Cretaceous Research: Paleolatitudes and Northward Migration of Crustal Fragments in the NW Pacific Inferred from Paleomagnetic Studies

Lateral migration of the Oshima and Sorachi-Yezo Belts within south central Hokkaido was quantitatively evaluated by means of paleomagnetic analyses in order to identify allochthonous blocks on the northwestern Pacific margin. The remanence stability of the Late Jurassic to Early Cretaceous voluminous igneous succession of the Kumaneshiri and Sorachi Groups and the overlying forearc sediments of the Cretaceous Yezo Group was evaluated through rock magnetic experiments. Twelve of the sites yielded characteristic primary components residing in mixtures of titanomagnetite and hematite having various mixing ratios. After an appropriate correction of inclinations’ shallowing of the post-depositional detrital remanent magnetization (pDRM) based on anisotropic acquisition experiments of the isothermal remanent magnetization (IRM), we confirmed significantly shallow inclinations even for the flattening-corrected data set, implying northward transportation after the emplacement. Based on comparisons to expected paleomagnetic directions calculated from contemporaneous reference poles, we conclude that the allochthonous blocks, including south central Hokkaido, migrated northerly during the Early Cretaceous. Previous investigations of paleomagnetism and numerical modeling of burial processes of sedimentary basins indicate that some crustal blocks in Hokkaido and NE Japan experienced delayed transportation and eventually amalgamated with the mother continent by the end of the Paleogene

Stratigraphic and Petrological Insights into the Late Jurassic– Early Cretaceous Tectonic Framework of the Northwest Pacific Margin

Late Jurassic–Early Cretaceous volcano‐sedimentary sequences in the Sorachi, Kumaneshiri, and Yezo groups are exposed in central Hokkaido. The sequences are considered to reflect the Late Mesozoic tectonic history of the northwest Pacific continental margin. Based on the stratigraphic and petrological characteristics of igneous and volcaniclastic rocks of the Sorachi, Yezo, and Kabato groups, Late Jurassic–Early Cretaceous tectonics in central Hokkaido can be divided into six stages. Stage I (Tithonian) is characterized by extensive eruption of tholeiitic basalt accompanied with andesitic volcaniclastic rocks and terrigenous deposits. Seafloor spreading or large igneous province formation occurred near an island arc and/or continent during this stage. In Stage II, island arc volcanic islands were constructed on the basaltic rocks formed during Stage I. Stage III (latest Berriasian‐Valanginian) is characterized by the formation of pull‐apart basins accompanied by seafloor spreading. Widespread upwelling of the asthenosphere below central Hokkaido may have occurred during this stage. After the cessation of in situ volcanism in Stage IV (Hauterivian), submarine island arc volcanism reoccurred in Stage V (Barremian). In Stage VI (Aptian–Campanian), typical active continental margin volcanism occurred and voluminous granitic batholiths were formed in western Hokkaido

Tectonic Synthesis: A Plate Reconstruction Model of the NW Pacific Region Since 100 Ma

Based on the results of interdisciplinary study from Chapters 1–4, a plate tectonic model of the northwestern Pacific region since 100 Ma is presented in this chapter. The evolution of the Pacific margin is viewed as a longstanding history of migration/amalgamation of allochthonous blocks onto the subduction zone. Such a process inevitably provoked diverse tectonic events, spatiotemporal positions of which have been discussed in this book. In order to reconcile paradoxical discrepancies in the docking process of arc fragments, the authors introduce a marginal sea plate with a spreading center that was alive in the Cretaceous. Oblique subduction of the ridge caused specific migratory igneous activity along the rim of the overriding plates, together with flips of shearing direction. Arc-trench systems on the eastern and western sides of the marginal sea plate developed following different timelines and were eventually mixed up during the plate’s closure that prompted formation of a coincident Oligocene clinounconformity widespread on the Eurasian margin. Since the demise of the hypothetical plate, the tectonic regime of the northwestern Pacific margin has been controlled by the growth, namely, the rotational history and modes of convergence of the Philippine Sea Plate

Distribution of Recent Benthic Foraminifera off Western Costa Rica in the Eastern Equatorial Pacific Ocean

Benthic foraminifera provide essential information for paleobathymetric reconstructions. However, the modern distribution of benthic foraminifera, especially at depths below 1000 meters below sea level (mbsl), is still obscure in the offshore regions near Central and South America. To characterize the bathymetric scale in the eastern equatorial Pacific Ocean, we examined the depth distribution of benthic foraminifera using piston core samples taken off the coast of Costa Rica. Foraminiferal assemblages vary according to water depth: 1) U1 (mainly composed of Ammonia beccarii, Cancris sagra, Elphidium tumidum, Hanzawaia concentrica, Pseudononion basispinata, and Planulina exorna) represent inner shelf faunas (shallower than 50 mbsl). 2) U2 (mainly composed of Ammobaculites foliaceus, Bolivina striatula, Cassidulina minuta, Hanzawaia concentrica, Uvigerina incilis, Bulimina denudata, and Cancris sagra) is correlated with mid-shelf depth assemblages, from 50 to 100 mbsl. 3) U3 (mainly composed of Uvigerina incilis, Hanzawaia concentrica, Angulogerina semitrigona, Bolivina acuminata, Bolivina bicostata, and Cibicorbis inflatus) is assigned to outer shelf assemblages from 100 to 200 mbsl. 4) U4 (mainly composed of Bolivina humilis, Bolivina seminuda, Bolivina subadvena, Cassidulina tumida, Epistominella obesa, Angulogerina carinata, and Cibicorbis inflatus) is the upper bathyal faunas (200–600 mbsl). 5) U5 (mainly composed of Brizalina argentea, Uvigerina peregrina, Uvigerina auberiana, Brizalina seminuda, Bulimina striata, Epistominella smithi and Globocassidulina subglobosa) is the mid-bathyal faunas (600–1000 mbsl). 6) U6 (mainly composed of Uvigerina auberiana, Uvigerina peregrina, Brizalina argentea, Bulimina mexicana, Cassidulina carinata, Epistominella smithi, and Lenticulina cushmani) represents the lower bathyal assemblage (1000–2000 mbsl). 7) U7 (mainly composed of Uvigerina auberiana, Brizalina argentea, and Eubuliminella tenuata) represent upper abyssal faunas (2000–3000 mbsl). 8) U8 (mainly composed of Glomospira sp. A, Lagenammina arenulata, Chilostomella oolina, Hoeglundina elegans, Melonis barleeanum, Nonion affine, Oridorsalis umbonatus, Pullenia bulloides, and Uvigerina proboscidea) is characterized by deep-water cosmopolitan faunas (deeper than 3000 mbsl). On the basis of a comparison with several environmental parameters, dissolved oxygen concentrations are likely to be the most effective factor controlling foraminiferal depth distributions in the eastern equatorial Pacific especially below the oxygen minimum zone (OMZ). Around OMZ, nitrate concentration also might be related with the benthic assemblage due to the nitrate respiration

Late Jurassic–Early Cretaceous intra-arc sedimentation and volcanism linked to plate motion change in northern Japan

The Sorachi Group, composed of Upper Jurassic ophiolite and Lower Cretaceous island-arc volcano-sedimentary cover, provides a record of Late Jurassic–Early Cretaceous sedimentation and volcanism in an island-arc setting off the eastern margin of the Asian continent. Stratigraphic changes in the nature and volume of the Sorachi Group volcanic and volcaniclastic rocks reveal four tectonic stages. These stages resulted from changes in the subduction direction of the Pacific oceanic plate. Stage I in the Late Jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back-arc basin. Slab roll-back caused rifting and sea-floor spreading in the supra-subduction zone along the active Asian continental margin. Stage II corresponded to the Berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island-arc chemical compositions. At the beginning of this stage, movement of the Pacific oceanic plate shifted from northeastward to northwestward. During Stage III, in the Valanginian, submarine basaltic volcanism was followed by subsidence. The Pacific oceanic plate motion turned clockwise, and the plate boundary between the Asian continent and the Pacific oceanic plate changed from convergent to transform. During Stage IV in the Hauterivian–Barremian, in situ volcanism ceased in the Sorachi–Yezo basin, and the volcanic front migrated west of the Sorachi–Yezo basin

Recent Advances in Research on Terrestrial and Marine sequences from the mid-Cretaceous Oceanic Anoxic Events (OAEs).

No abstract available

Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan

The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided into four zones extending NNW to SSE, from east (Zone I) to west (Zone IV), based on lithofacies and areal distribution. Zones I and III consist of aphyric tholeiite resembling back-arc basin basalt (BABB), while Zone II is characterized by the coexistence of BABB-like tholeiite along with high-Mg andesite. Zone IV has a different lithology from the other zones, and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite (IAT) type andesitic subaqueous pyroclastic deposits and terrigenous sediments. These stratigraphic and petrological characteristics of the Gokurakudaira Formation cannot be explained by the oceanic plateau or mid-oceanic ridge models, but they could correspond to the marginal sea model, as in the Lau Basin. Therefore, we conclude that the Horokanai Ophiolite was formed in the Late Jurassic in a marginal basin above a supra-subduction zone on the margin of the Asian continent