D2 Folds Research Articles

Sedimentology and structure of the Malvern Hills Coal Mine, Canterbury, New Zealand

The Malvern Hills Coal Mine provides an opportunity to gain insight into the stratigraphy and sedimentology of the Broken River Formation while addressing the complex deformation of range-front tectonics. An up-to-date geological map, stratigraphic columns and cross-sections have led to a greater understanding of both the mine and regional geology. The stratigraphy reflects a transgressive sequence: the Broken River Formation comprises fining-upwards cycles of flaser- and lenticular-bedded quartz sandstones and carbonaceous mudstones and coal seams representing a coastal swamp environment. Cross-bedded marine quartz sandstones of the overlying Conway Formation are typical of bar or dune settings, and lenses of tidal sedimentation reflect a lower intertidal–subtidal environment. NW–SE compression resulted in D1 tilting and block-faulting along range-front structures. Subsequently, D2 conjugate strike-slip shears bisect the mine forming a deformation wedge. Inside this, D3 folding about NE–SW hinges and D4 listric break-out faulting of the folds has resulted in sheared anticline-syncline pairs.

The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey

AbstractThe boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D1) was manifestly different in intensity and scale in the two formations, whereas the second (D2) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group.

Crustal thickening and attenuation as revealed by regional fold interference patterns: Ciudad Rodrigo basement area (Salamanca, Spain)

The structure of the Ciudad Rodrigo area (Iberian Massif, Central Iberian Zone) has been revisited in order to integrate new geological data with recent models of the evolution of the Iberian Massif. Detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the Variscan belt. The structural evolution shows, in the first place, the development of a regional train of overturned folds with associated axial planar foliation (D1). Towards the lower structural levels, the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone (D2), which extends at least to the deepest parts of the basement exposed in the study area. The amplification and rotation of D1 folds about a horizontal axis also occurred within this shear zone. The flat-lying character of the D2 structures accounts for the attenuation of the previously thickened crust, which developed following gravity gradients during thermal re-equilibration. Subsequent deformation led to the formation of two orthogonal sets of upright folds (D3), representing a new shift between crustal thinning and crustal thickening in the region.

Composite mesoscopic and magnetic fabrics of the Paleo-Proterozoic Wangtu Gneissic Complex, Himachal Himalaya, India: Implications for ductile deformation and superposed folding of the Himalayan basement rocks

The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhumation, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. 40Ar–39Ar dating of biotite from one site from the core of WGC gives an age of 9.3±0.3 (2σ)Ma. It is inferred that the doming of the WGC took place at ∼9Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.

Paleoproterozoic structural evolution of the southern segment of the Jiao-Liao-Ji Belt, North China Craton

The Paleoproterozoic Jiao-Liao-Ji Belt separates the Eastern Block of the North China Craton into two small sub-blocks: the northern Longgang and the southern Rangrim blocks. However, it still remains unknown or controversial about the subduction polarity, collisional deformation and kinematics between two sub-blocks. The southern segment of the belt consists of the Paleoproterozoic Fenzishan and Jingshan groups, and Paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the Jiaodong Complex. All of which are separated from the Jiaodong Complex of Neoarchean TTG gneisses by STZ1 ductile shear zones. Structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the Jiao-Liao-Ji Belt underwent three distinct episodes of folding (D1 to D3) and two stage of ductile thrust shearing (STZ1 coeval to D1 and D2, STZ2 between D2 and D3). The D1 deformation formed penetrative axial planar foliations (S1), bedding-parallel ductile shear zone, mineral stretching lineations (L1), and rarely preserved small isoclinal D1 folds in the Jingshan and Fenzishan groups. In the Jingshan Group, however, penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding-parallel ductile shear zones (STZ1) at a period of about 1956Ma to 1914Ma. The kinematic indicators of STZ1 in the Jingshan Group with resultant prograde peak metamorphism up to granulite facies grade and the Fenzishan Group with peak metamorphism up to amphibolite facies grade indicate NW-directed compression. D2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914–1893Ma. The D2 deformation produced NW-verging asymmetric and recumbent folds, interpreted to have resulted from basement-involved thicken-skin structures. The Jiaodong Complex was also involved into the development of WNW-verging asymmetric tight folds associated with D2 in the Jingshan and the Fenzishan groups. Ongoing collision led to the development of transpressional ductile shearing (STZ2), forming the transpressional Taipingzhuang dextral ductile shear zone between the Jingshan Group and the southern Archean Complex and the transpressional Tading-Xiadian sinistral ductile shear zone between the Jingshan Group and the northern Archean Complex. All three lithotectonic units were superposed during the late D3 deformation with amphibolite facies metamorphism. The D3 deformation developed WNW-trending open to tight upright folds at about 1893–1875Ma. The structural pattern resulting from superimposition of D2 and D3 is a composite synform in the Fenzishan and Jingshan groups. The structural events of D1 and STZ1, and D2 and STZ2 deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites. The structural patterns and deformational history of the Fenzishan and Jingshan groups suggest a southeastward-directed oblique subduction beneath the northwestern margin of the Rangrim Block, and that the final scissor-shaped closure of the rift led to collision between the two blocks to form the coherent basement of the Eastern Block of the North China Craton.

Reactivation of basement: example from the Anasagar Granite Gneiss Complex, Rajasthan, western India

Abstract Amidst the Meso- to Neoproterozoic South Delhi Fold Belt (SDFB) of Rajasthan, India, a sheet-like body of megacrystic Anasagar Granite Gneiss (AGG) embedded in a supracrustal unit consisting of metapelites, quartzite and calc gneiss is exposed. Detailed analyses of mesoscopic and microscopic structures identify four phases of deformation. Lithological relationships coupled with U–Pb dates of zircon indicate that the protolith of the AGG was emplaced within the supracrustal unit during D 1 folding at approximately 1.85 Ga. This event is significantly older than the age of volcanism in the SDFB (0.99 Ga) but probably synchronous with Aravalli Orogeny. Thrusts associated with the easterly-vergent D 2 folds have a ramp–flat geometry and are refolded by coaxial D 3 folds. The petrology of the metapelites indicates that porphyroblasts of staurolite and/or garnet were formed as a function of bulk-rock composition between D 1 and D 3 folding, at the time of the culmination of metamorphism (5.7±1.5 kbar, 560±50 °C). Combining the petrological and structural attributes, it is proposed that the AGG and its enveloping supracrustals might represent the basement of the Delhi Supergroup, which was folded, thrusted and domed up during the South Delhi Orogeny. The cause of the thermal perturbation that triggered the growth of the porphyroblasts in the metapelites is explored.

Constraints on long-lived Mesoproterozoic and Palaeozoic deformational events and crustal architecture in the northern Mount Painter Province, Australia

Meta-sedimentary and igneous rocks preserved in the hanging wall of the Paralana Fault, northern Mount Painter Province, record a protracted (ca. 1600Myr) multi-staged metamorphic and poly-deformational evolution related to Mesoproterozoic and Palaeozoic orogenic events. The Mount Painter Province was located near the margin of both the South Australian Craton and the North Australian Craton, which allows comparisons of the tectonic evolution of these margins and provides context for the evolution of the eastern Proterozoic Australia for both the Columbia and Gondwana supercontinents. Within the Mount Painter Province meta-sedimentary rocks form part of a basin system developed adjacent to the margins of the Gawler Craton and Curnamona Province following the Olarian–Wartakan orogenic system. Sedimentation was followed by ca. 1590–1552Ma ductile deformation (D1–D3) at mid to upper crustal levels preserving a rapid (ca 6Myr) burial and exhumation cycle. Initial deformation is characterised by the development of a layer parallel fabric (S1). S1 is overprinted by recumbent north-verging folds (F2) formed during mid to upper amphibolite facies peak metamorphism (M2), which are overprinted by tight, upright shallow NE–SW plunging folds (F3). D3 folds developed contemporaneous with sinistral oblique shearing and the development of proto-mylonitic to ultra-mylonitic fabrics along the Paralana Fault Zone. The timing of D3 deformation is constrained by the development of a penetrative steeply dipping foliation (S3) in the ca. 1585–1569Ma shallowly emplaced Mount Neill Granite and relatively undeformed granites dated at ca. 1552Ma. Further constraint on the absolute timing of Mesoproterozoic tectonism is provided through the dating of zircon growth from samples of the meta-sedimentary Radium Creek Metamorphics. This dating reveals a 1555±16Ma metamorphic event, interpreted to be the consequence of M3 amphibolite facies, regional metamorphism and accompanying metasomatism. The transition between D2 and D3 is interpreted to reflect a shift from ca. 1591–1585Ma N–S shortening to ca. 1585–1552Ma NW–SE shortening. This episode of orogenesis is correlated with the ca. 1585–1540Ma Kararan Orogeny in the northern Gawler Craton, which distinctly postdates the ca 1600–1590Ma Olarian Orogeny as previously interpreted. The Mesoproterozoic structural architecture was reactivated during Neoproterozoic extension (D4) and the ca. 500Ma Delamerian Orogeny. Delamerian structures are formed in a predominantly brittle regime. Early thin-skinned thrusting was followed by thick-skinned deformation (D5–D6); during which open F5 folds developed above oblique lateral ramps, and produced type 0 overprinting patterns with F3 Mesoproterozoic folds. Steeply plunging F6 buckle folds with SE-trending axial traces are the last phase of deformation recorded. Both Mesoproterozoic ductile deformation phases and the Palaeozoic ductile-brittle to brittle overprints were focussed along the Paralana Fault, emphasising the repeated long-lived evolution of this crustal scale fault system.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

The northwestwards-directed Eocene propagation of the Western Alpine orogen is linked with (1) compressional structures in the basement and the Mesozoic sedimentary cover of the European foreland, well preserved in the External Zone (or Dauphiné Zone) of the Western Alps and (2) tectono-sedimentary features associated with the displacement of the early Tertiary foreland basin. Three major shortening episodes are identified: a pre-Priabonian deformation D1 (N-S shortening), supposedly linked with the Pyrenean-Provence orogeny, and two Alpine shortening events D2 (N- to NW-directed) and D3 (W-directed). The change from D2 to D3, which occurred during early Oligocene time in the Dauphiné zone, is demonstrated by a high obliquity between the trends of the D3 folds and thrusts, which follow the arcuate orogen, and of the D2 structures which are crosscut by them. This change is also recorded in the evolution of the Alpine foreland basins: the flexural basin propagating NW-wards from Eocene to earliest Oligocene shows thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Locally, a steep submarine slope scar is overlain by kilometric-scale blocks slided NW-wards from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features are kinematically consistent with D2 in the Dauphiné foreland. Since ∼32Ma, the previously subsiding areas were uplifted and the syntectonic sedimentation shifted westwards. Simultaneously, the paleo-accretionary prism, which developed during the previous, continental subduction stage, was rapidly exhumed during the Oligocene collision stage due to westward indentation by the Adriatic lithosphere, which likely enhanced the relief and erosion rate. The proposed palinspastic restoration takes into account this two-stage evolution, with important northward transport of the distal passive margin fragments (Briançonnais) involved in the accretionnary prism before the formation of the western arc, which now crosscuts the westward termination of the ancient orogen. By early Oligocene, the Ivrea body indentation, which was kinematically linked with the Insubric line activation, initiated the westward escape and the curvature of the arc was progressively acquired, as recorded by southward increasing counter-clockwise rotations in the internal nappes. We propose that the present N-S trend of the Ivrea lithospheric mantle indenter which appears roughly rectilinear at ∼15km depth could be a relict of the western transform boundary of Adria during its northward Eocene drift. The renewed Oligocene Alpine kinematics and the related change in the mode of accomodation of Africa–Europe convergence can be correlated with deep lithospheric causes, i.e. partial detachment of the Tethyan slab and/or a change in motion of the Adria plate, and was enhanced by the E-directed rollback of the eastern Ligurian oceanic domain and the incipient Ligurian rifting.

Characteristics of pegmatoidal granite exposed near Bayalan, Ajmer district, Rajasthan

The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D2 deformation of the Delhi orogeny along the axial planes of D2 folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses.

Structural development of a high-pressure collisional accretionary wedge: The Samaná complex, Northern Hispaniola

The Samaná metamorphic complex exposes a segment of a high-pressure collisional accretionary wedge, built during Caribbean island arc-North America continental margin convergence. Combined detailed mapping, structural and metamorphic analysis, and 40Ar/39Ar geochronology show that the deformation can be divided into five main events. Early subduction-related D1 deformation and high-P/low-T M1 metamorphism under lawsonite blueschist (325–425°C/12–18 kbar; Rincon Marbles and Santa Bárbara Schists lower structural nappes) and eclogite facies conditions (425–450°C/18–20 kbar; Punta Balandra upper structural nappe), was followed by M2 decompression and cooling in the blueschist facies conditions during D2 folding, thrusting and nappe stacking. 40Ar/39Ar plateau ages and T-t/P-t estimations revealed Late Eocene to earliest Miocene retrograde M2 metamorphism in the different nappes for a consistent D2 top-to-the-ENE tectonic transport, which suggests a general northeastward progradation of deformation. The D3 event substantially modified the nappe stack and produced open to tight folds with amplitudes up to kilometer-scale and the D4 ductile to brittle normal shear zones and faults, and related subhorizontal folding, record a late extensional deformation, which also affects the whole nappe pile. Non-penetrative D3 and D4 fabrics indicate M3 cooling in and under the M3 greenschist-facies conditions. From the Miocene to the Present, the nappe pile was cut and laterally displaced by a D5 sinistral strike-slip and reverse fault system associated with the Septentrional fault zone.

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

The Valais units in Savoy (Zone des Brèches de Tarentaise) have been re-mapped in great detail and are subject of combined stratigraphic, structural and petrological investigations summarized in this contribution. The sediments and rare relics of basement, together with Cretaceous age mafic and ultramafic rocks of the Valais palaeogeographical domain, represent the heavily deformed relics of the former distal European margin (External Valais units) and an ocean–continent transition (Internal Valais unit or Versoyen unit) that formed during rifting. This rifting led to the opening of the Valais ocean, a northern branch of the Alpine Tethys. Post-rift sediments referred to as “Valais trilogy” stratigraphically overlie both External and Internal Valais successions above an angular unconformity formed in Barremian to Aptian times, providing robust evidence for the timing of the opening of the Valais ocean. The Valais units in Savoy are part of a second and more external mid-Eocene high-pressure belt in the Alps that sutured the Briançonnais microcontinent to Europe. Top-N D1-deformation led to the formation of a nappe stack that emplaced the largely eclogite-facies Internal Valais unit (Versoyen) onto blueschist-facies External Valais units. The latter originally consisted of, from internal to external, the Petit St. Bernard unit, the Roc de l’Enfer unit, the Moûtiers unit and the Quermoz unit. Ongoing top-N D2-thrusting and folding substantially modified this nappe stack. Post 35 Ma D3 folding led to relatively minor modifications of the nappe stack within the Valais units but was associated with substantial top-WNW thrusting of the Valais units over the Dauphinois units along the Roselend thrust during W-directed indentation of the Adria block contributing to the formation of the arc of the Western Alps.

Mid-Cimmerian, Early Alpine and Late Cenozoic orogenic events in the Shotur Kuh metamorphic complex, Great Kavir block, NE Iran

The Shotur Kuh complex, exposed in the NE part of the Great Kavir block, is composed of amphibolite facies metaigneous rocks and micaschist, and of lower-grade Permian–Miocene cover sequences that experienced four main deformation phases and at least two metamorphic events. The D1 deformation phase is associated with a prograde metamorphism that in the basement reached amphibolite facies conditions. This Barrovian-type metamorphism with field gradient of 20–22 °C/km was related to collision-induced crustal thickening. The D2 event corresponds to post-collisional exhumational upflow of middle crust, resulting in updoming of the basement core and its top-to-the-Northwest unroofing along a low-angle detachment shear zone at the basement/cover boundary. The D1 and D2 events are considered as Mid-Cimmerian in age because they also affected the Upper Triassic–Lower Jurassic Shemshak Formation and are sealed by the Middle Jurassic conglomerates. The D3 folding event, characterised by NE–SW shortening, also affected the Cretaceous limestones, and it is sealed by Paleocene conglomerates. Considering the Late Cretaceous age of this deformation, it is related to the Late Cimmerian–Early Alpine orogeny that resulted from the Cenozoic closure of the Neo-Tethys oceanic tract(s) and convergence between the Arabian and Eurasian plates. The D4 folding event, characterised by NW–SE shortening, also affected the Miocene conglomerates, suggesting its Miocene or post-Miocene age. This deformation event is associated with Late Cenozoic convergence between Arabia and Eurasia, and it could be combined with a left-lateral activity along the Great Kavir fault-bounding system.

Structure and evolution of the Lewisian Gairloch shear zone: variable movement directions in a strike-slip regime

Synopsis The Gairloch shear zone (GSZ) has long been regarded as a dextral strike-slip shear zone with a northwesterly plunging shear direction. However, the original strain analysis applies only to the northeastern third of the zone and the structure of the remaining two-thirds is considerably more complex. The 6.5 km wide GSZ was formed during late Laxfordian (D3) deformation at c . 1.7 Ga. It was a product of the interaction of D3 folding and dextral strike-slip shear under retrogressive greenschist-facies metamorphism on rocks previously intensely deformed by early Laxfordian (D2) deformation in amphibolite-facies. The GSZ is divided into four units across strike: the Northeastern shear zone with NW-plunging D3 linear fabric; the Flowerdale belt with SE-plunging D2 linear fabric and low D3 strain; the Ard belt with steeply plunging F3 folds; and the Southwestern shear zone, with SE-plunging D3 lineation. Thus the GSZ consists of two high-strain D3 shear zones on either side of a central low-strain belt. Although consistently dextral strike-slip throughout, the D3 shear regime varies from NE-up in the Northeastern shear zone through sub-horizontal strike-slip in the Ard belt to SW-up in the Southwestern shear zone. This variation may have been caused by local uplift or depression induced by variations in geometry of the walls of the strike-slip system. A revised very approximate minimum estimate of the horizontal component of dextral shear is c . 9.5 km with a net NE-up vertical component of c . 5.5 km. Similarities in structure between the GSZ and the South Harris belt strengthen the case for these two zones to have been part of the same structure.

Hot metamorphic core complex in a cold foreland

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Visean/Namurian boundary (≤320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D1), all structural levels shared the same tectonic evolution: D2 (extension and exhumation), D3 (refolding) and post-D3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M2). M2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D2 effects defines a bulk prolate strain in a “pinched pull-apart” setting. Ductile D2 deformation during M2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D1 is only represented by the inverted position of bedding (overturned limbs of recumbent D1 folds) and by refolded D1 folds. U–Pb monazite and zircon ages and K–Ar muscovite ages are in accord with Ar–Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at ≥330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shear-zone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward propagating rift of the Palaeotethys ocean, which helped to destroy the Variscan orogen.

Major nappe-like D2 folds in the Dalradian rocks of the Beinn Udlaidh area, Central Highlands, Scotland

ABSTRACTTwo regional-scale, recumbent folds control the structure of the Beinn Udlaidh area, Tyndrum, Perthshire. They reached their maximum development during D2, following the regional metamorphic peak, and are part of a stack of larger SE-facing recumbent folds formed during the ∼470 Ma Grampian Orogeny. The rocks belong to the Neoproterozoic–Lower Ordovician Dalradian Supergroup, and preserve a sedimentary transition between the Grampian Group and the overlying Appin Group. The latter occupies the core of the S-facing, recumbent Beinn Udlaidh Syncline (D2) which, with the underlying complementary Glen Lochy Anticline, is gently folded by a regional-scale structure, the Orchy Dome. The recumbent folds postdate an early fabric (S1), which is generally obliterated by the D2 imprint, but preserved as inclusion trails in regional metamorphic garnets, that are highly oblique to, and wrapped by, S2. It is concluded that the Dalradian rocks described here from below the Iltay Boundary Slide are in structural continuity with those of the Tay Nappe above, and that the Slide represents a structurally-modified disconformity between the Leven Schist (Appin Group) and the overlying Ben Lui Schist (Argyll Group). The Orchy Dome probably influenced the spatial distribution of minor intrusions and explosion vents of the lamprophyre suite.

Variscan structure in the eastern part of the Pallaresa massif, Axial Zone of the Pyrenees (NW Andorra). Tectonic implications

AbstractThe Variscan structures of the Axial Zone of the Pyrenees are traditionally separated into the “infrastructure” and the “suprastructure” domains. Three deformation events have been recognized within Cambro-Ordovician rocks in the eastern part of the Pallaresa massif. D1 consists of a slaty cleavage (S1), and no folds have been found associated with it. The D2 event is characterized by E-W trending and north-verging recumbent folds with an associated foliation (S2) that can be observed at all scales. The S2 foliation has a horizontal orientation, and is the main foliation in the northern and central parts of the study area. The D3 deformation event is distinguished by development of E-W trending, upright folds, of centimetric to metric scale. These folds are associated with a steep and rough crenulation cleavage (S3) in the southern part of the study area. Subvertical faults trending E-W are associated with D3 folds. Most of the structures recognized in the eastern part of the Pallaresa massif have developed under low metamorphic grade conditions, although close to the Aston and Hospitalet domes, in the vicinity of some igneous intrusions, the metamorphism is coeval with S2 foliation and may reach high grade conditions. A gradual transition can be observed from zones characterized by structures of type D2 to zones exhibiting a better development of D3 structures. This transition can be explained by a décollement level located in the Cambro-Ordovician succession. This level can be related to faults and D3 folds observed in the eastern part of the Pallaresa massif. Most of the structures can be explained in a contractional geological setting. In addition the relationships between the different Variscan structures of the eastern part of the Pallaresa massif are better described. The different models recently presented by some authors about the Variscan structure of the Pyrenees are discussed in the light of these new results.

Succession of structural events in the Goren greenstone belt (Burkina Faso): Implications for West African tectonics

Ten years after field investigations in the SE Goren greenstone belt (GGB) of Burkina Faso by the Sanmatenga J.V., sponsored field studies conducted in 2007 have significantly enhanced structural datasets. The studies in 2007 were conducted across an expanded area of the GGB that included both southwestern and northeastern domains, and portions of the Pissila batholith to the west of the GGB. A revision of tectonic models proposed by Hein et al. [Hein, K.A.A., Morel, V., Kagoné, O., Kiemde, F., Mayes, K., 2004. Birimian lithological succession and structural evolution in the Goren Segment of the Boromo-Goren Greenstone Belt, Burkina Faso. Journal of African Earth Sciences 39, 1–23] is now possible. Three deformation events characterise the Goren greenstone belt. The deformation, D1 (previously D3) resulted in the formation of NW to NNW-trending steeply-dipping dextral-reverse shear zones folds and a penetrative S1-C schistosity that formed during a period of NE–SW shortening. The event is termed the Tangaean Event because it can be correlated across NE Burkina Faso in the Boromo, Bouroum, Yalago and Oudalan-Gorouol greenstone belts. The deformation, D2 (previously D2) resulted in the progressive development of NNE to NE-trending macroscopic to mesoscopic folds and a penetrative axial planar cleavage (S2), which was followed by the formation of dextral- and sinistral-reverse shears and a pervasive schistosity (S2-C). The first-order crustal-scale Sabce Shear Zone, which traverses the northern portion of the study area, is associated with macroscopic anticlockwise drag rotation of NW to NNW-trending D1 shears and folds: (the macroscopic fold was previously classified as D1). D2 in the GGB corresponds with the Eburnean Orogeny at 2130–1980 Ma, as described by [Feybesse, J.-L., Billa, M., Guerrot, C., Duguey, E., Lescuyer, J.-L, Milesi, J.-P., Bouchot, V., 2006, The paleoproterozoic Ghanian province: geodynamic model and ore controls, including regional stress modelling. Precambrian Research, 149–196]. The deformation D3 (previously D4) is recognised throughout the GGB. It is characterised by the formation of kinks and chevron folds (F3), or crenulation cleavage (S3) that are hosted by narrow WNW-trending shear zones. These formed during a period of north–south shortening termed the Wabo-Tampelse Event that post-dates the Eburnean Orogeny.

The Timmins-Porcupine Gold Camp, Ontario: Anatomy of an Archean Greenstone Belt and Ontogeny of Gold Mineralization

New structural and geochronological data and interpretations from the Timmins-Porcupine gold camp, Abitibi subprovince, more precisely define the stratigraphy and the generation, timing, and effects of deformation. Tisdale (basalts) and Porcupine (volcaniclastic and turbiditic sedimentary rocks) assemblage rocks exhibit three major foliations (S2, S3, S4, plus a late constrictional fabric), of which S3 and S4 are also present in younger Timiskaming assemblage sedimentary rocks. Pre-Timiskaming folding has long been recognized in Tisdale assemblage rocks, but no foliation had previously been attributed to such an event. D1 regional uplift and extension produced a low-angle unconformity between the Tisdale and Porcupine assemblages, with partial excision of upper Tisdale stratigraphy and perhaps of the Blake River assemblage. D2 involved imbrication of a set of south-over-north thrust sheets rooted in the Porcupine-Destor deformation zone. D3 en echelon folds along the northern flank of the Porcupine-Destor deformation zone are a result of left-lateral strike-slip along this zone. The Timiskaming basin, lying along the curvilinear trace of the Porcupine-Destor deformation zone, formed as a D3 transtensional half graben, and the Dome fault formed its faulted margin. S4 foliation formed during subsequent right-lateral strike-slip movement and folding of S3 foliation. Intense D5 constrictional strain deformed earlier lineations, as the previously dilatational jog that hosts Timiskaming assemblage rocks acted as a compressional jog during right-lateral movement along the Porcupine-Destor deformation zone. These D2 to D5 phases represent an overall thrusting–strike-slip transpressional regime. There were several phases of gold mineralization in the Timmins-Porcupine gold camp. Clasts of ankerite veins occur in Timiskaming assemblage conglomerate at the Dome mine, and D3 foliation and folds crosscut early ankerite veining and alteration and some quartz veining. Thus, low-grade mineralization and possibly the quartz-fuchsite veins predate Timiskaming sedimentation. However, the bulk of gold postdates Timiskaming sedimentation. Quartz-carbonate-tourmaline-gold veins (Hollinger-McIntyre, Hoyle Pond, Dome, Aunor–Delnite mines) formed within Timiskaming assemblage and Tisdale assemblage rocks. These gold deposits developed largely as oblique slip and extensional vein arrays formed during north-south shortening and local strike-slip faulting. Younger deposits (Pamour) are associated with D4 dip-slip faulting and were weakly deformed during late D4. Thus, the largest known Archean lode gold camp formed throughout protracted orogenesis consisting of a broadly transpressional regime with unusually well developed early thrusting.

Structure and metamorphism of the Gondwanan basement in the Bariloche region (North Patagonian Argentine Andes)

In the Bariloche region, the Gondwanan basement consists of metamorphic and deformed igneous rocks intruded by Mesozoic and Tertiary granitoids. Metasediments and amphibolites have been affected by three main deformation events (D1, D2 and D3). D1 (S1 foliation) is well shown preserved in the microlithons of the S2 regional foliation and in albite and garnet porphyroblasts. Both foliations have been folded by upright open folds with associated crenulation (S3) locally developed in the D3 folds hinge zones. An early-stage (syn-D1) subduction episode can be invoked based on evidence of high pressure (HP) metamorphism in schists and amphibolites as well as on the presence of pre-collisional I-type granitoids. D2–D3 developed under regional intermediate pressure–low pressure (IP–LP) metamorphic conditions. High temperature (HT) conditions were reached, as deduced from regional migmatization and the intrusion of syn- to late-orogenic S-type granitoids. The transition from HP to IP metamorphic conditions could be associated with the emplacement of NE directed crustal-scale thrusts during D2.

U–Pb dating of granodiorite and granite units of the Los Pedroches batholith. Implications for geodynamic models of the southern Central Iberian Zone (Iberian Massif)

The first U–Pb geochronological results on the magmatic alignment of the Los Pedroches batholith are presented. The batholith is composed of a main granodioritic unit, several granite plutons and an important acid to basic dyke complex, all of them intrusive after the main Variscan regional deformation phase, D1, along the boundary between the Ossa-Morena and Central Iberian zones (SW Iberian Massif). Zircons from samples on both extremes of the granodiorite massif record nearly simultaneous magmatic crystallization at ca. 308 Ma, while the emplacement of granite plutons was diachronic between 314 and 304 Ma. The U–Pb results combined with new field and textural observations allow to better constrain the age of Variscan deformations D2 and D3 across the region, while the age of D1 remains imprecise. Transcurrent D2 shearing-tightening of D1 folds occurred around 314 Ma (lower Westphalian) in relation to the emplacement of the first granitic magmas. D3 faults and shear bands bearing a strong extensional component developed at ca. 308 Ma (upper Westphalian), associated to the intrusion of the main granodiorite pluton (granodiorite) of the batholith. Together with available geochemical and geophysical information, these results point to the Variscan reactivation of lithospheric fractures at the origin and subsequent emplacement of hybrid magmas within this sector of the Massif.