Polyphase Structural Evolution Research Articles

Variscan basement tectonics and Alpine shear zones in the external Balkanides: Structural data from the Vezhen Massif, Central Stara Planina Mts., Bulgaria

The Alpine Balkanides in Bulgaria are developed over Variscan orogenic fragments, which are variously affected by late-stage thick-skinned thrust tectonics. Often, these fragments hold both Variscan and Alpine structural records. А key area to shed light on important aspects of the tectonic evolution of the external parts of the Alpine Balkanide orogen is the Vezhen Massif in the Central Stara Planina Mountains. Although it is built exclusively of Paleozoic crystalline rocks, its structure was long considered as an example of intense mid Eocene to early Oligocene (Late Alpine) shortening. Detailed fieldwork in the area shows a more complicated tectonic framework and provides evidence of a polyphase structural evolution, involving an important stage of Variscan metamorphism and deformation and two stages of north-vergent Alpine shortening. The Stargel-Boluvanya Tectonic Zone that affects the western Vezhen Massif is one of the most important Variscan structures in the Balkan fold-and-thrust belt. Structural data indicate that the metamorphic basement records initial top-to-north thrusting, followed by further shortening, which led to folding and localized strike-slip deformation. The Variscan syn-metamorphic fabric is cross-cut by several post-kinematic igneous bodies of late Carboniferous to Permian ages. The tectonic history continues with a development of a network of greenschist facies north-vergent mylonitic zones for which an Early Alpine (post-late Permian but pre-Late Cretaceous) age is assumed. Late Alpine north-vergent thrusting is evident only on the eastern and northern flanks of the Vezhen Massif. In a broad structural context, the documented record in the basement rocks of the Vezhen Massif is comparable with those of the external massifs of the Alps and Iberia.

Litho-structural mapping and structural evolution of the Bocaranga pluton, northwest Adamawa-Yadé domain, Central African Republic

The Bocaranga pluton, in the northern domain of the Central African Orogenic Belt (CAOB) in the Central African Republic (CAR), is located in the Adamawa-Yadé domain, near the Mbéré Shear Zone (MBSZ). This study provides new results of the satellite image processing, the detailed structural and petrographic studies of Bocaranga pluton which helps to understand the emplacement mechanism of this pluton. Image processing of the satellite data and field showed the occurrence and spatial distribution of (1) amphibole-pyroxenites, (2) garnet-biotite rich amphibolite, (3) migmatite, (4) hornblende-biotite and hornblende-garnet gneiss, (5) hornblende-biotite granite (HBG), (6) two micas granite (TMG), (7) fine-grained biotite granite (FBG), and (8) coarse-grained biotite granite (CBG) rock types. The mapping of regional lineaments was carried out by automatic and manual lineament extraction methods. NE-SW direction corresponds to the direction of S2 and S3 foliations, faults, mineral lineation, quartz vein, dyke (aplitic granitic or pegmatitic). NW-SE direction also corresponds to the direction of S1 foliation, F1 folds, faults, quartz veins, dykes (aplitic granitic or pegmatitic). In the absence of absolute geochronological data in study area, it has been possible to propose a polyphase structural evolution, deduced from the relative chronology of the Bocaranga granitoid. Three phases of deformation can be defined in this study. D1 phase is represented by early NW-SE trending foliation (S1) that has been progressively rotated parallel to S2 overprinting foliation occurring in gneiss, amphibolites and hornblende-biotite granite (HBG) intrusion as well. D2 phase of ductile sinistral sense from NE-SW to ENE-WSW is responsible for an emplacement of Bocaranga granitic pluton. The scarcity of structure orientation in the TMG and FBG (except the contact zones), would imply that these granites would have been crystallized in the absence of any stress. D3 is a phase of brittle deformation with three main directions corresponding to the main direction of the shear zones in the Northern domain of the CAR Pan-African fold belt.The mylonitic deformation observed in study area illustrates the intensity of the deformation. The dominance of ellipsoidal structures observed in Bocaranga area suggests that the shear mechanism is the main controlling structure of Bocaranga granite emplacement, which can be correlated with the emplacement of Tamkoro-Bossangoa granitoid in CAR and the Adamawa-Yadé domain in Cameroon, Chad and NE Brazil.These results imply a syntectonic emplacement of Bocaranga pluton during sinistral simple shear regime related to deformation along the Central Cameroon Shear Zone (CCSZ).

Tectonostratigraphic evolution of the Slyne Basin

Abstract. The Slyne Basin, located offshore NW Ireland, is a narrow and elongated basin composed of a series of interconnected grabens and half-grabens, separated by transfer zones coincident with deep crustal structures formed during the Silurian- to Devonian-aged Caledonian Orogeny. The basin is the product of a complex, polyphase structural evolution stretching from the Permian to the Miocene. Initially, relatively low-strain rifting occurred in the Late Permian and again in the latest Triassic to Middle Jurassic, followed by a third phase of high-strain rifting during the Late Jurassic. These extensional events were punctuated by periods of tectonic quiescence during the Early Triassic and Middle Jurassic. Late Jurassic strain was primarily accommodated by several kilometres of slip on the basin-bounding faults, which formed through the breaching of relay ramps between left-stepping fault segments developed during earlier Permian and Early–Middle Jurassic rift phases. Following the cessation of rifting at the end of the Jurassic, the area experienced kilometre-scale uplift and erosion during the Early Cretaceous and a second, less severe phase of denudation during the Palaeocene. These post-rift events formed distinct regional post-rift unconformities and resulted in a reduced post-rift sedimentary section. The structural evolution of the Slyne Basin was influenced by pre-existing Caledonian structures at a high angle to the basinal trend. The basin illustrates a rarely documented style of fault reactivation in which basin-bounding faults are oblique to the earlier structural trend, but the initial fault segments are parallel to this trend. The result is a reversal of the sense of stepping of the initial fault segments generally associated with basement control on basin-bounding faults.

Poly-phase structural evolution of the northeastern Alxa Block, China: Constraining the Paleozoic-Recent history of the southern central Asian Orogenic belt

The Alxa Block in western China is a significant tectonic unit in the middle part of the southern Central Asian Orogenic Belt that was affected by multiple Paleozoic and Meso-Cenozoic deformation events. In this study, the results from detailed mapping and structural analysis coupled with new U-Pb zircon ages indicate that the Langshan region of the northeastern Alxa Block has experienced ten deformation events since the Late Devonian. Four separate structural domains are identified based on distinctive litho-tectonic relationships, and these domains contain intrusive and structural crosscutting relationships that allow the complex deformational history to be determined. Each deformation phase can be related to regional tectonic events associated with the consolidation of Central Asia’s crust and subsequent intraplate reactivation. The first three events (D1-D3) are tied to convergence between the Alxa Block, the North China and the Yangtze Cratons prior to and during closure of the Paleo-Asian Ocean in the Mid-Late Permian. Subsequently, sinistral displacement (D4) occurred between the Alxa Block and the North China Craton during the Triassic. Since the late Mesozoic, reactivation of the northeastern Alxa Block occurred repeatedly as an intraplate response to subduction of the Paleo-Pacific Plate, closure of the Mongol-Okhotsk Ocean (D5), retreating subduction of the Paleo-Pacific Ocean (D6), collision between the Qiangtang and Lhasa blocks and convergence between the East Asian margin and Western Philippine Block (D7), and later collision between India and Eurasia (D8-10). The Alxa Block provides a superb case study of how a continental interior region structurally records the multi-phase transition from plate margin tectonism to intraplate reactivation. The events documented in this study invite correlation with other Paleozoic-Recent tectonic events recorded elsewhere in the southern CAOB and adjacent cratonic blocks in Central Asia.

Constraining the Passive to Active Margin Tectonics of the Internal Central Apennines: Insights from Biostratigraphy, Structural, and Seismic Analysis

The polyphase structural evolution of a sector of the internal Central Apennines, where the significance of pelagic deposits atop neritic carbonate platform and active margin sediments has been long debated, is here documented. The results of a new geological survey in the Volsci Range, supported by new stratigraphic constraints from the syn-orogenic deposits, are integrated with the analysis of 2D seismic reflection lines and available wells in the adjacent Latin Valley. Late Cretaceous syn-sedimentary faults are documented and interpreted as steps linking a carbonate platform to the adjacent pelagic basin, located to the west. During Tortonian time, the pelagic deposits were squeezed off and juxtaposed as mélange units on top of the carbonate platform. Subsurface data highlighted stacked thrust sheets that were first involved into an initial in-sequence propagation with top-to-the-ENE, synchronous to late Tortonian foredeep to wedge-top sedimentation. We distinguish up to four groups of thrust faults that occurred during in-sequence shortening (thrusts 1–3; about 55–60 km) and backthrusting (thrust 4). During Pliocene to recent times, the area has been uplifted and subsequently extended by normal faults cross-cutting the accretionary wedge. Beside regional interest, our findings bear implications on the kinematic evolution of an orogenic wedge affected by far-traveled units.

GEOLOGIA DA FOLHA TOPOGRÁFICA GUAPIARA NA ESCALA 1:50.000

A região da Folha Topográfica de Guapiara situa-se no extremo sul do estado de São Paulo e compreende na porção central da Província Mantiqueira a faixa centro-sul do Cinturão Ribeira. O arcabouço geológico local envolve rochas da sequência metavulcanossedimentar do Supergrupo Açunguí, de idade meso- a neoproterozoicas deformadas e metamorfizadas no neoproterozoico, associadas a rochas granitoides geradas durante os episódios colisionais ligados ao Ciclo Brasiliano e à formação do Supercontinente Gondwana. As principais unidades litoestratigráficas são constituídas pelos metassedimentos da Formação Água Clara e dos grupos Votuverava e Itaiacoca de idade meso- a neoproterozoica, corpos graníticos neoproterozoicos representantes do Complexo Granito Três Córregos e do Maciço Granito Capão Bonito, rochas sedimentares do Grupo Itararé, efusivas básicas associadas ao Magmatismo Serra Geral e sedimentos recentes quaternários. O quadro estrutural-metamórfico é determinado pelo arranjo tectônico final neoproterozóico, evidenciado nas rochas epimetamórficas por uma evolução estrutural do tipo polifásica, com quatro fases de deformação (Sn, Sn+1, Sn+2 e Sn+3) e três eventos metamórficos (M1, M2 e M3), de caráter regional progressivo, contato e regional a localmente dinâmico de natureza retrometamórfica.
 Palavras-chave: geologia, estrutural, mapeamento, Guapiara.

Geological and geophysical evidences of the polyphase structural evolution of the Southern Precordillera (31°42′S-69°24′W), central-western Argentina

The evolution of the western margin of South America comprised different tectonic regimes responsible of the present geological composition and structure of the Central Andes. Remote sensing, geological and geophysical data (aeromagnetic and seismological data) have been processed to understand the polyphase structural evolution of the Argentine Southern Precordillera. The outcrops exposed in the Sierra de Barreal (31°42′S-69°24′W) show evidences of a complex structural evolution related to the superposition of three orogenies from the Early Paleozoic to nowadays. The Chanic orogeny (Late Devonian) is characterized by two systems of non-coaxial ductile folds that affect Early Paleozoic rocks of the Ciénaga del Medio Group. The recognized rotation in the shortening direction (from NW-SE to ENE-WSW) is interpreted here as a result of a sinistral transpressive regime. The San Rafael orogeny (Early Permian) is represented by back-thrusts and blind faults associated to asymmetric folding that involves the deformation of Upper Carboniferous rocks (Majaditas Formation). Finally, the compressive Andean orogeny associated to the Pampean flat-slab subduction segment has been locally controlled by inherited pre-Andean anisotropies. This is evidenced by: 1-the structural style of western-southern Argentine Precordillera, which in the study area is represented by faulted blocks and top-to-west vergent back-thrusts; 2- NW-SE deflections and associated magnetic anomalies in the Argentine Precordillera fold-and-thrust belt; 3-neotectonic structures developed in a sinistral transpressive system; 4-the crustal seismicity associated to NW-SE trending structures with left-lateral component motion.

An unusual Proterozoic petroleum play in Western Africa: the Atar Group carbonates (Taoudeni Basin, Mauritania)

Abstract The Taoudeni Basin is the largest sedimentary basin in Africa. This intracratonic basin, which forms the sedimentary cover of the West African Craton, records episodic sedimentation since the Proterozoic. It is a largely unexplored basin, with only eight exploration wells drilled to date, although its petroleum prospectivity was established in 1974 when gas and liquids were tested in the Abolag-1 well. This paper focuses on the identification and assessment of potential source rocks and reservoirs in the Taoudeni Basin in Mauritania and aims to establish a geochemical genetic link between the occurrence of petroleum and potential sources. Various disciplines and techniques were integrated to reach a better understanding of the petroleum systems potentially existing in this basin. We also define and describe the elements and processes of the Atar-Atar (!) petroleum system. This study is an example of the exploration of an unusual and high-risk play in some of the oldest sedimentary rocks on Earth, with a long, polyphase structural evolution, a complex thermal history (where burial might not be the only controlling factor), a massively diagenetized carbonate reservoir, and large uncertainties on the timing of generation, trap formation and preservation.

Magnetic fabrics and Pan-African structural evolution in the Najd Fault corridor in the Eastern Desert of Egypt

In order to assess the Pan-African structural evolution from early orogenic fabrics through Najd wrenching to the latest orogenic collapse/extension, the authors used field work, aided by aerial photographs and satellite images. This work is complemented by the study of the anisotropy of the magnetic susceptibility (AMS, or magnetic fabric). The Pan-African rock associations of the Um Gheig-Kadabora area can be divided into a lower tier composed mainly of amphibolite–migmatite and granitoid gneisses, and an upper tier of ophiolitic rocks, metavolcanics and their related volcaniclastics, and molasse-type Hammamat sediments. Both these units are intruded by late orogenic granitoid plutons and dykes. The lower tier is exposed in a domal structure in the El Sibai area, the upper tier forms a series of weakly to highly deformed thrust units, called Pan-African Nappes here, which are dissected by high strain shear zones. According to their age, these rock units are divided here into early and late-orogenic. The early orogenic rock association is characterized by medium–high metamorphic grades. The late orogenic rock association is characterized by low metamorphic grade. The rocks in the upper tier form a series of low angle thrust sheets, which are bounded by NW-striking high angle shear zones related to the Najd Fault System. The early orogenic rocks show a polyphase structural evolution with early folds, thrusts, and strike-slip shear zones. The late orogenic rocks show a relatively weaker deformation. The latest intrusives studied here are the dykes dissecting the late orogenic Kadabora granite. In the present work magnetic fabric data document the deformational features in detail and assess the role of the Najd Fault System in the deformational evolution. A strong variation in volume susceptibility of various rocks, due to their variations in mineral composition, is observed. Lower values are in the range of 10−6 SI units for late-orogenic alkaline granite and the dykes dissecting it, the highest susceptibilities exceed 7×10−2 SI units in magnetite-bearing serpentinite. Early orogenic rocks are characterized by relatively high anisotropies (P′ up to 1.7) and are deformed in numerous shear zones. Most of these shear zones can be related to the Najd Fault System. In contrast, late orogenic sediments and intrusives show mostly low anisotropies. However, magnetic lineations are still distinctly oriented parallel with the Najd Fault trend. The very latest Pan-African intrusives, the broadly N–S trending dykes crosscutting the Kadabora pluton, imply c. E–W directed extension. Such an extension is consistent with the magnetic fabric in some of the dykes. Therefore, the Kadabora dykes mark the end of Najd wrenching and a late stage of extension in this part of the Eastern Desert of Egypt. The other dykes display mostly primary fabrics, related to magma flow during their intrusion and are thus post-deformational with regard to the Pan-African orogeny.

Age and petrogenesis of the Neoproterozoic Chon-Ashu alkaline complex, and a new discovery of chalcopyrite mineralization in the eastern Kyrgyz Tien Shan

Neoproterozoic volcanics and granitoids formed at Rodinia margins within a time span of 880Ma–700Ma, are well-documented in many terranes of the southern Central Asian Orogenic Belt (CAOB). Ages younger than 550Ma corresponding to the opening of the Terskey Ocean are also common. However, so far, there were very few published ages in the range 700Ma–550Ma from the Kyrgyz Tien Shan. In this paper we present new data for the alkaline Chon-Ashu complex emplaced at the end of the Cryogenian Period of the Neoproterozoic (850–635Ma, Gradstein et al., 2012). The alkaline complex intrudes the Precambrian metamorphic rocks north of the Nikolaev Line which separates the Northern and Middle Tien Shan terranes in the eastern Kyrgyzstan. The undeformed shallow level alkaline rocks range from olivine gabbro to nepheline and cancrinite syenites and leucosyenites. The differentiated rock assemblage can be explained by fractional crystallization of high-silica mineral phases which drives nepheline-normative melts away from the silica saturation boundary. The alkaline rocks of Chon-Ashu are enriched in LILE and HFSE indicative of their origin from lithospheric mantle. An age of 678±9Ma (U–Pb, SHRIMP) was obtained for a protolith of country gneiss, and an age of 656±4Ma was obtained for the crosscutting alkaline rocks of the Chon-Ashu complex. Seven zircon grains recovered from gneiss and alkaline rocks had bright overgrown rims which yielded a cumulative age of 400±8Ma. A metamorphic event, followed by uplift and emplacement of shallow level alkaline complex, constrains the geodynamic setting. Alkaline rocks usually form in an extensional setting and originate from lithospheric mantle. The 690Ma xenoliths of mafic granulite from the NW Tarim have been interpreted to originate by mafic underplating. This mafic underplating may have been responsible for metamorphism in the middle crust prior to emplacement of the Chon-Ashu complex. The 670Ma–630Ma period of extension and emplacement of enriched alkaline rocks can be also traced on a regional scale through southern Kazakhstan and the northern Tarim. We tentatively interpret these events as a result of mafic underplating and subsequent rifting related to the break-up of Rodinia. During field work at Chon-Ashu, rich chalcopyrite mineralization has been discovered in carbonate veinlets in leucosyenite alkaline dikes and has also been found in the adjacent Cambrian gabbro and granites shown on the map as undivided Devonian–Silurian. Stockwork mineralization predominates though disseminated mineralization is also present. The Cu content reaches 16,184ppm and is associated with elevated concentrations of Pb, Zn and Ag. The polyphase structural evolution of the area suggests that mineralization could have formed in several genetically unrelated stages. Based on structural and mineralogical evidence we tentatively relate the earliest stage of chalcopyrite mineralization to the late magmatic CO2-rich fluids emanating from the Cryogenian alkaline complex. The Early Devonian thermal event registered by growth of new zircon at 400Ma has important metallogenic implications on a regional scale. However the origin of two zones of alteration in the undivided Silurian–Devonian granites is ambiguous because their age was not determined geochronologically. The 522±4Ma Cambrian gabbro of the Tashtambektor Formation is strongly foliated along the splays of the Nikolaev Line, indicating a Hercynian origin of the fabric. Superimposed mineralized stockwork postdates the foliation and suggests a late-Hercynian age of mineralization in gabbro. The new data enable a reassessment of the metallogenic potential of the Eastern Kyrgyz Tien Shan. Presence of not eroded high-level mineralized Neoproterozoic alkaline intrusions points to a previously underestimated metallogenic potential of pre-Hercynian granitoids which may host preserved porphyry systems, skarns and shear-related mineralization. Finally, the Devonian magmato-metamorphic event which caused formation of a number of ore deposits in central Kyrgyzstan and Kazakhstan could also create potential exploration targets in eastern Kyrgyzstan.

准噶尔盆地中部1、3区块J1s2储层发育特征及有利区带预测<br>The Reservoir Characteristic and Favorable Area Prediction of J1s2 Formation in Block 1 and 3 of Central Junggar Basin

准噶尔盆地具有“盆大、水浅、坡缓、振荡性构造运动”的特征，中部1、3区块侏罗系三工河组是准噶尔盆地腹部重要的勘探目的层系，该组沉积时期主要受北西向与北东向物源共同影响，发育浅湖背景下的三角洲沉积，主要发育三角洲前缘水下分支河道与河口坝砂体，具有较好的储集物性。成藏受控于异常高压带、断裂系统、沉积相带、古隆起等因素，其中，车–莫古隆起的发育和演化为该区创造了独特的油气运聚背景，决定了多期构造演化与多期成藏的特点。 Big area, shallow water, gentle slope and oscillating movement are the main characteristics of the Junggar basin. The J1s2 formation is the main exploration strata of the block 1 and 3. During the J1s2 sedimentary period, on the influence of the northeast and northwest provenance, it developed the delta front deposition under the mere background there. The subaqueous distributary channel and channel mouth bar reservoir have the better physical property. The res- ervoir formation is controlled by the abnormal pressure belt, fracture system, sedimentary facies tract and ancient uplift. Among them, the development and evolution of Chemo ancient uplift supply the peculiar migration and accumulation background for oil & gas, which determined the characteristics of polyphase structural evolution and reservoir formation.

Polyphase structural evolution of a fine-grained, fold-dominated end moraine, Brúarjökull surge-type glacier, Iceland

The glaciotectonic architecture and structural evolution of a fine-grained end moraine formed by the 1890 glacier surge of Brúarjökull is described from four excavated cross-sections. The end moraine ridge is a morphological expression of a marginal sedimentary wedge formed during the last days of the surge. The actual end-moraine ridge was formed on the last day of the 1890 surge when the glacier became coupled to the bed and ploughed into the reverse slope of the marginal sediment wedge. Ductile deformation, favoured by high porewater pressure, dominated the construction of the end moraine while brittle deformation was induced when porewater pressure decreased, particularly at the end of the surge. Thus, the deformation was polyphase, developing from open folding to multiple overfolding when porewater pressure was high and finally to overthrusting, faulting and shearing at the very end of the surge when porewater pressure dropped severely upon porewater blow-out in front of the moraine. This structural continuum is exhibited by the four cross-sections. The glaciotectonic stress was absorbed within a relatively narrow zone due to high friction along a basal décollement. A new model illustrates the structural evolution of a fine-grained, fold-dominated end moraine and may serve as an analogue to similar end moraines in modern and Pleistocene environments.

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex, Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D 1–D 2, D 3 and D 4. The garnet-bearing assemblages crystallized in course of the deformation stage D 1–D 2 which led to the formation of the regional main foliation S 2. In X Ca– X Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X Mg and variably decreasing X Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P– T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P– T trends. The P– T evolution started at ∼450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P– T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P– T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes.

Normal faulting and block tilting in Lofoten and Vesterålen constrained by Apatite Fission Track data

Mesozoic and possibly younger normal faulting and block tilting in the Lofoten–Vesterålen archipelago can be constrained by Apatite Fission Track data. Previous studies have documented the polyphase structural evolution of the archipelago, based on field data and onshore–offshore correlations. Our new AFT data document vertical movements on a more regional scale, with kilometer-scale offset on some faults. Below Sortlandsundet, the Hadselfjord Fault Zone forms the eastern limit of the Sortlandsundet half-graben and our data reveal latest Cretaceous and younger half-dome shaped uplift of the footwall. East of the Western Lofoten Border Fault, on Vestvågøya, southeastward tilting is indicated both by the pattern of AFT ages, ranging from 81 +/− 7 Ma to in the NW to 167 +/− 16 Ma in the SE, and tilted paleosurfaces. AFT ages immediately east of Sortlandsundet (72 +/− 5 Ma) and on Vestvågøya (81 +/− 7 Ma) are among the youngest ones found on the Norwegian mainland. The present-day landscape of the Lofoten–Vesterålen archipelago does not represent a re-exposed and glacially modified Mesozoic landscape. Although most of our data indicate cooling to below ∼ 60 °C took place during the Mesozoic, the youngest footwall ages are hard to explain without alluding to Cenozoic normal faulting. A Cenozoic structural control on the landscape can thus be inferred for Lofoten and Vesterålen.

Petrology and 40Ar–39Ar dating of shear zones in the Lanterman Range (northern Victoria Land, Antarctica): implications for metamorphic and temporal evolution at terrane boundaries

The Lanterman Fault Zone, a major terrane boundary in northern Victoria Land, displays a polyphase structural evolution. After west-over-east thrusting, it experienced sinistral strike-slip shearing. Sheared metabasites from the Wilson Terrane (inboard terrane) preserve a record of retrograde metamorphic evolution. Shearing took place under amphibolite-facies metamorphic conditions (roughly comparable to those reached during regional metamorphism) which later evolved to greenschist-facies conditions. In contrast, the Bowers Terrane (outboard terrane) preserves a prograde metamorphic evolution which developed from greenschist-facies to amphibolite-facies metamorphism during shearing, followed by greenschist-facies metamorphism during the late deformational stages. Laser step-heating 40Ar–39Ar analyses of syn-shear amphibolite-facies amphiboles yielded ages of 480–460 Ma, in agreement with a ∼480-Ma age obtained from a biotite aligned along the mylonitic foliation. These ages are younger than those (∼492 to ∼495 Ma) obtained from pre-shear amphibole relics linked to regional metamorphism of the Wilson Terrane. Results attribute the structural and metamorphic reworking during shearing to the late stages of the Cambrian-Ordovician Ross Orogeny and to the Middle-Late Ordovician probably in relation to the beginning of deformation in the Lachlan Orogen, thus precluding any appreciable impact of the Devonian-Carboniferous Borchgrevink event in the study area.

Polyphase structural evolution in the northeastern segment of the North Patagonian Massif (southern Argentina)

Structural analyses in the northeastern segment of the North Patagonian Massif (southern Argentina) show that the simply deformed and metamorphosed phyllitic succession of the Late Precambrian–Cambrian El Jagüelito Formation has been intruded by Ordovician granitoids that are not ductilely deformed. The unconformable cover of the Silurian–Lower Devonian Sierra Grande Formation suggests that the Early Paleozoic Famatinian deformation of western Argentina did not affect this sector of the North Patagonian Massif. The ∼NW–SE compression of this succession led to the formation of open fold structures combined with high-angle reverse and sinistral strike–slip faults. Deformation interfered with the cooling of the Laguna Medina granitoids and is assigned here to the Late Paleozoic interval (probably Permian). A comparable mechanism is assumed for the metamorphism in the Sierras Australes fold-and-thrust belt north of Patagonia. The ∼NE–SW compression in the area west of Mina Gonzalito led to the formation of mylonites in the Peñas Blancas and La Laguna granites. It is suggested that ductile deformation is Permian in age and took place along important shear horizons. On a regional scale, it is comparable to that of the Cerro de Los Viejos granite (La Pampa Province) and the Sierras Australes fold-and-thrust belt (Buenos Aires Province) north of the inferred suture between Patagonia and Gondwana South America. This suggests that, on both sides of the boundary, intense compression took place during the same Gondwanide period and that extra-Andean Patagonia collided with Gondwana South America. The deformation in the Sierra Grande area is interpreted as a second-stage event during the Gondwanide deformational and magmatic history.

Polyphase structural evolution of the southwestern Argentine Precordillera

The western and southwestern parts of the Argentine Precordillera display complex geometries which are not consistent with those of a typical high-level fold-and-thrust belt. They are the result of a polyphase structural evolution which spans the Early Paleozoic to Late Tertiary period. After an Early Paleozoic folding and shearing event under a greenschist facies metamorphism, uplift, erosion, and deposition of Late Carboniferous to Early Permian clastics were accompanied by extensional faulting. This was followed by a Permian folding and faulting event which led to a partial inversion of the Late Carboniferous-Early Permian graben fill. Permian to Triassic crustal extension was combined with block faulting and the deposition of a thick volcanic sequence. The subsequent Late Tertiary crustal shortening partly reactivated older fault lines. Excluding folds, a few thrusts, and reverse faults, the crustal shortening within the older blocks was accommodated by a dominant sinistral strike-slip faulting under a W-E compressive regime. Above a major décollement, the entire sequence of faulted and folded blocks was carried from west to east towards its present position. The regional situation indicates that this southern part of the orogen was transferred further to the east with respect to the central thin-skinned parts. The movements are interpreted to be related to an important thrust fault which obliquely cuts through the fold-and-thrust belt.

Deformation in the Caledonides of England, Ireland and Scotland

Summary The past few years have witnessed a remarkable change in our view of the British Caledonides resulting from radiometric work, application of plate tectonic models as well as basic field work. Overshadowing all else has been the acceptance of a later Proterozoic (probably Grenville) orogeny in the Moine rocks occurring between the Moine thrust and the Great Glen fault. The northern Moines are clearly polymetamorphic in character, but several matters are still controversial, for example the separation of Proterozoic and Caledonian fabrics and the areal extent of the older orogeny. This northern block was subjected to intense and repeated tectonism and, above all, large thrust translation during the Caledonian orogeny and the rocks are now juxtaposed with the older Proterozoic and Archaean gneisses of the Caledonian foreland. In Ireland small areas of Grenville basement are in slide contact with the younger Moine and Dalradian rocks. South of the Great Glen fault above a postulated Grenville basement, are late Proterozoic sediments (Central Highland Granulites and Dalradian) which were orogenically deformed in lower Ordovician times. In this area the essence of the structural pattern established some years ago stands unquestioned for the most part, but significant modifications have been made, notably the recognition of a slice of ‘Cadomian’ basement in the Dalradian pile of NE Scotland, and possible pre-Caledonian basement in W Ireland. In Ireland the structural pattern in the Scottish Highlands may be recognised in the north, but becomes considerably modified in the west where the strike becomes E-W. The picture of the Scottish Highlands as a metamorphic Alpine-type orogen, dating from the early Ordovician, and undergoing uplift during Ordovician and later times, is complemented by the recent recognition of the Southern Uplands zone (later, non-metamorphic Caledonides) as being an accretionary prism, on the Aleutian model, in which imbricate thrusts developed in series towards the south above the subducting oceanic crust of Iapetus. The Iapetus suture lies between the Southern Uplands and northern England and passes through central Ireland. The pattern of Ordovician vulcanicity in northern England, Wales and SE Ireland has been attributed to southerly directed subduction of the Iapetus crust beneath the opposing continental margin. Final closure of Iapetus in Silurian and Devonian times, seems to have been marked by relatively mild tectonic activity in the ‘metamorphic Caledonides’, but by considerable compression across the slate belts of the Lake District, Wales and SE Ireland with polyphase structural evolution.