Research ArticleSubhorizontal tectonic framework of the Horoman peridotite complex

and enveloping crustal rocks, south-central Hokkaido, Japaniar_714 458..469

HIROSHI YAMAMOTO,1* NATSUMI NAKAMORI,1 MASARU TERABAYASHI,2 HAFIZ UR REHMAN,1

MASAHIRO ISHIKAWA,3 YOSHIYUKI KANEKO4† AND TAKASHI MATSUI2

1Department of Earth and Environmental Sciences, Kagoshima University, Kagoshima 890-0065, Japan (email:hyam@sci.kagoshima-u.ac.jp), 2Department of Safety Systems Construction Engineering, Kagawa University,Takamatsu 761-0396, Japan, 3Graduate School of Environment and Information Sciences, Yokohama NationalUniversity, Yokohama 240-8501, Japan, and 4Faculty of Physical Sciences and Engineering, Meisei University,

Hodokubo 2-1-1, Hino-shi Tokyo 191-8506, Japan

Abstract Geological observations in the Horoman area, south-central Hokkaido, show thatthe Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about1200 m thick. The peridotite slab is intercalated into a gently east-dipping structure. Theunderlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes inthe sedimentary layers of the accretionary complex near the structural bottom of theperidotite slab indicate top-to-the-west (thrust) displacement. The overlying unit is com-posed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphicrocks). The boundary surface between the peridotite complex and metamorphic rocksforms a domal structure. Microstructures of sheared metamorphic rocks near the struc-tural top of the peridotite slab indicate top-to-the-east (normal) displacement. The resultscombined with previous studies suggest that the Horoman peridotite complex wasemplaced onto the Asian margin (Northeast Japan) during the collision between the Asianmargin and the Hidaka crustal block.

Key words: emplacement, Hokkaido, Horoman, peridotite, structure.

INTRODUCTION

‘Alpine-type’ peridotite bodies commonly occur inorogenic belts. The presence of Alpine-type peri-dotite indicates that dense peridotite bodies havebeen emplaced in the vicinity of less dense crustalrocks. The process and mechanism of peridotiteemplacement have widely been discussed frompetrological and structural aspects (e.g. Van derWal & Vissers 1996; Searle & Cox 1999; Platt et al.2003; Janák et al. 2006). The Horoman peridotitecomplex is a well-known peridotite body in theHidaka metamorphic belt, south-central Hok-kaido, Northeast Japan (Fig. 1). It has beenstudied in detail from mineralogical, petrological,

and microstructural points of view (e.g. Niida 1974;Takahashi 1992; Takazawa et al. 1992; Sawaguchi2004). However, these studies give little descrip-tion of the structural relationship between theperidotite body and its enveloping crustal rocks.Substantial observations on the periphery of theperidotite body are necessary before we give tec-tonic interpretations to its emplacement. We haveexamined structures of the boundaries betweenultramafic rocks of the Horoman peridotitecomplex and the crustal rocks. In this paper wepresent new data on the structural setup of theHoroman peridotite complex and discuss theirimplications to the tectonics of Hokkaido.

GEOLOGICAL SETTING

Hokkaido Island is considered to have beenformed by the collision and amalgamation of

*Correspondence.

†Present address: Faculty of Education, Meisei University, 2-1-1Hodokubo, Hino-shi, Tokyo 191-8506, Japan

Received 31 July 2009; accepted for publication 11 March 2010.

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several crustal blocks between the NortheastJapan Arc and the Kuril Arc (e.g. Jolivet et al. 1988;Kimura 1996; Arita et al. 1998). The Hidaka Moun-tains, together with the surrounding area, haveundergone collision-related deformation and modi-fication (cf. Arita et al. 1998). Thus, the south-central part of Hokkaido is called the Hidakacollision zone. This collision zone includes theHoroman peridotite complex and other metamor-phic rocks of the Hidaka metamorphic belt (Fig. 1).The Hidaka metamorphic belt extends NNW–SSEfor about 120 km, with a maximum width of about20 km. The general structure of the Hidaka meta-morphic belt is west-vergent and the metamorphicgrade increases from east to west up to granulite

facies pressure–temperature conditions (Komatsuet al. 1983; Osanai et al. 1991). In the southernmostpart of the Hidaka metamorphic belt, granulite-facies metamorphic zones are situated around theHoroman peridotite complex and their distributionextends about 10 km to the southeast (Shiba et al.1992). The western margin of the Hidaka metamor-phic belt is bordered by the Hidaka Main Thrustwhere the rocks of the Hidaka metamorphic belt onthe hanging wall are separated from an ophioliticsequence of the Poroshiri ophiolite (Miyashita1983; Jolivet & Miyashita 1985) and a Cretaceous–Paleogene accretionary complex of the Idon’nappubelt (Kiyokawa 1992; Ueda et al. 2000, 2001; Ueda2006). The eastern boundary of the Hidaka meta-morphic belt is not clear. Metasediments of theeastern part of the belt (structural upper) arelithologically continuous with the early Paleogenesedimentary sequence of the Nakanogawa Group(Nanayama 1992; Toyoshima et al. 1994). Komatsuet al. (1982) suggested that the Hidaka metamor-phic belt was a crustal fragment of either island arcor continent. Subsequent studies have consideredthis belt an exposed cross-section of magmatic arcand its tectonic evolution has been widely discussed(e.g. Osanai et al. 1991; Toyoshima et al. 1994).

Several separate bodies of ultramafic rocks aredistributed in the northern and southern areas ofthe Hidaka metamorphic belt (Komatsu & Nochi1966; Niida & Katoh 1978; Niida & Takazawa 2007).In the northern part of the Hidaka metamorphicbelt, five peridotite complexes (Uenzaru, Pankenu-ushi, Tottabetsu, Oku-Niikappu, Koibokushushibi-chari) are aligned from north to south along theHidaka Main Thrust (Niida & Takazawa 2007). Inthe southern part of the Hidaka metamorphicbelt, meanwhile, the Menashunbetsu and Horomanperidotite complexes are situated in the westernmargin of the metamorphic belt and three smallperidotite bodies (Pon-Nikanbetsu, Nikanbetsu,Abeyaki) are aligned from the eastern side ofthe Horoman peridotite complex to the southeastwithin the Hidaka metamorphic belt (Niida & Taka-zawa 2007). The Horoman peridotite complexis incomparably larger than the other ultramaficbodies in the Hidaka metamorphic belt.

The Horoman peridotite complex (Fig. 2) con-sists of a variety of ultramafic rocks (harzburgite,lherzolite, dunite, wehrlite) and minor gabbroicrocks. The lithological thickness of this complexis estimated to be more than 3000 m (Niida 1974)or 3700 m (Sawaguchi 2004). The ultramafic rocksof the Horoman peridotite complex have beenknown for their well-developed compositional

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layering and the limited influence of serpentini-zation. Many workers have conducted petrological,geochemical, and microtectonic studies on them(e.g. Niida 1984; Takahashi 1992; Takazawa et al.1992; Ozawa & Takahashi 1995; Sawaguchi &Takagi 1997; Yoshikawa & Nakamura 2000; Tora-maru et al. 2001; Sawaguchi 2004; Malaviarachchiet al. 2008). Despite numerous contributions to the

study of ultramafic rocks, the structural setup ofthe Horoman peridotite complex remains unclear.It is generally accepted that the Horoman peridot-ite complex is situated in the structural bottomof the Hidaka metamorphic belt (Komatsu et al.1982, 1983; Toyoshima et al. 1997). However, thereare specific interpretations discordant with thegeneral view. Several examples of geological maps

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and cross-sections show that the contact surfacesof the Horoman peridotite with the Hidaka meta-morphic rocks are high-angle faults (Niida 1974;Arita et al. 1978), medium to low-angle faults withthe Horoman peridotite on those hanging walls(Obata & Nagahara 1987; Komatsu et al. 1992;Sawaguchi 2004), or low-angle faults that sandwichthe Horoman peridotite on its top and bottombetween the Hidaka metamorphic rocks (Jolivet &Huchon 1989; Saheki et al. 1995). Discrepanciesamong these structural interpretations have notbeen discussed in previous papers.

CRUSTAL ENVELOPE OF HOROMANPERIDOTITE COMPLEX

TOP BOUNDARY

The Horoman peridotite complex is surroundedby the Hidaka metamorphic rocks on threesides (northeast, east, south) with intricate boun-daries (Fig. 2). The Hidaka metamorphic rocks inthe Horoman area consist of pelitic gneiss (biotitegneiss, garnet–biotite gneiss, garnet–pyroxenegneiss), felsic gneiss, and minor schistoseamphibolite intruded by mafic–felsic igneousrocks. A penetrative foliation and mineral lineationare present throughout felsic and pelitic gneisses.The foliation is defined by aligned flaky mica, ellip-soidal plagioclase, and flattened quartz aggre-gates. The lineation is defined by approximatelyparallel alignment of flaky mica, ellipsoidal plagio-clase, and fine-grained mineral aggregates (mainlyquartz). Dips of the foliation are generally shallowto moderate (mostly 60° or lower). The major dip-directions of the foliation are northeast and south-west with scattered distribution in other directions

(Figs 2,3). The mineral lineation plunges N, NE,SE, S, SW, or NW at shallow to moderate angles(Figs 2,3). The attitudes of foliation and lineationsuggest that felsic–pelitic gneisses form dome- orbasin-like structures (Fig. 3).

We have precisely located the boundaries ofultramafic rocks and pelitic–felsic gneisses in fourseparate locations (Fig. 4). Pelitic–felsic gneissesoverlie ultramafic rocks in all these localities.Based on the tracing of rocks of the hangingwall and footwall, the boundaries are inferredto be northeast-dipping (Fig. 4a) or south-dipping(Fig. 4b) low-angle faults. Direct contact of theserpentinized peridotite (footwall) and felsicgneiss (hanging wall) is exposed at HH80 (Fig. 2).The serpentinite near the boundary has under-gone brittle deformation and grades into a faultgouge toward contact with felsic gneiss (Fig. 4c).Another direct contact is exposed at HH140(Fig. 2). Peridotite on the footwall grades upwardinto brecciated serpentinite and comes into sharpcontact with mylonitic felsic gneiss on the hangingwall (Fig. 4d). The boundary surface betweenthe Horoman peridotite complex and the Hidakametamorphic rocks has been involved in anupright-gentle fold. The pelitic–felsic gneisses ofthe Hidaka metamorphic rocks are distributed inthe axial zones of synform and attitudes of foliationnear the boundary surface are conformable to thisupright-gentle fold (Fig. 2). Accordingly, the ultra-mafic rocks of the Horoman peridotite complex aredistributed in the axial zones of antiform (Fig. 2).

There is an exception to the above-mentionedtectonic relationship. A high-angle fault extendsfrom northwest to southeast between Pin’neshiriand Apoi-dake in the northeastern part of thestudy area (Fig. 2). The hanging wall of this fault

Fig. 3 Lower hemisphere equal-areaprojections of poles to foliation (total 37measurements) and orientations (trendsand plunges) of mineral lineation (total17 measurements) in the Hidaka meta-morphic rocks.

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carries ultramafic rocks of the Horoman peridotitecomplex. The east- or north-dipping high-anglesurface of the fault is not concordant with theattitudes of foliation in the Hidaka metamorphicrocks. Moreover, the fault obliquely cuts thesubhorizontal boundary between gneisses andultramafic rocks. Therefore, we regard it as a sec-ondary generated transverse fault. These observa-tions confirm that the Horoman peridotite complexunderlies the Hidaka metamorphic rocks and in abroad sense, the boundary surface forms a domalstructure.

Unfortunately, we could not carry out kinematicanalysis of the contact zone between the Horomanperidotite complex and the Hidaka metamorphicrocks because the rocks in the vicinity of thecontact are very fragile due to inhomogeneousbrittle deformation and alteration. The shear senseof ductile deformation was determined from threesamples from separate localities (HH14, HH84,HH154) near the boundary between the Horomanperidotite and the Hidaka metamorphic rocks(Fig. 2). Microscopic observations were made onoriented samples of pelitic gneiss, while thin-sections of samples were made on the section per-

pendicular to the gneissosity and parallel to themineral lineation. Several structures with mono-clinic symmetry were observed. Their geometricpatterns indicate a sense of shear (e.g. Passchier& Simpson 1986; White et al. 1986; Passchier &Trouw 1996). Representative microstructures andmovement directions are shown in Figure 5. Sets ofsubparallel shear bands occur in biotite gneiss. Theshear bands are oblique to gneissosity at a low-angle (Fig. 5a), and are regarded as C′-type shearbands (Passchier & Trouw 1996). The C′-type shearbands, which were observed at a locality HH154,indicate top-to-the-southeast displacements.Plagioclase porphyroclasts in biotite gneiss aremantled by asymmetric tails to form s-typeporphyroclasts (Passchier & Simpson 1986).The monoclinic symmetry of s-type porphyro-clasts with respect to the center of the porphyro-clast indicates top-to-the-southeast displacements(Fig. 5b, locality HH84). A s-type porphyroclast inthe sample HH154 is associated with shear bands(C′). The shear senses indicated by these coexistingstructures are consistent with each other. Plagio-clase porphyroclasts in garnet–pyroxene gneissare mantled by asymmetric tails to form d-type

Fig. 4 Photographs of boundaries between ultramafic rocks and pelitic–felsic gneisses. Broken lines indicate inferred (a, b) or actual (c, d) loci oftectonic contacts. See Figure 2 for locations. (a) Looking north, HH124. (b) Looking east, HH132. (c) Looking east, HH80. The attitude of contact surfaceis N35°W, 40°E. (d) Looking downward (top of view to the north), HH140. The attitude of contact surface is N73°W, 38°N.

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porphyroclasts (Passchier & Simpson 1986). Themonoclinic symmetry of d-type porphyroclastswith respect to the center of the porphyroclastindicates top-to-the-northeast displacement(Fig. 5c, HH14). These shear directions suggestnormal-fault movement in the Hidaka metamor-phic rocks near the contact surface with theHoroman peridotite complex. The easterly trend ofshear directions is consistent with the predominanttop-to-the-southeast shear movement determinedfrom mylonitic rocks of the Hidaka metamorphicbelt in the east of the Horoman area (Araki & Shiba1996).

BOTTOM BOUNDARY

The western–northwestern border of theHoroman peridotite complex runs along the foot-hills of Apoi-dake and Pin’neshiri (Fig. 2). Theboundary corresponds to the Hidaka Main Thrust.Unfortunately, this part of the study area is poorlyexposed. The rocks near the boundary are more orless covered by thick forest and alluvial deposits.Although direct contact is not exposed, the bound-ary is inferred as an east-dipping low-angle faulton the basis of the distribution of mafic–ultramaficrocks and non-metamorphic sedimentary rocks(Fig. 6, section A–B Fig. 2). A sedimentarysequence (sandstone and mudstone) of theIdon’nappu belt is structurally underlying theHoroman peridotite complex. The sandstone of

the Idon’nappu belt is fine- to medium-grainedlithic-feldspathic wacke and interbedded withmudstone. These layers generally strike north-west and dip moderately to the northeast. Theinterbedded sandstone and mudstone near thebottom boundary of the Horoman peridotitecomplex have been deformed in a brittle manner.At HH39, HH95 and HH98 locations in Figure 2,three examples of brittle shear zone (composed oftwo or three sets of shear planes) are observed(Fig. 7a–c). A conspicuous shear plane with thelargest separation is denoted by ‘Y’ in Figure 7.Subsidiary shear planes oblique to the Y plane arecomparable to the ‘R1’ and ‘P’ planes of Riedelshear (Logan et al. 1979; Rutter et al. 1986). TheR1 plane is a minor fault with slight opening. The Pplane is denoted by a subparallel arrangement oflenticular clasts of sandstone and mudstone. Themonoclinic alignment of these composite planescan be used as a shear sense indicator as explainedin Chester and Logan (1987) and Kano et al.(1991). The composite planes in three separatelocalities consistently indicate top-to-the-west dis-placement (Fig. 7d). On the basis of the microtec-tonic analysis of mylonitic biotite gneiss close tothe north of the Horoman peridotite complex,Shiba et al. (1997) suggests that the Hidaka meta-morphic rocks on the hanging wall of the HidakaMain Thrust were displaced toward the south-southwest or southwest. The displacement alongthe Hidaka Main Thrust in the Horoman area was

Fig. 5 Photomicrographs (openpolarized light) of sheared metamorphicrocks and stereographic projections offoliation, lineation, and shear direction.See Figure 2 for locations. (a) Shearbands (C′) in biotite gneiss (HH154)displacing gneissosity (S) indicatetop-to-the-southeast movement. (b)Asymmetric s-type porphyroclast of pla-gioclase (‘P’ encircled by broken line)in biotite gneiss (HH84) indicatestop-to-the-southeast displacement. (c)Asymmetric d-type porphyroclast of pla-gioclase (‘P’ encircled by broken line) ingarnet-pyroxene gneiss (HH14) indicatestop-to-the-northeast displacement. (d)Lower-hemisphere equal-angle projec-tions. Great circles represent the atti-tudes of gneissosity at each locality.Black dots on the great circles indicatethe attitudes of lineation. Arrows indicaterelative displacement directions of thehanging wall of foliation.

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generally west vergent, regardless of deforma-tions in ductile or brittle regimes. This suggestswestward tectonic emplacement of the Horomanperidotite complex together with the Hidaka meta-morphic rocks onto the Idon’nappu belt.

TECTONIC IMPLICATIONS FOR HIDAKACOLLISION ZONE

The three major lithological units in the Horomanarea form a subhorizontal tectonic pile (Fig. 8a).The structural lower unit is composed of aCretaceous–Paleogene accretionary complex of theIdon’nappu belt. The structural upper unit is com-

posed of felsic–pelitic gneisses and mafic–felsicintrusive rocks (the Hidaka metamorphic rocks).The Horoman peridotite complex is a tectonic slice,which is intercalated between the Idon’nappu beltand the Hidaka metamorphic rocks. Assuming thatthe bottom boundary of the Horoman peridotitecomplex in the section A–B of Figure 2 is extendedto the east, the structural thickness of the complexis inferred to be about 1200 m. The contact surfaceof the Horoman peridotite with the Hidaka meta-morphic rocks is considered to be primarily a low-angle (subhorizontal) normal fault or shear zone.The thin slab of peridotite complex dips gently tothe east and probably extends eastward under-neath the Hidaka metamorphic rocks. The resultsof this study exert a new constraint on the tectonicsof the Hidaka collision zone.

There is a separate ultramafic body, known asthe Nikanbetsu peridotite complex, about 4 kmeast from the eastern margin of the Horoman peri-dotite complex. The extent of the Nikanbetsu peri-dotite complex is about 2 km from east to west and1 km from north to south (Niida & Katoh 1978).Gabbro, diorite, amphibolite, mylonitic gneisses,and small bodies of ultramafic rocks are distrib-uted in the area between the Horoman and Nikan-betsu peridotite complexes (Takahashi et al. 2002).According to Niida and Katoh (1978) and Taka-hashi (2001), the rocks of this complex are petro-logically identical to those of the Horomanperidotite complex. Maruyama et al. (1991) reportan intense positive Bouguer anomaly related tothe dense rocks of the Nikanbetsu peridotite com-plex. A more detailed Bouguer anomaly map ofthe Horoman area, including exposures of theHoroman–Nikanbetsu peridotite complexes, indi-cates the presence of ultramafic rocks underneaththe Hidaka metamorphic rocks (Yamamoto et al.2001). Preceding petrological and geophysicalstudies suggest that the Horoman and Nikanbetsuperidotite complexes are the exposed sections of alarge slab of peridotite lying under the Hidakametamorphic rocks. This view is consistent withthe results of this study.

Arita et al. (1998) proposed the listric-shapedHidaka Main Thrust and a subhorizontal reflector(décollement) at a depth of 14 km on the basis ofseismic reflection and gravity surveys. Accordingto the seismic profile of Tsumura et al. (1999), theeastern edge of the Northeast Japan Arc beneaththe Hidaka Mountains is wedge-shaped. The crustof the Kuril Arc is delaminated in the mid-level ofthe lower crust (at a depth of about 24 km). Theupper part of the lower crust of the Kuril Arc is

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on the hanging wall of the east-dipping contactsurface to the Northeast Japan wedge. Theupward extension of this contact is regarded as theHidaka Main Thrust. The Hidaka Main Thrust inthese listric models does not extend to the depthsof the mantle. The lower part of the lower crust ofthe Kuril Arc is on the footwall of the west-dippingcontact surface to the wedge. Toyoshima et al.(1997) state that rocks equivalent to the lowestpart of the crust are not exposed in the Hidakametamorphic belt. This geological observation isapparently concordant with the delaminationhypothesis. Accordingly, the delamination-wedgestructure is applied to tectonic interpretations ofthe Hidaka metamorphic belt (e.g. Sawaguchi2004; Shimura et al. 2006). The listric model of theHidaka Main Thrust has been generally accepted;however, there seems to be a lack of careful con-sideration concerning the presence of a mantle-derived peridotite body under the Hidakametamorphic rocks. It is very difficult to carry alarge slab of mantle peridotite on the hanging wallof the Hidaka Main Thrust if the thrust has alistric shape, as shown in the seismic models byArita et al. (1998) or Tsumura et al. (1999). Crustaldelamination perhaps took place after the em-placement of the Horoman peridotite complex.

The listric model is not a unique solution ofseismic surveys in south-central Hokkaido. Theseismic velocity structure proposed by Miyamachiet al. (1994) shows the eastward subduction ofNortheast Japan under the Hidaka Mountains.Refraction/reflection surveys of Iwasaki et al.(2004) indicate gently east-dipping reflectors from

Fig. 7 Field photographs of shearedsedimentary layers and stereographicprojections of Riedel shear planes (P, R1,Y). See Figure 2 for locations. (a) HH39,attitudes of P and Y planes are N36°W,20°N and N78°W, 20°N, respectively.Striation lineation on the Y plane trendsN64°W and plunges to southeast.Looking north. (b) HH95, attitudes of R1,P and Y planes are N78°W, 12°S, N2°E,38°E, and N5°W, 59°E, respectively.Looking south. (c) HH98, attitudes ofP and Y planes are N4°W, 28°E andN72°W, 18°S, respectively. Lookingnorth. (d) Lower-hemisphere equal-angle projections. Great circles indicatethe attitudes of R1, P, and Y planes.Arrows indicate relative displacementdirections of the hanging wall of Y plane.

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Fig. 8 (a) Tectonostratigraphic column of the Horoman peridotitecomplex and adjacent geological units. Diagram not to scale. (b) Sche-matic projection cross-sections indicating emplacement process of theHoroman peridotite complex. Plate-tectonic framework is adopted fromKiyokawa (1992), Kimura (1996), Maeda and Kagami (1996), Miyashitaet al. (1997), and Ueda et al. (2001). Diagrams take no account of tectonicdisplacements in north–south direction and are not to scale.

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the crust of Northeast Japan in the western part ofthe Hidaka collision zone and the seismic structuredirectly underneath the Hidaka Mountains is notclearly described.

Paleogeographic maps of northeastern Japanshow the multiple collision of crustal blocksbetween the Asian margin (Sakhalin, West Hok-kaido) of the Eurasian Plate and the Kuril Arc of theOkhotsk Plate (Kimura et al. 1983; Kimura 1996;Kimura & Kusunoki 1997). The crustal blocks ofHidaka and East Hokkaido were situated near thesoutheastern end of the paleo Kuril Arc in theOkhotsk Plate in Eocene. The Hidaka block movedSSW relative to Sakhalin and West Hokkaido andwas then juxtaposed with West Hokkaido (includ-ing the Idon’nappu belt) in the Oligocene andMiocene (Kimura 1996; Kimura & Kusunoki 1997).A right-lateral strike-slip shear zone runningnorth–south in Hokkaido and Sakhalin and operat-ing during Eocene, Oligocene, and Late Miocenetimes is related to the southward movement of theHidaka block (Kimura et al. 1983; Jolivet &Miyashita 1985). This view has been revised bythorough structural study of the Hidaka metamor-phic belt by Toyoshima et al. (1994). They suggest atop-to-the-south movement of the subhorizontalcrustal slab and an associated right-lateral strike-slip on its western side. Sawaguchi and Takagi(1997), Sawaguchi et al. (2001), and Sawaguchi(2004) have described several subhorizontal shearzones in the ultramafic rocks of the Horoman peri-dotite complex. The shear zones underwent theearlier top-to-the-south shear, the following top-to-the-north shear, and the later top-to-the-southshear. The subhorizontal shear zones operatedunder high pressure–temperature conditions(more than 2 GPa and 950°C) and are probablyrelated to the top-to-the-south movement of theHidaka crust (Toyoshima et al. 1994) overlying theHoroman peridotite at that time. The within-peridotite subhorizontal shear deformation can becorrelated with the tectonic unroofing process ofthe mantle peridotite body (cf. Sawaguchi 2004),however, it can not be directly related to theemplacement of the peridotite body onto theshallow level of the crust. The geological map andcross-sections of Sawaguchi (2004) show that theHoroman peridotite complex overlies the Hidakametamorphic rocks and that there is no tectonicinterpretation to form the structure. In fact, theHoroman peridotite complex underlies the meta-morphic rocks.

According to Kimura (1996) and Kimura andKusunoki (1997), the collision between the forearc

of Kuril (including the Hidaka block) and WestHokkaido around the end of the Miocene or latercaused the exhumation of the Hidaka metamorphicbelt. The movement direction of the Kuril Arc atthe time is considered to be nearly perpendicularto the extension of the Hidaka metamorphic belt.The collision caused general west-vergent struc-ture of the Hidaka metamorphic belt. The meta-morphic grade increases from east to west(Komatsu et al. 1983) and the Horoman peridotitecomplex lies at the structural bottom (westernmargin) of the metamorphic belt (this study).The general west vergent thrusting in ductile(Shiba et al. 1997) and brittle regimes (this study)along the Hidaka Main Thrust suggests thatthe Horoman peridotite complex, together withthe Hidaka metamorphic rocks, emplaced onto theIdon’nappu belt in the later stages of the evolutionof the Hidaka metamorphic belt. To interpret thestructural setting of the Horoman peridotitecomplex during and after the collision, the east-ward subduction of the Asian margin under theHidaka block is preferable to the westward sub-duction of the Hidaka block (Fig. 8b). On the otherhand, structural studies of the Cretaceous–Paleogene accretionary complexes (Idon’nappubelt, Poroshiri ophiolite) suggest that accretionarycomplexes were formed by the westward subduc-tion of an oceanic crust under the Asian margin(Kiyokawa 1992; Ueda et al. 2001). We proposethat the oceanic crust between the Asian marginand the Hidaka block had subduction zones onboth eastern and western sides. The eastern andwestern subduction zones operated around thesame time or the eastern side was relativelyyounger. The Horoman peridotite complex is con-sidered to have flaked off from the mantle wedgeand thrust over the Asian margin (West Hokkaido)during the collision between the Asian margin andthe Hidaka block.

ACKNOWLEDGEMENTS

We thank the Hidaka Subprefectural office of Hok-kaido Government, Hidaka Forestry Office, andSamani Board of Education for their approval ofour field work in the Hidaka Sanmyaku–ElmoQuasi-National Park. We are grateful to H. Takagiand T. Morishita for critical and constructivereviews. This study was partly supported by Grantsfrom the Ministry of Education, Culture, Sports,Science and Technology, Japan (KAKENHI:16540420, 19340152).

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