Forearc Basin System Research Articles

Cretaceous forearc basin remnant sediments in the eastern Yarlung Zangbo suture zone, near the eastern Himalaya syntaxis

Low-grade metamorphosed Cretaceous sedimentary records of forearc basin systems in the eastern part of the Transhimalaya (Gangdese) arc system have not been reported yet, impairing understanding of the Mesozoic-Cenozoic convergent margin processes along the southern margin of Asia. This study incorporates observations from petrography, stratigraphy, sedimentology, and analyses of detrital-zircon geochronology and bulk-rock Sr-Nd isotope for a series of low-grade metamorphosed sedimentary rocks referred to as the Lilong succession in the eastern Yarlung Zangbo suture zone, east of Langxian, in southeastern Tibet, China. Field observations reveal that this unit has three different lithological associations (I, II, and III) including gravel-coarse sandy inner-middle submarine fan facies, sandy-muddy middle-outer submarine fan facies, and silty-muddy outer submarine fan and/or deep sea plain facies moving higher in the succession. Detrital-zircon geochronology indicates a maximum depositional age of ~110 Ma and the absence of younger zircons that are dominant in the Gangdese batholith north of the basin suggests the minimum depositional age ~ 85 Ma, comprising an interval of the Albian-Santonian age. Volcanic and granitic clasts in sandstones/conglomerates, εNd(0) values (−0.09 and 4.6) and 87Sr/86Sr ratios (0.704105–0.704770) of slates, and detrital zircon age spectra (mainly 100–140 Ma) demonstrate provenance from the central-northern and southern Lhasa Terrane. In addition, there is evidence for recycling of cherty beds of the Chongdui Formation. Similarities between provenance, lithofacies, and tectonic setting lead us to interpret that the newly identified remnant unit correlates to the Ngamring Formation in the Xigaze forearc basin. When taking into account the lower Bomi Group and Nyingchi complex, we propose that the Lilong succession and the metamorphosed composite in the eastern Himalayan syntaxis are counterpart remnants of the eastern Xigaze forearc basin system, which are different from the two forearc basin systems in the western Himalaya.

Geochemistry of Khor Um-Safi ophiolitic serpentinites, central Eastern desert, Egypt: Implications for neoproterozoic arc-basin system in the Arabian-Nubian shield

Serpentinites in the Eastern Desert of Egypt are the most distinctive lithological unit in the Arabian–Nubian Shield (ANS) ophiolite sequence which associated with major suture zones. Khor Um-Safi (KUS) serpentinites represent dismembered fragments of ophiolitic rocks located in the central Eastern Desert (CED) of Egypt.KUS serpentinites exhibit affinity to the typical metamorphic peridotites with harzburgitic protolith compositions. Their opaque mineral assemblage (pentlandite, heazlewoodite and magnetite) is similar to that observed in oceanic serpentinites and implies serpentinization under highly reducing conditions. They have refractory major element compositions with Al2O3 contents comparable to oceanic and active margin peridotites as well as Pan-African serpentinites. The Cr and TiO2 contents reflect evolution within a supra-subduction zone (SSZ) environment. This implication is confirmed by the Al2O3/SiO2 and MgO/SiO2 ratios which akin to ANS ophiolitic peridotites in fore-arc setting. Their enrichment in compatible trace elements (Cr, Ni and Co) reveals a depleted mantle peridotite protolith.Modelling trace elements indicates that they represent the mantle residues from 15 to 20 % melting of spinel peridotite at oxygen fugacity conditions of the QFM + 1 buffer. This range of melt extraction is consistent with the typical range of SSZ peridotite. Oxygen fugacity estimation suggests evolution under more oxidizing regime similar to modern fore-arc basin system. Moreover, this implication indicates that the KUS mantle represents arc lithosphere interacted with arc melt.

Late Paleozoic tectonic evolution of the Kangguer Shear Zone and Yamansu Arc Belt, Eastern Tianshan (NW China): Constraints from structure, petrogenesis and geochronology of granitoids

Despite abundant research efforts, no consensus exists about the Carboniferous tectonic evolution of the Eastern Tianshan. The tectonic environment of the Yamansu arc belt and the geodynamic mechanisms of the Kangguer shear zone are not fully understood. To refine the proposed tectonic history, we studied three granitoids with a variety of shear deformation characteristics and emplacement relationships in the Yamansu–Kangguer area. Bulk geochemistry indicates that the mylonitic granite porphyry is I-type, while the undeformed two-mica granite and quartz porphyry are S-type granites. Zircon UPb dating results give concordant ages of the granite porphyry, two-mica granite, and quartz porphyry at 336.4 ± 2.3 Ma (MSWD = 1.9, n = 16), 331.5 ± 6.1 Ma (MSWD = 2.6, n = 6), and 332.8 ± 1.7 Ma (MSWD = 1.5, n = 25), respectively, suggesting an emplacement age in the middle Carboniferous. The whole-rock SrNd and zircon Hf isotopic analyses of the three granitoids indicate that the granitic magmas were derived from a Mesoproterozoic to Neoproterozoic crustal basement. The whole rock geochemical and isotopic data of the studied granitoids indicate partial melting with insignificant crustal contamination (except the two-mica granites) and fractional crystallization during magma origin and evolution. These granitoids show enrichment of LILEs (e.g., Rb, Ba, K) and Pb, depletion of Nb, Ta, P, Ti, and P, with low Sr/Y ratios (3–73) and Sr contents (33–160 ppm), displaying affinities to arc magma, and they were classified as continental arc granites. By combining the structural analysis and geological evidence, we propose that southward subduction of the Kangguer Ocean in the middle–late Carboniferous resulted in the ductile shear deformation of strata in the Kangguer shear zone and voluminous magmatism in the Yamansu–Kangguer continental arc–fore-arc basin system.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.

The West Andaman Fault: A complex strain‐partitioning boundary at the seaward edge of the Aceh Basin, offshore Sumatra

AbstractAlong the Sumatran subduction zone, oblique strain is partitioned between the Sunda megathrust and the strike‐slip Sumatran Fault located near the volcanic arc. Additional strike‐slip motion may localize adjacent to the forearc basin system, forming the Mentawai and West Andaman Fault Zones. An alternative interpretation of the West Andaman Fault, as primarily a backthrust, has also been proposed. We combine a 2‐D multichannel seismic survey (SUMUT) acquired aboard the R/V Sonne in 2008 with previous bathymetry and seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Two‐dimensional Kirchoff prestack time migration of SUMUT profiles provides the clearest images to date of this portion of the West Andaman Fault. In these images we interpret inward dipping fault segments along the West Andaman Fault indicative of a transpressional flower structure. A reflector at the base of the Aceh Basin sediments (believed to be base Neogene in age) continues trenchward of the fault zone for >15 km and has geometry inconsistent with the geometry expected if a major backthrust were to daylight at the edge of the basin. Therefore, we conclude that the West Andaman Fault is predominantly strike‐slip in nature. As such, the West Andaman is likely part of a system of faults including the Sumatran Fault that accommodate the significant shear component of strain in this oblique subduction zone. Additionally, the location of the West Andaman Fault within the forearc may be controlled by the rheologic contrast between the marginal plateau and the forearc basin.

BSRs and Associated Reflections as an Indicator of Gas Hydrate and Free Gas Accumulation: An Example of Accretionary Prism and Forearc Basin System along the Nankai Trough, off Central Japan

Abstract. Multi‐channel seismic data obtained from the Nankai accretionary prism and forearc basin system has been studied to elucidate the migration and accumulation process of gas to the BGHS and examine the distribution pattern of BSRs and characteristic reflections associated with them.BSRs are distributed widely in the Nankai accretionary prism and associated forearc basins (33,000 km2) and 90 % of them have migration and recycling origins. The widest distribution of the BSRs can be seen at the prism. A correlation between the BSR distributions and prism size shows that the BSRs tend to be more well‐developed in a prism of large size. This suggests that a large prism may produce much amount of gas‐bearing fluids that migrate to the BGHS and form the BSRs (tectonic control), hi the forearc basins, the BSRs are identified at topographic highs, anticlines and basin margins (structural control).The upward migration of gas‐bearing fluids is carried out through permeable sand layers and as a result, the distribution of BSRs is confined to alternating beds of sand and mud facies (sedimentary control). However, if there is enough time for upward migration and accumulation of gas to the BGHS, the BSRs can be generated widely in low‐permeable mud facies (time control).Those results imply that structural, tectonic, sedimentary and time controls are primary factors to decide the distribution of BSRs in the Nankai Trough area.

Subduction-Accretion History of the Central Asian Orogenic Belt: Constraints from Mongolia

Despite abundant research efforts, no consensus exists about the Carboniferous tectonic evolution of the Eastern Tianshan. The tectonic environment of the Yamansu arc belt and the geodynamic mechanisms of the Kangguer shear zone are not fully understood. To refine the proposed tectonic history, we studied three granitoids with a variety of shear deformation characteristics and emplacement relationships in the Yamansu–Kangguer area. Bulk geochemistry indicates that the mylonitic granite porphyry is I-type, while the undeformed two-mica granite and quartz porphyry are S-type granites. Zircon UPb dating results give concordant ages of the granite porphyry, two-mica granite, and quartz porphyry at 336.4 ± 2.3 Ma (MSWD = 1.9, n = 16), 331.5 ± 6.1 Ma (MSWD = 2.6, n = 6), and 332.8 ± 1.7 Ma (MSWD = 1.5, n = 25), respectively, suggesting an emplacement age in the middle Carboniferous. The whole-rock SrNd and zircon Hf isotopic analyses of the three granitoids indicate that the granitic magmas were derived from a Mesoproterozoic to Neoproterozoic crustal basement. The whole rock geochemical and isotopic data of the studied granitoids indicate partial melting with insignificant crustal contamination (except the two-mica granites) and fractional crystallization during magma origin and evolution. These granitoids show enrichment of LILEs (e.g., Rb, Ba, K) and Pb, depletion of Nb, Ta, P, Ti, and P, with low Sr/Y ratios (3–73) and Sr contents (33–160 ppm), displaying affinities to arc magma, and they were classified as continental arc granites. By combining the structural analysis and geological evidence, we propose that southward subduction of the Kangguer Ocean in the middle–late Carboniferous resulted in the ductile shear deformation of strata in the Kangguer shear zone and voluminous magmatism in the Yamansu–Kangguer continental arc–fore-arc basin system.

Structure and tectonics of the Manila trench system, Western Luzon, Philippines

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The Manila Trench is associated with an east-dipping Benioff zone beneath Luzon, a well-developed forearc basin system and a volcanic arc. The Luzon Trough forearc basin system contains up to 4.5 km of Cenozoic sediments. Trench strata are folded and faulted at the base of the trench slope. The major décollement at the Manila Trench usually forms near the unconformity separating the pelagic from the turbidite sediments in the trench. The accretion of turbidite sediments deposited in the Manila Trench has resulted in the upbuilding of an accretionary prism which forms the seaward margin of the forearc basin system. The uplifted Zambales ophiolite and its offshore extension form the landward side of the forearc basin. Uplift of the forearc basins is confined to a narrow zone near the intersection of the relict spreading center within the South China Sea plate. The forearc north of Lingayan Gulf is disrupted by active, northerly trending fault systems that extend offshore from the Luzon Central Valley and from the southern Cordillera Central. The Early Miocene uplift and tilting of the Zambales ophiolite may mark the beginning of subduction at the Manila Trench. We infer that the long-term average convergence rate across the Manila Trench has been about 1 cm/yr and may be slowing in the north due to the collision of Taiwan with Eurasia.

Forearc basin development along Western Luzon, Philippines

Sediment distribution within the Manila Trench exerts a strong influence on the tectonic evolution of the accretionary prism and the forearc basins of this convergent plate margin. Multichannel seismic data show up to 2600 m of turbidite-style sediments overlying a thin pelagic/ hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. Manila Trench sedimentation is dominated by turbidite sources at the ends of the trench coupled with transport along the trench axis. In contrast, the forearc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the forearc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and is strongly correlated with the local thickness of turbidite sediments within the trench. More than 200 m of trench turbidites are probably needed to create net uplift of the trench-slope break through accretion. This general relationship may apply to other convergent plate margins. The uplift rate at the trench slope break varies along the trench and is largely independent of variations in the local sediment accumulation rates in the forearc basins. Several different types of forearc morphology are formed along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough forearc basin is remarkably uniform, with continuous seismic sequences for up to 110 km along strike. The unconformities that bound these seismic sequences result from a complex interplay of relatively long-term (~ 10 + m.y.) uplift of the accretionary prism and relatively short-term (~ 1−5 m.y.) variations in sediment supply to the forearc basin. The latter may be controlled by eustatic sea level changes.

A geophysical study of the Manila Trench, Luzon, Philippines: 1. Crustal structure, gravity, and regional tectonic evolution

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The trench trends northerly and is associated with a volcanic arc, an east dipping Benioff zone beneath Luzon, and a well‐developed fore arc basin system. The Luzon Trough fore arc basins lie landward of the Manila Trench and contain up to 4.5 km of Cenozoic sediments. Offscraping and accretion of turbidite sediments deposited in the Manila Trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system. The uplifted Zambales ophiolite and its offshore extension form the landward side, of the fore arc basin. Multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope. The major structural décollement at the Manila Trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments. Subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits. Faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the South China Sea basin have presumably been subducted. North of Lingayan Gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the Luzon Central Valley and from the southern Cordillera Central. Based on regional geological and tectonic observations, we infer that subduction probably began at the Manila Trench in late Oligocene to middle Miocene time. The long‐term average convergence rate at the Manila Trench is estimated to be between 10 and 20 mm/yr and may be slowing in the north owing to the collision of Taiwan with Eurasia.

A geophysical study of the Manila Trench, Luzon, Philippines: 2. Fore arc basin structural and stratigraphic evolution

The pattern of sediment distribution within the Manila Trench subduction zone west of Luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin. Multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic/hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. These turbidites were transported from the uplifted collision zone of Taiwan, southward along the Manila Trench by gravity‐controlled processes. Similarly, thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the North Palawan block and the central Philippines. Sedimentation within the Manila Trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis. In contrast, the fore arc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench. A thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region. This generalization may apply to other convergent plate margins as well. The uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins. This variability results in the formation of several different types of fore arc morphology along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough fore arc basin is remarkably uniform, with seismic sequences continuous for up to 110 km along strike. The unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long‐term (approximately 10 Ma) uplift of the accretionary prism and relatively short‐term (approximately 1–2 Ma) variations in sediment supply to the fore arc basin, modulated by eustatic sea level changes.

Evolution of the south-central segment of the Archean Abitibi Belt, Quebec. Part III: Plutonic and metamorphic evolution and geotectonic model

Textural criteria permit distinction between the pre-Kenoran and Kenoran phases of plutonism and metamorphism. The pre-Kenoran plutons and pre-Kenoran metamorphic phases are directly related to the volcanic evolution. Synvolcanic tonalite–trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes. The volcanic rocks underwent three types of pre-Kenoran metamorphism, namely, a pervasive alteration, a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons, and a high-intensity hydrothermal alteration that affected cross-cutting pipes in central volcanic complexes.Synkinematic Kenoran metamorphism resulted in the growth of minerals (chlorite, actinolite, etc.) parallel to schistosities. Synkinematic metamorphic grade ranges form the pumpellyite–prehnite facies to the amphibolite facies. Late- to post-kinematic metamorphic phases resulted in the growth of minerals across schistosities. Syn- to post-kinematic plutons are not voluminous in the part of the Abitibi Belt described here, but they underlie vast areas in the Bellecombe Belt. They range from gneissose early synkinematic plutons to late-kinematic plutons that have well preserved igneous textures.The paleogeographic, tectonic, plutonic, and metamorphic histories of the Abitibi and Bellecombe belts are reviewed and we conclude that the belts are analogous to an island arc – fore-arc basin system.