Crustal Scale Research Articles

Efficient and accurate modeling of ocean bottom seismometer data using reciprocity

Seismic experiments in which the number of sources is considerably larger than the number of receivers occur regularly. An important example is the collection of crustal scale seismic data using ocean bottom seismometers and marine sources. We describe a method to accurately and efficiently compute synthetic seismograms for such experiments by using finite differences and reciprocity. We show numerically how to decompose an explosive source into its equivalent body force components using the staggered-grid finite-difference technique with a fourth-order approximation for the spatial derivative and a second-order approximation for the temporal derivative. This decomposition results in a source configuration where the equivalent body forces are defined in 12 points around the point where the ex-plosive source is applied. We then use the derived equivalent body forces for the explosive source and seismic reciprocity theorems to convert the common receiver gather to a common shot gather. The method is tested on a structurally complex elastic model of the crust and the results show that it is accurate within floating point precision. The synthetic data are compared to data from a real ocean bottom seismometer experiment conducted across a continent-ocean transition zone. A good fit in terms of traveltime is observed for many of the prominent seismic phases. The amplitude fit of these arrivals is not always as good as the traveltime fit. This indicates that full-waveform modeling of such data can provide useful information about the subsurface that cannot be obtained from traveltime modeling. If enough data are available, the modeling method can be used in full-waveform inversion.

P- and S-wave velocity model along crustal scale refraction and wide-angle reflection profile in the southern Korean peninsula

The onshore seismic experiment, KCRT2004, explored the crustal velocity structure of the southern Korean peninsula. We present an interpretation of seismic data along this 340km long NNW–SSE trending profile. The crust was found to consist of three layers: the upper, middle, and lower crust with P- and S-wave velocities ranging 5.50 to 6.95km/s and 2.82 to 3.91km/s, respectively. The average P-wave velocity (6.26km/s) and Pn velocity (7.82–7.88km/s) are lower than the worldwide average of continental crust. Moho depths are 29.0–34.9km, gradually thickening toward south.The Vp/Vs ratio of crustal material is estimated to be 1.73 (σ=0.249) for the upper and middle crust and the ratio increases with the depth in the lower crust. The Gyeonggi massif in the north of the profile has a lower Vp/Vs ratio than other tectonic units. The average crustal Vp/Vs ratio of 1.74 (σ=0.253) is remarkably lower than the average value 1.78 (σ=0.27) for the bulk continental crust. The low average crustal Vp/Vs ratio is similar to that measured in eastern China. The empirical analysis using both P-wave velocity and Vp/Vs ratio shows that the upper and middle crust is dominantly felsic and the lower crust is intermediate in composition. The absence of the mafic material in the lower crust that is also found in eastern China contrasts with the generally accepted global model of the mafic lower crust.

Structural styles of mountain building: Controls of lithospheric rheologic stratification and extensional inheritance

The strength of the crust and the mantle lithosphere are strongly influenced by the temperature, the thickness and the composition of the crust and mantle lithosphere, and by inherited weaknesses. It consequently strongly depends on the geodynamic setting and is then expected to be different from one orogen to another, which might explain the diversity of deformation styles. Here we use 2D thermomechanical models at lithospheric scale to study the effect of the strength of the upper and middle crust on the geometry of contractional systems. The role of extensional inheritance is included by forward modeling the formation of rift basin or passive margin and using the resulting extensional structure as initial condition for lithospheric scale inversion. Our results demonstrate that: 1) crustal strength strongly controls the width and the elevation of orogenic wedge; 2) relative weak crust facilitates efficient propagating of thick‐skinned crustal scale thrusts; 3) contraction of a strong crust leads to the formation of long thrust sheets and an anti‐formal stack in the core of the orogen or the accretion of upper crust depending on the depth of the decoupling zone in the mid crust; 4) erosion processes favor the localization of shortening in a narrow crustal orogen; 5) rifting inheritance can explain the presence at shallower depth of lower crustal or mantle material previously upwelled and facilitates the propagation of the deformation in the external part of the chain. The range of predicted behaviors is compared to first order with the Zagros, the Alps and the Pyrenees, three natural examples for which the crustal structure is well constrained by geophysical data.

Anatomy of 2.57 - 2.52 Ga granitoid plutons in the eastern Dharwar craton, southern India: Implications for magma chamber processes and crustal evolution

We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton (EDC) corresponding to different crustal levels. At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite. On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement. The plutons are composite which comprises voluminous intrusive monzodiorite, quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery. Numerous xenoliths, Mafic Magmatic Enclaves (MME), disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic facies. The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively. Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins, phenocrysts and C-S fabrics. The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas. Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma Archaean magmatic accretion, reworking and cratonization. chambers. Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion, reworking and cratonization.

Plio-Plistocene in-sequence thrust propagation along the Main Central Thrust zone (Kumaon–Garhwal Himalaya, India): New thermochronological data

The Kumaon and Garhwal Himalayas, NW-India have similar geologic, tectonic, topographic and precipitation pattern. However, low-temperature thermochronological age pattern varies significantly along the strike as well as across the strike of major faults. Here, we interpret the new set of thermochronological age data across the strike of major faults namely Vaikrita Thrust (VT), Munsiari Thrust (MT), Berinag Thrust (BT) and the Baijnath nappe along Pindari valley in the Kumaon–Garhwal Himalaya. In the present work, 18 new apatite fission track (AFT) ages of samples collected along a north–south transect have been reported. Ages in the hanging wall of VT which range from 2.1±0.2 to 4.2±0.7Ma, have been found to be becoming younger linearly with distance from north to the VT. This trend crosses the VT with significant jump in ages. In the south of the VT, ages are lying between 0.3±0.1 and 3.9±0.8Ma, and show linear variation with distance from the VT to BT. No significant jump in ages across the MT is observed and the linear trend of younging southward is continuing till the BT. The break and displacement in age pattern with youngest ages close to thrusts possibly indicate progressive late thrust movement. It reflects sequential uplift and cooling towards the south from the VT to BT which is consistent with an in-sequence style of thrust propagation. We compare our new data to the published thermochronological data from its adjacent traverses along the Goriganga and Dhauliganga valleys (~25km away from the present traverse towards east and west respectively) in the Kumaon–Garhwal Himalaya in order to understand potential along-strike variations in the kinematics. Our observations indicate a dynamic coupling between tectonic and surface processes in the Garhwal–Kumaon Himalaya and support that the geometry of crustal scale faults and their associated kinematics control exhumation pathways of rocks.

From oceanic crust to exhumed mantle: a 40 year (1970–2010) perspective on the nature of crust under the Santos Basin, SE Brazil

Abstract The first comprehensive geological and geophysical surveys of the Brazilian continental margin during the 1970s recognized the crust in the SE Brazilian basins as ‘anomalous’ but models for the opening of the South Atlantic proposed at that time invoked a very narrow continent–ocean transition. Nevertheless, such studies established the presence of a thick sedimentary prism, including an extensive salt layer under the São Paulo Plateau. The earliest reconstructions for the South Atlantic invoked a seaward shift of the spreading axis to account for the asymmetric widths of the salt layer between the Brazilian margin and its conjugate in offshore Africa. Although our understanding of continent–ocean transition has progressed since then, direct seismic imaging at crustal scale has only been possible recently through long offset (10 km), deep recording (18 s), pre-stack depth migrated (PSDM) to 40 km, seismic-reflection data. These data allow us to generally image the Moho from under thick continental crust (>30 km) to thin oceanic crust ( c. 5 km). Although the nature of the transitional crust is still contested, these seismic data allow for constraints on various models for continent–ocean transition. Future integrated studies utilizing PSDM and refraction-seismic data will further refine these models.

Tectonic Amalgamation of Crustal Blocks along Gadag-Mandya Shear Zone in Dharwar Craton of Southern India

Abstract The crustal scale Shear Zone that can be traced from Gadag in the north to Mandya in the south in Dharwar Craton of southern India is considered as the boundary between two subcratonic blocks namely the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) in published literature. The present study on the Gadag-Mandya Shear Zone (GMSZ) in the Javanahalli-Hagalvadi sector has brought out a detailed account on the disposition, geometry and kinematics of the shear zone, and also the distinctive structural patterns of the two adjacent supracrustal belts, namely the Chitradurga schist belt (CSB) in the west and Javanahalli schist belt (JSB) in the east. The JSB has an overall N-S striking and gentle easterly dipping geometry, the structural features of which are indicative of a predominant non-coaxial deformation and westward transportation of the supracrustal assemblage. In contrast, deformation in the CSB, which is defined mainly by a flattening type of strain, has produced an overall verticality of the structures (dominant foliation, axial planes of regional folds). Difference in metamorphic grade is apparent from the mineral assemblages in the rocks of these two belts, while the JSB showing an amphibolite facies CSB is restricted to green schist facies mineral assemblages. The 2 to 3 km wide, N-S striking, prominent ductile shear zone, GMSZ, has affected both CSB and JSB rocks and also the syntectonically emplaced Bukkapatna granite. The geometric and kinematic evidences from both the belts and the GMSZ point to an E-W horizontal compressional regime. The gentle dipping, amphibolite grade JSB and its underlying gneisses acted as an allochthon which got amalgamated with the subvertical, green schist grade CSB by a top-W thrusting along the GMSZ. The subvertical CSB impeded further up-dip movement of the eastern block, resulting in the subsequent development of N-S trending sinistral strike slip shear along the contact of the two contrasting belts to accommodate the continued E-W compression. The linear Bukkapatna Granite, synkinematically emplaced along GMSZ shows evidences of both magmatic stage and solid state deformation in the shear zone.

Geochemistry of the late phanerozoic mafic dykes from the Moyar shear zone, South India, and its implications on the spatial extent of Deccan Large Igneous Province

The late Phanerozoic dykes of the Moyar shear zone mark a prominent intrusive structure in the Precambrian crystalline rocks of northern Kerala. The dykes, having variable strike length and width, show a predominant NW-SE trend and basaltic composition with SiO2 ranging from 48.59 % to 49.53 % and normative quartz/olivine. The chondrite normalized REE patterns are fractionated, parallel to sub-parallel, and are generally uniform but with negative Eu-anomalies. Chemical characteristics are typical of MORB or within-plate basalts and suggest derivation of melt from a fertile or plume-related mantle source with a considerable correlation to Deccan basalts. This is consistent with the regional geological setting including the volcanism, associated with a Proterozoic crustal scale shear zone, occurring long before the onset of seafloor spreading in the Indian Ocean. The possibility of redefining the southern limit of the Deccan Large Igneous Province is examined using the characteristic features of the dykes.

Geothermal and rheological regime in the Po plain sector of Adria (Northern Italy)

We present a thermo-rheological study of the Adriatic crust in the central Po plain (Northern Italy), which records the convergence of the Alpine and Apennine external fronts. The present thermal regime of the crust is inferred from geological and geophysical data from oil exploration. A set of temperature data from 37 deep boreholes (bottom hole, drill stem, and production test temperatures) is used to estimate undisturbed formation temperatures down to 6-7 km of depth. Best quality data are used to estimate upper crustal geotherms, which are subdivided into four groups taking into considerations the following factors: i) presence/absence of relevant fluid circulation; ii) conductive thermal refraction; iii) recent sedimentation, and iv) other disturbing tectonothermal processes (e.g., recent tectonics). Assuming average corrections for temperature data affected by recent sedimentation effects, geotherms fall within the same range and provide a homogeneous thermal signal. Temperature measurements in the upper ~7 km are used to constrain a 1-D thermal model at the crustal scale. The best fitting geotherm gives a thermal gradient of ~23 ± 5 mK m-1 (over 7 km of depth), a surface heat flow of ~46 ± 9 mW m-2, and an estimated Moho temperature of 620 ± 80 °C.Using the estimated crustal structure and composition and the best-fitting geotherm, a representative strength envelope is obtained.For hydrostatic and supra-hydrostatic pore pressures, the calculated total crustal strength of this sector of the Adria microplate (not including the upper mantle contribution) is in the 1.8-2.5 TN m-1 range. Considering also the lithospheric mantle contribution, the total lithospheric strength is estimated to be between 23 (wet peridotite)and 39 (dry peridotite) TN m-1.

Crustal structure and rift architecture across the Krishna–Godavari basin in the central Eastern Continental Margin of India based on analysis of gravity and seismic data

The Krishna–Godavari basin is a rifted passive margin basin that developed orthogonally to the NW–SE trending Pranhita–Godavari graben along the central Eastern Continental Margin of India in response to the continental rifting process and the subsequent seafloor spreading between India and eastern Antarctica during the early Cretaceous period. The 3-D gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin. The gravity-derived crustal models indicate that the crust at the eastern Indian shield margin is 39–41 km thick and thins to as much as 20–23 km at the Ocean Continent Transition (OCT) in the offshore. There is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin: the Cuddapah basin, Pranhita–Godavari Graben, and the Eastern Ghat Mobile Belt that indicates the lateral segmentation of the margin. A zone of high density (3.0 g/cm3) crustal material at the OCT separates the pure continental and oceanic crusts on either side. The Moho is shallowest along this zone and brought the upper mantle notably close to the surface, which indicates that it could be comprised of Proto-Oceanic Crustal rocks. Furthermore, the present study highlights a major basement structural high in the deep offshore area of the margin. The geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks, comprised of low density crust, and lies close to the OCT towards offshore. Based on the inferred tectonic reconstruction, we believe that this structural high could be a continental fragment that was left behind during the process of breakup between India and the Elan Bank.

Magnetic fabrics in characterization of magma emplacement and tectonic evolution of the Moyar Shear Zone, South India

The Moyar Shear Zone (MSZ) of the South Indian granulite terrain hosts a prominent syenite pluton (∼560 Ma) and associated NW-SE to NE-SW trending mafic dyke swarm (∼65 Ma and 95 Ma). Preliminary magnetic fabric studies in the mafic dykes, using Anisotropy of Magnetic Susceptibly (AMS) studies at low-field, indicate successive emplacement and variable magma flow direction. Magnetic lineation and foliation in these dykes are identical to the mesoscopic fabrics in MSZ mylonites, indicating shear zone guided emplacement. Spatial distribution of magnetic lineation in the dykes suggests a common conduit from which the source magma has been migrated. The magnetic foliation trajectories have a sigmoidal shape to the north of the pluton and curve into the MSZ suggesting dextral sense of shear. Identical fabric conditions for magnetic fabrics in the syenite pluton and measured field fabrics in mylonite indicate syntectonic emplacement along the Proterozoic crustal scale dextral shear zone with repeated reactivation history.

The crustal structure of the southern Argentine margin

SUMMARY Multichannel reflection seismic profiles, combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost Argentine margin, at the transition from a rifted to a transform margin and outline the extent of the North Falkland Graben. Based on these data, we establish a regional stratigraphic model for the post-rift sediments, comprising six marker horizons with a new formation in the Barremian/Lower Cretaceous. Our observations support that a N–S trending subsidiary branch of the North Falkland Graben continues along the continental shelf and slope to the Argentine basin. During the rift phase, a wide shelf area was affected by the E–W extension, subsequently forming the North Falkland Graben and the subsidiary branch along which finally breakup occurred. We propose the division of the margin in two segments: a N–S trending rifted margin and an E–W trending transform margin. This is further underpinned by crustal scale gravity modelling. Three different tectono-dynamic processes shaped the study area. (1) The Triassic/Early Jurassic extensional phase resulting in the formation of the North Falkland Graben and additional narrower rift grabens ended synchronously with the breakup of the South Atlantic in the early Valanginian. (2) Extensional phase related to the opening of the South Atlantic. (3) The transform margin was active in the study area from about Hauterivian times and activity lasted until late Cretaceous/early Cenozoic. Both, the rifted margin and the transform margin are magma-poor. Very limited structures may have a volcanic origin but are suggested to be post-rift. The oceanic crust was found to be unusually thin, indicating a deficit in magma supply during formation. These findings in combination with the proposed breakup age in the early Valanginian that considerably predates the formation of the Parana–Etendeka continental flood basalt provinces in Brazil and Namibia question the influence of the Tristan da Cunha hotspot during the initial formation of the South Atlantic.

Carbon isotope studies of auriferous quartz carbonate veins from two orogenic gold deposits from the Neoarchean Chitradurga schist belt, Dharwar craton, India: Evidence for mantle/magmatic source of auriferous fluid

Carbon and oxygen isotopic compositions of carbonates from auriferous quartz carbonate veins (QCVs) of two orogenic gold deposits – Ajjanahalli and Guddadarangavvana Halli (G.R. Halli) – from the Neoarchean Chitradurga schist belt of the Dharwar craton, southern India are examined to understand the origin of the mineralizing fluids. The average carbonate carbon (δ13Cpdb) and oxygen (δ18Osmow) isotope compositions of QCVs of Ajjanahalli are −5.5±1.3‰ and 14.1±2.7‰, respectively. The same ratios for the QCVs of G.R. Halli are −6.2±1.9‰ and 14.1±0.5‰. The corresponding average fluid δ13C and δ18O compositions are −5.81±1.14‰, 13.78±5.1‰ for Ajjanahalli and −4.64±0.7‰, −6.50±0.6‰ for G.R. Halli. The δ13Cpdb of syn-sedimentary carbonates of BIF of Ajjanahalli (−1.8±0.1‰), carbonated metabasalts of Ajjanahalli (−1.4‰) and G.R. Halli (−1.3‰) fall in the compositional range of marine carbonates (0±2‰). As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO2 with δ13C values similar to or more enriched than parent rock, the carbonate or fluid δ13C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO2 or carbonates) obtained in this work cannot be the result of metamorphism. It is proposed that gold mineralizing fluids were derived from juvenile magmatic melts and were channeled through crustal scale shear zones to give rise to the gold deposits.

The Moho: Boundary above upper mantle peridotites or lower crustal eclogites? A global review and new interpretations for passive margins

Abstract We have performed a global study of 2D crustal scale wide-angle profiles across passive margins, with regard to local elevations in the Moho which could possibly be interpreted as indicative of lower crustal eclogites. A total of 16 candidates have been found, mainly in the North Atlantic and around Australia. These cases make up c. 6% only of the total profile length studied, confirming the interpretation of the Moho generally representing the top of the mantle. The interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the Barents Sea, off mid-Norway, in the Newfoundland Appalachians and in the Yilgarn Craton, western Australia. It is also possible that there is a genetic link between the formation of Caledonian eclogites and the Jan Mayen Fracture Zone, which is the only major fracture zone in the North Atlantic. Several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain, such as coastlines and shelf edges, indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture. Interpreting the Moho beneath the Bedout High on the northwest shelf of Australia as the top of a body of lower crustal eclogites, may imply that the northern termination of the Lambert Shelf represents a paleo suture zone and that the western termination of the Broome Platform acted as a major transfer zone. The significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history, subsidence and hydrocarbon maturation modelling.

Detrital zircon U–Pb ages of metasedimentary rocks from Sierra de Valle Fértil: Entrapment of Middle and Late Cambrian marine successions in the deep roots of the Early Ordovician Famatinian arc

We obtained detrital zircon ages from three metasedimentary rocks with siliciclastic protoliths that were buried to paleodepths of 20–27 km during Early Ordovician Famatinian arc magmatism. In all three samples, U–Pb zircon age distributions are polymodal but the dominant peaks make well-defined clusters, matching the ages of orogenic systems that characterize West Gondwana. The most prominent peaks of constituent age populations at 520 Ma, 600 Ma, and 1050 Ma reflect derivation from sources that are characteristic of the West Gondwana margin. Moreover, grains with Meso- and Paleo-Proterozoic ages appear as small fractions (<5%). The siliciclastic protoliths to metasedimentary rocks from the deep Famatinian crust are dominantly composed of detritus derived from two sources: 1) a nearby magmatic arc or a rapidly exhuming orogen, and 2) sedimentary recycling of Neoproterozoic–Lower Cambrian sedimentary successions broadly grouped as Puncoviscana Formation. Our findings reveal that granulite–facies metasedimentary rocks in the deep crust of the Famatinian arc have detrital zircon patterns closely resembling those found in nearby Middle to Upper Cambrian quartz-rich turbidite sequences from central Famatina, the Cordillera Oriental and the Puna. At the regional scale, these sedimentary successions yield maximum depositional ages corresponding to the Middle Cambrian (<520 Ma). This indicates that metasedimentary rocks in the Sierra de Valle Fértil were rapidly buried to deep levels after deposition. Our results are also consistent with the existence of a crustal scale paleo-suture located at the western margin of the Sierras Valle Fértil and La Huerta that explains the differences of detrital zircon age patterns between metasedimentary rocks in the Valle Fértil and age-equivalent sedimentary units in the nearby Cuyania Terrane.

The Role of Fluids in Promoting Seismic Activity in Active Spreading Centers of the Salton Trough, California, USA

We interpret seismic activity in the active spreading centers of the Salton Trough to indicate 1) a magmatic intrusion in the lower crust beneath the active Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault systems; and 2) fluids in the upper crust that have been released from that magmatic body. The absence of a magmatic body and fluids at the location of fossil spreading centers along the Sand Hill and Algodones faults ndicated by little or no seismic activity in those areas. We show several lines of evidence to point out that both melt and fluids related to the seismic activity. In particular, receiver function analysis, Vp/Vs ratios, and tomographic data reveal low velocity zones coincide with the location of the active spreading centers. High Vp/Vs ratios and low velocity zones in the lower crust and upper mantle attributed to melt inclusion, while low Vp/Vs ratios in the upper crust are attributed water inclusions. Frequency-mag- nitude distributions characterized by high b-values in southern California; high b-values have also been associated with crustal fluids. A crustal scale model developed from the receiver functions, gravity, and magnetic data supports the existence of a magmatic intrusion within about 20 km of the surface southwest of the Salton Sea, that intrusion extends for 70 km in a SW-NE direction.

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

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

Uranium deposits associated with Na-metasomatism from central Ukraine: A review of some of the major deposits and genetic constraints

The paper reviews geological, structural, mineralogical, fluid inclusion, geochemical and stable isotope characteristics of U deposits related to Na-metasomatism in the Central Ukrainian Uranium Province (CUUP). The province contains numerous U deposits hosted in gneisses, migmatites and granites of the Ingul Paleoproterozoic Megablock and in iron formations along its margin with the Fore–Mid-Dnieper Mesoarchean Megablock. The richest U deposits are distributed along the nearly E–W striking Subotin–Mashorin tectonic zone, but they are tightly controlled by N–S and more rarely SW–NE striking crustal scale shear zones. Na-metasomatism and U-mineralisation were controlled by protracted tectonic activity along these shears zones from ductile to brittle regimes of deformation. A metasomatic zoning is developed from outermost unaltered lithologies inwards to dequartzified rocks (episyenites), aegirine–albitites (Na-metasomatites), andradite–epidote–Ca-amphibole–calcite-rich rocks (Ca–Mg-metasomatites), and ultimately to biotite–albitites (K-metasomatites). Main U-ore deposition, predominantly consisting of brannerite and uraninite, with lesser amounts of U-ferropseudobrookite, is associated with the Ca–Mg- and K-metasomatised zones superimposed on earlier aegirine–albitites. Na-metasomatites were formed at temperatures exceeding 500 °C possibly triggered by the onset of a plume in the mantle associated with the intrusion of the highly potassic Korsun–Novomyrgorod rapakivi granitic pluton at about 1750–1800 Ma, as constrained by U–Pb geochonologic data on the granites and uranium mineralisation. These intrusions have been partly reworked tectonically with recrystallisation under lower P–T conditions. The mineral paragenesis of Ca–Mg- and K-metasomatites crystallised during the cooling of the system down to 370–340 °C. Ca–Mg- and K-metasomatisms resulted in further dequartzification of the rocks and their enrichment mainly in Ca, Mg, K, U, Zr, Fe 2+ and Hf. The hydrothermal fluids were oxidised, moderately saline and highly alkaline. The high alkalinity of the fluids explains the dissolution of quartz, the deposition and stability of albite, the high Zr/Hf ratios (up to 224) in the metasomatites and the high solubility of U and Zr. U deposition was controlled by mixing with a reduced fluid which explains the increase of whole rock Fe 2+ concentration during uranium deposition. The last widespread hydrothermal event corresponds to a chlorite–epidote paragenesis superimposed on all previous mineral associations at temperatures down to less than 100 °C. H, O and C isotopic ratios in minerals and fluid inclusions extract from albitites suggest a surface/ formation water origin for the hydrothermal fluids producing Na-metasomatism and a mixed metamorphic-surficial/formation water origin for the fluids producing Ca–Mg- and K-metasomatism and the main U-mineralisation.

Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (∼2.2–2.0 Ga), western Burkina Faso

We have investigated three greenstone belts (Boromo, Houndé, and Banfora) and associated granitoid terrains, which form part of the Eburnean orogen, situated in western Burkina Faso. These belts expose tholeiitic basalts (basal unit) followed by calc-alkaline intermediate predominantly effusive volcanic and sedimentary suites geochemically reminiscent of present-day volcanic island arc environments. The basal mafic unit probably corresponds to a juvenile arc crust or oceanic plateau. It contains unusual megacrystic tholeiitic basalts, allowing us to correlate the western margin of the Boromo belt with the eastern margin of the Houndé belt. These two N–S trending belt-parallel tholeiitic units are interpreted as limbs of a crustal scale anticline, intruded and partially obliterated by tonalite–trondhjemite–granodiorite (TTG) and granite intrusions. Three deformation events (D1–D3) can be distinguished in western Burkina Faso. The first deformation phase (D1) operated under an E–W to WNW-oriented compression. Regional greenschist to lower amphibolite facies metamorphism and intense folding characterize early-Eburnean deformation phases, during which time the crust was thickened by lateral shortening of volcanic island arcs and concomitant magma input. The crustal-scale antiform between the Boromo and Houndé belts is attributed to the D1 event. Shallow water detrital Tarkwaian-type sediments were deposited during the late D1 event within the Houndé belt, in a belt-parallel basin extending for 400 km. The subsequent D2 phase overprints the structural grain of the study area, and is best visible in airborne magnetic data. It is characterized by N to NE-trending transcurrent shear zones, which are considered preferred host structures for gold mineralization. We suggest that the newly formed and thickened crust reached the maximum thickness supportable by a weak and hot mantle during the D1 phase, and the pure shear dominated compressional regime switched to simple shear dominated transpression during the subsequent D2 phase. Granitoid diapirism played an important role at all stages of the Eburnean crustal growth processes in particular through early volume addition to the newly formed orogen and through later accommodating part of the lateral shortening. Pluton emplacement contributed to the greenstone belt structuration at local scales; however, the regional scale system geometry was controlled by coaxial shortening of the viscous volcanic units (basalts, gabbros, and andesites) of the greenstone belts, supported by coeval magma input. The last D3 deformation, which is either late-Eburnean or perhaps even Pan-African in age, is characterized by shallow N or S dipping minor thrust faults or an E-W trending steeply dipping spaced crenulation cleavage and kink folds, occurring mainly in highly anisotropic lithologies across the study region.

FANTOM: Two- and three-dimensional numerical modelling of creeping flows for the solution of geological problems

A new finite element code for the solution of the Stokes and heat transport equations is presented. It has purposely been designed to address geological flow problems in two and three dimensions at crustal and lithospheric scales. A variety of rheologies has been implemented including nonlinear thermally activated creep and brittle (or plastic) frictional models. A cloud of particles is used to track materials in the simulation domain which allows to record the integrated history of deformation; its density is variable and dynamically adapted. The code is built on the Arbitrary Lagrangian–Eulerian kinematical description: the computational grid deforms vertically and allows for a true free surface while the computational domain remains of constant width in the horizontal direction. The code can be run in sequential or parallel mode. The parallelisation is based on the MPI paradigm and the domain decomposition algorithm is presented. The solution to the large system of algebraic equations resulting from the finite element discretisation and linearisation of the set of coupled partial differential equations to be solved is obtained by means of an efficient (sequential or massively parallel) direct solver. Details of implementation concerning plasticity, cloud handing, nonlinear convergence, strain accumulation and pressure smoothing are given. The sequential 2D version of the code is used to run the numerical sandbox benchmark at normal and high resolutions. The 2D parallel version is used in the case of a thermo-mechanically coupled extension experiment in which the mantle is present. The 3D parallel version is used to run a crustal scale orogeny experiment. The overall performance of the code, as well as the respect of the incompressibility constraint, of the yield stress criterion and of the volume conservation are discussed, along with parallel scalability. Benchmark results of scalar field advection, the Rayleigh–Taylor experiment and the falling block experiment are presented in Appendix.