Journal of Southeast Asian Earth Sciences, Vol. 4, No. 3, pp. 195-201, 1990 0743-9547/90 $3.00 + 0.00 Printed in Great Britain Pergamon Press plc

Back-thrusting in accretionary prisms: microtectonic evidence from the Cretaceous-Lower Tertiary Shimanto belt of southwest Japan

O. FABBRI, M . FAURE a n d J. CHARVET

D6partement des Sciences de la Terre, U.R.A./C.N.R.S. 1366, Universit6 d'Orl6ans, B.P. 6759, 45067 Orl6ans CEDEX 2, France

(Received 16 May 1989; accepted for publication 21 August 1989)

Abstract--Deformation styles of the Cretaceous-Tertiary Shimanto belt of southwest Japan have been examined in the eastern Kyushu and Kanto mountains. An early ductile deformation, which affects only the basal part of the Cretaceous formations, predates a widespread south-verging brittle deformation. The ductile deformation, which occurred between Lower Cretaceous and Lowermost Eocene time, is characterized by two conspicuous microstructures: a cleavage (S 1) and a stretching lineation (L I). The L 1 trend is transverse to the belt, averaging north-south. Strain analysis reveals a non-coaxial deformation regime related to synmetamorphic shearing directed from south to north. Such a landward vergence is thought to reflect back-thrusting at the rear of a Cretaceous accretionary prism formed along the SW margin of Japan.

INTRODUCTION

CLASSICAL models of accretionary prisms state that the accretion of sediments produces synthetic seaward- verging folds and thrusts along the base of the inner trench slope (e.g. Karig and Sharman 1975). But studies of modern prisms and ancient sub-aerial prisms have revealed the importance of landward-verging defor- mation structures, which consist mainly of thrusts and folds (Seely 1977, Westbrook 1982, Needham and Knipe 1986, Hibbard and Karig 1987, Silver and Reed 1988). Among these structures, back-thrusting seems to be particular!y important at the rear of the accretionary prism (Silver and Reed 1988). Back-thrusting may occur during accretion at the same time as front-thrusting, as predicted in the experimental model of Malavieille (1984). Modern examples of simultaneous landward and seaward verging thrusts include the Barbados Ridge Complex (Westbrook 1982) and the Sunda-Banda arc system (Silver and Reed 1988), for instance. Concurrent frontal and back-thrusting have also been described in collision belts, for example in Taiwan where opposite vergences were developed simultaneously (Suppe 1981, Pelletier and Stephan 1986, Faure et al. 1987a). In some other cases, deformation is dominantly of a landward- verging nature (Hibbard and Karig 1987, Byrne and Hibbard 1987).

The aim of this paper is to show that, in the Shimanto belt of southwest Japan, an early ductile deformation recorded a landward-thrusting stage. We interpret this stage as back-thrusting synchronous with accretion along the Eurasian margin during Cretaceous time.

GEOLOGICAL SETTING AND DEFORMATION TYPES

The Shimanto belt (Taira et al. 1982, Ogawa 1985, Sakai 1985) extends along the Pacific side of southwest Japan (Fig. 1). Landwards, it is overthrust by a Jurassic

olistostrome, belonging to the Jurassic Orogen (Faure et al. 1987b). The thrust, called here (A) correspondsto the Butsuzo tectonic line. The Shimanto belt is divided into two narrow sub-belts, the northern one consisting of Cretaceous rocks and the southern sub-belt of Paleo- gene rocks. The Cretaceous sub-belt is currently thrust upon the Paleogene sub-belt along a major brittle fault, labelled (B) in Fig. 1.

Detailed deformation analyses have been carried out in two areas: eastern Kyushu and the Kanto mountains (Fig. 1). For comparison, field studies were undertaken on the Okinawa and Amami-Oshima islands. The strata of the Shimanto belt are strongly deformed with the Cretaceous units showing two styles of deformation. The oldest deformation is ductile, occurred under meta- morphic conditions which reached greenschist facies (T = 200-300°C and P = 3-5 kb, Toriumi et al. 1986), and has strongly overprinted the previous structures. It is found all along the Cretaceous sub-belt except in Shikoku and in southern Kyushu. In both the eastern Kyushu and Kanto areas, the ductile deformation is developed in what appears to be the basal part of the Cretaceous sub-belt (see cross-sections of Figs 2 and 3). The transition between non-schistose and schistose for- mations is progressive and not faulted. Bed polarity criteria show that the entire sequence is in an upright position. The schistose formations are thus contempor- aneous with or older than the overlying Mid to Upper Cretaceous formations. On Amami-Oshima island, the schistose formations are cross-cut by 55-49 Ma granitic rocks (Fabbri and Charvet 1987). Thus the ductile deformation is of Cretaceous to Lower Eocene age. It pre-dates a widespread brittle deformation which con- sists of numerous shear bands, reverse faults, and recum- bent folds, all of which show a general southward (oceanward) vergence as shown by slickensides, drag folds and other common criteria.

The Paleogene formations also experienced southward verging brittle deformations, but lack any important ductile deformation. Slightly schistose flysch sequences

195

196 O. FABBRI et al.

Fig. 1. Modern setting of the Shimanto belt with location of the study areas. MTL means Median Tectonic Line, QA is the Butsuzo Tectonic Line, QB is the tectonic boundary between the Cretaceous and Tertiary

sub-belts.

are found locally, for instance in the Eocene Kitagawa Group (Fig. 2, Ogawauchi et al. 1984). In the study areas, this brittle deformation usually shows a south- ward, southwestward or southeastward vergence.

Some recent studies have suggested that in the Cre- taceous sub-belt, ductile and brittle deformations are two end-members of a progressive deformation history which occurred during underthrusting of the accretion- ary prism (Mackenzie et al. 1987, Needham and Mackenzie 1988). According to these authors, the

underthrust sediments experience ductile deformation, and the vergence is always oceanwards. But our investi- gations, based on the analysis of regionally widespread microstructures, show that the vergence of the ductile deformation is unexpectedly directed landwards.

GEOMETRY AND DESCRIPTION OF FABRIC

The fabric of the schistose part is defined by two microstructures, a cleavage S 1 and a lineation L 1, which have been formed contemporaneously. In eastern Kyushu (Fig. 2), S 1 gently dips to the northwest and L l generally trends between N 40°W and N 10°E. Schistose levels were found in two localities in the Kanto study area (Fig. 3). In the narrow and elongated area south of Okutama, S1 dips to the northeast and L 1 trends vary between N5 and N60°E. Northwest of Okutama, in a window through the Jurassic olistostrome, L 1 trends are more scattered. Two main directions are found, how- ever, one at N50°E similar to the region south of Okutama; the other is approximately perpendicular, that is to say N60°W. The reasons for this scattering are not clear, although the schistose formations are refolded by recumbent southwest-verging folds with Nl l0 to N130°E axes and by broad upright folds with NI20 to N140°E axes. These late folding stages may have scat- tered previous orientations of microstructures. In the Cretaceous sub-belt of Okinawa and Amami-Oshima islands (Fig. 1), the schists bear a lineation trending Nl15 to N145°E and N120 to N150°E, respectively. Thus, except in the window of the Kanto mountains, the lineation is transverse to the trend of the belt.

CRETACEOUS CRETACEOUS l ! SE PALEOGENE FORM. I SCHISTOSE F. F, / P ,

~, ~ ~ ...~ve ! ,,~ NW

A

h ~ ~ _'.lk ',

I0 km

N t

o , . . . "

7 .°

~/A GROUP (EOCENE)

65

O ~5 .. , ~ . . . . ' • ~1~ C~' . ~

lineallon LI

, , ,~wi th shear sense l unknown shear sense

20 km

Fig. 2. Simplified geological map and cross-section of eastern Kyushu area. No vertical exaggeration. Crosses indicate Mid Miocene granite complexes. Contoured area equals lower hemisphere projection of lineation (L 1); orientation is for the

whole schistose area. On inset arrows along thrusts indicate kinematics for the brittle motions.

Back-thrusting in accretionary prisms

N N

B' ~ ~A'~ C r e t a c e o u s f. \ B [

t i '~" " , ' 4 , p ~ ....

I | O k m ~ Fig. 3. Simplified geological map and cross-section of Kanto moun- tains area. No vertical exaggeration. Same symbols as Fig. 2. Stereonet A shows the L 1 lineation in schistose rocks of the window, stereonet B is for the L I lineation in the schistose rocks south of Okutama. 1, Mid to Upper Miocene granitoid; 2, Lower Miocene sediments; 3, Miocene basic volcanics (Tanzawa Mountains); 4, Cretaceous schis- tose formations; 5, L t lineation symbols (arrow if the sense of shear is known, bar if not). On inset arrows along thrusts indicate kinematics

for the brittle motions.

The tectonic fabric is strongly dependent on lithology and on a downward gradient of increasing metamorphic grade and strain intensity. Brittle deformation and sol- ution transfer mechanisms are important in a lowermost greenschist facies in the upper parts of the schistose sequence (Fabbri et al. 1987). In the lower parts, meta- morphic grade increases up to the middle greenschist facies and crystal plastic deformation is present (Toriumi et al. 1986, Mackenzie et al. 1987). Solution transfer, however, remains the predominant mechanism. More- over, it post-dates plastic deformation as shown by plastically deformed quartz veins (with undulatory ex- tinction) which are cut by thin seams of insoluble residue.

At outcrop scale in the coherent formations, S I is usually parallel to bedding. S1 is well developed in fine-grained shales and siliceous sediments which

197

become somewhat slaty in nature. In sandstones, S 1 is a spaced cleavage. In chaotic units, blocks and fragments of sandstones, lavas, and cherts are devoid o f any cleavage, though strongly fractured. But the cleavage is conspicuous in the surrounding matrix. The L 1 lineation in sandstones is defined by the linear arrangement of clasts and tails. In fine-grained chaotic formations, small fragments of sandstone or chert are usually aligned with their long axes parallel to L 1. Under the microscope, S 1 is defined by a wide variety of features, including continuous and/or parallel tamellae of phyllosilicates, flattened clasts flanked by mica overgrowths (mica "beards"), or undulatory or flat seams of residual dark materials. L 1 is a stretching lineation as shown by a comparison of thin sections cut respectively parallel or perpendicular to the lineation. Minerals are commonly outlined by pressure shadows and deformed "pull- apart" clasts are conspicuous in sections parallel to L 1 but scarce in perpendicular sections. In siliceous sedi- ments, the long axes of recrystallized radiolaria are parallel to L l, similar to those of pebbles in deformed microconglomerates.

KINEMATICS OF THE DUCTILE DEFORMATION

Bulk strain ellipsoid

Estimates of the strain ellipsoid shape used deformed radiolarians and volcanic clasts, which are both assumed to be initially spherical. The maximum elongation axis X of the strain ellipsoid is parallel to L 1. The maximum flattening plane X Y is parallel to the S 1 plane. In eastern Kyushu and the Kanto mountains to the south of Okutama, the Flinn parameter values are around 1 (Toriumi 1985, Yanai and Yamakita 1987). In the tectonic window north of Okutama, our measurements on radiolaria tests yield values ranging between 1 and 1.9. But solution transfer has affected recrystaUized radiolaria (they are surrounded by minute insoluble opaque minerals), so matter migration is possible and K values are thus lower estimates. Nevertheless, K values are roughly constant, suggesting an homogeneous finite deformation close to plane strain.

Deformation regime and sense of shear

Several criteria indicate the non-coaxiality of the deformation everywhere in the study areas. These cri- teria are found exclusively in X Z sections (Fig. 4). In agreement with previous studies (e.g. Escher and Watter- son 1974, Mattauer et al. 1981), we interpret the trans- verse lineation L1 as representing the direction of shearing and nappe transport during the ductile stage of deformation. In thin sections cut parallel to the X Z plane, the criteria used to determine the sense of shear consist of asymmetric pressure-shadows around clasts (e.g. Fig. 4a), and sigmoidal shape of phyUitic minerals or of deformed radiolaria (Fig. 4b). Abundant S-C

SEAES 4 / ~

198 O. FABBRI et al.

fabrics are in agreement with other microstructural criteria (Fig. 4c).

In eastern Kyushu, we examined 120 thin sections. Fifty of these sections show criteria suggesting north- ward-directed shear, while only 10 sections suggest an opposite southward-directed shear. The other sections do not yield any reliable criteria. Shear sense criteria in the Kanto mountains' window are less numerous. On a total of 88 thin sections, 29 indicate a shear directed to the north, northeast or northwest, depending on the lineation trend. Five sections give a southward sense. To the south of Okutama, Yanai and Yamakita (1987) mentioned microtectonic evidence for a northward- directed ductile shear. In the schistose rocks of Amami-Oshima island, a few samples (8) give indi- cations for a northward shear, though S1 is usually poorly developed (Fabbri and Charvet 1987). In sum- mary, according with previous local studies (Fabbri et al.

1987, Yanai and Yamakita 1987), in the study areas, the ductile deformation along the bottom of the Cretaceous formations corresponds to a shear directed from south to north.

The basal part of the Eocene Kitagawa Group (Fig. 2), which is slightly schistose, shows isoclinal folds having a weak axial planar cleavage. These folds exhibit southeastward vergence as indicated by both asymmetry of folds and facing of strata. On the contrary, in the Cretaceous schistose formations, isoclinal folds with S 1 as an axial planar cleavage are rare, small in size and do not give any indication of fold vergence.

DISCUSSION

The ductile deformation recorded in the schistose rocks of the Cretaceous sub-belt indicates a change of vergence during the shortening history and does not support a southeast-directed progressive shearing. Unlike the conclusion reached by Mackenzie et al. (1987) or by Needham and Mackenzie (1988) in eastern Kyushu, our more extensive dataset points out the existence of an early northward vergence.

Several hypotheses can account for the northward vergence. (1) It could be the result of a large northward nappe emplacement over the Cretaceous frontal arc area, but no geological evidence supports this hypothe- sis. (2) Ductile normal faulting (e.g. Malavieille 1988) could be a possible explanation as an abnormally thick accretionary prism may have been affected by a large- scale crustal extension. Such a phenomenon can occur through the way of brittle normal faults in the surface giving way to ductile normal faults at depth. The ductile deformation could reflect normal faulting at depth. But it should be accompanied, in more superficial levels, particularly in the overlying non-schistose Cretaceous formations, by normal brittle faulting of Paleocene or Eocene age. Such extensional features have not been detected in the study areas or elsewhere in the Shimanto belt. The normal faults which are encountered may be

S Sedimentary cover N

4 3\ 2\,, I ~.. 7..... § ~

Northward ductile deformation " "

Fig. 5. Schematic model for the evolution of southwest Japan margin during Cretaceous interpreted after the experiments of Malavieille (1984). Underplated and/or subducted sediments are omitted. The faults which appear in the shortened zone are numbered from I to 7,

depending on their formation order.

very recent because local Pliocene and Quaternary sedi- ments are affected, although age constraints are lacking. On the contrary, abundant reverse faults reflect a com- pressive strain field, at least until Lower Miocene. (3) The experimental models of asymmetric shortening provide a more plausible explanation (Malavieille 1984). In Malavieille's model, while the frontal shortened domain is sliced into thrust sheets younging outward according to a piggyback-type process, the rear of the wedge suffers from back-thrusting along a few faults working continuously. Our model proposes that, in the deep parts of the prism, where greenschist facies meta- morphism is developed, shear motion along the ductile seaward-dipping reverse faults can account for the northward ductile synmetamorphic shear in the deep levels of the Cretaceous prism (Fig. 5). This deformation, which is known to be pre-Eocene, may have occurred during Mid to Upper Cretacous times, when an oceanic plate (Kula or Pacific plate), moving northward, was subducting under the Eurasian margin. Due to the general southward thrusting of all formations, the initial position of the ductile seaward-dipping faults is not known. We suppose that they were located at the rear of the prism, where thickness is important enough to allow the development of greenschist facies metamorphism.

The transition from the idealized cross-section of Fig. 5 to the present sections (Figs 2 and 3) results from the evolution of the prism during Tertiary time and is out of the scope of this paper. That late evolution, well documented at least between Upper Eocene and Lower Miocene, is rather complex (Ogawa 1985, Sakai 1985, Charvet and Fabbri 1987, Sakai 1988). In particular, a possible Lower Miocene collision between the SW Japan margin and a thick-crust microblock, carried by the subducting Philippine Sea plate, could account for the widespread southward thrusting of the Shimanto prism, including the previously back-thrusted levels (Charvet et al. 1990). The colliding microblock, now not out- cropping any more, may have been subducted or under- plated beneath the Shimanto belt. The Mid Miocene igneous activity, mainly acidic, developed inside the Shimanto belt, would reflect the presence of the micro- block at that time (Stein et al. 1988).

Back-thrusting in accretionary prisms 199

Fig. 4. Examples of asymmetric criteria from thin sections cut in X Z plane. (a) Asymmetric pressure shadows around an albite crystal in a schistose basic lava of the Kanto mountains. Scale bar is 0.8 mm. (b) Asymmetric pressure shadows around recrystallized radiolaria in a schistose ribbon chert of Kyushu. Scale bar is 0.3 mm. (c) S-C fabric in a schistose siltstone

of the Kanto mountains. Scale bar is 0.1 ram.

Back-thrusting in accretionary prisms 201

Acknowledgements--This work has been supported by a grant from the Ministrre des Affaires Etrangrres for O. F. and from the Kaiko project for J. C. and M.F .M. Murata, A. Nishimura and M. D. Courme are thanked for their determinations of microfossils.

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