D2 Folds Research Articles

Petrogenesis and deformation history of the lawsonite‐bearing blueschist facies metabasalts of the Diamante‐Terranova oceanic unit (southern Italy)

AbstractThe Neotethyan oceanic Diamante‐Terranova unit (DIATU; southern Apennines–Calabria–Peloritani Terrane system) includes basic rocks that during the Cenozoic were subducted and metamorphosed to lawsonite‐blueschist facies conditions. Petrological and structural observations (both at the meso‐ and micro‐scale) show that lawsonite growth was continuous during three distinctive ductile deformation stages (D1–D3). These likely occurred close to the metamorphic peak, estimated at 350–390°C and 0.9–1.1 GPa, producing an equilibrium assemblage made of blue Na‐amphibole, lawsonite, chlorite and pumpellyite. Locally, pods dominated by quartz and epidote (plus chlorite, calcite and green Ca‐amphibole) developed at similar conditions (350–370°C, 0.8–0.9 GPa). Post‐peak evolution during the final exhumation of the DIATU along the subduction channel, also consisted of three deformation stages, defined by folding (D4) and normal faulting (D5) and finally by strike‐slip faulting (D6), affecting both the blueschist unit and the unconformably overlying Tortonian conglomerates. Vorticity analysis on syn‐tectonic lawsonite crystals indicates that severe flattening occurred during the D2 stage, with a significant secondary non‐coaxial strain component along the W–E plane. This is associated with an eastward tectonic vergence, consistent with the subsequent D3 and D4 folding stages characterized by a dominant ENE tectonic transport. It is suggested that exhumation started from the D2 stage and continued during D3 at similar HP/LT metamorphic conditions. The widespread occurrence of unreacted lawsonite crystals suggests that exhumation was very fast and supports the idea that concurrent ductile deformation might play a role in its preservation.

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

AbstractA new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2‐D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large‐scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite‐rich rocks. The lower layer consists of high‐grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large‐scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike‐slip shear zone (i.e., Juzbado‐Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

The Golden Mile in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, has produced 385 million tonnes of ore at a head grade of 5.23 g/t gold (1893–2016). Gold-pyrite ore bodies (Fimiston Lodes) trace kilometre-scale shear zone systems centred on the D2 Golden Mile Fault, one of three northwest striking sinistral strike-slip faults segmenting upright D1 folds. The Fimiston shear zones formed as D2a Riedel systems in greenschist-facies (actinolite-albite) tholeiitic rocks, the 700-m-thick Golden Mile Dolerite (GMD) sill and the Paringa Basalt (PB), during left-lateral displacement of up to 12 km on the D2 master faults. Pre-mineralisation granodiorite dykes were emplaced into the D2 shear zones at 2674 ± 6 Ma, and syn-mineralisation diorite porphyries at 2663 ± 11 Ma. The widespread infiltration of hydrothermal fluid generated chlorite-calcite and muscovite-ankerite alteration in the Golden Mile, and paragonite-ankerite-chloritoid alteration southeast of the deposit. Fluid infiltration reactivated the D2 shear zones causing post-porphyry displacement of up to 30 m at principal Fimiston Lodes moving the southwest block down and southeast along lines pitching 20°SE. D3 reverse faulting at the southwest dipping GMD-PB contact of the D1 Kalgoorlie Anticline formed the 1.3-km-long Oroya Shoot during late gold-telluride mineralisation. Syn-mineralisation D3a reverse faulting alternated with periods of sinistral strike-slip (D2c) until ENE-WSW shortening prevailed and was accommodated by barren D3b thrusts. North-striking D4 strike-slip faults of up to 2 km dextral displacement crosscut the Fimiston Lodes and the barren thrusts, and control gold-pyrite quartz vein ore at Mt. Charlotte (2651 ± 9 Ma).

Structural evolution of the semischist–schist transition in an accretionary complex: the Otago Schist section at Lake Hāwea, New Zealand

ABSTRACTPermo-Triassic, Rakaia Terrane, pelitic and psammitic schists at Lake Hāwea, Otago, show progressive metamorphism from TZII, pumpellyite–actinolite facies to TZIV, greenschist facies. The sequence shows evidence of five phases of deformation. D1 and D2 are progressive synmetamorphic phases, forming macroscopic nappe-like folds, interpreted to have formed by internal deformation within the Rakaia–Caples accretionary wedge during terrane collision. Thick S1 quartz veins are the form surface for definition of D2 folding, intensification of which progressively destroys evidence of D1 at higher textural and metamorphic grades. D3–5 are post metamorphic, less pervasive, kink folds (commonly conjugate) and brittle fault zones that predate the intrusion of lamprophyre magma and carbonatitic fluids associated with the Alpine Dike Swarm (c. 25 Ma). They relate to tectonic relaxation and exhumation of the schist in Cretaceous–Paleogene times. Fluid flow has been ubiquitous throughout the deformation, resulting in extensive quartz veining with diffusion of silica from the immediate country rock.

Contrasting Pan-African structural styles at the NW margin of the Congo Shield in Cameroon

Field, microstructural, and anisotropy of magnetic susceptibility (AMS, magnetic fabrics) studies assessed the Pan-African deformational history and strain geometry at the southern margin of the Central African Fold Belt (CAFB) against the older, cratonic basement of the Congo Shield (CS). Reflected light microscopy and thermomagnetic studies supported the identification of magnetic minerals. Data cover a low angle thrust margin (Mbengis-Sangmelima area) in the east and high angle shear zones cutting the margin (Kribi area) in the west, at the Atlantic coast. In the CS basement units, magnetic anisotropy is generally higher than in the low grade Pan-African units. In the latter, early D1/D2 shortening produced a flat-lying magnetic foliation parallel with the regional trend of the belt, a shallow magnetic lineation, and mostly oblate fabrics. Subsequent D3 deformation is only of local importance in the Mbengis-Sangmelima area. The magnetic lineation shows distinct maxima in NNE-SSW direction, parallel with the low angle tectonic transport direction.In the Kribi area, the NNE-SSW trending Kribi-Campo shear zone (KCSZ) affected both older rocks and Pan-African high grade metapelites of the Yaoundé unit together with their basal thrust. The early planar fabric (S1) was overprinted during D2 folding under relatively high T conditions, and subsequent D3 wrenching. Magnetic fabrics document a progressive change from oblate towards prolate ellipsoids towards the KCSZ. Magnetic foliations with medium to steep dips curve into the N-S to NE-SW orientation of the KCSZ, lineations follow the same trend with shallow to medium plunges. This fabric implies that the KCSZ is a Pan-African strike-slip shear zone with a subordinate component of compression.Strike-slip tectonics in the west (KCSZ) and thrusting in the east imply N-S to NE-SW convergence during Pan-African terrane assembly against the present northern margin of the CS. In addition, the KCSZ may separate the CS from the São Francisco Craton in Brazil and thus be the northern part of a link connecting the CAFB to the West Congo Belt in the south. This putative Pan-African link separated the São Francisco Craton from the Congo Shield prior to Mesozoic Gondwana break-up.

Structural architecture and glacitectonic evolution of the Mud Buttes cupola hill complex, southern Alberta, Canada

This paper presents the results of a detailed multidisciplinary study of the deformed bedrock and overlying Quaternary sediments exposed at the Mud Buttes in southern Alberta, Canada. This large, arcuate cupola hill is composed of intensely folded and thrust sandstones, siltstones and mudstones of the Cretaceous Belly River Group. Glacitectonism responsible for the development of this internally complex landform occurred at the margin of the newly defined Prospect Valley lobe of the Laurentide Ice Sheet. Analysis of the deformation structures reveals that construction of this landform occurred in response to at least two phases of south-directed ice sheet advance separated by a period of retreat. The first phase led to the formation of a forward propagating imbricate thrust stack leading to polyphase deformation of the Belly River Group. D1 thrusting led to the detachment of thrust-bound slices of bedrock which were accreted to the base of the developing imbricate stack. This process resulted in the structurally higher and older thrust-slices being progressively “back-rotated” (tilted), accompanied by D2 thrusting and folding. Further thrusting during D3 was restricted to the core of the Mud Buttes as the deforming sequence accommodated further compression imposed by the advancing ice. Minor oscillations of the ice margin led to localised brittle-ductile shearing (D4) of the bedrock immediately adjacent to the ice contact part of the thrust stack. The second phase of ice advance led to the accretion of a relatively simple thrust and folded sequence seen the northern side of Mud Buttes. The resulting composite thrust moraine was subsequently overridden by ice advancing from the NNW to form a dome-like cupola-hill. This readvance of the Prospect Valley lobe led to the formation of a thin carapace of Quaternary sediments mantling the Mud Buttes which include glacitectonite, till and an organic-rich clay-silt (?palaeosol).

Neoarchean orogenic, magmatic and hydrothermal events in the Kalgoorlie-Kambalda area, Western Australia: constraints on gold mineralization in the Boulder Lefroy-Golden Mile fault system

The Boulder Lefroy-Golden Mile (BLF-GMF) fault system is the most intensely mineralized structure (>2150 t Au to 2015) in the Archean Yilgarn Craton, Western Australia. The fault system links the Kalgoorlie and Kambalda mining districts in the Eastern Goldfields Province, a continental-margin orogen subdivided into the western Kalgoorlie ensialic rift and the eastern Kurnalpi volcanic arc. After rifting, the 2.73–2.66 Ga greenstone-greywacke succession in the Kalgoorlie-Kambalda area underwent five phases of orogenic deformation, predominantly during ENE-WSW shortening: D1 upright folding at ca. 2680 Ma, D2 sinistral strike-slip faulting at 2678–2663 Ma, D3 folding of late conglomerate-turbidite successions at 2665–2655 Ma, D4 dextral strike-slip faulting at 2655–2640 Ma and D5 east-northeast-striking normal faulting. Regional prehnite-pumpellyite to greenschist facies burial metamorphism took place during D1 and D3 crustal thickening, and amphibolite facies aureoles formed around granite batholiths during and after D3 at 400 ± 100 MPa pressure. The D2 BLF offsets D1 folds by 12 km SW-side south and contains a porphyry dyke (2676 ± 7 Ma) boudinaged by transtensional oblique-slip along a line pitching 21° southeast. The BLF is linked by transverse D2 thrusts to other sinistral faults recording strike-slip until 2663 ± 7 Ma. Late D2 strike-slip movement alternated with early D3 shortening. D3 thrusts accommodated strain in fault blocks of rigid mafic-ultramafic volcanic rocks consolidated during D1, while the sedimentary rocks in D3 synclines were foliated at high strain. Biotite-sericite alteration and gold-pyrite mineralization in the BLF-GMF system took place at 11 ± 4 km burial depth in faults active during D2 and D3. The Golden Mile (1708 t Au) and other deposits are associated with stocks and dykes of high-Mg monzodiorite-tonalite porphyry, part of a late-orogenic (2665–2645 Ma) mantle-derived suite of adakitic affinity. Hornblende and apatite compositions indicate that these intrusions are characterized by high water contents (5–6 wt% H2O in melt), by high oxidation states (δNNO +1.0 to +2.4 log units) and by igneous anhydrite. Some stocks contain pervasive anhydrite-pyrite mineralization of low gold grade (0.4 g/t). Biotite-sericite-pyrite ore bodies such as those at Kanowna Belle (140 t Au) also replace faulted metamorphic rocks above batholith domes cored by plutons of the monzodiorite suite. The D4 strike-slip faults are barren at Kambalda but control gold quartz-vein ore at Kalgoorlie (2651 ± 9 Ma), and Au-Ag breccia ore at Black Flag (<2648 ± 6 Ma).

Restoration of Late Neoarchean–Early Cambrian tectonics in the Rengali orogen and its environs (eastern India): The Antarctic connection

Geological mapping and P–T path reconstructions are combined with monazite chemical age and Secondary Ion Mass Spectrometric (SIMS) U–Pb zircon age determinations to identify crustal domains with distinctive evolutionary histories in the Rengali orogen sandwiched between two Grenvillian-age metamorphic belts, i.e. the Eastern Ghats Granulite Belt (EGGB) in the south, and the amphibolite facies Gangpur Schist Belt (GSB) in the north, which in turn forms a collar along the NW/W margins of the Paleo/Mesoarchean Singhbhum Craton (SC) north of the Rengali orogen. Anatectic gneisses in the orogen core exhibit multi-phase Neoarchean/Paleoproterozoic deformation, metamorphic P–T histories and juvenile magma emplacement events. The high-grade belt is inferred to be a septum of the Bastar Craton (BC). The flanking supracrustal belt in the orogen — dominated by quartz-muscovite schists (±staurolite, kyanite, garnet pyrophyllite), inter-bedded with poorly-sorted and polymict meta-conglomerate, and meta-ultramafic/amphibolite bands — evolved along P–T paths characterized by sub-greenschist to amphibolite facies peak P–T conditions in closely-spaced samples. The supracrustal rocks and the anatectic gneisses of contrasting metamorphic P–T histories experienced D1, D2 and D3 fabric-forming events, but the high-angle obliquity between the steeply-plunging D3 folds in the anatectic gneisses and the gently-plunging D3 folds in the supracrustal unit suggests the two lithodemic units were tectonically accreted post-S2. The supracrustal belt is inferred to be a tectonic mélange formed in an accretionary wedge at the tri-junction of the Bastar Craton, the Eastern Ghats Granulite Belt and the Singhbhum Craton; the basin closure synchronous with the assembly of EGGB and the Singhbhum Craton–Gangpur Schist belt composite occurred between 510 and 610Ma. Based on the available evidence across the facing coastlines of the Greater India landmass and the Australo–Antarctic blocks at ~500Ma, it is suggested that the EGGB welded with the Greater India landmass during the Pan African along an accretion zone, of which the Rengali orogen is a part, synchronous with the final assembly of the Gondwanaland.

Tectonic Imprints of the Hazara Kashmir Syntaxis on the Mesozoic Rocks Exposed in Munda, Mohmand Agency, Northwest Pakistan

AbstractTwo well‐developed mesoscopic folds, D2 and D3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D3 folds, which developed during SE–NW shortening. Micro‐ to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D3. The newly established NW–SE trending micro‐ to mesoscopic structures in Munda termed D2, which postdated F1/F2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D2 and D3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D2 and D3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.

Pre-Alpine extensional tectonics of a peridotitelocalized oceanic core complex in the Late Jurassic, high-pressure Monviso ophiolite (Western Alps)

The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed, eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere. The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex, which developed during the initial stages of the tectonic evolution of the Alpine Tethys. The NNW-striking, 20-25-km-long shear zone (Baracun Shear Zone) contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni-Cr concentrations and high Cl contents. Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds, providing a critical age constraint for the timing of its formation. Metabasaltic-metasedimentary rocks in the hanging wall increase in thickness away from the shear zone, characteristic of syn-extensional rock sequences in supradetachment basins. A Lower Cretaceous post-extensional sedimentary sequence unconformably cover the syn-extensional strata, the detachment shear zone, and the ophiolitic footwall, establishing a strong structural evidence for the intraoceanic, seafloor spreading origin of the tectonic fabric of the Monviso ophiolite, prior to the onset of subduction zone tectonics in the Alpine Tethys. The Monviso ophiolite and the Baracun Shear Zone represent a peridotite-localized oceanic core complex, which survived both the subduction and continental collision tectonic stages of the Alpine orogeny. Intraoceanic detachment faults and oceanic core complexes may play a significant role in subduction initiation, and hence their recognition in orogenic belts is an important step in reconstructing the record of ocean basin collapse and closure.

Metamorphic modifications of the Muremera mafic–ultramafic intrusions, eastern Burundi, and their effect on chromite compositions

The Muremera mafic–ultramafic intrusions were emplaced into metasedimentary rocks of the Karagwe–Ankole Belt in eastern Burundi, as part of the Mesoproterozoic Kibaran tectonomagmatic event. Igneous minerals of the Muremera intrusions have been partly altered to hydrous and carbonated metamorphic assemblages, although in most cases, the original igneous textures are well-preserved. Rounded, subhedral cumulus olivine has been partially and pseudomorphically replaced by lizardite–magnetite mesh-rim and lizardite–brucite mesh-centre assemblages, while anhedral interstitial plagioclase has been replaced by chlorite–tremolite. A later and localized event results in prograde alteration to antigorite–magnetite–chlorite–talc–carbonate and talc–carbonate–chlorite assemblages. The rocks are inferred to have undergone at least three separate metamorphic/alteration events resulting in: AS1 – an early alteration assemblage (mesh-rim lizardite–magnetite) characterized by very low fluid/rock ratios and widespread distribution; AS2 – a later, widespread low-temperature retrogressive (mesh-centre lizardite–brucite) assemblage associated with abundant close-spaced parallel veins; AS3 – later, prograde (antigorite–magnetite) and AT4 (talc–chlorite–carbonate) assemblages associated with more localized shearing and higher fluid/rock ratios. The AS1 assemblage most likely represents deuteric alteration that occurred soon after intrusion and cooling. The AS2 assemblage may relate to a continuation of this cooling, or may be correlated with the regional upright D2 folding event, while the AS3 and AT4 alteration assemblages are most likely correlated with the N–S oriented D3 faulting episode linked to the distal East African Orogeny. Euhedral to subhedral chromite grains are essentially unaltered where enclosed in primary unaltered olivine, pyroxene or plagioclase, as well as in AS1 lizardite–magnetite and AS2 lizardite–brucite altered olivine or pyroxene. In samples which show alteration to AS3 antigorite–magnetite and AT4 talc–carbonate–chlorite assemblages, the chromite grains are zoned to Mg–Al-poor ferritchromite rims. The width and the composition of these alteration rims are related to the degree of alteration of the silicate assemblages. However, it is concluded that chromite found in rocks that have avoided late/localized AS3–AT4 alteration preserve their magmatic to late-magmatic chemical and textural characteristics, and can therefore be used in regional geological studies and exploration.

The Cimmerian accretionary wedge of Anarak, Central Iran

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered “ophiolites” which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the “ophiolites” within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the “ophiolites” were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan, Sanandaj–Sirjan zone, Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D1. The main mylonitization event was during the D2 deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D3 folding and related mylonitization. We documented the systematic changes in the orientations of D2 linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.

Structural evolution and deformation kinematics of a subduction-related serpentinite-matrix mélange, Santa Elena peninsula, northwest Costa Rica

Detailed structural analysis of the Isla Pelada serpentinite-matrix mélange on Santa Elena Peninsula, Pacific coast of northwest Costa Rica, elucidates for the spatial distribution and temporal progression of a décollement-related deformation. This décollement zone constitutes the fault rock of a plate boundary in a subduction to collision setting. The décollement zone is defined by a thick tectonic mélange that includes blocks and belts of polymictic breccia, turbiditic greywacke, argillite, radiolarite, basalt, gabbro and serpentinized peridotite in a variably foliated serpentinite matrix. The chemical-mineral data of the peridotite blocks in the mélange indicate that they are fragments of abyssal mantle, different of the very depleted forearc mantle peridotites of the overlying Santa Elena ultramafic nappe.The structural evolution of the mélange can be divided into four main events. Early D1 deformation took place during underthrusting in the footwall of the décollement and is characterized by subhorizontal layer-parallel extension and subsequent by heterogeneous shearing of trench-fill deposits, oceanic volcanic rocks and variably serpentinized peridotites, resulting asymmetric boudinage, mesoscopic porphyroclasts and pervasive (S1) S–C fabrics. D1 is followed by layer-parallel contraction with early D2 folding and local development of an S2 pressure-solution cleavage, and late-D2 thrusting and large-scale duplexing. Therefore, D2 deformation evolves from pervasive coaxial subhorizontal contraction to a more localized non-coaxial strain (along relatively narrow thrust surfaces, in the mélange). D2 deformation mainly occurred in the hanging-wall of the décollement and was related to the downward migration of the subduction thrust. The result is the incorporation of slices of the subducted sequence by underplating into the overriding plate, typical of accretionary prisms.Kinematic indicators in the mélange are consistent with a general eastward-directed shearing during D1 and D2 deformations. This kinematics can be correlated with the direction of emplacement of the overlying Santa Elena ultramafic nappe during the early Upper Cretaceous. This also suggests the western-directed subduction of the oceanic lower plate. After the mélange was incorporated into the accretionary prism, the whole tectonic edifice was deformed in pre-late Campanian time by WNW–ESE trending D3 cylindrical upright folding and S-directed D4 narrow brittle thrusting. The recorded evolution of deformation in rocks already lithified, combined with the exotic origin of the oceanic lithologies and the serpentinite nature of the matrix points to disruption and mélange formation mainly during tectonic burial and underthrusting. In this context, the serpentinite matrix of the mélange is the result of the low-T hydration, metamorphism and deformation of mantle peridotites in a relatively shallow subduction setting.

Neoarchean continental crust formation and Paleoproterozoic deformation of the central Rae craton, Committee Bay belt, Nunavut

Integrated mapping, structural analysis, and U–Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic–ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper &lt;2.69 Ga clastic ± komatiite–quartzite sequence. Following an 80 million year hiatus, this succession was intruded by voluminous ca. 2.61–2.57 Ga granodiorite–tonalite–granite ± diorite, which do not appear to have thermally or tectonically affected the supracrustal belt. Instead, three generations of structures record polyphase Paleoproterozoic deformation of the region. D1 structures are consistent with a doubly vergent structural fan developed at ca. 2.35 Ga in response to the Arrowsmith orogeny that affected the western Rae margin. Penetrative D2 structures dominate the map pattern and include northeast-trending, southeast-dipping folds and fabrics within which gold is localized. The general southeast dips of S2 and inclined, northwest-vergent attitude of F2 reflect northwest-directed shortening at 1.84–1.82 Ga. The absence of syn-D2 plutonic rocks in the west and central Committee Bay belt support amphibolite-facies metamorphism as a response to crustal thickening, which, in turn, led to syn-D2 crustal melting in the east. Regionally extensive upright to northwest-vergent D2 structures and associated ca. 1.85–1.82 Ga tectonometamorphism across the Rae craton are attributed to an early stage of the Hudsonian orogeny involving microcontinent collision(s) with its southeastern margin. D3 folds and dextral shearing at ca. 1780 Ma accommodated localized, late-stage compressional strain during final amalgamation of Laurentia.

The Connemara Antiform and its possible Silurian history

The Connemara Antiform has iconic status in the geology of western Ireland. With its eastto-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle late Silurian deformation in the origin of the Connemara Antiform. Overview of the geology of Connemara Connemara was part of the south-eastern Laurentian margin Dalradian sequence that experienced a major short-lived early Ordovician orogenic episode, the Grampian Orogeny, caused by collision with, and obduction of, an arc that developed above oceanward-directed subduction (Chew 2009). The Connemara Terrane became detached from the Laurentian margin and now lies to the south of the obducted arc and the South Mayo Trough (Fig. 1). The translation of the Connemara Terrane from the Laurentian margin to its current position is indicated by an influx of coarse clastic rocks of Connemara provenance into the c. 464Ma Bunnacunneen Member of the Mweelrea Formation of the South Mayo Trough (McConnell et al. 2009). Dewey (2005) provides a succinct account of the local plate-tectonic, metamorphic and structural history of the Connemara Terrane and the South Mayo Trough during the Grampian Orogeny, and Graham et al. (1989), Leake and Tanner (1994), McConnell et al. (2002) and Long et al. (2005) provide a comprehensive overview of the detailed geology of the area. The Grampian Orogeny began with the subduction of rifted Laurentian margin sediments beneath a continent-facing oceanic arc c. 478Ma (Dewey 2005). The Connemara Dalradian sediments were buried in a thick nappe pile of D2 and D3 folds and experienced high-pressure, lowtemperature Barrovian metamorphism (Leake and Tanner 1994). Palaeomagnetic studies indicate that the D3 folds originally dipped c. 208 south (Robertson 1988). Between 474Ma and 470Ma (Friedrich et al. 1999) a suite of metagabbros and orthogneisses (the Metagabbro Orthogneiss Complex) were injected into the deforming nappe pile during D2 D3 in an east west belt that extends for more than 80km along the southern margin of the terrane (Fig. 1). The high-temperature metagabbros and orthogneisses drove sillimanite-grade, hightemperature, low-pressure Buchan metamorphism. Migmatites resulting from the metagabbro heat pulse formed at c. 6kbar but crystallised at much lower pressures of c. 3.5kbar, indicating rapid exhumation (Barber and Yardley 1985). Rapid exhumation may have been facilitated by extensional collapse. The Renvyle Bofin Slide, a lowangle post-D3 extensional detachment, has, on the Irish Journal of Earth Sciences 32 (2014), 71 78 doi: 10.3318/IJES.2014.32.71 # 2014 Royal Irish Academy

A new map and interpretation of the geology of part of Joyces Country, Counties Galway and Mayo

A new detailed (1:25,000) geological map, cross-sections and descriptions are presented of a large part of Joyces Country, consisting of Connemara Dalradian Schists unconformably overlain by both Silurian (Upper Llandovery to Wenlock) and Carboniferous (Tournaisian to Lower Visean) rocks; only the edge of the last has been mapped. The Dalradian rocks contain D2, D3 and D4 folds and are of staurolite to sillimanite grade with late andalusite and cordierite, constrained to between 470 and 463 Ma. Major D3 folds are folded by the D4 Connemara Synform and the complementary D4 Joyces Antiform, which predate the intrusion of the 462 Ma Oughterard Granite. The folding of the Silurian rocks, termed Scandian as it was intra-Silurian, is identified precisely as a short late Wenlock to early Ludlow episode, (430 to 424 Ma; Gradstein et al. 2012), probably Gorstian Stage (427 ± 3 Ma), and hence not end-Silurian (419 Ma). The mechanism whereby the Scandian open folding of the Silurian rocks took place above the relatively high-level rigid Dalradian basement in the northern part of the area is shown to be by closely-spaced nearly vertical E–W faulting and shearing along the strike of the sub-vertical foliation in the Dalradian Schists. This controlled the trend of the folds in the Silurian rocks. At deeper levels in the basement in the southern part of the area, far fewer, and more widely spaced faults involving larger movements are probably related to syn-Scandian folding. At least five major folds in the exposed Silurian are identified. There would have been more folds (now eroded away) in the south of the area above the Dalradian rocks. The area is crucial in exposing part of the brittle-ductile transition in the Dalradian during the Scandian folding and the solution of the reciprocal problems of how the basement controlled the folding of the Silurian rocks and how their folding affected the Connemara basement.

Polyphase Laramide Structures and Possible Folded Tertiary(?) Sills at Dagger mountain, Big Bend National Park, Texas

Dagger Mountain, in Sierra del Carmen within Big Bend National Park, Texas, is a 5 km-long, doubly-plunging, southwest-vergent anticline adjacent to a doubly plunging syncline. Dagger Mountain lies near the eastern margins of the Cordilleran orogen and the Basin and Range province. Mapping at 1:12,000 scale reveals details about three phases of Laramide and Basin and Range structures. Mapping and descriptive structural analysis complement previous mapping at 1:12,000 – 1:75,000 scales (Poth, 1979; Moustafa, 1988; Cooper and others, 2011; Turner and others, 2011; Maxwell and others, 1967). Four distinctive formations of Cretaceous age crop out on Dagger Mountain: Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation. At least two phaneritic, mafic, feldpathoid-rich sills intrude the Boquillas Formation. A similar, possibly correlative sill south of Dagger Mountain is dated at 32.47 ± 0.41 Ma ( 40Ar/39 Ar on groundmass; Morgan and Shanks, 2008). One well-exposed Dagger Mountain sill can be traced from one map-scale fold limb, through the hinge, and into the other limb. The Dagger Mountain anticline is a first-phase (D1) fold. D1 map- and outcrop-scale folds contain subvertical NNW-striking axial planes and subhorizontal fold axes. Second-phase (D2) folds produced NNW and SSE plunges of the DM anticline. D2 map- and outcrop-scale folds display subvertical NE-striking axial planes and subhorizontal fold axes. Third-phase (D3) high-angle faults strike NNW and NW and cross-cut D1 folds and Tertiary sills. Drag during D3 faulting produced D3 folds. Dagger Mountain structures are significant because: a) few polyphase folds have been documented in the Big Bend region, b) the west-verging Dagger Mountain anticline and other D1 folds show fault-propagation fold characteristics, c) the apparently folded Tertiary(?) sill suggests that Laramide deformation at Dagger Mountain is post-32 Ma and unusually recent. Alternatively, the sill could be as old as Cretaceous, or the sill could have intruded both limbs and hinge of an existing fold.

Pressure–temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S1 and S2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7mol% and an increase of pyrope component from 4 to 6mol% from core to rim. The grossular content firstly increases from the inner (Grs~21) to the outer core (Grs~27) and then decreases towards the outermost rim (Grs~15). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P–T pseudosections were calculated in the system Na2O–K2O–CaO–FeO–MnO–MgO–Al2O3–TiO2–SiO2–H2O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8GPa and temperatures of 460–500°C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540–570°C and 0.7–1.0GPa. These results suggest an HP-metamorphic imprint during the D1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D2 folding and shearing occurred at decreasing P–T conditions during the exhumation of the metamorphic complex.

Molecular Mechanism of SSR128129E, an Extracellularly Acting, Small-Molecule, Allosteric Inhibitor of FGF Receptor Signaling

The fibroblast growth factor (FGF)/fibroblast growth factor receptor (FGFR) signaling network plays an important role in cell growth, survival, differentiation, and angiogenesis. Deregulation of FGFR signaling can lead to cancer development. Here, we report an FGFR inhibitor, SSR128129E (SSR), that binds to the extracellular part of the receptor. SSR does not compete with FGF for binding to FGFR but inhibits FGF-induced signaling linked to FGFR internalization in an allosteric manner, as shown by crystallography studies, nuclear magnetic resonance, Fourier transform infrared spectroscopy, molecular dynamics simulations, free energy calculations, structure-activity relationship analysis, and FGFR mutagenesis. Overall, SSR is a small molecule allosteric inhibitor of FGF/FGFR signaling, acting via binding to the extracellular part of the FGFR.