D1 Structures Research Articles

Orogen-parallel tectonic transport in the Variscan belt of northeastern Sardinia (Italy): implications for the exhumation of medium-pressure metamorphic rocks

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse.

Structural History and Timing of Gold Mineralization in the Northern East Strelley Belt, Pilbara Craton, Western Australia

The McPhees and Iron Stirrup deposits, recent gold producers in the Pilbara Craton, Western Australia, are located within a succession of Early to Middle Archean metavolcanic rocks of the northern part of the East Strelley belt. Deformation occurred during five stages (D1-D5), with mineralization late in the tectonic cycle (D4). D1 structures only affected rocks below a regional, Early Archean unconformity. D2 deformation produced a ubiquitous, variably developed S2 foliation, defined by peak metamorphic minerals. Mineralization is hosted within, but postdates a 100- to 400 m-wide, D2 high-strain zone of talc-chlorite-carbonate schist (ultramafic protolith) that contains competent boudins of actinolite-chlorite schist (mafic protolith). D3 deformation produced localized open folds, outlined by lithological layering and S2 foliation. Regional, open folding, and sinistral shearing of reactivated D2 structures occurred during D4 deformation, which controlled synkinematic intrusion of granitic pegmatite at ca. 2880 Ma. At the McPhees deposit, narrow, sinistral, north-northeast–trending D4 shear zones were conduits for mineralizing fluids, which reacted with, and preferentially mineralized, mafic rocks. Gold occurs in altered schistose wall rock and within veins composed of actinolite, quartz, and pyrrhotite, and minor tourmaline, albite, talc, and magnesite; vein selvages consist of albite and tourmaline. Mineralization at Iron Stirrup was also controlled by north-trending D4 structures that reactivated D2 fabrics, and was concentrated at a sheared contact between serpentinite and talc-chlorite ± carbonate schist. Similar north-trending D4 shear zones overprint earlier structures and control ca. 2890 Ma gold mineralization nearby in the Mount York gold district, suggesting a single mineralizing event for the entire belt, which is the same age (within analytical error) as granitic pegmatite intrusions. Gold was deposited under amphibolite metamorphic conditions at Mount York and greenschist conditions at Iron Stirrup and McPhees, indicating a decrease in metamorphic conditions away from the western granite-greenstone contact, where granitic pegmatite dikes are abundant. Therefore, mobilization and transport of gold occurred in response to thermal perturbations related to granitic pegmatite dike emplacement during D4 deformation.

Palaeozoic to Mesozoic polyphase deformation of the Patuxent Range, Pensacola Mountains, Antarctica

The Patuxent Range forms the most southerly third of the Pensacola Mountains, East Antarctica. Largely unstudied since the original geological survey work of the 1960s, the Patuxent Range was thought to expose metasediments deformed by a single Precambrian event. However, new structural data collected from two geographically separate areas in the central Patuxent Range reveal the presence of three distinct generations of structures. A synthesis of the regional geology together with new data suggests that the Patuxent Formation was mildly deformed during end Cambrian times as part of the late stage Ross Orogeny. However, the most intense deformation, although poorly constrained in age, probably occurred during the Permo-Triassic Gondwanian Orogeny. A third phase of deformation predates the intrusion of 183 Ma lamprophyre dykes and involved an inferred vertical axis rotation of the pre-existing D1 and D2 structures and the localized development of a spaced foliation and mesoscale folding. These D3 structures may be the first evidence of an Early Jurassic deformation event in the Transantarctic Mountains, which correlates with the Peninsula and Rangitata I orogenies of the Antarctic Peninsula and New Zealand, respectively.

Anti-sm autoantibodies in systemic lupus target highly basic surface structures of complexed spliceosomal autoantigens.

Autoantibodies directed against spliceosomal proteins are a common and specific feature of systemic lupus erythematosus. These autoantibodies target a collection of proteins, including Sm B, B', D1, D2, and D3. We define the common antigenic targets of Sm D2 and D3 and examine their role in spliceosomal autoimmunity. Our results define nine major common epitopes, five on Sm D2 and four on Sm D3. These epitopes have significantly higher (more basic) isoelectric points than do nonantigenic regions. In fact, this association is of sufficient power to make isoelectric point an excellent predictor of spliceosomal antigenicity. The crystallographic structure of Sm D2 and D3 is now partially described. The anti-Sm D2 and D3 antigenic targets are located on the surface of the respective three-dimensional complexed proteins, thereby suggesting that these epitopes are accessible in the native configuration. All but one of these nine epitopes conspicuously avoid the specific regions involved in intermolecular interactions within the spliceosomal complex. One of the D3 epitopes (RGRGRGMGR) has significant sequence homology with a major antigenic region of Sm D1 (containing a carboxyl-terminal glycine-arginine repeat), and anti-D3 Abs cross-react with this epitope of Sm D1. These results demonstrate that spliceosomal targets of autoimmunity are accessible on native structure surfaces and that cross-reactive epitopes, as well as structural associations of various spliceosomal Ags, may be involved in the induction of autoimmunity in systemic lupus.

NEW GEOLOGICAL DATA FROM GORGONA ISLAND (NORTHERN TYRRHENIAN SEA)

The oceanic HP/LT metamorphic units cropping out in the Gorgona Island (Tuscan Archipelago) are an important element to define the Alps-Apennines geological boundary. In particular, they could represent a link between the Alpine Schistes Lustres of the Corsica Island and the similar oceanic metamorphic units of the inner part of the Northern Apennines (e.g. Argentario Promontory, Giglio Island, Elba Island). The Gorgona Island tectonic stack includes two main units (the Metasedimentary Unit and the overlying Metaophiolitic Unit) separated by a north-eastward dipping cataclastic-mylonitic shear zone. These units are imprinted by a polyphased tectono-metamorphic evolution ranging from an Oligocene blue-schists event to the ?Miocene re-equilibration and exhumation in the green-schists facies. The new 1:2.000 mapping and the litostratigraphical, structural, petrographical studies presented here allow to improve the geological knowledge of the island. In the Punta Gorgona Calcschists (Metasedimentary Unit) three lithofacies were distinguished and mapped. Their areal distribution and the structural data (e.g. “s” and “z” - type asymmetries of the parassitic folds, the relationships between bedding and cleavages) evidenced several NW-vergent folds in the overturned limb of the D2 mega-fold in the central-southern part of the island. Moreover, during the D2 event, which exhumed and deformed the D1 structures, shearing produced a mylonitic horizon between the two units. During the following extensional stages, an evident inversion of movement along the mylonitic horizon took place and produced a cataclastic texture. Later, weak folds, mainly E-W joints and possible high angle normal faults affected both units. The reconstructed metamorphic-tectonic evolution is compared with that defined for “Alpine Corsica” and for the Northern Apennines chain. From a stratigraphic point of view the data suggest that the Cala di Pancia Sandstones may represent the stratigraphic top of the Metasedimentary Unit, and its probable lateral-vertical transition to part of the Punta Gorgona Calcschists are inferred. The Authors also discuss the possible correlations of the Gorgona Units with other Ligurian-Piedmontese Units cropping out in Corsica, in the Tuscan Archipelago and in the Northern Apennines.

Synthesis and X-Ray Crystal Structures of Fe2Ru(CO)12−n(CNBut)n and FeRu2(CO)12−n(CNBut)n (n=1, 2)

The clusters Fe2Ru(CO)12−n(CNBut)n (3, n=1; 4, n=2), FeRu2(CO)12−n(CNBut)n (5, n=1, 6, n=2) and FeRu2(CO)11(CNCy) (5a) have been prepared by direct substitution from the parent carbonyl precursors Fe2Ru(CO)12 (1) and FeRu2(CO)12 (2). All compounds have been characterized spectroscopically and clusters 3, 4, 5, and 6 by single crystal X-ray determinations. In all cases, the isonitrile ligands adopt axial or pseudo-axial positions on a ruthenium atom. The structures of 3–5 are very similar to their parent clusters, but the extent of metal framework disorder is significantly less. Cluster 6 adopts the same C2v Fe3(CO)12 type structure as 4, and thus differs markedly from the parent compound 2, which has a D3 structure .

Transpressional deformation in northwestern Sardinia (Italy): insights on the tectonic evolution of the Variscan Belt

Structural investigations in northwestern Sardinia highlight the occurrence of a regional D2 transpressional deformation related both to NNE–SSW direction of compression and to a NW–SE shear displacement. The deformation is continuous and heterogeneous, showing a northward strain increase, indicated by progressively tighter folds and occurrence of F2 sheath folds. D2 transpression is characterised by the presence of a crustal-scale shear deformation overprinting previous D1 structures, related to nappe stacking and top-to-the-south and southwest thrusting. The L2 prominent stretching lineation points to an orogen-parallel extension and to a change in the tectonic transport from D1 to D2. It is attributed to the position of Sardinia close to the northeastern part of the Cantabrian indenter during the progressive evolution of the Ibero-Armorican Arc. D1 phase developed during initial frontal collision, whereas D2 deformation characterised the progressive effect of horizontal displacement during the increasing curvature of the arc. To cite this article: R. Carosi, G. Oggiano, C. R. Geoscience 334 (2002) 287–294. L'analyse structurale a mis en évidence dans le Nord-Ouest de la Sardaigne une déformation D2 transpressive, continue et hétérogène, qui augmente d'intensité vers le nord. Elle se superpose aux structures précédentes D1, liées à la mise en place des nappes vers le sud et le sud-ouest. La linéation d'allongement L2 met en évidence une extension parallèle à la chaı̂ne et un changement net, de la phase D1 à la phase D2, de la direction du transport tectonique. La phase D1 s'est développée durant la collision frontale initiale, alors que la déformation D2 est due à l'augmentation progressive de la composante en décrochement pendant le développement de l'arc Ibéro-Armoricain. Pour citer cet article : R. Carosi, G. Oggiano, C. R. Geoscience 334 (2002) 287–294.

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.

Pre‐Tabberabberan deformation in eastern Tasmania: A southern extension of the Benambran Orogeny

Recumbent folding in eastern Tasmania affected turbidites containing Lower to Middle Ordovician (Bendigonian Be1 to Darriwilian Da3) fossils, but not stratigraphically overlying turbidites containing Silurian (Ludlow) graptolites, and is of a timing consistent with Ordovician to Silurian Benambran orogenesis on the Australian mainland. Two subsequent phases of upright folding post‐date deposition of turbidites containing Devonian plant fossils but pre‐date intrusion of Middle Devonian granitoids, and are of Tabberabberan age. A closely spaced disjunctive cleavage (S2), associated with the first phase of Tabberabberan folding, everywhere cuts a slaty cleavage (S1) associated with the earlier formed recumbent folds. However, refolding associated with development of S2 is not always clear in outcrop and it is proposed that coincident tectonic vergence between the two events has resulted in reactivation of recumbent D1 structures during the D2 event. The transition to rocks not affected by recumbent folding coincides with a marked change in sedimentology from shale‐ to sand‐dominated successions. This contact does not outcrop but, from seismic data, appears to dip moderately to the east, and can only be explained as an unconformity. The current grouping of all pre‐Middle Devonian turbidites in eastern Tasmania into the one Mathinna Group is misleading in that the turbidite sequence can be subdivided into two distinct sedimentary packages separated by an orogenic event. It is proposed that the Mathinna Group be given supergroup status and existing formations placed into two new groups: an older Early to Middle Ordovician Tippogoree Group and a younger Silurian to Devonian Panama Group.

Evolution of transcript structure and base composition of rDNA expansion segment D3 in ticks.

Four to thirty-two copies of the rDNA 28S gene expansion segment D3 and flanking H14 stem were sequenced in six species of ticks (Ixodes: Ixodidae: Acari). Sequence match among species varied from 66% to 97%. Sequence length averaged 130 bases in I. persulcatus across eight Eurasian sites and averaged 186 bases in five other species across 19 Eurasian and North American sites. The difference in length represents one or more deletions totalling about 60 bases that correspond to stems S3 or S4 of the folded transcript. The typical transcript conformation was observed as one possible low energy structure in the five species of longer D3. The structure entails a basal loop with four stem/loop structures, S1-S4 (moving 5' to 3') atop stem H14. A secondary structure lacking S4 but possessing all other putative standard features of the D3 transcript is possible with the shorter I. persulcatus sequences. Interspecific sequence differences occur at higher frequency in loops and bulges vs. complementary pairing regions of stems. Insertion/deletion events (indels) and base substitutions accounted equally for sequence differences. Indels are flanked by similar sequences, suggesting that they occur by slippage during replication. The D3 of Ixodes species is composed of a degenerate set of subrepeats. Thus, unequal exchange among subrepeats may have caused the reduction in length of the I. persulcatus D3. Compensatory base substitution and compensatory insertion/deletion events are indicated by the failure of mutations to affect secondary structure. Transversions accounted for 64% of sequence differences and were biased toward the gain of G and U and the loss of A and C. This bias could re-establish intramolecular base pairing when disrupted by insertions or deletions that shift one side of a stem relative to the other. The distribution of sequence differences, biased substitution, and conservation of transcript conformation in D3 suggest selective constraint.

Structural evolution within the Luning–Fencemaker fold-thrust belt, Nevada: progression from back-arc basin closure to intra-arc shortening

The Luning–Fencemaker fold-thrust belt (LFTB) of central Nevada reflects major Mesozoic shortening in the western US Cordillera, and involved contractional deformation in Triassic and lower Jurassic back-arc basinal strata. Structural analyses in the Santa Rosa Range, in the northern LFTB, provide new insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D1 involved tight to isoclinal folding, cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW–SE shortening of ≧55–70% in the Early and/or Middle Jurassic. D2 structures are similar in orientation to D1 but involved much less overall strain and are well developed only to the southeast. D2 appears to be related to thrusting along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D3 deformation reflects very minor shortening (<5%) in a subvertical direction, and is tentatively interpreted to reflect stresses generated during initial intrusion of mid-Cretaceous plutons in the area. D4 deformation demonstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102Ma (U–Pb zircon) based on syntectonic relations between D4 structures and thermal metamorphism associated with intrusion, and an upgrade in D4 strain in the thermally softened metamorphic aureoles of the intrusions. This last phase of deformation reflects minor regional NE–SW shortening, coupled with localized strain associated with pluton emplacement.Formation of the LFTB structural province was accomplished during the D1 and D2 phases of deformation, and most shortening occurred during the D1 event. This Jurassic deformation led to structural closure of the back-arc basin by top-to-the-SE tectonic transport and development of a largely ductile fold-thrust belt. Subsequent deformation (D3 and D4) is >50m.y. younger and unrelated to development of the LFTB. The younger deformation reflects a combination of minor regional shortening, interpreted to be related to the Cretaceous Sevier orogeny, plus localized shortening related to emplacement of Cretaceous intrusions.

Tectonic history of the Ellsworth Mountains, West Antarctica: Reconciling a Gondwana enigma

Early Paleozoic orogenesis has been recognized along the southern African (Saldanian orogeny) and East Antarctic (Ross orogeny) sectors of the Gondwana paleo- Pacific margin. However, the absence of a contemporaneous orogenic event in the Ellsworth Mountains of West Antarctica, once a contiguous part of this margin, has resulted in their geology being considered enigmatic. In this contribution, widespread, detailed structural studies from all stratigraphic levels of the Ellsworth Mountains allow their tectonic evolution to be reassessed. Geochemical, stratigraphic, and structural data indicate that the Middle to Upper Cambrian Heritage Group developed in a continental rift basin. Multiple observations of the crucial end-Cambrian contact between this rift sequence and the overlying Crashsite Group reveal the contact to be regionally conformable; there are localized outcrop-scale unconformities. Furthermore, structural continuity can be demonstrated across this key boundary, thus precluding the possibility of an end-Cambrian orogenic event. The entire stratigraphic succession was affected by two post-Permian phases of deformation. D1 structures are locally developed and superimposed by the main dextral transpressive D2 Permian−Triassic Gondwanian deformation event. D2 is succeeded by an episode of extension orthogonal to orogenic strike. Recent geochronological data on the early Paleozoic evolution of the Cape fold belt reveal a strong tectonostratigraphic correlation with the Ellsworth Mountains, indicating that this sector of the Gondwana paleo-Pacific margin was affected by Middle to Late Cambrian rifting. A new tectonic model is proposed for the southwestern paleo-Pacific margin of Gondwana that accounts for the presence of rifting along a margin otherwise dominated by active subduction.

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.

Tertiary evolution of the central Menderes Massif based on structural investigations of metamorphics and sedimentary cover rocks between Salihli and Kiraz (western Turkey)

Augen gneisses, mica schists, and marbles of the Menderes Massif and its sedimentary cover rocks are exposed south of the Gediz graben. The augen gneisses form the structurally lowest part of the studied lithological sequence, and are overlain by a schist complex. The structurally highest part is formed by a series of marbles. The ages of this lithological sequence range from Precambrian to Early Paleocene. Furthermore, this sequence records the tectonic evolution since the Precambrian. The sedimentary cover of the Menderes Massif consists of two groups of sediments from Early Miocene to Quaternary. The lower group, the Alasehir group, consists of Early- to mid-Miocene-aged fluvial and limnic sediments which form the lower and the upper parts, respectively. The Alasehir group is overlain by mainly fluvial sediments of the Gediz group. Both the Alasehir and the Gediz groups are separated by an angular unconformity. Six deformational phases could be distinguished within the metamorphic rocks of the Menderes Massif and its Tertiary cover. The structures which were interpreted to belong to deformational events predating the Paleocene are summarized as deformational phase D1. D1 structures were nearly completely overprinted by the subsequent deformation events. The second deformational phase D2 occurred between Early Eocene and Early Oligocene. D2 occurred contemporaneously with a Barrovian-type regional metamorphism. The third deformational phase D3 is characterized by folding of the axial planes which formed at the end of Early Oligocene. The deformational event D4 occurred during the Late Oligocene and is related to an extensional period. The deposition of the sedimentary rocks which belong to the Tertiary cover of the Menderes Massif that started in the Early Miocene was interrupted by a compressional phase (D5) during the Late Miocene. Sediments which were deposited since the Early Pliocene record structures which were related to a young extensional phase (D6). This extensional phase has continued to the Present.

Dimeric Structure of the Coxsackievirus and Adenovirus Receptor D1 Domain at 1.7 Å Resolution

Background: The coxsackievirus and adenovirus receptor (CAR) comprises two extracellular immunoglobulin domains, a transmembrane helix and a C-terminal intracellular domain. The amino-terminal immunoglobulin domain (D1) of CAR is necessary and sufficient for adenovirus binding, whereas the site of coxackievirus attachment has not yet been localized. The normal cellular role of CAR is currently unknown, although CAR was recently proposed to function as a homophilic cell adhesion molecule. Results: The human CAR D1 domain was bacterially expressed and crystallized. The structure was solved by molecular replacement using the structure of CAR D1 bound to the adenovirus type 12 fiber head and refined to 1.7 Å resolution, including individual anisotropic temperature factors. The two CAR D1 structures are virtually identical, apart from the BC, C″D, and FG loops that are involved both in fiber head binding and homodimerization in the crystal. Analytical equilibrium ultracentrifugation shows that a dimer also exists in solution, with a dissociation constant of 16 μM. Conclusions: The CAR D1 domain forms homodimers in the crystal using the same GFCC′C″ surface that interacts with the adenovirus fiber head. The homodimer is very similar to the CD2 D1-CD58 D1 heterodimer. CAR D1 also forms dimers in solution with a dissociation constant typical of other cell adhesion complexes. These results are consistent with reports that CAR may function physiologically as a homophilic cell adhesion molecule in the developing mouse brain. Adenovirus may thus have recruited an existing and conserved interaction surface of CAR to use for its own cell attachment.

Theoretical study of fullerene C50 and its derivatives

The electronic and structural properties of three fullerene isomers of C50 and their derivatives are studied at the HF/3-21G and B3LYP/3-21G levels, respectively. Of the three structures, two have D5h symmetry [D5h(I)andD5h(II)], and the other has D3 symmetry. The results of the fully optimized calculations show the D3 structure and the D5h(I) structure are near isoenergetic isomers. Because the D3 isomer has a bigger HOMO–LUMO energy gap than the D5h(I) isomer, the D3 structure should be the preferable isomer. The D5h(I) structure is an unstable isomer. The energies and properties of negative and positive ions of the three C50 isomers are also investigated. Generally, the stability of negative ions is better than that of positive ions. The hydrogenated C50 molecules for the three isomers have also been studied, and the most active carbon atom sites for adding hydrogen are predicted for each isomer.

Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the Kmax axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C.

Thrusting in the lower crust: evidence from the Oygarden Islands, Kemp Land, East Antarctica

Layered orthogneisses of the Oygarden Islands preserve evidence for four high-grade deformation events (D1 to D4). Archaean D1 and D2 structures are only patchily preserved due to extensive recrystallization during D3 and D4, which represent effects of the c. 1000 Ma Rayner Structural Episode. Ductile thrusting at middle to lower crustal levels occurred during D3, which is separated into two mutually cross-cutting phases based on structural geometry; the two phases represent changes in finite strain that developed during progressive deformation. East-directed transport during D3a developed subhorizontal thrusts that contain co-axial, east-trending F3a folds and L3a lineations. Buckling as a consequence of constriction in thrust duplexes developed upright F3b folds coaxial to F3a folds, and steeply south-dipping D3b shear zones. Garnet–clinopyroxene- and garnet–orthopyroxene-bearing assemblages in mafic lithologies, and garnet–sillimanite-bearing assemblages in pelitic lithologies reflect D3 conditions of P=9 kbar and T=800–850 °C. The well-exposed D3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre-scale thrust stacking and ductile flow.

Late Archean structural and metamorphic history of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton

The Archean rocks of the Wind River Range in western Wyoming record a Late Archean history of plutonism that extends for more than 250 m.y. The range is dominated by granitic plutons, including the 2.8 Ga Native Lake gneiss, the 2.67 Ga Bridger batholith, the 2.63 Ga Louis Lake batholith, and late 2.54 Ga granites. These plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the Late Archean. Five deformation events are recorded. D1 is a penetrative deformation that occurred during the earliest granulite-facies metamorphism; D2 is a folding event, probably in amphibolite facies, that deforms porphyritic dikes that cut the D1 fabrics. Both D1 and D2 predate the intrusion of the ca. 2.8 Ga Native Lake gneiss. D3 is a folding event, accompanied by upper amphibolite to granulite metamorphism, that deformed the Medina Mountain sequence, a sequence of rocks that was either deposited or thrust upon the Native Lake gneiss. D4 is a fabric-forming event associated with the Mount Helen structural belt (MHSB). It is represented by mylonites in the MHSB, a penetrative fabric in the Bridger batholith, and folding of the D3 structures in the Medina Mountain sequence. We consider D3 and D4 to be coeval with the emplacement of the Bridger batholith, and hence to date at ca. 2.67 Ga. The latest structures (D5) are fabrics associated with the folding and thrusting of the 2.65 Ga South Pass sequence. We recognize at least four metamorphic events. M1 is associated with the D1 fabrics and occurred at high T (>750°C) and high P (∼7–8 kilobars). M2 (650–750 °C and 4–5.5 kilobars) is associated with the intrusion of the Bridger batholith and formation of the D3 and D4 structures. The D5 structures of the South Pass sequence record M3, which is low P (∼2–3 kilobars) and low T (∼500°C). The final metamorphism, M4, is a contact metamorphism around the Louis Lake batholith. In the south against the South Pass sequence, the metamorphism occurred at ∼3 kilobars and at temperatures <700 °C. In contrast, in the north where the Louis Lake batholith is charnockitic, the metamorphism occurred at 6 kilobars and 800°C. This pressure gradient is probably a reflection of tilting of the Wind River block during the Laramide orogeny. The composition of the plutons and the structural and metamorphic history of the Wind River Range indicate that during the Late Archean this area occupied the active margin of the Wyoming province. This tectonic environment is similar to the long-lived Phanerozoic margins of North America. The Wind River Range represents the best-documented active margin of Archean age.

Late Archean structural and metamorphic history of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton

Late Archean structural and metamorphic history of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton