Shear Belt Research Articles

Structural and litho-tectonic controls on Neoproterozoic base metal sulfide and gold mineralization in North Hamisana shear zone, South Eastern Desert, Egypt: The integrated field, structural, Landsat 7 ETM + and ASTER data approach

The Hamisana shear zone (HSZ), is an excellent example of high strain shear belt of the Neoproterozoic Arabian–Nubian Shield (ANS), situated in a position between three major Pan-African terranes (SE Desert, Gabgaba, and Gebeit terranes). The north Hamisana shear zone area (NHSZ), cuts across sequences of island arc-related metasediment-metavolcanic, which are structurally overlain by ophiolitic nappes and intruded by a number of granitoids and gabbroic rocks. Although it is interesting geological-tectonic setting for base metal sulfide and gold ores, there is a lack of detailed studies on structure, geology and mineralization in this area, which considered as poorly mapped remote region not only in Egypt but also in Sudan.For these reasons, we integrated image transformation methods include; Principal Components Analysis (PCA), Band Ratios (BR), False-Color Composite (FCC) and filtering on the spectral bands of two distinct datasets ETM+ and ASTER for detailed mapping of the NHSZ area. Spectral data fusion with structural data analysis and field observations provide a better understand of the temporal and spatial relationships between the litho-tectonic units and regional tectonic setting in the course of controls on base-metal sulfide and associated gold mineralization. Merging these different data sources in the study area suggests that, the base-metal sulfide and associated gold mineralization are restricted to quartzites, (member of island arc-related metasediments), and mostly controlled by D3 deformational event (main HSZ deformational event). This event characterized by transpressional stress regime, with σ1 trending E–W and σ3 trending N–S, activated conjugate NNW sinistral and NNE–SSW dextral strike-slip faulting, likely during syn-tectonic granodiorite emplacement.

Active tectonics along the Nebrodi–Peloritani boundary in northeastern Sicily (Southern Italy)

In the epicentral area of the seismic swarm of the June–September 2011, at southern edge of the Calabrian arc in NE Sicily, very recent extensional motions remobilised two main NW–SE dextral faults. The extensional reactivation of strike–slip faults responded to a new regional dynamic, also evidenced by GPS and seismological data. The inverted structures are aligned at the margin of a wide crustal block that is moving apart from the rest of the island and is uplifting faster than the adjacent regions. The active faults terminate to the northwest at the intersection with a prominent NNE trending fault that represents the western boundary of the mobile block. The vertical displacement along this border exactly matches the difference in elevation of the marine terraces resting inside and outside the block, respectively. On the contrary, only part of differential displacement of the marine terraces was actually accommodated as cumulative motion along the two NW oriented inverted faults, across the southwestern boundary of the block. Amounts of the vertical displacement were distributed on distinct fault planes of the previous dextral shear belts. The widespread fracturing is also the best explanation for the seismic swarm of the 2011, whose epicenters spread on a discrete rock volume rather than concentrated along a single fault plane. The diffuse fracturing seems to represent a peculiar style of deformation, connected to the tectonic inversion of previous strike–slip shear zones. Seismic swarm also affects the northern termination of the Calabrian arc where active extensional deformation reactivated previous strike–slip faults. The similarity of the two regions suggests that seismic swarm can be peculiar of extensional belts developed on previous strike–slip shear zones, along which the pre-existing geometry favours the dispersion of the tectonic motion on a network of small linked fault planes.

Permian clockwise rotations of the Ebro and Corso-Sardinian blocks during Iberian–Armorican oroclinal bending: Preliminary paleomagnetic data from the Catalan Coastal Range (NE Spain)

Paleomagnetic investigations of Early Permian and Triassic magmatic rocks from Catalan Coastal Ranges provide three magnetic directions which are interpreted as primary magnetizations and partly as overprints. The succession of magnetic directions is interpreted in terms of a succession of large-scale clockwise rotations, which bring the whole assembly of Pyrenees, Catalonia, Corsica-Sardinia and Maures-Esterel massifs from the southeast of the Massif Central to the west during Permian. This major movement is associated with clockwise rotation of northern limb of the Iberian orocline during Latest Carboniferous and Early Permian (~305–290Ma). Subsequently, Permian large-scale dextral transtensional–extensional shearing operated along Aquitanian Shear Belt between the Massif Central in the northeast and the Iberian Variscan massifs in the southwest. The latest phase of the rotation in the Late Permian-Early Triassic is associated with alkaline magmatism probably linked to Neo-Tethys opening activity in Western Europe.

K'Mudku A-type magmatism in the southernmost Guyana Shield, central-north Amazon Craton (Brazil): the case of Pedra do Gavião syenogranite

<p>The Mesoproterozoic K'Mudku event (1490 - 1147 Ma) is represented by a britlle-ductile shear belt that cuts across the Paleoproterozoic units in the southernmost Guyana shield, central-north Amazon craton. This event produced mylonitization and cataclasites at low/medium- to high-grade metamorphic, and local within-plate magmatism. In the Amazonas State, Brazil, A-type magmatism chronologically associated to K'Mudku has been reported for the Pedra do Gavião and Samaúma syenogranites. However, the spatial relationship between K´Mudku event and A-type magma generation are not yet adequately clarified in the region. The Pedra do Gavião syenogranite is a high-K alkaline, metaluminous, reduced A-type granite with a post-collisional to within-plate geochemical signature. It has U-Pb zircons crystallization age of 1218 Ma and inherited zircons with ages between 1820 and 1720 Ma, which, together with the Sm-Nd data, suggest melting of Paleoproterozoic basement rocks of the Cauaburi Complex (1810 - 1780 Ma) regional unit. These data demonstrate that the effects of the A-type magmatism associated to the end of the Grenvillian-Sunsas orogeny, reported primarily in the southwestern margin of the Amazon craton, may also be extended for the central-northern part of the Amazon craton. Probably the generation or emplacement mechanisms of A-type magma occurred with some degree of involvement in the final stages of the K´Mudku event. However, this tectonic framework conception still needs more geological and geophysical investigations. Therefore, these news data should instigate to the return of geological research in the region, as well as to debate on the tectonic evolution and A-type granites production during the Ectasian-Stenian period in the central-north Amazon craton.</p>

Review of the Senegalo-Malian shear zone system – Timing, kinematics and implications for possible Au mineralisation styles

The Kédougou Kénieba Inlier (KKI) of eastern Senegal forms a typical Paleoproterozoic greenstone belt characterised by low-metamorphic sequences of volcanic rocks and volcano-sediments that have been intruded at various stages by gabbroic suites and calc-alkaline granitoids. The main structures of the KKI comprise two anastomosing structures about N–S trending shear belts that are distinguished into the western Main Transcurrent Zone (MTZ) and the eastern Senegalo-Malian shear zone system (SMSZ). These shear belts are taken to define the limits between the western Mako, the central Diale-Kéniebandi and the eastern Daléma-Kofi domains even though transitions exist between their characteristic deposition sequences.Remote sensing analysis of airborne geophysics and satellite data (Landsat, ENVI, SRTM) in combination with geological field mapping, structural analysis and geochronology suggests that the SMSZ represents a Birimian structure that records a main stage of deformation that could characterise the major transcurrent Eburnean tectonics (D2) dated from 2105Ma (Feybesse et al., 2006a–e). This sinistral transpressive deformation marked by a major constraint oriented NNW–SSE, is accompanied by a NNE–SSW extension leading to the opening of dilational areas such as small pull-apart basins marked by local calk-alkaline volcanic sequences and several coeval intrusions of the Boboti Suite dated 2080±0.9Ma (Hirdes and Davis, 2002). A post-Birimian to pre-Neoproterozoic deformation post dates the transcurrent phase and is marked by regional N–S extension.Review of the existing Au-mineralisation models in combination with the spatial analysis of soil geochemical data suggest seven possible mineralisation styles that are related to the transpressional tectonics and coeval magmatism.

Strain and rotation rate patterns of mainland Greece from continuous GPS data and comparison between seismic and geodetic moment release

AbstractWe processed data from ~100 continuous GPS stations to provide new insights into the crustal motion and deformation of central and western Greece. We used the derived velocity field to evaluate two‐dimensional strain and rotation rate tensors, and we mapped the dilatation and maximum shear strain rates. In central Peloponnese and Epirus, we documented a 90° switch in the extension direction, which can be explained on the basis of the plate boundary configuration. Evidence for an extended deformation pattern in central Greece was found. Additionally, we detected two pairs of shear belts, one in Akarnania‐NW Peloponnese and one in North Aegean. We delineated two rotational domains that dominate the present‐day pattern. Moreover, we saw no geodetic evidence for North Anatolian Fault growth toward central Greece. We translated the geodetic strain rates into rates of seismic moment release and compared them with earthquake catalog‐based moment rates. In the central Ionian Sea, the geodetic strain is completely released seismically, which is indicative of a fully coupled seismogenic zone. However, for most of the study area, the geodesy‐based moment rates are at least 2 times higher than the earthquake‐based rates. We attribute this mainly to earthquake catalog representativity over the long‐term situation. However, for the Gulf of Corinth, it is unrealistic to associate the high ratio of geodetic to seismic moment rates only to incompleteness of the earthquake catalog; instead, long‐term aseismic deformation must be an important mechanism accommodating a considerable portion of the strain budget, especially at its western part.

The Rwenzori Mountains, a Palaeoproterozoic crustal shear belt crossing the Albertine rift system

This contribution discusses the development of the Palaeoproterozoic Buganda-Toro belt in the Rwenzori Mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori Mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south, the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal-scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori Mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal-scale nappe, whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle reverse faulting. The Palaeoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori Mountains are situated and where the Lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori Mountains and that this stiff belt may form part of the reason why the Rwenzori Mountains are relatively high within the rift.

Advances and Recent Patents about Cracking Walnut and Fetching Kernel Device

In recent years, many patents have been devoted to develop the walnut mechanical cracking and kernel fetching, kernel shell separation and walnut classification techniques that can significantly improve the walnut shell cracking rate and decrease the walnut kernel breaking rate. However, those machines adjust poorly to different sizes of walnuts, requiring size grading by human with high kernel breaking rate, low efficiency. This paper analyzes the patent on new walnut cracking, kernel fetching and shell kernel separation equipment in recent years. Based on the sphere walnut models and the advantages of variety of walnut shell machine, the walnut shearing, pressing and cracking device is designed by the author. Different sizes of walnuts are adjusted to these machines, including the conveyor belt shear extrusion flexible shell system, flexible blade hammer kernel fetching system and the air-flexible screw blade coupling walnut kernel shell roller two-way separation system, realizing the function of walnut crashing, kernel fetching and shell kernel separation, with the walnut shell cracking rate being 98%, the kernel breaking rate 2.9% and the kernel revealing rate 70%, improving the walnut shell cracking rate and reducing the kernel breaking rate. Keywords: Air-flexible screw blade, cracking the shell, flexible blade hammer, roller two-way separation, walnut, walnut classification screening.

An integrated approach to the late stages of Neoproterozoic post-collisional magmatism from Southern Brazil: Structural geology, geochemistry and geochronology of the Corre-mar Granite

The Neoproterozoic post-collisional stage in south Brazil is marked by intense granitic magmatism controlled by transcurrent shear zones related to the Southern Brazilian Shear Belt (SBSB). The SBSB has controlled the ascent and emplacement of magmas from crust and mantle. In such scenario, the Corre-mar Granite (CMG), represents a small intrusion emplaced in a low-strain zone located between two important segments of the SBSB: the Major Gercino and Itajaí-Perimbó shear zones. The CMG has a diagnostic subvertical S–C foliation pair present in all intrusions, regardless of their size. It was emplaced within a conjugate system where sinistral NNE shearing and NW–SE extension were both active, generating space along the NE direction. Solid-state deformation associated to the NNE shearing is attested to by microstructures as feldspar recrystallization and asymmetric recrystallization tails. The opening is attributed to the regional dextral transcurrent dynamics of the Major Gercino and Itajaí-Perimbó shear zones and magma emplacement was essentially conditioned by the NW extension component. Zircon U–Pb LA-MC-ICP-MS crystallization age of CMG at 615±4Ma, very close to other regional granites, as the 611Ma Serra dos Macacos (SMG) and 620Ma Rio Pequeno Granite (RPG) points these three granitic bodies as synchronous. The strong deformation features present in the CMG, as opposed to the other Neoproterozoic granites, demonstrate that space, rather than time, must be called upon to explain the difference in structural patterns identified in the CMG. Geochemical and Sr–Nd isotopic signatures, as the slightly peraluminous character, high K contents, high LREE/HREE ratios, moderate Rb, Nb, Zr, and REE contents to regular SiO2, together with low 87Sr/86Sri and the strongly negative ɛNdt values indicate that the CMG is derived from old crustal sources, possibly related to the Paleoproterozoic Camboriú Complex quartz-feldspathic orthogneisses. The coincidence of CMG Archean to Paleoproterozoic inheritance ages with the age of a migmatization event identified in the Camboriú Complex, and moreover the coincidence of the crystallization age intervals of the granites (∼615–611Ma) with the last migmatization event at 640–610Ma reinforces the genetic link between them. The Paleoproterozoic TDM ages, the geochemical and isotopic signatures, the zircon U–Pb crystallization and inheritance age, and the resemblance of the CMG and the SMG suggest that they represent comagmatic and contemporaneous granitic pulses, with a possible genetic link with the Neoproterozoic migmatization event in the Camboriú Complex.

Magnetotelluric characterization through the Ambargasta-Sumampa Range: The connection between the northern and southern trace of the Río de La Plata Craton – Pampean Terrane tectonic boundary

The South American Platform was part of the Western Gondwana, a collage of plates of different ages assembled in late Neoproterozoic to Cambrian times. The Transbrasiliano Lineament, a continental shear belt that transversely intersects this platform from NE to SW, has its southern expression in the tectonic boundary between the Río de La Plata Craton and the Pampean Terrane. Magnetotelluric long-period data in a W–E profile (29°30′ S) that crosses the Ambargasta-Sumampa Range and the Chaco-Pampean Plain were obtained to connect information of this mostly inferred tectonic boundary. A 2-D inversion model shows the Chacoparanense basin, Río Dulce lineament, Ambargasta-Sumampa Range and Salina de Ambargasta in the upper crust. At mid-to-lower crust and 40 km to the east of the Ambargasta-Sumampa Range, a discontinuity (500–2000 Ω m) of 20-km-wide separates two highly resistive blocks, the Río de La Plata Craton (6000–20,000 Ω m) in the east, and the Pampean Terrane (5000–20,000 Ω m) in the west. This discontinuity represents the tectonic boundary between both cratons and could be explained by the presence of graphite. The geometry of the Pampean Terrane suggests an east-dipping paleo-subduction. Our results are consistent with gravimetric and seismicity data of the study area. A more conductive feature beneath the range and the tectonic boundary was associated with the NE–SW dextral transpressive system evidenced by the mylonitic belts exposed in the Eastern Pampean Ranges. This belt represents a conjugate of the mega-shear Transbrasiliano Lineament and could be explained by fluid–rock interaction by shearing during hundreds of years. The eastern border of the Ambargasta-Sumampa Range extends the trace of the Transbrasiliano Lineament. The electrical Moho depth (40 km to the west and 35 km to the east) was identified by a high electrical contrast between the crust and upper mantle. The upper mantle shows a resistive structure beneath the Río de La Plata Craton that could have been originated by stationary delamination by the presence of hydrated lithosphere.

SHRIMP zircon U–Pb geochronology, geochemistry and H–O–Si–S–Pb isotope systematics of the Kanggur gold deposit in Eastern Tianshan, NW China: Implication for ore genesis

The Kanggur gold deposit is located in the southern margin of the Central Asia Orogenic Belt and in the western segment of the Kanggur–Huangshan ductile shear belt in Eastern Tianshan, northwestern China. The orebodies of this deposit are hosted in the Lower Carboniferous volcanic rocks of the Aqishan Formation and mainly consist of andesite, dacite and pyroclastic rocks. The SHRIMP zircon U–Pb age data of the andesite indicate that the volcanism in the Kanggur area might have occurred at ca. 339Ma in the Early Carboniferous, and that the mineralization age of the Kanggur gold deposit was later than the age of volcanic rocks in the area. Geochemically, the andesite rocks of the Aqishan Formation belong to low-tholeiite and calc-alkaline series and display relative depletions in high field strength elements (HFSEs; i.e. Nb, Ta and Ti). The δ18Ow and δDw values vary from −9.1‰ to +3.8‰ and −66.0‰ to −33.9‰, respectively, indicating that the ore-forming fluids were mixtures of metamorphic and meteoric waters. The δ30Si values of 13 quartz samples range from −0.3‰ to +0.1‰ with an average of −0.15‰, and the δ34S values of 18 sulphide samples range from −0.9‰ to +2.2‰ with an average of +0.54‰. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 10 sulphide samples range from 18.166 to 18.880, 15.553 to 15.635 and 38.050 to 38.813, respectively, showing similarities to orogenic Pb; these values are consistent with those of the andesite from the Kanggur area, suggesting a common lead source. All of the silicon, sulphur and lead isotopic systems indicate that the ore-forming fluids and materials were mainly derived from the Aqishan Formation, and that the host volcanic rocks of the Aqishan Formation probably played a significant role in the Kanggur gold mineralization. Integrating the data obtained from studies on geology, geochronology, petro-geochemistry and H–O–Si–S–Pb isotope systematics, we suggest that the Kanggur gold deposit is an orogenic-type deposit formed in Eastern Tianshan orogenic belt during the Permian post-collisional tectonism.

Brittle reactivation of ductile shear zones in NW Namibia in relation to South Atlantic rifting

Rifting has occurred worldwide along preexisting mobile belts, which are therefore thought to control rift orientation on a large scale. On a smaller scale, shear zones within mobile belts are reactivated as rift faults. In NW Namibia, shear zones of the Neoproterozoic Kaoko Belt run subparallel to the present-day continental passive margin and are inferred to have been reactivated during the opening of the South Atlantic Ocean. However, the extent of this reactivation and the influence of the reactivated shear zones on South Atlantic rifting are largely unknown. A combined remote sensing and field study was conducted to quantify offsets that are a direct function of shear zone reactivation. The shear zones of the Kaoko Belt are partly overlain by the Parana-Etendeka volcanic rocks, which were emplaced shortly before or simultaneously to the Atlantic rifting. Faulting within these volcanic rocks can be linked to synrift or postrift movements. Along the shear zones, downfaulting of the basalts is widespread along listric faults where half-graben form in the hanging wall. At three sites we could determine vertical offsets of ~1180 m, ~470 m, and ~70 m. Although many shear zones were reactivated as faults, these are isolated, and offsets are small, suggesting that reactivation occurred only as a side effect of the rifting and that the Kaoko Belt shear zones have not exerted a significant influence on the rift orientation.

Geochronological data from TTG-type rock associations of the Arroio dos Ratos Complex and implications for crustal evolution of southernmost Brazil in Paleoproterozoic times

U–Pb isotope analyses by LA-MC-ICPMS (Laser Ablation – Multi Collector – Inductively Coupled Plasma Mass Spectrometry) in zircon crystals from metatonalites, tonalites and granodiorite gneiss from the Arroio dos Ratos Complex (ARC) early magmatism in southernmost Brazil are presented. The ARC is located in the eastern portion of the Sul-rio-grandense Shield, occurring as septa and roof pendants on granitoids emplaced along the Southern Brazilian Shear Belt (SBSB). The SBSB corresponds to a translithospheric structure composed of several anastomosed shear zones of dominantly transcurrent kinematics whose syntectonic magmatism, of Neoproterozoic age, is characteristic of post-collisional environments. The studied rocks comprise TTG-type associations with coeval mafic magmatism, deformed and metamorphosed within a ductile shear zone. Zircon crystals obtained from six samples are interpreted as igneous given that the crystals are subhedral to euhedral, bipyramidal, with concentric zonation, have ratios Th/U between 0.13 and 0.81 and have restricted evidence of overgrowth. The oldest Association 1 (A1) has structures compatible with recrystallization under conditions of high temperature and an igneous age of 2148 ± 33 Ma, obtained in a metatonalite. The rocks of Association 2 (A2) have similar compositions, although with a more significant coeval mafic fraction. They are intrusive into A1 and also show high-temperature recrystallization features. However, they are less deformed and partly preserve their primary, igneous fabric. The igneous ages obtained from two A2 tonalites are 2150 ± 28 Ma and 2136 ± 27 Ma. Association 3 (A3) is represented by tonalitic to granodioritic gneisses whose structure, composition and metamorphic features are similar to those of A1 rocks, except for the absence of coeval mafic magmas in the former. Local features resulting from partial melting are present in A3 rocks. Three samples from A3 were dated. A tonalitic gneiss gives igneous age of 2099 ± 10 Ma and two granodioritic gneisses give igneous ages of 2081 ± 7 Ma and 2077 ± 13 Ma. Restricted to A1, inheritance is represented by one subhedral, zoned, gently rounded zircon crystal interpreted as igneous, of 2732 ± 40 Ma (207Pb/206Pb age), with discordance of 9% and 232Th/238U ratio of 1.17. A single Neoproteozoic metamorphic date value was obtained from the rim of a zircon crystal of Paleoproterozoic core. The age of 635 ± 6 Ma (207Pb/206Pb age), with Th/U ratio < 0.1 and 1% discordance, is interpreted as compatible with adjacent SBSB magmatism. The three associations are interpreted to represent the record of successive magmatic pulses that mark the evolution of a Paleoproterozoic continental magmatic arc. In the study area, these magmatic arc associations represent relict areas partly reworked and relatively well-preserved from Neoproterozoic tectono-magmatic post-collisional events during the construction of the Southern Brazilian Shear Belt.

Caracterização cinemática de uma zona de cisalhamento transcorrente de direção NNE marcada na evolução de seu magmatismo granítico sintectônico na região de Camboriú, SC

O Granito Corre-mar (GCM) localiza-se na região de Itapema-Camboriú, Santa Catarina, extremo nordeste do Cinturão Dom Feliciano. Uma zona de cisalhamento subvertical de direção NNE controla seu posicionamento, gerando um par S-C subvertical com cinemática sinistral. A foliação S (magmática), de direção NE-SW, é dada principalmente pela orientação de feldspatos e xenólitos, enquanto a foliação C (cisalhamento), de direção NNE-SSW, é marcada pelo estiramento dos cristais e formação de caudas de recristalização assimétricas nos feldspatos. Nas rochas encaixantes, próximo aos contatos, desenvolvem-se zonas de deformação de baixa temperatura paralelas à foliação C local do GCM. Os corpos do GCM têm uma sistemática variação na espessura, onde zonas mais largas mostram as foliações menos desenvolvidas. Microestruturas indicativas de deformação progressiva em temperatura decrescente são representadas por subgrãos em padrão tabuleiro de xadrez no quartzo e textura granoblástica em K-feldspato, as quais são superpostas por estruturas de mais baixa temperatura, como recristalização fina do quartzo, selamento de fraturas dos cristais maiores de feldspato por material da matriz finamente recristalizada e neoformação de grãos finos ao redor dos cristais de feldspato. A integração dos dados estruturais e petrológicos indica que o GCM se posicionou quando a zona de cisalhamento NNE estava ativa, e sua geometria indica uma componente de abertura na direção NE, típica de regime transtrativo. A estrutura transtrativa é compatível com sua posição em zona de baixa deformação transcorrente do Cinturão de Cisalhamento Sul-brasileiro, situada entre as zonas de mais alta deformação representadas pela zonas de cisalhamento Major Gercino e Itajaí, ativas no estágio pós-colisional do Ciclo Brasiliano no sul do Brasil.

One feature of the activated southern Ordos block: the Ziwuling small earthquake cluster

Abstract: Small earthquakes (Ms > 2.0) have been recorded from 1970 to the present day and reveal a significant difference in seismicity between the stable Ordos block and its active surrounding area. The southern Ordos block is a conspicuous small earthquake belt clustered and isolated along the NNW direction and extends to the inner stable Ordos block; no active fault can match this small earthquake cluster. In this paper, we analyze the dynamic mechanism of this small earthquake cluster based on the GPS velocity field (from 1999 to 2007), which are mainly from Crustal Movement Observation Network of China (CMONOC) with respect to the north and south China blocks. The principal direction of strain rate field, the expansion ratefield, the maximum shear strain rate, and the rotation rate were constrained using the GPS velocity field. The results show that the velocity field, which is bounded by the small earthquake cluster from Tongchuan to Weinan, differs from the strain rate field, and the crustal deformation is left-lateral shear. This left-lateral shear belt not only spatially coincides with the Neo-tectonic belt in the Weihe Basin but also with the NNW small earthquake cluster (the Ziwuling small earthquake cluster). Based on these studies, we speculate that the NNW small earthquake cluster is caused by left-lateral shear slip, which is prone to strain accumulation. When the strain releases along the weak zone of structure, small earthquakes diffuse within its upper crust. The maximum principal compression strees direction changed from NE-SW to NEE-SWW, and the former reverse faults in the southwestern margin of the Ordos block became a left-lateral strike slip due to readjustment of the tectonic strees field after the middle Pleistocene. The NNW Neo-tectonic belt in the Weihe Basin, the different movement character of the inner Weihe Basin (which was demonstrated through GPS measurements) and the small earthquake cluster belt reflect the activated southern margin of the Ordos block, which was generated through readjustment of the tectonic strees field after the middle Pleistocene.

Earthquake sequences in the southern block of the Pernambuco Lineament, NE Brazil: Stress field and seismotectonic implications

The Pernambuco Lineament (PeL) is an E–W-striking, 700km long shear belt located in the central part of the Borborema Province, Brazil. Seismic sequences have been monitored in the region since 1991, and they are clearly correlated with the main shear belt and its NE-trending branches. A new sequence of small earthquakes occurred in April 2010 away from the main belt in the southern block of the lineament. We monitored this seismicity using a ten station network for 154days and used the observations to determine the nature of the seismicity and the stress patterns. In addition, we used data from five previous local seismograph networks to determine the stress tensor. The results indicate that the seismicity is concentrated in three clusters, two of which are associated with normal faults: the first strikes 96°, dips 51°, and has a rake of −65°, and the second strikes 253°, dips 64°, and has a rake of −120°. These faults are 2.2km and 1.5km long and 2.0–3.0km and 3.0–3.5km deep, respectively, and are not correlated with the Pernambuco Lineament or its NE-trending branches. The inversion of focal mechanisms from our investigation and previous studies indicates E–W-trending compression and N–S-trending extension that affects both the northern and southern blocks of the PeL. The new data indicate that the fault behavior in the shear zone area is more complex than has previously been observed along reactivated mylonitic belts.

Oppositely dipping thrusts and transpressional imbricate zone in the Central Eastern Desert of Egypt

This paper documents the 40–60km wide ENE–WSW trending Mubarak–Barramiya shear belt (MBSB) in the Central Eastern Desert of Egypt by examining its structural styles, kinematics and geometry. Our study revealed the existence of prevalent dextral and minor sinistral conjugate shear zones. The MBSB is metamorphic belt (greenschist–amphibolite) characterized by at least three post-collisional (740–540Ma) ductile Neoproterozoic deformation events (D1, D2 and D3) followed by a brittle neotectonic deformation (D4). D1 event produced early top-to-the-northwest thrust displacements due to NW–SE shortening. D2 produced discrete zones of NNW-trending upright folds and culminated in initiation of major NW-trending sinistral shear zones of the Najd Fault System (NFS, at c. 640–540Ma ago) as well as steeply dipping S2 foliation, and shallowly plunging L2 lineation. NW-to NNW-trending F2 folds are open to steep and vary in plunge from horizontal to vertical. D2 deformational fabrics are strongly overprinted by D3 penetrative structures. D3 is characterized by a penetrative S3 foliation, steeply SE- to NW-plunging and shallowly NE-plunging stretching lineations (L3), asymmetric and sheath folds (F3) consistent with dextral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2–D3 represent a non-coaxial progressive event formed in a dextral NE- over NW-sinistral shear zone during a partitioned transpression in response to E–W-directed compression during oblique convergence between East and West Gondwana developed due to closure of the Mozambique Ocean and amalgamation of the Arabian–Nubian Shield in Cryogenian-early Ediacaran time.

Zircon ages defining deposition of the Palaeoproterozoic Soutpansberg Group and further evidence for Eoarchaean crust in South Africa

The precise age of the volcano-sedimentary Soutpansberg Group, which was deposited upon the Palala shear belt separating the Kaapvaal Craton from the Central Zone of the Limpopo Belt, has long been debated. The Soutpansberg Group is subdivided into a lower and upper succession, which are separated from each other by a prominent regional unconformity. Zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the Soutpansberg Group rocks.The zircon grains of the investigated samples from both successions yield a wide range of ages, spanning from 1831 to 3937Ma. Most of the zircon grains have rounded shapes, however it is not clear whether they are mainly xenocrystic or detrital, or have been rounded by resorption in a silicic magma chamber. The youngest zircon grain ages obtained come from the lower succession and are 1832±9 and 1831±15Ma. We interpret these youngest zircon grains as magmatic grains that have been rounded by resorption. This view is corroborated by the fact that no magmatic rocks of this particular age have been observed in the Kaapvaal Craton or the Central Zone of the Limpopo Belt, and that no apparent sedimentary admixtures are present in the well exposed pyroclastic rocks. We therefore conclude that deposition of the Soutpansberg volcano-sedimentary succession commenced around 1830Ma.The Soutpansberg rocks were deposited apparently over a lengthy period of time (ca. 230Ma), as provided by the published age of 1604Ma for pyroclastic rocks of the upper succession in Botswana. Zircon grain age spectra of our Soutpansberg samples show prominent peaks at 2.0, 2.6 and 3.2Ga, indicating the Central Zone of the Limpopo Belt as the source area, but excludes the adjacent northern part of the Kaapvaal Craton. The oldest zircon grain identified in the Soutpansberg samples has an age of 3937±4Ma, one of the oldest zircon grain ages yet reported from the African continent.

Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures–Tanneron Massif (SE France)

Abstract Synthesis of structural, petrological and geochronological data for the Maures–Tanneron Massif and its integration in the framework of adjacent massifs (i.e. Sardinia and Corsica) has allowed us to propose a new model of evolution for the southern Variscan belt. After Siluro-Devonian subduction associated with high-pressure–low-temperature (HP/LT) metamorphism M 0 ( c. 10–15 °C km −1 ) and subsequent Carboniferous nappes stacking, the belt underwent strong reworking related to back-thrusting. Nappes stacking and back-thrusting were associated with typical Barrovian metamorphism M 1 ( c. 20–30 °C km −1 ) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M 2 ( c. 40–60 °C km −1 ) and M 3 ( c. 60–80 °C km −1 ) during 330–300 Ma in the internal part of the belt. Progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root. Continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold-dome structures. The mass transfer is accommodated by orogen-parallel transpressive shearing synchronous with M 3 during Late Carboniferous time. The orogenic wedge is characterized by two main tectono-metamorphic units decoupled by a major shear belt: an Internal Zone with migmatites and syntectonic granitoids, where HP relicts have been exhumed, and an External Zone that escaped the late HT event and preserved precious structures.

Geologia do Grupo Paranoá na porção externa da Faixa Brasília

The Paranoa Group represents a psamo-pelitic-carbonated succession deposited in shelf conditions. The stratigraphy of this important lithostratigraphic sequence of the Brasilia Fold Belt was originally proposed by letter-code designation that includes 11 units with the following stratigraphic stacking: SM, R1, Q1, R2, Q2, S, A, R3, Q3, R4 and PC. The present paper formalizes these units and proposes the following denomination to the formations, named from base to top: Ribeirao Sao Miguel, Corrego Cordovil, Serra da Boa Vista, Serra Almecegas, Serra do Parana, Ribeirao Picarrao, Ribeirao do Torto, Serra da Meia Noite, Ribeirao Contagem, Corrego do Sansao and Corrego do Barreiro. The deposition of the Paranoa Group is considered to have been processed in the Mesoproterozoic (1,542 to 1,042 Ma). This age interpretation is supported by the stratigraphic position (as it occurs over the post-rift phase sediments of the Arai Group and under slates and carbonates of the Bambui Group), by the presence of conical stromatolite (conophyton) and by isotopic data. The sedimentation was controlled by transgressive-regressive cycles, including grading and traction processes, debris flows, tides, waves, storms and basin bottom paleogeography. The sediments were affected by low-grade metamorphism and the deformation resulted in the formation of monoclines, different styles of folds (chevrons, in box and cylindrical ones) and folding interference resulting in domes and structural basins. The regional deformation of the Paranoa Group is mainly controlled by the Parana and Rio Maranhao thrust systems and by the Ribeirao Sao Miguel shear belt.