Subhorizontal Shortening Research Articles

Deformation of the European Plate (58-0 Ma): Evidence from Calcite Twinning Strains

We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.

The effects of fault-zone architecture, wall-rock competence and fluid pressure variations on hydrothermal veining and gold mineralization along the Sheba Fault, Barberton Greenstone Belt, South Africa

Hydrothermal vein systems illustrate the spatial and temporal evolution of structural permeabilities along the Sheba Fault, the first-order fault structure in the Sheba-Fairview complex of gold mines in the Archaean Barberton greenstone belt. The fault juxtaposes competent Moodies Group metapsammites against rheologically weaker metaturbidites and intercalated serpentinites of the Fig Tree and Onverwacht groups. Strain localization into the weaker lithologies results in the pronounced asymmetric zonation of the fault into a fault core and a fault-damage zone. The fault core in the weaker Fig Tree and Onverwacht lithologies is characterized by the transposition of earlier bedding and pervasive foliation development. In contrast, hydrothermal veining, alteration and mineralization are confined to competent Moodies Group rocks that form the tens of meter wide damage zone in the immediate footwall of the fault core. Vein systems of the Sheba West ore body disclose the multistage evolution of the fluid flow system along the fault. An early phase of crackle-breccia formation and associated carbonate (dolomite) alteration is regionally widespread and largely pre-dates the main phase of gold mineralization. Crackle breccias are overprinted by successive sets of subhorizontal and subvertical extension (mode I) veins, steeply inclined shear veins and moderately-dipping late-stage cataclasites. The orientation and controls of the vein sets demonstrate their formation during (1) progressive NW-SE directed subhorizontal shortening, and (2) waning fluid pressures from lithostatic values (mode I veins) to sub-lithostatic fluid pressures (mode II shear veins and cataclasites). The economic-grade mineralization of the Sheba West ore body relates to an undulation of the controlling Sheba Fault that corresponds to a releasing bend during top-to-the-NW thrusting. The vein systems along the Sheba Fault illustrate the controls of permeability structures as a result of lithological and rheological contrasts, geometric variations (fault bends) and fluid-pressure fluctuations during faulting.

Syn-kinematic emplacement of granitic batholith and leucogranite along the extensional detachment shear zone system of the Tin Begane area, Laouni terrane (LATEA metacraton, Central Hoggar, Algeria)

The vast Tin Begane batholith (5000 km2) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 ± 3 Ma, U–Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 ± 4, U–Pb zircon) and a fine-grained monzogranite (591 ± 6 Ma, U–Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 ± 6 Ma, U–Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleoproterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 °C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 °C) not far from the emplacement of the Tin Begane batholith.A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.

Late Cenozoic transpression at the plate boundary: Kinematics of the eastern segment of the Dauki Fault Zone (Bangladesh) and tectonic evolution of the petroliferous NE Bengal Basin

The Sylhet Trough in the Bengal Basin (Bangladesh) hosts a number of gas fields. The evolution of the trough is closely associated presumably with the growth and the tectonics of the Dauki Fault Zone (DFZ). Hence understanding the transpression tectonics of the NE Bengal Basin is crucial in petroleum geoscience. The eastern segment of the DFZ in the NE Bengal Basin transpressed dextrally and activated since the latest Miocene up to the earliest Pliocene due to compression along the north, east and southeast. The structural and the tectonic elements of the area are assessed from geomorphic features and deformation structures. Nature of faults and comparison of fault-slip stress field with modern geodetic measurement allow establishing geometric and kinematic relationship between different tectonic elements with the DFZ. Bedding attitude measurements from the study area indicate sub-horizontal east–west-trending antiform fold axis gently plunging towards west confirming the existence of a large-scale monocline. Out of the two interpreted compression directions N–S and E-W, the former explains the orientation of the monocline while the later may cause the dextral transpression in combination with the former compression direction. Fault kinematics and incremental strain axes indicate a bulk north-trending subhorizontal shortening and vertical thickening of the fold and faults approximately perpendicular to the east-west oriented DFZ. The timing of the compressional deformation and fault activation in the study area is inconclusive. Paleostress analyses results match with the present day stress regime and this implies that all the deformations are genetically linked with the DFZ.

Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCl eq. in subhorizontal veins and 6–0 wt% NaCl eq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO 2 and CH 4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide. Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031

Multi-stage alteration, rheological switches and high-grade gold mineralization at Sheba Mine, Barberton Greenstone Belt, South Africa

Structural, petrographic and geochemical data from Sheba Gold Mine in the Barberton Greenstone Belt of South Africa indicate that gold mineralization formed at the end, and as the result of a multi-stage structural, fluid flow and alteration history. This multi-stage evolution is a prerequisite for the formation of high-grade (>40 g/t Au) ore bodies. The Sheba Mine is situated in well-bedded metaturbidites of the Mesoarchaean Fig Tree Group and structurally infolded ultramafic lavas in the first-order, refolded Ulundi syncline in the northern parts of the belt. Original komatiitic lava flows are pervasively altered to talc-carbonate greyschist. This regionally widespread alteration relates to an early phase of deformation and associated fluid flow. The reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall-rock sequence. This results in the localization of strain along the contact between talc-carbonate greyschist and overlying, massive greywacke and chert units during later-stage refolding and associated flexural slip. Pervasive quartz veining, silicification, K-enrichment and the progressive replacement of the earlier talc-carbonate assemblages by a high-variance quartz-fuchsite green schist alteration testify to the localized fluid flow along this contact. The newly formed, finely intergrown green schist assemblage leads, in turn, to a reaction hardening and embrittlement of the former greyschist-chert contact. High-grade gold mineralization is hosted by brittle quartz-carbonate vein networks and breccias developed along brittle-ductile structures that cross-cut the competent green schist at high angles. The siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks. The thrust kinematics of the late-stage controlling structure studied here indicate that the mineralized structure accommodated regional NW-SE subhorizontal shortening when further tightening of the Ulundi syncline passed the lock-up stage could no longer be accommodated by flexural slip. The results highlight that the formation of favourable conditions for high grade mineralization, such as those at Sheba Mine, may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region.

The early Palaeogene transition from thin-skinned to thick-skinned shortening in the Potosí uplift, Sierra Madre Oriental, northeastern Mexico

ABSTRACT The central Sierra Madre Oriental in Mexico developed primarily in the Late Cretaceous to early Palaeogene as a thin-skinned fold belt above a décollement hosted in Jurassic evaporite. In some regions shortening transitioned to sub-décollement structural levels, resulting in thick-skinned uplifts such as the Potosí uplift in Nuevo León. Thick-skinned deformation in the Potosí uplift involved folding of sub-décollement strata into an NNW-trending anticlinorium, cleavage development, thrust and conjugate strike-slip faulting, and extension fractures associated with barite mineralization. These structures consistently record subhorizontal shortening, directed ~52–65° in the southern uplift, ~69–72° in the northern uplift. Palaeocene to mid-Eocene zircon (U-Th)/He cooling dates record the timing of exhumation associated with thick-skinned uplift and suggest a continuation of shortening rather than a separate tectonic event. Zircon (U-Th)/He dates across the southern Potosí uplift and the Aramberri uplift, ~50 km to the south, range from ~66–53 Ma, whereas dates in the northern part of the Potosí uplift range from ~49–44 Ma. We attribute the transition to thick-skinned shortening to the mechanical strengthening of a planar décollement as rheologically weak evaporite was evacuated beneath synclinal keels of detachment folds. Along-strike differences in timing of exhumation and shortening directions may relate to differences in mechanical stratigraphy. Thicker intervals of evaporite in the northern uplift allowed thin-skinned shortening to continue while the southern uplift transitioned to thick-skinned shortening as the evaporite décollement was exhausted. As a result, stress-strain trajectories in the northern uplift refracted clockwise during continued deformation. Our findings provide new insight into the structural evolution of the Potosí uplift and may provide a framework for studying other thick-skinned uplifts in the orogen, and more generally orogenic belts that record a transition in deformation styles during progressive shortening.

Calcite twinning fabrics along the Middle America trench, Costa Rica and the Motagua sinistral fault, Honduras and Jamaica: Tectonic implications for the Caribbean plate

The Cretaceous oceanic crust of the Caribbean plate has a long and complex tectonic history as an indentor plate bounded by transcurrent faults (N and S) and subduction margins (W [east slab dip] and E [west slab dip]) with no strain data. Miocene forearc sediments and accreted ophiolites are exposed along the central Pacific coast of Costa Rica and preserve a variety of folding and faulting styles. These clastic sediments, deposited in the forearc as the Middle America trench evolved, contain calcite cements and are cross cut by calcite veins. Analysis of the mechanical twins in these two calcite groups (8 samples, n = 267 twins) at Punta Dominical and the Nicoya Peninsula ophiolite preserve a consistent sub-horizontal shortening strain that is oriented parallel to the current trench axis, as are most of the accretion-related fold axes. Differential stress magnitudes for the calcite twinning strain average −450 bars, and there is little evidence of a twinning strain overprint (low NEVs). Obduction along the Middle America trench involves, in part, a component of trench-parallel horizontal shortening.The Motagua fault originates in the Middle America trench and strikes east as the northern boundary of the Caribbean plate as a sinistral margin. The Roatan Islands are a series of islands composed of south-dipping rocks along the southern side of the Motagua fault. Recent, deformed clastic sediments are drapped over gneisses, schists (40Ar-39Ar biotite age of 14 Ma), conodont-free marbles and ophiolitic fragments of presumed Cretaceous age. Twinning studies (n = 6 samples, n = 425 twins) of these marbles preserve a N–S, horizontal shortening strain, normal to the sinistral plate boundary. Differential stress magnitudes for the calcite twinning strain average −36 MPa. Only in a combined sample (n = 189 grains) is a strain overprint (high NEVs; n = 47) present, which records horizontal shortening strain at 132°, 5°. Jamaica is also along the eastern Motagua fault (Cayman trough) and twinning strain studies in Eocene limestones and younger calcite veins (n = 4 samples; n = 136 twins) preserve a horizontal shortening strain normal to the sinistral plate boundary.Twinning strain results are compared to regional GPS, focal mechanism and SKS anisotropy results around the Caribbean plate, especially along the Middle America trench on the western margin of Costa Rica with some new insights into subduction dynamics.

Archean tectonics in the sangmelima granite-greenstone terrains, Ntem Complex (NW Congo craton), southern Cameroon.

Structural study of in the Sangmelima granite-greenstone terrains (SGB) provide a new overview of the evolution of the Ntem Complex (South Cameroon), the northwestern part of the Congo Craton. The regional deformation is marked by a strong steep dips foliation and steeply plunging mineral-stretching. D1-related structures are folding and shearing by a D2 deformation that is synchronous with extraction granitic dykes. D1–D2 structures were developed during regional E-W sub-horizontal shortening under ductile conditions associated with steeply dipping principal stretching in a transpressive regime, progressive strain localization along strike-slip shear zones. Syn-D1 mineral assemblages and high strain microstructures suggest that ductile deformation D1 occurred under high-grade metamorphic conditions, whereas D2 microstructures suggest a brittle deformation. The strain patterns in the SGB recorded a long-lasting Archean tectonic history within the Archean Ntem Complex (ca. 2.83–2.72Ga). Finally, some Archean structures were reactivated during a brittle deformation (D3), likely during the Eburnean event.

Time-space distribution of silicic plutonism in a gneiss dome of the Iberian Variscan Belt: The Évora Massif (Ossa-Morena Zone, Portugal)

In the Iberian Variscan Belt, polyphasic deformation has been recognized as comprising an early phase of crustal thickening, followed by an intermediate phase of crustal extension and doming, and a later phase of shortening. The Évora Massif is a gneiss dome of the westernmost domains of the Ossa-Morena Zone (SW Iberia), which provides a remarkable insight into the late Paleozoic deep crustal structure of the Variscan continental crust of northern Gondwana. In this study, we bring new structural and geochronological U-Pb data for the northern hanging-wall of the Évora Massif. We describe the existence of low-dipping D2 extensional shear zones associated with Buchan-type metamorphism (M2); this enables three tectono-metamorphic units to be distinguished: the Lower Gneiss Unit, the Intermediate Schist Unit, and the Upper Slate Unit. D2-M2 structures experienced subhorizontal shortening (D3) and were transposed by low-plunging folding, thrusting and strike-slip faulting. Zircon grains extracted from Pavia quartz-feldspathic gneiss of the Lower Gneiss Unit yielded a crystallization age of ca. 521 Ma (Cambrian Stage 2–3), which establishes a correlation with tectono-metamorphic units of the footwall and southern hanging-wall of the Évora Massif. U-Pb zircon dating of Divôr foliated quartz-diorite (339 ± 7 Ma), Malarranha weakly foliated biotite-rich granite (322 ± 3 Ma), and undeformed porphyritic granite of the Pavia pluton (314 ± 4 Ma) constrain the timing of emplacement of granitic magmas synchronously with doming. Carboniferous magmatism initiated with doming (ME1 - ca. 343–335 Ma), continued through D2-M2 (ME2 - ca. 328–319 Ma), and lasted until the waning stage of crustal extension (ME3 - ca. 317–313 Ma). The Évora Massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy-driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle-derived (i.e. dioritic-gabbroic) magmas rocks found in the footwall of the Évora Massif.

Regional folding, low-angle thrusting and permeability networks: Structural controls of gold mineralization in the Hope reef at Fairview Mine, Barberton Greenstone Belt, South Africa

The Hope reef forms part of a number of high-grade mineralized, but hitherto undocumented auriferous shear zones at Fairview Mine in the Barberton Greenstone Belt in South Africa, collectively referred to as the hanging-wall reefs. The reefs are hosted by steeply-dipping greywacke units of the Fig Tree Group on the western limb of the regional-scale Ulundi syncline. The currently mined shallow Hope reef represents a top-to-the northwest, low-angle thrust that formed as a secondary accommodation structure during continued NW subhorizontal shortening after the lock up of the Ulundi syncline. Northwest shortening strains correspond to the main phase of upright regional folding (D2/3) in the greenstone belt.The formation of shear and extensional vein sets documents fault-valve behaviour along the low-angle thrust, driven by close-to-lithostatic fluid pressures and under very low differential stresses (<5 Mpa). High-grade ore shoots in the plane of the shallow reef delineate lithologically and structurally controlled interconnected fault-fracture meshes. Bedding planes in Fig Tree greywacke units have provided suitably orientated anisotropies that promoted the development of a wider, anastomosing shear-zone system. The shear zones envelop lithons of relatively competent greywacke units. Slip transfer from shears into the competent lithons induced extensional and shear fractures and the formation of an interconnected fracture network in and between lithons recording a net dilation of up to 50-60 vol%. Individual lithons show extents of 20–25 m2, but ore shoots defined by imbricated lithons cover areas of several hundred square metres. Mutually cross-cutting extensional veins sets and local domains of normal slip point to transient stress switches related to periods of post-slip relaxation and rapid fluid pressure build up prior to the restoration of horizontal stresses and reverse slip along the shallowly dipping reef.The hanging-wall reefs at Fairview Mine underline the formation of mineralized reefs in the Barberton gold district with vastly different orientations, structural and lithological controls, either in sequence or broadly contemporaneous, but during the late-stage shortening of the greenstone belt. The absolute timing of this deformation and associated hydrothermal events remain unclear.

Mechanics of Fold‐and‐Thrust Belts Based on Geomechanical Modeling

AbstractWe use a large strain geomechanical model and critical state soil mechanics to study the evolution of stress and deformation in an evolving fold‐and‐thrust belt and its underlying footwall sediments. Both mean effective stress and deviatoric stress contribute to porosity loss within the wedge with 35% of the porosity loss resulting from increased shear. As a result, porosity increases abruptly across the décollement because both mean‐effective and shear stresses are much higher inside the wedge than in the footwall. As the basal friction coefficient (μb) increases, more shear stress is transmitted across the décollement, resulting in additional compaction of the footwall sediment and decrease in the porosity contrast across the décollement. As the internal friction coefficient (μs) increases, the wedge sediment is more compacted because it can withstand higher mean‐effective and deviatoric stresses. Inside the wedge, the sediment experiences subhorizontal shortening strain and subvertical elongation strain. We predict a 10–30 km wide “transition zone” in which the shear‐stress ratios and compaction curves change rapidly between compressional critical state failure and uniaxial strain (K0) state. Our model results agree with the taper angles and the stress orientations predicted by critical taper theory. This large‐strain drained modeling approach provides first‐order insights into the mechanical processes of loading and compaction in fold‐and‐thrust belts and a foundation for understanding field observations of pressure, stress, and deformation in thrust belt systems.

Twinning Strains in Synfolding Calcite, Proterozoic Sinian System, China

Synfolding calcite was precipitated between layers of Neoproterozoic sandy dolomite and striated parallel to the fold axis of an open anticline with a shallow plunge during folding. The fold had limb dips of 45° and plunged 20° to the south. The synfolding calcite had sub-horizontal grooves that trended parallel to the fold. Limb-hinge-limb calcite samples (3 samples; n = 100 grains) preserved a layer-parallel shortening strain that trended at an acute (45°) angle to the trend of the fold axis and fold lineations. Extension axes were vertical and there was no strain overprint (low NEVs). Shortening strain magnitudes were −2.9% and the differential stress responsible for twinning was −38 MPa. The commonly observed structures were layer-parallel slip striations normal to the fold axis: sub-horizontal interlayer slip surfaces parallel to a fold axis (parallel to bedding strike) were unreported; as was a sub-horizontal shortening strain at an acute angle to the axis of a plunging fold.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.

Extensional orogenic collapse captured by strike-slip tectonics: Constraints from structural geology and U[sbnd]Pb geochronology of the Pinhel shear zone (Variscan orogen, Iberian Massif)

The late Paleozoic collision between Gondwana and Laurussia resulted in the polyphase deformation and magmatism that characterizes the Iberian Massif of the Variscan orogen. In the Central Iberian Zone, initial continental thickening (D1; folding and thrusting) was followed by extensional orogenic collapse (D2) responsible for the exhumation of high-grade rocks coeval to the emplacement of granitoids.This study presents a tectonometamorphic analysis of the Trancoso-Pinhel region (Central Iberian Zone) to explain the processes in place during the transition from an extension-dominated state (D2) to a compression-dominated one (D3). We reveal the existence of low-dipping D2 extensional structures later affected by several pulses of subhorizontal shortening, each of them typified by upright folds and strike-slip shearing (D3, D4 and D5, as identified by superimposition of structures). The D2 Pinhel extensional shear zone separates a low-grade domain from an underlying high-grade domain, and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust, crustal thinning, decompression melting, and magma intrusion. Progressive lessening of the gravitational disequilibrium carried out by this D2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the Pinhel shear zone by strike-slip tectonics during renewed crustal shortening. High-grade domains of the Pinhel shear zone were folded together with low-grade domains to define the current upright folded structure of the Trancoso-Pinhel region, the D3 Tamames-Marofa-Sátão synform. New dating of syn-orogenic granitoids (SHRIMP UPb zircon dating) intruding the Pinhel shear zone, together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca. 331–311Ma, with maximum tectonomagmatic activity at ca. 321–317Ma. The capture and apparent cessation of movement of the Pinhel shear zone occurred at ca. 317–311Ma.

Calcite Twinning in the Ordovician Martinsburg Formation, Delaware Water Gap, New Jersey, USA: Implications for Cleavage Formation and Tectonic Shortening in the Appalachian Piedmont Province

A traverse across the Stone Church syncline in the Ordovician Martinsburg turbidites reveals an axial planar cleavage (N40°E, SE dips) in regional thrust-related folds (N40°E, shallow plunges) and five phases of sparry calcite. Calcite fillings are bedding-parallel, cleavage-parallel, and one vein set cross-cuts both earlier phases; the youngest calcite filling is a bedding-parallel fault gouge that crosscuts the cleavage and preserves top-down-to-the-southeast normal fault kinematics. Calcite veins unique to disharmonically-folded calcareous siltstones (Maxwell, 1962) were also analyzed. Stable isotopic analysis (O, C) of all of the calcite phases indicates a uniform fluid source (δ13C −2.0, δ18O −13.3 VPDB) and, potentially, a similar precipitation and mechanical twinning age. The twinning strains (n = 1341; average Δσ = −32 MPa; average ε1 = −2.9%) in the calcite suite are consistent with SE-NW thrust shortening, and sub-horizontal shortening perpendicular to evolving axial planar cleavage planes in the Stone Church syncline. Calcareous siltstone layers within the Martinsburg Fm. turbidites share concordant bedding planes and are unique, chemically (XRF), but folded and cleaved differently than the surrounding clay-rich Martinsburg turbidites. Neither sediment type yielded detrital zircons. Electron backscatter X-ray diffraction (EBSD) and calcite twinning results in a folded calcareous siltstone layer preserving a layer-normal SE-NW shortening strain and Lattice Preferred Orientation (LPO). Shortening axes for the five-phase calcite suite trends ~N40°W, consistent with tectonic transport associated with crystalline nappe emplacement of the Reading Prong within the Piedmont province.

Geochronological evidence for the Alpine tectono-thermal evolution of the Veporic Unit (Western Carpathians, Slovakia)

Tectono-thermal evolution of the Veporic Unit was revealed by multiple geochronological methods, including 87Rb/86Sr on muscovite and biotite, zircon and apatite fission-track, and apatite (U–Th)/He analysis. Based on the new data, the following Alpine tectono-thermal stages can be distinguished: The Eo-Alpine Cretaceous nappe stacking (~135–95Ma) resulted in burial of the Veporic Unit beneath the northward overthrusting Gemeric Unit and overlying Jurassic Meliata accretionary wedge. During this process the Veporic Unit reached metamorphic peak of greenschist- to amphibolite facies accompanied by orogen-parallel flow in its lower and middle crust. The subsequent evolution of this crust is associated with two distinct exhumation mechanisms related to collision with the northerly Tatric–Fatric basement. The first mechanism (~90–80Ma) is associated with internal subhorizontal shortening of the Veporic Unit reflected by large-scale upright folding and heterogeneous exhumation of the Veporic lower crust in the cores of crustal-scale antiforms. This led to juxtaposition of the higher and lower grade parts of basement, all cooled down to ~350°C by ~80Ma. The second mechanism is associated with the overthrusting of the Veporic Unit over the attenuated Fatric crust. This led to a passive en-block exhumation of the Veporic crust from ~350°C to 60°C between ~80 and 55Ma followed by erosion (~55–35Ma). The erosion processes resulted in formation of planation surface before the Late Eocene transgression. After erosion and planation, a new sedimentary cycle of the Central Carpathian Palaeogene Basin was deposited with the sedimentary strata thickness of ~1.5–2.0km (~21–17Ma). The early to middle Miocene is characterised by destruction tectonic disintegration and erosion of this basin (~20–13Ma) and formation of the Neogene Vepor Stratovolcano (~13Ma). The final shaping of the area has been linked to erosional processes of the volcanic structure since the Late Sarmatian with accelerated processes during the Plio-Quaternary.

From steep feeders to tabular plutons – Emplacement controls of syntectonic granitoid plutons in the Damara Belt, Namibia

Granitoid plutons in the deeply eroded south Central Zone of the Damara Belt in Namibia commonly show tabular geometries and pronounced stratigraphic controls on their emplacement. Subhorizontal, sheet-like pluton geometries record emplacement during regional subhorizontal shortening, but the intrusion of spatially and temporally closely-related granitoid plutons at different structural levels and in distinct structural settings suggests independent controls on their levels of emplacement. We describe and evaluate the controls on the loci of the dyke-to-sill transition that initiated the emplacement of three syntectonic (560–530 Ma) plutons in the basement-cover stratigraphy of the Erongo region. Intrusive relationships highlight the significance of (1) rigidity anisotropies associated with competent sedimentary packages or pre-existing subhorizontal granite sheets and (2) rheological anisotropies associated with the presence of thick ductile marble horizons. These mechanical anisotropies may lead to the initial deflection of steep feeder conduits as well as subsequent pluton assembly by the repeated underaccretion of later magma batches. The upward displacement of regional isotherms due to the heat advection associated with granite emplacement is likely to have a profound effect on the mechanical stratification of the upper crust and, consequently, on the level at which granitoid pluton emplacement is initiated. In this way, pluton emplacement at progressively shallower crustal depths may have resulted in the unusually high apparent geothermal gradients recorded in the upper crustal levels of the Damara Belt during its later evolution.

Upper-plate deformation in response to flat slab subduction inboard of the aseismic Cocos Ridge, Osa Peninsula, Costa Rica

Along the Middle America Trench in southern Costa Rica, flat slab subduction of the aseismic Cocos Ridge has uplifted and exposed the outer forearc, shortened the Terraba forearc basin sequence in the inner forearc (i.e., the Fila Costena thrust belt), and uplifted the magmatic arc. The Osa Peninsula, an outer forearc high ∼20 km inboard of the Middle America Trench and ∼3 km to ∼10 km above the plate interface at its trenchward edge, is deforming in response to variations in the bathymetry of the subducting aseismic Cocos Ridge where relief locally exceeds 1 km. Modern topography of the Osa Peninsula, elevation of the basement rocks (Early to Middle Tertiary Osa melange), elevations of Quaternary marine deposits (Marenco formation), and distribution of late Quaternary uplift rates directly mirror the bathymetry on the Cocos Ridge outboard of the Middle America Trench. Rates of late Quaternary uplift are calculated from eight new radiocarbon ages, five new optically stimulated luminescence ages, and 10 previously published radiocarbon ages. Rates of uplift range from 1.7 m/k.y. to 8.5 m/k.y. The Osa Peninsula is fragmented into small (∼5 km), independently deforming blocks bounded by trench-parallel and trench-perpendicular, subvertical, normal and reverse faults that extend down to the plate interface, allowing for greatly different deformation histories over short distances. Quaternary deformation on the Osa Peninsula is modeled as a thin, outer-margin wedge that deforms in response to subduction of short-wavelength, high-relief asperities on the downgoing plate. Permanent deformation is largely accomplished by simple shear on a complex array of subvertical faults that allow the upper plate to adjust to variations in the slope of incoming ridges and seamounts. Currently, permanent deformation of the outer forearc does not appear to involve significant subhorizontal shortening of the margin wedge, although the global positioning system velocity field records elastic shortening related to locking of the plate interface. Permanent uplift and uplift rates in the outer forearc in southern Costa Rica are driven, to the first order, by the bathymetry associated with the subducting Cocos Ridge and not by the basal shear stress on the plate interface.

Sinistral transpression along the Main Boundary Thrust in Amritpur area, Southeastern Kumaun Himalaya, India

Outcrop mapping and structural analyses in a 100m thick fault damage zone elucidate geometry and kinematics of the Main Boundary Thrust in the study area. The north-dipping fault damage zone is sandwiched between the Proterozoic Amritpur granite in the hangingwall and the Neogene Siwalik sediments in the footwall. The Amritpur granite is overlain by the volcanic-sedimentary sequence along a top-to-the-south thrust. The footwall sediments are deformed into gentle upright and steeply-plunging folds. By contrast, the hangingwall granite is generally massive. The km-scale folds in the Sub Himalayan sediments are geometrically discordant to the folds in the Lesser Himalayan volcanic-sedimentary sequence.The damage zone contains both the Sub Himalayan sediments and the Lesser Himalayan amphibolite. It consists of an imbricate structure, tectonic wedges and pressure solution bands. The imbricate structure is made up of seven fault-bounded lithotectonic domains that record the imprints of coaxial refolding and subsequent modification of the folds into sheath-like structures. The geometry and kinematics of mesoscopic structures and the results of paleostress analysis reveal a sinistral wrench accompanied N-S to NNE-SSW directed subhorizontal shortening in the damage zone. The aforesaid evidences also help distinguish between pure contractional, pure wrench, contraction-dominated and wrench-dominated deformation in the different domains and the tectonic wedges. The sinistral transpression in the damage zone is ascribed to the occurrence of an oblique/lateral ramp along the Main Boundary Thrust in the study area.