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A structural and stratigraphic study in Liuyuan area, Southern Beishan orogenic collage, Northwest China
Get PDFA stratigraphic and structural study of the Permian North Shuangbaotang Complex offers new evidence constraining the timing of the final closure of the Paleo-Asian Ocean to the late Permian. The fold and thrust belt of turbidite sequence in the North Shuangbaotang Complex is interpreted as a syn-tectonic sedimentary deposit in a retro-foreland basin on the upper plate recording the termination of the final stage of accretionary orogenesis in the Beishan orogenic collage.
A reconstruction of stratigraphy and structural analysis in this study identified that the North Shuangbaotang Complex is composed of two fault- bounded sedimentary packages and one cover package, which are subjected by two phases of deformations. Package 1 is considered as a part of an accretionary wedge derived from the Liuyuan Complex and package 2 is a part of a forearc basin derived from the Dundunshan terrane. The northward subduction of the oceanic crust and early collision between the Dundunshan terrane and the Dunhuang terrane led to D1 deformation characterized by southwesterly thrusting. The continuous collision thickened the orogenic wedge and promoted the uplift-driven erosion of packages 1 and 2. Hence a syn-tectonic foreland basin formed and unconformably overlied imbricated packages 1 and 2. D2 deformation is represented by the north-verging fold and thrust belt in the retro-foreland basin and constrains the final stage of shortening in the southern Beishan orogenic collage to late Permian
Age constraints on the tectonic evolution and provenance of the Pie de Palo Complex, Cuyania composite terrane and the Famatinian orogeny in the Sierra de Pie de Palo, San Juan, Argentina
Get PDFNew U-Pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the Pie de Palo Complex represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust. A gabbroic pegmatite, interpreted to have formed during arc rifting or subsequent back-arc spreading, yielded a U-Pb age of 1204 +5.3/-4. 7 Ma. Highly tectonized ultramafic-mafic cumulates, occurring at the structural base of the Pie de Palo Complex and previously interpreted to represent remnants of a primitive arc phase, prior to rifting and back-arc spreading, could not be dated, but should be older than 1204 Ma if these inferences are correct. Tabular, sill-like bodies of leucogabbro /diorite and calc-alkaline tonalite/granodiorite sills yielded ages of 1174±43 and 1169+1/-7 Ma respectively. They may represent a younger, more evolved arc phase established after arc rifting or a younger, tectonically unrelated Mesoproterozoic arc. SHRIMP-analysis of metamorphic zircon rims with low Th/U ratios in VVL 110 gave a 206Pb/238U age of 455 ± 10 Ma, similar to lower intercept dates determined by discordia lines. Combined, these data indicate that the bulk of the amphibolite facies metamorphism present in the Pie de Palo Complex was generated during the Famatinian Orogeny. Analysis of six single detrital zircon grains in a metasedimentary, quartzofeldspathic garnet-mica schist, tectonically interleaved with the igneous rocks of the Pie de Palo Complex, and tentatively correlated with the Difunta Correa metasedimentary sequence of other workers, yielded three age populations: 1150-1160 Ma; 1050-1080 Ma and 665 Ma, indicating that these sedimentary rocks were deposited during the late Neoproterozoic or Early Paleozoic. In addition, they confirm structural evidence that intercalation of rocks of the Pie de Palo Complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications. These ages are also consistent with a Laurentian provenance, and earlier interpretations that these rocks once represented a sedimentary cover to the Pie de Palo Complex. The zircon population of 1050-1080 Ma could be derived from Grenville-age felsic plutons identified elsewhere in the Pie de Palo Complex by other workers. However, no evidence has been found in our samples for a Grenville-age orogenic event, invoked previously to explain accretion of the oceanic Pie de Palo Complex to Laurentia prior to the late Neoproterozoic/Early Cambrian rifting and drift of Cuyania.Fil: Vujovich, Graciela Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: van Staal, Cees R.. Geological Survey of Canada; CanadáFil: Davis, William. Geological Survey of Canada; Canad
A structural-metamorphic approach to deciphering the exhumation of the Gubaoquan eclogite in the Beishan Orogenic Collage, NW China
Get PDFThe Ordovician Gubaoquan eclogite is situated in the southern part of the Palaeozoic
Beishan Orogenic Collage of the Central Asian Orogenic Belt in NW China. Mapping of an
extensive section of the metamorphic tectonite belt around the eclogite and optical petrography
have uncovered a high degree of fabric preservation, not recognised by earlier studies.
The study area is underlain by a highly deformed belt of metamorphic tectonites, intruded
by syn-tectonic granitoids. The belt largely consists of D3 retrograde fabrics, with only local
preservation of D1 eclogite-facies fabrics in the cores of mafic boudins. Fabrics associated with D1-
D4 document a clockwise metamorphic trajectory, characterised by eclogite-facies metamorphism
followed by a large decrease in pressure and small decrease in temperature, high-temperature lowpressure
conditions and greenschist-facies metamorphism. D2-D4 is associated with widespread NS
shortening and local extension.
SHRIMP U-Pb dating of zircon indicates felsic orthogneiss in the area primarily has
Neoproterozoic protoliths, whereas metasedimentary rocks and metabasites primarily have
Mesoproterozoic protoliths. Metamorphic zircon ages indicate a protracted metamorphic history
from ~470 to 420 Ma.
We propose that the metamorphic tectonite belt hosting the Gubaoquan eclogite represents
Meso-/Neo-Proterozoic crust that underwent Ordovician-Silurian, north-directed subduction to
various depths. The eclogite and its host were detached from the down-going slab, probably due to
slab break-off, which also caused extensive syn-tectonic magmatism. Uplift and exhumation
probably occurred in an extruding-wedge-type setting. Local convergence continued until the
emplacement of the Silurian-Devonian granitoids. Afterwards, the area was tectonically quiescent
until the Carboniferous-Triassic, when the area was exhumed
TECTONIC EVOLUTION OF THE BAIE VERTE MARGIN, NEWFOUNDLAND
No full textLocated along the Early Paleozoic Laurentian continental margin in Newfoundland, the Baie Verte Margin tectonistratigraphy and tectono-metamorphic evolution have been controversial for decades. Here, the results of a detailed field, petrological and geochronological study are presented, where Baie Verte Margin is subdivided into three tectono-metamorphic units separated by tectonic contacts: the East Pond Metamorphic Suite (EPMS) basement, EPMS cover, and the Fleur de Lys Supergroup (FdLS). Each unit exhibits a distinct metamorphic and structural evolution recorded during the subduction, exhumation, and post-collisional history of this ancient margin. The combination of thermodynamic modelling, petrochronology, and structural analysis provided insights into the P-T-t-d paths of the studied units, allowing a better understanding of their role during the evolution of the Taconic subduction system. High-pressure (HP) to ultra-high-pressure (UHP) conditions were reached between 483 and 475 Ma during the D1 phase, with the EPMS cover recording eclogite-facies metamorphism at ~2.8 GPa and 620°C. Subsequent decompression resulted in a β-shaped pressure-temperature-time (P-T-t) path, with near-isothermal decompression to ~2 GPa and heating to 860°C during exhumation. A multi-stage exhumation model is proposed for the EPMS eclogites: 1) buoyant rise through a low-density mantle wedge and 2) subsequent ascent at shallower crustal levels, facilitated by external tectonic forces and slab break-off, as evidenced by Late Taconic magmatism.
While the EPMS cover re-equilibrated at UHP conditions, the EPMS basement and FdLS experienced decompression and Barrovian metamorphism during late-D1, indicating decoupling of the units during this stage. Coupling between the units occurred along a D2 shear zone during retrograde metamorphism, spanning 475–452 Ma.
Two exhumation scenarios are proposed to explain the tectonic evolution of the margin: (i) Following late D1 detachment, the EPMS basement and FdLS were exhumed to crustal levels while the EPMS cover was subducted deeper into the mantle. Tectonic extrusion along D2 shear zones, potentially aided by melt weakening, then emplaced the EPMS cover between the two units. (ii) Alternatively, sequential detachment occurred from the top to the bottom of the slab, resulting in deeper subduction of lower units, followed by their exhumation through back-folding and crustal wedge thrusting.
The Silurian F3 folding deforms both D1-2 structures in each unit and the D2 shear zones that bound them, suggesting that the continental wedges, which recorded different tectono-metamorphic paths after early D1, were juxtaposed before the onset of deformation associated with the Salinic Orogeny. Later deformation phases, D4 and D5, are probably related to tectono-metamorphic activity related to the Acadian and Neo-Acadian orogenies.
This research improves our understanding of the dynamic tectono-metamorphic evolution of the Baie Verte Margin, emphasizing the role of fluids, thermal perturbations, and deformation in driving metamorphic reactions, and exhumation. The findings contribute to understanding the mechanisms controlling HP-UHP terrain evolution in subduction zones and highlight the complex interactions between subduction, exhumation, collision, and magmatism throughout the Taconic orogeny
PETROLOGY, TECTONIC SETTING, AND REGIONAL IMPLICATIONS OF THE OPHIOLITIC LIUYUAN COMPLEX, NW CHINA
Get PDFLocated along the southern margin of the Central Asian Orogenic Belt (CAOB), the mafic 290-280 Ma Liuyuan Complex stratigraphy and tectonic setting have been controversial for decades. Here, the results of a detailed field and petrological study are presented, where the Liuyuan Complex is subdivided into troctolite, melatroctolite, layered gabbro, olivine gabbro, podiform olivine gabbro, podiform hornblende gabbro, intrusive hornblende gabbro, plagiogranite, sheeted dyke, and mafic tectonite in addition to previously identified and studied basalt and chert. All contacts between the igneous facies are intrusive, with gabbroic rocks separated from the overlying basalt by a newly discovered, well-developed and laterally continuous sheeted dyke complex. Based on their trace element signature and thermodynamic modeling, the basalts of the Liuyuan Complex formed by 10-17% partial melting of a spinel lherzolite mantle source in a back-arc basin setting. The parental melt of the suite followed a typical tholeiitic liquid line of descent, fractionating olivine, plagioclase, clinopyroxene, and ilmenite. Stratigraphic relations, mineral chemistry, and trace element inversions further indicate that the gabbroic rocks in the Liuyuan Complex are comagmatic with the overlying basalts. The formation of the Liuyuan Complex as a back-arc basin ophiolite is inconsistent with previously proposed tectonic models for the southern CAOB, which required the Liuyuan Complex to have formed as a continental rift or a fore-arc ophiolite. Based on recent field investigations in the Liuyuan area, a tectonic model where the Liuyuan Complex formed as a back-arc to the recently identified Ganquan Arc is proposed. The back-arc basin was then consumed by a north-dipping subduction zone beneath the active margin of Composite Siberia starting ca. 281 Ma. The magmatic center of this arc migrated southwards, likely caused by slab roll-back, with the Liuyuan Complex becoming the basement of this arc. Exhumation of the Liuyuan Complex took place by 267 Ma, marking the closure of the southernmost and youngest known tract of the Paleo-Asian Ocean
EVOLUTION OF MULTI-ARC ACCRETION SYSTEMS: A CASE STUDY FROM THE SOUTHERN BEISHAN OROGEN, CENTRAL ASIAN OROGENIC BELT, NW CHINA
No full textThe Central Asian Orogenic Belt (CAOB) is characterized as an accretionary orogen, evolved for ca. 800 m.y from late Mesoproterozoic to early Mesozoic. Its terminal collision and accretion generally commenced in the west and terminated in the east, creating the Tien Shan suture and Solonker suture, respectively. The Beishan Orogen is a southern-central segment of the Central Asian Orogenic Belt, located in Northwest China. It connects the two sutures and contains some of the youngest ophiolitic complexes, arc terranes, and sedimentary basins in the CAOB. Hence, investigations of the southern Beishan Orogen can contribute to correlating the geology on both sides and establishing an integrated tectonic history of the Paleo-Asian Ocean’s closure.
Through detailed mapping at 1:25000 scale in southern Beishan, five distinctive tectono-stratigraphic packages have been identified, from south to north, including the Shiyaozi Complex, the Baidunzi Complex, the Ganquan Complex, the Liuyuan Complex, and a cover sequence forming the Heishankou Basin. The Baidunzi Complex comprises a mixture of sheared and transposed metasedimentary and syn-kinematic meta-plutonic rocks, ranging from greenschist to amphibolite facies. The metasedimentary sequences contain Carboniferous-Permian marble and meta-calcareous clastic rocks, which were intruded by syn-tectonic arc magmatic rocks (Baidunzi Intrusive Suite). Collectively these rocks represent a continental arc setting. A massive sheet of bedded quartzite in this complex, displaying no age signatures younger than 1.0 Ga, stands as the most probable candidate for Precambrian basement, with an inferred affinity to the Tarim Craton.
The Baidunzi Complex was overthrusted by the Ganquan Complex, which primarily consists of two sequences separated by unconformities and thrust faults. The lower sequence exhibits minimal reworking and lacks continent-derived materials in its stratigraphy. U-Pb zircon dating of this sequence yielded ages ranging from 295 to 285 Ma, without zircon inheritance. In contrast, the upper sequence is dominated by epiclastic sedimentary rocks, structurally and/or unconformably overlying rocks of the lower sequence. Both sequences display geochemical characteristics typical of island arcs, with an increase of εHf values over time. They correlate temporally and tectonically with the adjacent back-arc ophiolitic rocks within the Liuyuan Complex, an ophiolite mélange formed in a backarc setting. Together they represent an arc-backarc system, with arc-trench migration leading to the progressive incorporation of juvenile components in the source.
A cover sequence forming a Permian foreland basin, with typical fold-and-thrust style deformation, developed unconformably on the Ganquan and Liuyuan Complexes. From bottom to top, it consists of black shale, an unconformably overlying flysch-like sedimentary unit, and a syn-orogenic conglomerate with clasts of rocks from all the underlying sequences. The sedimentary record and structural analyses of this basin suggest south-verging deformation, associated with a collisional event involving the Ganquan arc-Liuyuan Backarc system over the Baidunzi Complex. Subsequently, this thrust-and-fold belt experienced a phase of north-verging deformation, aligning with the amalgamation of the Shiyaozi Complex from the southernmost region toward the Baidunzi Complex.
The overall field relationships record a transition from a passive to an active margin (Baidunzi Complex), formation and accretion of a relatively juvenile arc, and collision during the Permian
The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block
Get PDFTIMS and SHRIMP U-Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540-535. Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian-Pampean orogeny to have occurred between ca. 530. Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian-Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530. Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530. Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530. Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530. Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U-Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600. Ma.Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570. Ma, following an earlier departure of Amazonia (∼650. Ma?). The separation of Arequipa-Antofalla from Amazonia and Laurentia opened the Puncoviscana and Iapetus oceans respectively.Fil: Escayola, Monica Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: van Staal, Cees R.. Geological Survey of Canada; CanadáFil: Davis, William J.. Geological Survey of Canada; Canad
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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