Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area consist of five lithological units (B-, MN-, MH-, PT- and T-units), which are combined into two geologic complexes : the greenstone-dominant Naizawa Complex and the clastic-dominant Horobetsugawa Complex. Radiolarian ages were determined for each unit. The Naizawa Complex is an alternating stack of slabs of tholeiitic greenstone (B-Unit), and fragments of Triassic seamount with Triassic - Lower Cretaceous ocean floor to trench-fill deposits (MN-Unit). The Horobetsugawa Complex is a pile of Upper Cretaceous ocean floor to trench-fill deposits (PT-Unit), with intercalated Lower Cretaceous to Upper Cretaceous submarine slide and debris flow deposits (MH-Unit) and latest Cretaceous - Paleocene slope basin deposits (T-Unit).Original disposition and relative tectonic movement of each unit were determined using illite crystallinity compared with fossil and K-Ar ages. These show post-accretion uplift, thrusting, and rearrangement by later right-lateral duplexing. A near-vertical section of the entire accretionary complex is exhumed in the Idonnappu Zone.The Naizawa Complex was formed by Early Cretaceous collision of subducting seamount with forearc ophiolite. The Horobetsugawa Complex formed as a clastic-dominant accretionary body in the Late Cretaceous, synchronous with the wane of forearc-basin sedimentation.
Stratigraphic horizons marked by the last occurrence of Spirosigmoilinella compressa and first occurrence of Miliammina echigoensis (benthic foraminifera) have been used as important biohorizons in the Neogene of the Sea of Japan region. We have elucidated the ages of these horizons on the basis of radiolarian biostratigraphy in seven onshore sections of Akita Prefecture. Age of the last occurrence of S. compressa is determined as 5.4 Ma, while that of the first occurrence of M. echigoensis as 5.2 Ma. We further reexamined the biostratigraphic ages of published data regarding cores from the deepest part of the Sea of Japan obtained by ODP Legs 127/128. The results are consistent with the ages which we obtained. Furthermore, these determined ages are close to the age of the Miocene / Pliocene boundary, 5.32 Ma. Therefore these foraminiferal biohorizons are significant in recognizing the Miocene/Pliocene boundary in the Sea of Japan region.Based on compilation of previous studies, the upper depth limit distribution of S. compressa is assumed to have been deeper than the Lower Middle Bathyal Zone, and than that of M. echigoensis from the Middle Bathyal Zone to the deepest part, which indicates their overlapping paleobathymetric ranges. Our compilation also indicates that S. compressa occurred preferably in strata deposited under a suboxic paleoenvironment, while M. echigoensis under an oxic one. Consequently, the disappearance of S. compressa and subsequent appearance of M. echigoensis suggest a change of the bottom water condition in the Sea of Japan. This paleoenvironmental change was presumably caused by the post-Messinian raised sea level in the earliest Pliocene, which enhanced the exchange of seawater between the Pacific and the Sea of Japan.
Fault-generated pseudotachylytes showing reddish, greenish and grayish color have recently been found from the mylonitized Ryoke Granites along the Median Tectonic Line (MTL). These pseudotachylytes are suggested to have a melt origin on the basis of petrographic observations of typical melt-quenched microstructures such as microlites and amygdales. Preferential melting of low melting point minerals is the most probable melting process. The absence of lithic fragments of hornblende and biotite, and systematic decrease of bulk SiO2 contents in the pseudotachylytes compared with their host rocks support this interpretation. These pseudotachylytes were formed after mylonitization and were post-dated by weak cataclasis resulting from movement of the MTL. Similar orientation of the MTL, fault veins of pseudotachylytes, and mylonitic foliation indicate that the deformation sequence of mylonite, pseudotachylyte, and cataclasite formation probably progressed under a single tectonic stress field. These pseudotachylytes are examples of the remnants of ancient faulting with frictional melting indicating seismic activity of the MTL.
We describe results of a paleomagnetic study for sedimentary rocks of the Plio-Pleistocene Sasaoka Formation in the Gojome area in Akita Prefecture, northeast Japan. Samples were collected from 26 sites over the Gojome Syncline, an open syncline with a nearly horizontal fold axis. Progressive thermal demagnetization revealed high-temperature characteristic remanences from 11 sites ; 5 sites on the lower part of the measured section had normal polarity, and 6 sites on the upper part were reversely magnetized. The incremental fold test showed that the best grouping of the remanence directions occurred when the syncline was 80% unfolded. Unfolding it at the same percentage climaxed the anti-parallelism of a set of normal remanences and that of reversed ones. The possibility that the rocks acquired the remanences during folding (synfolding remanence) seems to be unlikely from the stratigraphically ordered polarity sequence. Alternatively, it is suggested that strain has altered the initial angular relationship between the bedding plane and the remanence. For this reason the discussion on vertical-axis crustal rotation becomes only qualitative, but little rotation may be deduced from the northerly mean directions at all stages of unfolding. The normal-to-reversed polarity-reversal horizon at the middle part of the measured section is correlated, with the aid of available biostratigraphic information, to the Gauss-Matuyama chron boundary at about 2.6 Ma.
The uppermost part of the Yezo Supergroup in the Nakatonbetsu area, northern Hokkaido witnesses to the lithostratigraphy and magafossil biostratigraphy from the lower Campanian to Paleocene. It is subdivided into the Kotobuki Formation of the Upper Yezo Group (offshore siltstone), the Kamikoma (siltstone intercalated with fine sandstone layers), Hcitarozawa (sandy siltstone), Oku-utsunai (fine sandstone) and Utsunaigawa (sandy siltstone) Formations of the Hakobuchi Group in ascending order. The Hakobuchi Group is dominated by offshore sandy siltstone, in contrast with the Ashibetsu, Oyubari and Hobetsu areas in central Hokkaido representing the westerly marginal shallow-marine sandy facies of the Yezo forearc basin along the Northwestern Pacific rim. The shallowest facies is represented by HCS sandstone of the lower parts of the Heitarozawa and Oku-utsunai Formations formed on storm-dominated lower shoreface. In terms of newly collected ammonites and inoceramids, we can establish three inoceramid zones with the geologic age as follows ; Kotobuki and lower part of Kamikoma Formations : Sphenoceramus schmidti Zone (lower Campanian), upper Kamikoma Formation : Inoceramus shikotanensis Zone (lowest Maastrichtian), uppermost Kamikoma to lower Heitarozawa Formations : Sphenoceramus hetonaianus Zone (upper part of lower Maastrichtian), upper Heitarozawa Formation : an unnamed ammonite zone above the inoceramid extinction horizon (upper Maastrichtian). A sharp erosional surface between the Heitarozawa and Oku-utsunai Formations in thought to be an unconformable sequence boundary. The surface itself was represented by ravinement surface and covered by transgressive lags containing Glycymeris shell beds of shallow-marine storm origin. The Oku-utsunai and Utsunaigawa Formations show a transgressive facies succession, consisting of the lag deposits, HCS sandstone, bioturbated silty sandstone and sandy siltstone. This facies succession and the previous micropaleontological control suggest that the strata of the uppermost Maastrichtian to Danian including the K/T boundary seem to have been eroded away.
Elongated orthopyroxene porphyroclasts commonly form stretching lineations in deformed peridotites or metabasic rocks. Internal obliquity of (100) exsolution lamellae with respect to elongation direction is a candidate for a shear sense indicator in these rocks. However, it requires statistical analyses of shape and lattice preferred orientation of orthopyroxene porphyroclasts with reference to another reliable shear-sense indicator such as lattice preferred orientation (LPO) of olivine. We measured three geometrical parameters ; R : aspect ratio, θ : long axis orientation, φ : (100) exsolution lamellae orientation on the XZ (foliation normal, lineation parallel) plane, for 1100 porphyroclasts in 160 thin sections selected from 5 structural zones in the Horoman Peridotite Complex, Hokkaido, Japan.Bimodal distributions of internal obliquity are shown in R-φ and θ-φ diagrams of all structural zones. The equigranular zone and the internal shear zone (ISZ) show high contrast of aspect ratio between opposite internal obliquity. This asymmetry is explained by a higher R being attained by synthetic slip on the (100) slip plane, whereas the other peak consists of grains which were deformed by antithetic slip on (100) and rotated towards an unfavorable orientation for slip. The asymmetric features of geometry of elongated orthopyroxene porphyroclasts and LPO of olivine consistently indicate a top-to-the-south sense of shear. On the other hand, high R symmetric bimodal distribution of internal obliquity in the basal shear zone (BSZ), where LPO and shape preferred orientation (SPO) of olivine show apparent asymmetry (a top-to-the-north sense of shear), can not be explained by a single progressive shear deformation. But it implies inverted shear movement in the basal shear zone.Statistical analyses of internal obliquity and aspect ratio of elongated orthopyroxene porphyroclasts can be used as an independent shear-sense indicator for deformed peridotites. Furthermore, these analyses compared with the LPO of olivine can decipher a more complex kinematic history which can not be revealed only by olivine fabrics.
Stratigraphic, sequence stratigraphic and sedimentary environment analyses on the late Pleistocene-Holocene sediments of the eastern side of Osaka Bay, central Japan was carried out. The Suminoe cores taken from the Suminoe area on the western side of Osaka plain were used. Sedimentological, tephrochronological, AMS14C dating, paleontological and micro-paleontological methods were applied. We found seven sedimentary units (Sm-7 to Sm-1) and grouped them into two formations. The Uemachi Formation is composed of marine clay deposits (Sm-6) with shore face deposits at the basal part (Sm-7) with fluvial terrace deposits at the top part (Sm-5). The Namba Formation is a Holocene marine sandy shoreface deposit (Sm-3) with fluvial deposits probably of the last glacial period at the basal part (Sm-4) and delta-front deposits at the upper part (Sm-2). We recognized two sedimentary systems, such as bay-delta system (Sm-7 to Sm-6 and Sm-3 to Sm-2) and conglomeratic braided stream system (Sm-5 and Sm-4). Furthermore we grouped these cores into three depositional sequences (DS1 to DS3) since 130 ka.