Tectonic accretion of a subducted intraoceanic remnant arc in Cretaceous Hokkaido, Japan, and implications for evolution of the Pacific northwest

Abstract   An accretionary complex, which contains fragments of a remnant island arc, was newly recognized in the Cretaceous accretionary terranes in Hokkaido, Japan. It consists of volcanics, volcanic conglomerate, intermediate to ultramafic intrusive rocks with island-arc affinity including boninitic rocks, accompanied by chert and deformed terrigenous turbidites. Compared with the results of modern oceanic surveys, the preserved sequence from island-arc volcanics to chert, via reworked volcanics, is indicative of intraoceanic remnant arc, because the sequence suggests an inactive arc isolated within a pelagic environment before its accretion. The age of a subducting oceanic crust can be discontinuous before and after a remnant-arc subduction, resulting in abrupt changes in accretion style and metamorphism, as seen in Cretaceous Hokkaido. Subduction of such an intraoceanic remnant arc suggests that the subducted oceanic plate in the Cretaceous was not an extensive oceanic plate like the Izanagi and/or Kula Plates as previously believed by many authors, but a marginal basin plate having an arc–back-arc system like the present-day Philippine Sea Plate.     

Evolution history of the Hidaka-oki (offshore Hidaka) basin in the southern central Hokkaido, as revealed by seismic interpretation, and related tectonic events in an adjacent collision zone

Off the southern coast of Hokkaido the Hidaka-oki (offshore Hidaka) basin has developed on the western flank of a collision suture under the influence of long-standing compressional plate motion and provoked tectonic stresses around the northwestern Pacific rim throughout the late Cenozoic. The basin forming history of the Japan arc and Kuril arc collision zone is described on the basis of seismic reflection data interpretation. We identify two stages of basin formation: the older (late Oligocene-Miocene) faulted en echelon graben (pull-apart basin) and younger (Plio-Pleistocene) regional downwarping. Paleoenvironmental changes recorded within the fore-arc sediments indicate that the older basin filled up by the late Miocene. We inferred the volumes of the distinctive basins from the depth-conversion of seismic data, which suggest episodic uplifts and massive erosion of the Hidaka Mountains in the middle-late Miocene and the Plio-Pleistocene. Estimated sediment supply rates into the basins have a similar level for the both stages. Cause of an episodic uplift in the older stage is attributed to the delayed opening of the Japan Sea. The eastern Eurasian margin underwent N-S right-lateral faulting at 25 Ma as a result of rifting of the Kuril back-arc basin. Formation of the Japan Sea back-arc basin since the early Miocene (ca. 20 Ma) caused eastward motion of the western Hokkaido block and transpressive regime along the pre-existing N-S shear deformation zone.     

Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks,south‐central Hokkaido,Japan

Geological observations in the Horoman area, south‐central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east‐dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top‐to‐the‐west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top‐to‐the‐east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.     

Formation and deformation processes of late Paleogene sedimentary basins in southern central Hokkaido,Japan: Paleomagnetic and numerical modeling approach

Formation and deformation processes of the late Paleogene sedimentary basins related to a strike–slip fault system in southern central Hokkaido are described by a combination of paleomagnetic study and numerical analysis. After correction of the Miocene counter‐clockwise rotation associated with back‐arc opening of the Japan Sea, paleomagnetic declination data obtained from surface outcrops in the Umaoi and Yubari areas show significant easterly deflections. Although complicated differential rotation is anticipated as a result of recent thrust movements, clockwise rotation in the study areas is closely linked with development of the Paleogene Minami‐naganuma Basin as a pull‐apart depression along the north–south fault system. Numerical modeling suggests that 30 km of strike–slip is required to restore the distribution and volume of the Minami‐naganuma Basin. The relative slip rate on the long‐standing fault system is about 10 mm/yr, which corresponds to global‐scale plate motion. It has inevitably caused regional rearrangement of the eastern Eurasian margin. A rotation field simulated by simplified dextral motion using dislocation modeling basically accords with the paleomagnetic data around the pull‐apart basin.     

Tectonic implications of paleomagnetic data from upper Cretaceous sediments in the Oyubari area, central Hokkaido, Japan

Abstract   Paleomagnetic studies provide constraints on the geometric configuration of the eastern Eurasian margin on geological time scales. Characteristic remanent magnetization components were isolated from eight sites by progressive demagnetization executed on samples from 25 sites in the Oyubari area, central Hokkaido where the Late Cretaceous Yezo Group is distributed. After tilt-correction, all sites show normal polarity site-mean directions, and well-clustered directions pass a positive fold test and a correlation test. Planktonic foraminifera indicate an age range of Cenomanian to Turonian, and the studied section is correlated to the geomagnetic polarity chron C34n. Reliable formation-mean directions that have been corrected for post-depositional shallowing (D = 7.5°, I = 65.9°, α₉₅ = 6.6°) are characterized by inclination data indicative of no significant latitudinal translation since the Late Cretaceous. Central Hokkaido has, therefore, been situated adjacent to easternmost Mongolia including Sikhote Alin around the present latitude since the Late Cretaceous. Declination data require significant differential rotation between Hokkaido and the eastern Asian margin, which may be indicative of rearrangement of crustal blocks along the continental margin.     

Tectonic and sedimentary processes at the submarine Antique Ridge and the accretionary wedge of Negros (Sulu Sea, Philippines): Results of an underwater television and photographic survey by RV 'Sonne'

Abstract Numerous Neogene/Quaternary marl outcrops of the submarine Antique Ridge and southern Negros accretionary complexes (Sulu Sea, Philippines) were formed by an oversteepen-ing of the slope by the collision with the Cagayan Ridge and Cuyo Platform and also by erosion.
The outcrops exhibit distinct joint systems that were developed under compressional stress parallel to an east-northeast subduction of the southeast Sulu Basin complex under the Panay-Negros Fore-Arc and Arc Complexes during the Late Miocene/earliest Pliocene. Typical bc-(longitudinal) joints following the axial trend of the subduction zone, hkO (diagonal) shear joints, and ac-(transverse) joints were formed. The regional stress in south-southeast, which has changed to northeast since the Early Pliocene, has caused an uplift of the accretionary complexes and a clockwise rotation of the subduction/collision zone axis of the Antique Ridge complex from a more northern direction to NNE. Consequently the pre-existing joint system has also rotated for 10° to 20°. A strike-slip motion parallel to this axis as a consequence of the NE collision may have been accommodated within the accretionary complex by the bc-joints.
Some bedding-plane parallel white veins or layers may be related to calcium carbonate precipitation via oxidation of methane which was probably carried by migrating fluids along shear zones.
Downslope, sediment transport as well as trench-parallel sediment transport in southerly directions is still going on, indicating active tectonic oversteepening of the slopes of the accretionary complexes as well as flowing water, possibly of intermediate water from the Northwest Sulu Basin into the Southeast Sulu Basin via the Panay Canyon.     

Debris flow and slide deposits at the top of the Internal Liguride ophiolitic sequence, Northern Apennines, Italy: A record of frontal tectonic erosion in a fossil accretionary wedge

Abstract In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a late Jurassic–early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite and can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current-derived deposits, whereas the second group consists of thin-bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of materials, from the accretionary wedge front, that was reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. This interpretation can be also applied for the origin of analogous deposits of Western Alps and Corsica.     

A comparison of stream bank erosion processes on forested and moorland streams in the Balquhidder Catchments,central Scotland

Stream bank erosion rates measured over a two-year period on a moorland and a forested stream in the Institute of Hydrology's Balquhidder Paired Catchments in central Scotland were compared. Bank erosion rates are generally higher on the mainstream of the moorland catchment and highest in wintger on both streams. Bank erosion is correlated with the incidence of frost: minimum temperatures measured on stream banks of the forested stream were an average of 3·7°C higher than on stream banks both outside the forest and on the moorland stream. This makes the incidence of frost on forested stream banks half as frequent. Volumes of material eroded from the mainstreams were combined with bulk density measurements and it is estimated that erosion of the mainstream banks is contributing 1·5 and 7·3 per cent of the sediment yield of the forested and moorland catchments, respectively. Analysis of the vertical distribution of erosion on the banks of both streams suggests an undercutting mechanism which is more pronounced in the moorland stream. The influence of trees on bank erosion and possible implications for the management of forest streams are discussed. © 1997 by John Wiley & Sons, Ltd.     

Early exhumation of the collisional orogen and concurrent infill of foredeep basins in the Miocene Eurasian–Okhotsk Plate boundary, central Hokkaido, Japan: Inferences from K–Ar dating of granitoid clasts

Abstract   Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na₂O + K₂O/Al₂O₃ in molecule) versus A/CNK (Al₂O₃/CaO + Na₂O + K₂O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido.     

Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid-oceanic ridge basalts

Abstract   The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene-rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid-oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower-Ti trend than MORB in an FeO*/MgO-TiO₂ diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO₂−10MnO−10P₂O₅ diagram. In a MORB-normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid-oceanic ridge basalt (N-MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N-MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO₂ and TiO₂−10MnO−10P₂O₅ diagrams.     

Zircon SHRIMP U-Pb ages and trace element geochemistry of the Kuhai gabbro and the Dur''''ngoi diorite in the southern east Kunlun tectonic belt, Qinghai, Western China and their geological implications

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A'nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur'ngoi diorite in the Kuhai-A'nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur'ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur'ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A'nyêmaqên suggests that the southern margin of the "Qilian-Qaidam-Kunlun" archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.