G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 10, Number 4 7 April 2009 Q04007, doi: /2008gc ISSN: Click Here for Full Article Horizontal shortening versus vertical loading in accretionary prisms Hugues Raimbourg and Shibata Tadahiro Japan Agency for Marine Earth Science and Technology, Natsushima-cho, Yokosuka , Japan (hraimbou@jamstec.go.jp) Yamaguchi Asuka Department of Earth and Planetary Science, Faculty of Science, University of Tokyo, Tokyo , Japan Yamaguchi Haruka Japan Agency for Marine Earth Science and Technology, Natsushima-cho, Yokosuka , Japan Gaku Kimura Department of Earth and Planetary Science, Faculty of Science, University of Tokyo, Tokyo , Japan [1] The analysis of two-phased deformation of subducted turbidites within the Shimanto Belt on Kyushu (Japan) shows strongly contrasted kinematics between an earlier, noncoxial fold-and-thrust -type stage and a later, metamorphic, coaxial vertical shortening stage. The deep deformation is characterized by the development of a subhorizontal muscovite-chlorite metamorphic foliation, associated with pure shear deformation, which overprinted the structures inherited from the earlier stage. We associate the metamorphic deformation stage with the lower levels of accretionary prism landward segment (inner wedge), which was therefore deformed by vertical loading and horizontal extension, much in contrast with prism trenchward segment (outer wedge), which often shows fold-and-thrust structures indicative of horizontal shortening. A possible explanation, based on Wang and Hu s (2006) model, for these longitudinal variations in deformation kinematics of the prism may lie in the contrasted rheology of its basal surface, strain weakening in the seismic portion below the inner wedge while strain strengthening in the aseismic portion below the outer wedge. The vertical shortening active within the inner wedge revealed by this study possibly represents a large contribution to the exhumation of the deeper levels of accretionary prisms. Components: 8370 words, 11 figures, 1 table. Keywords: accretionary prism; stress orientation; metamorphism; subduction zone. Index Terms: 3060 Marine Geology and : Subduction zone processes (1031, 3613, 8170, 8413); 3660 Mineralogy and Petrology: Metamorphic petrology; 8011 Structural Geology: Kinematics of crustal and mantle deformation. Received 15 October 2008; Revised 10 December 2008; Accepted 2 February 2009; Published 7 April Raimbourg, H., S. Tadahiro, Y. Asuka, Y. Haruka, and G. Kimura (2009), Horizontal shortening versus vertical loading in accretionary prisms, Geochem. Geophys. Geosyst., 10, Q04007, doi: /2008gc Copyright 2009 by the American Geophysical Union 1 of 17

2 1. Introduction [2] The internal structure of accretionary wedges is often decomposed into an outer wedge with a steep slope concentrating much of the contractile deformation through fold and thrusting and an inner wedge with a gentler slope, less deformation and possibly overlain by fore-arc basins. Incorporating strain and stress evolution over a seismic cycle into the critical taper model of fold-and-thrust belts [Dahlen, 1984, 1990; Davis et al., 1983], Wang and Hu [2006] have related the prism longitudinal segmentation to variations in the mechanical behavior of the plate interface. [3] In such models, as well as the general framework of the critical taper theory, the properties of the basal surface of the prism exert a large control on prism geometry and dynamics. The precise position of this surface and its properties depend on the sediments on top of the subducting plate and vary as such sediments are progressively dewatered, lithified, metamorphosed and deformed. In particular, the décollement at the base of the Nankai accretionary prism steps down in its portion between outer and inner wedge [Bangs et al., 2004; Park et al., 2002], resulting in duplex formation and underplating of subducted sediments [Kimura et al., 1996]. This step down, which coincides with the updip limit of the seismogenic zone, may result from the progressive hardening of subducted sediments [Hyndman et al., 1993; Moore and Saffer, 2001] and is often associated with the formation of tectonic mélanges sometimes involving slivers of oceanic crust, as documented in many subduction zones such as Alaska or Shimanto Belt in Japan [Byrne, 1984; Fisher and Byrne, 1987; Ikesawa et al., 2005; Kimura and Mukai, 1991; Moore and Byrne, 1987; Onishi et al., 2001; Vrolijk, 1987]. On the other hand, the further burial of sediments within the seismogenic zone is less understood, probably as a result of the scarcity of samples and the possible obliteration of prograde deformation by more intense peak and retrograde metamorphism. [4] Such deep evolution is nevertheless of prime importance, in order to understand the trigger of earthquakes along the plate interface down to the lower limit of the seismogenic zone [Oleskevich et al., 1999; Peacock and Hyndman, 1999] as well as the general dynamics of the inner domains of accretionary prisms. For this purpose we have studied the Kitagawa group of the Shimanto accretionary complex on Kyushu (Japan), showing a two-stage prograde deformation with contrasted kinematics: While early deformation is characterized by folds and thrusts, locally accommodated by mud injections, these structures are overprinted at depth by a strong subvertical shortening leading to the development of a dense metamorphic cleavage. We associate the latter stage with the deformation of the sediments below and within the deeper levels of the inner wedge of the accretionary prisms. Such deformation kinematics shows that the maximum compressive stress is close to vertical in the inner segment of accretionary prisms, much at variance with the horizontal compression observed in the outer segment. Moreover, vertical shortening in the distal region of accretionary prisms may be responsible for part of deep metamorphic rocks exhumation. 2. Geological Setting [5] The Shimanto Belt is an ancient accretionary prism distributed along southwest Japan, which was formed through the subduction of Pacific Plate in Cretaceous and Tertiary times (Figure 1). Northern and Southern Shimanto belts are separated in eastern Kyushu by a major north dipping boundary fault, the Nobeoka Thrust, interpreted as a out-ofsequence thrust, responsible for the exhumation of deeper Morotsuka and Kitagawa groups in the north onto the shallower Hyuga group in the south [Kondo et al., 2005; Okamoto et al., 2006; Okumura et al., 1985]. [6] The Kitagawa unit, on which this study focuses, is composed of a Eocene turbiditic sequence of alternating layers of sandstones and mudstones whose thickness varies from a few millimeters to a few centimeters [Ogawauchi et al., 1984; Sakai et al., 1984]. It lies through the Nobeoka Thrust over the Hyuga group, which is a mélange unit composed of intensively fractured blocks of sandstones embedded in a shale matrix [Kondo et al., 2005]. 3. Geological Evolution of the Unit in the Subduction Zone: Deformation Stages 3.1. Fold-and-Thrusting Deformation [7] Exposures all along the coast north of the Nobeoka Thrust show that the turbiditic sequence, with a NE-SW trend and low dip to the north, is consistently deformed by numerous NE-SW strik- 2of17

3 Geosystems G 3 raimbourg et al.: accretionary prism stress conditions /2008GC Figure 1. Map of the Kitagawa unit in the Northern Shimanto Belt on Kyushu, Japan [after Kondo et al., 2005; Okumura et al., 1985]. Gray and dark diamonds samples correspond to slightly and strongly developed metamorphic foliation, respectively (see Figure 3). ing thrusts, whose movement generate folds (Figures 1, 2, 3a, and 3b). The lineation, mostly visible on folded quartz vein surfaces, strikes NW-SE, perpendicular to folds axes. In several fold hinges, we also observed the infiltration of mudstone into sandstones layers (Figure 3a) Metamorphic Foliation Development [8] The second deformation stage involves the development of a metamorphic foliation of muscovite and chlorite (Figure 3c) [Imai et al., 1971; Toriumi and Teruya, 1988]. This foliation, absent from the upper levels of the unit, is heterogeneously developed in the lower levels of the Kitagawa unit, from weakly deformed samples with few discrete metamorphic planes slightly offsetting the sedimentary bedding to strongly deformed ones where closely spaced metamorphic planes have completely replaced the sedimentary structures (Figure 4) Final Exhumation [9] Following this two-stage prograde history, later deformation is present only in the vicinity (a few hundreds of meters) of the Nobeoka Thrust and consists of tight, sometimes kink-like folding of the 3of17

4 Figure 2. Structural features: The inherited sedimentary foliation ((a) poles to foliation) is deformed in a first stage by thrusts (see Figure 1) and folds ((b) fold axes). The top-to-the-se thrusting movement is principally indicated by quartz veins dragged and deformed by thrusts ((c) lineation axes on quartz veins). In a second deformation stage, the structures of the first stage are overprinted by the development of a subhorizontal metamorphic foliation ((d) poles to foliation). metamorphic foliation itself, with a direction NW- SE, i.e., parallel to the transport direction on the Nobeoka Thrust [Kondo et al., 2005]. The interpretation of this late stage deformation is not yet clear, but its geographical distribution and its geometry leads us to associate it with the retrograde movement along the Nobeoka Thrust. 4. Development of a Metamorphic Foliation at Depth 4.1. Microscopical Processes [10] Characterization of the metamorphic foliation by electronic microprobe revealed that it is mainly composed of very small grains (d mm) of muscovite and chlorite. Maps of element distribution (Figure 5) in juxtaposed domains of preserved original sedimentary structure and metamorphic foliation shows that the development of the metamorphic minerals is accompanied by a strong reduction in the Si concentration in the metamorphized domains. Additionally, the Ti concentration, apart from isolated large Ti-rich minerals in sandstones layers, is largely increasing within the metamorphic domains. [11] To confirm these patterns of variation, we performed XRF bulk rock analyses of samples variously affected by metamorphism (Figure 6 and Table 1): Nonmetamorphized samples do not show any metamorphic cleavage, in slightly deformed samples the metamorphic cleavage constitutes the mechanical fabric of the rock although sedimentary bedding can still be recognized, while the sedimentary fabric has been completely erased in strongly deformed samples. In average, samples affected by metamorphic recrystallization (slightly and strongly deformed samples) show a lower Si and a higher Ti content than samples where the metamorphic foliation is not apparent. Within recrystallized samples, the intensity of the deformation (slightly versus strongly), which is 4of17

5 Figure 3. Structures associated with the two stages of deformation. Stage I: (a) Macroscopic folding; close up shows mud injection in fold hinges. (b) Large-scale top-to-the-southeast thrust with associated fold in the footwall. Stage II: (c) Apparition of a metamorphic foliation at angle with the bedding, disrupting the sandstone beds. (d) As metamorphic deformation increases, foliation crosscutting the sedimentary layers disrupts the beds into lenses, so that inherited folds cannot be recognized any more except for their thicker hinges. Intense deformation results in the parallelization of inherited fold axial plans and metamorphic foliation. (e) Quartz vein folded as a result of metamorphic deformation. characterized macroscopically, does not seem indicative of variations in bulk composition. [12] Ti is often considered as an immobile element whose concentration variations are used to estimate the mass fluxes in the rocks (i.e., the isocon method from [Grant, 1986; Gresens, 1967]). As a result of this immobility, Ti concentration varies relatively as a function of the flux of the other 5of17

6 Figure 4. Samples showing the heterogeneous development of metamorphic foliation. (a) In weakly deformed samples, the metamorphic foliation is present as scarce thin planes slightly offsetting the sedimentary bedding. (b) In contrast, in strongly deformed samples, metamorphic planes are closely spaced and the original sedimentary fabric has been completely erased. elements. An increase in Ti implies therefore that metamorphic recrystallization was accompanied by dissolution and net mass loss of the order of few tens of percent. As these sedimentary rocks are very rich in SiO 2 and in quartz, most of the mass flow results from quartz dissolution and SiO 2 transport out of the rock. This can be confirmed by the negative correlation between SiO 2 and TiO 2 content (Figure 6). Nevertheless, the variations only in SiO 2 are not sufficient to account for the relative TiO 2 variations, and outflow of other elements is required. The mineral reactions and element transport involved cannot be precisely determined, but a plot of the concentration in other major elements with respect to Si content shows that Fe, Mg, Al and K are negatively correlated Figure 5. Electronic microprobe maps of the metamorphic foliation, with scales on the right showing the count rates. While Si is by far the most abundant element, its concentration strongly decreases within the foliation. Relatively to the untransformed rock, Al, Fe, and Mg are more abundant within the foliation, while Ca and Na, which are not incorporated in either muscovite or chlorite, are less abundant. 6of17

7 Figure 6. Evolution of the bulk composition as a result of metamorphic recrystallization. Samples affected by metamorphic recrystallization (either slightly or strongly deformed samples ) show, on average, a higher Ti content than unrecrystallized samples (i.e., no metamorphism ). The increase in Ti is indicative of a mass decrease during metamorphism, essentially operated through Si transport out of the rock, as can be seen in the very good negative correlation between Si and Ti concentrations. Al, Fe, Mg, and K, less mobile than Si, are relatively concentrated in recrystallized rocks, in contrast with Ca and Na, which are more or less uncorrelated with Si content. The coefficient R 2 indicates the goodness of the linear fit. with Si content while Ca and Na do not show any correlation. From these conjoint evolution patterns, it can be inferred that Fe, Mg, Al and K are less mobile than Si, and are therefore progressively enriched when fluids flow through the rocks, while Ca and Na, whose concentration is independent of Si concentration, are much more mobile and not affected by relative increase when Si flows out of the rock. [13] It must be noted that the two elements not correlated with Si, considered therefore as very mobile, are also the two ones that are not incorporated within metamorphic minerals (chlorite and muscovite) forming the foliation. The mobility of Ca and Na can therefore be rather interpreted as Table 1. XRF Bulk Rock Analysis of Samples Variously Affected by Metamorphic Deformation and Recrystallization SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total Nonmetamorphized Samples 13_ _ _ HN18_ HN HN HN Average Slightly Deformed Samples 16_ _ HN HN Average Strongly Deformed Samples HN HN HN HN HN _ Average of17

8 Figure 7. Temperature estimates of the metamorphic foliation development, based on the four reactions (two independent) chlorite + Qzt + H 2 O, for P = 2 kbar (pressure conditions from Kondo et al. [2005]) and optimized X Fe 3+. The error bar corresponds to the dispersion between the four curves. Only estimates with an error smaller than 100 C have been plotted. The average temperature is estimated as T = 272 C. The calculations were done using the software developed by O. Vidal, available at perso/ovidal/downloads/downloads.htm. The theoretical grounds of the method are described by Vidal and Parra [2000] and Vidal et al. [2001, 2005]. the result of absence of incorporation into developing metamorphic minerals, in contrast with Fe, Mg, Al and K, which, if dissolved at some point in the fluid, are for a large proportion trapped into these growing minerals. [14] In summary, Si and Ti variations support therefore the idea that the development of the metamorphic foliation is the result of combined chemical reactions (crystallization of muscovite and chlorite) and mass flux out of the rock (dissolution of quartz and transport of SiO2), in a way relatively similar to the pressure solution mechanisms observed at lower grade conditions in accretionary prisms [e.g., Kawabata et al., 2007] Peak Temperature Deformation [15] Using the equilibrium Chlorite + Qtz + H2O [Vidal and Parra, 2000; Vidal et al., 2001], we have estimated the average temperature conditions of the development of the metamorphic foliation as T = 272 C (Figure 7). Such temperatures are in agreement with the estimations by Toriumi and Teruya [1988] of metamorphic parageneses equilibrium conditions as T = C and P =3 5 kb. Our estimations, highly sensitive to chlorite compositions, show a large scatter from one sample to another one. Nevertheless, we could not observe any trend of temperature variation across the unit. We suppose therefore that even if the intensity of metamorphism strongly varies spatially, associated temperature conditions are similar throughout the unit. Consequently, we interpret the heterogeneous development of the metamorphic foliation as related to variable intensity of the deformation and not to variations in the P-T conditions of metamorphism. [16] The temperature conditions inferred from chlorite paleothermometer are very similar to estimates by Kondo et al. [2005] ranging from C, using vitrinite reflectance. This method, which yields the highest temperature of the P-T path, shows that the metamorphic foliation development coincides with the stage of the deformation where temperature was maximum Relation Between Metamorphic Foliation Development and Fold-and- Thrusting Deformation [17] The low angle between the bedding and the metamorphic foliation makes the interpretation of the latter a bit difficult, and Mackenzie et al. [1987] described it as a cleavage associated with folding, similarly to other examples in the Shimanto Belt [DiTullio and Byrne, 1990]. Nevertheless, the metamorphic foliation developed with a relatively constant subhorizontal orientation throughout the unit, irrespective of the geometry of the folds that it crosscuts. For example, at outcrop scale, the subhorizontal foliation developed within a tight fold (Figure 8a) is almost perpendicular to the vertical fold axial plane. Similarly, in map-scale folds (Figure 8b), the same geometrical relationship between subhorizontal foliation and steeply dipping axial plane shows that the metamorphic plane cannot be interpreted as the fold axial plane. When metamorphic foliation is intense, sedimentary beds are cut into mm-scale lenses, so that folds are erased, except in their thicker hinges (Figure 3d). Consequently, the two deformation stages recognized, i.e., fold and thrusting and metamorphic foliation development, are not concomitant and metamorphic deformation is interpreted as a later stage overprinting deformation structures inherited from an earlier folding and thrusting stage Kinematics of Metamorphic Foliation Development [18] The kinematics associated with the development of the metamorphic foliation is best revealed by the deformation of the abundant quartz and 8of17

9 Figure 8. Relation between the metamorphic foliation and the fold-and-thrusting deformation. At both (a) outcrop and (b) map scale (locations on Figure 1), the subhorizontal metamorphic foliation crosscutting the folds is strongly inclined with respect to fold axial planes, so that it cannot be interpreted as metamorphic fold cleavage. Consequently, the prograde deformation is divided into two stages: earlier fold-and-thrusting stage and later development of a metamorphic foliation which crosscuts structures inherited from the previous stage. calcite veins cutting through the turbidites. In slightly deformed domains, the incipient deformation affecting veins consists of low-amplitude folds that coincide with the restricted zones where the foliation is developed (Figure 9b). In strongly deformed domains, vein deformation is much more intense, as is apparent in their very tortuous shape (Figure 9a). Even in these domains, the foliation is heterogeneously developed at thin-section scale. As a result of this, veins are parallelized to the foliation within the well-foliated zones, while they keep a higher angle to the foliation in the zones more preserved from deformation. Following a single vein, the apparent rotations and offsets that 9of17

10 Figure 9. Kinematics of deformation associated with the development of the metamorphic foliation. (a) In strongly deformed samples, where the sedimentary bedding has been completely erased, inherited veins are intensely folded and locally offset along planes where the foliation, seen as dark planes, is developed. Both senses of shear can usually be found, showing that the deformation is globally coaxial. (b) In slightly deformed samples, quartz veins as well sedimentary bedding are shortened perpendicular to the metamorphic foliation. At the thin-section scale, deformation is heterogeneous and the domains of shortening, where folds appear, coincide to zones where the foliation is developed. (c) At outcrop scale, the kinematics of foliation-perpendicular shortening are apparent in the differential deformation of folded veins and sedimentary bedding. In zones where veins and bedding are at low angle to the foliation, they are undeformed, while they are strongly shortened by small length-scale folds when orientated perpendicular to the foliation. result from this heterogeneous deformation indicate both senses of shear, showing that macroscopically deformation is coaxial. The micromechanism of deformation is apparent in vein fold microstructures (Figure 9b), where the foliation grows preferentially in shortened domains, showing that its development results from localized dissolution. At the outcrop scale, similar structures of veins showing symmetric folds are abundant (Figure 3e). Another evidence of the kinematic regime of the metamorphic deformation can be found in the domains where the metamorphic deformation has not completely erased the bedding and the largescale folds that affected it in the first deformation stage. In such occurrences, early stage veins subparallel to the bedding that were folded with it show a differential deformation depending on their local orientation with respect to the metamorphic foliation: when the vein is close to parallel to the metamorphic foliation, it is virtually undeformed, while when it is at high angle to the foliation, it is strongly shortened by small length-scale folds (Figure 9c). [19] These observations at microscopic and outcrop scale show that the metamorphic foliation developed mainly by shortening perpendicular to the foliation plane. This pure shear deformation is consistent with the interpretation of the metamorphic foliation planes as domains of preferential dissolution, which are therefore orientated perpendicular to the shortening axis. Simple shear does not seem to play a major role, as also attested by the absence of clear asymmetric fabrics. This is quite surprising, considering that simple shear deformation is a major component of deformation in subduction zones and that the lower boundary of the Kitagawa unit is a large-scale out-of-sequence thrust. Although some component of simple shear is present, the deformation associated with the development of a metamorphic foliation in the nearby Makimine Group presents similarly a large component of layer-perpendicular pure shear [Fabbri et al., 1987]. [20] In order to characterize the deformation kinematics by an independent method, we carried out the Anisotropy of Magnetic Susceptibility (AMS) analysis of samples with variable intensity of metamorphic foliation development. Roughly classifying the samples between slightly and strongly affected by metamorphic foliation development, magnetic anisotropy seems to increase as metamorphic deformation gets stronger (Figure 10b). Additionally, for several of these samples the orientation of metamorphic foliation and bedding, 10 of 17

11 Figure 10. Magnetic fabric associated with the metamorphic foliation. Light gray symbols stand for samples where the metamorphic foliation is only slightly developed, and dark ones stand for strongly foliated samples. (a) Stereographic plot of magnetic axes after restoration of the metamorphic foliation to horizontal position. Minimum axes (K3) are concentrated near the vertical axis, while intermediate (K2) and maximum axes (K1) are within a horizontal girdle. (b) The magnetic susceptibility ellipsoid is defined by a anisotropy factor (P 0 1) and a shape factor ( 1 T 1, sketches on the right show the shape of the ellipsoid for the two extreme values, i.e., problate and oblate shapes, for T = 1 and T = 1, respectively) [Jelinek, 1981]. As the metamorphism intensity increases, the magnetic anisotropy (P 0 ) also increases, and the ellipsoid tends toward a oblate shape, for which K 3 < K 1 = K 2. Such a shape of the magnetic ellipsoid, as well as the orientation of the K3 axes perpendicular to the metamorphic foliation are fully consistent with the pure shear kinematics observed on the field. when it has not been erased, differs significantly. When rotating the axes so that the metamorphic foliation is horizontal (Figure 10a), magnetic axes and especially minimum axis K3, are orientated systematically with respect to the metamorphic foliation and not with respect to the bedding. The magnetic signal corresponds therefore to the metamorphic event and any inherited fabrics, if there was any, was erased during the metamorphism. [21] The kinematics of the deformation can be analyzed through the shape of the magnetic ellipsoid as well as the orientation of the magnetic axes. The magnetic ellipsoid is close to a pure oblate shape, with K1 and K2 of equal intensity and larger than K3. K3 is orientated perpendicular to the metamorphic foliation, while K1 and K2 are randomly orientated within the foliation plane. [22] Both magnetic ellipsoid and magnetic axes orientation are consistent with a deformation axisymmetric around an axis perpendicular to the metamorphic foliation plane, i.e., in agreement with pure shear with shortening perpendicular to the foliation and isotropic extension in the foliation. In contrast with other AMS studies of tectonic mélange units in the Shimanto Belt [Kitamura et al., 2005; Ujiie et al., 2000], where K1 axes concentrates parallel to the shear direction, there is no preferential orientation of K1 in our samples, showing that the component of simple shear of the deformation, although present [Fabbri et al., 1987; Mackenzie et al., 1987], was probably of much lower intensity than the pure shear component. 5. Accretionary Prism Stress Conditions and Dynamics 5.1. Tectonic History of Kitagawa Unit Metamorphic Deformation in the Lower Levels of the Inner Wedge [23] The evolution of the subducted turbidites of the Kitagawa Group can be decomposed into progressive burial, accretion and exhumation. The correspondence between these phases and the distinct tectonic stages observed in the rocks is somehow complicated by the lack of constraints on the P-T conditions of each of these stages. [24] The highest temperature conditions recorded both by vitrinite reflectance and chlorite paleothermometers indicate values around 300 C for the development of the metamorphic foliation. If the prism is in steady state regime, the highest temperatures are reached for the deepest burial. Nevertheless, in the Shimanto accretionary prism, the subduction of the Kula-Pacific Ridge in Eocene time [Maruyama and Seno, 1986; Taira et al., 1988] resulted in a large-scale thermal overprint [Ohmori et al., 1997; Sakaguchi, 1996, 1999] that 11 of 17

12 may offset peak temperature with respect to peak burial. The development of the metamorphic foliation is dated as 48.4 ± 1 Ma [Mackenzie et al., 1990], i.e., possibly occurred concomitantly with the thermal event and later than the peak burial, after emplacement into the accretionary wedge. [25] This complex thermal history of the Shimanto Belt prevents us to ascertain where exactly the metamorphic foliation developed, but the amplitude of the temperatures enables to give estimates of the minimum depths reached. According to Sakaguchi [1999], the average geothermal gradient corresponding to the Eocene thermal heating event is of the order of 50 C/km on Shikoku island. In the Hyuga mélange unit next to the Kitagawa unit studied here, the geothermal gradient was estimated as C/km [Kondo et al., 2005]. These values can be considered as maximum estimates for the regional gradient, as they correspond to the intensely fractured zone along the Nobeoka Thrust that was affected by large circulation of fluids. Considering a geothermal gradient of 50 C/km, the depth of development of the metamorphic foliation is at least 6 km and the peak burial depth may even be larger. [26] Such large depths provide important clues as regards the tectonic history of the Kitagawa unit. Seismic reflection profiles across modern accretionary prisms such as Nankai [Moore et al., 1990, 2007], Alaska [von Huene and Klaeschen, 1999] or Oregon [Cochrane et al., 1994] show that the outer part of the accretionary wedge, deformed by insequence fold and thrusting as a result of horizontal contraction, is at most 4 5 km thick and much thinner on toward the trench where the frontal accretion of sediments occur. It is therefore unlikely that Kitagawa sediments have been frontally accreted and that subsequent vertical thickening resulted in their burial down to more than 6 km. We propose therefore that the Kitagawa unit was transported near the plate subduction interface until reaching the lower levels of the inner wedge of accretionary prism, where it was affected by the metamorphic deformation and started exhuming (Figure 11) Fold-and-Thrusting Deformation: A Two-Step Burial [27] In the scenario proposed above, the second phase of deformation affecting Kitagawa sediments, i.e., the development of a metamorphic foliation, is associated with the peak of burial. Accordingly, the fold-and-thrusting, earlier stage must have occurred at shallower levels during prograde evolution. [28] Deformation recorded in underthrusted sediments usually shows two distinct kinematic patterns. First, as a result of the velocity difference between subducting and overriding plate, simple shear is a major component of the deformation along the plate interface, as illustrated by the formation of tectonic mélanges by intense shearing and stratal disruption of subducted sediments and sometimes underlying oceanic crust [e.g., Needham, 1987]. Second, layer parallel extension resulting in extensive vein formation is also sometimes recorded in underthrusted sediments [e.g., Byrne and Fisher, 1990; Kitamura et al., 2005]. The sharp contrast in deformation pattern across the décollement between the overlying wedge with horizontal contraction and underthrusted sediments with horizontal extension is often accounted for by the presence of highpressure of fluids within the décollement, resulting in a very low coefficient of friction and strain decoupling [Byrne and Fisher, 1990; Housen et al., 1996]. [29] Such kinematics, observable in underthrusted sediments, are disagreeing with the first phase of the deformation recorded in the Kitagawa sediments, which nevertheless occurred presumably during burial. Rather than typical underthrusted sediments deformation, the fold-and-thrusting structures described here are similar to the deformation within the wedge itself, either observed in tectonic mélanges, in relation with underplating onto the prism [Hashimoto and Kimura, 1999; Kimura et al., 1996; Onishi et al., 2001], or in sediments frontally accreted. The large temperatures recorded during the metamorphic stage (see discussion above), as well as the involvement of numerous quartz veins in the folds, somehow preclude that the fold-and-thrusting deformation occurred by shallow frontal accretion. We propose therefore to relate the fold-and-thrusting deformation of the Kitagawa sediments to underplating below the outer wedge (Figure 11). [30] As the fold-and-thrusting deformation stage was overprinted by a later deformation stage at higher temperatures, underplating must have been followed by further burial. In analog models by Gutscher et al. [1998a], structures accreted or underplated onto the prism may be remobilized later by an out-of-sequence thrust and transported further down near the plate interface. Such a scenario of burial in two steps, i.e., first underplating resulting in fold-and-thrusting deformation, then 12 of 17

13 Figure 11. Interpretative sketch of accretionary prism dynamics based on the deformation phases recorded in Kitagawa sediments. In an earlier phase, sediments were underplated below the outer wedge and affected by a strong fold-and-thrusting deformation attesting to the horizontal compressive stress active in the outer wedge of the prism. Later, these sediments were further brought down in the footwall of an out-of-sequence thrust and underplated onto the lower levels of the inner wedge. They were affected by the development of a horizontal metamorphic foliation, showing that within the inner wedge, the maximum compressive axis is vertical. The two-stage deformation of the Kitagawa unit therefore illustrates the very large longitudinal variations within accretionary prism of the stress orientation. further burial as a result of the movement along a later out-of-sequence thrust, is consistent with the deformation stages we analyzed (Figure 11) Stress Conditions and Deformation Kinematics Within Accretionary Prisms An Inner Wedge Under Vertical Loading? [31] The metamorphic foliation described here developed subhorizontally with a low angle with respect to the bedding except in strongly folded domains. Assuming that the deep structures were not too strongly rotated during exhumation, which seems reasonable as both bedding and foliation are only slightly tilted, the metamorphic foliation developed under the action of a maximum compressive stress close to vertical. [32] The metamorphic foliation described here is a form of slaty cleavage, which is widely reported within the Shimanto Belt. In Muroto Peninsula, such slaty cleavage was interpreted as axial fold cleavage [DiTullio and Byrne, 1990], but the kinematics of the deformation associated with slaty cleavage development are much different in other areas. The metamorphic foliation developed in mélange units on Okinawa Islands, Japan [Ujiie et al., 2000], show a strongly flattened oblate magnetic ellipsoid, associated by the authors to the development of a metamorphic foliation by uniaxial vertical shortening after the accretion at depth of the mélange onto the overlying accretionary prism. Similarly, the strain geometry determined by deformed shapes of radiolarians patterns in various metamorphic terranes of the Shimanto Belt shows for a large fraction of them deformation by layer perpendicular shortening [Toriumi and Teruya, 1988]. The development of a slaty cleavage/metamorphic foliation seems therefore in many occurrences, including the Kitagawa sediments described here, to result from vertical shortening. [33] The temperature conditions associated with such metamorphic deformation are relatively variable but usually exceed 200 C. In the Kitagawa unit, metamorphic temperature is of the order of 300 C, i.e., conditions compatible with the seismogenic 13 of 17

14 zone conditions [Hyndman et al., 1993], which is thought to constitute the base of the inner wedge [Kimura et al., 2007; Wang and Hu, 2006]. The deformation recorded by the metamorphic foliation development indicates that vertical shortening/ horizontal extension is active in the lower levels of the inner wedge (Figure 11) Model of Longitudinal Segmentation of Accretionary Prisms [34] On the basis of variations in the tectonic style revealed by seismic profiles [Moore et al., 2007; Park et al., 2002], Nankai accretionary prism can be decomposed in distinct sections in the longitudinal direction: In contrast of the pervasive normal faulting visible within in the Kumano fore-arc basin on top of the inner wedge (landward segment), the structure of the outer wedge (trenchward segment) as revealed by seismic studies is characterized by intense fold-and-thrusting deformation. Sharp differences between outer and inner wedge are not restricted to the Nankai prism and have been described in many distinct areas [von Huene and Klaeschen, 1999]. In the outer wedge of prisms, the deformation is usually quite strong and is characterized by fold-and-thrusting deformation and underplating with duplex formation where the décollement steps down [Silver et al., 1985]. In contrast, the absence of contractile permanent deformation is a widespread characteristic of the inner wedge [von Huene and Klaeschen, 1999]. [35] To account for the large longitudinal variations in tectonic style and taper angle in accretionary prism [Kimura et al., 2007], Wang and Hu [2006] developed the critical wedge taper theory [Dahlen, 1984; Davis et al., 1983] in order to include the effects of short-term variations during seismic cycle. In their model, the difference in mechanical regime between inner and outer wedge results from variations in the rheology of the basal surface of the prism, characterized by strain-weakening behavior below the inner wedge and strain strengthening below the outer wedge. [36] As a result, large differences in stress orientation and amplitude arise both in space (inner/outer wedge) and time (coseismic/interseismic time). The deformation and state of stress recorded in the outer wedge results from the transmission during coseismic time of the strain due to seismic rupture below the inner wedge and from the increase in the basal friction of the outer wedge caused by this strain. Such transient state of stress in the outer wedge, with horizontal maximum compressive stress, is not directly correlated with the long-term, interseismic stress in the inner wedge, which controls the metamorphic deformation. Consequently, it is therefore possible that while the outer wedge is affected by recurrent short period of critically compressive state, where thrusts form as a result of horizontal compression, the inner wedge is in noncritical, extensional state of stress during the interseismic time. Such a configuration with the inner wedge in non critical, extensional state of stress during the interseismic time would be in agreement with the kinematics of the ductile deformation described here: Even if the prism is in stable state, i.e., deviatoric stress is too low for brittle deformation to occur, the ductile deformation would record the vertical direction of the maximum compressive stress. The model by Wang and Hu [2006] provides therefore a possible framework to interpret the deformation structures we analyzed in a deep unit of the Shimanto accretionary prism (Figure 11) Implications on the Prism Dynamics Steady State Regime of Prism Dynamics [37] As ductile strain rates are very low, the extensional state of the inner wedge needs to be persistent over a long time scale for the deformation to develop. Furthermore, horizontal ductile extension leads to a decrease in the vertical load, therefore tends to annihilate itself. For horizontal extension to persist over long time, a competing phenomenon must permanently contribute to increase thickness of the prism and the vertical load. [38] Prism evolution is in general very irregular and often alternates growth and erosion [e.g., Kimura, 1994; von Huene et al., 1996]. Moreover, its growth involves several distinct mechanisms among which frontal accretion and underplating. Analogical experiments by Gutscher et al. [1998b] showed episodic growth with phases of frontal accretion of short imbricate thrust slices, alternating with underplating of long, undeformed sheets. While frontal accretion of sediments increase the length of the prism, underplating of long and thin slices of sediments under the inner wedge is necessary to increase its thickness and consequently the vertical load. Underplating at depth, suggested from seismic profiles of the Nankai prism [Park et al., 2002], is therefore required for extension in 14 of 17

15 the rear of the accretionary prism to be sustained over long time scales Exhumation Within the Accretionary Prism [39] The exhumation of metamorphic rocks in collision and convergence zones is still a source of controversy and distinct and sometimes discrepant models have been designed to account for this exhumation. The exhumation of deep metamorphic rocks from the high-pressure and ultrahigh-pressure domains is often accounted for by an upward circulation of material along the plate interface in subduction channel models [England and Holland, 1979; Raimbourg et al., 2007; Shreve and Cloos, 1986]. In accretionary prism, there is no evidence of such an upward flow localized near the plate interface, but rather than the exhumation occurs within the accretionary prism after accretion of the sediments onto it. [40] A major structural feature of the Nankai accretionary prism is the out-of-sequence thrust extending from the ocean floor to the plate interface [Moore et al., 2007; Park et al., 2002]. Thrusting movement along major thrusts may contribute to exhumation, but as pointed out by Platt [1993], in itself thrusting results in increasing the thickness/topography, but not in net decrease of the depth. It must be necessarily be accompanied by an additional mechanism such as erosion, which is enhanced by the increase in topography associated with thrusting. [41] Considering not the deformation on a single thrust but the whole circulation within the wedge, Platt [1993] proposes a different model, based on the wedge mechanics: material accretion along the base of the prism tends to increase the taper angle and is compensated by horizontal extension/vertical shortening within the prism, which maintains the taper angle at equilibrium value. The vertical shortening at depth unraveled in this study, tending to reduce the thickness of the prism, is compatible with Platt s [1993] model of exhumation. Vertical thinning as a mechanism of exhumation is therefore not restricted to postorogenic lithospheric thinning [Dewey, 1988], but may be active at the same time as convergence. 6. Conclusion [42] Our structural and metamorphic study of the Kitagawa unit of the Shimanto accretionary prism shows a complex prograde history, composed of two phases with strongly contrasted kinematics. The later phase of deformation is characterized by the development of a subhorizontal metamorphic foliation as a result of vertical compression, much different from the folds and thrust of the earlier phase. We interpreted the metamorphic deformation by vertical compression as illustrating the stress conditions within the deep levels of the inner wedge, which are consequently much different from the horizontal compression often observed within the outer wedge of accretionary prisms. The longitudinal segmentation of the prism, in terms of stress orientation, between inner and outer wedge may be related to the variations in the mechanical behavior of the plate interface, as proposed in Wang and Hu s [2006] model. Acknowledgments [43] This work was supported by two grants of the Japanese Society for Promotion of Science (JSPS): Grant-in-Aid for JSPS Fellows P05323 and Grant-in-Aid for Creative Scientific Research 19GS0211. The authors are grateful to H. Yoshida and O. Vidal for their support. References Bangs, N. L., et al. (2004), Evolution of the Nankai Trough décollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging, Geology, 32(4), , doi: /g Byrne, T. (1984), Structural evolution of melange terranes in the Ghost Rocks Formation, Kodiak Island, Alaska, in Melanges, Their Origin and Significance, edited by L. Raymond, Spec. Pap. Geol. Soc. Am., 198, Byrne, T., and D. Fisher (1990), Evidence for a weak and overpressured décollement beneath sediment-dominated accretionary prisms, J. Geophys. Res., 95(B6), , doi: /jb095ib06p Cochrane, G., et al. (1994), Multichannel seismic survey of the central Oregon margin, Proc. Ocean Drill. Program Initial Rep., 146, Dahlen, F. A. (1984), Noncohesive critical Coulomb wedges: An exact solution, J. Geophys. Res., 89(B12), Dahlen, F. A. (1990), Critical taper model of fold-and-thrust belts and accretionary wedges, Annu. Rev. Earth Planet. Sci., 18, 55 99, doi: /annurev.ea Davis, D. M., et al. (1983), Mechanics of fold-and-thrust belts and accretionary wedges, J. Geophys. Res., 88(B2), , doi: /jb088ib02p Dewey, J. F. (1988), Extensional collapse of orogens, Tectonics, 7(6), , doi: /tc007i006p DiTullio, L., and T. Byrne (1990), Deformation paths in the shallow levels of an accretionary prism The Eocene Shimanto Belt of southwest Japan, Geol. Soc. Am. Bull., 102(10), England, P. C., and T. J. B. Holland (1979), Archimedes and the Tauern eclogites: The role of buoyancy in the preservation of exotic eclogite blocks, Earth Planet. Sci. Lett., 44, , doi: / x(79) of 17

16 Fabbri, O., et al. (1987), Phase ductile a vergence nord dans la zone Shimanto de Kyushu (Japon SW), C. R. Acad. Sci., Ser. II, 304, Fisher, D., and T. Byrne (1987), Structural evolution of underthrusted sediments, Kodiak Islands, Alaska, Tectonics, 6, , doi: /tc006i006p Grant, J. A. (1986), The Isocon diagram: A simple solution to Gresens equation for metasomatic alteration, Econ. Geol., 81(8), Gresens, R. (1967), Composition-volume relationships of metasomatism, Chem. Geol., 2, 47 65, doi: / (67) Gutscher, M. A., et al. (1998a), Material transfer in accretionary wedges from analysis of a systematic series of analog experiments, J. Struct. Geol., 20(4), , doi: / S (97) Gutscher, M.-A., et al. (1998b), Episodic imbricate thrusting and underthrusting: Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge, J. Geophys. Res., 103(B5), 10,161 10,176, doi: /97jb Hashimoto, Y., and G. Kimura (1999), Underplating process from melange formation to duplexing: Example from the Cretaceous Shimanto Subbelt, Kii Peninsula, southwest Japan, Tectonics, 18(1), , doi: /1998tc Housen, B. A., et al. (1996), Strain decoupling across the décollement of the Barbados accretionary prism, Geology, 24(2), , doi: / (1996)024<0127: SDATDO>2.3.CO;2. Hyndman, R. D., et al. (1993), Tectonic sediment thickening, fluid expulsion, and the thermal regime of subduction zone accretionary prisms: The Cascadia Margin off Vancouver island, J. Geophys. Res., 98(B12), 21,865 21,876, doi: /93jb Ikesawa, E., et al. (2005), Tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin: An example from the Cretaceous-early Tertiary Mugi Melange, the Shimanto Belt, Japan, Tectonophysics, 401(3 4), , doi: /j.tecto Imai, I., et al. (1971), Geologic structure and metamorphic zonation of the northeastern part of the Shimanto terrane in Kyushu, Japan, J. Geol. Soc. Jpn., 77, Jelinek, V. (1981), Characterization of the magnetic fabric of rocks, Tectonophysics, 79, T63 T67, doi: / (81) Kawabata, K., et al. (2007), Mass transfer and pressure solution in deformed shale of accretionary complex: Examples from the Shimanto Belt, southwestern Japan, J. Struct. Geol., 29, , doi: /j.jsg Kimura, G. (1994), The latest Cretaceous early Paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern Pacific margin, J. Geophys. Res., 99(B11), 22,147 22,164, doi: /94jb Kimura, G., and A. Mukai (1991), Underplated unit in an accretionary complex: Melange of the Shimanto Belt of eastern Shikoku, southwest Japan, Tectonics, 10, 31 50, doi: /90tc Kimura, G., et al. (1996), Well-preserved underplating structure of the jadeitized Franciscan complex, Pacheco Pass, Calif. Geol., 24(1), Kimura, G., et al. (2007), Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone, Earth Planet. Sci. Lett., 255(3 4), , doi: /j.epsl Kitamura, Y., et al. (2005), Mélange and its seismogenic roof décollement: A plate boundary fault rock in the subduction zone An example from the Shimanto Belt, Japan, Tectonics, 24, TC5012, doi: /2004tc Kondo, H., et al. (2005), Deformation and fluid flow of a major out-of-sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan, Tectonics, 24, TC6008, doi: / 2004TC Mackenzie, J. S., et al. (1987), Progressive deformation in an accretionary complex An example from the Shimanto Belt of eastern Kyushu, southwest Japan, Geology, 15(4), , doi: / (1987)15<353:pdiaac>2.0. CO;2. Mackenzie, J. S., et al. (1990), Cleavage dating by K-Ar analysis in the Paleogene Shimanto Belt of eastern Kyushu, S.W. Japan, J. Mineral. Petrol. Econ. Geol., 85, Maruyama, S., and T. Seno (1986), Orogeny and relative plate motions: Example of the Japanese islands, Tectonophysics, 127, , doi: / (86) Moore, G. F., et al. (1990), Structure of the Nankai Trough accretionary zone from multichannel seismic-reflection data, J. Geophys. Res., 95, , doi: / JB095iB06p Moore, G. F., et al. (2007), Three-dimensional splay fault geometry and implications for tsunami generation, Science, 318(5853), , doi: /science Moore, J. C., and T. Byrne (1987), Thickening of fault zones: A mechanism of melange formation in accreting sediments, Geology, 15, , doi: / (1987) 15<1040:TOFZAM>2.0.CO;2. Moore, J. C., and D. Saffer (2001), Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress, Geology, 29(2), , doi: / (2001)029<0183: ULOTSZ>2.0.CO;2. Needham, D. T. (1987), Asymmetric extensional structures and their implications for the generation of melanges, Geol. Mag., 124(4), Ogawauchi, Y., et al. (1984), Stratigraphy and geologic structures of the Shimanto supergroup in the northeastern part of Nobeoka city, Miyazaki Prefecture, Japan, Bull. Fac. Sci. Kagoshima Univ., 17, Ohmori, K., et al. (1997), Paleothermal structure of the Shimanto accretionary prism, Shikoku, Japan: Role of an outof-sequence thrust, Geology, 25, , doi: / (1997)025<0327:PSOTSA>2.3.CO;2. Okamoto, S., et al. (2006), Earthquake fault rock indicating a coupled lubrication mechanism, eearth, 1, Okumura, K., et al. (1985), Geology of the Kamae District, Geol. Surv. of Jpn., Tsukuba, Japan. Oleskevich, D. A., et al. (1999), The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile, J. Geophys. Res., 104(B7), 14,965 14,991, doi: / 1999JB Onishi, C. T., et al. (2001), Deformation history of tectonic melange and its relationship to the underplating process and relative plate motion: An example from the deeply buried Shimanto Belt, SW Japan, Tectonics, 20(3), , doi: /1999tc Park, J. O., et al. (2002), A deep strong reflector in the Nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release, J. Geophys. Res., 107(B4), 2061, doi: / 2001JB of 17