Turbidite Basin Research Articles

River-fed basin-fill and hyperpycnites in an island-arc setting: The Silurian–Devonian Kekexiongkuduke Formation from Western Junggar, China

Sediments from land are primarily transported into basins through hyperpycnal flow, which plays a significant role in the process of basin filling. Various models of hyperpycnites have been proposed. This study examines a Silurian–Devonian basin-fill sequence in NW China to validate existing models and presents a visually compelling case demonstrating the process of basin filling through flood-dominated river discharge.The Utubulak Member of the Kekexiongkuduke Formation can be subdivided into three parts, representing basin floor, slope and shelf environments, collectively representing a regressive basin fill sequence. The lower part consists of flood-related turbidites contain or encompass material of terrestrial and marine origin, their sustained and pulsed velocity structures are similar to river discharge. These turbidites may result from basinal hyperpycnal flow or retrogressive failures of the shelf-edge (hyperpycnal) delta. The middle (slope deposits) contains bi-graded hyperpycnite bed interpreted commonly attributed to deposition by river floods exhibiting the classical waxing-waning configuration. Additionally, fine-grained deposits resulting from river-generated, dilute hyperpycnal or hypopycnal plumes are observed interbedded with the basin turbidites or slope background deposits. In the upper part, the presence of thick-bedded sandstones indicates that sustained hyperpycnal flow can transport sands to the shelf-edge/shelf, facilitating deep-water sand delivery. The vertical distribution of the various flood-related facies in basin floor, slope and shelf strata emphasize the complexity of hyperpycnites in different settings, and particularly highlighting challenges associated with their recognition in basin deposits.The investigated succession accumulated in an island-arc setting which provided a favorable condition for a river-fed turbidite system to develop. The narrow shelf facilitated transport of sediments by hyperpycnal flows directly into the basin, while erosion of high mountains promoted a high sediment supply for hyperpycnal flow. And a river-fed turbidite system commonly developed during the late stages of basin filling.

Testing turbidite conceptual models with the Kaikōura Earthquake co-seismic event bed, Aotearoa New Zealand

ABSTRACT The 2016 Mw7.8 Kaikōura Earthquake in Aotearoa New Zealand provides an opportunity to test widely applied turbidite sedimentation models because it triggered a co-seismic turbidity current. The resultant Kaikōura event bed (KEB), interpreted as a turbidite, is sampled for ∼ 1300-km down-flow along the depositional system. Sediment core lithologies, computed tomography (CT), and particle-size data are used to test event-bed thickness, silt content, facies distribution, and stacking patterns against the foundations of the turbidite conceptual model of Bouma (1962). KEB thickness is variable to ∼ 100 km down-flow distance and attains a maximum thickness at ∼ 700 km down-flow distance before thinning distally, similar to the predicted bell-shaped proximal to distal trend. Silt content is high throughout the KEB from canyon to fan. The KEB is dominated by laminated Td facies and Te facies that evolve down-system from laminated, then graded, to homogeneous muds. CT and granulometry data are key to differentiating subtle density and textural variations in fine-grained deposits and reveal that KEB Td and Te facies in the KEB that are often not preserved or readily observed in older deposits. The KEB highlights a fine-grained sedimentary system that contrasts with more widely studied sandy turbidite basins. In particular, the KEB example reveals that Td and Te facies are ubiquitous in this fine-grained, silt-rich system. A varied conceptual model developed from the KEB may be applicable to many modern deep-sea turbidite systems and crucial for understanding present-day particulate transport to the deep sea and interpreting evidence from the stratigraphic record.

Faulting, sediment loading, and flow of underlying ductile units: A case study from the Western Ionian Basin Offshore Eastern Sicily

Tectono-stratigraphic analysis coupled with digital 3D surface modelling derived from high-resolution seismic profiles is performed along a narrow turbidite basin offshore E-Sicily in order to increase understanding on the processes that contributed to the shaping of the Western Ionian Basin. Seismic-reflector patterns of the identified Pliocene-Quaternary sequence point to syn-depositional deformation during the Pliocene associated with the simultaneous activity of regional faults and underlying ductile units. Long-wavelength sediment fanning results from the extensional activity of the Malta Escarpment faults. Conversely, internal reflector architecture and lateral terminations indicate localized subsidence associated with the growth of uprising structures in the easternmost part of the basin. Lateral shifting of basin depocenters is in line with withdrawal effects observed in basins floored by ductile units (salt or shale). 3D modelling of time-reference surfaces highlights sub-circular depressions associated with nearby structural culminations. This pattern is similar to salt-withdrawal minibasins commonly reported in evaporite-floored basins. Accordingly, salt migration/flow triggered by sediment loading, locally enhanced by fault activity, is proposed as the process controlling basin evolution during the Pliocene in the Western Ionian Domain. Nevertheless, the possibility of shale/mud tectonics as the ductile source of deformation cannot be discounted.

Paleocene A-type igneous suites in the Sikhote-Alin (the East Asian continental margin): Petrological, geochronological, isotopic, and geodynamic constraints

Global geodynamic reconstruction models of the Pacific margin of Asia refer to materials collected throughout Japan, Korea and northeastern China, but they lack data on southern Sikhote-Alin. Therewith, the Sikhote-Alin orogenic belt (NE Asia) constituted a single eastern margin of the paleo-Asian continent with the abovementioned territories in the Paleocene. New isotopic, geochemical, and geochronological data show that Paleocene igneous activity (∼61–55 Ma) is widely developed in southern Sikhote-Alin. Bulk rock compositions of the igneous rocks of this period yield ferroan, peraluminous, calc-alkaline to alkaline affinities, highly abundant LILE and HFSE (with pronounced Ba, Sr, Eu, and Ti negative anomalies) and depleted HREE. The initial melts, displaying Zr + Y + Ce + Nb > 350 ppm and 10,000 × Ga/Al > 2.6, derived from an OIB-like mantle source crystallised under fairly reducing conditions (below FMQ buffer), and yield high temperature of zircon saturation (>850 °C), indicating typical A-type granite and related volcanic rock features. It is logical to associate variations in A-type rock geochemical composition with an enrichment of the upper part of the magma chamber with fluid-mobile components involving a redistribution of major and trace elements through fluid-magmatic differentiation. Strong depletion in Ca, Mg, Ba, Sr, Eu in the A-type rocks is caused by an outflow of these elements by an oxidizing, initially reduced, acidic fluid beyond the zone of magma generation. Whole-rock Sr-Nd isotope data argue for the generation of the A-type rocks by melting of dominantly a juvenile mantle component with a subordinate component of the ancient crust. Isotope variations of igneous rocks of the reference area: 87Sr/86Sr(t) (0.7024–0.7118), εNd(t) (−0.9 to −5.1) and TDM2 (934–1277 Ma), result from the mixing of the OIB-like mantle source with selective melts or from the metapelite contamination of the Samarka terrane accretionary prism and of the Zhuravlevka-Amur turbidite basin, later followed by fluid-magmatic differentiation that led to the formation of anatectic or hybrid melts. We further suggest that the origin of the A-type granites and related volcanic rocks is the result of the oblique interaction of oceanic and continental plates. This interaction accounts for the simultaneous formation of tears in the slab, enabling sub-slab asthenospheric upwelling, and strike-slip fault-related extensional structures in the overriding continental plate.

Generating clusters for turbidite probability maps using machine learning methods

Turbidite deposits are of great importance because of their correlation with the existence of oil reservoirs. Therefore, the spatial distribution of these deposits is frequently studied, considering the turbidite probability maps. The uncertainty of the original turbidite data, captured indirectly by sonars, is quantified by generation of possible scenarios of the probability map. This research aims, from the use of statistical characteristics and the creation of groups, through unsupervised machine learning algorithms, to identify the possible scenarios, generated by geostatistical methods, that are more similar to the original data considered in the study. Through the results, in addition to quantifying the uncertainties generating possible scenarios, it is verified that the methodology developed is capable of creating differentiated groups and identifying the group that has similar characteristics to the reference data, helping in the generation and identification of possible scenarios of turbiditic basins. The main novelty of this work is to combine geostatistical and machine learning methods, in addition to creating groups of images that have similar statistical characteristics.

Depositional record of confined turbidites in syn-subduction intraslope basin: insight from the Tufiti di Tusa Formation (Southern Apennines, Italy)

A detailed lithostratigraphy and facies analysis of a type section of the Tufiti di Tusa Formation, including deep-marine clastic successions with syn-orogenic volcanic detritus and deposited in the late Eocene - early Miocene basin system at the front of the growing Maghrebian - Southern Apennines orogen, is discussed in the paper. Based on facies analysis and composition, the study section was subdivided into the following units, from bottom to top: Unit I, mostly formed by contained-reflected beds (including a bed similar to the Contessa megabed of the Marnoso-arenacea Formation in the Northern Apennines), with the ratio of sandstone intervals to mudstone intervals (S/M) of 0.6 and with mostly-calciclastic sandstone to siltstone fraction; Unit II, recording a moderate decrease in contained-reflected beds and a moderate increase in slurry beds, with S/M ratio of 0.9, and with mostly-siliciclastic sandstone to siltstone fraction; Unit III, recording a further decrease in contained-reflected beds and an evident increase in slurry beds and very-thick beds with a basal massive very coarse to coarse-grained sandstone, with S/M ratio of 2.5, and with mostly-volcaniclastic sandstone to siltstone fraction.In accordance with the depositional models for the infill of confined turbidite basins, Units I and II are here interpreted as representing a flow ponding depositional phase, while Unit III as iconic of a flow stripping depositional phase. The compositional variation from Unit I to Unit II records cutoff of calciclastic supply from underplate sources, possibly tied to tectonic uplift of the external basin margin; while that from Unit II to Unit III records sudden availability of volcaniclastic sediment possible due to burial of morphological high(s) between the internal volcanic arc (source of the volcaniclastic sediment) and the depositional basin, and/or establishment of tectonically-controlled conduits cutting the above high(s). This study may improve the knowledge not only of infilling evolution of confined turbidite basins, but also of the depositional setting of the late Paleogene Southern Apennines subduction margin in the Central Mediterranean.

Geochronology and petrogenesis of Triassiclow‐Timafic intrusions in Funing: Implications for the tectonic evolution of Nanpanjiang Basin,SWChina

The Nanpanjiang Basin, one of the largest Triassic turbidite basins in the South China Block, has long been disputed with respect to its tectonic affinity. To better understand the tectonic processes of the basin, we conducted geological mapping, geochronological, and geochemical studies on the mafic intrusions (the Anding group) in Funing emplaced within the southern Nanpanjiang Basin. The Anding group mafic intrusions in Funing consist of gabbronorite and gabbroic diorite, and they are characterized by low TiO2(0.54–1.18 wt%) and Ti/Y ratios (162–222), low (La/Yb)CNratios (3.28–4.19), flat REE patterns, pronounced negative Nb, Ta and Ti anomalies. These mafic intrusions can be further divided into two subgroups: the Group 1 at 244 ± 3 Ma and the Group 2 at 224 ± 2 Ma. The Group 1 exhibits negative Eu anomalies, high Th/Hf ratios (1.61–1.91), high (87Sr/86Sr)iratios (0.708710–0.711843), and lowεNd(t) values (−6.70 to −7.82), while the Group 2 exhibits positive Eu anomalies, lower Th/Hf ratios (1.05–1.07), lower (87Sr/86Sr)iratios (0.708397–0.708425) and higher εNd(t) values (−3.28 to −3.21). Both the Group 1 and Group 2 are derived from partial melting of spinel‐bearing lithospheric mantle and formed at an intracontinental back‐arc setting. These results, combined with regional considerations, support a new model that the Triassic Nanpanjiang Basin is an intracontinental back‐arc extensional basin triggered by the westward subduction of the Palaeo‐Pacific Plate, and the Triassic magmatism in Funing was a response to the extension.

The mudstone composition as reflected in the sedimentary evolution of a turbidite basin: The example of the Agnone Flysch (Molise, Italy)

Chemical and mineralogical signatures of mudstones from the Agnone Flysch turbidite successions (late Tortonian to early Messinian) deposited in the Lagonegro-Molise foredeep basin reveal new insight for the sedimentary evolution of the southern Apennines during late Miocene. Geochemical and mineralogical signatures of the Verrino and Poggio Villanelle members of the Agnone Flysch mark a complex sedimentation history testifying to a multi-source area probably due to structural changes and depositional basin physiography. The studied mudstones show chemical variations in term of Ca vs. Si and Al. Mineralogical analyses confirm a decrease of carbonate minerals and an increase of silicate phases passing from Verrino Member to Poggio Villanelle Member samples. Compositional data of the mudstone samples of both members mainly record a derivation from felsic rocks due to the Calabrian terranes unroofing. Poggio Villanelle Member samples further show an enrichment in Cr, Ni, Fe e Mg in accordance with higher chlorite content suggesting a mafic supply probably related to an ophiolitic source successively exposed. Thus, those trends based on chemical and mineralogical data suggest a change in relative abundance of a detritus characterized by different compositions through time and space. Whole rock geochemistry of the studied mudstones indicate that they are from the first-cycle sediments.Palaeoweathering processes in source areas were moderate as shown by CIA (Chemical Index of Alteration) and CIA’ (modified Chemical Index of Alteration) values of the mudstones of both members (Verrino and Poggio Villanelle members). The trends observed in the paleoweathering diagrams suggest source areas characterized by moderate weathering in non-steady-state conditions which have changed toward the upper portion of the sedimentary succession. Those changes are mainly related to the geodynamic conditions of the source areas where active tectonism allows erosion of all zones within weathering profiles developed on source rocks.The paleocurrent data and the compositional analysis of the mudstones and sandstones indicate a provenance (supply) from the western, north-western, and south-western sectors of the basin where carbonate platforms and magmatic, metamorphic and sedimentary rocks cropped out.

Tectonic and eustatic control of Mesaverde Group (Campanian–Maastrichtian) architecture, Wyoming-Utah-Colorado region, USA

AbstractWe describe and analyze the depositional history and stratigraphic architecture of the Campanian and Maastrichtian succession of the southern greater Green River basin of Wyoming, USA, and surrounding areas to better understand the interplay between tectonic and eustatic drivers that built the stratigraphy. By integrating new measured sections with published outcrop, well-log, and paleogeographic data, two new stratigraphic correlation diagrams, 35 new paleogeographic reconstructions, and six new tectonic diagrams were created for this part of the Western Interior Seaway. From this work, two time-scales of organization are evident: (1) 100–300 k.y.-scale, mainly eustatically driven regressive-transgressive shoreline oscillations that generated repeated sequences of alluvial-coastal plain-shoreline deposits, passing basinward to subaqueous deltas, then capped by transgressive estuarine/barrier lagoon deposits, and (2) 3.0–4.0 m.y.-scale, tectonically driven groups of 10 to 15 of these eustatically driven units stacked in an offset arrangement to form larger clastic units, which are herein referred to as clastic wedges. Four regional clastic wedges are recognized, based on the architectures of these clastic packages. These are the: (1) Adaville, (2) Rock Springs, (3) Iles, and (4) Williams Fork clastic wedges. Pre-Mesaverde deposition in the Wyoming-Utah-Colorado (USA) region during the Middle Cretaceous was characterized by thickening of the clastic wedge close to the thrust-front, driven primarily by retroarc foreland basin (flexural) tectonics. However, a basinward shift in deposition during the Santonian into the early Campanian (Adaville clastic wedge) signaled a change in the dominant stratigraphic drivers in the region. Shoreline advance accelerated in the early to middle Campanian (Rock Springs clastic wedge), as the end of activity in the thrust belt, growing importance of flat-slab subduction, and steady eastward migration of the zone of dynamic subsidence led to loss of the foredeep and forebulge, with the attendant formation of a low-accommodation shelf environment. This “flat-shelf” environment promoted large shoreline advances and retreats during sea-level rise and fall. During the middle to late Campanian (Iles clastic wedge), deep erosion on the crest of the Moxa Arch, thinning on the crests of the Rock Springs and Rawlins uplifts, and subsequent Laramide-driven basin formation occurred as the Laramide blocks began to partition the region. The next clastic package (Williams Fork clastic wedge) pushed the shoreline over 400 km away from the thrust belt during the late Campanian. This was followed by a very large and persistent marine transgression across the region, with the formation of a Laramide-driven deepwater turbidite basin with toe-of-slope fans into the early Maastrichtian. The Mesaverde Group in the Wyoming-Utah-Colorado region is thus characterized by: (1) a succession of four tectonically driven classic wedges, each comprised of a dozen or so eustatically driven packages that preserve large basinward and landward shoreline shifts, (2) broad regional sand and silt dispersal on a low-accommodation marine shelf setting, (3) a progressive, tectonically driven, basinward shift of deposition with offset, basinward stacking of successive clastic wedges, and (4) the gradual formation of various uplifts and sub-basins, the timing and sizes of which were controlled by the movement of deep-seated Laramide blocks. The Mesaverde Group in the Wyoming-Utah-Colorado region provides an outstanding opportunity to study the dynamic interaction among the tectonic control elements of a subducting plate (crustal loading-flexure, dynamic subsidence/uplift, and regional flat-slab basin partitioning), as well as the dynamic interaction of tectonic and eustatic controls.

ХЕЙЛУНЦЗЯНСКИЙ КОМПЛЕКС – ФРАГМЕНТ ЮРСКОЙ АККРЕЦИОННОЙ ПРИЗМЫ В ТЕКТОНИЧЕСКИХ ОКНАХ ПЕРЕКРЫВАЮЩЕЙ КОНТИНЕНТАЛЬНОЙ ПЛИТЫ: МОДЕЛЬ ПЛОСКОЙ СУБДУКЦИИ

The Circum-Pacific Late Albian-Cenomanian orogenic belts (including the Sikhote-Alin-Western-Sakhalin belt) were formed as a result of the deformation of mainly epioceanic terranes – fragments of Jurassic-Early Cretaceous accretionary wedges with ophiolites and other fragments of oceanic crust, turbidite basins, and island-arc systems. To the west of the Sikhote-Alin - Northern-Sakhalin belt and orthogonally, there are structures of earlier consolidation, which include the Bureya-Jiamusi-Khanka fragment of the orogenic belt of the Late Cambrian-Early Ordovician consolidation of the Late Proterozoic - Cambrian complexes. Within this belt, four isolated outcrops of the Heilongjiang complex are mapped, combining metamorphites of the epidote-amphibolite and glaucophane-schist facies and representing a fragment of an accretionary wedge of the Jurassic age. It was assumed that these outcrops marked the suture, i.e., represented the remains of the closed Mudanjiang paleoocean, separating the original Jiamusi terrane (and the Bureya-Jiamusi-Khanka belt) located to the west of Central Asia structures. The proposed article provides data that the Heilongjiang complex does not mark a suture, but is an underground near-horizontal continuation of the marginal continental accretionary wedge of the Nadanhad - Bikin terrane (flat subduction model) brought to the surface at the site of the antiform bending. The unity of the compared parts of the accretionary wedge is emphasized by the close matrix age, the similarity of detritus zircon populations, and the similarity of the composition and age of allochthonous inclusions (limestone, chert, Late Paleozoic and Early Mesozoic basalt). An important common feature is that Late Paleozoic and Early Mesozoic basalts from allochthonous inclusions in both cases are represented by the N-MORB and OIB types, with no traces of suprasubduction volcanism in the matrix. The Heilongjiang complex forms, according to this interpretation, a tectonic window among the more ancient formations of the Jiamusi terrane. There is no need to assume the existence of the Mudanjiang Ocean to explain the formation of the Heilongjiang complex. The features of the structure of this complex and the conditions of its occurrence can be explained by the processes of flat subduction of the Pacific slab in the Jurassic and its deformation in the Early Cretaceous.

Fabric and internal architecture of Permian Basin turbidites indicated by unsupervised machine learning analysis of P-P and SV-P images

The Permian Basin of west Texas spans two major subbasins — the Midland Basin and the Delaware Basin. Both basins contain Wolfcampian- to Leonardian-age turbidites that form thick sections of prolific unconventional reservoirs. For the past several years, the most active drilling targets in the United States have been the Wolfberry turbidite interval of the Midland Basin and the Wolfbone turbidite interval in the Delaware Basin. We have used two new technologies to examine the internal architecture and fabric of a thick interval of these unconventional drilling targets with seismic reflection seismology. First, we used seismic interpretation software that uses unsupervised machine learning (ML), so that a higher level of detail could be extracted from seismic images. Second, we complemented our P-P imaging of Wolfberry turbidites with a new seismic imaging option, that being SV-P (or converted-P) imaging. Because vertical vibrators, particularly arrays of vertical vibrators, produce downgoing P and downgoing SV illuminating wavefields, SV-P reflections can usually be extracted from the same vertical-geophone responses as are P-P reflections. The combination of these two images essentially doubles the amount of information that can be extracted from data generated by P sources and recorded with vertical geophones. SV-P imaging with P sources has been ignored by reflection seismologists for decades, so we felt an obligation to illustrate the value of this ignored seismic mode. These two new tools — SV-P imaging and interpreting P-P and SV-P images via unsupervised ML — expanded our insights into the internal architecture and fabric of Wolfberry turbidites. Our work provides interpreters a much-needed example of applying unsupervised ML technology in a joint interpretation of P- and S-wave data.

Cyclic stacking pattern, architecture and facies of the turbidite lobes in the Macigno Sandstones Formation (Chattian-Aquitanian, northern Apennines, Italy)

This study presents a detailed facies analysis of an 800 m-thick stratigraphic succession in the basinal turbidites of the Macigno Sandstones Formation (MSF) recording the first foredeep in the northern Apennines (Italy). This stratigraphic succession shows an evident stacking pattern that, besides the basal part (200 m thick) dominated by fine grained and thin beds, is characterized by seven magnificently exposed sandstone lobes (from 90 to 19 m thick) separated by well-developed fine-grained interlobe deposits. Thanks to the detailed facies analysis, not only a well-defined architectural hierarchy has been identified, but a facies scheme has also been proposed, whereby cyclic recurring of three facies associations has in turn been identified. Each facies association records a well-determined evolutionary stage of lobe development, where an early depositional stage of the lobe building phase is followed by a flow impact and bypass stage recording flow decelerations and hydraulic jumps induced by the morphology created by the underlying depositional lobe. The latter phase promotes a switching and channel feeder avulsion that heralds the third phase of abandonment with the deposition of fine-grained interlobes. The facies stacking pattern and the evolutionary model of the MSF - also supported by other similar field and experimental works - are intended as a contribution to the understanding of intrinsic autogenic mechanisms at the basis of sand-rich depositional lobe development.

Multi-stage tectonic events of the Eastern Kunlun Mountains, Northern Tibetan Plateau constrained by fission track thermochronology

The present Tibetan plateau is a collage of several terranes, including the Lhasa, Qiangtang, Songpan-Ganzi and Kunlun-Qaidam terranes. Following closure of the Palaeo- and Neo-Tethyan oceans, the terranes of the Tibetan plateau migrated northward, and the deformation timing of the northern part of the Songpan-Ganze terrane between Golmud and Dulan in the Eastern Kunlun Orogenic Belt (EKOB) is still poorly understood. Fission track (FT) analysis of zircon and apatite from two localities between Golmud and Dulan has yielded new information about the timing of deformation in EKOB. Samples collected from Aikeng and Boguole yielded zircon and apatite FT central age ranges of ~193–133 Ma and ~103–29 Ma respectively. Thermal history modelling of FT data revealed five major episodes of cooling in these regions at ~200–170 Ma, ~170–100 Ma, ~100–50 Ma, ~50–18 Ma, and ~18–0 Ma. Stage one represents the collision between the Qiangtang and Kunlun tectonic blocks and the closure of the Songpan-Ganzi-Bayan Har Triassic turbidite basin. In stage two, the northward subduction of the Neo-Tethys oceanic plate accentuated sediment transport to the actively closing ocean basin between the Lhasa and Qiangtang terranes, the far-field effect of collision caused the cooling of the EKOB. Stage three is characterised by tectonic quiescence, which ended at ~50 Ma due to the collision of India and Asia. In stage four, the establishment of a strike-slip fault system and fold-thrust belt in the EKOB resulted in exhumation. Stage five saw rapid cooling in response to the most recent compressional event associated with fault development in the EKOB.

Accretion of the Anuy Zone, Tectonic Zonation, and Evolution of the Samarka Accretionary Complex: Details of Evolutionary Scenario of the Sikhote-Alin Segment of the East Asian Continental Margin

The Sikhote-Alin Orogen in the southeast of Russia is a collage of geological terranes of various age and tectonic origin, which formed along the East Asian continental margin as a result of the Jurassic–Early Cretaceous subduction of the Pacific oceanic plates. The Jurassic Samarka accretionary complex (AC) and the Early Cretaceous Zhuravlevka turbidite basin in the southern part of the orogen are considered indicators of subduction continental margin and transform plate boundary regimes, respectively. The regime conversion was assumed at the end of the Jurassic, when subduction stopped. Our biostratigraphic study of radiolarians from siliceous and fine-clastic sedimentary rocks has revealed the latest oceanic deposits and the youngest, Early Cretaceous fragment of the Samarka AC in its less studied northeastern part, which is ascribed to the Anuy tectono-stratigraphic element. The well-preserved radiolarians allow accurate dating of cherts, siliceous mudstone, and mudstone. This and other available biostratigraphic data allow the reinterpretation of the stratigraphy of the accreted oceanic sedimentary rocks. The refined stratigraphy is interpreted as a subsequent change in depositional settings on an oceanic plate, which moves to the convergent plate margin. Cherts accumulated in an oceanic pelagic zone from the Middle Triassic to the Late Jurassic (Early Oxfordian). Siliceous mudstone were deposited in a hemipelagic zone in the early Oxfordian–middle Tithonian. Mudstone and siltstones deposited on an external slope of a deep trench in the late Tithonian–Berriasian. The sandy deposits were deposited in an axial part of the trench in the early Valangian, which best corresponds to the timing of accretion. Thus recognized early Valanginian accretion episode shows that subduction underneath the continental margin lasted longer than previously suggested. The onset of the transform continental boundary regime occurred later, probably, in the late Valanginian. This further details the Mesozoic evolutionary scenario, which was previously proposed for the Sikhote-Alin segment of the East Asian continental margin. We also refine the tectonic zonation and evolution of the Samarka AC.

Alb-Cenomanian granitoid magmatism and copper ore genesis of the Sikhote-Alin

Late Albian-early Cenomanin epoch of Au-Cu porphyry mineralization has been distinguished within the Sikhote-Alin. It is associated with the Alb-Cenomanian granitic rocks which emplacement coincided with the processes of orogeny and neoformation of continental lithosphere caused by compressive stress in the setting of transform continental margins of that time. The intrusion of the granitic magma into the crust of Jurassic accretionary wedge terranes and Early Cretaceous terrane of epicontinental turbidite basin provoked development of Au-Mo-Cu and Cu-Au-W ore genesis, respectively. U-Pb dating of zircons from host granites of the Malmuzh Au-Cu deposit yielded Alb-Cenomania age of 100-95 Ma. This age harmonizes with the age data reported by other researchers on the granitic rocks of East and Southeast Asia which are productive for hydrothermal mineral deposits of copper, gold, tin and other metals. Such age consistency suggests that there is Albian-Cenomanian metallogenic megabelt extending throughout the entire East Asian continental margin.

The Lazurnoe deposit in the Central Sikhote-Alin, Eastern Russia: Combined shoshonite-related porphyry Cu-Au-Mo and reduced intrusion-related Au mineralization in a post-subduction setting

The Lazurnoe porphyry Cu-Au-Mo deposit (>1Mt Cu-eq.) in the Mesozoic Central Sikhote-Alin orogenic belt (Eastern Russia) is located in an Early Cretaceous turbidite basin associated with a crustal-scale strike-slip fault zone, and was formed under the continental transform margin regime after the cessation of active subduction. The deposit is related to a magnetite-series, high-K calc-alkaline to shoshonitic, Early Cretaceous igneous suite that occurred in this tectonic setting, possibly due to asthenospheric mantle upwelling. The fertile magma was sourced from a subduction-modified mantle, under a low degree of partial melting, and was subjected to intense amphibole fractionation during its crystallization, the latter facilitating water and metal saturation. The parental igneous suite at Lazurnoe was coeval to ilmenite-series calc-alkaline igneous suites accompanied by reduced intrusion-related Au(±W) to W-Au deposits as well as to ilmenite-series, high-K calc-alkaline to shoshonitic suites accompanied by Sn-polymetallic mineralization.The deposit comprises magnetite-rich, quartz-K-feldspar to quartz-K-feldspar-biotite (potassic) alteration assemblages, containing bornite and chalcopyrite, which evolve to quartz-chlorite ± epidote ± amphibole (propylitic) assemblages, containing dominant pyrite with magnetite and chalcopyrite. Molybdenite-chalcopyrite-pyrite paragenesis dominates in quartz-sericite (phyllic) alteration assemblages that overprint and replace potassic and propylitic alteration. Potassic alteration assemblages formed from hot (~530–510 °C), high-salinity (45–48 wt% NaCl and 9–10 wt% CaCl2), sodic-calcic aqueous-chloride fluid. Gradual fluid cooling and dilution (from 29 to 31 wt% NaCl and 18–19 wt% CaCl2 through 24–25 wt% NaCl and 16–17 wt% CaCl2 to 12 ± 0.5 wt% NaCl and 12 ± 0.5 wt% CaCl2) is recorded for propylitic alteration assemblage. Another cycle of fluid exsolution from crystallizing magma can be suggested for phyllic alteration assemblages formed from a cooler (~390–310 °C), high-salinity (30–37 wt% NaCl and 5 ± 0.5 wt% CaCl2) aqueous-chloride fluid. The porphyry-style mineralization was overprinted by the late auriferous quartz-sulfide veins containing arsenopyrite, pyrrhotite and other sulfide minerals, together with native gold (locally > 100 g/t Au) and tellurides. These veins were formed from boiling aqueous-carbonic fluid that was characterized by elevated CO2 and CH4 contents, with subsequent separation of high-carbonic and aqueous-chloride phases, and the fluid cooling from ~390 to 250 °C. These veins can be related to another, more reduced, plutonic suite, which crystallized at a greater depth.

Late Triassic tectonic inversion in the upper Yangtze Block: Insights from detrital zircon U–Pb geochronology from south‐western Sichuan Basin

AbstractThe Sichuan Basin and the Songpan‐Ganze terrane, separated by the Longmen Shan fold‐and‐thrust belt (the eastern margin of the Tibetan Plateau), are two main Triassic depositional centres, south of the Qinling‐Dabie orogen. During the Middle–Late Triassic closure of the Paleo‐Tethys Ocean, the Sichuan Basin region, located at the western margin of the Yangtze Block, transitioned from a passive continental margin into a foreland basin. In the meantime, the Songpan‐Granze terrane evolved from a marine turbidite basin into a fold‐and‐thrust belt. To understand if and how the regional sediment routing system adjusted to these tectonic changes, we monitored sediment provenance primarily by using detrital zircon U‐Pb analyses of representative stratigraphic samples from the south‐western edge of the Sichuan Basin. Integration of the results with paleocurrent, sandstone petrology and published detrital zircon data from other parts of the basin identified a marked change in provenance. Early–Middle Triassic samples were dominated by Neoproterozoic (~700–900 Ma) zircons sourced mainly from the northern Kangdian basement, whereas Late Triassic sandstones that contain a more diverse range of zircon ages sourced from the Qinling, Longmen Shan and Songpan‐Ganze terrane. This change reflects a major drainage adjustment in response to the Late Triassic closure of the Paleo‐Tethys Ocean and significant shortening in the Longmen Shan thrust belt and the eastern Songpan‐Ganze terrane. Furthermore, by Late Triassic time, the uplifted northern Kangdian basement had subsided. Considering the eastward paleocurrent and depocenter geometry of the Upper Triassic deposits, subsidence of the northern Kangdian basement probably resulted from eastward shortening and loading of the Songpan‐Ganze terrane over the western margin of the Yangtze Block in response to the Late Triassic collision among Yangtze Block, Yidun arc and Qiangtang terrane along the Ganze‐Litang and Jinshajiang sutures.

GRAIN SIZE STATISTICAL PARAMETERS AND PALAEOFLOW VELOCITY MEASUREMENTS OF THE TERTIARY PINDOS FORELAND BASIN TURBIDITES

Pindos foreland is a tertiary turbiditic basin fill trending parallel to the external Hellenides (Aubouin, 1959). The basin is bounded to the east by the Pindos thrust and to the west by the Ionian thrust. Apart of these two major thrusts, minor thrusts separate the basin into linear narrow sub-basins, trending also parallel to the basin axis. For the grain size statistical analysis 35 sandstone samples were collected from sandstone beds in three sections: Metsovo, Amphilochia and Palaiopyrgos. The thickness of the beds ranges from 8 to 25 cm, and comprise Ta, Tb and Te Bouma sequence subdivisions. The samples were smashed in small pieces and then they were disaggregated using acetic acid solution. Then the samples were washed with deionized water and prepared for sieve analysis. The results of the sieve analysis were plotted in grain size cumulative diagrams in order to estimate the statistical parameters. Sorting, skewness and kyrtosis were calculated and also the samples were plotted in CM, FM and LM diagrams (Passega, 1957; Passega, 1964) The palaeoflow velocity measurements were estimated using Komar's model (1985). The results of the above analysis provided the following conclusions: a) Sorting values are decreasing at the top of Metsovo and Palaiopyrgos sections indicating an increase of the sediments immaturity, b) The asymmetry values range from positive to very positive with a trend to increase at he top in the sections of Metsovo and Amphilochia, which shows a dominance of the coarser fraction in the selected samples, c) C-M, F-M, L-M affirms that the sediment was transported to the deeper parts of the basin by turbidity currents. Mean flow velocities at the time of deposition range between 1,86 and 26,59 cm/sec. These values are very much in agreement with those proposed for low-density turbidity currents (<25cm/sec) (Nelson and Nilsen, 1984). A similar velocity range is refereed also by Avramidis (1999) who studied the turbidites of the Middle Ionian zone. In Metsovo and Palaiopyrgos mean flow velocity values increase towards the top of the stratigraphy.

The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.

U‐Pb Dating and Lu‐Hf Isotopes of Detrital Zircons From the Southern Sikhote‐Alin Orogenic Belt, Russian Far East: Tectonic Implications for the Early Cretaceous Evolution of the Northwest Pacific Margin

AbstractThe Sikhote‐Alin orogenic belt in Russian Far East is comprised of several N‐S trending belts, including the Late Jurassic to Early Cretaceous accretionary prisms and turbidite basin which are now separated by thrusts and strike‐slip faults. The origin and collage of the belts have been studied for decades. However, the provenance of the belts remains unclear. Six sandstone samples were collected along a 200 km long east‐west traverse across the major belts in the southern Sikhote‐Alin for U‐Pb dating and Lu‐Hf isotope analysis to constrain the provenance and evaluate the evolution of the northwest Pacific margin at this time. The result reveals that the sediments from the main Samarka belt was mainly from the adjacent Bureya‐Jiamusi‐Khanka Block (BJKB); the eastern Samarka belt and the Zhuravlevka turbidite basin were supplied by detritus from both the North China Craton (NCC) and the BJKB; the Taukha belt was mainly fed by sediments from the NCC; whereas the data from the Sergeevka nappes are insufficient to resolve their provenance. In the Late Jurassic to Early Cretaceous, collision and subduction was important in the initial collage of most belts in Sikhote‐Alin. However, merely E‐W trending collage cannot explain the increasing importance of the NCC provenance from west to east. It is proposed that the main Samarka belt was located adjacent to the BJKB when deposited, whereas the other belts were farther south to accept the materials from the NCC. Sinistral strike‐slip faulting transported the eastern belts northward after their initial collage by thrusting.