West-directed Thrust Research Articles

U–Pb apatite geochronology shows multiple thermal overprints within the Neoarchean foreland basement of the Faroe–Shetland Terrane

The Neoarchean foreland basement of the Faroe–Shetland terrane (FST) displays abundant evidence for isotopic resetting of U–Pb systems in apatite between c. 1800 and 200 Ma, interpreted to result from episodic heating pulses associated with regional-scale tectonic events. Major apparent age peaks of c. 1800–1600 Ma broadly correspond to the timing of Nagssugtoqidian–Laxfordian orogenesis >225 km further south. These are thought to reflect widespread heating during late- to post-orogenic delamination that affected a wide area of the orogenic foreland and resulted in a low to middle greenschist-facies static overprint, affecting much of the FST basement. Late- to post-orogenic delamination might also account for major apparent age peaks at c. 1300–1100, 800 and 500–400 Ma, corresponding to, respectively, Grenvillian, Knoydartian and Caledonian orogenic events. However, east-dipping seismic reflectors in the basement west of Shetland may represent the northward extension of the Grenvillian Outer Hebrides Thrust (Zone) and/or Caledonian thrusts and so perturbation of isotherms during west-directed thrusting could therefore also account for these apparent age peaks. Minor apparent age peaks of c. 700, 600, 350 and 200 Ma are most easily interpreted as resulting from enhanced heat flux that accompanied periods of crustal extension prior to and following the Caledonian orogeny.

Plate-tectonic processes at ca. 2.0 Ga: Evidence from >600 km of plate convergence

Abstract We addressed when plate-tectonic processes first started on Earth by examining the ca. 2.0 Ga Limpopo orogenic belt in southern Africa. We show through palinspastic reconstruction that the Limpopo orogen originated from >600 km of west-directed thrusting, and the thrust sheet was subsequently folded by north-south compression. The common 2.7–2.6 Ga felsic plutons in the Limpopo thrust sheet and the absence of an arc immediately predating the 2.0 Ga Limpopo thrusting require the Limpopo belt to be an intracontinental structure. The similar duration (∼40 m.y.), slip magnitude (>600 km), slip rate (>15 mm/yr), tectonic setting (intracontinental), and widespread anatexis to those of the Himalayan orogen lead us to propose the Limpopo belt to have developed by continent-continent collision. Specifically, the combined Zimbabwe-Kaapvaal craton (ZKC, named in this study) in the west (present coordinates) was subducting eastward below an outboard craton (OC), which carried an arc equivalent to the Gangdese batholith in southern Tibet prior to the India-Asia collision. The ZKC-OC collision at ca. 2.0 Ga triggered a westward jump in the plate convergence boundary, from the initial suture zone to the Limpopo thrust within the ZKC. Subsequent thrusting accommodated >600 km of plate convergence, possibly driven by ridge push from the west side of the ZKC. As intracontinental plate convergence is a key modern plate-tectonic process, the development of the Limpopo belt implies that the operation of plate tectonics, at least at a local scale, was ongoing by ca. 2.0 Ga on Earth.

Petrogenesis and Geodynamic Evolution of the Paleoproterozoic (~1878 Ma) Trout Lake Volcanogenic Massive Sulfide Deposit, Flin Flon, Manitoba, Canada

The Trout Lake deposit (1878 ± 1 Ma) is a rhyolite-hosted, bimodal-mafic, volcanogenic massive sulfide (VMS) deposit. It is one of the largest VMS deposits in the Flin Flon mining district, containing 21.8 Mt of Cu-Zn rich ore. The deposit underwent greenschist facies metamorphism and polyphase synvolcanic to postvolcanic hydrothermal alteration. Footwall and hanging-wall lithostratigraphic units of the Trout Lake deposit occur as an upright, steeply NE dipping, homoclinal succession of thrust-repeated volcanic and siliciclastic sedimentary units. The deposit consists of several massive sulfide ore lenses hosted within a felsic volcanic unit that represents a localized felsic eruptive center composed of flows, synvolcanic sills, and volcaniclastic lithofacies with an FIIIb-type tholeiitic to transitional rhyolite composition (Zr = 213–385 ppm; Ybcn = 47–100; La/Ybcn = 0.97–2.33). The felsic unit occurs along strike of an andesitic unit comprising an intercalation of high-MgO andesite (MgO > 5.0 wt %) and Nb-enriched andesite (Nb = 10–13 ppm) flows that are underlain by a footwall basaltic unit composed of flow lithofacies with a low-Ti tholeiitic basaltic composition (TiO2 < 0.48 wt %; Zr = 15–17 ppm; Al2O3/TiO2 = 37–45). Postvolcanic gabbros with back-arc basin basalt composition intrude the volcanic units, which are unconformably covered by a younger argillite unit deposited at ca. <1843 ± 9 Ma. The felsic volcanic and argillite units were repeated and structurally interleaved during west-directed thrusting. The presence of low-Ti tholeiitic basalts indicates high-temperature melting (1,200°–1,500°C) of strongly depleted refractory mantle sources (Nb/Yb 1.45) variably affected by slab-derived hydrous melts. These petrologic characteristics suggest a geochemically heterogeneous mantle wedge that underwent metasomatism by slab-derived components. In addition, zircon saturation thermometry indicates that FIIIb-type rhyolites formed at temperatures >900°C during partial melting of mafic lower crust. Collectively, the occurrence of high-temperature melts indicates that the Trout Lake VMS deposit formed in a hot, intraoceanic, extensional-arc geodynamic setting. Intra-arc rifting, asthenospheric upwelling, and subduction of young and hot oceanic crust are mechanisms that explain the high-temperature magmatism and are features consistent with the formation of the massive sulfide ore lenses. The back-arc basalt geochemical signatures of the postvolcanic gabbros suggest that the tectonic environment of the Trout Lake area evolved from an intra-arc rift, during the deposition of the ore lenses, to an incipient back-arc basin during the intrusion of the gabbros.

High spatial resolution 40 Ar/ 39 Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK)

Abstract: Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40 Ar/ 39 Ar UV laserprobe technique. It has been possible to measure 40 Ar/ 39 Ar age and ‘apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40 Ar/ 39 Ar dating methods. We report a range of complexities including significantly younger narrow (&lt;1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argon contamination. We also report pseudotachylite veins of varying magnitudes that have been subject to later thermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite of these it is still possible to determine geologically meaningful 40 Ar/ 39 Ar ages for pseudotachylite-forming events. In Lewis the c . 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and South Uist a 1300–1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded at c . 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-field effects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland. Supplementary material: 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18372 .

Detrital zircon geochronologic tests of the SE Siberia-SW Laurentia paleocontinental connection

Abstract. Strikingly similar Late Mesoproterozoic stratigraphic sequences and correlative U-Pb detrital-zircon ages may indicate that the Sette Daban region of southeastern Siberia and the Death Valley region of southwestern North America were formerly contiguous parts of a Grenville foreland basin. The Siberian section contains large numbers of detrital zircons that correlate with Grenville, Granite-Rhyolite, and Yavapai basement provinces of North America. The sections in both Siberia and Death Valley exhibit west-directed thrust faults that may represent remnants of a Grenville foreland thrust belt. North American detrital-zircon components do not occur in Siberian samples above a ~600 Ma breakup unconformity, suggesting that rifting and continental separation blocked transfer of clastic sediment between the cratons by 600 Ma. Faunal similarities suggest, however, that the two cratons remained within the breeding ranges of Early Cambrian trilobites and archeocyathans.

Structure and tectonics of subophiolitic mélanges in the western Hellenides (Greece): implications for ophiolite emplacement tectonics

The Jurassic Vourinos ophiolite is part of the Western Hellenide ophiolite belt in Greece and rests tectonically on the Pelagonian microcontinent. The Vourinos and coeval Pindos ophiolite to the west display suprasubduction-zone geochemical affinities, and represent remnants of oceanic lithosphere formed in a rifted incipient arc–forearc setting within the Pindos Basin. In structurally descending order, and from west to east, the subophiolitic mélange beneath the Vourinos ophiolite contains the Agios Nikolaos Formation (ANF) and a rift assemblage, both of which display ENE-vergent thrust faults, shear zones, and folds. The ANF comprises schistose mudstone with pebbles, cobbles, and boulders of arenite and wacke derived from the crystalline basement of Pelagonia. Imbricated along ENE-directed thrust faults and metamorphosed up to lower amphibolite facies, the ANF represents continental rise deposits of the rifted Pelagonian margin. The rift assemblage includes blocks of basaltic lavas, ribbon chert, micritic cherty limestone, metagabbro, dolerite dikes, and serpentinite breccia that are commonly in thrust contact with each other and are tectonically imbricated with the Pelagonian carbonates; however, primary intrusive and depositional contacts are locally well preserved. Gabbro and dolerite dikes are locally intrusive into the recrystallized carbonates and metapelitic rocks of the Pelagonian microcontinent. Lavas display mid-ocean ridge basalt within plate basalt affinities and represent Upper Triassic rift units that erupted during the separation of Pelagonia from Apulia. Gabbro, dolerite, and serpentinite breccia are the products of a magmatic rifting episode prior to the onset of seafloor spreading in the Pindos Basin. The Vourinos subophiolitic mélange thus consists of passive margin and rift assemblages that were tectonically overridden by the Vourinos ophiolite in the Middle Jurassic. The Avdella mélange beneath the Pindos ophiolite to the west includes a chaotic matrix including ophiolitic clasts and blocks, reefal to pelagic limestones, and olistostromal turbidites that range in age from Triassic to Jurassic–Cretaceous. The Avdella mélange rests tectonically on a Cretaceous–Eocene flysch unit and the Cretaceous–Eocene shelf and slope deposits of the Apulian margin along west-directed thrust faults. The evolutionary history of the Vourinos and Avdella subophiolitic mélanges indicates diachronous tectonic emplacement of the Mesohellenic oceanic slab, first in the east as a result of a trench-continent (Pelagonia) collision in the Middle Jurassic, then in the west due to the Eocene collision of Apulia with Eurasia.

Triassic and Jurassic Tectonics in the Eastern Yanshan Belt, North China: Insights from the Controversial Dengzhangzi Formation and Its Neighboring Units

The Jurassic geology of the Niuyingzi–Dengzhangzi area of western Liaoning province south of Lingyuan has challenged geologists for many years. Its most controversial structural element is a narrow (0.1–1.5 km), subvertical, fault-bounded panel of Middle Proterozoic Changzhougou quartzite and, locally, higher Changcheng System units. The 35 km-long, NNE-striking panel is bordered on the west by the subvertical Daqizi fault (new name) and the steep, largely east-overturned Late Triassic–Early Jurassic Dengzhangzi Formation. It is bordered on the east by the subvertical Zhuzhangzi fault (new name) and west-dipping Middle Jurassic Guojiadian Formation strata. Previous structural interpretations have favored ideas that the Daqizi fault is a steep normal fault with Dengzhangzi side down, and that the Zhuzhangzi fault is a steep reverse fault with Guojiadian side down. We disagree with both interpretations and present evidence that both faults have formed during previously unrecognized Jurassic tectonic events. The Daqizi fault is interpreted as a late Early Jurassic to early Middle Jurassic west-directed thrust fault placing Changcheng strata across lower-plate Dengzhangzi strata. Both upper and lower plates were then folded. The Zhuzhangzi fault was originally a normal fault bounding a Guojiadian half-graben that formed during an episode of previously unrecognized Middle Jurassic extension. The fault was localized along the folded, east-dipping contact between Changzhougou quartzites and overlying shales of the Chuanlinggou Formation. As the fault developed, detached hanging wall Chuanlinggou and higher Changcheng strata above the Changzhougou quartzite were dropped down and buried beneath the half-graben fill. The geometry of the Dengzhangzi–Changzhougou–Guojiadian structural assemblage was subsequently modified by late Middle Jurassic or early Late Jurassic contraction. Among its effects were (1) reverse fault inversion of the Zhuzhangzi normal fault and (2) eastward Yangzhangzi–Wafangdian thrusting of Jixian strata across the Changcheng fault-bounded panel and its neighboring Jurassic units. Steepening and eastward overturning of Dengzhangzi strata occurred at this time. The results of our study indicate that the Jurassic tectonic history of the western Liaoning area was considerably more complicated than believed earlier.

Permian–Triassic amalgamation of Asia: Insights from Northeast China sutures and their place in the final collision of North China and Siberia

The Central Asia Orogenic Belt (CAOB) corresponds to the domain where Siberia and Mongolia were welded to North China. The eastern extension of the CAOB in Northeast China is disputed, since both suture location and timing are poorly documented. This paper reports for the first time the recognition of two suture zones in the southern part of Northeast China (Manchuria), between the Fushun Mishan and Yilan-Yitong faults. In the Jilin Province, west-directed thrust sheets involving successively, from west to east, passive continental margin rocks, metamorphic rocks and ophiolites, block-in-matrix formations and arc plutons indicate a Permian–Early Triassic collision. In the Liaoning Province, arc plutonism and top-to-the-north ductile shearing, coeval with the emplacement of an ophiolitic nappe, suggest a Palaeozoic collision. These two sutures are correlated with the Ondor Sum and Solonker sutures, described in Inner Mongolia. A new geodynamic model involving rifting and collision of the southern part of the Xilinhot Block with North China is proposed.

Mirdita Zone ophiolites and associated sediments in Albania reveal Neotethys Ocean origin

The Mirdita Ophiolite Zone in Albania is associated with widespread melanges containing components of up to nappe-size. We dated matrix and components of the melange by radiolarians, conodonts, and other taxa. The components consist of radiolarites, pelagic limestones and shallow-water limestones, all of Triassic age, as well as ophiolites. Triassic radiolarite as a primary cover of ophiolite material proves Middle Triassic onset of Mirdita ocean-floor formation. The melange contains a turbiditic radiolarite-rich matrix (“radiolaritic flysch”), dated as Late Bajocian to Early Oxfordian. It formed as a synorogenic sediment during west-directed thrusting of ophiolite and sediment-cover nappes representing ocean floor and underplated fragments of the western continental margin. The tectonic structures formed during these orogenic events (“Younger Kimmeridian or Eohellenic Orogeny”) are sealed by Late Jurassic platform carbonates. The geological history conforms with that of the Inner Dinarides and adjoining areas; we therefore correlate the Mirdita-Pindos Ophiolite Zone with the Vardar Zone and explain its present position by far-distance west-directed thrusting.

Structural geology and Zn-Pb mineral occurrences of northeastern Cornwallis Island: implications for exploration of the Cornwallis Fold Belt, northern Nunavut

Abstract The Stuart River and Caribou Lake areas of northeastern Cornwallis Island, Nunavut are dominated by Middle Ordovician to Upper Silurian carbonate and shale, and Lower to upper Middle Devonian carbonate and sandstone. These units have been thrust faulted and have undergone multiple phases of folding. Four deformational events are recognized. D1 develops NNW-SSE trending F1 folds and associated thrusts in Ordovician and Silurian strata. D1 is correlative with movement on the Boothia Uplift (Late Silurian-Early Devonian). D2 develops F2 folds and west-directed thrust faults that are co-axial with F1 but affect late Middle Devonian and older strata. D3 exhibits east-west trending F3 folds and is correlated with the Ellesmerian Orogeny (Late Devonian to Early Carboniferous). F3 folds re-fold F1/F2 folds creating interference patterns at a variety of scales. D4 is defined by northwest-southeast trending F4 folds. F4 folds overprint F1/F2 folds, and are unaffected when axial surfaces of F3 are crossed. D4 may possibly be associated with the Paleogene Eurekan Orogeny. Hydrothermal dolomite and Zn-Pb mineralization are strongly controlled by pre-exisiting structures. Specifically, they correlate most commonly with thrusts (preferentially east-directed thrusts) and along the hinge surface traces of regional F3 folds.

From N-S collision to WNW-directed post-collisional thrusting and folding: Structural study of the Frontal Penninic Units in Savoie (Western Alps, France)

The Frontal Penninic Units (FPU) south of Moutiers (Savoie), are an important geometrical link between the internal and highly metamorphic Penninic units and the external fold-and thrust-belt of the Dauphinois. They were subjected to large-scale poly-phase deformation, analysed in detail by this work. The deformation history of the FPU, after correlation with that of the more internal Penninic units and the external Dauphinois units, reveals three stages of the tectonic evolution, additionally constrained by recent metamorphic and fission track studies. A first pre- to syn-collisional stage of subduction and nappe stacking during the Eocene led to detachment of the Houiller (“Zone Houillere”) and Subbrianconnais units, followed by subsequent poly-phase (D1 and D2) isoclinal large-scale folding within an accretionary wedge that formed in front of the internal Brianconnais and above the previously subducted Valais ocean. In the frontal parts of this wedge, made up of the Cheval Noir flysch unit, sedimentation was contemporaneous with this deformation until final collision with Europe at the end of the Priabonian. Top-north thrusting in a sinistrally transpressive tectonic scenario is inferred from kinematic indicators. This oblique convergence indicates a lateral position of the Dauphinois during and before collision with the Subbrianconnais-Brianconnais ribbon continent and the Adria plate. The onset of the second stage at around the Eocene-Oligocene boundary reveals an important change towards top-WNW thrusting, related to WNW-directed indentation of the Adria plate. During Oligocene to Early Miocene times the nappe stack of the FPU became re-folded by large-scale F3 folds, together with the more internal Penninic units. Simultaneously, the entire Penninic nappe stack was carried onto the Dauphinois unit along the WNW-directed Roselend Thrust. Hence, the French-Italian Western Alps, characterised by this west-directed thrusting, underwent a different tectonic evolution from that of the Swiss-Italian Alps, were top N-NW thrusting continued during this second stage. Decoupling occurred by dextral movements along the Insubric- Simplon-Rhone fault system. After a period of quiescence within the FPU during the Late Miocene, associated with progressive foreland-propagation of post-collisional deformation into the external massifs of the European foreland deformation, deformation in the FPU resumed again during a third stage, which initiated about 5 Ma ago. This stage is presently still ongoing and characterized by normal faulting.

Structural evidence for the Permo-Triassic tectonic evolution of the Yidun Arc, eastern Tibetan Plateau

Deformation across the Yidun Arc occurred as a two-phase process; (i) Early Triassic deformation of Paleozoic sequences resulting from closure of the Jinsha Jiang Suture, and (ii) Late Triassic deformation of Middle to Late Triassic rocks as a result of final basin closure. Deformation in the western Yidun Arc is dominantly west-vergent. A north–south striking west-directed thrust, the Jinsha Thrust, forms the boundary between the western Yidun Arc and the eastern Qiangtang Block. The Jinsha Thrust incorporates within it ophiolitic material of the Jinsha Jiang Suture, and the structural geometry across the Jinsha Thrust suggests that Early Triassic subduction was east-dipping along the Jinsha Jiang Suture. Deformation within Middle to Late Triassic sequences of the eastern Yidun Arc, eastern Qiangtang Block and western Songpan Garzê Fold Belt is thin-skinned, being restricted largely to a single generation of upright folds and associated thrusts. Late Triassic westwards subduction underneath the combined western Yidun Arc–Qiangtang Block resulted in arc volcanism in the eastern Yidun Arc. Coeval arc volcanics in the northern margin of the Qiangtang Block suggest that the suture along the northern margin of the Qiangtang Block may be correlated with the Garzê–Litang Suture rather than the Jinsha Jiang Suture.

Segmentation of the Laramide Slab—evidence from the southern Sierra Nevada region

During the latest Cretaceous-early Paleogene Laramide orogeny, the lithosphere beneath the southernmost Sierra Nevada batholith and the adjacent Mojave Desert region batholith was sheared off and displaced deeper into the mantle. The lithosphere beneath the greater Sierra Nevada batholith to the north was left intact until mid-Miocene time, when fragments of it were entrained as volcanic xenoliths. The Laramide slab was evidently segmented into a shallow flat segment to the south and a deeper segment to the north. Shearing off of the upper mantle to the south was followed by the tectonic underplating of schists derived from Franciscan subduction complex and possibly forearc basin protoliths. The overlying batholithic crust was deformed, deeply denuded, and tectonically breached westward across its forearc region while the schists were underplated. Westward breachment resulted from a combination of west-directed thrusting and extensional collapse. The westernmost breached rocks were tectonically removed by a combination of trench linked transform and subduction erosion processes. Subsequent tectonic erosion by the Neogene San Andreas transform has left the Salinia and the San Gabriel terranes as dispersed residual fragments of the westward breached arc segment. The shallow slab segment appears to have been ;500 km in width, measured along the plate edge. To the south, the slab descent appears to have remained deep beneath the Peninsular Ranges batholith. Classic Laramide structures of the craton are concentrated in a corridor that corresponds to the shallow slab segment as defined by plate edge relations and the corresponding trajectory of Farallon‐North American relative plate motions when viewed on a pre-Neogene palinspastic base. The plate interior is suggested to have been deformed first by end loading as the shallow slab segment initially descended beneath the plate edge, and then by greater basal traction components as the shallow segment progressed beneath the cratonic region. The subcontinental mantle lithosphere beneath the cratonic deformation zone remained intact through Laramide time. Similar segmented slab topologies have been resolved beneath the modern Andean orogen where the shallow segments are correlated with the subduction of aseismic ridges. A number of researchers have suggested that the Laramide orogeny arose from the subduction of a counterpart of the Hess-Shatsky large igneous province of the northwest Pacific basin. Fragments of rock assemblages that are correlative to the Hess-Shatsky province that were accreted to the Franciscan complex in Laramide time support this view. Recognition of the resulting shallow slab segment and its trajectory beneath North America explains the geographic focusing of Laramide deformation, relative to the rest of the Cordilleran orogen, and the relationships between plate edge and plate interior deformational regimes.

Thrust-related accretion of an Archaean greenstone belt in the Midlands of Zimbabwe

Detailed structural data from the Midlands greenstone belt show that, in contrast to pre-existing models, the evolution of the greenstone belt involved an early episode of thin-skinned thrusting affecting separate lithological domains. These display different sedimentary and structural-metamorphic histories prior to their juxtaposition across steep, west-directed thrust zones. The domains include 3.5Ga old gneiss, 2.67–2.88Ga, mafic–felsic volcanic units and early-syn-tectonic, clastic sedimentary sequences. Concomitant with thrusting along domain boundaries, upright folding and ‘minor’ shear accommodated strain within domains.The early thin-skinned, and later, steeper, thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence, possibly associated with under-plating or low-angle subduction. The clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons, and may have formed on alluvial fans developing in front of west-moving nappes. Strike-slip motion on the main shear zones in the Midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements. A shift of σ1, from E–W to N–S during the late stages of Archaean deformation of the greenstone belt can be attributed to extensional collapse following E–W shortening and crustal thickening of the Zimbabwe craton.

Neogene basins of the northern Rio Grande rift: partitioning and asymmetry inherited from Laramide and older uplifts

Three asymmetric Neogene basins in the northern Rio Grande rift of New Mexico and Colorado — the San Luis basin, the upper Arkansas River graben, and the Blue River graben — are tilted toward large flanking normal faults and lie astride the similarly asymmetric Late Cretaceous–early Tertiary (Laramide) San Juan–San Luis, Sawatch, and Front Range–Gore Range uplifts, respectively. The steep, thrust-faulted side of each uplift is on the same side as the down-rotated side of each of the Neogene basins. In addition, the direction of stratal tilt changes northward across the Villa Grove accommodation zone from east in the San Luis basin to west in the upper Arkansas River graben. This accommodation zone coincides approximately with the northward change from the east-directed San Juan–San Luis uplift to the west-directed Sawatch uplift. These observations, supported by seismic-reflection studies across the San Luis basin and studies of several other superimposed pairs of rift basins and Laramide uplifts, suggest that the basin-bounding normal faults are listric and merge at depth with the older thrusts, which are also listric and root into the crust at about 15–16 km. The Blue River graben is complicated by lack of basin fill and a thrust history along the west side of the Gore Range that is at least as old as late Paleozoic. Nonetheless, the Neogene valley is demonstrably tilted west and lies astride an overall west-directed thrust system, similar to other thrust-and-basin relationships in the northern Rio Grande rift.

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.

Large-scale Hercynian west-directed tectonics in southeastern Sardinia (Italy)

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded.The floor thrust of the Sarrabus unit, the Villasalto thrust, is a westdirected thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerreì, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units.

Large-scale Hercynian West-directed tectonics in southeastern Sardinia (Italy)

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded. The floor thrust of the Sarrabus unit, the Villasalto thrust, is a west- directed thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerrei, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units. © Elsevier, Paris

A late orogenic extensional origin for the infracrustal gneiss domes of the East Greenland Caledonides (72–74°N)

The Caledonides in the Fjord Region of East Greenland are made of a NS-trending basement dome of Archaean to Mesoproterozoic migmatites and gneisses, bordered to the east and to the west by Neoproterozoic (Eleonore Bay Supergroup) metasediments. Emplacement of younger rocks on older ones, abrupt break in metamorphic grade, and kinematic indicators, demonstrate that the east-dipping high-strain zone along the eastern margin of the dome is an extensional detachment (the Fjord Region Detachment) and not a west-directed thrust. The detachment zone cuts across contractional structures in the hanging wall. Amphibolite-grade mylonites, up to 1–2 km thick, developed during the initial stage of movement on the Fjord Region Detachment. P-T estimates from less deformed lenses or layers within the mylonite zone indicate that the mylonite protolith originated from a depth of more than 25 km (>7 kbar). Subsequent deformation occurred under progressively lower P-T conditions. A 10–50-m-thick brecciated, ultracataclasite zone marks the upper part of the high-strain zone. Stratigraphical evidences and isotopic data ( 40Ar 39Ar ) on lamprophyres suggest that extension started in early Late Silurian times (ca. 420 Ma) and continued into the Upper Devonian. The Middle to Upper Devonian molasse basin is an upper crustal response to the extensional movement on the Fjord Region Detachment. The structural and metamorphic relationships observed along and across the Fjord Region Detachment in central East Greenland is comparable to the situation encountered below the Neoproterozoic metasediments in the Ardencaple Fjord region to the north (75–76°N). In the light of this similarity we suggest that the Fjord Region Detachment (or some other extensional shear zone) continues northward to the Ardencaple Fjord region. If so, it seems reasonable to relate exhumation of the eclogitic basement terrain around Dove Bugt to a late Caledonian extensional event. Such a scenario is comparable to existing models for the Western Gneiss Region in the Scandinavian Caledonides, where formation of the Devonian basins is related to orogenic extensional collapse and exhumation of coesite-bearing eclogites. Neither a high-strain zone comparable to the Fjord Region Detachment nor a metamorphic or structural break was observed along the eastern boundary of the gneiss dome. Instead a gradual downward increase in metamorphic grade is observed in the Neoproterozoic metasediments (Peterman Bjerg Group) reaching high amphibolite grade, with local migmatization, at the contact.

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting. The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.