Shearing Event Research Articles

A dynamic atlas of interference patterns in superimposed, opposite sense ductile shear zones

Ductile shear zones that reactivate a coplanar shear zone with opposite shear sense are known from a variety of tectonic environments. Recognition of reactivation requires understanding the interference patterns that form when microstructures produced in the first event (D1) are modified by the second event (D2). We use numerical modelling to demonstrate the effect of D1 structures on the development of shear zone interference patterns during a coplanar, opposite sense D2 ductile shearing event. Seven models were generated from increasing D1 dextral simple shear strains (γdextral = 2–14) and we then superimposed D2 sinistral shearing (γsinistral = 10). The interference patterns produced are highly variable with geometric relationships between weak layers and strong lithons determining deformation style. Shear zones with high D1 strain can more easily accommodate D2 strain because more strain is localised into long, weak phase C planes, which are readily inverted and reused during sinistral shear. Interference structures in models and naturally deformed rocks include rotated σ-clasts, folded D1 S planes, disharmonic and hook folds, and cuspate layers of weak phase. We present a dynamic atlas of interference patterns produced due to overprinting shear zones to facilitate identification of these zones in nature.

Late Paleozoic tectonic transition from subduction to collision in the Chinese Altai and Tianshan (Central Asia): New geochronological constraints

The evolution of the largest accretionary orogen in the world, the Central Asian Orogenic Belt (CAOB), involved a prolonged accretion history since the Neoproterozoic, followed by a collisional phase in response to the closure of the Paleo-Asian Ocean in the latest Paleozoic. The exact process for the tectonic transition from subduction to collision is still poorly constrained. Here we address this issue by investigating the late Paleozoic tectonic evolution of the Chinese Altai and Tianshan orogens in the western CAOB. We provide new geochronological data from two areas of the Chinese Altai and Tianshan orogens, which allow us to link polyphase deformation with orogenic processes. In the Fuyun area of the Chinese Altai Orogen, we conducted monazite U-Pb dating on four samples that show pervasive foliations with the originally sub-horizontal orientation (DS2/DQ2). The monazite U-Pb ages cluster at ∼284 to 281 Ma, which interpreted to represent the time of sub-horizontal foliations (DS2/DQ2) that may result from orogen-parallel extension related to the collision of the Chinese Altai Orogen with the East Junggar Terrane. Farther south, in the Gangou area of the Chinese Tianshan Orogen, we obtained a muscovite 40Ar/39Ar plateau age of 256.6 ± 0.6 Ma for a mica schist from the dextral South Central Tianshan Shear Zone. This age confirms the Permian activity of dextral strike-slip deformation (DCT4) in the Chinese Tianshan Orogen. In contrast, three mylonitic schist/granitoid samples from the dextral Main Tianshan Shear Zone are characterized by 40Ar/39Ar plateau ages of 353.9 ± 1.9 Ma (biotite), 353.9 ± 1.5 Ma (biotite) and 352.1 ± 0.7 Ma (muscovite). We interpret these early Carboniferous ages to either represent a pre-Permian dextral shearing event, or to record an early Carboniferous tectono-thermal event with recrystallized micas not reset during the Permian strike-slip deformation (DCT4). An additional 40Ar/39Ar plateau age of 280.9 ± 0.5 Ma (hornblende) from a mafic dike (dolerite) that crosscuts macroscopic folds (DST2) in the southern Chinese Tianshan Orogen, provides a minimum time constraint for these folds. This age supports the simultaneous folding deformation (DST2) with dextral shearing (DCT4) in the Chinese Tianshan Orogen. Combined with a comprehensive synthesis of available geological and geochronological data, we argue that orogen-parallel extension and transpressional tectonics might have played a significant role in the late Paleozoic arc/continental amalgamation of the western CAOB.

Re-regeneration to reduce negative effects associated with tail loss in lizards

Many species of lizard use caudal autotomy, the ability to self-amputate a portion of their tail, regenerated over time, as an effective anti-predation mechanism. The importance of this tactic for survival depends on the degree of predation risk. There are, however, negative trade-offs to losing a tail, such as loss of further autotomy opportunities with the regenerated tail vertebrae being replaced by a continuous cartilaginous rod. The common consensus has been that once a tail has been autotomised and regenerated it can only be autotomised proximal to the last vertebral autotomy point, as the cartilage rod lacks autotomy planes. However, anecdotal evidence suggests that although the regenerated portion of the tail is unable to autotomise, it can re-regenerate following a physical shearing event. We assessed re-regeneration in three populations of the King’s skink (Egernia kingii), a large lizard endemic to south-west Western Australia and surrounding islands. We show that re-regeneration is present at an average of 17.2% across the three populations, and re-regenerated tissue can comprise up to 23.3% of an individual’s total tail length. The ability to re-regenerate may minimise the costs to an individual’s fitness associated with tail loss, efficiently restoring ecological functions of the tail.

CT scanning and flow measurements of shale fractures after multiple shearing events

A shearing apparatus was used in conjunction with a Hassler-style core holder to incrementally shear fractured shale cores while maintaining various confining pressures. Computed tomography scans were performed after each shearing event, and were used to obtain information on evolving fracture geometry. Fracture transmissivity was measured after each shearing event to understand the hydrodynamic response to the evolving fracture structure. The digital fracture volumes were used to perform laminar single phase flow simulations (local cubic law with a tapered plate correction model) to qualitatively examine small scale flow path variations within the altered fractures. Fractures were found to generally increase in aperture after several shear slip events, with corresponding transmissivity increases. Lower confining pressure resulted in a fracture more prone to episodic mechanical failure and sudden changes in transmissivity. Conversely, higher confining pressures resulted in a system where, after an initial setting of the fracture surfaces, changes to the fracture geometry and transmissivity occurred gradually. Flow paths within the fractures are largely controlled by the location and evolution of zero aperture locations. A reduction in the number of primary flow pathways through the fracture, and an increase in their width, was observed during all shearing tests.

Tectonic and chemical implications of cathodoluminescent microstructures in quartz, Parry Sound domain, Ontario, Canada

Cathodoluminescence (CL) imaging of quartz in orthogneiss of the Parry Sound domain, Grenville Province, Ontario, Canada, reveals four dark microstructures—medium-dark grains, mantles, sinuous lines, and straight lines. The Parry Sound domain experienced granulite-facies deformation and metamorphism followed by variable degrees of amphibolite-facies retrogression associated with the development of retrograde shear zones. We integrate our observations of CL-visible quartz microstructures with structures observable with optical and scanning transmission electron microscopy, analyses of water species concentration, and trace element distribution in quartz, as well as with microstructures in plagioclase, to determine their characteristics and elucidate their timing and mechanisms of formation relative to the deformational history. Medium-dark grains are primarily located in and near kilometer-scale shear zones at the margins of the Parry Sound domain, have notably darker cores than most samples in the interior, and may have higher water contents than other samples in the interior. These grains formed during late shearing, at a lower temperature than CL-bright grains; they thus provide evidence that the large shear system narrowed with time. Dark mantles occur at grain boundaries and correlate with higher concentrations of Fe. They formed after the first shearing event, but while still at amphibolite facies, and they provide evidence for post-kinematic alteration at grain boundaries. Dark sinuous lines correspond to subgrain boundaries that formed after retrograde shearing, but while still at amphibolite facies. Dark straight lines correspond to fluid inclusion trails and most likely formed at low temperatures and pressures during exhumation.

Metamorphic evolution of blueschists, greenschists, and metagreywackes in the Cretaceous Mt. Hibernia Complex (SE Jamaica)

A suite of samples collected mainly from river cobbles in the Mt. Hibernia Complex in SE Jamaica includes metamorphosed basaltic and clastic sedimentary rocks, which are interpreted here as part of a Cretaceous accretionary prism. The metabasic rocks occur as blueschists and greenschists, whereas the metasedimentary rocks are former greywackes s.l ., or psammopelites, and can now be characterized as phyllites. Eight characteristic mineral assemblages can be defined within the metabasic sample suite, involving amphibole (Na amphibole and/or actinolite), pumpellyite, lawsonite, epidote, chlorite, titanite, phengite, albite and quartz. Augite is an important relict phase of the magmatic source rocks. White mica, chlorite and quartz predominate in the phyllites in variable proportions, but Na amphibole, epidote, albite and titanite may also be important. Calculated P–T pseudosections show that the assemblage glaucophane–pumpellyite–chlorite occurs only in a restricted field along the lower P–T limit of the blueschist facies. Pseudosections also indicate that peak P–T conditions for the blueschist samples studied here lie in the range 6.5–7.2 kbar and 310–340°C, that is at the upper stability limit of pumpellyite and lawsonite, and in the range 6.4–7.8 kbar and 340–380°C for greenschist samples. The P–T range for phyllites overlaps with both. Thus, blueschists and greenschists represent comparable pressure but different temperature conditions of formation. Literature data on the mapped distribution of metabasic rock types suggests an isobaric metamorphic field gradient in the Mt. Hibernia Complex, with temperatures increasing slightly but significantly from blueschist to greenschist zones. We interpret this gradient as reflecting either increasing temperatures in the accretionary complex away from the subducting slab or thermal re-equilibration in a cooling subduction zone. Sodium-Ca amphiboles, predominantly winchite, are commonly observed as rims around Na amphibole and formed together with albite porphyroblasts during decompression. Comparison with published information on the miscibility gap between Na and Ca amphiboles shows that many of these compositions fall within this gap for temperatures around 350°C. Thus, these amphiboles either grew metastably below the critical temperature of the gap, or some as yet undefined parameter must be responsible for drastically altering the shape of the gap. Published descriptions of deformation in the Mt. Hibernia Complex show that coaxial pure shear initially predominates, as is common in accretionary complexes; however, a late shearing event, postulated here to be caused by the collision between the Great Arc of the Caribbean and the continental margin of North America at 75–70 Ma, is held responsible for the rapid exhumation of the combined blueschist–greenschist sequence.

The relationship between gold mineralization, exhumation of metamorphic core complex and magma cooling: Formation of the Anjiayingzi Au deposit, northern North China Craton

The Anjiayingzi gold deposit is hosted by the Jiguanzi Quartz Monzonite in the footwall of the Kalaqin Metamorphic Core Complex (MCC) at Chifeng in the northern part of the North China Craton (NCC). An earlier (ca. 135–125Ma) top-to-the-NE ductile–brittle shear and a later (ca. 121Ma) top-to-the-NW and SE brittle detachment normal faulting are recognized around the Kalaqin MCC. The Jiguanzi Quartz Monzonite intruded the footwall of the Kalaqin MCC at ca. 133±1Ma with crystallization temperatures of 640–691°C, pressures of 167–271MPa and an average depth of 8.6km. Gold veins in the Anjiayingzi gold deposit are hosted by the monzonite and controlled by the NE-trending brittle–ductile and brittle fault zones. These ore-bearing faults are products of a regional sinistral shearing event coinciding with the earlier top-to-the-NE ductile–brittle shear at the Kalaqin MCC. The ore of the Anjiayingzi deposit is characterised by its low Au/Ag ratio (<1) and high sulphide content (4–30vol.%). The assemblage associated with gold mineralization consists of: (1) quartz–coarse-grained pyrite; (2) quartz–fine-grained pyrite; (3) quartz–polymetallic sulphide; and (4) quartz–carbonate. The ore formed at 245–358°C from a H2O–CO2–NaCl fluid system with salinities of 1.3–15.6wt.% NaCl equiv. Trapping pressures of ore fluids were 50–110MPa, corresponding to an average depth of ~2.5km under a lithostatic load and ~7.5km under a hydrostatic load. Fluid mixing and immiscibility are potential causes for the rapid precipitation of sulphides carrying Au. The stable isotope δD (−80.3 to −96.5‰) and δ18OH2O (3.7 to 5.5‰) of the ore fluids, the consistent Pb isotope compositions between the ore-bearing sulphides and K-feldspar from the Jiguanzi Quartz Monzonite, and a >2billionyear time gap between the gold mineralization and metamorphism of the basement all indicate that the mineralizing fluids and metals are derived from a concealed intrusion near the major eastern Louzidian Fault in the Kalaqin MCC. This proposal is supported by the presence of mafic magmatic enclaves and rhyolite dykes in the Jiguanzi Quartz Monzonite, and higher average Thtotal (322°C) and δ34S (2.4‰) of Eastern veins than those of Western veins (with a Thtotal of 287°C and δ34S value of 0.3‰). The ca. 133–126Ma gold mineralization at Anjiayingzi was formed during rapid cooling of ca. 133–126Ma Jiguanzi Quartz Monzonite at a cooling rate of 42–71°C/Ma, and fast exhumation and cooling of the Kalaqin MCC between ca. 135 and 121Ma at a cooling rate of 30°C/Ma. This process was accompanied by fluid immiscibility and mixing within the structural transition from ductile–brittle to brittle deformation with resultant fluid-pressure fluctuations between the supra-hydrostatic and hydrostatic regimes.

Evidence for Cool Extrusion of the North Indochina Block along the Ailao Shan Red River Shear Zone, a Diancang Shan Perspective

Much debate surrounds the temporal-thermal-structural evolution of the >1000-km NW-SE-trending Ailao Shan–Red River (ASRR) shear zone. This mainly reflects the contradictory interpretations of the timing and P-T conditions for the occurrence of left-lateral shearing event due to a diverse data set of mineral ages. Our new microstructural, petrological, and geochronological data from the southern section of the Diancang Shan (DCS) block along the west wall of the ASRR support slow cooling from ∼300°C to ∼150°C from the Middle Eocene to the Late Miocene, accompanied by left-lateral shearing activity. This conclusion is based on the following: (1) The lower greenschist facies assemblages of biotite replaced by muscovite or chlorite, the albitized K-feldspar, and the stable association of K-feldspar + chlorite rather than muscovite + biotite are universally observed for the synkinematic Sn+1 and mylonitic S/C fabrics from both quartzofeldspathic and amphibolite domains. (2) New 40Ar/39Ar data for K-feldspar and muscovite of various structural domains such as the K-feldspar phenocrysts (∼20 to ∼40 Ma) from the early Triassic magmatic protolith (zircon U-Pb ages of 230–250 Ma), pre- to synkinematic muscovite (∼17 to ∼23 Ma) in mylonites, and K-feldspar (∼5 to ∼10 Ma) in postmylonitic cross-cutting veins produce an age spectrum that increases from ∼5 to ∼40 Ma under lower greenschist facies. Together these data indicate that the DCS block was slowly extruded along the ASRR shear zone through much of the Tertiary. These results contradict earlier interpretations of rapid cooling from synkinematic amphibolite metamorphic conditions caused by shear heating based in part on the assumption that all Miocene ages were cooling ages rather than crystallization ages.

Kinematic evolution of the Fodjomekwet-Fotouni Shear Zone (West-Cameroon): Implications for emplacement of the Fomopéa and Bandja plutons

The Fodjomekwet-Fotouni shear zone (FFSZ) belongs to the western part of the Central Cameroon Shear Zone (CCSZ). From the border to the core, the FFSZ is made of: (i) protomylonites, (ii) garnet-sillimanite mylonites and (iii) garnet-sillimanite ultramylonites. Structural data indicate (1) a N40°E mylonitic foliation trend (strongly dipping toward NE or SW) with strike varying between N34°E–78°NW at Fotouni and N52°E–61°SE at Fodjomekwet; (2) a stretching lineation (plunging toward NE or SW) with best line varying between 224/4 at Fotouni and 232/10 at Fodjomekwet; (3) sinistral (N–S) and dextral (NE–SW) kinematic markers, respectively correlated to the D2 and D3 of the CCSZ deformation phases. The presence of relic sinistral shear markers wrapped by dominant dextral shear markers rendered difficult the understanding of the FFSZ evolution. Anisotropy of magnetic susceptibility in the FFSZ shows that magnetic fabrics are characterized by dominant NE–SW and locally N–S to NNE–SSW trending foliation and lineation. These directions show that mylonitisation was initiated in the N–S direction during the sinistral syn-D2 shearing, evolved and reoriented toward NNE–SSW and NE–SW between 613 and 590Ma. The sinistral shearing event was thereafter followed by a NE–SW trend dextral syn-D3 reactivation of the N50°E fault between 580 and 552Ma. The sinistral phase seems to have favoured the evolution of fractures in pull-apart structures. The pull-apart opening of fracture enables the upwelling of magma and the emplacement of the Fomopéa and the Bandja plutons (FBP). This synkinematic emplacement is witnessed by the elongated shape and the NE–SW trending direction of these massifs. Kinematic emplacement of FBP controlled by active shear zone is comparable to syntectonic plutonism in eastern Nigeria and NE Brazil.

Metamorphism, anatexis, zircon ages and tectonic evolution of the Gongshan block in the northern Indochina continent—An eastern extension of the Lhasa Block

The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~ 22 Ma. Leucosome and S-type granite of 22–53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages. This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore, the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~ 53–22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.

Middle Jurassic tectono-magmatic evolution in the southwestern margin of the Gyeonggi Massif, South Korea

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates and geochemical and isotopic (Sr and Nd) data were obtained for Middle Jurassic granitoids from the Buyeo area along the southwestern margin of the Gyeonggi Massif, South Korea. Foliated 179 Ma granitoids are dominated by granite and diorite, non-foliated 167 Ma granitoids include granite to alkali-feldspar granite, and the youngest, non-foliated 165 Ma granitoids consist of alkali-feldspar granite. The 179 Ma and 167 Ma rocks are geochemically typical of I-type granitoids, and their LILE enrichment and negative Nb-Ta-Ti-P anomalies are common features of subduction-related granitoids. In contrast, 165 Ma rocks are A-type granitoids that show HREE-enriched patterns with significant depletions of Sr, P, Eu, and Ti. I-type granitoids dated at 179 Ma and 167 Ma have I Sr = 0.7092–0.7158, 0.7206–0.7263 and ɛNd(T) = −13.96 to −13.55, −21.47 to −19.51, respectively. A-type, 165 Ma granitoids have relatively lower I Sr (0.7052) and strongly negative ɛNd(T) (−20.47), reflecting melt derivation from the lower crust. These results suggest that the formation of granitoids along the southwestern boundary between the Gyeonggi Massif and the Okcheon Belt took place at ca. 179–167 Ma by the subduction of the paleo-Pacific plate. The 165 Ma post-orogenic A-type granites intruded in an extensional setting, providing a critical time constraint on the termination of Jurassic plutonism in South Korea. The 179 Ma foliated granitoids are similar to syn-kinematic granitoids in and around the Honam shear zone, indicating that ductile shear movement may have occurred at ca. 179 Ma. Biotite K-Ar ages of deformed Precambrian rocks and Middle Jurassic granitoids from the Buyeo area generally yield ages of 144–152 Ma and 124–135 Ma. These results probably reflect pervasive reactivation during Latest Jurassic to Early Cretaceous deformation that postdated the main ductile shearing event along the southern boundary of the Gyeonggi Massif.

Precambrian mafic magmatism in south Indian granulite terrain

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.

Progressive soft sediment deformation within a subglacial shear zone—a hybrid mosaic–pervasive deformation model for Middle Pleistocene glaciotectonised sediments from eastern England

This paper presents a hybrid ‘pervasive’ and ‘mosaic’ model for the progressive deformation of a stratified diamicton complex in eastern England within an active subglacial shear zone. Sedimentary evidence from undeformed low-strain zones towards the base of the sediment pile indicates that the tectonised sediments were originally deposited as a series of subaqueous flows in a glaciolacustrine basin. These sediments have subsequently been deformed during a progressive subglacial shearing event. This event is divided into three stages: ductile folding and sediment remobilisation (D1), brittle faulting in the form of Reidel shears (D2) and hydrofracturing and sediment remobilisation (D3). The dominant control on the pattern and style of deformation appears to relate to the rate of thrust-induced till accretion, and the aquifer properties and pore water content and/or pressure of the deforming sediments.

Late Sinistral Shearing along Gondwana’s Paleo‐Pacific Margin in the Ross Orogen, Antarctica: New Structure and Age Data from the O’Brien Peak Area

New isotopic data from O’Brien Peak, Antarctica, provide constraints on the timing of deformation within the Ross Orogen in the Queen Maud Mountains. U‐Pb data, in conjunction with structural data from a mylonitic granite, indicate that sinistral, orogen‐parallel ductile shearing occurred after ∼489 Ma, the granite’s crystallization age, and $$\mathrm{before}\,\,478\pm 3\,\mathrm{Ma}\,\,\mathrm{to}\,\,473\pm 3\,\mathrm{Ma}\,$$, the 40Ar‐39Ar cooling ages of undeformed and synkinematic white mica, respectively. These data provide evidence for the youngest sinistral shearing event within the Ross Orogen to date and indicate that left‐oblique subduction possibly continued into the Early Ordovician along Gondwana’s paleo‐Pacific margin.

Alpine Metamorphism in the Aspromonte Massif: Implications for a New Framework for the Southern Sector of the Calabria-Peloritani Orogen, Italy

Structural, petrologic, and thermobarometric data presented in this paper contribute to our understanding of the tectono-metamorphic evolution of the lowest tectonic slices of the Aspromonte Massif (southern Calabria, Italy), which crop out in three main tectonic windows. Despite previously being considered different units, they exhibit the following similar features: the same tectonic evolution, analogous blasto-deformation relationships, and absence of Hercynian mineralogical assemblage relics. Similar P-T paths indicate early HP-LT Meso-Alpine metamorphism (400-600° C at 0.95-1.35 GPa), evolving in the Oligocene-Miocene toward a subsequent retrograde shearing event ranging from 480° to 610°C and 0.50 to 0.95 GPa. The latest retrograde evolution is characterized by 350-480°C and 0.32-0.62 GPa. In this new tectonic framework, it is proposed to group the metapelite sequences defining the Madonna di Polsi Unit. Data presented herein suggest that the pre-Alpine geodynamic setting of southern Calabria was a thinned continental margin made up of Hercynian basement and Mesozoic terrigenous-carbonate sedimentary cover. This continental margin evolved during the early Meso-Alpine stage into a subduction zone beneath the European plate, followed by Neo-Alpine syn-convergent exhumation along a deep-seated mylonitic shear zone. These processes are responsible for the Alpine metamorphic overprint on the Hercynian terranes, as well as for Alpine metamorphism of their Mesozoic cover.

Fabrics of pre- and syntectonic granite plutons and chronology of shear zones in the Eastern Borborema Province, NE Brazil

We used the fabrics of two granite plutons and U/Pb (SHRIMP) zircon ages to constrain the tectonic evolution of the E-trending Patos shear zone (Borborema Province, NE Brazil). The pre-tectonic Teixeira batholith consists of an amphibole leucogranite locally with aegirine-augite. Zircons from a syenogranite yielded crystallization ages of 591 ± 5 Ma. The batholith fabrics were determined by anisotropy of magnetic susceptibility (AMS) and mineral shape preferred orientation. The fabrics support pre-transcurrent batholith emplacement, as evidenced by: (i) magmatic/magnetic fabrics in low susceptibility (<0.35 mSI) leucogranites highly discordant to the regional host rock structure, and (ii) concordant magnetic fabrics restricted to high susceptibility (>1 mSI) corridors connected to shear zones branching off from Patos. One of these satellite shear zones controlled the syntectonic emplacement of the Serra Redonda pluton, which yields a crystallization age of 576 ± 3 Ma. This late shearing event marks the peak regional deformation that, south of Patos, was coupled to crustal shortening nearly perpendicular to the shear belt. The chronology of the deformational events indicates that the major shear zones of the eastern Borborema are late structures active after the crustal blocks amalgamated.

El Sibai gneissic complex, Central Eastern Desert, Egypt: Folded nappes and syn-kinematic gneissic granitoid sheets – not a core complex

The El Sibai area of the Central Eastern Desert (CED) of Egypt consists of an ophiolitic association of arc metavolcanics, ophiolitic rocks, mélange, metasediments and minor mafic intrusions; and a gneissic association of amphibolite, gneissic diorite, tonalite, granodiorite and granite. Previous studies of the El Sibai area have identified the gneissic association as a lower crustal infrastructure in sheared contact with upper crustal ophiolitic association suprastructure, and have presented it as an example of a metamorphic or magmatic core complex. Detailed structural remapping of the El Sibai area reveals that the gneissic association rocks are not infrastructural but form a unit within the ophiolitic association nappes. Furthermore, the El Sibai structure is not domal in shape, and is not antiformal. The main gneissic association rocks are tabular intrusions roughly concordant with the shears dividing the ophiolitic association into nappes, and are syn-kinematic with the nappe stacking event (∼700–650 Ma). The gneissic granite tabular intrusions and their ophiolitic host were later folded about upright NW–SE trending mainly open folds during a NE–SW directed shortening event (∼625–590 Ma). Subsequently, NW–SE regional extension effects became evident including low angle normal ductile shear zones and mylonites. The latest gneissic red granites are syn-kinematic with respect to these shear zones. Probably continuing from the low-angle shearing event were steep normal faults, and sinistral WNW and N–S trending transcurrent faults (∼590–570 Ma). The normal faults mark the southeastern and maybe also the northwestern limits of the El Sibai gneissic association rocks. The El Sibai complex is not a core complex, but exemplifies the overlap of NW–SE folding and NW–SE extensional which is a significant theme of CED regional structure.

Structural and stratigraphic controls on the origin and tectonic history of a subducted continental margin, Oman

Eclogites and blueschists exposed in Saih Hatat, Oman, record the subduction and exhumation of continental crustal material beneath the Cretaceous Semail Ophiolite during ophiolite obduction. The eclogite-bearing lower plate, originally part of Oman's distal continental margin, is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle, high strain, décollement structure. A major tectonic break, currently poorly exposed, records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote-blueschists. The subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint. The décollement truncates structures and the metamorphic field gradient in the lower plate, but does not significantly truncate structures or stratigraphy in the upper plate. It is not responsible for the exhumation of the high pressure rocks to mid-crustal levels. Most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active, and post ophiolite emplacement onto the platform carbonate sequences. A revised tectonic model is presented which accounts for the structural, geochronological and metamorphic observations.

40Ar/ 39Ar Dating on Mylonites in the ductile shear zones from South Tan-Lu fault belt in Anhui Province, East China

The area for this study is in the joint part of Tan-Lu fault belt with the East Qinling-Dabie orogenic belt in east part of China (Mattauer et al., 1985). Tan-Lu fault belt is an important deep fault belt in east China and Asian continent, and its southern part is the key part for geology, where the mylonites were developed in the ductile shear zones since its shifting movement, and the plastic deformation are commonly occurring in the mylonites. In this study the 40 Ar/ 39 Ar age of several kinds of deformed minerals have been obtained to illustrate the evolution of the ductile shear zones and formation of mylonites in this region. Methods The biotite and plagioclase samples separated from the mylonites were determined by 40 Ar/ 39 Ar fast neutron activation technique in the Institute of Geology and Geophysics, CAS. Some deformed minerals such as biotite and feldspar can record the chronological information since the period of ductile shearing event, these ages from the deformed minerals may represent the important period of the history of deformation caused by the shifting movement of Tan-Lu fault belt. Results Finally, five ages from single deformed minerals were obtained with the plateau ages of 98Ma, 109Ma, 104Ma, 106Ma and 113Ma, respectively. These ages represent the cooling time accompanying formation of mylonites, when the large scale of shifting movement of Tan-Lu fault belt occurred after the collision of Yangtze block and North China block since Triassic, which accords with the former studies in Dabie-Sulu region.

Structural evolution and 40Ar/39Ar dating of the Zanhuang metamorphic domain in the North China Craton: constraints on Paleoproterozoic tectonothermal overprinting

Abstract The Zanhuang domain in the hinterland of the North China Craton, consists of Archaean, Paleoproterozoic, Mesoproterozoic and Palaeozoic rocks, and was previously considered a Mesozoic metamorphic core complex. Our reappraisal of regional structural history and mineral 40 Ar/ 39 Ar geochronology shows that the domain was subjected to a number of deformation events and had a heterogeneous uplift-cooling history during the Paleoproterozoic. Four primary deformation events have been recognized in the domain. The first event reflects the earliest deformation in the region, however, only closed and rootless folds are preserved. D2 was dominated by WNW-ESE shortening and top-to-ESE thrusting in a collisional environment. D3 was a divergent extensional and ductile shearing event, driven by the post-collisional extensional collapse and exhumation of the thickened crust in the domain. D4 was characterized by localized ductile shearing with the development of three NNE-trending ductile shear zones. The 40 Ar/ 39 Ar plateau ages derived from the thermochronology analyses of the biotite concentrates from the metamorphic rocks are interpreted as cooling ages through biotite 40 Ar/ 39 Ar closure temperature (300 °C) and this allows us to constrain the timing of deformation events. In conjunction with the evidence from structural analyses and previous geochronological data, we conclude that 1870 and 1870–1826 Ma define the timing of D1 and D2 events, respectively, while 1826–1793 Ma represents the age of D3. The last event (D4) occurred at 1689–1633 Ma, as constrained by 40 Ar/ 39 Ar plateau ages of biotite concentrates from the mylonites within the ductile shear zones. These data, together with K-feldspar 40 Ar/ 39 Ar geochronology, allows us to depict the cooling history of the region during the Paleoproterozoic and Mesoproterozoic. It appears that the uplift-cooling rate varies temporally and spatially in the region.