Mesoscopic Folds Research Articles

Kinematic and tectonic implications of crenulation cleavage, kink bands, and mesoscopic folds in the Baraboo Syncline, Wisconsin (∼1.45 Ga Picuris Orogen)

The Baraboo Syncline of Wisconsin is a south-verging, regional-scale fold that formed during the ∼1.45 Ga Picuris Orogeny. It involves Mesoproterozoic quartzite and phyllite of the Baraboo Quartzite. Phyllite layers preserve two deformational assemblages. D1 includes the regional-scale flexural-slip to flexural-flow syncline and associated mesoscopic parasitic folds. It also includes spaced cleavage in quartzite layers and north-dipping phyllitic cleavage in phyllite layers. On the gently north-dipping south limb of the regional syncline, parasitic folds verge south (out-of-the-hinge). On this limb, D2 includes north-verging monoclinal kink bands as well as south-dipping asymmetric crenulation cleavage; kink bands have not been found on the north limb. Crenulation growth micro-folded phyllitic cleavage domains into asymmetric sigmoids that are shaped like an ‘S' on the Baraboo Syncline's south limb and like a ‘Z' on the fold's north limb (as viewed looking east). The origin of D2 structures has long been controversial. Some authors attribute them to a post-syncline phase of extensional tectonism on north-dipping shear zones, whereas others consider them to be a consequence of progressive crustal shortening that continued after D1 fabrics had developed. Our structural analysis supports the second hypothesis and therefore implies that the manifestation of the Picuris Orogeny in southern Wisconsin did not include a post-syncline extensional phase. This interpretation is compatible with a tectonic model in which the Baraboo Syncline formed during inversion of a rift or pull-apart basin during north-south crustal shortening. Specifically, we found no north-dipping normal-sense shear zones, and we demonstrate that the kink bands and asymmetric crenulation of the south limb accommodated a component of north-south shortening in the plane of phyllitic cleavage. Published analog models show that the north-verging kink bands of the south limb could have developed in the same deformation regime as did south-verging parasitic folds. The geometry of crenulation in the north limb indicates that it could be an extensional crenulation accommodating stretching of the limb once it was steep. Late-stage flattening at a high angle to bedding on the south limb locally rotated the crenulation cleavage domains to a sub-horizontal dip. Outcrops in the Baraboo Syncline emphasize that asymmetric crenulation is a fungible fabric, in that examples with similar appearance can accommodate different strains and displacements depending on local context.

Combined X-Ray Computed Tomography and X-Ray Fluorescence Drill Core Scanning for 3-D Rock and Ore Characterization: Implications for the Lovisa Stratiform Zn-Pb Deposit and Its Structural Setting, Bergslagen, Sweden

Abstract We present the results of a pilot study that integrates automated drill core scanning technology based on simultaneous X-ray computed tomography (XCT) and X-ray fluorescence (XRF) analyses to provide high-spatial-resolution (<0.2 mm) information on 3-D rock textures and structures, chemical composition, and density. Testing of its applicability for mineral exploration and research was performed by scanning and analyzing 1,500 m of drill core from the Paleoproterozoic Lovisa stratiform Zn-Pb sulfide deposit, which is part of a larger mineral system also including Cu-Co and Fe-(rare earth element) mineralization, hosted by the highly strained West Bergslagen boundary zone in south-central Sweden. The obtained scanning data complements data derived from structural field mapping, drill core logs, and chemical analysis as well as from multiscale 3-D geologic modeling at Lovisa. Data integration reveals macro- and mesoscopic folding of S0/S1 by asymmetric steeply SE-plunging F2 folds and N-striking vertical F3 folds. Stretching lineations, measured directly from the scanning imagery, trend parallel to F2 fold hinges and modeled ore shoots at the nearby Håkansboda Cu-Co and Stråssa and Blanka Fe deposits. The textural character of the Lovisa ore zones is revealed in 3-D by XCT-XRF scanning and highlight remobilization of Zn and Pb from primary layering into ductile and brittle structures. The downhole bulk geochemical trends seen in scanning and traditional assay data are generally comparable but with systematic variations for some elements due to currently unresolved XRF spectral overlaps (e.g., Co and Fe). The 3-D deformation pattern at Lovisa is explained by D2 sinistral transpression along the West Bergslagen boundary zone in response to regional north-south crustal shortening at ca. 1.84–1.81 Ga. Local refolding was caused by D3 regional east-west crustal shortening resulting in dextral transpression along the West Bergslagen boundary zone, presumably at ca. 1.80–1.76 Ga. Based on polyphase ore textures and modeled ore shoots aligned to F2 fold hinges, we postulate that D2 and D3 transpressive deformation exerted both a strong control on ore remobilization and the resulting orebody geometries at Lovisa and neighboring deposits within the West Bergslagen boundary zone. We conclude that the combined XCT-XRF drill core scanning technique provides a valuable tool for 3-D ore and rock characterization, generating continuous downhole data sets, with the potential for increasing precision and efficiency in mineral exploration and mining.

Transpression in the Sanandaj-Sirjan Zone (Zagros Orogen, Iran) during the Jurassic and Early Cretaceous: Evidence from the North Shahrekord Shear Zone

Deformation style associated with the early stage of subduction of the Neo-Tethys under Iranian microcontinent is poorly understood. Utilizing field observations, microscopic thin-section interpretation, and recently published 40Ar-39Ar ages, we conducted a structural analysis of the North Shahrekord Shear Zone. The study area is located in the central Sanandaj-Sirjan Zone in western Iran, in an area that includes high-grade eclogite-bearing and low-grade metamorphic units of the North Shahrekord Metamorphic Complex. Two generations of deformation were observed in the study area, including D1 and D2. D1 structures are rare and involve folds and axial-plane foliations (S1), which have been almost obliterated by D2. D2 is characterized by a dextral transpressional zone, where strain is partitioned into distinct simple shear and pure shear domains. The pure shear domain is observed mainly in low-grade zone and some high-grade metamorphic rocks, and is characterized by NW-striking close to isoclinal folds (F2) and axial-plane foliations (S2). Folds are mostly inclined and their vergence is towards the southwest and northeast in the northern and southern parts of the study area, respectively. In the central part of the area, F2 includes approximately upright folds. These mesoscopic folds are associated with map-scale folds. The simple shear domain is restricted to the high-grade metamorphic rocks of the North Shahrekord Shear Zone, defined by NW-striking steeply dipping mylonitic foliation with low-angle stretching lineation. Kinematic indicators, such as quartz sigmoids, feldspar porphyroclasts, mica fish, and C- and C′-type shear bands, reveal a dextral strike-slip movement with a minor reverse component along the North Shahrekord Shear Zone. Recrystallized phengite from mylonitic eclogite and metagranite have been recently dated (40Ar-39Ar) at 178–170 Ma and ~110 Ma, respectively, indicating that the Sanandaj-Sirjan Zone was dominated by dextral transpression phases in the Early-Middle Jurassic and Early Cretaceous. The transpressional phases are interpreted to have occurred in response to the oblique subduction of the Neo-Tethys Ocean under the Iranian microcontinent.

Fold pattern analysis around Kanjamalai Salem district Tamilnadu

Kanjamalai one of the fascinating location in Southern Granulite Terrain (SGT) for studying Archaean geology and structures as the entire hill is made up of variety of rock types like two pyroxene granulite, amphibolites, quartzo - feldsapthic gneisses, banded iron formation, and intrusive rocks like dunite, peridotite and pegmatite and beautifully carved structures. The entire hill resembles a canoe shape with doubly plunging fold structure with E-W elongation. The entire hillock seems to sit pretty on mylonitised hornblende biotite gneisses which also have a common N70-95 degree trend and sub vertical dip with NE plunge which is in contradiction to centrally plunging lineations of the hill. The SW part of Kanjamalai near Chinasrirangapadi was displaying beautiful fold structures, with interference pattern out of which six domains were selected for detailed study and analysis. The multiple generation folded structure will have a clue in reconstructing the deformation history of this Kanjamalai. The observed f1, f2 and f3 folds show significant Type III interference pattern as that of Ramsay and 01 and 03 type folds of Bernhard Grasemann. Wavelength –amplitude analysis was made to generalize and regroup the observed folds in to high amplitude, high wavelength or open folds, low wavelength and Mesoscopic folds. And visual harmonic analysis was made to analyse the symmetry of the folds and analyze the geometry, symmetry and harmony and genesis of the fold in terms of relative timing of the events.

Extension-related buttress-like folds, the western side of the Gulf of Suez rift, Egypt

The wedge-shaped St. Paul block, western side of the Gulf of Suez rift, exposes Late Cretaceous beds that are folded into transverse folds. The block is bounded from the east and west by east- and southeast-dipping rift-related normal faults, respectively. This study reveals that the transverse folds are extension-related, and formed due to the buttress-like effect that was created during the movement along the rift-related normal faults. The more competent Eocene hanging-wall block of the N–S-striking fault buttressed against the moving less competent Cretaceous hanging-wall block of the NE–SW-striking fault. A localized zone of shortening developed between the two fault trends causing the intervening wedge-shaped block to be crumpled and folded into a series of kilometer-scale, gentle SSE-plunging folds. The buttressing, within block internal localized strain caused by the zigzag geometry of the NE–SW-striking fault, and the effect of the minor cross fault explain the SSE-to-ESE plunges of the mesoscopic folds. This study turns the attention to the buttress-like effect as one of the mechanisms of extension-related folds during rifting.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

The North Cascades orogen (northwestern USA) provides an exceptional natural laboratory with which to evaluate potential temporal and kinematic links between processes operating at a wide range of crustal levels during collapse of a continental arc, and particularly the compatibility of strain between the upper and lower crust. This magmatic arc reached a crustal thickness of ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional transtension was marked by magmatism, migmatization, ductile flow, and exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core coeval with subsidence and rapid deposition in nonmarine basins adjacent to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is the larger of two regions of exhumed deep crust with Eocene cooling ages in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to 53 Ma; the youngest orthogneisses overlap 40 Ar/ 39 Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional domains, and strong stretching parallel to lineation of hinges of mesoscopic folds in the Skagit Gneiss Complex are compatible with transtension linked to dextral-normal displacement of the Ross Lake fault zone, the northeastern boundary of the Cascades core. The other deeply exhumed domain, the 9–12 kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging lineation and gently to moderately dipping foliation, which are associated with top-to-the-north noncoaxial shear. This gneiss is separated from overlying metamorphic rocks by a folded detachment fault. The Eocene Swauk and Chumstick basins flank the southern end of the Cascades core. In the Swauk basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, and then covered by ca. 49 Ma Teanaway basalts and intruded by associated mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin subsided rapidly. Extension directions from these dikes and from Eocene dikes that intruded the Cascades core are dominantly oblique to the overall trend of the orogen (275°–310° versus ∼320°, respectively) and to the northwest-southeast to north-south ductile flow direction in the Skagit and Swakane rocks. This discordance implies that coeval extensional strain was decoupled between the brittle and ductile crust. Strain orientations at all depths in the Cascades core contrast with the approximately east-west extension driven by orogenic collapse in coeval metamorphic core complexes ∼200 km to the east. Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part from dextral shear along the plate margin and from along-strike gradients in crustal thickness and temperature.

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.

Fold dating: A new Ar/Ar illite dating application to constrain the age of deformation in shallow crustal rocks

We propose a deformation dating method that combines XRD quantification and Ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay-bearing layers. Two folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB), which was deformed from Late Cretaceous to Eocene, are used to illustrate the method and its future potential.Variations in mineral composition, illite-polytype, crystallite-size (CS) and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds. This analysis examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, versus possible regional thermal overprints. The Ar total-gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M1 (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5 ± 0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9 ± 2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. The method offers a novel approach to radiometric dating of clay-bearing folds formed at very low-grade metamorphic conditions, and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts.

Deformation style in the Munsiari Thrust Zone: a study in the Madlakia–Munsiari–Dhapa section in north-eastern Kumaun Himalaya

The Main Central Thrust demarcates the boundary between the Lesser Himalaya and the Higher Himalaya in the Himalayan orogen. Several definitions of the Main Central Thrust have been proposed since it was originally described as the southern boundary of the crystalline rocks (the Main Central Thrust mass) in the Kumaun-Garhwal Himalaya. The long-held contention that the Munsiari Thrust represents the Main Central Thrust has been negated by recent isotopic studies. One way to define the Main Central Thrust is that it is a ductile shear zone that is delimited by the Munsiari Thrust (MCT-I) in south and the Vaikrita Thrust (MCT-II) in north. The alternative proposition that the Vaikrita Thrust represents the Main Central Thrust is fraught with practical limitations in many parts of the Himalaya, including the study area. In the metamorphic rocks bounded between the Vaikrita Thrust and the Munsiari Thrust, the isoclinal folds of the earliest phase are routinely ascribed to the pre-Himalayan orogeny, whereas all subsequent folding phases are attributed to the Himalayan orogeny. This article elucidates the structural characteristics of the kilometre-thick Munsiari Thrust Zone and revisits the issue of pre-Himalayan orogenic signatures in the thrust zone. With the help of high-resolution field mapping and the analyses of mesoscopic scale structures, we demonstrate that the Munsiari Thrust is a typical fault zone that is made up of a fault core and two damage zones. The fault core traces the boundary between the quartzite and the biotite-gneiss. The damage zones consist of the low-grade metasedimentary rocks in the footwall and the gneiss-migmatite in the hanging wall. The entire fault zone shares an essentially common history of progressive ductile shearing. Successively developed mesoscopic folds trace various stages of progressive ductile shearing in the damage zones. Two recognizable stages of the shearing are represented by the early isoclinal folds and the late kink folds. As the strain during progressive deformation achieved the levels that were too high for accommodation by ductile flow, it was released by development of a tectonic dislocation along a mechanically weak boundary, the Munsiari Thrust. The isoclinal folds and the Munsiari Thrust were developed at different stages of a common progressive deformation during the Himalayan orogeny. Contrary to the popular notion of consistency with respect to orientation, the stretching lineations show large directional variability due to distortion during the late folding.

Neoproterozoic structural evolution of SE Sinai, Egypt: II. Convergent tectonic history of the continental arc Kid Group

The Neoproterozoic Wadi Kid Group includes volcano-sedimentary sequences deposited in active continental margin back-arc, remnant arc and intra-arc settings. The ENE–WSW to E–W arc trend and arrangement of arc-related basins suggests subduction to the N or NNW. The four stage deformation history (events D1–D4) of the northern Wadi Kid area is described in the companion paper (part I). This contribution (part II) reveals three deformation events (D1–D3) in the southern Wadi Kid area. These three events can be correlated with D1–D3 in the northern Wadi Kid area, with differences in D1–D3 intensity and orientation patterns between the northern and southern areas. In the southern Wadi Kid area D1 probably involved NW-ward thrust stacking of beds with lesser folding effects. D2 structures include gently dipping foliation, semi-recumbent mesoscopic folds, and SSE-vergent thrust faults. D3 is a macroscopic folding event with mainly gentle upright WNW–ESE trending folds. The Quneia Diorite intruded syn-D2 and is also D3 affected. For the Wadi Kid area as a whole, recent workers interpreted D1 as the main crustal thickening event, followed by D2 gravitational collapse, regional extension and core complex exhumation. Our investigation finds both D1 and D2 to be folding and thrusting events (i.e. both convergent tectonic events) that emplaced high T schists in the north over the lower T arc metavolcanics in the south. The core complex and thrust tectonic interpreted models are both consistent with clockwise metamorphic P– T–t histories, however, they have distinct P– T–t–D diagrams.

The Monte Penna thrust (southern Tuscany, Italy): geometry and kinematic of a collisional structure affecting the Tuscan Nappe during the Northern Apennines orogenic building

In this paper we describe the geometry and kinematics of a contractional structure, consisting of a thrust and related minor structures, which doubled the Tuscan Nappe cropping out in the southern side of the Monte Amiata geothermal region (Selvena-Castell’Azzara area), during the building of the Northern Apennines. Such a thrust (hereafter the Monte Penna thrust, MPT) is documented, in this area, for the first time. The integration of fieldwork with mine data from two different cinnabar/stibnite mines helped us to better define the thrust geometrical setting both at the surface and underground. Thrusting took place during the Early Miocene and produced the tectonic repetition of the Tuscan Nappe succession. The thrust gave rise to two tectonic subunits (SU1 and SU2) separated by a contractional shear zone up to 3m thick. Such a shear zone consists of three structural domains characterised by different structures such as folds, a tectonic foliation, reverse faults and Riedel shear fractures. They indicate a top-to-the ESE sense of shear, coherently with the general staking pattern of the Northern Apennines belt at least for the study area and surroundings. The thrust-sheet was deformed by cartographic- to mesoscopic folds, N135° to N220° striking on average, NE- and SE-verging, respectively. The tectonic setting recognised in the study area is coherent with that of the Tuscan Nappe described for the Monte Amiata region and the whole southern Tuscany, resulting characterised by duplex system. Such a duplex system was affected by extensional structures since the Middle Miocene. Presently, the thrusts consist of relic structures preserved within extensional horses. The whole evidence possibly make the Tuscan Nappe duplex system comparable with the structure of the Tuscan Metamorphic «basement», in terms of geometry, stacking pattern and timing, suggesting a probable common structural evolution even if a different structural levels.

Transected folds with opposite patterns in Terena Formation (Ossa Morena Zone, Portugal): anomalous structures resulting from sedimentary basin anisotropies

The Terena Formation is located in the central part of the Ossa-Morena Zone (OMZ) and outcrops in the core of a latter (D3) first order syncline. This Formation is a Lower Devonian flysch and shows an unusual “Z” shape, with a central sector trending nearly N-S, and the tips trending NW-SE. This central sector is crossed by the cleavage (NW-SE) showing an apparent dextral (clockwise) transection pattern, anomalous and opposite to the regional widespread sinistral (anti-clockwise) transpression. The same sector with cartographic dextral transection, shows at outcrop scale, mesoscopic folds with a sinistral transection. During the Lower Devonian a N-S trending basin was developed as an effect of an early tectonic deformation phase. This trough was filled with turbidites and its elongated geometry determined the shape of the main syncline. We propose that the dextral transection pattern, at cartographic scale, result from the superposition of the NW-SE upright S3 cleavage on this major regional structure controlled by a sedimentary trough. The mesoscopic folds, observed on the upper levels of the sedimentary sequence were not influenced by the topographic anisotropy of the basin, and therefore they developed a left transection, according to the regional deformation mechanisms. The “Z” shape of the syncline could be explained as a consequence of two major tectonic shear zones situated along the north and south boundaries of the OMZ, respectively the Tomar-Badajoz-Cordoba Shear Zone and the South Iberian Suture, lined by the Beja-Acebuches Ophiolitic Complex. Both shear zones have a sinistral transpressive character and were active during late Variscan tectonic events.

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.

Messinian-Early Pliocene crustal shortening along the Tyrrhenian margin of Tuscany, Italy

This paper illustrates the results of structural studies carried out in the western margin of Tuscany along a major crustal structure. Surface deformation of sediments filling different basins aligned on top of this major structure (from north to south: the Fine Basin, the Sassa-Guardistallo basin, the Rio Guardigiano area in the Lustignano basin) allow us to date its tectonic activity to the Messinian-Early Pliocene. In these areas, structures such as reverse and strike-slip faults and mesoscopic folds are widely developed. Structural analysis determined a compressive stress field with the σ1 oriented from E-W to NE-SW active from Messinian to Early Pliocene. At the southern end of this crustal structure, the Gavorrano antiform and the granitic pluton (radiometric age of granite ∼4.4 Ma) coring this fold correlate with a thrust ramp anticline at depth, and thus constrain thrust activity to the Early Pliocene. These data document a Messinian-Early Pliocene compressive activity that contrasts with models invoking continuous extensional tectonics affecting the hinterland since the Late Oligocene-Middle Miocene in the frame of a back-arc-slab retreating process. The results presented therefore raise the question of which geodynamical model could account for such a complex structural evolution of Northern Apennines hinterland.

Thrusting and multiple folding in the Neoproterozoic Pan-African basement of Wadi Hodein area, south Eastern Desert, Egypt

Detailed field mapping and structural studies of the area around the mouth of Wadi Hodein, some 20 km west of Shalatein at the Red Sea coast in the south Eastern Desert of Egypt, revealed four phases of structural deformation (D1–D4) affecting the Neoproterozoic Pan-African basement rocks. D1 is related to arc–arc collision and is represented by ENE–WSW oriented megascopic upright open folds associated with low angle thrusts and mesoscopic tight, overturned and recumbent F1 folds. Kinematic indicators indicate thrusting towards the SSE. D2 is represented by NNW–SSE oriented megascopic and mesoscopic folds, which are tight, verge towards the WSW and display a left-stepping en echelon pattern. D3 includes major NNW–SSE trending sinistral shear zones that show subordinate reverse fault components and dip steeply towards the ENE. These sinistral shear zones are comparable with the Najd Fault System, as they display a similar sense of movement and relationships to earlier structures. Therefore, they are interpreted to be the continuation of the Najd Shear System in southern Egypt. D2 and D3 are related to accretion of east and west Gondwana. D4 is represented by E–W oriented dextral faults with left-stepping segments. The first three deformation events are in agreement with the general evolutionary model for the East African Orogen in the Arabian–Nubian Shield that begins with NNW–SSE shortening, followed by ENE–WSW compression and subsequent deformation by the NNW–SSE striking Najd Fault System. The E–W dextral faults may be the conjugate shear fractures to the D3 NNW–SSE oriented sinistral wrench faults or are related to a subsequent event, D4. NW–SE oriented gold-bearing quartz veins originated during D1 and were subsequently deformed by D2–D4 events.

The high P/T Sambagawa extrusional wedge, Japan

A metamorphic inversion along the Asemigawa section of the Besshi unit in the Sambagawa high P/T metamorphic zone has been explained previously by S-vergent major recumbent folding or thrusting. Through careful structural field investigations, however, we recognize the D2 Asemigawa detachment fault at the boundary of the highest-grade (oligoclase–biotite zone) thermal culmination in the belt. We have also discovered a series of normal faults in the hanging wall of the major detachment in the upper domain and a series of thrust faults bounding every metamorphic mineral zone of the footwall in the lower domain. Outcrop-scale D2 asymmetric folds are closely associated with axial-planar pressure-solution cleavages, and are genetically related to the above D2 normal faulting and thrusting as fault-propagation folds. Northward fold vergence is found only in the upper domain and southward vergence is found only in the lower domain. Exceptions to this rule occur only for parasitic folds in the overturned limbs of mesoscopic folds, which indicates flexural slip as a likely fold mechanism. We propose an extrusion wedge model for the inverted metamorphism and accordingly, this mechanism may also account for a large part of the exhumation of the Sambagawa metamorphic rocks, although westward shear at the time of D1 stretching lineation has partly contributed to the exhumation by counter-flow in the accretionary wedge. We infer that M1 with its retrograde actinolite schists represents a D1 ductile shear zone, but it contributes as much as a component of the D2 Asemigawa detachment, other normal and thrust faults, and asymmetric folds, as reactivated brittle shear zones. Thus, shear direction had changed orthogonally and drastically between ductile D1 stretching and ductile–brittle transition to brittle D2 extrusion. We infer that this transition reflects the collision of an actively spreading mid-ocean ridge and is a consequence of increased coupling of plates and orogen-normal stress, followed by oblique subduction. D0 and D −1 record eastward down-flow during the oblique underthrusting of oceanic plate, before peak metamorphism M1.

Structure of the Buller and Takaka Terrane rocks adjacent to the Anatoki Fault, northwest Nelson, New Zealand

Structures and microstructures within flanking lithologies of the Buller and Takaka Terrane boundary, the Anatoki Fault, record three pre‐Cenozoic deformation events in northwest Nelson. Each deformation event recognised in the Buller Terrane has structures that can be matched in style, orientation, and timing of development with that in the adjacent Takaka Terrane. D 1 is represented by north‐trending, upright or overturned to the west, large‐scale folds with an axial planar slaty cleavage. D 1 is mid Devonian in age and relates to amalgamation of the Buller and Takaka Terrane. D 2foliation occurs in a zone of ductile deformation adjacent to the Anatoki Fault near Boulder Lake. Rb‐Sr geochronology, and the relationship between the foliation and metamorphism associated with the adjacent c. 111 Ma Mt Olympus Pluton, suggests D 2 formed in the Early Cretaceous following pluton emplacement. D 3 is represented mainly by mesoscopic folds with an axial planar crenulation cleavage. D 3 structures crenulate and refold both D 1 and D 2 structures and are mid Cretaceous in age. Both D 2 and D 3 relate to an extremely active and changing tectonic period of New Zealand in the Early to mid Cretaceous. The east‐dipping Anatoki Fault in northwest Nelson records a complex history of ductile and brittle movement. Tectonites from central segments record ductile/brittle east‐over‐west reverse‐slip associated with D 1. To the north, D 2 tectonites from the Boulder Lake area record Early Cretaceous ductile dextral‐slip reactivation. To the south, tectonites from the Crow River and Mt Benson area record ductile/brittle dextral‐normal slip which, in the Crow River area, represents reactivation that postdates the intrusion of the c. 137 Ma Crow Granite. From the Crow River southwards, the Anatoki Fault has also undergone late Cenozoic brittle reactivation.

Structural controls on Tertiary orogenic gold mineralization during initiation of a mountain belt, New Zealand

Two types of structurally controlled hydrothermal mineralization have occurred during folding of fissile schist in southern New Zealand: fold-related mineralization and normal fault-related mineralization. Both types have the same mineralogy and textures, and are dominated by quartz–ankerite veins and silicified breccias with ankeritic alteration. Most mineralized zones are thin (centimetre scale), although host schist is commonly impregnated with ankerite up to 20 m away. Thick (up to 5 m wide) mineralized zones are generally gold-bearing and contain pyrite and arsenopyrite with stibnite pods locally. Some of these auriferous zones have been extensively mined historically despite rugged topography and difficult access. Mineralization occurred during regional tectonic compression in the initial stages of development of the Southern Alps mountain belt at the Pacific–Australian plate boundary in the Miocene. Most of the gold-bearing deposits occur in east to south-east, striking normal faults that cut across mesoscopic folds in a belt that coincides with the southern termination of a regional-scale north trending antiform. Mineralized zones have similar structural control and relative timing to a nearby swarm of Miocene lamprophyre dykes and carbonatites. Limited stable isotopic data (C and O) and trace element geochemistry suggest that there was probably no genetic link between the igneous activity and gold mineralization. However, these two types of fluid flow have been controlled by the same tectonically created crustal plumbing system. This Miocene hydrothermal activity and gold deposition demonstrates that orogenic (mesothermal) mineralization can occur during the inception of an orogenic belt, not just in the latter stages as is commonly believed. These Miocene structures have been preserved in the orogen because the locus of uplift has moved northwards, so the early-formed gold deposits have not yet been structurally overprinted or eroded.

The structure and kinematics of an imbricate stack of oceanic rocks in the Jurassic accretionary complex of Central Japan: an oblique subduction model

The Jurassic accretionary complex of the Mino Belt in the Takayama area, Central Japan includes a major unit characterized by abundant bedded chert and basalt slabs. This unit has an imbricate structure, represented by the repetition of stratigraphic layers composed of sandstone, black mudstone, felsic tuff, bedded chert, siliceous claystone, limestone and basalts. Shear zones subparallel to the trend of the slabs occur in the siliceous claystones and basalts. Deformation structures in the shear zones suggest that phyllosilicate-rich layers within the accretionary complex were preferentially weakened due to grain size reduction by cataclasis and pressure solution-accommodated deformation. Restored shear directions in the siliceous claystones and basalts indicate dextral kinematics subparallel to the trench axis. In contrast, mesoscopic asymmetric folding of the bedded chert, indicated from the restored shear, was oblique or normal to the trench axis. To interpret the two distinct shear directions, a model is proposed in which the constituent rocks of this unit were obliquely subducted dextrally along the East Asian continental margin. Due to the oblique subduction, slip vectors were apparently partitioned into trench-orthogonal components in the shallower part of the subduction zone and trench-parallel components in the deeper part.

Late-Svecofennian transpressive deformation in SW Finland—evidence from late-stage D3 structures

Late-Svecofennian deformation was investigated in the Bjensböle area of SW Finland. The deformation history was revealed by following marker horizons of amphibolite, marble and black schist, in addition to microscopic studies and U-Pb monazite geochronology. The D1 structures are transposed and overprinted by D2 structures. The mesoscopic F2 folds are tight to isoclinal with fold axes plunging 20-40[ddot] NE-ENE. Regional large-scale open F3 folds formed during the main D3 phase, and the macroscopic structure in Bjensböle is an F2-F3 interference pattern formed by N-S compression. The area consist of low P/high T amphibolite facies sillimanite gneiss and low P/low T amphibolite facies quartz schist, separated by a low P/low T amphibolite horizon in the central part of the area. The sillimanite gneiss yielded a U-Pb monazite age of 1824±5 Ma. This age gives a timing for the peak metamorphism and the D3 folding on the Kemiö Island. After the peak metamorphism some deformational structures developed. A NE-SW directed crenulation cleavage in the quartz schist and the amphibolite, small open folds in the marble, a foliation in the sillimanite gneiss and a muscovite orientation in the quartz schist all have the same direction. These structures are interpreted as late-stage D3 (D3L). The angle between the NE-SW directed D3L structures and the locally predominant S0 + S1 + S2 schistosity is ca. 25[ddot]. The structural evidence related to the D3L point to a dextral sense of shearing along roughly E-W directed zones with a NW-SE compression. This is related to a regional ductile shearing signifying the vaning stages of the late-Svecofennian transpressive deformation in SW Finland.