Granitoid rocks of the southern Menderes Massif (southwestern Turkey): field evidence for Tertiary magmatism in an extensional shear zone

Recent field campaign in the southern Menderes Massif in southwestern Turkey revealed that the so-called ‘core of the massif’ comprises two distinct types of granitoid rocks: an orthogneiss (traditionally known as augen gneisses) and leucocratic metagranite, where the latter is intrusive into the former and the structurally overlying ‘cover’ schists. These differ from one another in intensity of deformation, degree of metamorphism and kinematics. The orthogneiss display penetrative top-to-the-N–NNE fabrics formed under upper-amphibolite facies conditions during the Eocene main Menderes metamorphism (MMM), whereas foliation and stretching lineation exists in the leucocratic metagranites but are not strongly developed. The leucocratic metagranites show evidence of syn- to post-emplacement deformation in a series of weakly developed top-to-the-S–SSW fabrics formed under lower greenschist-facies (?) conditions. Leucocratic metagranite bodies occur all along the augen gneiss–schist contact in the southern Menderes Massif; they are emplaced as sheet-like bodies into country rocks (previously deformed and metamorphosed during a top-to-the-N–NNE Alpine orogeny) along a ductile extensional shear zone, located between orthogneisses and metasediments, which was possibly active during emplacement. The data presently available indicate that emplacement and associated ductile extensional deformation occurred during Late Oligocene–Early Miocene time. These results confirm previous contentions that there are Tertiary granites in this part of the Menderes Massif.     

Tectonic significance of deformation patterns in granitoid rocks of the Menderes nappes, Anatolide belt, southwest Turkey

Deformation fabrics in Proterozoic/Cambrian granitic rocks of the Çine nappe, and mid-Triassic granites of the Bozdag nappe constrain aspects of the tectonometamorphic evolution of the Menderes nappes of southwest Turkey. Based on intrusive contacts and structural criteria, the Proterozoic/Cambrian granitic rocks of the Çine nappe are subdivided into older orthogneisses and younger metagranites. The deformation history of the granitic rocks documents two major deformation events. An early, pre-Alpine deformation event (D_PA) during amphibolite-facies metamorphism affected only the orthogneisses and produced predominantly top-to-NE shear-sense indicators associated with a NE-trending stretching lineation. The younger metagranites are deformed both by isolated shear zones, and by a major shear zone along the southern boundary of the Çine submassif. We refer to this Alpine deformation event as D_A3. D_A3 shear zones are associated with a N-trending stretching lineation, which formed during greenschist-facies metamorphism. Kinematic indicators associated with this stretching lineation reveal a top-to-south sense of shear. The greenschist-facies shear zones cut the amphibolite-facies structures in the orthogneisses. ²⁰⁷Pb/²⁰⁶Pb dating of magmatic zircons from a metagranite, which crosscuts orthogneiss containing amphibolite-facies top-to-NE shear-sense indicators, shows that D_PA occurred before 547.2ǃ.0 Ma. Such an age is corroborated by the observation that mid-Triassic granites of the Çine and Bozdag nappes lack D_PA structures. The younger, top-to-south fabrics formed most likely as a result of top-to-south Alpine nappe stacking during the collision of the Sakarya continent with Anatolia in the Eocene.     

The Rand Granite in the southern Schwarzwald and its geodynamic significance in the Variscan belt of SW Germany

The Rand Granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the Moldanubian Zone at the southern margin of the Central Schwarzwald Gneiss Complex. It is a largely mylonitized elongated body and is thrust over the Badenweiler-Lenzkirch Zone forming a nappe with shear zones along its northern and southern boundaries. It comprises meta-granites, meta-trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites. Mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions. Nappe thrusting, which also affected the South Schwarzwald Gneiss Complex, occurred in Visean time during high-temperature / low-pressure metamorphism. Kinematic indicators in the mylonites document E- to ESE-directed nappe transport, highly transpressive relative to the trend of the nappe boundaries and the foliation. The trondhjemites formed at 351 +5/-4 Ma, predating the Variscan HT metamorphism. They have initial _Nd-values of +6.6 to +6.7 and relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7042 to 0.7063), indicating a predominant mantle origin. The granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 Ma, suggested by U-Pb zircon model ages. They are different from the trondhjemites with low initial _Nd-values (–4.7 to –3.3) and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7068–0.7077), indicating that large part of the Rand Granite originated from anatexis of continental crust. Internal structure of zircons from the Rand Granite reveals mixing of magmas derived from both mantle and crust sources. These new data support an interpretation that the Rand Granite developed along an active continental margin and therefore represents a possible root of a Variscan magmatic arc.     

阿尔卑斯旋回中喜马拉雅山链和阿尔卑斯山链的主要变形特征

<正> 特提斯—阿尔卑斯山系是当今世界两大造山带之一,它自加勒比海开始,横贯欧亚大陆南部,直达印度尼亚西。这一山系的形成经历新特提斯洋打开至闭合的整个过程。特提斯洋是在华力西造山运动之后(T—J)联合大陆由西太平洋开始剪切张开而形成的;晚侏罗世至老第三纪时期,由于大西洋由南往北张开造成特提斯洋闭合。与此同时大洋内及洋壳和陆壳间发生俯冲或仰冲,以至大洋消亡,南北大陆板块碰撞(有些地段无碰撞)。     

Transverse lineation and large-scale structures related to Alpine obduction in Corsica

In Alpine Corsica, the major tectonic event during the late Cretaceous was the thrusting to the west of an ophiolitic nappe and its sedimentary cover upon the Variscan basement and its Mesozoic cover. A detailed field survey shows that the basal contact of the nappe corresponds to a pluri-kilometric scale shear zone. Thus gneissified basement slices have been tectonically emplaced in the ophiolitic nappe. The thrusting was responsible for small scale structures: foliation, lineation and folds, initiated in a HP/LT metamorphic context. The deformation analysis shows that the finite strain ellipsoid lies in the constriction field close to that for plane strain. Moreover occurrences of rotational criteria in the XZ planes (sigmoidal micas, asymmetric pressure shadows, quartz C-axes fabrics) are in agreement with shear from east to west. All structural data from microscopic to kilometric scales, of which the most widespread is a transverse stretching lineation, can be interpreted by a simple shear model involving ductile synmetamorphic deformation. At the plate tectonic scale the ophiolitic obduction is due to intraoceanic subduction blocked by underthrusting of continental crust beneath oceanic lithosphere.     

The structure of the Palaeozoic schists in the Southern Menderes Massif,western Turkey: A new approach to the origin of the Main menderes metamorphism and its relation to the Lycian Nappes

The Early Eocene to Early Oligocene tectonic history of the Menderes Massif involves a major regional Barrovian-type metamorphism (M₁, Main Menderes Metamorphism, MMM), present only in the Palaeozoic-Cenozoic metasediments (the so-called “cover” of the massif), which reached upper amphibolite faciès with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels. It is not present in the augen gneisses (the so called “core” of the massif), which are interpreted as a peraluminous granite deformed within a Tertiary extensional shear zone, and lie structurally below the metasediments. A pronounced regional (S₁) foliation and approximately N-S trending mineral lineation (L₁) associated with first-order folding (F₁) were produced during D₁ deformation coeval with the MMM. The S₁ foliation was later refolded during D₂ by approximately WNW-ESE trending F₂ folds associated with S₂ crenulation cleavage. It is now commonly believed that the MMM is the product of latest Palaeogene collision across Neo-Tethys and the consequent internal imbrication of the Menderes Massif area within a broad zone along the base of the Lycian Nappes during the Early Eocene-Early Oligocene time interval. However, the meso- and micro-structures produced during D₁ deformation, the asymmetry and change in the intensity and geometry of the F₂ folds towards the Lycian thrust front all indicate an unambiguous non-coaxial deformation and a shear sense of upper levels moving north. This shear sense is incompatible with a long-standing assumption that the Lycian Nappes were transported southwards over the massif causing its metamorphism. It is suggested here that the MMM results from burial related to the initial collision across the Neo-Tethys and Tefenni nappe emplacement, whereas associated D₁ deformation and later D₂ deformation are probably related to the northward backthrusting of the Lycian nappes.     

黄河源地区磁组构特征及其地质意义

黄河源区位于青藏高原东北部,区内主要为三叠系沉积地层,发育一系列由北向南的推覆构造带,间有早期近直立的韧性剪切带。笔者对黄河源地区巴颜喀拉山群沉积岩进行了磁组构分析,结果显示岩石磁化率各向异性度P值和磁化率百分率各向异性度H值均不大,反映该地区总体韧性变形较弱,较强韧性变形仅发育于局部地段;岩石磁组构具有磁面理发育、磁线理不发育、磁化率椭球呈压扁形椭球体的特点,反映在挤压应力作用下,岩石发生了压扁变形,主应力方位主要为NNE-SSW(近SN)向,其次为NE-SW向。根据岩石磁组构分析认为黄河源地区存在两条韧性剪切带,韧性剪切带与现今湖泊水体的展布有一定的耦合关系;北部韧性剪切带沿现今黄河河谷分布,控制着扎陵湖、鄂陵湖和玛多"四姐妹湖"的展布;南部韧性剪切带沿岗纳格玛错—野牛沟一线展布,控制着岗纳格玛错和尕拉拉错等残余湖泊的分布。     

Relationship between shear zones and igneous activity: The Closepet Granite of Southern India

The late Archaean Closepet granite of southern India is bounded by N-S trending shear zone. At the southern end of the granite both charnockite and granite veins are spatially associated with ductile shears. These shears continue further north and are confined to the contact zones in the central part of the granite outcrop. The main component of the shear zone are highly deformed granite sheets, augen gneisses and mylonites. Field observations and microstructural fabric of mylonites indicate a dextral sense of shear movement. Field evidence suggests that shear deformation was active throughout the evolution of the Closepet granite     

Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D₁) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D₁ migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D₁ fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.     

Constraining P–T conditions during thrusting of a higher pressure unit over a lower pressure one (Gran Paradiso,Western Alps)

Identifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large‐scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso‐ and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18–20 kbar, 480–520 °C and of ~13–18 kbar, 500–530 °C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high‐P part of the albite stability field at P–T conditions of ~12.5–14.5 kbar and 530–560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na‐bearing phases (glaucophane, paragonite) have been overprinted by the low‐P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid‐solution model not incorporating a pyrophyllite component.     

Extensional shear band development on the outer margin of the Alpine mylonite zone,Tatare Stream,Southern Alps,New Zealand

Schistose mylonitic rocks in the central part of the Alpine Fault (AF) at Tatare Stream, New Zealand are cut by pervasive extensional (C′) shear bands in a well-understood and young, natural ductile shear zone. The C′ shears cross-cut the pre-existing (Mesozoic—aged) foliation, displacing it ductilely synthetic to late Cenozoic motion on the AF. Using a transect approach, we evaluated changes in geometrical properties of the mm–cm-spaced C′ shear bands across a conspicuous finite strain gradient that intensifies towards the AF. Precise C′ attitudes, C′-foliation dihedral angles, and C′–S intersections were calculated from multiple sectional observations at both outcrop and thin-section scales. Based on these data the direction of ductile shearing in the Alpine mylonite zone during shear band activity is inferred to have trended >20° clockwise (down-dip) of the coeval Pacific-Australia plate motion, indicating some partitioning of oblique-slip motion to yield an excess of “dip-slip” relative to plate motion azimuth, or some up-dip ductile extrusion of the shear zone as a result of transpression, or both. Constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary (SZB). Across all examined parts of the shear zone, the mean dihedral angle between the C′ shears and the mylonitic foliation (S) remains a constant 30 ± 1° (1σ). The aggregated slip accommodated on the C′ shear bands contributed only a small bulk shear strain across the shear zone (γ = 0.6–0.8). Uniformity of per-shear slip on C′ shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain-hardening rheology, such that they locked up at a finite shear strain (inside C′ bands) of 12–15. Shear band boudins and foliation boudins both record extension parallel to the SZB, as do the occurrence of extensional shear band sets that have conjugate senses of slip. We infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with W_k < 0.8, and perhaps even as low as <0.5. The C′ shear bands near the AF formed in a thinning/stretching shear zone, which had monoclinic symmetry, where the direction of shear-zone stretching was parallel to the shearing direction.     

Role of plagioclase and reaction softening in a metagranite shear zone at mid‐crustal conditions (Gotthard Massif,Swiss Central Alps)

A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre‐scale shows an evolution from a weakly deformed metagranite (Qtz_I–Kfs_I–Ab_I–Bt_I ± Pl_II–Zo_I–Phg_I–Grt) to a fine banded ultramylonite (Qtz_II–Kfs_II–Ab_II–Pl_II–Bt_II–Phg_II ± Grt–Zo_II). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K‐feldspar. The continuous recrystallization of plagioclase during deformation into a very fine‐grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed‐system conditions with the destabilization of metastable Ab_I–Zo_I porphyroclasts into fine‐grained (20–50 μm sized) Ab_II–Pl_II aggregates, and with minor crystallization of phengite at the expense of K‐feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H₂O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine‐grained Ab_II–Pl_II aggregate into a more Pl_II‐rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K‐feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one‐third of the bulk rock volume in granitic systems and forms a stress‐supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P–T conditions and/or bulk composition must be taken into account, together with quartz and K‐feldspar, in order to understand strain localization processes in granites.     

Structure and metamorphism of the Gran Paradiso massif, western Alps, Italy

The pressure-temperature-time trajectory and structural history of high-pressure rocks presently exposed in the Gran Paradiso massif provide constraints on the processes that caused their thermal evolution and exhumation. High-pressure metamorphism of the rocks is found to have culminated at temperatures around 525 °C and pressures of 12 to 14 kbar. After high-pressure metamorphism, the rocks cooled during initial decompression, while undergoing top-to-the-west shear on chlorite-bearing shear bands and larger scale shear zones. Biotite-bearing shear bands and larger shear zones related to top-to-the-east deformation affected the Gran Paradiso massif during reheating to temperatures of around 550 °C at 6 to 7 kbar. Further exhumation occurred at relatively high temperatures. A potentially viable explanation of the observed stage of reheating before final cooling and exhumation is breakoff of a subducting slab in the upper mantle, allowing advective heat transfer to the base of the crust. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-001-0357-6.     

The Rila-Pastra Normal Fault and multi-stage extensional unroofing in the Rila Mountains (SW Bulgaria)

The Rhodope Metamorphic Province represents the core of an Alpine orogen affected by strong syn- and postorogenic extension. We report evidence for multiple phases of extensional unroofing from the western border of the Rila Mountains in the lower Rila valley, SW Bulgaria. The most prominent structure is the Rila-Pastra Normal Fault (RPNF), a major extensional fault and shear zone of Eocene to Early Oligocene age. The fault zone includes, from base to top, mylonites, ultramylonites and cataclasites, indicating deformation under progressively decreasing temperature, from amphibolite-facies to low-temperature brittle deformation. It strikes E–W with a top-to-the-N-to NW-directed sense of shear. Basement rocks in the hanging wall and footwall both display amphibolite-facies conditions. The foliation of the hanging-wall gneisses, however, is discordantly cut by the fault, while the foliation of the footwall gneisses is seen to curve into parallelism with the fault when approaching it. Two ductile splays of the RPNF occur in the footwall, which are subparallel to the foliation of the surrounding gneisses and merge laterally into the mylonites of the main fault zone. The concordance between the foliation in the footwall and the RPNF suggests that deformation and cooling in the footwall occurred simultaneously with extensional shearing, while the hanging-wall gneisses had already been exhumed previously. The RPNF is associated with thick deposits of an Early Oligocene, syntectonic breccia on top of its hanging wall. Integrating our results with previous studies, we distinguish the following stages of extensional faulting: (1) Late Cretaceous NW–SE extension (Gabrov Dol Detachment), exhumation of the present day hanging wall of the RPNF; (2) Eocene to Early Oligocene NW–SE to N–S extension (RPNF); (3) Miocene to Pliocene E–W extension (Western Border Fault), formation of the Djerman Graben; (4) Holocene to recent N–S to NW–SE extension (Stob Fault), reactivating the SW part of the Western Border Fault.     

Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps

High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.     

U–Pb zircon data of Variscan meta-igneous and igneous acidic rocks from an Alpine shear zone in Calabria (southern Italy)

The paper deals with the U–Pb data of zircon separated from three samples representative of mylonitic leucogranites, trondhjemites and pegmatites occurring along the Alpine tectonic zone between the Castagna and Sila Units in northern Calabria. These mylonites are associated to Variscan granitic-granodioritic biotite-rich augen gneisses derived from Neo-Proterozoic-Early Cambrian protoliths. Apparent ages ranging from Early Cambrian to post-Variscan have been obtained. Th, U and rare earth elements have been determined in two zircon domains of mylonitic leucogranite and trondhjemite giving different ages in order to get information relative to their geological significance. The pegmatite preserves intrusive contact with the augen gneisses and with the other mylonites; it turns out to be emplaced at 290–300 Ma, like the Variscan plutonites of the Castagna Unit. The deformation masks the original contacts of the mylonitic leucogranite and trondhjemite with the biotite-rich augen gneisses. The age-group averaging 540 Ma is interpreted as indicative of the emplacement of the protoliths and it coincides with the age previously determined for the emplacement of the protoliths of the biotite-rich augen gneisses. Zircon from the mylonitic pegmatite includes domains showing concordant and discordant ages younger than 290 Ma, thus reflecting various degrees of partial resetting and Pb-loss caused by post-Variscan events. Zircon from the mylonitic leucogranite and trondhjemite includes apparent ages between 300 and 280 Ma as well as ages younger than 250 Ma. Perturbation of U–Pb system by Alpine shearing appears evident; however, possibile effects caused by thermal input and hydrothermal fluid infiltration from the Variscan plutonites cannot be excluded.     

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe,SW Highlands,Scotland

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D₁ spaced cleavage was responsible for the formation of the D₂ spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D₂ microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D₁ microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D₂ flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.     

Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine-Austroalpine boundary zone in the Western Alps

Mylonitic structures related to two orogenic events are described from the upper and lower contacts of the Combin zone and the immediately overlying upper Austroalpine Dent Blanche nappe/Mont Mary klippe and the directly underlying lower Austroalpine Etirol-Levaz slice. The first event, Late Eocene in age, commenced during blueschist facies P-T conditions, but pre-dated the peak of subsequent greenschist facies overprint. The second event, Early Oligocene in age, took place during retrograde greenschist facies conditions. Most sense of shear indicators associated with the retrograde mylonites indicate top SE shearing, but subordinate top NW displacing shear sense indicators have also been mapped. Mylonitic top SE shearing appears to be restricted to the Combin zone and its upper and lower contacts. Within the Dent Blanche nappe and Mont Mary klippe and at the base of the Etirol-Levaz slice, structures were observed which developed during blueschist/greenschist facies conditions and are, in conjunction with the P-T-t history of these rocks, inferred to be older. Associated kinematic data indicate a top NW shear sense. Comparable blueschist/greenschist facies shear sense indicators have not been observed in the Combin zone. Nonetheless, the foliation in the Combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions. This indicates that the Combin zone and the immediately over- and underlying Austroalpine units shared a common tectono-metamorphic evolution since the Late Eocene. Finite strain data reveal oblate strain fabrics, which are thought to result from a true flattening strain geometry. Flow path modelling reveals a general non-coaxial deformation régime and corroborates significant departures from a simple shear deformation. In the study area, mylonitic top SE shearing in the Combin zone is attributed to Early Oligocene backfolding and backthrusting of the Mischabel phase. Temperature-time curves suggest slight reheating in the Monte Rosa nappe underneath and cooling in the Dent Blanche nappe above the Combin zone, hence confirming a thrust interpretation for this event. The top NW displacing structures are thought to result from Late Eocene emplacement of the Dent Blanche nappe and the Combin zone onto the Middle Pennine Barrhorn series along the Combin fault. As related structures initiated during mildly blueschist facies conditions in the Dent Blanche nappe and the underlying Combin zone and both were emplaced together onto the greenschist facial Barrhorn series, it is concluded that the structures developed as the nappes moved upward relative to the earth's surface. Thus the Combin fault is regarded as a thrust. The geometry of this structure indicates that the Combin fault is an out of sequence thrust that locally cut down section. Hence, top NW out of sequence thrusting caused local thinning of the metamorphic/structural section in association with horizontal shortening. Out of sequence thrusts cutting down section, and back-thrusts, offer the possibility of explaining the pronounced break in the grade of metamorphism across the Combin fault, i.e. the contact between the eclogite facial Zermatt-Saas zone and the overlying lower grade Combin zone, by contractional deformation.     

Structural zones and continental collision, Central Alps

The traverse of the Central Alps between Lake Constance and Lake Como (eastern Switzerland, northern Italy) allows the reconstruction of a cross-section through a collision belt some 140 km wide and 40 km deep. It can be described in terms of a series of structural zones (A–F), defined by the age and character of the latest phase of penetrative deformation affecting both basement and cover rocks, each zone showing a characteristic structural history. These zones do not coincide with the well-known tectono-stratigraphic Alpine subdivisions (Helvetic, Pennine, Austroalpine) which are based on gross geometry, facies and petrography. Zones A and B, in the north, developed during late Oligocene and Miocene times, affecting the Helvetic realm and the already overlying Pennine and Austroalpine units. Zone A is characterized by a steeply dipping penetrative cleavage S_A, zone B by the same cleavage later modified by nappe-forming movements. Zone F, in the south, also developed during the late Oligocene and Miocene, first as a monoclinal flexure, later as a steeply dipping zone of mylonitization and cataclasis (foliation S_f), affecting Pennine and Austroalpine units. The final manifestation of these movements was the Tonale line and their net result was the uplift of the region to the north by about 20 km. Between these two belts lay an area in which late Oligocene-Miocene movements had little effect — structural zones C (Pennine), D (Pennine-Austroalpine transition) and E (Austroalpine). In zones C and D, the latest phase of penetrative deformation, resulting in large recumbent fold structures and a penetrative foliation S_c zone C, can be dated as late Eocene-early Oligocene. This seems to be related to the overriding of the Austroalpine nappe complex (zone E), which already showed the effects of a late Cretaceous orogeny.Unravelling these events backwards, reveals, at the Eocene—Oligocene boundary, a southward dipping subduction zone in the process of locking. Its mouth is full of upper Cretaceous-Eocene flysch; its throat is choked by the Pennine nappe complex, undergoing the s_c ductile deformation. Before subduction, the Pennine nappe complex can best be described as a mega-mélange-a tectonic mixture of large fragments of continental basement, oceanic basement, trough-facies cover and platform-facies cover, already showing a complicated structural history. It is supposed that collision started in mid-Cretaceous times, not at a single subduction suture (trench), but by complicated surficial processes across a wide zone, as non-matching, rifted and thinned continental margins approached and small oceanic remnants were obducted. Post-mid-Oligocene events are essentially intra-plate compressional effects, combined with isostatic response.