Extensional Shear Zones Research Articles

Structural observations adjacent to a large-scale extensional detachment zone in the hinterland of the Norwegian Caledonides

Structural and lithological observations in a N-S traverse across Bremangerlandet, western Norway, demonstrate that the region is separated into two plates by the E-W-striking, S-dipping ductile-brittle Vetvika Shear Zone. The Vetvika Shear Zone places Caledonian high-grade gneisses, with a Precambrian protolith age of the lower plate, in tectonic contact with heterogeneous gneisses of the upper plate. The Vetvika Shear Zone is the uppermost part of the Nordfjord-Sogn Detachment Zone. A gently W-plunging stretching lineation within the S-dipping lower-plate augen gneisses is interpreted as having formed within a several kilometers thick ductile W-dipping extensional shear zone that has undergone later rotation around an E-W axis. A highly attenuated sequence of nappes occur structurally on top of a heterogeneous basement and cover quartzite of the upper plate. Pre-Caledonian structures are preserved locally in the gneissic upper plate basement. The nappes at Bremangerlandet are correlated with the Caledonian Middle and Upper Allochthons, representing Baltic/suspect and exotic terranes, respectively. One large and several medium- to small-scale, brittle, top-to-the-south extensional faults are superimposed on ductile contractional structures in the upper plate. Some of the extensional faults merge downward with the Vetvika Shear Zone. The contractional structures include E-W-trending folds and S-directed thrusts in the cover quartzite, and are superimposed on a pronounced W-E stretching lineation. Conjugate sets of fractures and faults related to E-W extension represent the youngest structures in the upper plate. The Old Red Sandstones (Devonian) of the Hornelen basin rest unconformably on top of extensionally faulted upper plate rocks on Bremangerlandet. The unconformity is partly faulted at Bremanger, and truncated by a regionally extensive late, brittle fault along the basin margin further east. This suggests a long-lived tectonic history for the Vetvika Shear Zone and its eastward continuation, with movement pre- and post-dating the filling of the collapse basin, possibly as an eastward migration of the break-away fault.

1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology

Research Article| March 01, 1994 1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology J.W.F. Ketchum; J.W.F. Ketchum 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar R. A. Jamieson; R. A. Jamieson 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar L. M. Heaman; L. M. Heaman 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar N. G. Culshaw; N. G. Culshaw 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar T. E. Krogh T. E. Krogh 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J.W.F. Ketchum 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada R. A. Jamieson 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada L. M. Heaman 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada N. G. Culshaw 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada T. E. Krogh 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (3): 215–218. https://doi.org/10.1130/0091-7613(1994)022<0215:GGITSG>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J.W.F. Ketchum, R. A. Jamieson, L. M. Heaman, N. G. Culshaw, T. E. Krogh; 1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology. Geology 1994;; 22 (3): 215–218. doi: https://doi.org/10.1130/0091-7613(1994)022<0215:GGITSG>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In the southwestern Grenville province, the parautochthonous Britt domain includes a variety of pre-Grenvillian metamorphic and plutonic rocks that were reworked at upper amphibolite facies during the Grenvillian orogeny. Near Pointe-au-Baril, Ontario, a crustal block containing pre-Grenvillian granulite facies mineral assemblages and pre-Grenvillian to early Grenvillian tectonic fabrics has been identified. The block is bounded on the northwest and southeast by extensional shear zones that may have isolated it from regional late Gren- villian deformation. Multiequilibria pressure-temperature (P-T) calculations for orthopyroxene-bearing mafic rocks suggest conditions of 625-700 °C and 7.2-8.4 kbar for the pre-Grenvillian metamorphism. The granulite facies assemblages were locally overprinted during higher pressure Grenvillian metamorphism, which peaked at 720-775 °C and 10.8-11.5 kbar. U-Pb zircon data from migmatitic, mafic supracrustal gneiss indicate metamorphism and leucosome development at ca. 1450-1430 Ma, in agreement with other pre-Grenvillian metamorphic ages for the Central gneiss belt and Grenville Front tectonic zone. An expanding data base on pre-Grenvillian events in the southwestern Grenville province indicates that high-grade metamorphism at ca. 1450-1430 Ma affected a large region of crust and was coeval with widespread felsic to intermediate plutonism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structure of the Eleonore Bay Supergroup at Ardencaple Fjord, North-East Greenland

The Caledonian structures in the late Proterozoic Eleonore Bay Supergroup of the Ardencaple Fjord area were controlled by the position and shape of the contacts with older basement rocks. Those contacts that are not late faults are likely to have been extensional shear zones of Vendian age which were reactivated in compression during the Caledonian orogeny. This interpretation may aIso be applied to the other large tracts of low grade Eleonore Bay Supergroup sediments in East Greenland, and explains their preservation at the present erosion level, surrounded by Precambrian basement rocks.

Reaction of Precambrian high-grade gneisses to mid-crustal ductile deformation in western Dove Bugt, North-East Greenland

Mid-crustal deformation of an Early Proterozoic high-grade gneiss complex in western Dove Bugt gave rise to at least two sets of nappes. Structures in mylonites in low-angle ductile shear zones associated with the younger nappes indicate north-easterly-directed displacements. The nappes and mylonites are folded by upright to inclined folds that verge north-west and which appear to be associated with decollements that dip south-east. Hornblende, sillimanite and anatectic partial melts that developed with the nappes, mylonites and younger folds show that deformation took place under amphibolite facies conditions. Several lines of evidence suggest that the younger nappes, the mylonites and the upright to inclined folds formed during the Caledonian orogeny. Some pre-Caledonian deformation may be represented by the oldest isoclinal folds. Numerous, small-scale, ductile extensional shear zones and more brittIe fractures that were superimposed across the Caledonian structures are believed to have formed during orogen-parallel collapse which may be related IO Devonian basin development farther south in central East Greenland. Younger fauIts and major joints are correlated with Carboniferous, Mesozoic and Tertiary basin development in North-East Greenland.

The tectonic evolution of the Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract The structural framework of the Quadrilatero Ferrifero is the result of two main deformational events. The first is an extensional event of Palaeoproterozoic age (between 2 100 and 1700 Ma, the Transamazonian Event), which resulted in the formation of Archaean granite-gneiss domes and the nucleation of regional synclines in the overlying Rio das Velhas and Minas supergroups strata. Such regional synclines are continuous with each other and are bordered by structural granite-gneiss highs. At the contact between these units, ductile-brittle to ductile extensional shear zones are developed, representing the dislocation surfaces of Archaean blocks. This tectonism is probably related to the evolution of a metamorphic core complex, formed in the hinterland to the west of a Transamazonian collision zone, which structured the Paramirim Craton. The second event is compressive and associated with the closure of the Pan-African/Brasiliano proto-ocean (650–500 Ma), situated to the east of Quadrilatero Ferrffero (QF). During this event, a west-verging fold-and-thrust belt (FTB) developed, causing inversion, amplification, translation and rotation of the basinal synclines. The FTB affected mainly the eastern portion of QF and obliterated many of the tectonic features of the extensional event.

Late Orogenic extension in mountain belts: Insights from the basin and range and the Late Paleozoic Variscan Belt

From two examples of orogenic domains, some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized. The Basin and Range province and the Variscan belt in the French Massif Central have both suffered important compressional orogenic crustal thickening, and the results of late orogenic processes can be observed in the field. Both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness. Late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed. At the scale of the orogenic domains, the most prominent tectonic features are the metamorphic core complexes (MCC) which expose deformed rocks from the middle crust generally affected by high‐temperature, low to medium pressure metamorphism, partial melting, and widespread granite emplacement. In these MCC, large‐scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations. They show a complete evolution from early deep‐seated ductile deformation (generally achieved under high‐temperature, low to medium pressure metamorphism) to a late shallow brittle stage characterized by cataclastic deformation. The late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits. Two main geometries of MCC are defined that are characterized by differing geometry and kinematics of low‐angle shear zones. In the first case, two low‐angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical MCC (often one system is dominant over the other). The second geometry characterizes asymmetrical MCC bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation. Detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow. Deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain. Strain distribution within the lower crust is less well constrained by field observation; however, analogies between COCORP and ECORS deep seismic reflection profiles give important constraints on crustal structure. Wide zones of highly subhorizontally layered lower crust and a flat high‐amplitude reflection Moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension. A kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains. A weak, hot upper mantle allows large‐scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation. Heterogeneous strain is accommodated by low‐angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust. Combined pure and simple shear occurs along localized shear zones, whereas at the scale of the whole lithosphere, deformation nearly corresponds to a vertical pure shear. Such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics. Thus, as much in the Andean‐type west American Cordilleran belt as in the collision‐type Variscan belt, late orogenic processes produced similar extensional features.

Evidence for extensional shear zones in the mantle, offshore Britain, and their implications for the extension of the continental lithosphere

Deep seismic profiles recorded in the seas around the British Isles have revealed bands of seismic reflections in the uppermost mantle. On the basis of their appearance, geophysical characteristics, and association with major crustal faults, these features are best interpreted as mantle shear zones. Furthermore, in the North Sea, the mantle reflectors appear to follow an extensional trend and, when interpreted as extensional shear zones, help explain the regional stratigraphy of the overlying basin. The crosscutting relationships between mantle reflections to the west of Scotland and a major strike‐slip fault, the Great Glen Fault (GGF), suggest that the mantle structure postdates the GGF, and hence is probably related to subsequent extension. Furthermore, kinematic analysis of lithospheric deformation to the north of Scotland is consistent with the structure observed on the seismic data if the Flannan mantle reflection is an extensional shear zone. Reconstruction of the lithosphere prior to Permian extension restores the Flannan to the edge of the Rockall Trough, a rift probably initiated in the Carboniferous. Thus the Flannan Fault may have developed during Carboniferous rifting. It is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle, and the role of such structures during lithospheric extension is considered.

Structural evidence for extensional exhumation of blueschist‐bearing serpentinite matrix melange, New England Orogen, southeast Queensland, Australia

The footwall of a normal fault in the North D'Aguilar block in eastern Australia contains blueschist blocks within a serpentinite matrix melange. The melange contains mass‐flow deposits and was derived by erosion and redeposition of oceanic lithosphere exposed on the ocean floor by faulting. In mid‐Carboniferous time, the melange was metamorphosed in the epidote‐blueschist facies, mylonitized, and underplated to the hanging‐wall of a subduction zone. An S‐type granitoid, intruded at ∼306–307 Ma, was probably derived by partial melting of subducted metasediments during the peak of a greenschist facies overprint that accompanied transposition of an early high‐P/low‐T fabric. Mylonitic rocks below the normal fault absorbed a component of top‐to‐the‐west simple shear related to movement on that originally west dipping fault, and resulted in juxtaposition of very low‐grade accretionary sequences above polydeformed blueschists in a metamorphic core complex. At deeper levels in the footwall of the fault, bulk deformation was approximately irrotational. Later, the low‐angle fault was folded, and its displacement was transferred westward into a more planar fault splay. The juxtaposition of high‐P serpentinite melange against upper plate rocks probably reflects the mechanical weakness of serpentinite in deformed oceanic sequences, and the role serpentinite plays in strain softening that concentrates deformation along major extensional shear zones.

Exhumation of high‐pressure metamorphic rocks during crustal extension in the D'Entrecasteaux region, Papua New Guinea

Abstract The D'Entrecasteaux Islands of eastern Papua New Guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea‐floor spreading in the west Woodlark Basin. The complexes are defined by mountainous domes (&gt;2500 m high) of fault‐bounded, high‐grade metamorphic rocks (including eclogite facies) intruded by 2–4‐Ma granodiorite plutons. Garnet–clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730–900° C. The jadeite component of omphacite indicates minimum pressure of 21 kbar, suggesting depths of &gt;70 km.The metamorphic rocks have undergone widespread retrogression to amphibolite facies. Retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes. P–T conditions in the early stages of shear zone activity, determined using the garnet–biotite exchange thermometer and the GASP and GRIPS barometers, were 570–730° C and 7–11 kbar. A second phase of re‐equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons. One re‐equilibrated gneiss indicated maximum temperature of 730° C at estimated pressures of approximately 4 kbar. This late, high‐temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite.Two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes. Both closely coincide spatially with high‐temperature metamorphic rocks, the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement. These observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the D'Entrecasteaux Islands metamorphic core complexes.

Deformation resulting from regional extension during pluton ascent and emplacement, central Sierra Nevada, California

Solid-state foliation, lineation, small-scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the Mt Givens pluton (MGP). Mylonite is common throughout much of the ∼1–2 km wide, 10 km long shear zone, with ultramylonite best developed near the contact with the MGP, which itself lacks significant solid-state deformation. Migmatization accompanies ultramylonite formation in the northern half of the zone, but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area. Strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear/pure shear towards the MGP. Microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of ∼3.5 kb and ∼680°C respectively, during shear zone formation, at least at its late stages of development. Zircon Pb/U and 40Ar/ 39Ar ages constrain timing of the high-temperature movement on the shear zone to ∼90 Ma, essentially the age of the MGP, although movement immediately prior to that time appears likely. We speculate that a regional extensional shear zone was developing prior to the emplacement of the MGP, which, as it ascended, heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone. The MGP was near its critical melt fraction during the last several kilometers (?) of its ascent, and could have possessed sufficient viscosity (strength) to impose a weak shear strain on the shear zone rocks, although most of the foliation and extensional features in the zone are probably related to the regional strain field. Late-stage folding of the foliation is attributed to shouldering aside of the wallrock by the MGP during the last increment of its ascent and final emplacement.

Different P-T-t paths and leucogranite occurrences along the High Himalayan Crystallines : Implications for subduction and collision along the northern Indian margin

Different P-T-t paths and a heterogeneous distribution of Tertiary leucogranites are recorded along the High Himalayan Crystallines (HHC), in the collided Indian plate. Rapid subduction with eclogite formation during the early-Eocene is apparently restricted to part of the HHC of northern Pakistan, that is the zone of the first collision between the narrow NW Indian spur and the Kohistan island arc. In northern Pakistan rapid exhumation occurred along a cooling path and the crustally derived leucogranites of Tertiary age are rare or absent. East of the spur the delayed India-Eurasia collision occurred at lower rates of convergence and determined less rapid and deep continental subduction. Here exhumation of the HHC occurred at lower rates and at increasing T, giving a relatively warm thickened crust which underwent extensive anatexis during uplift in Oligocene-Miocene times. In eastern Himalayas, anatexis at medium- and low-P produced numerous cordierite-andalusite-bearing leucogranites which emplaced along extensional shear zones at high structural levels during the Miocene.

Field relations, petrogenesis and emplacement of the Bhagirathi leucogranite, Garhwal Himalaya

Abstract The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions. The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining. The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the y-z plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.

Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya

Abstract Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south. Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted P-T gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab. Cooling rates for the upper MCT zone determined from 40 Ar/ 39 Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende 40 Ar/ 39 Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above c . 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.

Basement–cover relationships in the East Greenland Caledonides: evidence from the Eleonore Bay Supergroup at Ardencaple Fjord

AbstractThe Eleonore Bay Supergroup (EBG) is a 16 km-thick shallow-water sequence of Neoproterozoic age that is preserved within the East Greenland Caledonides in several tracts, surrounded by crystalline gneisses and schistose supracrustal rocks. The apparent downward transition from non-metamorphic EBG into gneiss gave rise to the classic ‘stockwerke’ hypothesis, in which all the metamorphism was regarded as Caledonian, and differences in grade were ascribed to the ascent of a migmatite front to different levels within the orogen. Field and isotopic studies in the 1970s however revealed that the underlying gneisses and schists had undergone orogenic reworking in mid-Proterozoic time; the EBG–basement contact was then interpreted as an approximately bedding-parallel décollement with apparent lag geometry, that is with EBG cover rocks in its hangingwall.Recent work in the northernmost EBG tract, at Ardencaple Fjord, has shed light on the problems posed by the basal relationships of the EBG, and together with regional structural and stratigraphic data leads to the following interpretation. There are two regionally important basement-cover interfaces within the East Greenland Caledonides. The earlier one is between Archaean/early Proterozoic gneisses and early Proterozoic supracrustal rocks, which were pervasively deformed in mid-Proterozoic time and form the basement to the Neoproterozoic Eleonore Bay cover sequence. This was deposited on a vast, continually subsiding shelf that is now preserved in East and NE Greenland and Svalbard, and contains Grenville detritus. EBG deposition was terminated by major extensional faulting of Vendian age; the succeeding Tillite Group is interpreted as a syn-rift sequence, presumably associated with the opening of Iapetus.The EBG–basement contacts that are not late faults are inferred to be extensional shear zones of Vendian age. These were reactivated in compression during the Caledonian orogeny in the Silurian, with metamorphic and fabric convergence, which accounts for the apparent downward transition from sedimentary rocks through schists into gneisses. Caledonian shortening was not large; inversion of the Vendian grabens was incomplete, so that the marginal shear zones retained their lag geometry and large tracts of low grade Eleonore Bay sediments are preserved at the present erosion level, surrounded by Proterozoic basement rocks, within the Caledonian belt of East Greenland.

Metamorphic zonation, migmatization and leucogranites along the Everest transect of Eastern Nepal and Tibet: record of an exhumation history

Abstract A polyphase evolution and a thermal and mineral zonation characterize metamorphism of the High Himalayan Crystallines (HHC) in the Everest transect. After an early Barrovian-type metamorphism (M1: T = 550–680° C; P = 8–10 kbar) caused by subduction and crustal thickening during India-Eurasia collision, exhumation of the HHC occurred at rates which allowed the isotherms to rise and the thickened crust to suffer partial melting. In the upper structural levels, the thermal conductivity contrast between crystalline and upper sedimentary rocks together with the infiltration of metamorphic fluids released during exhumation, favoured migmatization and formation of leucogranite magmas through partial melting of muscovite-bearing metasediments chiefly at medium (M2: T = 650–750° C; P = 4–7 kbar) but also at low-pressure (M3: T = 600–700° C; P = 2–4 kbar). The leucogranite melts segregated as dykes and kilometre-sized bodies into extensional shear zones situated at the uppermost levels of the HHC and at the base of the overlying Tibetan Series. During exhumation, a metamorphic zonation was imposed on the HHC in response to the complex interplay between rise of the isotherms in the thickened crust, emplacement of leucogranites in the uppermost levels of the HHC and conductive cooling near the ‘cold’ Lesser Himalaya and Tibetan Series coupled with thrust imbrication.

Structural evidence for the relationship between thrusts, extensional faults and granite intrusions in the Variscan belt of Galicia (Spain)

New structural and petrofabric data are presented from the Hombreiro pluton, a synkinematically emplaced granite located in the northern part of the Variscan belt of Spain. This pluton shows the imprint of two plastic deformations: the first is related to the motion of the Mondoñedo Nappe, with a top-to-the-east shear sense, and the later one to the Vivero Fault, an extensional shear zone with a top-to-the-west motion. These deformations initiated at high temperatures, close to the granite solidus as shown by the preservation of ⋎c⋎-slip fabrics in quartz. This fact indicates that extensional faulting was activated soon after the thrusting of the Mondoñedo Nappe during which the Hombreiro Granite was emplaced. We propose that the position of the Vivero Fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault.

Relations between sill intrusions and bedding-parallel extensional shear zones in the Mid-continent Rift System of the Lake Superior region

The Mid-continent Rift System is a continuous U-shaped feature extending from Kansas north through the Lake Superior region and then south through Michigan. The Logan Sills on the northwest side of the lake were intruded during the rifting stage (1109 Ma). Deformation features in the country rock adjacent to the Logan sills indicate two episodes of shear strain. The first consists of minor thrusts and mesoscopic shear bands, both suggesting top-to-the-south (southwest) movement parallel to the bedding. The second rotates the previously formed shear bands in the opposite sense, top-to-the-north (northeast) and is localized at the basal contact between the sills and the country rock. We attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding-parallel shear zones that are the surface expression of a south-dipping detachment plane. The detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting. In this scenario the first episode of shear strain might be related to bedding-parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south. The polarity of both events is supported by small scale kinematic indicators such as veins, fiber growths and foliation.

The interaction between oblique and layer-parallel shear in high-strain zones: Observations and experiments

The Scandinavian Caledonides consist of a rigid basement, a weak décollement zone, and a series of crystalline nappes. The basement is cut by oblique, west-dipping extensional shear zones, associated with west-verging folds in the overlying décollement zone. The oblique shear zones may, theoretically, have formed after, prior to, or during post-contractional, top-to-the-west shear movements. Physical modelling of this geometrical/mechanical situation shows that each of the three deformation histories results in a characteristic structure. If the oblique, extensional shear zones postdate the layer-parallel (décollement) shear, a normal-type, oblique shear zone structure occurs. If the layer-parallel shear occurs after the development of oblique shear zones, the resulting structure is characterized by a low-strain region above the oblique shear zone. Some folds typically develop above this “protected” region. However, simultaneous oblique and layer-parallel shear provides abundant asymmetric folds with gently dipping axial planes and sub-horizontal, shear zone-parallel fold axes in the region above the oblique shear zone. A neutral surface may be encountered above, and parallel to the oblique shear zone, separating a lower, thin zone of layer-parallel extension from the main, contractional region. The differences between the three types can be used to interpret field observations of this type of structures. The large-scale, post-contractional structures in the Scandinavian Caledonides are consistent with layer-parallel shear related to back-movement of nappes, synchronous with oblique shear zones in the basement. Similar geometries are found as meso- and micro-scale structures in ductile high-strain zones, where oblique shear bands or shear zones affect more competent layers within the tectonites. Also, these structures compare well with those produced experimentally by simultaneous layer-parallel and oblique shear, and are consistent with the general assumption that oblique shear bands form contemporaneous with the general shear in shear zones.

Petrological constraints and PT path of Devonian collapse tectonics within the Scandian mountain belt (Western Gneiss Region, Norway)

In Norway, post-Caledonian extension controlled the formation and sedimentation of Middle Devonian sedimentary basins. These ‘collapse basins’ were deposited along a low-angle extensional detachment fault on top of Caledonian high-grade metamorphic rocks of the Western Gneiss Region. This paper concerns the petrological analysis of eclogite and high-pressure micaschist collected within the extensional shear zone. Study of gneisses and micaschists surrounding these high-pressure rocks allows description of the complete PT path which characterized this area. Application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high-pressure conditions (ll-15kbar at 500-600°C) was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism. The shape of the PT path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift. We can observe an increase in temperature during the first stages of decompression smaller than predicted by the models. Considering the evidence for extensional tectonic in the study area, we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late-orogenic heating of the Caledonian thickened crust.

Tectonic setting and U‐Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor‐Odin ‐ Pinnacles Area, Southern Omineca Belt, British Columbia

The Thor‐Odin ‐ Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite‐facies rocks that were deformed during Mesozoic‐Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep‐seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U‐Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee décollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal‐scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late‐synkinematic to posttectonic plutons. U‐Pb systematics reveal that zircons in high‐temperature shear zones may have suffered high‐temperature Pb loss, perhaps due to deformation‐ or fluid‐enhanced diffusion, and that monazite systematics from samples from high‐grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age.