D2 Phase Research Articles

A Formação Salinas, Orógeno Araçuaí (MG): história deformacional e significado tectônico

The Salinas Formation, made up of metasandstones, metapelites and metaconglomerates, occurs mainly in the surroundings of the town of Salinas, northern Minas Gerais. In this region, rocks of the Salinas Formation lie unconformably on top of the Macaubas Group on the west, and are intruded by Neoproterozoic and Cambrian granites on the east. Four generations of deformational structures were documented in this region, three of them affecting the Salinas rocks. The structures of the oldest phase, D D, representing an episode of syn-sedimentary deformation, include folds, faults and convolute bedding. The subsequent phases, D1, D2 and D G are related to the development of the Aracuai orogen. The D1 phase, represented by folds, shear zones, and a whole series of small-scale elements, took place during the Brasiliano event (585 to 560 Ma) and was associated to regional metamorphism in the green schist to amphibolite facies conditions. The D2 phase, corresponding to the extensional collapse of the orogen (520 to 500 Ma), is recorded by fold trains that cascade down to ESE. These structures are associated to a crenulation cleavage and normal-sense shear zones, which affect only the metasediments of the Macaubas Group. The D G phase, related to the emplacement of granite bodies, caused folding and contact metamorphism in the Salinas rocks. The nature, age, deformation history and tectonic setting of Salinas Formation in the Aracuai orogen point out to a deposition in a syn-collisional basin - a flysch basin -, developed between a passive margin and an orogenic front.

A Formação Salinas, Orógeno Araçuaí (MG): história deformacional e significado tectônico

The Salinas Formation, made up of metasandstones, metapelites and metaconglomerates, occurs mainly in the surroundings of the town of Salinas, northern Minas Gerais. In this region, rocks of the Salinas Formation lie unconformably on top of the Macaúbas Group on the west, and are intruded by Neoproterozoic and Cambrian granites on the east. Four generations of deformational structures were documented in this region, three of them affecting the Salinas rocks. The structures of the oldest phase, D D, representing an episode of syn-sedimentary deformation, include folds, faults and convolute bedding. The subsequent phases, D1, D2 and D G are related to the development of the Araçuaí orogen. The D1 phase, represented by folds, shear zones, and a whole series of small-scale elements, took place during the Brasiliano event (585 to 560 Ma) and was associated to regional metamorphism in the green schist to amphibolite facies conditions. The D2 phase, corresponding to the extensional collapse of the orogen (520 to 500 Ma), is recorded by fold trains that cascade down to ESE. These structures are associated to a crenulation cleavage and normal-sense shear zones, which affect only the metasediments of the Macaúbas Group. The D G phase, related to the emplacement of granite bodies, caused folding and contact metamorphism in the Salinas rocks. The nature, age, deformation history and tectonic setting of Salinas Formation in the Araçuaí orogen point out to a deposition in a syn-collisional basin - a flysch basin -, developed between a passive margin and an orogenic front.

Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development

Abstract The lower part of the St. George Group of western Newfoundland consists of Tremadocian shallow marine platform carbonates of the Boat Harbour (about 180 m thick) and the underlying Watts Bight (about 60 m thick) formations. In the Boat Harbour Formation, dolomitization is pervasive at the top of most shallowing-upward, metre-scale, peritidal hemicycles. Petrographic examination of the Boat Harbour Formation carbonates suggests that the succession has been affected by at least three phases of dolomitization, which influenced the final rock porosity. These phases have crystal-size ranges of about 4 to 40 μm (earliest dolomite D1), 50 to 150 μm (D2), and 300 μm to 20mm (saddle dolomite D3), respectively. They occur as both replacements and cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The occurrence of near-micritic size dolomites (about 4–40 μm) may suggest that dolomitization started at low temperatures during early stages of diagenesis. The lack of evaporite interbeds in the formation and the depleted δ18O values (−6.2±0.8‰ VPDB) as well as the low Sr contents (168±45ppm) of the earliest dolomites likely exclude a brine origin from evaporated seawater. The Sr/Ca molar ratios (0.0069 to 0.0017), calculated for the earliest dolomitizing fluid, suggest a mixture of marine and meteoric waters possibly in a mixing zone environment. The petrographic features and geochemical attributes of D2 and D3 phases, such as their depleted δ18O values (−6.9±1.5‰ and −8.3±0.9‰ VPDB, respectively) and Sr contents (177±76 and 117±33ppm, respectively), suggest that they were formed under relatively deeper burial conditions and possibly from hydrothermal fluids which is supported by homogenization temperatures (up to 135° C) and estimates of salinities (up to 23 wt. % NaCl) in the latest dolomites (D3). Based on visual estimates from thin sections, the porosity varies from <1% in most of the formation to about 10% in a dolomitized algal lime mudstone bed in the upper part of the formation, a few metres below the Boat Harbour Disconformity. Except for some vugs, the majority of pores are intercrystalline and associated with D2. The dolomitization events recorded by the Boat Harbour Formation carbonates resemble, in their succession and geochemical signatures, those previously documented in the Aguathuna Formation. However, significant differences are noted with the dolomitization events recorded in the Lower Ordovician Romaine Formation on Anticosti Island; these differences are tentatively associated with the very distinct tectonic settings of each case.

Deformation history of the eclogite‐ and jadeitite‐bearing mélange from North Motagua Fault Zone, Guatemala: insights in the processes of a fossil subduction channel

AbstractIn Guatemala, along the northern side of the Motagua Valley, a mélange consisting of blocks of eclogite and jadeitite set in a metaserpentinitic and metasedimentary matrix crops out. The metasedimentary rocks display a complex deformation history that includes four tectonic phases, from D1 to D4. The D1 phase occurs only as a relic and is characterized by a mineral assemblage developed under pressure temperature (P–T) conditions of 1.00–1.25 GPa and 206–263°C. The D2 phase, characterized by isoclinal folds, schistosity and mineral/stretching lineation, developed at P–T conditions of 0.70–1.20 GPa and 279–409°C. The following D3 and D4 phases show deformations developed at shallower structural levels. Whereas the D1 phase can be interpreted as the result of underplating of slices of oceanic lithosphere during an intraoceanic subduction, the following phases have been acquired by the mélange during its progressive exhumation through different mechanisms. The deformations related to the D2 and D3 phases can be regarded as acquired by extrusion of the mélange within a subduction channel during a stage of oblique subduction. In addition, the structural evidences indicate that the coupling and mixing of different blocks occurred during the D2 phase, as a result of flow reverse and upward trajectory in the subduction channel. By contrast, the D4 phase can be interpreted as related to extension at shallow structural levels. In this framework, the exhumation‐related structures in the mélange indicate that this process, probably long‐lived, developed through different mechanisms, active in the subduction channel through time. Copyright © 2009 John Wiley &amp; Sons, Ltd.

TECTONO-METAMORPHIC HISTORY OF THE DUARTE TERRANE (JARABACOA AREA, HISPANIOLA ISLAND): INSIGHTS ON THE TECTONIC EVOLUTION OF THE NORTHERN RIM OF THE CARIBBEAN OCEANIC PLATEAU

The northern margin of the Caribbean Plate is characterized by a transpressional belt consisting of an assemblage of several terranes derived from Late Cretaceous to Tertiary oblique convergence between the Northern American and Caribbean Plates. Fragments of the oceanic plateau belonging to the Caribbean Plate are incorporated in these belts as strongly deformed and metamorphosed sequences, as recognized in the Jarabacoa area of the Hispaniola Island. In this area the Duarte Terrane consists of two different units showing different metamorphic grades. The lower unit consists of greenschist facies metabasites, whereas the upper one is represented by amphibolite facies metabasites topped by metasedimentary rocks, whose protoliths were clastic deposits supplied by a magmatic arc. Both units suffered four deformation phases, from D1 to D4, that can be strictly correlated. In both units the D1 phase is characterized by a syn-metamorphic foliation associated to a mineral lineation and very rare rootless isoclinal folds. The D2 phase is characterized by a well developed foliation and isoclinal folds acquired under retrograde metamorphic conditions. The D1 and D2 phases developed in the Cenomanian-Turonian time span. The following D3 and D4 phases produced weak deformations without metamorphic imprint. The D3 and D4 phase are probably connected with transpression tectonics of Early Oligocene to Early Miocene age. All these deformations developed in an arc setting connected with southward oblique subduction of the oceanic lithosphere of the North America Plate beneath the Caribbean oceanic plateau.

Indosinian high‐strain deformation for the Yunkaidashan tectonic belt, south China: Kinematics and40Ar/39Ar geochronological constraints

Structural and40Ar/39Ar data from the Yunkaidashan Belt document kinematic and tectonothermal characteristics of early Mesozoic Indosinian orogenesis in the southern part of the South China Block. The Yunkaidashan Belt is tectonically divided from east to west into the Wuchuang‐Sihui shear zone, Xinyi‐Gaozhou block, and the Fengshan‐Qinxi shear zone. Indosinian structural elements ascribed to the Indosinian orogeny include D2and D3deformation. The early D2phase is characterized by folding and thrusting with associated foliation and lineation development, related to NW‐SE transpression under amphibolite‐ to greenschist‐facies conditions. This event is heterogeneously overprinted by D3deformation characterized by a gentle‐dipping S3foliation, subhorizontally to shallowly plunging L3lineation, some reactived‐D2folds and low‐angle normal faults. The D3fabrics suggest a sinistral transtensional regime under greenschist‐facies metamorphism. The timing of the D2and D3events have been constrained to the early to middle Triassic (∼248–220 Ma) and late Triassic (∼220–200 Ma) respectively on the basis of40Ar/39Ar geochronology and regional geological relations. The change from oblique thrusting (D2) to sinistral transtension (D3) may reflect oblique convergence and crustal thickening followed by relaxation of the overthickened crust. In combination with the regional relations from Xuefengshan to Yunkaidashan and on to Wuyishan, the early phase of the Indosinian orogeny constituted a large‐scale positive flower structure and is related to the intracontinental convergence during the assembly of Pangea in which the less competent South China Orogen was squeezed between the more competent North China and Indosinian Blocks.

Marble dykes emanating from marble layers in an amphibolite-facies, multiply-deformed carbonate succession, Troms, northern Norway

In one of the higher nappes of the Caledonides of Troms, northern Norway, rare thin dykes of calcite marble lie subparallel to the axial surfaces of folds in multilayered marble beds. The dykes emanate from pure calcite marble layers mostly in or close to the hinge zones of these flat-lying, tight to sub-isoclinal, F3 folds. In many cases, they appear to penetrate along the axial surfaces of these same folds, and there is an up to 10° angular disparity between dyke orientation and axial surfaces. The dykes are considered to have originated, and been expelled, from parent calcite marble layers either at or shortly after the peak of amphibolite-facies metamorphism between the D2 and D3 deformation phases of the Scandian orogeny. The dykes display a dyke-parallel, dark grey to white, colour banding, the origin of which is uncertain at present. In thin-section, an equigranular texture is dominated by 2–3 mm, equant, unstrained, calcite grains with straight boundaries and low-T twin lamellae, denoting a late-stage recrystallization. A weak, oblique, calcite grain-shape orientation fabric curves into a dyke-parallel alignment along the finer-grained margins, reflecting the simple-shear deformation imposed on the host rock. Regarding the initiation of these marble dykes, it seems likely that a combination of P–T conditions and possibly partial melting processes was conducive to generating extreme ductility/superplasticity, at least locally, and that this led to the sudden expulsion of a partially molten carbonate aggregate which had momentarily attained a critical pressure and intruded preferentially along available, transient fractures prior to the imposition of the D3 simple-shear regime.

Tectono-metamorphic history of the ophiolitic Lento unit (northern Corsica): evidences for the complexity of accretion-exhumation processes in a fossil subduction system

The Alpine Corsica (Corsica Island, France) is characterized by a stack of continent- and ocean-derived tectonic units, known as Schistes Lustrés complex. This complex is affected by deformation and metamorphic imprint achieved during Late Cretaceous – Early Tertiary subduction- related processes connected with the closure of the Ligure-Piemontese oceanic basin and subsequent continental collision. In the Schistes Lustrés complex, the Lento oceanic unit is characterized by four deformation phases, from D1 to D4 phase. The D1 phase, characterized by blueschist metamorphism, is regarded as related to coherent underplating in a subduction zone at a depth of about 25-30 km. The subsequent deformation phases can be referred to exhumation history, as suggested by the continuous decrease of metamorphic conditions. The transition from accretion to exhumation is represented by the D2 phase, achieved during the development of a duplex structure of accreted units. The D3 phase is in turn achieved by a further horizontal shortening, whereas the D4 phase is developed during an extensional event representing the final exhumation of the Lento unit. On the whole, the data collected for the Lento unit suggest an history that include an accretion by coherent underplating followed by exhumation, more complex than previous described.

The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure

Most of the tectonic units cropping out in Western Tuscany are fragments of the Jurassic oceanic crust, ophiolitic successions, overlaid diachronously by Upper Cretaceous-middle Eocene carbonate and siliciclastic flysch successions with their Cenomanian-lower Eocene shalycalcareous basal complexes. These units, so called Ligurian, have been emplaced during the closure of the Ligurian-Piedmont Ocean. Ophiolite bearing debris flows are common in the flysch basins and their relationship with ophiolitic tectonic slices points to a strong relation between tectonics and sedimentation from the early compressive events of the Late Cretaceous. The tectonic activity reflects in a rough morphology of the ocean floor. It progressively influences the distribution and sedimentology of the turbidites. During middle Eocene this relationship begun very important and a paleogeographic reconstruction with prominent linear ophiolitic reliefs that bounded some turbiditic basins can be done. In our reconstruction the sedimentary and structural evolution can be framed in the context of strain partitioning, developed during the ocean closure, between subduction processes and ancient weakness zones crosscutting both the ocean and the Adria continental margin and reactivated in compressive regime. These weakness zones can be interpreted as transform faults of the Ligurian-Piedmont Ocean with prolongations in the Adria passive margin. The weakness zones crosscut the oceanic lithosphere and the Adria continental margin and interfered with the subduction processes. The activity of the weakness zones is reflected in the Ligurian Units architecture where two main structural strike trends of thrusts and folds axial planes occur. The first trend is WSW-ENE oriented and it is connected with the reactivation of the weaknesses zones. This first orientation developed progressively from Late Cretaceous to Pliocene, from oceanic to ensialic convergence (D1, D2, and D4 deformation phases). The second trend is NNE-SSW oriented and is related to the late Eocene continental collision and the subsequent translation to the NE of the oceanic units onto the Adria continental margin (D3 deformation phase).

Tectonic evolution of the Yaoundé segment of the Neoproterozoic Central African Orogenic Belt in southern Cameroon

The deformation history of the Neoproterozoic Central African Orogenic Belt in southern Cameroon is well recorded in the low- to high-grade rocks outcropping in the area around Yaoundé. The fabrics in these rocks are consistent with two main ductile deformation events D1 and D2. D1 predated emplacement of calc-alkaline dioritic bodies and caused the formation of nappes that resulted in high-pressure granulite metamorphism of soft sediments. A strong overprinting of these nappes during D2 symmetric extension, probably associated with large-scale foliation boudinage and (or) gneissic doming and intense magmatic underplating, gave rise to regional flat-lying fabrics. The latter were further buckled by D3 and D4 folding phases defining a vertical constriction occurring with a major east–west to NW–SE shortening direction. The corresponding F3 and F4 folds trend north–south to NE–SW and east–west to NW–SE, respectively, and represent the main regional strain patterns. Based on the east–west to NW–SE maximum shortening orientation indicated by F3 folds, it is proposed that the nappe-stacking phase D1 occurred in the same direction. The deformation history in the area can thus be described as corresponding principally to alternating east–west to NW–SE contractions and north–south to NE–SW orogenic-parallel extensions. At the regional scale, this could be due to the Transaharan east–west collisional system.

Tectono‐metamorphic history of the Tacagua ophiolitic unit (Cordillera de la Costa, northern Venezuela): Insights in the evolution of the southern margin of the Caribbean Plate

Abstract The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid‐oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep‐sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4. Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel (D2 and D4 phases) and normal (D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike‐slip and pure compressional or pure extensional tectonics occurred through time.

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D1, D3, D4, and D5, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D1 and syn- to post-D3. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D1 veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D1) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D3 and D4 phases. Little is known about the nature of the D1 ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D1 indicates a high temperature for the fluid. The D3 hydrothermal phase coincided with peak metamorphism. D4 fluids were hypersaline, high temperature, CO2-poor, and H2S-poor.

Structural setting and petrogenesis of the Ben Vuirich Granite Pluton of the Grampian Highlands: a pre-orogenic, rift-related intrusion

Synopsis The Ben Vuirich intrusion is a small, elongate body of monzogranite that occurs in the Tummel Steep Belt, Perthshire. It was emplaced into Dalradian rocks (Appin Group) at 590 Ma, prior to the D1 phase of the Grampian Event (Caledonian Orogeny), and was strongly deformed during D2. Locally preserved cordierite- and andalusite-bearing hornfelses were altered to garnet ± kyanite-bearing assemblages during post-D2 regional metamorphism. A new study of the lower-grade hornfelses shows that the protolith was an undeformed, fine-grained, parallel-laminated sedimentary unit, confirming that the pluton is pre-orogenic with respect to the Grampian Event. Whole-rock and trace element analyses of 33 samples of the intrusion, together with rare earth elements, Rb–Sr and O-isotope data, show that it is an A 2 -group monzogranite. This finding supports the hypothesis that the granite, emplaced at a depth of 7–14 km, formed in the same extensional tectonic setting as the Tayvallich lavas at 600 Ma. Geochemical and isotope parameters point to a largely crustal source. The intrusion belongs to a swarm of rift-related, A-type granitoids that originally stretched from the Appalachians to Scotland, and includes foliated granitoids in the Moine. The granitoids formed in response to the early break-up of Rodinia, c. 50 Ma before the development of the Iapetus Ocean.

Polyorogenic deformation history recognized at very shallow structural levels: the case of the Antola Unit (Northern Apennine, Italy)

The Antola Unit is a Ligurian Unit occurring at the top of the nappe pile of the Northern Apennines (Italy). Structural analysis indicates that the Antola Unit has been involved in a complex polyphase deformation history developed at a shallow structural level. The successive deformation phases belong to both the Alpine and Apenninic orogenic cycles. Therefore the Antola Unit is a good example of a polyorogenic tectonic unit deformed at a very shallow structural level. The older part of the structural evolution is assigned to the Meso-Alpine tectonic stage and consists of two deformation phases (D1 and D2) characterized by opposite tectonic transport directions. The D1 and D2 structures were sealed by the middle Eocene-Miocene deposits of the Tertiary Piedmont Basin. The younger part of the deformation history instead can be referred to the Northern Apennines tectonics, and consists of two phases (D3 and D4) both involving the Tertiary Piedmont Basin succession. The structural data indicate that the D1 deformations developed in mostly unlithified rocks, and that these structures can be related to the westward emplacement of the Antola Unit in the Alpine orogenic wedge. The D2 phase is characterized by folds and low-angle extensional faults, and is referable to a gravity-driven deformation. Both the D3 and D4 phases, characterized by folds and thrusts, are due to the compressive regime related to the development of the Northern Apennines.

Constrictive deformation in transpressional regime: field evidence from the Cauvery Shear Zone, Southern Granulite Terrain, India

The paper describes a typical field example of rarely preserved complex structural patterns related to constrictional strain from the deeply eroded central part of the Neoproterozoic crustal-scale Cauvery shear zone system, southern India. The area exposes intense migmatisation and a distinctive map pattern of a broad domal structure with complex strain patterns. The field features such as interfering fold structures, extreme hinge line variation, domes and basins, highly asymmetrical and amoeboid shape structures, geometry of foliation trajectories and radial stretching lineations could be related to a single phase of D2–M2 constrictive deformation. Our observations suggest a close spatial and temporal relationship between transpressive deformation, deep crustal melting, assent of granite magmas and migmatisation synchronous with D2–M2.

Syn‐deformational migmatites and magmatic‐arc metamorphism in the Xolapa Complex, southern Mexico

AbstractThe Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.

Controls of gold-quartz vein formation during regional folding in amphibolite-facies, marble-dominated metasediments of the Navachab Gold Mine in the Pan-African Damara Belt, Namibia

The Navachab gold mine in central Namibia is situated in amphibolite-facies, marble-dominated metasediments of the Neoproterozoic Damara Sequence in the Pan-African Damara Belt. Gold mineralization is hosted by two main, and at least three minor, sets of auriferous quartz-veins that are developed on the subvertical northwestern limb of the regional-scale, shallow doubly-plunging anticline of the Karibib dome. The orientation, relative timing and progressive deformation of quartz veins indicate that veining occurred during folding and fold amplification of the Karibib dome. This deformation forms part of the main phase of northwest-directed collisional tectonics in the Damara belt at ~540 to 550 Ma. The two main vein sets include (1) bedding-parallel, shallowly-plunging ore lenses and shoots situated at the base of a prominent marble unit, and (2) a laterally extensive swarm of shallowly-dipping quartz veins that truncate the host rocks at high angles. The bulk of the bedding-parallel lenses represent dilational jogs that opened during flexural flow along bedding-parallel slip planes during the amplification of the Karibib dome. Flexural flow and associated bedding-parallel fluid infiltration were concentrated close to the contact between marbles and the underlying siliciclastic formations. This lithological contact, in particular, represented a pronounced rheological contrast resulting in increased slip rates during fold amplification. The highly discordant sets of shallowly-dipping, sheeted quartz veins were emplaced during the fold lock-up stage of the Karibib dome. Quartz veining occurred when the northwestern limb of the dome was rotated to subvertical attitudes, so that bedding-parallel flexural slip ceased to be active. Extensional fracturing and veining was facilitated by the presence of transiently supralithostatic fluid pressures. The high-amplitude fluid pressure cycling associated with extensional fracturing is likely to have triggered gold precipitation in the shallowly-dipping veins. Although bedding-parallel veins and discordant veins are broadly contemporaneous, related to the regional-scale D2 phase of fold-and-thrust tectonics, cross-cutting relationships between the two main vein systems indicate that the two vein sets have represented separate entities and succeeded each other rather than forming an interconnected fracture mesh.

Late-Svecofennian transpressive deformation in SW Finland—evidence from late-stage D3 structures

Late-Svecofennian deformation was investigated in the Bjensböle area of SW Finland. The deformation history was revealed by following marker horizons of amphibolite, marble and black schist, in addition to microscopic studies and U-Pb monazite geochronology. The D1 structures are transposed and overprinted by D2 structures. The mesoscopic F2 folds are tight to isoclinal with fold axes plunging 20-40[ddot] NE-ENE. Regional large-scale open F3 folds formed during the main D3 phase, and the macroscopic structure in Bjensböle is an F2-F3 interference pattern formed by N-S compression. The area consist of low P/high T amphibolite facies sillimanite gneiss and low P/low T amphibolite facies quartz schist, separated by a low P/low T amphibolite horizon in the central part of the area. The sillimanite gneiss yielded a U-Pb monazite age of 1824±5 Ma. This age gives a timing for the peak metamorphism and the D3 folding on the Kemiö Island. After the peak metamorphism some deformational structures developed. A NE-SW directed crenulation cleavage in the quartz schist and the amphibolite, small open folds in the marble, a foliation in the sillimanite gneiss and a muscovite orientation in the quartz schist all have the same direction. These structures are interpreted as late-stage D3 (D3L). The angle between the NE-SW directed D3L structures and the locally predominant S0 + S1 + S2 schistosity is ca. 25[ddot]. The structural evidence related to the D3L point to a dextral sense of shearing along roughly E-W directed zones with a NW-SE compression. This is related to a regional ductile shearing signifying the vaning stages of the late-Svecofennian transpressive deformation in SW Finland.

Post collisional transpressive tectonics in northern Sardinia (Italy)

The aim of this work is to review and synthetize the geological and structural analysis performed in the Variscan Basement of Northern Sardinian during the last ten years and to add new preliminary data on the Anglona-SW Gallura area. A transpressive crustal- scale deformation (D2), is documented in the Variscan Basement of northern Sardinia. A shear deformation parallel to the belt, overprinting previous D1 structures related to a top- to-the S and SW nappe stacking, has been recognized. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2 deformation phases. D1 phase developed during initial frontal collision whereas D2 defor- mation was characterized by dextral shearing. In this sector of the Variscan belt exhumation is due to continuing compression with an increasing component of horizontal displacement developed in a regime of decreasing pressure. The D2 transpressional deformation enhanced telescoping of the Barrovian isogrades and the exhumation of the low- to medium-grade metamorphic rocks. 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 re- garded as a general mechanism affecting the exhumation of rocks and preventing the overthickened and thermally softened collisional crust from undergoing a diffused gravita- tional collapse. The rotation of nearly 90° of the tectonic transport in Sardinia during collisional and post-collisional stages could be related to paleoposition of the Corsica-Sar- dinia block, close to southeast France and northeast Spain, and to the development of the Ibero-Armorican arc.

LITHOSTRATIGRAPHY AND DEFORMATION HISTORY OF THE LAGO NERO UNIT (CHENAILLET MASSIF, WESTERN ALPS)

In the Western Alps, the Schistes Lustres includes ophiolitic units that, even if affected by HP metamorphism, are relatively poorly deformed. Among them, the Lago Nero unit displays a thick and well-developed sedimentary cover, that allows a comparison with the analogue successions of the Northern Apennines. This sedimentary cover includes hemipelagic deposits, represented by Radiolarite Fm., Metalimestone and Replatte Fm., showing a transition to turbiditic (Gondran Flysch) and mass-gravity deposits (Rocher Renard complex). This succession, characterized by a coarsening upward trend, records the approaching of an oceanic floor section to the accretionary wedge. In this picture, the Rocher Renard complex can be regarded as a formation related to a frontal erosion of the accretionary wedge, whereas the Gondran Flysch is interpreted as a turbidite deposits supplied by the continental margin of the Europe plate. The following deformation history includes four deformation phases. The first phase D1 deformations, characterized by HP metamorphism (lawsonite±Na-amphibole±phengite), were achieved during the coherent underplating in the alpine subduction zone. The D2, D3 and D4 phases was developed under retrograde metamorphic conditions, from grren-schist to very-low grade, during the progressive exhumation of the Lago Nero unit. All the collected data suggest that the succession of the Lago Nero unit can be correlated with those of the Internal Liguride units where the same sequence of hemipelagic to turbiditic and mass-gravity deposits has been recognized.