Subduction Complex Research Articles

Structures, strain analyses, and 40Ar/39Ar ages of blueschist‐bearing Heilongjiang Complex (NE China): Implications for the Mesozoic tectonic evolution of NE China

The high‐pressure Heilongjiang Complex belongs to a Jurassic accretionary complex in Yilan area, NE China. The complex is located in the junction of three orogenic systems in the NE Asia: the E‐W‐striking Central Asian Orogenic Belt (CAOB), the N‐S‐striking Paleo‐Pacific system, and the ENE‐WSW‐striking Mongol‐Okhotsk system. Thus, a detailed tectono‐metamorphic analysis coupled with accurate radiometric dating of the Heilongjiang Complex provided in this study represent important evidence for understanding the complex interactions between the three orogenic systems in the Late‐Palaeozoic to Mesozoic. New structural and microstructural data combined with the strain analysis highlight the presence of three distinct deformation events affecting the high‐pressure Heilongjiang Complex. The first deformation event D1 is related to development of blueschist facies prolate to plane strain fabrics that were subsequently reworked by the blueschist‐amphibolite facies event D2 that transposed all previous structures by the main west‐dipping schistosity. This planar fabric was folded by upright F3 folds and heterogeneously reworked by greenschist facies E‐W‐striking vertical cleavage during large scale N‐S shortening. Each deformation event was related to specific strain fabrics and metamorphic conditions highlighting dynamic evolution of the subduction complex during burial, exhumation, and final shortening. The first D1 event is related to westward directed prograde constrictional flow and progressive dismembering of competent mafic, ultramafic layers, and quartzites in highly stretched meta‐sedimentary matrix. This mélange was folded by recumbent detachment folds resulting in emplacement of the exhumed unit progressively reaching amphibolite facies conditions atop of W‐WSW‐dipping channel. This event is associated with dominant flattening implying increasing mechanical coupling between the colliding Songliao and Jiamusi blocks. Subsequently, the accreted subduction complex was reworked during N‐S‐directed horizontal shortening. This event was associated with an important post‐buckle flattening of fold limbs and rotation of competent blocks with inherited D1 and D2 strain ellipsoids along F3 fold axes. Our new 40Ar‐39Ar plateau ages on muscovite and phengite (173–177 Ma), together with previous radiometric data, are interpreted as reflecting exhumation of subduction channel during the blueschist‐amphibolite facies event while the N‐S shortening (D3) probably operated in Late Jurassic as indicated by some previously published cooling ages (~140–160 Ma). The westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex followed by N‐S‐directed upright folding are interpreted to reflect transition from E‐W Pacific subduction regime to N‐S‐directed shortening related to the closure of the Mongol‐Okhotsk Ocean.

SHRIMP U–Pb zircon geochronology of the granitoids of the Imiter Inlier: Constraints on the Pan-African events in the Saghro massif, Anti-Atlas (Morocco)

This study presents new U–Pb zircon ages for three granitoid bodies in the Imiter inlier at the eastern part of the Moroccan Anti-Atlas chain, which is located in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 538 ± 6 and 575 ± 10 Ma for the Igoudrane massif, 567 ± 6 Ma for the nearby Bou Teglimt granodiorite, and 582 ± 6 Ma for the Bou Fliou granite. These new data reveal that the Igoudrane massif is younger than previously thought (677 Ma), and that all the granitoids in the Imiter area are Ediacaran in age and are related to the Ouarzazate Group. Sm-Nd isotopic data yield negative εNd(t) values (−4.5 to −0.2) and TDM model ages of 1.04–1.82 Ga. These data indicate the presence of an inherited Paleoproterozoic to Archaean component in some zircon grains and show that generation of the parent magmas involved a Neoproterozoic juvenile contribution and recycling of an older crustal component.These new data allow reinterpreting the geochemical and isotopic data of the granitoids, and revisiting the structural significance to the Igoudrane massif, which was assumed until now to be coeval with the major Pan-African event of the Central Anti-Atlas. They also bring significant constraints to discuss the context and age of deposition of the Saghro Group, which is still a matter of debate. All these data provide evidence of an emplacement of the Imiter granitoids in a subduction complex, which affected all of the Anti-Atlas chain during the Late Proterozoic time.

Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex

Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex

A 1.9‐Ga Mélange Along the Northern Margin of the North China Craton: Implications for the Assembly of Columbia Supercontinent

AbstractThe involvement and location of the Neoarchean‐Paleoproterozoic North China craton in the supercontinent Columbia remains enigmatic, and the tectonic history along its margins impacts our understanding of connections between North China and other continents. Here we present structural observations from a Paleoproterozoic mélange that is located along its northern margin and that we refer to as the Bayan Obo mélange. It is composed of a structurally complex tectonic mixture of metapelites and metasedimentary rocks mixed with exotic blocks of ultramafic‐mafic rocks, metabasalts, metacarbonate and alkaline rocks, and tonalite‐trondhjemite gneisses. New zircon geochronology of the various constituent blocks suggest that it formed, and was subsequently deformed, at ca. 1.9 Ga. The oldest intramélange blocks are 2.45‐ to 2.54‐Ga tonalite‐trondhjemite‐granodiorite rocks and granitoids that signify the stabilization of the northern North China in the Neoarchean‐Paleoproterozoic. A ca. 2.45‐Ga plagiogranite block probably originated by partial melting of the older tonalite‐trondhjemite‐granodiorite rocks. We suggest that this mélange formed in a sedimentary setting near the subduction trench, on the basis of mixing of upper and lower plate volcanic rocks and textural relationships. The Bayan Obo mélange thus represents one of the oldest documented sedimentary mélanges on Earth, yet its fundamental characteristics strongly resemble Phanerozoic subduction complexes. Based on similar ages and styles of deformation, this zone may represent the ca. 1.9‐Ga collisional zone between North China and the southwestern margin of the Siberia craton. In this context, the North China craton became an integral component of the supercontinent Columbia starting at ca. 1.9 Ga.

Opposite Subduction Polarity in Adjacent Plate Segments

AbstractActive and fossil subduction systems consisting of two adjacent plates with opposite retreating directions occur in several areas on Earth, as the Mediterranean or Western Pacific. The goal of this work is to better understand the first‐order plate dynamics of these systems using the results of experimental models. The laboratory model is composed of two separate plates made of silicon putty representing the lithosphere, on top of a tank filled with glucose syrup representing the mantle. The set of experiments is designed to test the influence of the width of plates and the initial separation between them on the resulting trench velocities, deformation of plates, and mantle flow. Results show that the mantle flow induced by both plates is asymmetric relative to the axis of each plate causing a progressive merging of the toroidal cells that prevents a steady state phase of the subduction process and generates a net outward drag perpendicular to the plates. Trench velocities increase when trenches approach each other and decrease when they separate after their intersection. The trench curvature of both plates increases linearly with time during the entire evolution of the process regardless their width and initial separation. The interaction between the return flows associated with each retreating plate, particularly in the interplate region, is stronger for near plate configurations and correlates with variations of rollback velocities. We propose that the inferred first‐order dynamics of the presented analog models can provide relevant clues to understand natural complex subduction systems.

A comparison between mid-Paleozoic New England, USA, and the modern western USA: Subduction of an oceanic ridge-transform fault system

A detailed comparison is made between the mid-Paleozoic Norumbega fault system (NFS) in Maine, USA, and the San Andreas fault system (SAFS) of coastal California, USA, and their tectonic settings. The SAFS formed following subduction of an oceanic ridge-transform system, and the NFS is interpreted as having formed the same way. The parallel evaluation of mid-crustal processes associated with the NFS and surface processes associated with the SAFS gives a unique perspective on modern and inactive crustal-scale strike-slip fault systems.The SAFS is separated from the Cascadia subduction zone by the Mendocino triple junction. Similarly, the mid-Paleozoic NFS is interpreted as having been separated from a convergent zone to the southwest by the interpreted Norumbega triple junction. The SAFS cuts a subduction complex, the Franciscan Complex, that formed when the margin was still convergent. No equivalent subduction complex associated with the NFS has been recognized, perhaps because it was eroded away or displaced, or because it never formed. The SAFS experienced periods of transpression and transtension, whereas the NFS shows transpressive structures only. Slab window magmatic rocks exist along the SAFS. Devonian to earliest Carboniferous plutonic rocks along the NFS may in part be slab window magmatic rocks, although the effects of the Acadian and prior orogenies in the Appalachians complicate their identification. Magmatic rocks along the SAFS are offset due to the high rate of displacement (20–40 mm/yr). Magmatic rocks along the NFS show little offset, consistent with the low rate of displacement (~3 mm/yr) in the model. Transverse structures are associated with both the SAFS and NFS.Other Mendocino-type triple junctions may have existed in the past, but may be difficult to recognize. Where a past crustal-scale intracontinental strike-slip fault system terminates on an along-strike contemporaneous convergent system, such a fault-fault-trench triple junction may be considered.

Metacarbonate-terrigenous complex of the Derba block (East Sayan): petrogeochemical and isotope parameters, metamorphism, and time of formation

The Derba block is one of the largest Precambrian terranes of the Sayan-Yenisei accretionary belt in the southwestern margin of the Siberian Platform. It is composed of metamorphosed terrigenous-carbonate rocks of the Sayan Group, injected by granitoids. The geochemical features of gneiss-schist associations indicate the low maturity of their sedimentary protoliths corresponding in composition mainly to graywackes and terrigenous-carbonate rocks (marls). According to the results of U-Pb (LA-ICP-MS) dating of detrital zircons from gneisses and schists, the sedimentary protolith formed in the Vendian. Neoproterozoic subduction complexes were probably the major provenance for terrigenous material, and Early Precambrian rocks made a limited contribution. The Ar-Ar and U-Pb isotope data testify to nearly coeval and multistage events of metamorphism (up to the amphibolite facies) and granitoid magmatism (~510-500 and 480-465 Ma) in the Derba block. These processes were reflective of the Early Caledonian orogenic processes in the structures of the Central Asian Orogenic Belt. The similarity in the composition, time of sedimentation, and provenances of metaterrigenous-carbonate complexes of the Derba block (Sayan Group), West Sangilen block of the Tuva-Mongolian massif (Erzin and Moren complexes), and the Khamar-Daban terrane (Slyudyanka Group) suggests that these structures were a single Vendian continental margin with lateral variations in depositional environments and the sources of terrigenous material.

Does subduction of mass transport deposits (MTDs) control seismic behavior of shallow–level megathrusts at convergent margins?

We present a critical appraisal of the role of subducted, medium (10–1000 km2) to giant (≥1000 km2) and heterogeneous, mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a “chaotic” internal fabric (i.e., sedimentary mélanges or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T < 250 °C) of a subduction plate interface.

The Skipjack Island fault zone: An active transcurrent structure within the upper plate of the Cascadia subduction complex

The Skipjack Island fault zone has been mapped in the San Juan Islands between Vancouver Island, Canada, and the Washington State mainland, USA. A decade ago, interpretation of multibeam sonar seafloor imagery revealed that Skipjack Island, an east–west striking sedimentary bedrock outcrop, was a fault-controlled structural feature. A major fault separates Skipjack Island from a deformed sedimentary bedrock outcrop on the seafloor to the north. Recently the Skipjack Island fault zone's morphology and extent have been explored both to the west and east of the island using multibeam echosounder bathymetry, seismic reflection profiles, and sediment cores. The character of the Skipjack Island fault zone is well defined locally by the interpreted seismic-reflection profiles, which show active faults that displace sediments deposited since the Last Glacial Maximum. The central part of the fault zone, near Skipjack Island, appears as a near-vertical structure in the shallow subsurface that has been subjected to left-lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault, the result of drag from fault motion. Interpretations of recent geophysical data suggest an extension of the Skipjack Island fault zone farther to the west, where it trends towards, and possibly connects with, the Fulford Fault on Vancouver Island, and to the east, where it either terminates north of Lummi Island, or continues eastward onto the mainland. The Skipjack Island fault zone is interpreted to be the northern boundary of the San Juan Archipelago with the Devils Mountain fault zone being the southern boundary. Both fault zones represent the longest continuous faults of the San Juan Archipelago. This newly mapped fault zone and its zone of deformation to the north may represent the northern boundary of the Cascadia forearc. Potential sesimicity of the Skipjack Island fault zone and its proximity to the steep unstable northeastern side of Orcas Island and the southern front of the Fraser River Delta poses a geohazads problem in the form of earthquakes, landslides, submarine slides, and tsunamis.

Early Onset of Franciscan Subduction

AbstractThe Franciscan subduction complex of California is considered a type example of a subduction‐accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high‐grade blocks within mélange. Many of the high‐grade blocks record counterclockwise pressure‐temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high‐grade blocks show evidence for clockwise pressure‐temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U‐Pb ages from inclusions in garnet and Lu‐Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at ~176 Ma. Matrix zircon U‐Pb ages and Lu‐Hf estimates of final garnet growth ages record a barroisite‐amphibolite assemblage overprint of eclogite at ~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

Discovery of Upper Cretaceous Neo-Tethyan trench deposits in south Tibet (Luogangcuo Formation)

Sedimentary basins within the Transhimalayan arc-trench system provide paleotectonic and paleogeographic information on the evolution of the late Mesozoic–early Cenozoic Neo-Tethyan subduction zone along the southern Asian margin. This paper presents detailed stratigraphic, sedimentologic, petrographic, detrital-zircon geochronologic and Hf isotopic data from the Luogangcuo Formation exposed as part of the Xiukang Mélange in south Tibet. The Luogangcuo Formation was deposited (ca. 88–81 Ma) in a trench environment on a deep-sea fan fed from the Lhasa block through a submarine canyon. Dominant chert and subordinate sandstone and limestone clasts in conglomerate beds indicate recycling from the Neo-Tethyan subduction complex during repeated episodes of gravitational failure. The interbedded turbiditic sandstones were sourced directly from the Gangdese magmatic arc and central Lhasa terrane. Detrital volumes from the active Asian margin increased markedly at ca. 88 Ma as a result of uplift of central Lhasa, leading to deltaic progradation, filling of the Xigaze forearc basin, and bypassing of sediments funneled via canyons across the subduction complex to reach the Luogangcuo trench basin at abyssal depths.

Slab interactions in 3-D subduction settings: The Philippine Sea Plate region

The importance of slab–slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific–Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu–Bonin–Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu–Bonin–Mariana slabs. When weak back-arc regions are included, slab–slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab–slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations.

Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at ~640–680 °C and ~13 kbar. This pressure-temperature estimate indicates that metagranitoids underwent high-pressure metamorphism but equilibrated at significantly lower pressures than associated eclogite-facies rocks, and, therefore, they do not necessarily share a common high-pressure metamorphic evolution. The new data show that the Chuacús complex in the study area represents a Proterozoic (1.1–1.0 Ga) to Late Triassic (220 Ma) continental basement that was subducted, and consequently metamorphosed under high-pressure conditions, during the Cretaceous evolution of the North America-Caribbean plate boundary.

Triassic alkaline magmatism and mineralization in the Xiong'ershan area, East Qinling, China

The Qinling orogenic belt is a complex subduction–accretion–collision orogen that welded the North China Craton and the Yangtze Craton during the final continental collision in the Triassic. The Xiong'ershan area, located in east Qinling, exposes a typical Triassic syenite pluton and several contemporaneous Mo, Au, and Cu deposits. The aegirine–augite syenites and syenites from the Mogou pluton are characterized by alkalic to peralkalic (total alkali Na2O + K2O = 13.95–14.63 wt.%, CaO = 0.06–2.87 wt.%), and shoshonitic features (K2O = 11.86–14.34 wt.%). Zircon LA–ICP–MS U–Pb dating of the aegirine–augite syenite and syenite yield emplacement ages of 232.5 ± 0.6 and 221.8 ± 0.7 Ma, indicating multiple pulses of magmatism. Evidence from zircon Hf isotopes; occurrence of mafic microgranular enclaves; heterogeneous peralkaline composition; and wide ranges of MgO, Ni, and other trace elements suggest that the parental magma was mainly sourced from partial melting of Archean to Paleoproterozoic crustal sources, mixed with juvenile mantle‐derived mafic magmas. The Mogou pluton was probably emplaced in the tectonic transition from syn‐collision to post‐collision settings and accompanying slab break‐off process, from the commencement of collision at approximately 245 Ma and post‐collisional extension at approximately 210 Ma. Gold, molybdenum, and copper deposits formed during the interval of 255–208 Ma, and the close temporal and spatial relationship between these Triassic polymetallic deposits and the Mogou alkaline pluton invokes a genetic linkage. The heat source for magmatism and related metallogeny is correlated to a hot upwelling asthenospheric mantle that caused partial melting of the Archean to Paleoproterozoic crustal basement, resulting in magma mixing between the two end‐members.

Why are diamonds preserved in UHP metamorphic complexes? Experimental evidence for the effect of pressure on diamond graphitization

ABSTRACTThe preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО3 (1550°С) and К2СО3 (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450–1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.

Immature intra-oceanic arc-type volcanism on the Izanagi Plate revealed by the geochemistry of the Daimaruyama greenstones in the Hiroo Complex, southern Hidaka Belt, central Hokkaido, Japan

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.

Successive Rifting Events in Marginal Basins: The Example of the Coral Sea Region (Papua New Guinea)

AbstractReactivation of extensional structures is commonly inferred during rift evolution. In that context, we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the Coral Sea region, Papua New Guinea. The first event (R1), poorly documented, occurred during the Triassic along an older N‐S Permian structural fabric. During the Jurassic, extensional faults were reactivated through a second extensional episode (R2), which formed small (~10/20 km) basins bounded by N‐S, NE‐SW, and E‐W listric faults. Extension prolonged during the Lower Cretaceous with seafloor spreading in the Owen Stanley Oceanic Basin, now incorporated in the Papuan fold and thrust belts. A third Late Cretaceous extensional phase (R3) gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent‐ocean transition. Seafloor spreading in the Coral Sea followed from Danian to Ypresian. This extensional system is sealed by unequally preserved Eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from Oligocene onward. This overall evolution suggests various extensional systems that are geographically and temporarily defined the one another. The early rifting of the crust is controlled by preexisting continental features resulting in the local Pangaea breakup. In contrast, the Coral Sea propagator cuts through the rifted margin and is controlled by a subduction complex in accordance with the Tasman Sea opening. This evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around Australia.

Petrographic Studies of the Vihowa Formation, Sulaiman Range, Pakistan: Implications for Provenance

The Vihowa Formation of late to early Miocene is investigated for mineral composition and petrographic characteristics to understand the provenance, diagenesis and depositional environments. Ten representative sandstone samples were collected from various lithological units for petrographic studies. Petrographic results show that the quartz is a major constituent; while the accessory minerals are chlorite, chamosite, palagonite, sphalerite, staurolite, aragonite, hypersthene, glaucophane, monazite, kyanite, epidote, olivine, rutile, ilmenite, hematite, magnetite, zircon, tourmaline and garnet. The tourmaline and rutile showing an igneous source however, the presence of epidote, staurolite and garnet expresses a metamorphic source for studied sandstones. The sandstone is medium to coarse grained having grains dominantly from the meta-sedimentary rocks whereas minor amount of grains from igneous rocks is also observed. Monocrystalline quartz is present in sands derived either from gneiss or granitic rocks, but polycrystalline quartz are from slate and schist. The inclusion of mica and garnet within quartz is observed which indicates a metamorphic source. On the QFL triangular diagram the sandstone of Vihowa Formation is mainly sub-lithic arenite and partly quartz arenite. The Qm-P-K plots indicate that feldspar and quartz are from granitic rocks along with small amount from metamorphic and volcanic rocks. The provenance of sandstone indicates that sediments were mostly deposited by transitional recycled orogeny and mixed magmatic subduction complex.

Oligocene-Miocene burial and exhumation of the southernmost Gangdese mountains from sedimentary and thermochronological evidence

The Kailas conglomerates crop out ubiquitously along the southernmost boundary of the Gangdese batholith. They unconformably overlie the Gangdese batholith and are displaced by the Great Counter thrust (GCT) fault, forming a fault contact with the Xigaze forearc basin, the associated subduction complex and the Tethyan Himalayan sequence. These strata furnish a record of uplift and paleoenvironmental change in the Indus-Yarlung suture zone during the Oligocene-Miocene. Our new and previously published low-temperature thermochronometric data from the Gangdese batholith and the Kailas conglomerates indicate a period of rapid exhumation beginning approximately 17–15Ma centered on the southern margin of the Gangdese batholith, whereas regional uplift commenced significantly earlier during the deposition of the Kailas conglomerates, based on the presence of an abrupt facies transition from deep-water lacustrine deposits to red alluvial fan or fluvial deposits. The period of rapid exhumation probably lagged behind the initiation of faster uplift, while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy. Subsequently, the Kailas conglomerates were buried in association with the development of the north-directed Great Counter thrust, while rapid exhumation was facilitated by efficient incision by the paleo-Yarlung river at approximately 17–15Ma.

A refined late-Cryogenian – Ediacaran earth history of South China: Phosphorous-rich marbles of the Dabie and Sulu orogens

The late-Cryogenian – Ediacaran geological framework for South China is constructed principally from sedimentary successions preserved in the central and western regions of the Yangtze Block. New stratigraphic and carbonate-carbon isotope data allow us to extend that framework into the exhumed HP-UHP subduction complexes of the eastern Dabie and Sulu orogens that separate the South and North China cratons. Those data show that marble and phosphorous-rich (P-rich) units in those complexes were originally part of an Ediacaran shallow-marine shelf-carbonate platform. The basal pebbly schist (metadiamictite) and lowermost P-rich marble of the Jinping Formation (Haizhou Group) in the Sulu Orogen matches in both facies character and C-isotope profile that of the Marinoan-equivalent glacial-cap carbonate couplet of the Nantuo and Doushantuo formations. The Daxinwu Formation (Susong Group) in the eastern Dabie Orogen contains a marble unit that has, for several hundreds of metres, a strikingly uniform C-isotope profile of low δ13C positive values and is overlain by a P-rich graphitic schist; these features match those of the late Ediacaran to early Cambrian Dengying Formation. These correlations establish that the HP-UHP metasedimentary rocks, many of which were once considered to be Palaeo- to Mesoproterozoic in age, are a Neoproterozoic-age cover sequence of the continental margin of the Yangtze Block. Further, their widespread development limits their utility as indicators of offset across the Tan-Lu fault zone and, instead, favours tectonic models that interpret that feature as a continental-scale tear fault formed during the Mesozoic collision and suturing of the North and South China cratons.