Arc-trench Gap Research Articles

“Where, when and why” for the arc-trench gap from Mesozoic Paleo-Pacific subduction zone: Sabah Triassic-Cretaceous igneous records in East Borneo

Establishing the location (where), timing (when) and reasons for the development (why) for the Mesozoic subduction along the western Paleo-Pacific that extended from Coastal South China to SE Vietnam has proven problematic due to the difficulty in the identification of the fore-arc igneous rocks. This paper firstly presents a set of zircon U-Pb and whole-rock 40Ar/39Ar geochronological, zircon in-situ Lu-Hf isotopes, and whole-rock elemental and Sr-Nd-Pb isotopic data for the Mesozoic igneous rocks from the Sabah ophiolites and Segama non-ophiolitic basement in NE Borneo. Our work documents major phases of the formation of these rock units at ∼185–140 Ma, ∼135–112 Ma and ∼130–85 Ma for the Telupid, Kudat and Darvel Bay ophiolites in Sabah, respectively. The mafic rocks within the ophiolites are classified into the MORB-, high-Nb, Nb-enriched and arc-like rocks, with the similar Sr-Nd (εNd(t) = +6.4–+10.2) and Atlantic-Pacific Ocean MORB-like Pb isotopic compositions. Their generation is related to input of slab-derived melt in the MORB mantle wedge source in an arc-trench gap setting. The Segama non-ophiolitic igneous rocks were dated at 251–178 Ma, with zircon in-situ εHf(t) = +9.5–+17.5, and are classified as high-Si and low-Si adakite, and high-mg andesite. They are the fractional products of the wedge-derived magma with the source being modified by slab-derived fluids. The magmatic flare-ups at ∼251–203, ∼185–153 Ma, ∼135–112 Ma and ∼95–85 Ma in Sabah are comparable with those in Coastal South China and West Borneo, indicating the Jurassic-Cretaceous accretionary orogenesis in Sabah. Sabah was tectonically located at the East Cathaysia margin of South China in the Mesozoic. It was a long-lived (>150 Ma) Andean-type active continental margin with multi-staged pulsed subduction and rollback in East Asia. Such a subduction system initiated no later than the earliest Triassic (∼251 Ma) and continued till the Late Cretaceous (∼85 Ma).

Tectonic setting for Tsunemori Formation in the Permian accretionary complex of the Akiyoshi Belt, Southwest Japan

The purpose of this study is to reexamine the age, depositional environment, and tectonic setting of the Tsunemori Formation. This study provides a new tectonic model on the formation of Tsunemori Formation, which is a key formation to understand the Late Permian subduction and accretionary processes in Japan. The sedimentation age of the Tsunemori Formation is late Middle Permian to early Late Permian, based on radiolarians, such as Follicucullus cf. scholasticus Ormiston and Babcock reported in this study. This paper disproves the previous theory of atoll carbonates collapse at the trench for the Akiyoshi Limestone. Most of sedimentary rocks of the Tsunemori Formation is not pervasively sheared, are less deformed compared with sedimentary rocks of the typical accretionary complexes. There is no layer parallel or sub-parallel thrusting in the turbidite sequences of the Tsunemori Formation. Mudstone of turbidite and pebbly mudstone is lack of fissility and scaly cleavages. The styles of deformation, occurrence of reworked fossils, presence of calcarenite and limestone breccia suggest that the Tsunemori Formation was not a part of accretionary wedges, but possibly was fore arc or slope basin deposits. Presence of reworked fossils, calcarenite, limestone breccia suggest that a part of the provenance of Tsunemori Formation are limestone exposed at the arc trench gap, which was moved upward to reach the arc trench gap where the Akiyoshi Limestone provides its fragments and blocks into forearc and/or slope basins. Therefore, Tsunemori Formation is a deposit in the forearc basin and/or slope basin rather than trench deposits.

Andean Adakite-like high-Mg Andesites on the Northern Margin of the Chilean–Pampean Flat-slab (27–28·5°S) Associated with Frontal Arc Migration and Fore-arc Subduction Erosion

Glassy, plagioclase phenocryst-free, ∼7–3 Ma, andesitic lavas erupted at the southern end of the Andean Central Volcanic Zone (CVZ) at 27–28·5°S are argued to contain a component from continental crust that was incorporated into the sub-arc mantle in a major pulse of fore-arc subduction erosion that removed ∼50 km of fore-arc. The 7–3 Ma amphibole-bearing Pircas Negras (54–64% SiO2) and ∼7·7 Ma pyroxene-bearing Dos Hermanos lavas erupted as the frontal arc was displaced some 50 km eastward over a developing bend in the Wadati–Benioff zone at the northern margin of the Chilean–Pampean flat-slab region. Their chemistry differs from that of older Miocene and younger CVZ arc lavas in the region in having more pronounced high field strength element (La/Ta = 40–100; Ba/Ta = 800–2400) and heavy rare earth element (Sm/Yb ∼ 4–9) depletions and an extreme adakitic-like character (∼600–1400 ppm Sr; Sr/Yb ∼ 400–1350). They also differ from older Miocene lavas in having higher 87Sr/86Sr and lower 143Nd/143Nd at the same wt % SiO2, while still showing trends with increasing wt % SiO2 that are best attributed to contamination in the ∼65–70 km thick crust. Overall, the trace element and isotopic features of the Pircas Negras lavas are consistent with modeling that attributes differences from older and younger lavas to enrichment of the arc mantle wedge by partial melts of tectonically eroded fore-arc crust, with the modeled eroded component being an outcrop and geophysically constrained mixture of 80–90% Jurassic mafic magmatic rocks and 10–20% silicic Paleozoic crust. An adakitic-type partial melt of this eroded crust generated at >2 GPa, reacted with mantle peridotite, can explain the high Mg# values (50–61) and Cr (100–350 ppm) and Ni (40–70 ppm) contents in some 5–3 Ma Pircas Negras lavas. Pre-eruption temperatures over 1060°C, calculated from mineral thermometry and the MELTS program, permit these magmas to subsequently melt and react with overlying eclogitic crust. Magma storage in this thick crust then led to amphibole crystallization and suppression of plagioclase fractionation. Quartz and feldspar xenocrysts were acquired from locally pooled mid-crustal magmas and the silicic upper crustal basement during final rapid ascent to the surface. Given a constant arc–trench gap of 300 km over the last 10 Myr, ∼198 km3 Myr–1 km–1 of fore-arc crust needs to be removed by subduction erosion at 8–3 Ma to account for the material balance of the fore-arc crust.

Tectonic evolution of the Dunnage Mélange tract and its significance to the closure of Iapetus

The Dunnage Mélange comprises a chaotic, heterogeneous assemblage of unsorted blocks set in a fine-grained matrix that is situated along the main Iapetus suture zone in Newfoundland Appalachians. The origin and tectonic setting of the mélange has been controversial and it has been variously interpreted as an olistostrome and a tectonic mélange that formed in arc–trench gap or backarc settings. The mélange preserves abundant evidence of tectonic and later soft-sediment deformation, indicating that tectonic deformation is a major component in the genesis of the Dunnage Mélange. The tectonic setting of the Dunnage Mélange is herein investigated through the Coaker Porphyry (469±4Ma), a peraluminous, syn-tectonic, plutonic suite that is exclusively emplaced into the mélange. The Coaker Porphyry contains a typically peri-Gondwanan inheritance indicating derivation from the adjacent Cambro-Ordovician Victoria–Penobscot Arc and its basement. The xenoliths contained in the Coaker Porphyry are indicative of an arc origin and there is no evidence for derivation from the Iapetus Ocean lithosphere. The mélange is thus interpreted to have formed above Victoria–Penobscot arc crystalline forearc basement. The emplacement of the Coaker Porphyry coincides with regional rift-like magmatism in the adjacent Victoria Arc. We interpret the mélange formation, rift-like magmatism and generation of Coaker Porphyry to be related to a Middle Ordovician ridge–trench collision, slab window formation and inversion of the forearc strata.

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis. At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting. The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis. Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.

The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block

Abstract TIMS and SHRIMP U–Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540–535 Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian–Pampean orogeny to have occurred between ca. 530 Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian–Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530 Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530 Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530 Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530 Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U–Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600 Ma. Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570 Ma, following an earlier departure of Amazonia (∼650 Ma?). The separation of Arequipa-Antofalla from Amazonia and Laurentia opened the Puncoviscana and Iapetus oceans respectively.

Lower Silurian subduction-related volcanic rocks in the Chaleurs Group, northern New Brunswick, CanadaGeological Survey of Canada, Contribution No. 2008-0166.Contribution to Natural Resources Canada’s Targeted Geoscience Initiative 3 (2005–2010).

Early Silurian volcanic and subvolcanic rocks are preserved in the lower part of the Chaleurs Group at two locations in northern New Brunswick. At Quinn Point, mafic to intermediate rocks are hosted by sedimentary rocks of the Weir Formation, and at Pointe Rochette, a bed of felsic tuff occurs near the base of the Weir. These rocks are interpreted as the first evidence in New Brunswick of magmatism associated with Late Ordovician – Early Silurian subduction of Tetagouche–Exploits back-arc oceanic crust. At Quinn Point, mafic rocks include a thick basaltic flow or sill and intermediate to mafic cobbles in overlying conglomerate beds. The in situ mafic rocks and the conglomerate clasts are chemically alike and display subduction-related affinities on tectonic discrimination diagrams. At Pointe Rochette, fine-grained felsic tuff contains elevated Th and U and depleted high-field-strength elements, consistent with a subduction-influenced setting, although rare-earth element (REE) abundances are low and the REE profile is relatively flat. A U–Pb (zircon) age of 429.2 ± 0.5 Ma was obtained from the tuff, consistent with the late Llandovery to early Wenlock age of the overlying La Vieille Formation and coinciding with the latter stages of development of the Brunswick subduction complex. Volcanic rocks were emplaced in the arc to arc-trench gap region, probably reflecting local step-back of the magmatic axis due to accretion of continental back-arc ribbons. The low volume of Early Silurian subduction-influenced rocks is probably related to the relatively narrow width of the back-arc basin and the young, “warm” character of back-arc crust.

Supra–subduction zone extensional magmatism in Vermont and adjacent Quebec: Implications for early Paleozoic Appalachian tectonics

Metadiabasic intrusions of the Mount Norris Intrusive Suite occur in faultbounded lithotectonic packages containing Stowe, Moretown, and Cram Hill Formation lithologies in the northern Vermont Rowe-Hawley belt, a proposed Ordovician arc-trench gap above an east-dipping subduction zone. Rocks of the Mount Norris Intrusive Suite are characteristically massive and weakly foliated, have chilled margins, contain xenoliths, and have sharp contacts that both crosscut and are parallel to early structural fabrics in the host metasedimentary rocks. Although the mineral assemblage of the Mount Norris Intrusive Suite is albite 1 actinolite 1 epidote 1 chlorite 1 calcite 1 quartz, intergrowths of albite 1 actinolite are probably pseudomorphs after plagioclase 1 clinopyroxene. The metadiabases are subalkaline, tholeiitic, hypabyssal basalts with preserved ophitic texture. A backarcbasin tectonic setting for the intrusive suite is suggested by its LREE (light rare earth element) enrichment, negative NbTa anomalies, and Ta/Yb vs. Th/Yb trends. Although no direct isotopic age data are available, the intrusions are broadly Ordovician because their contacts are clearly folded by the earliest Acadian (Silurian‐Devonian) folds. Field evidence and geochemical data suggest compelling along-strike correlations with the Coburn Hill Volcanics of northern Vermont and the Bolton Igneous Group of southern Quebec. Isotopic and stratigraphic age constraints for the Bolton Igneous Group bracket these backarc magmas to the 477‐ 458 Ma interval. A tectonic model that begins with east-dipping subduction and progresses to outboard west-dipping subduction after a syncollisional polarity reversal best explains the intrusion of deformed metamorphosed metasedimentary rocks by backarc magmas.

Adakitic lavas in the Central Luzon back‐arc region, Philippines: lower crust partial melting products?

Abstract Volcanism in the back‐arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side‐main volcanic arc igneous rocks. The back‐arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite‐like geochemical characteristics of these back‐arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet‐bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.

Mineralogical and Chemical Studies of Volcanic-RelatedArgillaceous Industrial Minerals of the Central American Cordillera(Western El Salvador)

The western provinces of El Salvador, which are covered almost exclusively by igneous rocks, offer favorable conditions to study the entire suite of volcanic-related, nonmetallic deposits involving accumulations of zeolite-feldspar, smectite, kaolinite, and diatomite. Formation of nonmetallic deposits, which is still going on in the active geothermal field near Ahuachapan, began during the late Pliocene. This age is based on micropaleontological data obtained during the study of lacustrine sediments laid down in volcanogenic depressions. Judging by their regional distribution and geological setting, the deposits may be subdivided into regional deposits (T <100°C) and structurally controlled deposits (T = 140°–250°C). Both types of volcanic deposits can be further subdivided into various stages:(1) zeolite-feldspar stage, (2) smectite stage, (3) quartz-kaolinite stage, (4) kaolinite-cristobalite stage, and (5) kaolinite stage. Stages (4) and (5) are characterized by the presence of alunite-, woodhouseite-, and crandallite-group minerals and some minerals of the aluminite-alunogen group. Within the deposits, gradual changes (progressive vs. regressive trends) may be attributed to various alteration processes. The progressive trends are correlative with the differentiation of the mineralizing fluids throughout hydrothermal activity and supergene alteration. The regressive trends are caused by hot-spring diagenesis. Knowledge of the minerostratigraphy in a geothermal field and of the intracrystalline processes within minerals such as siliceous compounds and aluminum-phosphate-sulfate minerals may help to answer the question of whether hypogene or supergene processes have triggered nonmetallic mineralization. The major and trace element compositions (Ag, As, Be, Co, Cr, Cs, Cu, Ge, Hf, Hg, Mo, Nb, Ni, Pb, Rb, Sb, Se, Sr, Tl, Th, U, V, Y, Zn, Zr, and REE) for several argillaceous deposits are presented in this paper. Cross plots using the couples Ba + Sr vs. Ce + Y + La, Ti + Fe vs. Cr + Nb, and Zr vs. Ti may help discriminate hypogene and supergene kaolin concentrations in these epithermal argillaceous deposits. The nonmetallic deposits represent a subtype of high-sulfidation-type deposits . The Salvadorian deposits formed distally to the arc-trench gap and belong to the most recent stages of the Cenozoic Ando-Caribbean mineralizing processes.

Geological–tectonic framework of Solomon Islands, SW Pacific: crustal accretion and growth within an intra-oceanic setting

The Solomon Islands are a complex collage of crustal units or terrains (herein termed the `Solomon block') which have formed and accreted within an intra-oceanic environment since Cretaceous times. Predominantly Cretaceous basaltic basement sequences are divided into: (1) a plume-related Ontong Java Plateau terrain (OJPT) which includes Malaita, Ulawa, and northern Santa Isabel; (2) a `normal' ocean ridge related South Solomon MORB terrain (SSMT) which includes Choiseul and Guadalcanal; and (3) a hybrid `Makira terrain' which has both MORB and plume/plateau affinities. The OJPT formed as an integral part of the massive Ontong Java Plateau (OJP), at c. 122 Ma and 90 Ma, respectively, was subsequently affected by Eocene–Oligocene alkaline and alnoitic magmatism, and was unaffected by subsequent arc development. The SSMT initially formed within a `normal' ocean ridge environment which produced a MORB-like basaltic basement through which two stages of arc crustal growth subsequently developed from the Eocene onwards. The Makira terrain records the intermingling of basalts with plume/plateau and MORB affinities from c. 90 Ma to c. 30 Ma, and a contribution from Late Miocene–present-day arc growth. Two distinct stages of arc growth occurred within the Solomon block from the Eocene to the Early Miocene (stage 1) and from the Late Miocene to the present day (stage 2). Stage 1 arc growth created the basement of the central part of the Solomon block (the Central Solomon terrain, CST), which includes the Shortland, Florida and south Isabel islands. Stage 2 arc growth led to crustal growth in the west and south (the New Georgia terrain or NGT) which includes Savo, and the New Georgia and Russell islands. Both stages of arc growth also added new material to pre-existing crustal units within other terrains. The Solomon block terrane collage records the collision between the Alaska sized OJP and the Solomon arc. Initial contact possibly first occurred some 25–20 Ma but it is only since around 4 Ma that the OJP has more forcefully collided with the Solomon arc, and has been actively accreting since that time, continuing to the present day. We present a number of tectonic models in an attempt to understand the mechanism of plateau accretion. One model depicts the OJP as splitting in two with the upper 4–10 km forming an imbricate stack verging to the northeast, over which the Solomon arc is overthrust, whilst deeper portions of the OJP (beneath a critical detachment surface) are subducted. The subduction of young (<5 Ma), hot, oceanic lithosphere belonging to the Woodlark basin at the SSTS has resulted in a sequence of tectonic phenomena including: the production of unusual magma compositions (e.g. Na–Ti-rich basalts, and an abundance of picrites); an anomalously small arc–trench gap between the SSTS and the Quaternary–Recent arc front; calc-alkaline arc growth within the downgoing Woodlark basin lithospheric plate as a consequence of calc-alkaline magma transfer along leaky NE–SW-trending faults; rapid fore-arc uplift; and rapid infilling of intra-arc basins. The present-day highly oblique collision between the Pacific and Australian plates has resulted in the formation of rhombohedral intra- and back-arc basins.

Chromite from the Bela Arc-Basin Ophiolite, Pakistan

The Bela arc-basin ophiolite (BABO) is a complex of ophiolite and melange outcrops that formed in the tectonic setting of an oceanic volcanic arc and its adjoining forearc basin. The BABO contains typically podiform segregations of chromite, whose microprobe compositions display a variation trend separate from that of the chromian spinel from surrounding peridotites, but similar to that in the latter igneous differentiates (pyroxenites and an andesite). The podiform chromitite and associated dunite were derived by mantle-melt reaction and cumulate-melt crystallization. Both exhibit conformable relations and are cogenetic. Chromite characteristics conform to the suprasubductionzone setting of the BABO, with two components distinguished from the whole-rock geochemistry, field relations, and chromite chemistry—a remnant forearc basin toward the southwest and an adjacent arc-trench gap toward the northeast. Individual ophiolite massifs within the BABO display differences in degree of partial melting and may h...

Sedimentology of trench-arc sediments and its relation to ophiolite obduction

The eastern Himalaya in Xigaze District of southern Tibet has well developed Cretaceous-early Paleogene trench-arc sediments deposited coevally with Gandise magmatic/volcanic arc phase, generated by Tethyan subduction. The Congdu Formation, composed of ophiolitic greywacke turbidite interbeded with volcanic tuff and pelagic radiolarian chert, represents typical trench-floor sediments deposited on the top of a younger oceanic crust composed of pillow lava with thin-bedded chert cover of late Albian to early Cenomanian age. The Cretaceous olistostromes (Zongzhuo Formation) probably of Albian age might be earlier trench-slope deposits. The trench turbidite together with slope olistostromes may constitute the residual accretionary prism of a trench-subduction complex formed before collision. The well known Xigaze Flysch, partly submarine and partly subaerial, rests on the continental crust and was accumulated in the forearc basin as Arc-trench gap sediments during late Aptian to Maestrichtian. It is unlikely that a presumed oceanic crust would be the base of Xigaze Basin. The Yarlung Zangbo ophiolite is suggested to be an older oceanic crust and mantle of upper Jurassic to pre-Albian age because of the radiolarian microfossils associated with the thick-bedded red chert (Lhaze Group). Ophiolites occurring as large thrusted ultramafics, dismembered altered basalts and radiolarite slices, obducted along the suture and formed the tectonic ophiolite melange. The Zangbo Suture Zone involves mainly the Congdu trench turbidite and Zongzhuo olistostromes in addition to obducted ophiolite melange. In other words, the Zongbo Suture is mainly a trench-subduction complex with obducted ophiolite melange system. No evidence exist to suggest that the Xigaze forearc basin sediments were involved either in the ophiolite belt or in the suture zone. Qiuwu Molasse of Eocene age, which unconformably overlies the Gandise granitoids, indicates that the collision occurred during the end of the Cretaceous or at the beginning of the Paleocene. The red subaerial Liuqu Conglomerate of Oligocene to Miocene age that uncomformably overlying the ophiolite, further suggests the ophiolite obduction to be pre-Oligocene. It is nearly contemporary with the continental collision of the early stage. Gandise granitoids in the Trans- Himalaya might be the root of the magmatic arc that followed the present Suture Zone to the north. These granitoids range in age from 120 to 40 Ma and are coeval with intermediate to acidic volcanics of Lingzizong Formation occurring to the north of the Gandise plutons and may correspond to Dras island arc volcanics in Ladakh.

Polyphase tectonics at the southern tip of the Manila trench, Mindoro-Tablas Islands, Philippines

The southern termination of the Manila trench within the South China Sea continental margin in Mindoro is marked by a complex polyphase tectonic fabric in the arc-trench gap area. Onshore Southern Mindoro the active deformation front of the Manila trench is marked by parallel folds and thrusts, grading southward to N50° W-trending left-lateral strike-slip faults. This transpressive tectonic regime, active at least since the Late Pliocene, has overprinted the collision of an Early Miocene volcanic arc with the South China Sea continental margin (San Jose platform). The collision is postdated by deposition of the Late Miocene-Early Pliocene elastics of the East Mindoro basin. The tectonic and geological framework of this arc, which overlies a metamorphic basement and Eocene elastics, suggests that it was built on a drifted block of the South China Sea continental margin.

Tethyan Cretaceous sediments in the Northwest Himalaya

Biostratigraphic studies indicate that the Tethyan Cretaceous sediments of the NW Himalaya were deposited in different sedimentary domains of the Indian and Eurasian plates. Those of the Indian plate are represented in the Lesser Himalaya by the Bansi Formation, and in the South Zanksar belt by, in ascending order, the Spiti Shales, Giumal Sandstone, Chikkim Limestone, Kangi La Flysch and Spanboth Limestone. Similar litho-units occur in the North Zanskar belt with the Lamayuru Unit on the northern margin of the Indian plate. The Bansi Formation was deposited in a shallow basin trending east-west which existed from the Cretaceous onwards. The sediments of South Zanskar and North Zanskar are continental shelf platform deposits, while the Lamayuru Unit in the latter was deposited on a continental slope. The sediments of the Eurasian plate mainly comprise (i) the Nindam Formation, deposited in a palaeosubduction zone, (ii) the Indus Formation, deposited in an arc-trench gap basin of the Indus Suture Zone, and (iii) the Shyok Group, deposited in a marginal (back arc) basin of the Shyok Suture Zone. The faunal and facies variations in the different parts of the NW Himalaya are discussed together with the palaeobiogeography of the NW Himalayan Tethys during the Cretaceous.

Indus tsangpo suture zone in ladakh—its tectonostratigraphy and tectonics

The Indus Tsangpo suture zone in Ladakh lies between the Phanerozoic sequence of the Zanskar Zone of Tethys Himalaya in the south and Karakoram zone in the north. The five palaeotectonic regimes recognized in the suture zone are: The Indus palaeosubduction complex, the Ladakh magmatic arc, the Indus arc-trench gap sedimentation, the Shyok backarc and the Post-collision molasse sedimentation. The Ladakh magmatic arc, comprising intrusives of the Ladakh plutonic complex and extrusives of the Dras, Luzarmu and Khardung formations, owes its origin to the subduction of the Indian oceanic plate underneath the Tibet-Karakoram block. The Indus Formation, lower Cretaceous to middle Eocene in age, was laid down in a basin between the magmatic arc and the subduction complex. The Shergol and Zildat ophiolitic melange belts exhibit green-schist and blue-schist facies metamorphism and show structural geometry and deformation history dissimilar to that of the underlying and overlying formations. The melange belts and the flysch sediments of the Nindam Formation represent a palaeosubduction complex. The Shyok suture zone consists of tectonic slices of metamorphics of the Pangong Tso Crystallines, Cretaceous to lower Eocene volcanics and sedimentaries, together with ultramafic and gabbro bodies and molasse sediments. This petrotectonic assemblage is interpreted as representing a back-are basin. Post-collision molasse sedimentaries are continental deposits of Neogene age, and they occur with depositional contact transgressing the lithological and structural boundaries. Two metamorphic belts, the Tso Morari crystalline complex and the Pangong Tso Crystallines, flank to the south and north respectively of the Indus suture zone in Eastern Ladakh. Three generations of fold structures and associated penetrative (and linear) structures, showing a similar deformation history of both the metamorphic belts, are developed. The shortening structures developed as a result of collision during the postmiddle Eocene time.

Plate tectonic interpretation of the Western Himalaya

The Western Himalaya, which includes the regions of Kumaun, Garhwal, Himachal, Jammu and Kashmir in India displays a complete cross-section of the Himalayan orogenic belt encompassing the principal tectonic zones of Outer Himalaya, Lesser Himalaya, Higher Himalaya, Tethys Himalaya, and Trans Himalaya. The rock formations of the Lesser Himalaya, which are correlatable to those of the northern part of the Indian Peninsular shield, represent deposits on the northern edge of the Indian craton. The Phanerozoic sequence of the Tethys Himalaya zone is interpreted as representing sedimentary deposits of the continental margin. The Central Crystallines of the Higher Himalaya zone, underlying the Phanerozoic sequence of the Tethys Himalaya zone, have been exposed in their present position as a result of uplift and southward movement along the Main Central Thrust. The Trans-Himalaya zone consists of the Indus Suture, Ladakh magmatic arc, Shyok Suture and Karakoram subzones. The Nidar ophiolite and the Shergol and Zildat ophiolitic melanges of the Indus Suture are the tectonised remnants of the Tethys ocean floor. The Ladakh plutonic complex, the Dras volcanics and other related Cretaceous volcanics belong to the plutonic-volcanic series of the Ladakh magmatic arc. The Shyok Suture is interpreted as a back-arc basin, and the Palaeozoic-Mesozoic sequence of the Karakoram subzone represents continental margin deposits of the Karakoram block. The molasse sedimentary deposits of the Outer Himalaya zone are interpreted as representing post-collision sedimentation in a trough that developed in front of the rising Himalaya. The tectonic evolution of Northwestern Himalaya has been involved in the following sequence of events. 1. (a) Subduction of the Indian oceanic plate under the Karakoram (Tibet) block in the Cretaceous, giving rise to the Ladakh magmatic arc and arc-trench gap sedimentation. 2. (b) Collision of the Indian plate with the Ladakh magmatic arc in the middle Eocene forming the Indus palaeosubduction zone. 3. (c) Closure of the Shyok back-arc basin and collision of the Ladakh magmatic arc with the Karakoram block. 4. (d) Suturing of the lithospheric blocks as a result of collision. The penetrative and non-penetrative deformation, regional metamorphism, generation of S-type granites in the Higher Himalaya zone and the formation of nappes and thrusts owe their origin to crustal shortening in the Oligocene-Miocene.

Synthesis of Arc-Derived DSDP (ODP) Modal Sand Compositions Using Multivariate Analyses: ABSTRACT

Sands derived from Quaternary and late Tertiary arc systems can be directly correlated with the tectonic setting of the marine basin in which they were deposited (trench, arc-trench gap, back arc, etc.), adjacent arc type continental margin versus intraoceanic; dissected versus undissected), and degree of arc/basin development at the time of deposition. Rigorous compositional classifications of these sediments are useful in interpreting tectonic setting of ancient sandstones of unknown or speculative origin. Representative arc-related sand samples from DSDP (ODP) sites form the basis of this investigation; regions sampled include the Mariana, Japan, Aleutian, North American, Middle American, Southwestern Pacific, Caribbean, and Mediterranean arc-related basins. Petrographic data were uniformly gathered using the Gazzi-Dickinson point-counting method. Modal sand compositions for each arc are unique, but marked differences along certain arc systems reflect the variable tectonic history of individual arc segments. Quartzo-feldspathic sands from the Mexican segment of the Middle American trench system sharply contrast with volcanic-rich sands from the Central American segment; more subtle changes occur along the sands from the Central American segment; more subtle changes occur along the Japan and North American arc systems. In general, multivariate analyses of petrographic data indicate (1) modal compositions of sands from magmatic-arc settingsmore » can be subdivided into intraoceanic arcs, continental arcs, and intraoceanic arcs with continental influence and (2) the percentage of continental components including potassium feldspar, quartz, mica, and sedimentary and metamorphic lithic grains increases toward continental margins. This synthesis represents the most complete study of modern arc sand compositions and provides information essential to studies of ancient arc-related sandstones.« less

Extensional tectonics in the Honshu fore-arc, Japan: integrated results of DSDP Legs 57, 87 and reprocessed multichannel seismic reflection profiles

Summary Leg 57 drilling on the Honshu fore-arc revealed late-Oligocene arc volcanism at Site 439 just 90 km from the axis of the present Japan Trench, on a basement palaeo-high. This, the Oyashio landmass, is defined by outboard- and inboard-onlapping reflectors in the thick (up to &gt; 2 km) inner trench-slope section. The Neogene section at Site 439 documents subsidence of the Oyashio landmass from a late-Oligocene position at or near wavebase, and subduction erosion was invoked by Leg 57 scientists to explain the anomalously narrow late-Oligocene arc-trench gap. Hole 584, drilled during Leg 87 just upslope from the mid-slope terrace above the outer edge of the Oyashio landmass, produced equally unexpected results. Though it failed to reach basement, penetration of 940 m of slope strata down to middle Miocene revealed below 200 m subbottom a progressive down-hole increase in dips to as much as 70°. Palaeomagnetic data indicate that dips are trenchward. Spot-coring in holes 500 m upslope and 700 m downslope revealed a similar peculiar geometry. However, lithostratigraphic and biostratigraphic markers are subhorizontal or only gently inclined trenchward when correlated between the three holes. Landward-dipping listric normal faults, not imaged in the seismic records available at the time of drilling, are hence required within the slope section. Numbers of such faults, giving step-wise displacements of limited overall vertical displacement, would explain the sub-horizontal markers. This extensional geometry is substantiated by healed, predominantly normal, microfaults which occur throughout the tilted section. Detailed structural analysis reveals that tilting occurred progressively from middle Miocene through earliest Pliocene, was preceded by development of a pervasive vein structure, and was both preceded and accompanied by extensional microfaulting. Microstructural observations, and the apparent restriction of kindred structures in other active margin cores to non-accretionary forearcs, lead us to ascribe the vein structure to an initial response of semi-lithified slope sediments to extension, rather than to hydrofracturing alone. The multichannel record on which Site 584 drilling was based showed slope-parallel reflectors, and gave no clue to the structure actually encountered with the drill. However, a line through Site 439 some 70 km upslope reveals landward-dipping normal faults, albeit without the rotations required to explain the geometry at Site 584. Subsequent reprocessing of a seismic record crossing the equivalent position of Site 584 40 km S shows considerable small-scale faulting like that around Site 439. This, in concert with the core-scale structural data, suggests tectonic jostling with an extensional fragmentation ranging from microstructure to blocks 300–500 m across. Such tectonism increases in intensity towards the mid-slope terrace until coherent reflections can no longer be resolved. Basement is either a Cretaceous-Palaeogene accretionary complex akin to the Shimanto Belt of SW Japan (more likely), or old crystalline rocks akin to those exposed onland in the Kitikami massif (less likely). Whatever its nature, low-angle landward-dipping reflectors (LDRs) are a common feature of the basement rocks. The listric landward-dipping faults generated in the slope sediments during Miocene-early-Pliocene subduction erosion may accordingly be explained in two ways. If the LDRs in the basement are old thrusts, they could conceivably have been reactivated as normal faults which propagated up into the cover as it accumulated. Alternatively, the faults in the cover may sole on low-angle detachment faults within the slope section. Present data only allow a first look at what must be a highly complex three-dimensional extensional setting.

Causes of compression and extension behind trenches

Abstract Three factors have hampered attempts to find the causes of compression and extension within overriding plates of subduction zones: 1. (1) adjacent subduction zones are often similar in many subduction parameters, 2. (2) proposed causal variables are themselves correlated, and 3. (3) physical models are limited by uncertainties in physical quantities and complexity of the subduction process. Nevertheless, modern subduction zones provide a rich source of observations that help to distinguish among models. We consider ten variables or combinations of variables which may control strain regime behind trenches: convergence rate (Vc), slab age (As), shallow slab dip (Ds), absolute motion (Voa), asthenospheric flow, slab pull force, As + Vc, As + Voa, Vc + Ds + Voa, and Vc + Ds + As. Ten observations are used to test these models. A key prerequisite to these tests is the semiquantitative classification of the strain regime of each modern subduction zone into one of seven strain classes, from active back-arc spreading (class 1) through active folding and foreland thrusting (class 7). Statistical analysis indicates that the modern variance in strain classes is not adequately accounted for by any single or dual-variable model, but the combination of Vc, Ds, and either AsorVoa can account for 77% of this variance (R = 0.88). Rapid changes in strain class both with time and along a subduction zone are similarly more consistent with the 3-variable models. The role of Vc is also supported by maximum compression orientations parallel to Vc; the role of Ds is also supported by the tendency for oceanic overriding plates to be more extensional than continental overriding plates. Two factors suggest that the Vc + Ds + As model is more likely than Vc + Ds+ Voa: 1. (1) the dominance of one or more of the variables Vc, As, andDs in other subduction relations (slab length, earthquake moment, trench relative depth, deep dip, and arc-trench gap), and 2. (2) the correlation between strain class and maximum earthquake moment (which is affected by Vc and As). However, no confident discrimination between the two 3-variable models is yet possible.