K–Ar age–chemistry–fabric relations of phengite from the Sanbagawa high-pressure schists, Japan

The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and cooling. This study examines the effects of high-strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation-induced grain-size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin-section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age.     

Phengite geochronology of crystalline schists in the Sakuma–Tenryu district,central Japan

The Sakuma–Tenryu district consists mainly of pelitic and basic schists. Its metamorphic sequence has been divided into two units, the Shirakura and the Sejiri units. We carried out K–Ar analyses of phengite separates and X‐ray diffraction analyses of carbonaceous materials from the pelitic schists of both units. The age–d₀₀₂ relationships show that the ages become older (66–73 Ma) in the Shirakura unit and younger (57–48 Ma) in the latter with increasing metamorphic temperature. The former has a positive relationship observed in the Sanbagawa meta‐Accretionary Complex (meta‐AC) (Sanbagawa metamorphic belt sensu stricto) in central Shikoku and the latter, a negative one in the Shimanto meta‐AC (a subunit of traditional Sanbagawa belt) of the Kanto Mountains. These contrasting age–temperature relationships are due to different tectonic styles relating to the exhumation of the metamorphic sequences. The duration from the peak metamorphism to the closure of the phengite K–Ar system was significantly different between the two metamorphic sequences: longer than 31 my in the Sanbagawa meta‐AC and shorter than 13 my in the Shimanto meta‐AC. The different natures of subducted plate boundaries may cause the different exhumation processes of metamorphic belts.     

Ridge collision and in situ greenstones in accretionary complexes: An example from the Late Cretaceous Ryukyu Islands and southwest Japan margin

Abstract Mélange units containing greenstones are common throughout the Cretaceous-Miocene Shimanto Supergroup in the Ryukyu Is and southwest Japan. Most greenstones in the accretionary complex originated in oceanic spreading ridges and seamounts, and they formed far from the convergent margin. Some mélange-like units in the supergroup, however, contain greenstones that were extruded upon and intruded into unconsolidated fine-grained terrigenous clastic sediments. It is inferred that eruption of the in situ greenstones resulted from igneous activity in the trench area. Geochemical signatures indicate that the greenstone protoliths were similar to mafic lavas generated at spreading ridges. Fossil ages of the strata containing in situ greenstones become younger over a distance of 1300 km eastward from Amami-Oshima (Cenomanian-Turonian) in the Ryukyu Is to central Japan (Late Maestrichtian-earliest Paleocene), implying that a site of igneous activity in the trench area migrated eastward along the Ryukyu Is and southwest Japan margin. Plate reconstructions of the northwest Pacific Ocean suggest the presence of the Kula-Pacific ridge near Late Cretaceous to early Paleogene Japan. In this context, it is suggested that the greenstones formed in response to Kula-Pacific ridge-forearc collision.
Ancient ridge-forearc collisions are best recognized by the presence of mid-ocean ridge basalt (MORB) extruded on sediments inferred to have accumulated in the trench area. Diachronous occurrences of the strata associated with these MORB in an orogenic belt are useful for documenting the ridge collision through time.     

Parentage of low‐grade metasediments in the Sanbagawa belt,eastern Shikoku,Southwest Japan,and its geotectonic implications

This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.     

Anomalous structural evolution of the Shimanto Accretionary Prism at Murotomisaki, Shikoku Island, Japan

Abstract The Late Oligocene-Early Miocene Nabae Sub-belt of the Shimanto Accretionary Prism was created coevally (ca 25-15 Ma) with the opening of the Shikoku back-arc basin, located to the south of the southwest Japan convergent margin. The detailed geology of the sub-belt has been controversial and the interaction of the Shimanto accretionary prism and the opening of the Shikoku Basin has been ambiguous. New structural analysis of the sub-belt has led to a new perception of its structural framework and has significant bearing on the interpretation of the Neogene tectonics of southwest Japan. The sub-belt is divided into three units: the Nabae Complex; the Shijujiyama Formation; and the Maruyama Intrusive Suite. The Nabae Complex comprises coherent units and mélange, all of which show polyphase deformation. The first phase of deformation appears to have involved landward vergent thrusting of coherent units over the mélange terrane. The second phase of deformation involved continued landward vergent shortening. The Shijujiyama Formation, composed mainly of mafic volcanics and massive sandstone, is interpreted as a slope basin deposited upon the Nabae Complex during the second phase of deformation. The youngest deformational pulse involved regional flexing and accompanying pervasive faulting. During this event, mafic rocks of the Maruyama Intrusive Suite intruded the sub-belt. Fossil evidence in the Nabae Complex and radiometric dates on the intrusive rocks indicate that this tectonic scheme was imprinted upon the sub-belt between ～23 and ～14 Ma. The timing of accretion and deformation of the sub-belt coincides with the opening of the Shikoku Basin; hence, subduction and spreading operated simultaneously. Accretion of the Nabae Sub-belt was anomalous, involving landward vergent thrusting, magmatism in newly accreted strata and regional flexing. It is proposed that this complex and anomalous structural history is largely related to the subduction of the active Shikoku Basin spreading ridge and associated seamounts.     

The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

One of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.     

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district,central Shikoku,southwest Japan

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).     

Detrital zircon multi‐chronology,provenance, and low‐grade metamorphism of the Cretaceous Shimanto accretionary complex,eastern Shikoku,Southwest Japan: Tectonic evolution in response to igneous activity within a subduction zone

Detrital zircon multi‐chronology combined with provenance and low‐grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench‐fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil‐bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission‐track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low‐grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out‐of‐sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.     

Petrography and geochemistry of upper Triassic sandstones from the Tumengela Formation in the Woruo Mountain area,North Qiangtang Basin,Tibet: Implications for provenance,source area weathering,and tectonic setting

The petrography and major and trace element concentrations of the sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, are studied to determine their provenance, intensity of weathering and tectonic setting. The detrital compositions of the Tumengela sandstone samples are dominated by quartz (58.0–70.1 %, average 64.7 %) and lithic fragments (21.8–35.9 %, average 27.3 %), but low in feldspar content (4.9–12.9 %, average 8.0 %). The sandstones can be classified as litharenite and feldspathic litharenite according to their detrital compositions, which is consistent with the geochemical data. The detrital modal compositions reflect that these sandstones are probably derived from a recycled orogenic source. The index of chemical variability (ICV) and SiO₂/Al₂O₃ ratio values suggest that the compositional maturity and recycling were moderate. The weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and Al₂O₃–(CaO* + Na₂O)–K₂O (A–CN–K) diagram indicate that the intensities of weathering in the source area were moderate. The Al₂O₃/TiO₂, Th/Co, La/Sc, La/Co, Th/Sc, Cr/Th ratio values and the discriminant function of the Tumengela sandstones indicate that the sediments were mainly derived from felsic source rocks, while also mixed with intermediate source rocks. The comparison of rare earth element patterns and its Eu anomalies to the probable source rocks infer that the sandstones were derived from the combination of granite, rhyolite, dacite, and gneisses. The proximal central uplift belt was probably the primary provenance area as evidenced by the petrographical and geochemical features of the Tumengela sandstones. The multidimensional tectonic discrimination diagram based on major elements show a collision setting (80 %) combined with a rift setting (20 %) for the Tumengela sandstones, which is consistent with the general geology of the study areas.     

Tectono-metamorphic history of the Tacagua ophiolitic unit (Cordillera de la Costa, northern Venezuela): Insights in the evolution of the southern margin of the Caribbean Plate

Abstract   The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid-oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep-sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4 . Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4 ) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel ( D2 and D4 phases) and normal ( D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike-slip and pure compressional or pure extensional tectonics occurred through time.     

Episodic exhumation of accretionary complexes: Fission-track thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan

Abstract Apatite and zircon fission-track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (T_e) generally indicate ~10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C T_e exhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion-burial-exhumation process. Zircon data suggest two cooling episodes: ~70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and ~15 Ma cooling in the central Kii Peninsula. The former is consistent with ⁴⁰Ar/³⁹Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano-plutonic complexes contemporaneously placed inland. Available geo- and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphism concepts.     

Strength and deformation behavior of the Shimanto accretionary complex across the Nobeoka thrust

A rapid reduction in sediment porosity from 60 to 70 % at seafloor to less than 10 % at several kilometers depth can play an important role in deformation and seismicity in the shallow portion of subduction zones. We conducted deformation experiments on rocks from an ancient accretionary complex, the Shimanto Belt, across the Nobeoka Thrust to understand the deformation behaviors of rocks along plate boundary faults at seismogenic depth. Our experimental results for phyllites in the hanging wall and shale‐tuff mélanges in the footwall of the Nobeoka Thrust indicate that the Shimanto Belt rocks fail brittlely accompanied by a stress drop at effective pressures < 80 MPa, whereas they exhibit strain hardening at higher effective pressures. The transition from brittle to ductile behavior in the shale–tuff mélanges lies on the same trend in effective stress–porosity space as that for clay‐rich and tuffaceous sediments subducting into the modern Nankai subduction zone. Both the absolute yield strength and the effective pressure at the brittle–ductile transition for the phyllosilicate‐rich materials are much lower than for sandstones. These results suggest that as the clay‐rich or tuffaceous sediments subduct and their porosities are reduced, their deformation behavior gradually transitions from ductile to brittle and their yield strength increases. Our results also suggest that samples of the ancient Shimanto accretionary prism can serve as an analog for underthrust rocks at seismogenic depth in the modern Nankai Trough.     

Volcanic resurfacing and the early terrestrial crust: Zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland

The oldest known bona fide succession of clastic metasediments occurs in the Isua Greenstone Belt, SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for > 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon. Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3.71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age.The mica schist hosted zircons have U/Pb ages, Th / U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed.Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic-ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking of Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt outpourings. This process causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope compositions potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth, from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments.     

贺兰山中段奥陶系米钵山组砂岩地球化学及构造背景分析

本文利用奥陶系米钵山组砂岩地球化学分析,结合区域地质研究,探讨贺兰山中、晚奥陶世的构造环境。贺兰山中段奥陶系米钵山组砂岩的地球化学研究表明,砂岩的siO2平均含量为81．3％;A120。／Si02值0．07～0．11,平均值为0．08;K20／Na20值变化较大,最大60．7,一般介于4．79～7．81;Fe2O3＋MgO含量较低,介于2．1％～2．81％。砂岩微量元素Nb丰度及V／（V＋Ni）与Ce／La、Sr／Ba值均较高,说明砂岩沉积于湿热、还原、低盐度环境,具有大陆型沉积特征。砂岩稀土元素富集,含量在116×10^-6～195×10^-6之间,平均值为158×10～;8Eu为0．52～0．58,具显著的负铕异常。这些数据指示了米钵山组具有重力流快速堆积的特征和大量陆源补给,浊流沉积作用是重力流携带陆源物质的主要途径。通过多种构造环境判别图解分析,显示物源区地质构造具有被动大陆边缘性质。     

Cenozoic stratigraphy and sedimentation history of the northern Philippine Sea based on multichannel seismic reflection data

Abstract To clarify the regional distribution and characteristics of the sedimentary deposits in the northern part of the Philippine Sea, multichannel seismic reflection surveys of 26 864 km in total length were performed. The seismic reflection data were interpreted and correlated with available Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) data and a general stratigraphic framework of the area was established. The sedimentary deposits in this area were divided into five layers; Units I, II, III, IV and V in ascending order. Their approximate geological ages are the Early Eocene, Middle to Late Eocene, Oligocene, Miocene and Pliocene‐Pleistocene, respectively. Seismic records were classified into three seismic facies, Facies A, B and C, on the basis of their characteristics. They were judged to represent pelagic and hemipelagic sediments of non‐volcanic origin, fine pyroclastic sediments and coarse pyroclastic or volcanic sediments, respectively, by comparing them with lithological data in the DSDP/ODP holes. From the thickness and facies distributions of these sediments, a sedimentary history in the area was reconstructed as follows. The oldest sediments in the study area, Unit I, interfinger with some parts of the Daito Ridge (acoustic basement) in the Minami Daito Basin. The geological age of the unit is estimated by microfossils in the sediment and supports the idea that this part of the Daito Ridge is composed of the Early Eocene oceanic basalt. Later, a fair amount of sediments were deposited in the Minami Daito Basin in the Middle to Late Eocene age. A large volume of volcanic materials was supplied from the Paleo‐Kyushu‐Palau Ridge in the Kita Daito Basin in the Eocene and Oligocene ages. The eastern part of the Shikoku and Parece Vela basins is characterized by volcanic sediments supplied from the Nishi Shichito and West Mariana Ridges in the Miocene age. However, pelagic and hemipelagic sediments prevail in the northern part of the Shikoku Basin in the Miocene or later. In short, the area of principal sedimentation has generally shifted from west to east through geological time, reflecting the evolution of the island arc systems with the same trend in the northern Philippine Sea.     

Dewatering and extensional deformation of the Shikoku Basin hemipelagic sediments in the Nankai Trough

The Shikoku Basin hemipelagic sequence, which underlies the Nankai Trough wedge, S.W. Japan, thins by 50% between the outer edge of the trench wedge and DSDP Site 582, 14 km arcward. A sedimentation model, which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe, indicates that 57% of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites. Burial-induced consolidation beneath the wedgeshaped turbiditic overburden, accounts for the remaining 43% of arcward thinning within the hemipelagic unit. Rapid dewatering, modeled as one-dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering. Thus, high pore water pressures should not be assumed to occur universally in convergent margin settings. Normal faults and vein structures in the hemipelagites suggest near-horizontal extension in addition to vertical consolidation. The estimates of excess pore water pressures together with fault geometries and horizontal extensional strains, determined from the geometry of the subducting oceanic plate, can be used to constrain the stress conditions at failure. The expulsion of hot water from the rapidly dewatering sediments in the Nankai Trough may help to explain anomalously high heat flow in the central part of the trough.     

Petrochemistry of sandstones from Neoproterozoic basins of the Bastar craton, Central Indian Shield: Implications for paleoweathering, provenance and tectonic history

Petrographic analysis and chemical analysis of major and trace elements including rare earth elements of the Neoproterozoic sandstones from the Chandarpur Group and the Tiratgarh Formation have been carried out to determine their provenance, tectonic setting and weathering conditions. All sandstone samples are highly enriched in quartz but very poor in feldspar and lithic fragments. Petrographically and geochemically these sandstones are classified as subarkose, sublitharenite and arenite. The Chemical Index of Alteration (CIA mean 68) and Th/U ratios (mean 4.2) for these sandstones suggest their moderate weathering nature. Generally, all sandstone samples are strongly depleted in major elements (except SiO₂), trace elements (except Zr) and REE in comparison with Post Archean Australian Shale (PAAS) and Upper Continental Crust (UCC). Their mineralogy and mean of elemental ratios suitable for determination of provenance and tectonic setting, e.g. Al₂O₃/SiO₂ (0.02), K₂O/Na₂O (10), Eu/Eu* (0.67), (La/Lu)n (10.4), La/Sc (3), Th/Sc (1.2), La/Co (0.22), Th/Co (0.08), and Cr/Th (7.2), support a felsic source and a passive margin tectonic setting for these sandstones. Also these key elemental ratios do not show much variation over a range of SiO₂. Thus we attest their significance in determining source rock characteristics of quartz rich sandstones. Chondrite‐normalized REE patterns with LREE enrichment and a strong negative Eu anomaly are also attributed to felsic source rock characteristics for these sandstones. The source rocks identified are granite and gneiss of the Bastar craton. Minor amounts may have been derived from older supracrustals of the Bastar craton. However, the major element data of the Paleoproterozoic Sakoli schists when compared with those of the Neoproterozoic sandstones indicate that the schists were derived from a mafic source and deposited in an active continental margin tectonic setting. There is, however, little difference in CIA values between the Paleoproterozoic Sakoli schists and Neoproterozoic sandstones, indicating prevailing of similar (moderate‐intense) weathering conditions throughout the Proterozoic in the Bastar craton. Our study also suggests a change in the provenance and tectonic setting of deposition of sediments from dominantly a mafic source and an active continental margin in the Paleoproterozoic to dominantly granite and gneiss (felsic source) and a passive continental margin in the Neoproterozoic in the Bastar craton.     

Phengite K-Ar ages of schists from the Sanbagawa southern marginal belt, central Shikoku, southwest Japan: Influence of detrital mica and deformation on age

Abstract K–Ar age determinations were carried out on phengite separates from pelitic schists collected systematically from the Sanbagawa southern marginal belt and the associated area. The petrography and phengite chemistry by electron probe micro-analyzer (EPMA) revealed the existence of detrital white micas in the schist that have an extremely older age (108 Ma) in comparison with the neighboring schists (88 Ma) without any detrital mica. The ages become gradually older from the north ( ca 78 Ma) to the south ( ca 90 Ma) except for some samples that contain detrital micas and/or have been reactivated thermally by intrusives. The age is interpreted as an exhumation-cooling age that has been controlled by the ductile deformation of the host rocks that have never experienced a culmination temperature higher than 350°C which corresponds to the closure temperature of the K–Ar phengite system. The southward aging of the recorded ages in the extensive chlorite zone of the central Shikoku, from the Dozan river area of the north ( ca 65 Ma) to the study area of the south ( ca 85 Ma) through the Asemi river area ( ca 75 Ma), is explained in terms of increasing exhumation/cooling rates of the host rocks from north to south. The phengite K–Ar ages in the pelitic schists from the Kyomizu tectonic zone, which is classically considered as a remarkable thrusting shear zone, have no significant difference in comparison with that of the neighboring schists. This fact suggests that the latest stage of brittle deformation during exhumation/uplift has not significantly affected the ages of phengite in the schists.     

Existence of multiple units with different accretionary and metamorphic ages in the Sanbagawa Belt,Sakuma–Tenryu area,central Japan

U–Pb ages of detrital zircons and white mica K–Ar ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the Sakuma–Tenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95 Ma, and the youngest age in the detrital zircons is 94.0 ± 0.6 Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 ± 0.9 Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica K–Ar ages of T1 and T5 are 69.8 ± 1.5 Ma and 56.1 ± 1.2 Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the Sakuma–Tenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high‐P/T Sanbagawa Metamorphic Belt.     

Stratigraphic Columns Modeling and Cyclicity Analysis of the Misoa Formation,Maracaibo Lake,Venezuela, using Markov Chains

A stochastic characterization of a hydrocarbon reservoir, constituted by a sedimentary sequence of sandstones interbbeded with siltstones and shales, has been performed. The stratigraphic unit studied here mainly comprises the C4 sands of the Misoa Formation, located in the Lama Field, Maracaibo Lake (Venezuela). A Markov Chain algorithm, based on the definition of genetic lithofacies relationships along stratigraphic columns, was developed. The application of the Monte Carlo stochastic method using this algorithm, to log data from 11 wells, allowed the generation of pseudo sequences at 20 new locations. This algorithm was able to properly model pseudo stratigraphic sequences and to quantify the relative facies percentage, showing a 82% confidence level related to the proportional content of sediments at a test well. The net sand map obtained integrating the stratigraphic columns, derived from the well information, and the Markov pseudo-columns, suggests the presence of sand bodies with a northeast-southwest orientation that agree with previous geological studies in the area. This map could help in the definition of prospective zones in the field. The existence of stratigraphic memory along the evaluated columns was recognized after applying the algorithm. The embedded Markov method used in the cyclicity analysis of the whole area indicates cyclic transitions just from sandstones to siltstones and from shales to siltstones. Hence for the study area, on average, fining upward and coarsening upward processes can be identified with the Markovian approach, as was expected for the tide-dominated deltaic system associated to the analyzed reservoir.