Extensional Shear Zones Research Articles

Post-collisional orogen-parallel large-scale extension in the Eastern Alps

Large-scale extension affected the Eastern Alps during post-collisional lateral tectonic extrusion in Early and Middle Miocene times. The Tauern window was mainly tectonically exhumed by the 160-km pullapart of the rigidly behaving Austroalpine basement blocks forming the tectonic lid of the Penninic contents of the window. An evaluation of the syn-extrusion fault pattern reveals displacements of several tens of kilometers along the low-angle extensional shear zones at the western and eastern margins of the window, and along the important strike-slip fault zones north and south of the window. Large-scale shear is reflected by discontinuities in mineral cooling patterns across important shear zones. The cooling histories of the tectonic blocks show that the window boundaries during exhumation coincide only partly with the classical boundaries of the window defined by thrust units. Reconstruction of the exhumation history shows an asymmetric evolution of the Tauern window. We argue that the eastern and western low-angle extensional shear zones formed in sequence, which is reflected by different evolutions of the fragmentation and uplift history of the Austraolpine tectonic blocks to the east and west of the window. In the Central and Eastern Alps, three structural domes, the Lepontin dome in the Central Alps and the Tauern and Rechnitz domes in the Eastern Alps, were formed by large-scale extension in Miocene times. Total E–W stretch amounts to more than 300 km and results in >70% extension. We propose that the Central and Eastern Alps are part of an extensional province that also includes the Pannonian basin and has a counterpart, separated by the Moesian indenter, in the Rhodope Mountains, the Aegean Sea and the Menderes Massif in Asia Minor. In these regions a number of mainly tectonically exhumed domes or deep sedimentary basins formed by Miocene large-scale extension.

Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States

Application of thermochronological techniques to major normal fault systems can resolve the timing of initiation and duration of extension, rates of motion on detachment faults, timing of ductile mylonite formation and passage of rocks through the crystal‐plastic to brittle transition, and multiple events of extensional unroofing. Here we determine the above for the top‐to‐the‐east Raft River detachment fault and shear zone by study of spatial gradients in40Ar/39Ar and fission track cooling ages of footwall rocks and cooling histories and by comparison of cooling histories with deformation temperatures. Mica40Ar/39Ar cooling ages indicate that extension‐related cooling began at ∼25–20 Ma, and apatite fission track ages show that motion on the Raft River detachment proceeded until ∼7.4 Ma. Collective cooling curves show acceleration of cooling rates during extension, from 5–10°C/m.y. to rates in excess of 70–100°C/m.y. The apparent slip rate along the Raft River detachment, recorded in spatial gradients of apatite fission track ages, is 7 mm/yr between 13.5 and 7.4 Ma and is interpreted to record the rate of migration of a rolling hinge. Microstructural study of footwall mylonite indicates that deformation conditions were no higher than middle greenschist facies and that deformation occurred during cooling to cataclastic conditions. These data show that the shear zone and detachment fault represent a continuum produced by progressive exhumation and shearing during Miocene extension and preclude the possibility of a Mesozoic age for the ductile shear zone. Moderately rapid cooling in middle Eocene time likely records exhumation resulting from an older, oppositely rooted, extensional shear zone along the west side of the Grouse Creek, Raft River, and Albion Mountains.

The Kyonggi Shear Zone of the Central Korean Peninsula: Late Orogenic Imprint of the North and South China Collision.

The crustal-scale Kyonggi shear zone of central Korea has been identified as a major boundary between the Precambrian Kyonggi massif in the south and the Imjingang belt in the north. The latter is an eastward extension of the Qinling-Dabie-Sulu collisional belt of China. Field observations and microstructural analysis indicate that the extensional shear zone evolved from a deep crustal ductile regime to a shallow crustal brittle regime, associated with a rapid uplift of the Kyonggi massif following the Late Permian-Early Triassic collision between the Sino-Korean and Yangtze cratons. A Rb-Sr muscovite age (226+/-1.2 Ma) of the mylonite suggests that the extensional ductile shearing occurred during the Late Triassic.

U–Pb and 40 Ar/ 39 Ar constraints on the Fjord Region Detachment Zone: a long-lived extensional fault in the central East Greenland Caledonides

The high strain zone separating infracrustal ortho- and paragneisses from supracrustal cover rocks in the East Greenland Caledonides has been interpreted variously as (1) a combined Grenvillian and Caledonian thrust, (2) a Vendian extensional shear zone reactivated as a Caledonian thrust, (3) a Caledonian extensional detachment reactivated as a late Caledonian thrust or (4) a late Caledonian extensional detachment. In this study we present new kinematic and geochronological data (U–Pb and 40 Ar/ 39 Ar) from a well-studied segment of the high strain zone in Kejser Franz Joseph Fjord, demonstrating top-to-the-east, normal-sense displacement. Syntectonic peraluminous granites in the infrastructure are interpreted to have formed by decompressional anatexis ( c . 430–425 Ma) and were later deformed to ultramylonites, cataclasites and psudotachylites along the high strain zone. Extension related fabrics overprint earlier and are synchronous with contractional top-to-the-NW thrust faults and associated structures. Muscovite records progressively younger 40 Ar/ 39 Ar cooling ages (from 408 to 388 Ma) downward in the extensional footwall, whereas feldspar from the mylonites yield a closure age of 349 Ma. Foreland-directed shortening in the East Greenland Caledonides lasted until at least Early Devonian times, while orogenic extensional collapse initiated along the zone of high strain during the Late Silurian in an overall collisional setting. Upper crustal thinning continued well into the Devonian along increasingly more narrow and brittle extensional faults.

Detachments and low-angle faults in the northern North Sea rift system

Abstract Several master faults in the North Sea basin tend to flatten to give low dips at depth, and in this sense form detachments in the rift system. Such low angle faults are identified in the western flank of the Viking Graben (Tampen Spur area), where they occur as both intra- and supra-basement detachments. Interference between detachments and steeper faults results in ramp-flat-ramp geometries. In the eastern part of the Gullfaks fault block, a supra-basement detachment is probably associated with anomalously high late Jurassic extension in the Gullfaks Field area. The low-angle Gullfaks detachment also helps explain the presence of sets of parallel east-dipping faults (domino systems), a common feature in the collapsed hanging wall to low-angle detachments. Similar detachments probably exist beneath the Gullfaks Sør block and SE of the Visund fault block. All of these are interpreted as late Jurassic collapse structures directly related to active late Jurassic extensional tectonics. Strong indications of intra-basement detachments are also found in the Tampen Spur area. These detachments are formed by major normal faults that flatten in the basement, as seen beneath the Visund fault block. This geometry may to some extent be related to fault rotation during repeated phases of extension in the Palaeozoic-Early Mesozoic period. However, abrupt flattening of some of the faults in the basement indicates that the master faults follow some of the many pre-existing mechanically weak zones in the basement, primarily low-angle Devonian extensional shear zones or Caledonian thrusts.

Pluton emplacement in the Northern Tyrrhenian area, Italy

Abstract The post-orogenic extensional processes that affected the inner sector of the Northern Apennine orogenic wedge (i.e. the Northern Tyrrhenian region) were accompanied by the emplacement of chiefly anatectic intrusive rocks of Late Miocene to Mid-Pleistocene age. In this paper, we compare geological and structural data from Messinian-Pliocene monzogranitic intrusions located both offshore (Monte Capanne, Porto Azzurro, Montecristo and Giglio) and onshore (Gavorrano and Botro ai Marmi) in the Northern Tyrrhenian region to constrain modes of pluton emplacement. Offshore, eastward non-coaxial extensional shear zones active both in ductile and brittle conditions accompanied the emplacement of the monzogranitic intrusions, and accommodated extension oriented E-W to WNW-ESE. Onshore, N-S dextral strike-slip faulting was active both during and after the late stage of emplacement of both Botro ai Marmi and Gavorrano plutons, and controlled their rise in coincidence with releasing bends. In our interpretation, the N-S, Late Miocene-Pliocene strike-slip faulting constitutes a secondary shear feature in a context of generalized post-orogenic extension, accommodating in the brittle upper crust the non-coaxial ductile extension in the lower crust. In this framework, N-S strike-slip faults localized the rise of early anatectic magma, generated during regional post-orogenic extension and residing at the base of the extending crust.

New zircon ages and regional significance for the evolution of the Pan-African orogen in Madagascar

New 207 Pb/ 206 Pb single zircon evaporation ages for granulites, gneisses and granites in southern and central Madagascar record a widespread Pan-African metamorphic and magmatic event in the period c. 650–556 Ma, but also earlier ages in the range 1890–1710 Ma, inherited from protolith material and reflecting heterogeneous crustal sources. South of the Ranotsara shear zone, metasedimentary gneisses and granulites contain an early population of detrital zircons with ages in the range 1890–1740 Ma; a detrital grain with an age of 899 ± 2 Ma suggests that some sedimentary protoliths were deposited later than c. 900 Ma. Metamorphic zircons have a mean age of 564.2 ± 0.9 Ma. North of the Ranotsara shear zone, our data provide information on the age of source material of metamorphic rocks: 788.6 ± 0.7 Ma for the time of emplacement of the granitic precursor of a granulite-facies charnockite and 650.9 ± 0.9 Ma for the protolith age of an amphibolite-facies migmatitic gneiss. A structurally conformable alkali granite sheet with a crystallization age of 568.7 ±1.6 Ma contains xenocrystic zircons, one of which has an age of 1229.6 ± 1.0 Ma, inherited from the source of the anatectically derived material. The post-tectonic, alkalic Carion granite has an emplacement age of 556.0 ± 1.7 Ma and provides a minimum age for granulite- and amphibolite-facies metamorphism. Our field data indicate that extensional shear zones are common in central Madagascar, locally controlling amphibolite-facies retrogression of granulite-facies rocks and the emplacement of crustal melt granites. These events record the widespread extensional collapse of the Pan-African orogen in Madagascar.

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.

Fluid circulation in the Apuane Alps core complex: evidence from extension veins in the Carrara marble

Abstract In the Apuane Alps (AA) metamorphic core complex, syn-metamorphic mineral deposits are mainly restricted to extensional shear zones in the Lower Plate Palaeozoic basement. By contrast, the extension structures at upper levels, such as the detachment fault, that are typically the seat of fluid circulation and mineralization in other core complexes, are barren in the AA. Extension veins hosted by the Jurassic Carrara marbles are among the few examples of (minor) mineralization located in the upper levels of the AA core complex. Calcite–dolomite geothermometry and fluid inclusion data suggest that the mineralizing process in these veins began under pressure (P)-temperature (T) conditions close to the metamorphic peak (about 400°C, 3 kbar). Progressive cooling and mixing between metamorphic and late stage meteoric fluids were probably responsible for most of the mineral deposition. Batches of relatively saline fluids presumably resulted from interaction with evaporitic levels located along the detachment fault. In agreement with previous estimates, fluid inclusion constraints on the P—T synmetamorphic path of the AA suggest a relatively rapid cooling of the core complex as a result of uplift. However, the maximum estimated geothermal gradient (about 35°C/km) is considerably lower than in other core complexes, where large-scale hydrothermal circulation was associated with extension and uplift. Hence, in the AA, fluid circulation at shallow levels and mixing among fluids of different origin were not favoured, thus precluding the formation of mineral deposits along major extensional structures.

Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures exposed there are crucial to understanding processes of how the Baltic basement and its cover allochthons responded to continental lithospheric subduction and subsequent continental separation. Relatively little is published about the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these spatially isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were affected by Caledonian regional metamorphism, and structures record tops-east contraction and later extension related to late- to post-Caledonian basement exhumation. Tops-west extension is preferentially developed in meter to km-scale ductile shear zones containing west-dipping extensional shear bands, west-verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging, sinistral-oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport. Mesoscopic backfolds are locally developed within the tops-west shear zones and further document west-directed transport of structurally higher rocks. the ductile extensional shear zones locally are cross cut by low-angle, tops-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-striking, high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed Precambrian basement with the Caledonian nappe sequence. ^40^ Ar/ ^39^ Ar data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at approximately 430 Ma) and continued episodically until at least Permian time. Rocks passed through the brittle/ductile transition rapidly at approximately 275 Ma. Recent paleomagnetic data also point to later phases of brittle faulting in Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of the Norwegian Sea. The magnitude and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basins and exposures of Caledonian ultra-high pressure assemblages in the Western Gneiss Region (WGR) of southern Norway. Differences in the styles of extension between northern and southern Norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the Caledonian orogeny.

Deep burial of the footwall of the northern Snake Range decollement, Nevada

Metapelitic Eocambrian strata from eastern exposures of the footwall of the northern Snake Range decollement experienced Mesozoic amphibolite facies metamorphism and greenschist facies overprint accompanying extreme ductile thinning. Samples from the Hampton Creek area where the overprint is weakest contain the assemblage quartz + muscovite + biotite + garnet + plagioclase + opaque oxides + staurolite ± kyanite ± tourmaline ± apatite. Textural relations and consistency of rim analyses among multiple domains in each sample indicate equilibrium. Rim pressure-temperature estimates were obtained using the garnet-biotite Fe-Mg exchange thermometer and the garnet-muscovite-plagioclase-biotite and garnet-aluminosilicate-plagioclase barometers. The results suggest final equilibration at 610 ± 50 °C and 810 ± 70 MPa, or a depth of 30 ± 3 km, about a factor of 3 greater than the stratigraphic depth. To the west, in the Schell Creek Range, correlative strata are contiguous with unmetamorphosed upper Paleozoic and Tertiary strata. From west to east, exposures of Eocambrian rocks in the two ranges collectively show increasing intensity of crystal-plastic deformation, metamorphic recrystallization, and metamorphic grade from subgreenschist to amphibolite facies, and a monotonic decrease in ^(40)Ar/^(39)Ar muscovite ages from Mesozoic age to about 23 Ma. In light of these relations, the pressure-temperature data suggest that Eocambrian strata were inclined eastward during Mesozoic metamorphism with ∼15–20 km of structural relief. We interpret the eastward tilting and burial to be the result of west-directed thrusting, as expressed by folding to the north in the Deep Creek Range. Unroofing of the deeply buried strata may have occurred partly in Cretaceous or early Cenozoic time, with final unroofing in Oligocene and Miocene time along the northern Snake Range decollement. These results exclude the hypothesis that the northern Snake Range decollement initiated as a brittle-ductile transition zone within Cambrian strata of a little-deformed miogeoclinal section, and support the hypothesis that it is a major low-angle extensional shear zone.

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 facies with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels, t 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 produce during D1 deformation coeval with the MMM. The S1 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 wit in 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 o 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 back- thrusting of the Lycian nappes. © Elsevier, Paris

Oligocene-Miocene Bu Khang extensional gneiss dome in Vietnam: Geodynamic implications

A large Oligocene-Miocene extensional gneiss dome in Vietnam has implications for the kinematics of the Ailao Shan–Red River shear zone and rifting of the South China Sea. A major northeast-dipping extensional shear zone separates the Bu Khang dome from less-metamorphosed units above. Shearing deformation occurred during the exhumation of high-pressure rocks. 40 Ar- 39 Ar ages on micas reveal Oligocene-Miocene ages, from 36 to 21 Ma. After a high-pressure episode of probable Oligocene age, the area was subjected to northeast-southwest extension during the opening of the South China Sea. A kinematic model involving left-lateral shear and block rotation within a north-south dextral shear zone integrates our observations in the Bu Khang dome with South China Sea rifting.

The Northern Sacramento Mountains, southwest United States. Part I: Structural profile through a crustal extensional detachment system

Abstract Miocene exhumation of the Sacramento Mountains metamorphic core complex in the Colorado River extensional corridor, southwest United States, produced a range of deformational fabrics: pervasive, synthetic ductile deformation (with associated syntectonic intrusion) occurred at depth, discrete antithetic ductile through to brittle extensional shear zones developed at intermediate crustal levels and brittle cataclasis occurred at upper crustal levels. The region divides into three structural domains on the basis of these fabrics: (1) the eastern domain is dominated by a penetrative, ductile fabric with a consistent mineral alignment and stretching lineation, and sense of shear synthethic with detachment fault motion (top-to-northeast); (2) the central domain is characterized by a continuum of overprinting ductile through to brittle extensional shear zones with motion antithetic (topto-southwest) to detachment faulting; and (3) the western domain, where deformation occurred entirely by brittle cataclasis. Structural and petrofabric data from the Sacramento Mountains suggest that increasing structural depth/temperature is exposed in the direction of tectonic transport (N60E), consistent with detachment faulting. This region is interpreted as a profile through a crustal-scale shear zone in which the central domain of discrete antithetic shear zones formed in response to a combination of non-coaxial shear and vertical shortening, and represents an exhumed brittle-ductile transition. Rapid uplift and cooling of the northern Sacramento Mountains in the Miocene facilitated the preservation of this relatively transient regime.

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 (M 1, 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 (S 1) foliation and approximately N-S trending mineral lineation (L 1) associated with first-order folding (F 1) were produced during D 1 deformation coeval with the MMM. The S 1 foliation was later refolded during D 2 by approximately WNW-ESE trending F 2 folds associated with S 2 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 D 1 deformation, the asymmetry and change in the intensity and geometry of the F 2 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 D 1 deformation and later D 2 deformation are probably related to the northward backthrusting of the Lycian nappes.

Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation

The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551±1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Contraction under greenschist to amphibolite facies conditions resulted in the imbrication of the Core and Cover Series and generated an inverted metamorphic sequence by north-directed thrusting. During Alpine extension, most of the south-dipping thrust faults were reactivated as extensional shear zones under decreasing greenschist facies conditions.

Evolution of the Appalachian Laurentian margin: Lithoprobe results in western Newfoundland

The Humber Zone of the western Newfoundland Appalachians represents the early Paleozoic Laurentian margin established by Neoproterozoic rifting. After a period of passive margin thermal subsidence, Taconian deformation began in the Early Ordovician with westward thrusting. Subsequently, an extensive foreland basin developed beneath the Gulf of St. Lawrence. It records rapid Late Ordovician to Early Silurian subsidence; mid-Silurian erosion; and renewed Late Silurian to Devonian subsidence. The Humber Zone was traversed by Lithoprobe seismic reflection profiles. Within the external part of the orogen, seismic reflections in the upper crustal section appear more coherent where seismic lines are parallel to fold hinges. Some subhorizontal reflectors are interpreted as thrust sheets of shelf limestone, but others probably represent intrabasement structures. A group of moderately northwest-dipping reflections probably represents late extensional shear zones. On the Baie Verte Peninsula, low-angle reflections passing beneath the Baie Verte Line are probably also late extensional shears, possibly reactivating earlier thrusts. Tectonism in the Humber Zone probably began with attempted eastward subduction of the Laurentian margin. Deep burial of the margin, accompanied by eclogite-facies metamorphism, probably coincided with rapid subsidence in the foreland basin. Later Barrovian metamorphism was associated with cleavage development and east-directed shear, and with dextral oblique slip, in Baie Verte Peninsula. Later Silurian sinistral transpression with thrusting east of the Baie Verte Line was followed by dextral transpression to transtension. "Acadian" thrusting dominated the western margin of the orogen in the Devonian and possibly earliest Carboniferous.

Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c. 1.65 Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42 Ga and probably during collisional assembly of island arc crust into new (1.7–1.6 Ga) continental lithosphere. Supracrustal rocks record 650–750 °C, 6–8 kbar metamorphism (M2); these high temperatures may have been reached during sandwiching between c. 1.65 Ga granitoids. However, the early history has been obscured by renewed tectonism at c. 1.4 Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4 kbar (M3). Metamorphic temperature variations from uppermost‐amphibolite to amphibolite facies rocks may be associated with c. 1.65 and/or 1.4 Ga plutonism, but not to a 1.4 Ga extensional shear zone as previously proposed. Syn‐ and post‐1.4 Ga contraction is suggested by high‐ and low‐temperature microstructures showing top‐to‐the‐south‐east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c. 1.65 Ga, but that the present fabric also records 1.4 Ga tectonism involving high‐T metamorphism and fabric reactivation.

Detachments in high-pressure mountain belts, Tethyan examples

Using the case examples of the phyllite–quartzite nappe of Crete, the Bündnerschiefer nappe of the Engadine window in the Alps, and the infraophiolitic units of the Saih Hatat window in the Oman Mountains, we describe the tectono-metamorphic signature of large-scale extensional detachments in high-pressure belts. In these three cases retrograde P–T paths were accompanied by the formation of large-scale extensional shear zones that contributed to the exhumation and extreme thinning of the metamorphic pile. The direction of transport along later detachments was parallel to the direction of convergence, detachments being first localized within or near the brittle–ductile transition zone. Below the major shear zone, or between the shear zones in Oman, the deformation regime was more coaxial and involved a significant component of vertical shortening. Metamorphic data, mostly derived from the petrology of metapelites, show that domains characterized by coaxial strain have nearly isothermal decompression paths (type II), while those immediately below the detachments were subjected to more efficient cooling during exhumation (type I). The detachments thus affect both the deformation history and the P–T evolution of the lower plate. The existence of the two types of cooling paths is a good test of the extensional nature of a tectonic contact.

Change from thrusting to syncollisional extension at a mid-crustal level: an example from the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland)

The Palaeoproterozoic Nagssugtoqidian Orogen in West Greenland represents a mid- to deep-crustal section through a collisional orogen with a complex intrusive, tectonic, and metamorphic history. In the northeastern central part of the orogen, in the Ussuit area, Palaeoproterozoic intrusive and supracrustal rocks are sandwiched between slices of Archaean rocks forming a stack of lithotectonic units. Juxtaposition of these units occurred during west- to northwest-vergent thrusting along ductile shear zones (= D1) associated with a foliation formed at upper amphibolite facies conditions. D1 structures were folded (= FA) and then reactivated and locally omitted by localized east- to east-northeast-vergent extensional ductile shear zones (= D2) at near-peak metamorphic conditions. Shallowly east-plunging, transport-parallel upright folds (= FB) affect but are also truncated by D2 extensional shear zones, suggesting an interplay between FB folding and D2 shearing, compatible with a scenario of overall shortening perpendicular to fold axial surface during simultaneous extension parallel to fold axes. Consequently, the structures preserved in the Ussuit area document a deformation history which results from a change from thrusting to syncollisional extension occurring in mid- to lower crustal levels at peak metamorphic conditions.