Hidaka Mountains Research Articles

Delamination structure imaged in the source area of the 1982 Urakawa‐oki earthquake

The Kuril arc collides with the northeast Japan arc in the southern part of Hokkaido, Japan. 3‐D tomographic inversion of data from a dense network of sensitive ocean‐bottom seismographs and land stations has allowed imaging of previously unseen details of the arc‐arc collision structure. A low velocity body dips gently southwestward, at depths of 35 to 45 km, from east of the Hidaka Mountains to the source area of the 1982 Urakawa‐oki destructive earthquake (Ms 6.8). The low velocity body is the lower half of the lower crust of the Kuril arc, which must have been delaminated by the collision. We believe that the continuing collision of the delaminated lower crust with the northeast Japan arc resulted as an episode of aseismic slow slip prior to the 1982 Urakawa‐oki earthquake as well being the reason for the high seismic activity in this region.

北海道日高山脈ポロシリ岳周辺における山岳永久凍土環境 気温と地表面温度観測からの考察

北海道日高山脈ポロシリ岳周辺における山岳永久凍土環境 気温と地表面温度観測からの考察

Research on the relation between Cross-dykes and Salvelinus fishes in northeast of Hidaka mountains

Research on the relation between Cross-dykes and <i>Salvelinus</i> fishes in northeast of Hidaka mountains

日高衝突帯‐前縁しゅう曲・衝上断層帯の地殻構造

The collision of the Kuril arc against the Northeast Japan arc has made a conspicuous crustal structure from the Hidaka mountain range to the Ishikari-Tomakomai lowland, Hokkaido, Japan, since Miocene. Recent advance of deep seismic reflection studies has revealed that the Kuril arc lithosphere is delaminated at about 23km deep in the lower crust in the Hidaka collision zone. The upper half of the lithosphere (upper crust+upper portion of the lower crust) is thrust westward on the Northeast Japan arc, whereas the lower half (lower portion of the lower crust+upper mantle) descends down. The wedge of the Northeast Japan arc lithosphere intrudes eastward into the delaminated Kuril arc lithosphere. The structure is called as a “delamination-wedge structure”. In the western foreland area of the Hidaka mountain range, the west-verging fold-and-thrust belt occurs more than 70km wide involving the pre-Tertiary strata. The activity of the belt has shifted westward since the initiation of the collision. The shortening length in the foreland fold-and-thrust belt is about 60km, which is nearly equal to the delamination-wedge length toward the colliding direction. The two lines of evidence mentioned above, the westward shift of the activity and the coincidence of both lengths, indicate that the fold-and-thrust belt has been growing associated directly with the formation of the delamination-wedge structure.

Delamination‐wedge structure beneath the Hidaka Collision Zone, central Hokkaido, Japan inferred from seismic reflection profiling

In the Hidaka Collision Zone of Hokkaido, northern Japan, the Kuril island arc collides with the northeast Japan arc. In order to better understand the collision process, new high resolution information about lithospheric structure was obtained by means of a series of seismic reflection surveys. These seismic profiles reveal distinct zones of seismic lamination in the lower crust of the Kuril arc. The upper portion of the lower crust is characterized by numerous east‐dipping reflectors. In contrast, west‐dipping reflectors dominate the lower part of the lower crust. From this reflector configuration, the lower crust of the Kuril arc is interpreted to be delaminated by the collision. The geometry of delamination is consistent with other geophysical data, as well as the peak metamorphic grades exposed in the Hidaka mountains. As earthquakes indicate that delamination here is ongoing, the Hidaka Collision Zone represents an active model for continental growth by arc accretion with delamination of lower arc crust.

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of ∼7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.

Collision structure in the upper crust beneath the southwestern foot of the Hidaka Mountains, Hokkaido, Japan as derived from explosion seismic observations

The P-wave velocity structure of the upper crust beneath a profile ranging from Niikappu to Samani in the southwestern foot of the Hidaka Mountains, Hokkaido, Japan was obtained through analysis of refraction and wide-angle reflection data. The mountains are characterized by high seismicity and a large gravity anomaly. The present profile crosses the source region of the 1982 Urakawa-oki earthquake ( M s 6.8). The length of the profile is 66 km striking northwest and southeast. Along the profile, 64 vertical geophones were set up and 5 shot points were chosen. For each shot, a 400–600 kg charge of dynamite was detonated. The studied area is composed of four major geological belts: Neogene sedimentary rocks, the Kamuikotan belt, the Yezo Group, and the Hidaka belt. The measurement line crosses these geological trend at an oblique angle. The structure obtained is characterized by remarkable velocity variations in the lateral direction and reflects the surface geological characteristics. A thin, high-velocity layer (HVL) was found between low-velocity materials in the central part of the profile, beneath the Kamuikotan Metamorphic Belt, at a depth ranging from 0.5 to 6 km, overthrusting toward the west on the low-velocity materials consisting of Neogene sedimentary rocks, and forming gentle folds. Outlines of the velocity structure of the Hidaka Mountains yielded by other studies have shown a large-scale overthrust structure associated with the collision of the Outer Kurile and the Outer Northern Honshu Arcs. The shallow velocity structure inferred by the present study showed a similar (although small scale) overthrust structure. The obtained structure shows that the composite tectonic force, comprising westward movement of the Outer Kurile Arc and northward movement of the Outer Northern Honshu Arc, plays an important role in the evolution of the tectonic features of the crust and upper mantle in a wide depth range beneath the Hidaka Mountains.

北海道日高山脈周辺のコーダ&lt;i&gt;Q&lt;/i&gt;値

北海道日高山脈周辺のコーダ&lt;i&gt;Q&lt;/i&gt;値

Magmatic Processes Observed in a Cross Section of Juvenile Continental Crust Exposed in the Hidaka Mountains, Hokkaido, Japan

Magmatic Processes Observed in a Cross Section of Juvenile Continental Crust Exposed in the Hidaka Mountains, Hokkaido, Japan

Collision orogeny at arc‐arc junctions in the Japanese Islands

Abstract In the Japanese Islands, collision tectonics are operating at arc‐arc junctions in three regions: Hokkaido, Central Japan and Kyushu. Hokkaido is situated at the junction of the Kuril and Northeast Japan Arcs. The Kuril fore arc sliver collides with the Northeast Japan Arc, and the lower crust of the Kuril Arc thrusts upon the fore arc of the Northeast Japan Arc in Hokkaido. Outcrops of the lower crust are observed in the Hidaka Mountains in the fore arc of the junction area. Central Japan is in the juncture area among the Northeast Japan, Izu‐Bonin, and Southwest Japan Arcs. The Izu‐Bonin arc is colliding against the Honshu mainland, which has been bent by the collision. Kyushu is a juvenile collision area between the Southwest Japan and Ryukyu Arcs. The fore arc of the Southwest Japan Arc is starting to underthrust beneath the Kyushu islands along the Bungo Strait, where shallow seismicity within the crust is active in terms of the collision. Collision tectonics are observed at most of the arc‐arc junctions in the circum‐Pacific orogenic belts and may be an important process contributing to the relatively rapid growth of new continental crust in subduction zones.

2.5-Dmodelling of elastic waves using the pseudospectral method

SUMMARY The pseudospectral method is a high-accuracy numerical modelling technique which requires less memory and CPU time than traditional numerical modelling techniques, such as finite difference. However, full 3-D modelling is still computationally intensive. As we must compromise between realism and computational efficiency, we have developed a 2.5-D pseudospectral scheme for calculating 3-D elastic wavefields in media varying in two dimensions. To demonstrate the feasibility of the scheme described in this paper, we use data from a refraction survey carried out in 1984 by the Explosion Seismology Group in the southeastern foothills of the Hidaka Mountains, Hokkaido, Japan.

A collision boundary imaged by magnetotellurics, Hidaka Mountains, central Hokkaido, Japan

Magnetotelluric (MT) data were acquired across the Hidaka Mountains, which are thought to have formed by collisional orogenic processes. A regional electrical strike of N18°W, consistent with surficial geology, was derived from impedance tensor decomposition. Two‐dimensional resistivity models were constructed by forward model fitting with preference for reducing the misfit of transverse magnetic mode. The final modeling imaged a collision boundary between Cretaceous forearc basin rocks and continental crust. The major features of the model are (1) a 5‐km‐thick surficial conductive layer corresponding to Cretaceous forearc basin and the underlying resistive basement that shallows and is exposed to the east where Sorachi formations, characterized by oceanic crustal rocks, crop out. Therefore we interpret the underlying resistive layer as oceanic crust. (2) A thin (500 m thick) conductive (3 Ω m) anomaly corresponds to serpentinite exposure in the Kamuikotan metamorphic belt. (3) Exposed deep crustal rocks in the Hidaka belt showed high resistivity (1000–2000 Ω m) at the surface. The subsurface westward extension inferred from the MT data is supported by seismic refraction data. (4) In the Hidaka Mountains, a relatively conductive (500–1000 Ω m) layer implying accretionary prism sequences, was inferred at 2‐to 10‐km depth beneath a more resistive (1000–2000 Ω m) surface layer. (5) A wedge‐shaped resistivity anomaly was found at 5‐to 50‐km depth beneath the Hidaka belt. The resistivity of this wedge decreases eastward from 30,000 Ω m to 2000 Ω m, consistent with the surface decrease of metamorphic grade. The spatial distribution is also consistent with a seismic high‐velocity anomaly. (6) The western side of the resistive wedge is fringed by a dipping conductor. Correlations with a low‐velocity anomaly and high seismicity suggest that this conductor is an enhanced porosity region, and that it may be working as a detachment zone at the collision boundary. The high resistivity contrast between oceanic crust and high‐temperature metamorphosed continental crust permitted us to image the interfingered complex structure beneath the Hidaka Mountains from 300‐m to 50‐km depth.

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

We have applied an inverse method to P and S wave arrival time data observed at 52 seismic stations for 349 local earthquakes in order to estimate a three-dimensional (3-D) velocity structure beneath northern Japan. The method is characterized by a simultaneous determination of the two-dimensional depth distributions of velocity boundaries, 3-D velocity distribution and station corrections as well as hypocenter parameters of earthquakes. The Moho discontinuity under eastern Hokkaido is in-clined from a depth of 32 km beneath the Hidaka Mountains to 20 km beneath the Konsen Plateau, while the thick crust with a thickness of 30-36 km distributes widely in western Hokkaido and the Tohoku region. The dip direction of the Moho in eastern Hokkaido is approximately perpendicular to the range of the Hidaka Mountains. The upper boundary of the subducting Pacific plate distributes in the depth range from 50 to 170 km in the surveyed area and the dip angle of the plate boundary in eastern Hokkaido (the Kurile arc) is slightly larger than that in the Tohoku region (the northeastern Japan arc). Estimated P (S) wave velocities are 5.8-7.0 km/s (3.4-4.1 km/s) in the crust, 7.4-8.0 km/s (4.2-4.5 km/s) in the upper mantle, and 8.1-8.6 km/s (4.6-4.9 km/s) in the plate, respectively. The velocities in the crust beneath northern Hokkaido are lower than those beneath eastern Hokkaido and the Tohoku region. The upper mantle has velocities which gradually increase with depth, and the subducted plate contains a high velocity zone with a P wave velocity faster than 8.3 km/s. VP/VS ratios derived from P and S wave velocities generally range from 1.75 to 1.80 in the crust and upper mantle, while the ratios in the high velocity zone within the plate are less than 1.75. These results suggest that the plate is composed of two layers: the first layer is a low-velocity and high-VP/VS zone with a thickness less than 20 km, and the second layer is a thick high-velocity and low VP/VS zone. The estimated plate thickness is about 90 km in the Kurile arc and about 110 km in the northeastern Japan arc. The relocated hypocenter distribution beneath Hokkaido clearly shows the double seismic plane in the subducting plate. The upper seismic plane is located in the first layer with a low velocity and the lower seismic plane is in the second layer with a high velocity. The seismic activity in the upper seismic plane is high in the depth range from 60 to 90 km, while that in the lower seismic plane is high at depths deeper than 100 km. The double seismic plane joins at depths from 90 to 130 km.

Geology of the Idonnappu Belt, central Hokkaido, Japan: Evolution of a Cretaceous Accretionary Complex

The Cretaceous Idonnappu Belt, located along the western Hidaka Mountains of central Hokkaido Island in Japan, records evidence of west to northwest directed underthrusting of oceanic crust. The Idonnappu fault divides the Idonnappu Belt into two subbelts; the western Oku‐niikappu (ON) subbelt and the eastern Koiboku (KO) subbelt. The ON subbelt is dominated by a melange facies. It includes various thicknesses of pillow basalts, bedded radiolarian cherts, limestones, and greenish siliceous shales, all of which are intermixed with a highly sheared shaley matrix that displays a scaly cleavage. The KO subbelt is dominated by a thick flysch sequence and alternations of sandstone and shale with thin tectonic melange. The stratigraphic sequences within these belts usually young toward the west and display westward vergent structures. Outcrop‐ to microscopic‐scale structures in the melange zone of the Idonnappu Belt, however, suggest eastward vergence. Detailed biostratigraphic studies show that structural packages young toward the east ranging in age from Lower to Upper Cretaceous. These observations are consistent with an accretionary prism model in which oceanic crust is underthrust toward the west or north west. In the middle Miocene, a change to westward vergence was caused by uplift of the Hidaka Mountains.

A Rb-Sr geochronological study on the Early Cretaceous Monai volcanic rocks from Okushiri Island, Northeast Japan Sea.

Rb-Sr whole rock isochron age has been determined on the Early Cretaceous Monai volcanic rocks occurring in Okushiri Island, Northeast Japan Sea. Five samples of dolerite, andesite and rhyolitic welded tuff give a well defined whole rock isochron with an age of 101.3±4.8 Ma and an initial 87Sr/86Sr ratio of 0.70532±0.00007 (2σ). These results almost agree with the mineral and whole rock isochron ages and the initial Sr isotopic ratios determined by the previous workers for the pre-Tertiary granitoids occurring in Okushiri Island and the western margin of southwest Hokkaido. This facts indicate the concurrence of the pre-Tertiary volcanism and plutonism in these areas and the similarity in isotopic compositions of the source materials for the Monai volcanic rocks and these granitoids. The initial Sr isotopic ratio of the Monai volcanic rocks are much higher than those of the Early Cretaceous volcanic rocks from the region to the west of Hidaka Mountains and the eastern margin of the Kitakami Mountains. Comparative investigations of initial Sr isotopic ratios reported by the previous and present workers for the Early Cretaceous volcanic rocks and granitoids from the NE Japan arc and Hokkaido indicate that initial Sr isotopic ratios for these igneous rocks increase from the eastern tholeiitic volcanic zone to the western calc-alkaline volcanic-plutonic zone, in contrast to the across-arc variation of the Sr ratios of the Quaternary volcanic rocks from the NE Japan arc.

Arc deformation and marginal basin opening: Japan Sea as a case study

We discuss the opening mechanism of the Japan Sea in Miocene time using (1) tectonic and published paleomagnetic data along the eastern margin from the north of Hokkaido Island to Sado Island, (2) a mechanical model which is tested by small‐scale physical modeling, and (3) crustal structure and bathymetric features in the Japan Sea which constrain our kinematic model and preopening reconstructions. Our main conclusions are the following. The eastern margin of the Japan Sea was, as a whole, a dextral shear zone about 100 km wide. This conclusion is supported by the existence of a ductile dextral shear zone in Central Hokkaido (Hidaka Mountains) and associated brittle deformation in western Hokkaido and northeastern Honshu. The stress field during the opening (which ended about 12 Ma ago at the end of the middle Miocene) changes from right‐lateral transpression in the north to right‐lateral transtension in the south. The western margin, along the Korean peninsula, during the same period, also was an active dextral shear zone. Paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north. We propose a model of right‐lateral pull‐apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one. Small‐scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary (Pacific side) is free of stress. The opening stopped and compression subsequently began about 12 Ma ago. Finally, we show that the dextral shear, which is distributed over the whole Japan Sea area, is accommodated by N‐S trending right‐lateral faults and rotation of blocks located between these right‐lateral faults.

北海道日高山脈周辺地域の第四紀後期の地殻変動 河成地形面を用いた隆起量の算定の試み

The purpose of this paper is to investigate the amount and mode of crustal movement in the Hidaka mountains, Hokkaido Island. It is known that longitudinal proflies of almost all rivers have changed in accordance with periodic changes in climate and sea level during the Quaternary. In the upper reaches, an erosional river bed formed in the Last Interglacial Age correspond to the bottom of the Last Glacial fill terrace deposit, and its longitudinal profile is often parallel to that of the present one. Therefore the author hypothesized that homologic river profiles were formed in two periods with similar climatic conditions and sea level heights, corresponding to the Last Interglacial Age and the Post Glacial Age. Based on this hypothesis, the difference in height between the Last Interglacial buried valley bottom and the present river bed is explained by the amount of uplift during this period.Fluvial and marine terraces are well developed, and their surfaces are covered with a number of marker tephra layers around the Hidaka mountains (Table 2). Figure 9 shows the difference between the height of the Last Interglacial buried valley bottom and that of the present river bed in the study field. The difference in height between the Last Glacial terrace surface and the Penultimate Glacial terrace surface (Fig. 10) is also useful for cross-checking of the data on height differences between interglacial profiles.The characteristics of crustal movement in the study field are summarized as follows. Almost all of the region on the east side of the Umaoi hills has a tendency to uplift. The uplift rate of the mountainous and hilly region on the west side of the Hidaka mountain range is estimated to have been 0.2-0.4mm/yr., and the rate increases toward the western Hidaka coast. This tendency is also supported by evidence that the shoreline heights of the Last Interglacial marine terrace along the Hidaka coast decrease toward the Erimo cape, where the present coast traverses the axis of the Hidaka mountain range. The east side of the Hidaka mountain range and the Tokachi plain have slowly upheaved at the rate of 0.1mm/yr. Short wave deformation of fluvial terrace surfaces accompanied by rapid uplift (more than 0.7mm/yr.) is expected around the Yubari mountains.

日高山脈とその周辺の3次元Q構造

The three-dimensional Q structure down to 150 km beneath the Hidaka Mountains, Hokkaido, Japan was determined using an inverse method. Apparent Q values were determined along ray paths from 456 earthquakes to the eight stations operated by Research Center for Earthquake Prediction of Hokkaido University. The hypocenters of these earthquakes were located beneath the Hidaka Mountains, with magnitude of 3.4-4.0. This region was divided into 600 blocks and then absolute Q values of the individual blocks were determined from these apparent Q values by using the inverse technique with least-squares method and constraints.In order to determine the Q values, the pulse broadening method (PB method) was used. We did not use spectral methods which need a time window with, at least, a few seconds from P arrivals, because seismograms observed at the eight stations have many phases of unknown origins which appear just after P arrivals. On the other hand, the PB method requires only one pulse of the first P arrivals; the Q values obtained by the PB method are hardly affected by scattering and reflections. The Q values determined by the PB method, however, involve considerable errors because of the narrow time window. We introduced some constraints on the Q values in the inversion process.The result shows that an extremely low-Q (Q=50) region is located beneath the area from the western part to the central part of the Hidaka Mountains and it extends to the depth of 100 km, and that relatively high-Q material exists beneath the summit of the Hidaka Mountains. Comparisons of the obtained three-dimensional Q structure with a three-dimensional Vp structure and with a seismic activity map, show the low-Q zones correspond to low-Vp and high seismicity zones.It is concluded that the low-Q zone is not caused by high temperature but composed of highly fractured rocks because no volcanic activity is found and rather low heat flow was observed in this region.

Teleseismic P-wave travel time and amplitude anomalies observed in Hokkaido region, Japan.

P-wave travel time and amplitude anomalies from 59 teleseismic earthquakes are analyzed for 17 stations of a seismic network in Hokkaido region, Japan. Relative travel time and log amplitude variations within the network are parameterized by an azimuth-independent term (average), and first and second azimuthal terms. For the travel time, the azimuth-independent terms, which represent shallow near-station velocity anomalies, show an obvious geographical variation and a significant correlation with gravity (Bouguer) anomalies. The azimuth-independent terms vary from - 0.73 to 0.58 s. The slow directions of the first azimuthal terms for stations surrounding the Hidaka Mountains point consistently toward an area near the axis of the mountains, suggesting the existence of a low-velocity zone in the uppermost mantle beneath the Hidaka Mountains. For the amplitude, the azimuth-independent terms have large positive values (amplification) at stations where the geological setting is young, i.e., Quaternary. The first azimuthal terms for stations surrounding the Hidaka Mountains show some indication of large amplitudes for the incidence azimuth in the direction of the mountains. If the azimuth-independent terms are subtracted from the observed values, the travel time residuals and log amplitudes show some correlation, suggesting the focusing and defocusing of rays by the heterogeneity in the uppermost mantle beneath the network.

北海道山麓の地表気温に対する日高山脈の影響

北海道山麓の地表気温に対する日高山脈の影響