Lithotectonic Units Research Articles

Timing of magmatism and migmatization in the 2.0–1.8 Ga accretionary Svecokarelian orogen, south-central Sweden

The Palaeoproterozoic (2.0–1.8 Ga) Svecokarelian orogen in central Sweden consists of a low-pressure, predominantly medium-grade metamorphic domain (central part of Bergslagen lithotectonic unit), ...

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.

Geodynamics of Late Carboniferous–Early Permian forearc in north Chile (28°30′–29°30′S)

A large section of the Late Palaeozoic forearc is exposed along the coastal ranges of north–central Chile (28°–29°30′S). This is characterized by three lithotectonic units: (1) the Punta de Choros Metamorphic Complex (basal accretion series), composed mostly of micaschists and metabasites; (2) the Chañaral Epimetamorphic Complex (frontal accretion series), formed by metaturbidites and metasediments; (3) the Llano del Chocolate Beds (forearc basin deposits), composed of a sedimentary sequence of clastic sedimentary rocks with minor limestones and acidic volcanic rocks. Within the basal accretion series, two distinctive blocks of garnet-bearing schists with amphibolite-facies metamorphism have been preserved, recording early stages of the subduction system. The stratigraphic record and the U–Pb dating of igneous (291–318 Ma) and detrital zircons (maximal deposition ages between 273 and 292 Ma) in the forearc basin deposits, coupled with 40 Ar/ 39 Ar ages for metamorphic rocks (319–280 Ma), indicate that forearc sedimentation was broadly contemporaneous with metamorphism and exhumation of the basal accretion series. The radiometric dating on garnet-bearing rocks indicates that the onset of the subduction system took place in the Mississippian, at a high thermal gradient. These conditions were responsible for generation of limited volumes of acidic melt, now recognized as acidic domes and tuffs within the forearc basin stratigraphy. Supplementary material: Tables of sedimentary facies, U-Pb geochronological analyses and Ar-Ar geochronological data are available at: http://doi.org/10.6084/m9.figshare.c.2868307 .

Time constraints on magmatism, mineralisation and metamorphism at the Falun base metal sulphide deposit, Sweden, using U–Pb geochronology on zircon and monazite

U–Th–Pb (zircon and monazite) ion probe data have provided constraints on the timing of emplacement and metamorphism of magmatic rocks close to the Palaeoproterozoic, Falun base metal sulphide deposit in the Bergslagen lithotectonic unit, Fennoscandian Shield, Sweden, and, thereby the timing of mineralisation. Hydrothermal alteration and mineralisation at Falun are constrained to a short interval of several million years between a 207Pb/206Pb weighted average age of 1894±3Ma for a rhyolitic sub-volcanic rock in the felsic volcanic to sub-volcanic host rock suite, and a 207Pb/206Pb weighted average age of 1891±3Ma for a post-sulphide, porphyritic dacite dyke. Magmatism also included the emplacement of granite plutons with igneous crystallization ages of 1894±3, 1894±2Ma and 1893±3Ma. The felsic sub-volcanic to volcanic activity and the emplacement of dacite dykes and granite plutons overlap in age within their respective analytical uncertainties, indicating hydrothermal alteration and sulphide mineralisation inside a narrow time span of intense magmatic activity, and burial of the supracrustal rocks. Two distinct patchy and homogeneous metamorphic monazite types in a felsic volcanic rock around and hydrothermally altered rocks at the Falun deposit yield 207Pb/206Pb weighted average ages of 1831±8Ma and 1822±5Ma, respectively. These ages fall well within the temporal range of a younger 1.84–1.81Ga (M2) metamorphic episode during the 2.0–1.8Ga Svecokarelian orogeny, with the older episode (M1) inside the Bergslagen lithotectonic unit at around 1.86Ga. This shows the major influence of the M2 event in the north-western part of this unit, leading to a complete resetting of the U–Th–Pb isotope system in monazite.

3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

The Falun pyritic Zn-Pb-Cu-(Au-Ag) deposit, situated in the Palaeoproterozoic (1.9–1.8 Ga) Bergslagen lithotectonic unit in the south-western part of the Fennoscandian Shield, is one of the major b ...

Delineating and characterizing the boundary of the Cathaysia Block and the Jiangnan orogenic belt in South China

The Jiangshan-Shaoxing fault zone lies along the SE margin of the Jiangnan belt, and delineates the northeastern margin of the Cathaysia Block of the South China Craton. At Shijiao in NE Zhejiang, the fault zone consists of hornblende schist intruded by migmatized quartz diorite. It constitutes a shear zone delineating the welded boundary between the Neoproterozoic Shuangxiwu Group of Jiangnan belt to the north and the Chencai Complex of Cathaysia Block to the south. The Shuangxiwu Group is composed mainly of basalt, andesite and flysch, whereas the Chencai Complex contains magmatic and sedimentary rocks that experienced amphibolite facies metamorphism. Zircons from quartz diorite and gabbro from the fault zone at Shijiao yield ages of 854±6 Ma, 857±5 Ma and 860±5 Ma, with positive ɛHf(t) values of 7.81–11.8 and 4.57–10.39. The quartz diorite and mafic-ultramafic rock samples display minor LREE enriched pattern with obvious depletion of Nb, Ta, Rb, Ba and Ti, compared to their neighboring elements and plot in the volcanic arc field on geochemical diagrams, similar to that of volcanic rocks from Shuangxiwu Group. Overall relationships within the Jiangshan-Shaoxing fault zone at Shijiao suggest the ca. 860–850 Ma rock suites were generated in a convergent plate margin and are part of the Jiangnan belt, and not the Cathaysia Block, thus constraining the location of the suture between the two lithotectonic units in NE Zhejiang area during Neoproterozoic.

The water content and hydrogen isotope composition of continental lithospheric mantle and mantle-derived mafic igneous rocks in eastern China

The water contents of minerals and whole-rock in mantle-derived xenoliths from eastern China exhibit large variations and are generally lower than those from other on- and off-craton lithotectonic units. Nevertheless, the water contents of mineral and whole-rock in Junan peridotite xenoliths, which sourced from the juvenile lithospheric mantle, are generally higher than those elsewhere in eastern China. This suggests that the initial water content of juvenile lithospheric mantle is not low. There is no obvious correlation between the water contents and Mg# values of minerals in the mantle xenoliths and no occurrence of diffusion profile in pyroxene, suggesting no relationship between the low water content of mantle xenolith and the diffusion loss of water during xenolith ascent with host basaltic magmas. If the subcontinental lithospheric mantle (SCLM) base is heated by the asthenospheric mantle, the diffusion loss of water is expected to occur. On the other hand, extraction of basaltic melts from the SCLM is a more efficient mechanism to reduce the water content of xenoliths. The primary melts of Mesozoic and Cenozoic basalts in eastern China have water contents, as calculated from the water contents of phenocrysts, higher than those of normal mid-ocean ridge basalts (MORB). The Mesozoic basalts exhibit similar water contents to those of island arc basalts, whereas the Cenozoic basalts exhibit comparable water contents to oceanic island basalts and backarc basin basalts with some of them resembling island arc basalts. These observations suggest the water enrichment in the mantle source of continental basalts due to metasomatism by aqueous fluids and hydrous melts derived from dehydration and melting of deeply subducted crust. Mantle-derived megacrysts, minerals in xenoliths and phenocrysts in basalts from eastern China also exhibit largely variable hydrogen isotope compositions, indicating a large isotopic heterogeneity for the Cenozoic SCLM in eastern China. The water content that is higher than that of depleted MORB mantle and the hydrogen isotope composition that is deviated from that of depleted MORB mantle suggest that the Cenozoic continental lithospheric mantle suffered the metasomatism by hydrous melts derived from partial melting of the subducted Pacific slab below eastern China continent. The metasomatism would lead to the increase of water content in the SCLM base and then to the decrease of its viscosity. As a consequence, the SCLM base would be weakened and thus susceptible to tectonic erosion and delamination. As such, the crust-mantle interaction in oceanic subduction channel is the major cause for thinning of the craton lithosphere in North China.

Role of initial basin width in partitioning total shortening in the Lesser Himalayan fold-thrust belt: Insights from regional balanced cross-sections

Published, regional, balanced cross-sections constructed across various transects along the Himalayan fold-thrust belt (FTB) suggest significant lateral variations in the magnitude and partitioning of total minimum shortening among various lithotectonic units. The variation in shortening is greatest in the Lesser Himalayan sequence (LHS). Western Nepal Himalaya records the highest shortening. Shortening variation shows non-uniform spatial distribution along the length of the FTB, with regions of lower shortening estimates lying in between regions of higher shortening estimates.We measured the initial and final lengths of the Lesser Himalayan sequence (LHS) from existing published balanced cross-sections. There is a direct correlation between the initial width of the LHS and the total minimum shortening distribution accommodated in all the lithotectonic units of the FTB. This indicates that the initial width of the LHS controlled the lateral variation in the total minimum shortening, and provides a new interpretation for minimum shortening variation in the FTB. The initial width of the LHS also controlled the lateral variation in structural architecture of the LHS by affecting the geometry and total number of LHS imbricates and horses along the FTB. The variation in structural geometry of the LHS along the Himalayan FTB resulted in non-uniform distribution of lateral variation in initial and current LHS outcrop widths.

Continuous Neoproterozoic to Ordovician sedimentation at the East Gondwana margin — Implications from detrital zircons of the Ross Orogen in northern Victoria Land, Antarctica

The Ross–Delamerian orogenic belt was formed along the eastern side of the Australian–East Antarctic continent during west-directed subduction of the Palaeo-Pacific Ocean in the early Palaeozoic. Northern Victoria Land (NVL) in Antarctica was located at a central position of the Ross-Delamerian system. Its metamorphic basement is formed by three lithotectonic units formerly interpreted as terranes: the Wilson, Bowers and Robertson Bay terranes (from west to east). Dating of detrital zircons from 14 metasedimentary samples of these terranes combined with petrographical and whole-rock geochemical studies give new insights into the stratigraphic and tectonic evolution of NVL. All samples show very similar zircon age spectra with two main intervals, a Ross/Pan-African-age interval (470–700Ma) and a Grenville-age interval (900–1300Ma), as well as subordinate craton-related ages dispersed over the range of ca. 1600–3500Ma. The Ross/Pan-African-age zircon population tends to get more dominant from the Priestley Formation of the Wilson Terrane to the Molar Formation of the Bowers Terrane, and finally to the Robertson Bay Group, whereas the number of craton-related ages diminishes in this direction. A common East Antarctic source area is indicated for all analyzed samples. The Priestley Formation was deposited on the Palaeo-Pacific passive continental margin of East Gondwana in the late Neoproterozoic after Rodinia breakup. The sequence was subsequently metamorphosed and intruded by the Granite Harbour Intrusives during the Ross Orogeny. The Molar Formation of the Bowers Terrane is interpreted as a turbiditic sequence deposited in an accretionary setting on the active continental margin in the Late Cambrian during and after accretion of the Glasgow island arc allochthon. The thick, homogeneous sequence of the Robertson Bay Group resulted from continuous turbiditic sedimentation in an accretionary wedge in front of the Ross Orogen after docking and imbrication of the Glasgow island arc in the Early Ordovician.

ACTIVE DEFORMATION DELINEATED BY GEOMORPHIC AND SEDIMENTARY RESPONSE OF RIVERS, CENTRAL ZAGROS FOLD – THRUST BELT, SW IRAN

The mountain generation in Iran is because of continental collision between the Arabian and Eur-asia plate. Southwestern Iran shows active shorten that its evidences is deformation of crust and frequent earthquakes. At depth, active basement of the Zagros fold-thrust-belt in southwestern Iran, which are covered by folding of the Phanerozoic sediments, affected by some blind thrusted faults that have seismic nature. The Zagros fold-thrust-belt can be divided into 4 lithotectonic units including Sanandaj-Sirjan Zone (SSZ), Imbricate Zone (IZ), Zagros Fold Belt (ZFB), and Molasse Cover Sequence (MCS); this dividing and classification is based on geomorphology landscape, drainage pattern, rate of tectonics and stratigraphic records. Each tectonic unit characterized by especial abnormal forces in river systems. Active tectonics has the most important role to control the river systems by changing of channels incline. Change in the drainage pattern, channels cut, longitudinal profile, anomalous changes of sinuosity, changing of the side form and forming of terrace, change of river direction, compact meanders, cutting of meanders and geomorphology features of the rivers are responds to the active tectonics of region that are studied using remote sensing, DEM and field observations. These parameters are used to understand the vertical movement in the study area. Existing structures, especially growing anticlines and blind thrusted faults in the Zagros fold-thrust-belt, which cut the river channels and sometimes put them in parallel, are used in the study of their effect on the longitudinal and transverse tilt of morphological changes.

Toward a unified model of Altaids geodynamics: Insight from the Palaeozoic polycyclic evolution of West Junggar (NW China)

The Altaid tectonic collage extends over Central Asia, exposing numerous accretionary orogens that can account for the Palaeozoic continental crust growth. A pluridisciplinary approach, using geochronological, geochemical, structural and palaeomagnetic tools was carried out to unravel the architecture and the evolution of West Junggar (Northwestern China), a segment of the Altaid Collage. A polycyclic geodynamic evolution is inferred and includes: (1) an Early Palaeozoic cycle, characterized by the closure of two oceanic basins bounded by island-arc systems; (2) an Early Devonian subduction jamming resulting in a minor-scale collision documented by thrusting, syntectonic sedimentation and subsequent crutal thinning associated with alkaline magmatism; (3) a Late Palaeozoic cycle, driven by the evolution of two opposite subduction zones developed upon the Early Palaeozoic basement. Detailed structural analysis and paleomagnetic data provide constraints for the late evolution of Junggar in the frame of the development of the Late Palaeozoic Kazakh orocline, which led to oblique subduction and transpression in the West Junggar accretionary complex. Progressive buckling of the Kazakh orocline further resulted in Late Carboniferous to Permian wrench tectonics, and lateral displacement of lithotectonic units. Block rotations that continued after the Late Triassic are due to diachronous intraplate reactivation. This scenario mirrors the Palaeozoic geodynamics of the Altaid Collage. Multiple Early Palaeozoic collisions of intra-oceanic arcs and micro continents have contributed to the formation of the Kazakhstan Microcontinent. Since the Late Palaeozoic, subductions formed around this microcontinent and the final oblique closure of oceanic domains resulted in the transcurrent collage of Tarim and Siberia cratons. Palaeozoic strike-slip faults were later reactivated during Mesozoic intracontinental tectonics.

A Quantitative Analysis of the Ramganga Drainage Basin and Structural Control on Drainage Pattern in the Fault Zones, Uttarakhand

Abstract The present study incorporates quantitative analysis of drainage of the Ramganga basin, Garhwal Himalaya, Uttarakhand. The drainage parameters (linear, areal and relief) of the Ramganga basin from Dudhatolidhar peak its point of origin to Kalagarh from where the river Ramganga enter into the plain, are covered. The impact of the morphological characters on the terrain is reflected by the drainage basin of the area. The calculated values of all the morphometric parameter of 38 Vth order intrabasins for different lithotectonic units were computed under four sectors viz., NE of NAT, between NAT and SAT, between SAT and MBT and downstream of MBT. In the lower reaches of Ramganga basin the various morphometric parameters shows existence of proportionality between stream length and catchment area while such proportionality is non-existence in the crystalline zone. Structural control on the drainage patterns of the Ramganga river and its tributaries are observed in the form of straight course, wide valleys with thick fluvial deposits and relatively gentle stream gradient along the transverse faults to incised valleys; less fluvial deposits in the banks and steeper stream gradient with knick points in the hanging wall of thrusts.

Detrital zircon U–Pb geochronology of the Siwalik Group of the Nepal Himalaya: implications for provenance analysis

This paper deals with the provenance analysis of the Neogene foreland basin sediments of the Siwalik Group in the Nepal Himalaya. This study adopts the techniques of the optical petrography and detrital zircon U–Pb ages from two river sections: the Koshi Nadi in eastern Nepal and the Surai Khola in western Nepal Himalaya. The optical petrography data and resulting QFL plot show a “recycled orogeny” field for the studied sandstone samples, indicating northern lithotectonic units; Tethys Himalaya, Higher Himalaya and Lesser Himalaya as the source of the foreland basin sediments. The detrital zircon geochronological data set has clearly revealed that the cluster ages are younger than ~1000 Ma; however, the older grains (>1000 Ma) are significantly fewer. The obtained age spectrum is similar to the Tethys Himalaya and the upper Lesser Himalaya, but the lower Lesser Himalayan rocks were not distinct, which indicates that sediments in the Neogene foreland basin of the Nepal Himalaya were primarily sourced from the Tethys Himalaya and upper Lesser Himalaya. The minor subordinate scattered peaks that roughly correspond to the age of the Higher Himalaya and lower Lesser Himalaya may indicate that a lower proportion of the sediments might have a link with the Higher Himalaya and lower Lesser Himalaya. Therefore, the provenance of the Siwalik Group in the Nepal Himalaya might have witnessed a mixed type of provenance similar to the northwestern Himalaya.

Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0–1.8 Ga accretionary orogen, Sweden

Absolute ages of migmatization and protolith formation, and constraints on the timing of ductile deformation in two major lithotectonic units in the south-western part of a 2.0–1.8Ga orogenic belt in the Fennoscandian Shield, south-eastern Sweden, have been determined using U–Pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite. Detrital and xenocrystic zircon in paragneiss and garnet-bearing leucogranite, respectively, in the Bergslagen lithotectonic unit show ages of 2.1–2.0Ga and 1.9Ga. Deposition of the sedimentary material occurred during or after a subduction-related magmatic event at 1.91–1.87Ga. Two orthogneiss protoliths formed during this magmatic event around 1.88Ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1.85Ga igneous protolith. A polyphase tectonothermal evolution with anatexis under low-P metamorphic conditions around 1.86Ga (M1) and 1.84–1.81Ga (M2) is inferred for the migmatitic gneisses in the Bergslagen unit; garnet-bearing leucogranite crystallised around 1.84–1.83Ga, close in time to major folding of the M1 gneissic fabric. A previously unrecognised 1.86–1.85Ga ductile deformational event under medium-grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south (Småland lithotectonic unit), close in time to the M1 event in the Bergslagen unit to the north.By constraining the timing of anatexis and comparing with information bearing on crustal thickness, excess mass at depth and the character and age of magmatic activity, it is inferred that anatexis in the Bergslagen lithotectonic unit is related to pulses of mafic underplating, during the early stages of two separate, subduction-related magmatic episodes after the 1.91–1.87Ga magmatic event. It is suggested that each pulse was related to intra- or back-arc spreading above a subduction boundary, which had entered a retreating mode with separate, long periods (20–50Ma) of extension or transtension. This study challenges the need to invoke crustal thickening related to plate collision at 1.9–1.8Ga as a mechanism to explain high-grade metamorphism in the southern part of the 2.0–1.8Ga orogen. Instead, a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred.

Multiple reactivation and strain localization along a Proterozoic orogen-scale deformation zone: The Kongsberg-Telemark boundary in southern Norway revisited

Structural analysis defines a multiphase Sveconorwegian tectonic evolution for the boundary zone between the Kongsberg and Telemark lithotectonic units in S Norway, referred to as the Kongsberg-Telemark Boundary Zone (KTBZ). This large-scale weakness zone developed predominantly within and at the margin of a c. 110km long granitic belt, the intrusion of which is dated between 1170±11 and 1146±5Ma by U–Pb SIMS zircon geochronology. The oldest KTBZ ductile fabric formed during the Sveconorwegian orogenic cycle (c. 1140–900Ma) as a penetrative top-to-the-W shear fabric, which was subsequently reactivated selectively by sinistral transpression that formed characteristic mylonitic shear zones within the granitic belt. Later folding affected the area at the northern end of the Kongsberg lithotectonic unit. Analysis of the subregional foliation trajectories unravels the occurrence of a large-scale fold structure, the “Norefjell-Hønefoss Fold”. All these structures are in turn cut by late-Sveconorwegian, E-dipping shear zones and normal faults, which accommodated a distinct phase of exhumation of the Telemark lithotectonic unit in the footwall of the KTBZ. This extensional detachment widens toward the north, where it might have controlled the emplacement of the late-orogenic Flå granite. Since late Sveconorwegian times, the KTBZ was repeatedly reactivated in a brittle fashion forming complex fault patterns, extensive quartz vein networks and leading to the generation of the so-called “Store Rivningsbreksje”, a 100km long brittle fault zone that follows the trend of the KTBZ and that locally juxtaposes blocks with different ductile precursor histories. The newly established deformation history helps to refine existing models for the orogenic evolution of the central Sveconorwegian orogen. The characterization of the Norefjell-Hønefoss fold structure provides a new perspective on Sveconorwegian geometries and fabrics in the area. The reactivation history established for the KTBZ helps to better understand the dynamics of long-lived weakness zones of Precambrian origin in general.

A Paleozoic Japan-type subduction-accretion system in the Beishan orogenic collage, southern Central Asian Orogenic Belt

Magmatic arcs ascribed to oceanic lithosphere subduction played a dominant role in the construction of the accretionary Central Asian Orogenic Belt (CAOB). The Beishan orogenic collage, situated between the Tianshan Orogen to the west and the Inner Mongolia Orogen to the east, is a key area to understanding the subduction and accretionary processes of the southern CAOB. However, the nature of magmatic arcs in the Beishan and the correlation among different tectonic units along the southern CAOB are highly ambiguous. In order to investigate the subduction-accretion history of the Beishan and put a better spatial and temporal relationship among the tectonic belts along the southern CAOB, we carried out detailed field-based structural geology and LA-ICP-MS zircon U-Pb geochronological as well as geochemical studies along four cross-sections across crucial litho-tectonic units in the central segment of the Beishan, mainly focusing on the metamorphic assemblages and associated plutons and volcanic rocks. The results show that both the plutonic and volcanic rocks have geochemical characteristics similar to those of subduction-related rocks, which favors a volcanic arc setting. Zircons from all the plutonic rocks yield Phanerozoic ages and the plutons have crystallization ages ranging from 464±2Ma to 398±3Ma. Two volcanic-sedimentary rocks yield zircons with a wide age range from Phanerozoic to Precambrian with the youngest age peaks at 441Ma and 446Ma, estimated to be the time of formation of the volcanic rocks. These new results, combined with published data on ophiolitic mélanges from the central segment of the Beishan, favor a Japan-type subduction-accretion system in the Cambrian to Carboniferous in this part of the Paleo-Asian Ocean. The Xichangjing-Niujuanzi ophiolite probably represents a major suture zone separating different tectonic units across the Beishan orogenic collage, while the Xiaohuangshan-Jijitaizi ophiolitic mélange may represent a Carboniferous back-arc basin formed as a result of slab rollback ascribed to northward subduction of the Niujuanzi oceanic lithosphere. Subduction of this back-arc basin probably took place in the early Carboniferous, generating the widespread arc-related granitoids including adakitic plutons, and overlapping earlier arc assemblages. The Beishan orogenic collage is not the eastern extension of the Chinese Central Tianshan, but it was generated by the same north-dipping subduction system separated by the Xingxingxia transform fault, as revealed by available regional data. This contribution implies that in addition to fore-arc accretion, back-arc accretion ascribed to opening and closure of a back-arc basin may also have been a common process in the construction of the CAOB, resembling that of the Mesozoic-Cenozoic subduction-accretion system in the SW pacific.

Distinct deformational history of two contrasting tectonic domains in the Chinese Altai: Their significance in understanding accretionary orogenic process

The Chinese Altai, a key component of the western Central Asian Orogenic Belt, is considered to be formed through multiple accretions of different terranes. However, the deformational histories of each terrane (tectonic domain), i.e. structural records before and after the accretion, are rarely studied, which has hindered our understanding of the accretionary processes. To fill the gap, a systematic macro- and microscopic structural analysis was carried out on two contrasting litho-tectonic units, i.e. the early Paleozoic low-grade Alegedayi Ophiolitic Complex (AOC) juxtaposed to the high grade Tarlang Granitic Massif (TGM). Selected rock samples were analyzed using zircon U–Pb isotopic dating to constrain the timing of polyphase deformation. Our structural and geochronological data suggest that the two litho-tectonic units were initially detached and located in different crustal levels and experienced distinct phases of deformation under contrasting P-T conditions. They were mutually accreted with each other in the early Devonian and jointly underwent a WNW-ESE-directed shortening deformational event (D1) at ∼390 Ma. The change of tectonic regime was further enhanced by a subsequent NNE-SSW-directed shortening deformation (D2) after ∼ 380 Ma. The shortening process ended before the crustal-scale sinistral strike-slip shearing deformation along the Erqis fault zone at 290 - 240 Ma. Results of this study provide solid field-based evidence for a model that the Chinese Altai initially underwent a nearly E–W-oriented subduction-accretional event in the middle Paleozoic, before it was reoriented to a nearly N–S-oriented convergence.

Geological analysis and updating velocity model by restoration of a cross section in Zagros fold–thrust belt in the southwest of Iran

The Zagros region (SW Iran) contains dominant parts of the remaining Iranian natural resources. Covering Abadan Plain, Dezful Embayment, and Izeh zone, this paper studies a structural cross section constructed across the Zagros belt using 2D and 3D seismic data, well data, surface and subsurface geological maps, satellite images, and field reconnaissance. Restoration assists us for validation and modification of the constructed cross section, better understanding of geology and stratigraphy of the Zagros basin, and computing an updated velocity model in seismic time to depth conversion. In the area of interest, Hormuz basal decollement and Gachsaran detachment play the most significant role in structural style and deformation of the Zagros. More complication links with interval decollements such as Triassic evaporites, Albian shales, and Eocene marls. Estimation of shortening is complex because of a variety of lithotectonic units and detachment surfaces. Here, we are about to introduce a new approach to update seismic velocity model by means of deduced balanced depth model by restoration and seismic two-way time. This is valuable in the case of thick sedimentary cover and steep dip of faults, where acquired seismic images are ambiguous and unreliable. The seismic velocity of Asmari and Gachsaran formations mainly depends on lateral facies and thickness variations within the overburden sediments and generally increases across the Zagros belt from foreland towards hinterland.

Deformation, Stratigraphy, Structures and Shortening of the Zagros Fold‐Thrust Belt in Southwest Iran Analysis by Restoration

AbstractA structural cross‐section constructed across the Zagros Fold‐Thrust Belt covering the Abadan Plain, Dezful Embayment, and Izeh Zone applied 2D and 3D seismic data, well data, surface and subsurface geological maps, satellite images and field reconnaissance. Besides validation and modification of the cross‐section, restoration allows better understanding of the geology, structural style and stratigraphy of the Zagros basin. In the area of interest, the Hormuz basal decollement and the Gachsaran detachment play the most significant roles in the structural style and deformation of the Zagros belt. More complexity is associated with interval decollements such as Triassic evaporites, Albian shales and Eocene marls. A variety of lithotectonic units and detachment surfaces confound any estimation of shortening, which generally decreases with increasing depth. Deformation completely differs in the Abadan Plain, Dezful Embayment and Izeh Zone because of different sedimentation histories and tectonic evolution; gentle and young structures can be interpreted as pre‐collisional structures of the Dezful Embayment before the Late Cretaceous. After the Late Cretaceous, the Mountain Front Fault is the main control of sedimentation and deformation in the Zagros Basin, and this completely characterizes fold style and geometry within the Dezful Embayment and the Izeh Zone.

Structural, metamorphic and geochronological record in the Goszów quartzites of the Orlica–Śnieżnik Dome (SW Poland): implications for the polyphase Variscan tectonometamorphism of the Saxothuringian terrane

This integrated study on the pressure–temperature–deformation‐time record of the Goszów light quartzites from the Młynowiec–Stronie Group (Sudety Mts., SW Poland) provides new data that improve our understanding of the structure and geodynamic development of the Orlica–Śnieżnik Dome (OSD) as a Gondwana‐derived unit involved in the formation of the Variscan orogen. The structural and metamorphic record of the Goszów light quartzites, when compared to the under‐ and overlying rock formations, indicates that the whole Młynowiec–Stronie Group in the eastern part of the Saxothuringian terrane functioned as a single, integral lithotectonic unit with no visible structural or metamorphic discontinuities. The sequence of structures and thermodynamic modelling indicate that the light quartzites underwent the same polyphase tectonometamorphic evolution as the adjacent rocks belonging to the Młynowiec–Stronie Group. The development of tight, N–S‐trending folds and axial penetrative metamorphic foliation was related to metamorphic progression from 500 °C to 640 °C at 6–7 kbar. Subsequently, under the retrogressive conditions below 540 °C, the foliation was reactivated as a result of subsequent N–S‐directed ductile shearing and extension. Therefore, the study of the light quartzites exemplifies the penetrative structures in the OSD, and the metamorphic foliation and N–S‐trending lineation are composite structures. The monazite metamorphic ages of ca. 364 Ma and 335 Ma may be related to the approximately E–W‐ and N–S‐oriented tectonic movements, respectively, which occurred during the amalgamation of the Saxothuringian terrane with Brunovistulia. In contrast, the previously unknown early Palaeozoic monazite age of ca. 494 Ma is interpreted as the protolith age of the light quartzites. Copyright © 2015 John Wiley & Sons, Ltd.