates areas that escaped >650°C metamorphism during the 2.7 Ga event. Younger monazite occurs in a migmatite-grade rock (2688, 2672 Ma) and staurolite-zone schists (2679–2642, 2648, 2637, and 2642–2628 Ma), similar to titanite ages in greenstones (2643–2550 Ma) and 40 Ar/ 39 Ar hornblende ages (2.64–2.56 Ga). These post-peak metamorphic ages in the greenstone belts based on monazite and titanite are attributed to hydrothermal growth from fluids channeled along permeable zones; they may reflect late reheating events observed throughout the Superior Province as leucogranitic magmatism, metamorphism in the deep crust, and mineral growth in shear zones.

Polyphase Folding in Nugrus-Sikeit Area South Eastern Desert, Egypt

lithological constitution of Nugrus-Sikeit area comprises a sequence of dismembered ophiolites, tectonic melange association and arc assemblage. These older rocks are intensively deformed and intruded by intracratonic association, which within the mapped area, is represented by gabbros and the associated hornblendite together with younger granites. Planar and linear mesoscopic structures exhibited by the rocks of Nugrus-Sikeit area indicate that these rocks are involved in superimposed folding events and at least three fold generations (F1, F2 & F3) can be recognized. The F1 folds resulted in isoclined folding of S0, and other primary structures around WNW-ESE axes. This phase represents the main folding that affected the area. Besides establishing the WNW-ESE pattern of rock units, it was also responsible for the transposition of S0 into penetrative surfaces S1 accompanying low to medium grade regional metamorphism. The F2 folds refolded the F1 folds into upright to overturned and even recumbent folds about NW-SE axes together with the crenulation of S1 surfaces producing crenulation cleavage S2? The F3 folds are of mild intensity compared to the F1 and F2 generations. It neither appeared to disturb the orientation of the earlier structures nor accompanied by strong transposition foliation. Associated folds are of the mesodimension and maintain a fairly constant NNE-SSW trend.

Structural development of the Snow Lake Allochthon and its role in the evolution of the southeastern Trans-Hudson Orogen in Manitoba, central Canada

The Snow Lake Allochthon is a zone of tectonic interleaving of sedimentary rocks of an inverted marginal basin (Kisseynew Domain) with island-arc and oceanic rocks. It is located in the southeastern part of the exposed internal zone of the Paleoproterozoic Trans-Hudson Orogen in Manitoba, Canada, near the external zone (Superior collision zone or Thompson Belt), which constitutes the local boundary between the Trans-Hudson Orogen and the Archean Superior Craton. The Snow Lake Allochthon formed, was deformed, and was metamorphosed up to high grade at low to medium pressure during the Hudsonian orogeny as a result of the collision of Archean cratons ~1.84-1.77 Ga. Four generations of folds (F1-F4) that formed in at least three successive kinematic frames over a period of more than 30 Ma are described. Isoclinal to transposed southerly verging F1-2 structures are refolded by large, open to tight F3 folds and, locally, by open to tight F4 folds. The axes of the F1-2 folds are parallel or near parallel to the axes of F3 folds, owing to progressive reorientation of the F1-2 axes during south- to southwest-directed tectonic transport, followed by F3 refolding around the previous linear anisotropy. A tectonic model is presented that reconciles the distinct tectono-metamorphic developments in the Snow Lake Allochthon and the adjacent part of the Kisseynew Domain on the one hand, and in the Thompson Belt on the other, during final collision of the Trans-Hudson Orogen with the Superior Craton.

Comportement du système U-Pb dans la monazite et chronologie de la déformation et du métamorphisme des metasédiments du domaine de Kisseynew, orogène trans-hudsonien (Manitoba, Canada)

The Kisseynew domain is the central unit of the Reindeer Zone of the Paleoproterozoic Trans-Hudson Orogen, in Manitoba and Saskatchewan. The southern flank of the domain is a transition zone between the greenschist facies of the volcano-plutonic assemblage of the Flin Flon - Snow Lake belt and the upper amphibolite facies of Kisseynew paragneisses. The Jungle Lake area, in the southern flank of the Kisseynew Domain, comprises mainly quartzofeldspathic gneisses representing continental clastic units of the Missi suite and migmatitic metagraywackes of the Burntwood suite. The area was affected by several phases of deformation, metamorphism, and migmatisation. Detailed mapping and U-Pb geochronology were carried out in order to establish the timing of the deformational and metamorphic phases. The oldest leucosome contains sillimanite formed during peak metamorphism and is associated with F2 folding and S2 fabric. Five single monazites from this leucosome yield ages between 1809 and 1803 Ma taken as the best estimate for the duration of peak metamorphism. Biotite schlieren in diatexites in the Burntwood suite show a S2 fabric folded by F3. Zircon from one of these diatexites yield a crystallization age of 1798+3-2 Ma, considered as the lower limit for the F2 event. Single monazites from the same rock yield ages between 1812 and 1789 Ma, the oldest of which are considered to be inherited. The youngest mobilisate is a pegmatite crosscutting F2 and F3 fabrics and yielded single monazite ages between 1875 and 1788 Ma. The youngest age is taken as the age of the pegmatite and is a minimum age for F3 fabrics. These results, together with those from other areas of the southern Kisseynew Domain, indicate a ca. 30 million year period (1818 and 1785 Ma) of continuous deformation and metamorphism. The data also show the presence of monazite crystals of different ages in the same rock illustrating the need to analyse single grains to obtain geologically meaningful ages.

Structure and stratigraphy of the south flank of the Kisseynew Domain in the Trans-Hudson Orogen, Manitoba: implications for 1.845-1.77 Ga collision tectonics

On the south flank of the Kisseynew Domain, orthogneisses derived from 1.92-1.85 Ga volcano-plutonic rocks are overlain by paragneisses (Burntwood and Missi groups) derived from 1.855-1.84 Ga marine turbidite and 1.845-1.83 Ga terrestrial clastic and volcanic rocks. The sediments in these groups are interpreted as having been shed into the Kisseynew paleobasin from an active margin bordering the Flin Flon Belt. The sedimentation apparently followed early microcontinental collision and accompanied the last arc magmatism in the Trans-Hudson Orogen. The sedimentary rocks and their basement were deformed into a complexly refolded stack of large recumbent folds. Premetamorphic F1 structures represent a fold and thrust system initiated during the sedimentation. These structures are interpreted as transported toward the Kisseynew Domain in the northeast and the hinterland in the southwest. F2 structures (~1.82 Ga) comprise westerly transported nappes. During 1.82-1.80 Ga high-grade metamorphism, the early structures were overturned, amplified, and refolded. Basement-cored culminations and sheet-like synforms of paragneiss were horizontally attenuated and transported south and southwest. North- and northeast-trending F4 folds and F5 faults formed after 1.79 Ga. The whole cycle of deformation is related to stages of continental collision between the internal (juvenile) zone of the Trans-Hudson Orogen and the three surrounding Archean cratons (Sask, Superior, and Hearne). The F4 upright folds and steep F5 faults are interpreted as the record of intracontinental transpression, strongly controlled by the Superior Craton boundary.

Tectono-metamorphic history of Neoproterozoic rocks in eastern Eritrea

The Neoproterozoic juvenile crust of Eritrea lies within the East African Orogen that marks a megasuture along which the East and West Gondwana converged. In eastern Eritrea, repeatedly deformed Neoproterozoic rocks of this orogen consist of two lithotectonic terranes, Ghedem and Nacfa. The Ghedem Terrane consists of amphibolite facies metamorphic rocks with dominantly subhorizontal to moderately dipping fabrics. The structurally overlying Nacfa Terrane consists of greenschist facies metamorphic rocks with steeper fabrics. Three deformation phases are distinguished that can be related to E–W lateral compression and E–W orogenic collapse of Neoproterozoic age. D1 structures are represented by an originally N–S/NNE–SSW steep and penetrative S1 foliation which is axial planar to F1 isoclinal upright folds. D1 structures were refolded by recumbent folds and nappes during D2, when deformation was dominated by subhorizontal shear zones accompanied by recumbent meso-microscopic NNW to NNE trending F2 folds. D3 deformation formed open upright F3 folds co-axial with F2 and a steep S3 spaced axial planar foliation accompanied by strike–slip shear zones. The D1–3 structures and fabrics are interpreted as phases of a long-lived Neoproterozoic lateral compression. Orogenic collapse during D2–3 was characterized by structures that were ductile at depth and semi-brittle at shallow levels. P– T conditions of metamorphism were determined from the mineral chemistry of pelitic–semipelitic rocks in the Ghedem Terrane. Mineral assemblages included within garnets and matrix assemblages in textural equilibrium with garnet rims define two P– T fields. Estimated P– T conditions of metamorphism for the inclusion assemblages in garnet cores range between 3.9 and 5.3 kbar and 516–584°C whereas those for the rim–matrix assemblages are between ∼5–11 kbar and 526–637°C. Microstructural inferences and the P– T conditions obtained from the mineral assemblages within garnets indicate that prograde M1 greenschist facies metamorphism was contemporaneous with D1 deformation. Similarly, the P– T equilibrium conditions constrained for the rim–matrix assemblages relate to late M2 amphibolite facies metamorphism during late D2 deformation. Textural relationships reveal that M3 metamorphism was retrogressive during D3. Inclusion and rim–matrix geothermobarometry depict a family of anticlockwise P– T paths indicating heating and contemporaneous loading during M1/D1 at intermediate crustal levels and nearly isothermal loading during early M2/D2 at deeper crustal levels followed by a return path during late D2 and D3 deformations. These rocks represent a segment of the laterally shortened and subsequently extended mid-lower continental crust of the East African Orogen that was exhumed from as deep as 48 km (12 kbar) during late Neoproterozoic interaction between orogenic collapse, erosion and deep isostatic root recovery. The preliminary interpretation is that the relationships between the P– T conditions of metamorphism and deformation in the Ghedem Terrane rocks of eastern Eritrea indicate a continental orogen related to the late Neoproterozoic terminal convergence between East and West Gondwana.

Evolution of the late Paleozoic accretionary complex and overlying forearc‐magmatic arc, south central Chile (38°–41°S): Constraints for the tectonic setting along the southwestern margin of Gondwana

Stratigraphic, structural, metamorphic, and geochronologic studies of basement rocks in the Andean foothills and Coast Ranges of south central Chile (39°–41°S) suggest a protracted late Paleozoic to middle Mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern Gondwana margin. In the study area the late Paleozoic paired metamorphic belt, coeval magmatic arc, and overlying Triassic sedimentary units preserve a record of Late Carboniferous to Early Permian subduction and arc magmatism, subsequent deep exhumation of the Western Series subduction complex, and diminished uplift and erosion of the Eastern Series arc‐forearc region by the Late Triassic. Late Paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism, collectively referred to as Dl, are largely erased in the Western Series by the dominant D2 schistosity and lower greenschist grade metamorphism. D1 structural features, as well as original sedimentary textures, are relatively well preserved in the less penetratively deformed Eastern Series. The regional distribution of late Paleozoic arc magmatism suggests that the late Paleozoic convergent margin deviated from a N–S trend north of this area to a NW–SE trend near this latitude and faced an open marine environment to the southwest. A transition from F2 isoclinal folding to more open, larger‐scale F3 folds, interpreted as change in ductility during differential uplift of the Western Series, is not apparent in the Eastern Series. Despite a lesser degree of uplift during the main exhumational D2 event, delineation of unconformities and U‐Pb dating of detrital zircons and intrusions into the Eastern Series allow tighter constraints to be placed on timing of uplift and denudation of the Eastern Series than on that in the Western Series. A regional unconformity exposed in the Lake District that separates more highly deformed Eastern Series lithologies from Late Triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the D2 event. We propose that dextral‐oblique convergence, initiated during the middle Permian along this segment of the Gondwana margin, resulted in the transpressional uplift and juxtaposition of high pressure/temperature (P/T) Western Series against low P/T Eastern Series lithologies and culminated with deposition of Late Triassic, continental to shallow marine, coarse clastic sedimentary rocks in fault‐bounded strike‐slip basins adjacent to the exhumed Western Series. Large‐scale dextral transpression and northward displacement of the accretionary complex during Late Permian to Late Triassic time along the Chilean margin of Gondwana are synchronous and kinematically compatible with widespread regional transpression, extension, and silicic magmatism inboard of the southern Gondwana margin at this time.We thank C. Mpodozis, M. Gardeweg, and J. Muñoz of the Servicio de Geología y Minería de Chile (SERNAGEOMIN) for their support of this work. Fruitful discussions with N. Blanco, F. Hervé, H. Moreno, C. Mpodozis, and F. Munizaga have aided in our understanding of the geology of the region. The hard work by the staff of SERNAGEOMIN’s Puerto Varas office is graciously appreciated. We thank J.D. Walker and W.R. Van Schmus at the University of Kansas for allowing MWM use of their U‐Pb and mass spectrometer facilities and J. Vargas and the staff of SERNAGEOMIN’s geochemistry laboratory for their assistance in this project. F. Munizaga allowed us to cite an unpublished 40Ar‐39Ar date. We thank G. Ya˜nez for access to aeromagnetic data. T. Kato wishes to thank W. G. Ernst. Comments by I. Dalziel, S. Kay, and V. Ramos helped clarify ideas presented in this paper and are greatly appreciated. This work is dedicated to our friend and colleague Alberto Campos C., who died in a climbing accident on Calbuco Volcano, 1996.

Timing of plutonism, deformation, and metamorphism in the Yellowknife Domain, Slave Province, Canada

U-Pb zircon, titanite, and monazite ages for plutonic rocks are used to constrain the timing of post-2660 Ma deformational and metamorphic events in the Yellowknife Domain, southern Slave Province. Diorite and tonalite plutons of the Defeat Suite yield ages of 2628 ± 3 and 2624 ± 3 Ma, older than previous, less precise estimates for this suite of ca. 2618-2620 Ma. The Defeat Suite plutons crosscut and postdate upright F1 folds in turbidites of the ca. 2680-2660 Ma Burwash Formation. D1 deformation is therefore bracketed between 2660 and 2630 Ma. The D1 event significantly predates deposition of polymict conglomerates of the Jackson Lake Formation. A second folding event (F2), represented by northwest- to north-trending steeply plunging folds of bedding and D1 structures, affected the area after intrusion of the Hidden Lake pluton at 2606 ± 2 Ma, and synchronously with intrusion of the 2596 ± 2 Ma Sparrow Lake pluton, a member of the two-mica granite Prosperous suite. The time of initiation and the duration of D2 deformation are not well defined. It may have affected units as young as the 2586 ± 2 Ma Morose Granite. Metamorphic isograds are spatially related to Defeat, Prosperous, or even younger plutonic complexes, and therefore formed at different times over a 30 Ma period in different parts of the Yellowknife Domain. The ca. 2620-2630 Ma Defeat-type plutonism started in the Yellowknife Domain 10-15 Ma earlier than similar intrusive events in the central and northern Slave Province and highlights diachronous development of plutonism in the period from 2630 to 2610 Ma. In contrast, the ages of younger granites in the Yellowknife Domain (Prosperous, Morose) are within the range of intrusive ages documented for similar plutonic suites elsewhere in the Slave Province.

Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks

Fifty million years of Archean evolution is recorded in the Manitouwadge greenstone belt and the Wawa-Quetico boundary region, from ca. 2720 Ma volcanism and subvolcanic plutonism associated with massive sulphide deposits and hydrothermal alteration zones, to 2689-2687 Ma and 2680-2677 Ma synkinematic plutonism. In the greenstone belt, greywackes were deposited after 2693 Ma, post-dating local volcanism by at least 25 Ma, and requiring that the volcanic-sedimentary contact is an unconformity or a fault. In migmatitic greywackes in the Quetico subprovince, detrital zircons limit the depositional age to <2690 Ma, permitting correlation of greywackes across the Wawa-Quetico subprovince boundary. Upward-facing inclined F2 folds that deform the volcanic-sedimentary contact are bracketed by the 2687 ± 2 Ma Loken Lake pluton, which shows strong D2 fabrics, and by 2680+4-3 Ma foliated granite which cuts D2 fabrics. Dextral transpression producing regional F3 folds and the overall east-west trends of the Wawa-Quetico boundary region post-dated the 2680 ± 2 Ma Nama Creek pluton. Field relationships and isotopic ages support correlation of greywackes across the subprovince boundary, and demonstrate that most or all of the ductile deformation post-dated sedimentation. Ductile structures, especially those associated with dextral transpression, are not directly related to juxtaposition of the Wawa and Quetico subprovinces, as these were already contiguous, either through sedimentation on a volcanic substrate or as a result of earlier cryptic structures. Our results imply that the belt-like configuration of the subprovinces, emphasized in accretionary models of the Superior Province, is a relatively late feature that overprints older, tectonically significant structures.