Main Deformation Phases Research Articles

Crustal shortening estimates across the north Indian continental margin, Ladakh, NW India

Abstract A structural and lithological map has been produced covering the Spontang ophiolite and the north Indian continental margin from the Indus Suture Zone in the north to the high-grade metamorphic rocks and granites of the High Himalaya in the south. Cross-section balancing techniques have been used to identify, quantify and sequentially restore three major phases of deformation (D1–3) affecting the north Indian continental margin resulting in > 85 km (280%) shortening. D1 in the late Cretaceous involved obduction of the Spontang ophiolite, associated Neo-Tethyan thrust sheets and Mesozoic continental slope deposits onto the outer passive margin. D1 was responsible for 200% shortening by internal folding and duplex formation within stratigraphic units in the outer shelf, but did not affect the innermost parts of the Indian passive margin. Restoration of later structures suggests that the allochthonous thrust sheets were emplaced a minimum of 70 km onto the continental margin. D2 from the early Eocene to Oligocene was the main phase of deformation associated with the collision of India and Asia. Re-thrusting places the Spontang ophiolite and associated mélanges over the Maastrichtian to Lower Eocene neo-autochthonous cover which accommodated 140–160% shortening in the hanging wall. D2 progressed with the propagation of thrusting down section and towards the foreland causing crustal thickening and Barrovian metamorphism. The thick, argillaceous late Cretaceous Kangi La Formation decoupled deformation in higher and lower structural levels in outer shelf areas. D3 backthrusting and break-backthrusting in the late Tertiary formed a pop-up structure at the northern edge of the continental margin associated with a further 190–230% shortening and inversion of structures in the Indus Suture Zone. A major anticlinal structure also initiated across the southern edge of the Indus Suture Zone. South of Spontang reactivation of D2 thrusts as late D2/D3 normal faults was associated with gravitational collapse of the High Himalaya to the south. Extensional movement on these structures was probably concomitant with shortening in the pop-up structure to the north. A reduction in present day and restored thickness of the Tethyan Himalaya and an increase in absolute shortening from east to west probably reflects a variation in the partitioning of deformation across the whole width of the orogen. This may be associated with the indentation of India into Asia to the northwest.

Relationships between magmatism, metamorphism and deformation in the Fraser Complex, Western Australia: Constraints from new SHRIMP U–Pb zircon geochronology

SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b).

Petrological and structural significance of the Santa Elena-Nicoya ophiolitic complex in Costa Rica and geodynamic implications

Detailed structural and petrological investigations were carried out on the Santa Elena-Nicoya ophiolitic complex in Costa Rica, which is represented by a classic association of mantle tectonites (Santa Elena Peninsula), a cumulitic gabbroic-plagiogranite sequence, and basaltic volcanics and sills (Matapalo and Esperanza Units of the Nicoya Peninsula), with discontinuous radiolarian covers. Despite biostratigraphically different ages (Late Jurassic for Matapalo and Mid-Late Cretaceous for Esperanza) in both units, the major and trace element data, Sr-Nd isotope systematics, and mineral phase crystallization order show identical tholeiitic fractionation trends, where the marked Fe-Ti enrichment of the volcanics is reflected in the gabbroic-Fe gabbroic-plagiogranite intrusives which, in turn, represent the cumulitic counterparts of an essentially analogous magmatic system. Parental basaltic magmas for the Nicoya complex share geochemical features with both Pacific MORB and oceanic plateaus, as indicated by the characteristic flat REE patterns, Zr/Y and Zr/Nb ratios, and Sr-Nd isotopic data ( 87 Sr/ 86 Sr: 0.70295-0.70323; 144 Nd/ 143 Nd: 0.51309-0.51275). On the other hand, basalts and gabbros from Santa Elena display typical N-MORB affinity, as shown by their LREE-depleted patterns. These data, together with the unusual abundance of Fe-Ti differentiated rocks, suggest that the Santa Elena-Nicoya ophiolites may have been generated at oceanic spreading centres in a mantle plume region, analogous to the present Galapagos ridge/hot spot system. The present fore-arc structural setting of the deformed Costa Rica ophiolites is the result of several tectogenetic events recorded by three main ductile deformation phases (from Late Cretaceous to Tertiary): the oldest d1 related to W-E convergent tectonics, followed by d2 related to N-S compression, and the youngest d3 related to NE-SW convergence. The oceanic lithosphere represented by the Costa Rica ophiolites may have been generated at the westernmost termination of the spreading system, leading to the formation of the Caribbean plateau, and later involved in complicated intra-oceanic convergence and rotation processes. These resulted in the progressive building and insertion of the Chorotega block (present Costa Rica) between the Maya-Chortis and Choco-South America continental blocks.

Geology and U-Pb geochronology of rocks of the Eokuk Uplift: a pre-2.8 Ga basement inlier in the northwestern Slave Province, Nunavut, Canada

Rocks of the Eokuk Uplift have been mapped in detail along the coast of Coronation Gulf and 10 key units have been dated by U-Pb analysis of zircon, monazite, and titanite. The combined data indicate that this inlier of the Slave Province has a >3.2 Ga crustal component, evidence of a granulite-grade orogenic event predating 2.8 Ga and a lack of evidence for any significant orogenic activity corresponding to the 2.7-2.6 Ga events common in the rest of the Slave Province. The oldest rocks in the study area are a succession of granitoid and supracrustal gneisses that have been metamorphosed to amphibolite to granulite facies. From field relationships, the oldest rock is a granodiorite to tonalite orthogneiss, with a zircon crystallization age of 3254+13-6 Ma. A granite gneiss, which may be a small felsic intrusion or an anatectic melt of the tonalite gneiss, yields a zircon age of 3216+14-13 Ma. A K-feldspar megacrystic monzogranite gneiss contains old, discordant, possibly inherited zircons with 207Pb/206Pb ages ranging from 3103 to 3039 Ma, together with coexisting 2879 ± 3 Ma zircon and monazite. These high-grade gneisses are intruded by two megacrystic granite plutons, dated at 2887 ± 2 and 2881+4-3 Ma. The absence of extensive recrystallization and complex structures in these plutons indicates that this igneous event postdated the high-grade metamorphism. An amphibolite-grade synplutonic metamorphic event is dated at ~2880 Ma by new monazite in the older gneiss units. A series of variably deformed mafic to felsic dykes and pegmatites intrude both the granites and gneisses and constrain the end of penetrative deformation in the area. Of these, a boudinaged diorite dyke, with a strong internal foliation parallel to the regional fabric, is dated at 2877 ± 3 Ma. A younger granodiorite dyke that crosscuts the regional fabric at a high angle and has only a weak internal foliation yields an age of 2864+3-9 Ma. An undeformed syenogranite pegmatite, which represents a suite that intrudes all other units in the study area, has a combined zircon-monazite age of 2852 ± 3 Ma. The varying degrees of deformation in these minor intrusive rocks constrains the end of deformation in the study area to ca. 2850 Ma. This contrasts with data from the rest of the Slave Province, where the main phases of deformation, metamorphism, and synmetamorphic plutonism have been dated at ca. 2.62-2.59 Ga. Metamorphic titanite ages from the diorite and granodiorite dykes indicate two lower amphibolite to greenschist facies metamorphic events: one at ca. 2705 Ma and one at ca. 2646 Ma. The youngest Archean magmatic event in the area is represented by granite intrusions at 2594+3-2 Ma, coeval with crystallization of titanite at greenschist-grade conditions in some of the older gneissic and intrusive rocks.

Basin analysis and mineral endowment of the Proterozoic Itajaí Basin, south‐east Brazil

A basin evolution synthesis and an integration of geological and geophysical data, relevant as guides to the exploration of gold and lead–zinc deposits in the Itajaí Basin, are presented in this paper. The Itajaí Basin is interpreted as a collision‐related foreland basin consisting of weakly metamorphosed sediments deposited between the structural front of the Dom Feliciano fold and thrust belt and the proximal flank of the cratonic forebulge. Its sediments represent a second‐order depositional sequence deposited during a foreland transgression–regression cycle related to flexural subsidence. After deposition, the basin underwent a main late‐collisional compressional deformation phase followed by an extensional post‐orogenic relaxation. Known gold and lead–zinc deposits are associated with late‐orogenic faulting of the Itajaí Basin sediments. The gold‐bearing quartz veins are of filonean hydrothermal affiliation, while the lead–zinc deposits were formed by solution‐remobilization in a meteoric–connate–magmatic mineralizing fluid. Major trends of favourability for such deposits are recognized. The most favourable sites for lead–zinc deposits are near known mineralized areas and also along a NE‐orientated fault at the margin adjacent to the Dom Feliciano metamorphic belt. The higher favourability for gold deposits is assigned to an area along the same NE trend, and also around a small known deposit near the cratonic margin.

Wide‐angle seismic transect across the Torngat Orogen, northern Labrador: Evidence for a Proterozoic crustal root

A refraction/wide‐angle reflection seismic transect across the Labrador peninsula covers the Core Zone of the SE Churchill Province, the Paleoproterozoic Torngat Orogen, and the Archean Nain Province including a portion of the Labrador continental margin. An airgun array was used as source, and 11 ocean‐bottom seismometers and 16 land stations recorded the shots. Forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the Torngat Orogen, with a crustal thickness of >49 km and with P‐wave velocities of 6.9–7.0 km/s. The geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the Nain crust. Gravity modeling suggests a correlation of the crustal root with a gravity low that extends ∼100 km in an E‐W direction and ∼200 km from north to south. The preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the SE Churchill Province. A discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation. West of the Torngat Orogen, the crustal thickness in the Core Zone of the Churchill Province varies between 35 and 38 km (P‐wave velocities of 5.8–7.0 km/s). East of the orogen, the crystalline crust in the Nain Province is ∼38 km thick (velocities from 5.8 to 6.9 km/s) but thins to 9 km in the shelf area of the Labrador margin, where it is covered with up to 8 km of sediments. No high‐velocity zone was found beneath the thinned continental crust at the margin.

Relations between folds and stretching lineations in the Verrucano of Pisani Mounts (northern Apennines)

Structural analysis of the Triassic siliciclastic Verrucano sequence of the Pisani Mounts (northern Apennines) revealed the presence of two main ductile deformation phases: an earlier deformation phase (D1), attributed to continental collision, followed by a second deformation phase (D2) related to the tectonic collapse of the tectonic units. The D1 phase is characterized by F1 folds, sometimes cleavage transected, by an S1 penetrative axial plane foliation and by a stretching lineation L1, that is parallel to F1 fold axes. Structural analysis and finite strain determinations allow the authors to discuss some models that could explain this parallelism in the Pisani Mounts.

The tectonometamorphic history of the peridotitic Chiavenna unit from Mesozoic to Tertiary tectonics: a restoration controlled by melt polarity indicators (Eastern Swiss Alps)

Occurrences of mantle and lower crustal rocks are often preserved in mountain ranges. When they occur in the internal part of collision belts, their initial geometry and palaeogeography are sources of debate. The mafic–ultramafic Chiavenna unit is one example in the Pennine units located in the Eastern Swiss Alps. Because of strong deformation and metamorphism during Tertiary collision tectonics, the pre-collision geometry of this unit can only be restored using a qualitative geometrical analysis of the successive ductile deformation phases (D1–D4). In-situ melting and melt migration during the main Alpine deformation phase (D2) is a way-up indicator and is of special importance to this restoration. The restored Prealpine geometry of the Chiavenna unit indicates that a sub-continental mantle close to a thinned continental margin becomes denuded during the Mesozoic extension and subsequently covered by carbonate sediments. Taking into account the position of the Chiavenna unit in the nappe pile and its PT-path, a northern Briançonnais situation during Mesozoic time becomes most probable.

Hercynian late-post-tectonic granitic rocks from the Fornos de Algodres area (Northern Central Portugal)

The Fornos de Algodres Complex (FAC) comprises several intrusions of late-post-tectonic Hercynian granitic rocks ranging in composition from hornblende granodiorites and quartz monzodiorites, through coarse porphyritic biotite granites and two-mica granites (coarse-, medium- and fine-grained), to muscovite-rich leucogranites. Field and regional constraints show that the emplacement of this large, composite, batholithic complex post-dates the main Variscan regional deformation phases (D 1+D 2+D 3) and associated metamorphic events. Field, petrographic and geochemical data suggest a strong genetic relationship between most of the members of the FAC. However, their Rb–Sr and Sm–Nd isotopic signatures appear to rule out any genetic process involving a single homogeneous source and/or closed-system fractional crystallization of the same parental magma. A model involving hybridization of mantle-derived basaltic liquids with crustal anatectic melts followed by further contamination and fractional crystallization is proposed to explain the isotopic and geochemical variation trends defined by the FAC granitic rocks.

The effects of brittle-plastic transitions in basement-involved foreland belts: the Central Southern Alps case (N Italy)

The Central Southern Alps (CSA), a 100-km-wide basement-involved fold-and-thrust belt, formed during the Europe-Africa convergence from the Cretaceous onward. Two main phases of deformation have been recognized in the Central Southern Alps. The first is a pre-Late Oligocene thrusting of pre-Alpine basement units upon Triassic cover sequences which is sealed by the Adamello intrusion (42-30 Ma) in the northern sector of the belt. The folds and thrust faults of this phase are locally overprinted by a post-Adamello deformation phase, possibly corresponding to the Middle-Late Miocene (16-6 Ma) Milano phase, recorded by the Oligo-Miocene sediments buried under the Plio-Pleistocene Padane plain. Several lines of evidence indicate that compressive deformations, originating from the current relative convergence between Eurasian and Adriatic plates, are still occurring within the Southalpine wedge, which can therefore be modelled as a critical wedge. A simple brittle rheology is not adequate to explain the slope and plateau topographic features of the belt which compare more favourably with those predicted by a critical taper model including the effects of temperature-dependent deformations. Low-angle surface slopes, typical of frontal sectors of wedges deforming with a purely brittle behaviour, are interrupted by relatively steeper slopes, where the wedge base crosses the brittle-plastic (B/P) transition and starts creeping while the basal décollement, deforming at higher strain rates, still behaves frictionally. Further to the rear, where both the wedge base and the décollement are plastic, the surface slope flattens (plateau). Calculated brittle-plastic transition depths along the basal décollement fit well with observed topographic changes. The location of slope-to-plateau topographic changes in the CSA is strongly influenced by surface heat-flow distribution, and the occurrence within the wedge and in particular at the wedge base of rock types characterised by different plastic rheological parameters, is responsible for repeated topographic changes, each related to a different B/P transition. The transition from brittle to plastic behaviour at the base of the wedge is responsible for a significant weakening within the basal décollement, which in turn has dramatic consequences on the dynamics and hence on the kinematics of the foreland fold-and-thrust belt. The maintenance of the critical state of a wedge with a weak plastic basal décollement necessitates a lower amount of internal deformation than that required by completely brittle wedges. Moreover, these deformations are strongly localised in the lower weak portions of the wedge and do not pervasively affect the upper brittle portions. These characteristics have been ascertained in the rear part of the CSA. B/P transitions commonly develop not only within wide mountain belts (e.g., the Andes) but also in narrow ones (e.g., CSA). This means that B/P transitions must be taken into account when modelling the mechanics of foreland fold-and-thrust belts at all scales. The occurrence of B/P transitions must be tested, particularly when topographic features suggest their existence, since only an appropriate rheological behaviour can account for the structural evolution and kinematics derived from field studies.

Analysis of a complex high-strain zone at Cap de Creus, Spain

The structural analysis of a high-strain zone developed in medium- to high-grade metamorphic micaschists from the Cap de Creus area, Spain provides an example of the complex relationships between geometry, strain and kinematics to be found in deep crustal shear zones. This high-strain zone is composed of E-W trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes. The main phase of deformation discussed here, D 2, affects steeply dipping bedding, boudinaged quartz veins and S 1 developed parallel to bedding. D 2 deformation of these features and of syn-D 2 pegmatite dykes gives rise to fold/boudin structures. In map view, a D 2 high-strain zone coincides with a km-scale dextral flexure of S 0, S 1 and S 2, although all small-scale structures on outcrop surfaces close to horizontal indicate a prevalent sinistral shear sense. In addition, a subvertical stretching lineation is present approximately parallel to the axis of the foliation-deflection and thus normal to the apparent ‘displacement’ direction in the high-strain zone. It is proposed that this high-strain zone did not form by horizontal dextral simple shear with a flow plane parallel to the boundaries of high- and low-strain zones but acted as a zone of highly vortical sinistral non-coaxial flow with a strong vertical extension component. The dextral flexure of S 0, S 1 and S 2 can be explained by considering that sinistral non-coaxial flow in the low-strain zones was either slower or earlier that in the high-strain core. This example stresses the necessity of being careful when interpreting displacement directions from the deflection of older fabric elements in deep-seated high-strain zones.

The Caledonian deformation of the Brabant Massif and the Early Palaeozoic in northeast Germany: compared

The Caledonian deformation of the Brabant Massif and the Rügen Early Palaeozoic has been compared by structural analysis including strain determinations, X-ray texture analysis, illite crystallinity and vitrinite reflectivity. Both areas have experienced weak internal deformation under anchimetamorphic to very low-grade metamorphic conditions. The deformation of the Brabant Massif is characterized by (a) a pronounced diagenetic foliation which resulted from compaction and weak extensional tectonics; (b) tilting or weak folding of the Cambrian strata around the Cambrian/Ordovician boundary without internal deformation; (c) a main phase of Caledonian deformation in the Lower Devonian which was associated with open folding and an axial-plane pressure solution cleavage; (d) bending of Caledonian structures from northwest–southeast to northeast–southwest. The internal deformation appears homogeneous with decreasing strain values towards upper structural levels. The strain ellipsoid is predominantly prolate with the main axis of extension parallel to the bedding–cleavage intersection lineation. It is interpreted as the result of superposition of compaction, tectonic shortening and volume loss through pressure solution. The Rügen Early Palaeozoic is less deformed and less metamorphosed under anchizonal conditions. While the upper structural level is almost undeformed, towards the deeper structural levels a spaced and then a continuous slaty cleavage is developed which is successively crenulated by a fracture cleavage. The internal deformation appears inhomogeneous due to several thrust zones with somewhat higher strain values. Outside the thrust zones strain values increase towards lower structural levels, but not significantly. The strain ellipsoid is prolate with a strongly varying orientation of the X-axis. The structural inventory of both areas is comparable with other fold-and-thrust belts. While the Rügen Palaeozoic represents an imbricated thrust-fan of an upper structural level with discrete thrusts and inhomogeneous strain, the Brabant Massif represents a deeper structural level dominated by folding and more homogeneous internal deformation.

Polyphase deformation in the Telupid area, Sabah,Malaysia

The Telupid area located in central Sabah consists of Mesozoic ophiolitic basementoverlain by Cretaceous-Oligocene sediments. Detailed mapping of the area has recognised at least three main phases of deformation. The first deformation is characterised by folding and thrusting of basement rock and older Paleogene sediments trending N70E, with associated N-S left lateral horizontal faults. The second deformation is characterised by imbrication of the basement rock and overlying sediments to the NE, with associated NE-SW left lateral horizontal faults. The third deformation is characterised by thrusting of the earlier deformed basement rock and overlying sediments to the NW. The timing of deformation is uncertain, but based on regional consideration they are interpreted to have occurred during the Middle Eocene, early Lower Miocene and early Middle Miocene, respectively, related to major NW-SE compression in the Borneo region.

Late Oligocene‐early Miocene sedimentary evolution of the foreland basins in the Sicilian mobile belt: the example of the Peloritani area

ABSTRACTIn the northeastern corner of Sicily (Peloritani Mountains) thin bodies of hercynian crystalline basement, covered by Meso‐Cenozoic veneers of sedimentary rocks, represent the highest and innermost Africa‐vergent group of thrust units of the Sicilian Belt. The Peloritani tectonic edifice consists of a set of prevalently middle‐ to high‐grade crystalline rocks (so‐called Fondachelli Unit, Mandanici Unit and Aspromonte Unit) and thrusts over a thin tectonic wedge made of prevalently Mesozoic to Tertiary sedimentary covers overlying pre‐Triassic low‐grade metamorphic rocks (Longi‐Taormina Unit). The tectonic bodies of the Peloritani thrust system are overlain by thick clastic sequences of late Oligoceneearly Miocene age (the so‐called Stilo‐Capo d'Orlando Formation). Previous work has pointed out the ‘molassic’ character of these clastic sequences, which postdate the main deformation phase of the Peloritani belt, started during Oligocene time. New structural data on the crystalline and sedimentary terrains, sedimentological analysis of the outcropping Oligo‐Miocene foreland clastic deposits and their geometric relationships with the substrate, make it possible to recognize the syn‐tectonic character and the timing of deformation of these basin‐fill deposits, which are expressed by prograding clastic fans in the active margin of a foreland‐foredeep system. This system has progressively been involved in the accretion of the Sicilian Belt and migration during the early Miocene towards the more external areas represented by the Sicilide sector. Seen in this light, three different lithological units have been distinguished to prdvide a framework for a review of the palaeotectonic significance of the overall Oligo‐Miocene terrigenous covers of the Peloritani Thrust belt

Geodynamic evolution of the Continental Allochthonous Terrane (CAT) of the Bragança Nappe Complex, NE Portugal

The Bragança Massif, in particular its Continental Allochthonous Terrane (CAT), is a keystone in the comprehension of the pre‐Variscan and Variscan geodynamic evolution of the Eurasian plate, in particular of the high‐grade metamorphic evolution. The Bragança CAT comprises three types of granulites (recording quite distinct metamorphic histories) and paragneisses with enclosed eclogite boudins. It has undergone five characterizable, main deformation phases during two orogenic cycles, separated by the extensive intrusion of mafic/ultramafic layered bodies (types 2 and 3 granulites) into a pervasively deformed granulitic lower continental crust (type 1 granulites). Geochronological data obtained on high‐grade metamorphic rocks from Bragança and other similar rocks in the Variscan fold belt have been typically interpreted in two different ways: polycyclical evolution, Precambrian and Paleozoic (Variscan), or monocyclical evolution (Variscan). This second hypothesis is in clear contradiction with field evidence, with tectonometamorphic data, and even with some geochronological data, as shown in this article. Thus we suggest that the tectonometamorphic evolution of the Bragança CAT, and, by comparison, of other similar CAT in the Variscan fold belt, took place during two orogenic cycles, one Precambrian (Grenvillian? Pan‐African/Cadomian?) and another one Paleozoic (Variscan).

Pressure‐temperature‐time paths and the relationship between collision, deformation and metamorphism in the north‐west Himalaya

AbstractTertiary collision in the NW Himalaya was between the Indian plate and the Kohistan island arc, the leading edge of the Indian plate being subducted beneath the arc along the Main Mantle Thrust (MMT). Porphyroblast‐matrix relationships show that metamorphism within Indian plate cover sediments exposed on the footwall of the MMT was synchronous with ductile shearing which marks the main deformation phase. The P‐T‐t paths, derived from inclusion assemblages and forward modelling of garnet zonation patterns, show that this metamorphism was along a path of increasing pressure. These data are consistent with Indian plate metamorphism occurring during active subduction beneath the Kohistan island arc. Geochronological data date the metamorphic peak as pre‐50 Ma. As initial collision cannot have pre‐dated 65 Ma, there was thus insufficient time for the long time‐scales of metamorphism generally predicted by conductive models of post‐thickening thermal relaxation. The cover sediments show a metamorphic inversion, argued previously as the result entirely of post‐metamorphic thrusting and imbrication. By contrast, basement gneisses exposed beneath the MMT have largely been unaffected by Tertiary metamorphism. The relationship between ductile shearing and metamorphism on the footwall of the MMT during active subduction, and the concentration of the highest metamorphic grades in the stratigraphically youngest rocks, suggests that early Tertiary metamorphism was controlled by dissipative shear heating distributed across the uppermost cover sediments of the subducting Indian plate. This generated a primary, inverted metamorphic sequence shortly after collision, an inversion reinforced by subsequent thrusting associated with accretion of the metamorphic pile onto the base of the over‐riding arc. This conclusion stresses the importance of dissipative shear heating in controlling metamorphism within thrust systems and provides a mechanism by which areas of apparently large regional extent can undergo regional metamorphism over short time‐scales, but in the absence of any magmatic heating.

Origins and ages of Proterozoic granitoids in the Bothnian Basin, central Sweden; isotopic and geochemical constraints

The Paleoproterozoic Bothnian Basin in central Sweden and south-central Finland is dominated by a 10 km thick sequence of metasediments, mainly metaturbidites. These have been intruded by granitoids, which have traditionally been classified as early, late and postorogenic relative to the c. 1.9-1.8 Ga S vecofennian Orogeny. The early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic, while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites. The postorogenic group forms large massifs of mainly granitic composition, and associated pegmatites are rare. An early orogenic tonalité yields a U-Pb zircon age of 1877 ± 6 Ma, which is within the age range for S vecofennian granitoids of this tectonic position elsewhere in Sweden and Finland. A late orogenic granite belonging to the Härnö granites from the southern part of the Bothnian Basin gives a U-Pb monazite age of 1822 ± 5 Ma. This provides a minimum age for the main phases of Svecofennian deformation in the region, and a correlation with the 1.84-1.83 Ga old granite-migmatite zone of southern Finland is suggested. A postorogenic granite belonging to the Revsund granitoids gives a U-Pb zircon age of 1798 ± 8 Ma. Together with previously published age data, this indicates that the Revsund granitoids were emplaced successively during an age interval of at least between 1.80 and 1.77 Ga. REE patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface. A contribution from the early orogenic granitoids is possible but not discerned by these calculations. The postorogenic granites, on the other hand, have too high concentrations of Sr, Ba, Y and HREE to be derived from such a metasedimentary source. The initial ε Nd values for the early orogenic granitoids range between + 2 and −1. Corresponding values for both late and postorogenic granites range between −1 and −3, which is on average slightly higher than those for surrounding metasediments. The initial Sr isotope ratios for the Härnö granites are intermediate between those for the metasediments and the early orogenic granitoids, while the initial Sr isotope ratios for the Revsund granites are distinctly lower. The geochemical and isotopic data support a model in which the Härnö granites were derived mainly from Bothnian Basin metasediments, with a contribution from early orogenic granitoids. The composition of the Revsund granitoids is, however, inconsistent with such a derivation. For these rocks a deeper, as yet unknown crustal source with a significant pre-1.9 Ga crustal component is indicated.

Stratigraphy and palaeodepositional environment of the Palaeoproterozoic volcano-sedimentary Konse Group in Tanzania

The Konse Group is a volcano-sedimentary assemblage of Palaeoproterozoic age located in central Tanzania between the Tanzanian Archaean Craton (to the west) and the high-grade Palaeoproterozoic Isimani Suite (to the east). The Konse Group is unconformably deposited on the high-grade Isimani Suite and it is only mildly metamorphosed to greenschist facies. Although the Konse Group is quite old, its deformational pattern is not complex and the order of superposition of its constituant lithologies can be outlined. There are also abundant primary sedimentary and volcanic structures which are still preserved in it. The Konse Group is therefore one amongst the very few oldest (if not the oldest) sedimentary basins in eastern Africa with preserved stratigraphy and primary structures, hence the basin serves as an important site for palaeodepositional environment, palaeoclimatology, and palaeotectonic studies in Africa and Gondwanaland as a whole. The Konse Group is made up of six lithologically distinct, conformable units. Its basal unit is a well-sorted orthoquartzite (the Mkulula Formation), which is shallow sea or epicontinental sands deposited on the sea shore. The basal orthoquartzite is overlain by a matrix-supported polymictic conglomerate (the Ruaha River Formation), which is either a lag gravel, point bar or channel bar deposit. The conglomerate is, in turn, overlain by thinly laminated silts and muds (the Kilimbe Formation) representing off-shore or shelf zone (distal) facies. The Kilimbe Formation is overlain by the Kikuyu Formation, which is made up of subaqueous volcanics of basic pillow lavas and basic tuffs with lapilli pockets which show low angle, cross-stratification. The subaqueous volcanics are overlain by dolomitic marble (the Ihumbirisa Formation) of the shallow water carbonate shelves fades and they are in turn overlain by the Mhwana Formation, which constitute the top-most unit of the Konse Group. The Mhwana is dominated by fine arenites intercalated with a banded quartz-Fe formation and a Mn-rich quartz formation. The Konse basing is interpreted as a peripheral foreland basin formed at the margin of the Tanzanian Archaean Craton during the :main phase of the Usagaran deformation.

Electrical conductivity and carbon in metamorphic rocks of the Yukon‐Tanana Terrane, Alaska

Electrical conductivity of a water‐saturated quartz‐mica‐garnet‐schist, collected from a surface outcrop near the Denali Fault Zone in the Yukon‐Tanana terrane of east central Alaska, increases slightly with pressure to about 200 MPa. This behavior is unlike that exhibited by other Yukon‐Tanana samples or by most rocks from other locations. Detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 μm thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation. The stringer is probably responsible for the anomalous conductivity change with pressure, making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature. The carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast. The deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism. We interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer, possibly during the main phase of metamorphism and deformation, and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts. Together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth (i.e., >10 km). Brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer, explaining in part why the rock does not exhibit anomalously high conductivity at 0.1 MPa (1 atm) pressure. The brittle deformation is interpreted to have been caused by unloading due to uplift. The observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic, throughgoing fractures.

The role of hybridization in the genesis of Hercynian granitodis in the gredos massif, Spain: inferences from Sr-Nd isotopes

The Gredos massif is one the better exposed granitoid complexes of the Iberian massif. It is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes. The massif also contains mega-enclaves of migmatites with which the granitoids show transitional contacts. Two major magmatic associations have been distinguished in this study: (1) One comprises the granitoids with microgranular enclaves, the enclaves, and basic rocks; (2) the other is formed by leucogranites, intrusive into the former series and free of microgranular enclaves. Field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process. The Sr-Nd isotopic study reveals that this process is complex, relating different end-members of mantle and crustal affinities, and occurred around 295 Ma ago, late with respect to the main deformation phases of the Hercynian orogeny. The granitoids with microgranular enclaves (GME) are part of an overall mixing trend involving Palaeozoic mantle-derived magma and melts of older crustal material. Amphibole-bearing GME, in general, contain greater proportions of the mantle-derived component than the cordierite-bearing GME. The actual mixing processes took place on a variety of scales, sometimes between melts which were themselves hybrids. On a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke. The isotopic signatures of the basic rocks are probably, to a large degree, the result of interaction with crustal melts, though additionally the presence of an enriched mantle source cannot be elmininated. Microgranular enclaves and their immediate hosts have differing initial Sr and Nd isotopic signatures, indicating that isotopic equilibrium was not attained. This suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system. The enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees. Late-stage peraluminous leucogranites have similar initial Nd isotopic compositions to the evolved GME; a crustal source with a radically different Nd isotopic composition or age does not need to be invoked in their petrogenesis.