Forearc Research Articles

Seafloor morphology along the active margin in Guerrero, Mexico: Probable earthquake implications

Submarine geomorphic features on the active margin along the Mexican Pacific coast are the result of subduction process. We analyze bathymetric data and present an overview of the continental slope geomorphology in the Guerrero seismic gap. For the first time we identify and describe submarine landforms in this area, including small-scale forearc basins, trench basin segmentation, narrow accretionary wedge and seamounts. We propose that the presence and formation of forearc basins are related to seamount subduction as well as the origin of higher relief above subducted seamounts. Here, forearc basins and trench sediments are sourced from submarine canyons. Notably, the thickness of sediments along the trench changes due to the presence of seamounts. Some parts of the trench seem to contain little sedimentary fill, which may increase friction along these segments of the subduction zone.

Crustal structure beneath Simeulue Island, Indonesia: Preliminary study from a joint inversion of receiver function and surface wave dispersion

Simeulue Island is located on the subduction zone between Indo-Australian and Eurasian plates. The Island is in the forearc basin west of Sumatra Island. Due to subduction process of Indo-Australian plate beneath Eurasian plate, high seismic activities occur around Simeulue Island. Two devastating megathrust earthquakes of 2004 Mw 9.2 Sumatra-Andaman and 2005 Mw 8.7 Nias earthquakes occurred close to the Island. Information of crustal structure may provide better understanding about tectonic characteristics. In this preliminary study, we investigated crustal structure beneath Simeulue Island from three temporary broadband seismometers. Crustal structure was derived from joint inversion of receiver function and surface wave dispersion. S-wave velocity profiles obtained from joint inversion may suggest that the crustal thickness beneath Simeulue Island is at about 20-25 km. S-wave velocity decrease is observed at depth of about 50-60 km depth. This low velocity decrease may be related to the subducting slab of Indo-Australian plate.

Stratigraphic and provenance analysis of Triassic rock units between 28-29° S, northern Chile: implications on the tectonic and paleogeographic evolution of the southwestern margin of Gondwana

Triassic rock units of northern Chile (28-29° S) record the transition, both in time and space, between two major orogenies that affected the southwestern margin of South America, the Gondwanian and Andean orogenies. The geodynamic configuration of the margin during this transition is still a matter of debate, particularly whether subduction was interrupted or continued under different parameters in between the orogenies. In order to evaluate these hypotheses by understanding the paleogeographic evolution of the margin, this work synthesizes recent stratigraphical, structural and geochronological data from northern Chile (28-29° S), along with detrital zircon analysis and detritus characterization of the two main siliciclastic Triassic basins present in the area. A detailed study of the evolution of the San Félix and the Canto del Agua basins, their source areas, and exhumation processes of the margin recognizes two stages of intra-arc/forearc basins system development separated by a Carnian unconformity. The first stage (Lopingian-uppermost Middle Triassic) develops an eastern intra-arc basin, which is represented by the volcaniclastic rocks included in the Guanaco Sonso Formation and the roots of the volcanic arc represented by Chollay Plutonic Complex, bounded to the east by a Pennsylvanian-Cisuralian basement block. The forearc basin for this stage is constituted by two graben depocenter, separated by a topographic high, of marine to transitional depositional environment and proximal sediment sources. The eastern graben is filled by conglomerates and turbiditic rocks grouped in Members M1 to M4 of the San Félix Formation, and the western graben, by sedimentary and volcanic rocks of the lower section of the Canto del Agua Formation. The second stage (Norian-Rhaetian) involves an eastern intra-arc basin, represented by the volcanic rocks of the La Totora Formation that seals the exhumed roots of the magmatic arc developed in the previous stage, and a marine to transitional forearc basin to the west, represented by the sedimentary rocks of M5 member of the San Félix Formation and the upper section of the Canto del Agua Formation. These two successions show basal fluvial conglomerates unconformably overlying Anisian prodelta deposits of the first stage, recording a major base level drop of the forearc basin.

Subduction erosion: contributions of footwall and hanging wall to serpentinite mélange; field, geochemical and radiochronological evidence from the Eocene HP-LT belt of New Caledonia

Slivers of mixed and metamorphosed ultramafic, mafic and sedimentary rocks crop out within the Eocene high pressure-low temperature metamorphic complex of northern New Caledonia. These units consist of: (1) slices of the serpentinite sole of Peridotite Nappe (upper plate), which characteristically contain fragmented supra-subduction dykes; (2) Intermediate Mélange units composed of elements of the serpentinite sole mixed with basalt and red chert clasts, and supra-subduction magmatic rocks that represent the interface of Peridotite Nappe and Poya Terrane (lower plate); and (3) the Pouébo Terrane, an eclogite-blueschist facies mélange, which crops out over 150 km along the northeast coast of the island. Intermediate Mélange units are transitional between shallowly accreted oceanic crust rocks of the Poya Terrane and the deeply subducted Pouébo Terrane. Their metamorphic grade is similar to that of the enclosing metasedimentary units of the Diahot Terrane and steeply increases northeastward. The composition of blocks in the Pouébo Mélange suggests that they come from: (1) subducted oceanic material (Poya Terrane Basalts, abyssal argillite/chert, passive margin sedimentary and magmatic components, terrigenous turbidites, limestone and dolerites) and (2) supra-subduction components (mantle wedge serpentinite, 55–52 Ma supra-subduction dykes, forearc basin), which characteristically refer to the lower plate and upper plate of the subduction/obduction system, respectively. KEY POINTS Metamorphosed serpentinite mélanges of New Caledonia contain elements from the footwall and the hanging wall of the subduction zone. Elements of the forearc crust and mantle formed immediately after subduction inception have been dragged into the subduction zone. Many blocks in the serpentinite mélange come from supra-subduction dykes that intruded the serpentinised hanging wall.

Early Neoproterozoic assembly of the Yangtze Block decoded from metasedimentary rocks of the Miaowan Complex

The present Yangtze Block was proposed to be formed as a unified block in the Mesoproterozoic-Neoproterozoic transition, but the detailed timing and process are poorly constrained. Here we report integrated in situ U–Pb and Hf–O isotopic analyses of detrital zircons from the Xiaoxikou metamorphosed sedimentary rocks of the Miaowan Complex in the Yangtze Block, as well as U–Th–Pb and Nd–O isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks. U–Pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 Ma. The 970 Ma zircons are dominantly characterized by mantle-like δ18Ozir values (4.5–5.8‰) and highly depleted εHf(t) values (+5 to + 14). The 900 Ma zircons mostly have O and Hf isotopic features similar to the 970 Ma zircons, except relative small proportion of grains with high δ18O values (6.0–9.2‰) and variable εHf(t) values (–33 to + 7). Monazites from the vein are dated at ca 850 Ma and have uniform δ18O values (5.4–6.6‰), but significantly enriched εNd(t) values (–14.5 to –12.6). Based on the new isotopic results and immature lithology, the Xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 and 850 Ma in a fore-arc basin, and clastic materials derived from ca 970–900 Ma arc rocks were related to the northward subduction (present-day coordinates) of the Miaowan oceanic plate. The 850 Ma leucocratic vein shows geological and isotopic characteristics consistent with local 860–840 Ma post-tectonic granitic plutons, and may represent a fractionated product of local coeval granitic magmas. We propose that the closure of the Miaowan oceanic basin and formation of the current unified Yangtze Block occurred between 900 and 860 Ma, i.e., later than the deposition of the fore-arc metasedimentary rocks while earlier than the emplacement of the post-tectonic plutons.

Geochemical characteristics and petrogenesis of Miocene high Sr/Y rocks in Xigatze fore-arc basin, southern Tibet

The Xigatze fore-arc basin, one of the well-preserved fore-arc basins worldwide, consists of interesting nearly E-W trending dike swarms of basaltic to granitic compositions. Mid-Miocene more felsic dikes to the responses of fore-arc lithosphere provide important records to the post-collisional processes. Miocene (~15–11 Ma, zircon UPb ages) high Sr/Y magmatic rocks from the Xigatze area in this study are characterized by (1) rich in Na as compared with K contents (Na2O/K2O = 1.75–5.62) and high Al2O3 content (> 16 wt%); (2) high Sr but low Y contents, thus high Sr/Y (>81.0) ratios, resembling those of adakites; (3) enrichment in LREE and depletion in HREE with weakly negative or no Eu anomalies (Eu/Eu⁎ = 0.80–1.02); (4) small range in 87Sr/86Sr(t) (0.705080–0.706031) and εNd(t) (−3.1 − +0.4) values, indicating that the Xigatze Miocene magmatic rocks are derived from partial melting of juvenile lower crust with minor contributions from the ancient crust in the Lhasa terrane; (5) thickened crust with a crustal thickness of ~52–69 km estimated from (La/Yb)N values. As compared with those in the Gangdese high Sr/Y suite, the Xigatze ones have lower SiO2 and K2O contents, but higher Na2O/K2O, Sr/Y, and K/Rb ratios, the higher Sr/Y and K/Rb ratios implying a weaker effect by amphibole either as a residual in the source or as a fractionation phase during the magma ascent than that in the Gangdese suite.

Triassic turbidites in the West Qinling Mountains, NW China: Part of the collisional Songpan-Ganzi Basin or an active forearc basin?

Lower to Middle Triassic clastic rocks in the West Qinling Mountains along NE margin of the Qinghai-Tibet Plateau are generally regarded as part of the Songpan-Ganzi flysch Basin. However relatively little attention has been paid to the age and provenance of these units. New petrological and geochemical results demonstrate that these sediments accumulated along an active continental margin and are dominated by feldspathic litharenite and lithic arkose with low mineral and compositional maturity. They were derived primarily from a continental arc source dominated by intermediate to felsic igneous rocks, with a minor contribution from older metamorphosed and sedimentary sources. Turbidite samples yielded two primary detrital zircon U-Pb age populations of ca. 273 Ma and ca. 435 Ma, which is different from the Early Triassic (ca. 252 Ma) and Middle Silurian (ca. 427 Ma) age populations that dominate the Songpan-Ganzi Basin. These data together with paleocurrent results indicate that the South Qilian Belt was the primary source origin because this belt contains both early Paleozoic Andean-type igneous and magmatic rocks and has a basement of Precambrian metamorphosed rocks. Regionally, an Andean-type arc traverses the South Qilian belts and extends into the West Qinling Mountains and Kunlun Orogen, which formed by north-directed subduction of the Paleo-Tethyan Ocean during the Early to Middle Triassic. Voluminous detritus that originated from the Qilian and Kunlun orogens as they were uplifted and eroded was transported to the south and deposited in the forearc area in front of the A’nimaqen-Mianlue suture.

Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (southwestern China): Evidence from sandstone geochemistry and detrital zircon U-Pb ages of the Hengdan Group

The Hengdan Group in the northwestern part of the Bikou Terrane preserves sedimentary archives for the debated early to middle Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (YB), while its depositional age, provenance and tectonic setting remain controversial. This research has reported the geochemical and geochronological studies on the sandstones in the Hengdan Group. Our work indicates that the sandstones in the Hengdan Group experienced low degree of chemical weathering and sedimentary recycling, and are mainly sourced from acidic to intermediate igneous rocks. In addition, their elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., LaN/YbN and Eu/Eu*) are comparable to those deposited in an active continental margin setting. All samples yield similar age distribution, having a dominated age range at ca. 850 to 720 Ma. Together with the dominantly angular to subangular morphology and clear oscillatory or broad zoning internal structure of these aged detrital zircons, indicating that the Hengdan Group should be proximally sourced from the early to middle Neoproterozoic igneous rocks. In addition, the youngest group of detrital zircon U-Pb ages of the sandstones in the Hengdan Group suggest that the onset and ending of the deposition of this group is later than ca. 741 Ma and ca. 722 Ma, respectively. Combining with previous work, the Hengdan Group was most likely deposited during ca. 741 to 717 Ma in a forearc basin setting at the northwestern margin of the YB, and with its primary provenance probably from nearby acidic to intermediate igneous rocks. In addition, all these above further supports that the northwestern margin of the YB should be a long-lived active continental margin setting during early to middle Neoproterozoic (ca. 835 to 717 Ma) and, the South China Block should be located at the relative peripheral location of the Rodinia supercontinent.

Large-scale gravity-driven phenomena in the Crotone Basin, southern Italy

The aim of this study is to better define the scale, timing and driving mechanisms of a seaward-gliding landmass currently affecting the Crotone Basin, a forearc basin on the Ionian side of Calabria (southern Italy). An integrated approach of seismic, well and field data, together with literature information, have revealed that the slide includes an onshore updip extensional domain compatible with the presence of a curved seaward-dipping fault system to the NW, which is connected southward to an offshore downdip compressional domain through a buried detachment surface lying at a Messinian halite layer. The landmass, which involves Messinian-Pleistocene deposits, has started to slide since the Zanclean, and experienced a paroxysmal episode between late Zanclean and early Piacenzian linked to transpressional tectonics. After a phase of inactivity during the Calabrian, the gravitational collapse underwent a second reactivation since middle Pleistocene due to regional uplift, which seems to be restricted to the offshore sector, the Crotone peninsula and probably the eastern coastal area. • A better definition of the scale, timing and driving mechanisms of the Crotone Megalandslide is provided. • A first phase of activity started during the Zanclean and ended during the Gelasian, and involved the whole basin. • A second phase of activity persists since the middle Pleistocene and involved only the Crotone peninsula and the offshore area.

Boreal Molluscan Records Around the Jurassic–Cretaceous Boundary in East Asia Provide Clues for the Paleobiogeographical Reconstruction in the Mid-Latitudes of the Northwest Pacific

Studying marine paleobiogeographical conditions in the mid-latitudes of the Northwest Pacific around the Jurassic–Cretaceous boundary would be expected to contribute to a better understanding of the paleoclimatic and/or paleoenvironmental background of the evolution of the Late Mesozoic terrestrial ecosystem in East Asia. However, uncertainty about the paleogeography of the eastern margin of the Asian Continent has meant difficulties for paleobiogeographical discussion. In this paper, the strata containing Boreal faunal elements, Buchia, and cylindroteuthidid belemnites in East Asia (Far East Russia, Heilongjiang in northeastern China, and Japan) and their tectonic settings are reviewed. The Uda and Torom (northern Sikhote-Alin), Suibin (Heilongjiang), and Tetori (northern Central Japan) regions were located from north to south in the eastern margin of the already amalgamated Asian Continent around the Jurassic–Cretaceous boundary and can be considered the “fixed points for paleobiogeographical reconstruction.” On the other hand, Buchia-bearing strata in the Komsomolsk (northern Sikhote-Alin) and Dong'an (Heilongjiang) regions can be considered to have been deposited in the fore-arc basin or trench slope basin on the accretionary complex along the East Asian continental margin. The strata around the Jurassic–Cretaceous boundary in the Partizansk Basin (southern Sikhote-Alin) contain both Buchia and Tethyan ammonoids and were deposited on the Paleozoic continental basement or block (Sergeevka Belt). The paleo-position of these three regions around the Jurassic–Cretaceous boundary is highly debated.The Tethyan–Pacific ammonoids, Boreal belemnites, and Tetori bivalve fauna, showing some similarities with those in the Boreal Realm and Early Cretaceous strata in Heilongjiang, are present in the late Tithonian–Berriasian Mitarai Formation of the Tetori Group in the Tetori Region. This unit, deposited in the eastern margin of the North China Block, provides evidence that Boreal faunal elements reached the mid-latitudes of the Northwest Pacific. The position of the ecotone of the Boreal and Tethys realms in the Northwest Pacific can be discussed based on the comparison of the faunal elements among almost coeval strata in the Tetori Region (fixed point), the Sergeevka Belt, and the South Kitakami Belt (Pacific side of Northeast Japan), which is usually correlated with the Sergeevka Belt but contains only Tethyan faunal elements. Further studies of the records of Tethyan and Boreal taxa in the “fixed points” and other localities could provide clues to reveal the paleoclimatic and/or paleoenvironmental background of the evolution of the terrestrial and marine ecosystems in East Asia around the Jurassic–Cretaceous boundary.

Slab-controlled elemental–isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy

Supra-subduction zone (SSZ) ophiolites formed in forearc settings and the Izu–Bonin–Mariana (IBM) in-situ forearc show conversions from normal mid-ocean ridge basalt (MORB) to boninitic affinities, which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism (SIM). Nevertheless, forearc sequence generated by subduction initiation need not always include boninitic rocks. Here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the Central Palawan and Amnay (CPA) ophiolites of the west Philippines to show diversity in melt evolution of SIM. Zircon U–Pb dating on the MORB-type Amnay gabbros reveal a 10 m.y. of seafloor spreading from 33 to 23 Ma. These ages together with the progressively enriched zircon in-situ oxygen isotopes (δ18O= 1.62–10.1‰) and whole rock elemental and Nd–Hf isotopic signatures ([La/Yb]N= 0.19–3.45; Nb/Yb= 0.07–5.24; εNd= 2.1–10.2; εHf= 11.2–24.1) indicate that the depleted MORB mantle (DMM)-type source of the Amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and Nb-enriched slab melts in the later stage. This distinct mantle evolution together with higher εHf at a given εNd of the Amnay mafic samples than the contemporaneous South China Sea (SCS) oceanic samples do not support previous interpretation that the Amnay ophiolite was a fragment of the SCS. However, the 33–23 Ma Amnay ophiolite can be the younger part of the 34 Ma forearc-type Central Palawan ophiolite, and both (i.e. the CPA ophiolites) represent on-land fragments of the NW Sulu Sea. We propose that the 34–23 Ma CPA ophiolites and the NW Sulu Sea constitute forearc basin of the late Oligocene–middle Miocene island arc–backarc basin duo of the Cagayan Ridge and SE Sulu Sea, associated with subduction of the Cretaceous to Eocene Proto-SCS since 34 Ma. The CPA ophiolites lack boninite but show geochemical progression from normal to enriched MORB affinities, whose melt evolution differs from that of the IBM SIM. We attribute the differences in SIM and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the Proto-SCS versus an older slab with thin pelagic sediment cover of the Pacific Plate.

Forearc Basin Stratigraphy Resulting From Syntectonic Sedimentation During Accretionary Wedge Growth: Insights From Sandbox Analog Experiments

AbstractForearc basin stratigraphy is expected to record a detailed history of the deformation and growth pattern of an accretionary wedge. However, the relationship between syntectonic basin sedimentation and growth of a wedge remains poorly understood, including how deformation of the wedge modifies the basin stratigraphy and how syntectonic sedimentation influences deformation of the wedge. This study reproduced accretionary wedges with and without syntectonic sedimentation by analog sandbox experiments. The results show that basin stratigraphy varied with changes of the growth pattern of the accretionary wedge. In the case that wedge growth was dominated by frontal accretion phase, the depositional area migrated trenchward. In contrast, underthrusting phase caused the sediment layers to be tilted landward and the depocenter to migrate landward. A prolonged underthrusting can result in combining a retrowedge basin with a wedge top basin and yield a wide area of accommodation space. The occurrence of two types of basin stratigraphy (trenchward and landward migration of the depocenter) reflects the two different types of deformation (frontal accretion phase and underthrusting phase), and these differences possibly depend on a contrast in strength of the basal shear resistance between the inner and outer parts of the wedge due to temporal and spatial variations of sedimentation on the wedge. A change in the magnitude of normal stress acting on the wedge base likely influenced the mode of deformation of the wedge. These results suggest that forearc basin stratigraphy is influenced by the growth pattern of accretionary wedges, which is itself affected by syntectonic sedimentation.

Contrasting P-T-t paths reveal a metamorphic discontinuity in the New Quebec Orogen: Insights into Paleoproterozoic orogenic processes

The large scale Paleoproterozoic Trans-Hudson Orogen exposed in Canada is recognized to record one of the first complete Wilson cycle. Although it is considered as a prototype of modern accretionary orogen, the strain pattern, thermal state, and architecture in its wedge are still poorly constrained. Part of it, the accretionary New Quebec Orogen (NQO, 1.82–1.77 Ga) resulted from oblique collision between the Superior Craton lower plate and a composite upper plate including the Kuujjuaq Domain (KD) and the Core Zone, and exposes supracrustal sequences of the Rachel-Laporte Zone (RLZ), a continental forearc basin that evolved in a collisional foreland basin upon collision. We performed an integrated analysis, using petrography, phase equilibria modelling and Lu-Hf garnet, and U-Pb zircon, monazite and rutile chronology on supracrustal rocks in the central segment of the New Quebec Orogen. On the one hand, the RLZ experienced a tight clockwise metamorphic evolution, with a peak at 650 °C/0.7 GPa. Prograde garnet and monazite growth is estimated at 1804 ± 8 Ma and 1796 ± 4 Ma respectively. On the other hand, the metamorphic peak for restitic granulites from the KD is estimated at 790 °C/0.72 GPa and follows isothermal decompression. Geochronology yields ages of 1836 ± 5 Ma for prograde garnet growth, and 1807 ± 4 Ma for crystallization of zircon from anatectic melt, which was followed by rapid cooling recorded by rutile at 1798 ± 16 Ma. We propose that the Lac Turcotte Fault that separates both domains has acted as a tectonometamorphic discontinuity that limits contrasting P-T-t paths. We suggest sequential burial of supracrustal sequences along an apparently continuous Barrovian sequence, with progressive recycling of inner zones in the foreland basin and subsequent involvement of the latter in the orogenic wedge.

Fragmented Ancient Fore-arc Basin: Cretaceous–Paleogene Sandstones Sporadically Found in Kanto and Southern Tohoku, Japan, as Eastern Extensions of the Izumi Group

Fragmented Ancient Fore-arc Basin: Cretaceous–Paleogene Sandstones Sporadically Found in Kanto and Southern Tohoku, Japan, as Eastern Extensions of the Izumi Group

Distributions of gas hydrate and free gas accumulations associated with upward fluid flow in the Sanriku-Oki forearc basin, northeast Japan

Abstract We applied automated velocity analysis to a 3D seismic data volume to obtain a high-resolution seismic velocity model that we used to investigate the influence of fluid behavior on the subsurface distribution of gas in the Sanriku-Oki forearc basin, northeast Japan. We identified free gas accumulations as zones of low P-wave velocity separated from overlying high-velocity gas hydrates by the clear seismic boundary of the bottom-simulating reflection (BSR) between them. We then used conductive modeling to map upward heat flow in our study area from the depth of the BSR derived from the velocity model. The estimated heat flow demonstrates that upward fluid flux has considerable influences on the distributions of both gas hydrate and free gas. The areas of high heat flow (representing high fluid flux) correspond to underlying permeable geological features such as gas chimneys, faults, and the edges of porous slumps, suggesting that these features provide fluid migration pathways from Eocene to Oligocene source rocks to free gas and hydrate accumulations in overlying sedimentary rocks. The techniques we have employed can contribute to the high-resolution mapping of gas hydrate and free gas accumulations in similar deep-water reservoirs off the eastern continental margin of Japan and at tectonically similar plate subduction margins in other regions.

Effects of reactive dissolution of orthopyroxene in producing incompatible element depleted melts and refractory mantle residues during early fore-arc spreading: constraints from ophiolites in eastern Mediterranean

Compositional variations of peridotites from the Cretaceous ophiolites in southern Turkey and Northern Cyprus are presented to document the nature of partial melting and possible effects of reactive dissolution of primary mantle phases during fore-arc spreading. The peridotites overall exhibit a range of 187Os/188Os ratios from 0.1171 to 0.1266 and appear to represent a mantle region that preserves a record of ancient melt depletion. The samples are depleted in 187Os/188Os compared to the ambient oceanic upper mantle (187Os/188Os ~0.127), suggesting that they are representatives of a shallow fore-arc mantle where transport of radiogenic 187Os during slab dehydration was limited. Chemical variations of primary mantle minerals indicate that the peridotites are the residues of moderate to high degrees (>16%) of partial melting and have experienced significant modal and chemical compositional modification through interaction with oxidizing hydrous basaltic melts. Interacting melts, which appear to be similar in composition to primitive arc tholeiite, are likely to have originated from sub-lithospheric lower part of the mantle wedge during early stages of fore-arc spreading and migrated upward to react with variably depleted harzburgites to induce further melting in the overlying lithospheric mantle through open-system reactive flow. This second stage melting resulted in (1) common occurrence of reactive harzburgites and dunites by incongruent melting of orthopyroxene and crystallization of olivine through interaction with olivine saturated melt; and (2) local development of refertilized peridotites by shallower melt impregnation that involves interaction with olivine + clinopyroxene saturated melt. The dissolution of orthopyroxene caused the reacting melt to be enriched in silica and diluted in incompatible elements which led to the production of the final melts similar in composition to fore arc basalt and boninite. Involvement of compositionally variable mantle and melt components with different rates of melt influx therefore appears to explain the generation of fore-arc crust with a range of diverse rock suites including temporally and spatially associated arc tholeiites and boninites with significant depletion in incompatible elements.

Evolution of the Ecuador offshore nonaccretionary-type forearc basin and margin segmentation

Forearc basins have developed along accretionary and nonaccretionary convergent margins. Here, we investigate the evolution of an offshore nonaccretionary-type forearc basin that has developed along the Ecuadorian convergent margin. We used 2D-Multichannel Seismic Reflection profiles to recognize five seismic units overlying the acoustic basement. The units were dated by correlation with wells and regional stratigraphy. They recorded the structural evolution of the margin into North, Central-North and Central-South margin segments identified from Gravity Free Air Anomalies. Intersegment transition zones accommodated deformation related to the margin segmentation, contributing to the development of shelf forearc depocenters. An offshore forearc basin (subunit L1a) developed on accreted oceanic terranes during the middle-to-late Eocene. The basin was affected by normal faulting during the Oligocene to early-middle Miocene (subunit L1b). This extension stage initiated during the rifting and oblique subduction of the Farallon plate beneath the Ecuador margin. The basin underwent an early-middle Miocene uplift outlined by unconformity U1 potentially associated with the subduction of the young Nazca plate, in a less oblique plate convergence environment. During the early-Pliocene, the Central-South margin segment individualized by uplift during the formation of unconformity U2. The Manta anticline, which developed in the intersegment transition zone, accommodated the segmentation. This event is associated with the arrival of the Carnegie ridge in the subduction. The North segment individualized by uplift during the Pliocene early-Pleistocene formation of the Pedernales structural high, which may coincide with a reactivation of the Canandé fault. Synchronously, unit L3 sediment recorded the subsidence of the Central-North segment, and strike-slip tectonics in the area of the Jama-Bahía depocenter. A mosaic of middle-to-late Pleistocene unit L5 depocenters that alternate with uplifted zones along the shelf likely recorded the subduction of the topographically irregular Carnegie ridge. The study provides a remarkable example of the evolution of a non-accretionary-type forearc basin.

Early Cretaceous subduction initiation beneath southern Tibet caused the northward flight of India

Collision between the Indian and Eurasian plates formed the ~2500 ​km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 ​Ma and 130 ​Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arc-related igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.

Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

Abstract The along-strike variability in sediment provenance within the Nanaimo basin is important for understanding the tectonic evolution of North America’s Late Cretaceous Pacific margin, providing context for paleogeographic reconstructions. Here, we provide 35 point-counted sandstone samples and 22 new detrital zircon samples from the Nanaimo basin. These new detrital zircon samples compose a portion of a basin-wide data set (N = 49, n = 10,942) that is leveraged to discern spatio-temporal changes in sediment provenance. Provenance data demonstrates that the majority of Nanaimo basin strata were sourced from regions within and east of the Coast Mountains Batholith, while only the southernmost Nanaimo basin, exposed in the San Juan Islands, was supplied sediment from the North Cascade thrust system. Additionally, near-identical age modes and synchronous changes in detrital zircon facies are used to hypothesize a correlation between the Nanaimo Group and the protolith of the Swakane Gneiss. These observations, along with previously identified events in the Cordillera, are used to define two basin-wide events that affected the Nanaimo basin: the first at 84 Ma and the second at 72 Ma. The first event is correlated to the onset of Kula-Farallon spreading, which affected basin subsidence, introduced Proterozoic detrital zircon to the central and southern Nanaimo basin, and uplifted the North Cascade thrust system. The second basin-wide event, which is speculated to have been driven by increased rates of subduction and obliquity, resulted in localized high-flux events in the arc, increased exhumation of the Cascade Crystalline Core, underplating of the Swakane Gneiss, and coarse-grained sedimentation across the basin. The data presented here provides added context for the evolution of the basin and provides insight into the protracted geodynamics of forearc basins undergoing oblique subduction.

Structural-morphological and sedimentary features of forearc slope off Miyagi, NE Japan: implications for development of forearc basins and plumbing systems

Multibeam (MB) and subbottom profile (SBP) data along the forearc slope off Miyagi, Japan, are combined to investigate structural-morphological and sedimentary structures along the forearc slope of the Japan Trench subduction zone. In addition to dip-oriented slope gullies, the MB data image a nearly dip-perpendicular slope trough bounded by a fault scarp landward of the trench-slope break. Seaward of the trench-slope break, the subbottom mostly contains normal faults that dip in opposite directions. The SBP data show not only unconformity and sliding surfaces, but also underfilled (forearc trough) and filled structures that may reflect the most recent forearc subsidence and basin filling. We propose these forearc trough and associated filling structures may indicate the earliest developments of modern isolated basins. Thus, a model of forearc basin development from slope gully to slope trough, isolated basins and forearc basin, transferred by active structures, could be proposed. Observed seafloor liquefaction in the SBP data may represent a near-surface seep structure of a plumbing system that enables hydrosphere-mantle fluid migration plausibly activated by earthquake cycles.