TECTONICS, VOL. 24, TC5008, doi: /2005tc001823, 2005

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1 TECTONICS, VOL. 24,, doi: /2005tc001823, 2005 Structural characteristics of shallowly buried accretionary prism: Rapidly uplifted Neogene accreted sediments on the Miura-Boso Peninsula, central Japan Y. Yamamoto Institute of Geosciences, Shizuoka University, Shizuoka, Japan H. Mukoyoshi Department of Sciences, Kochi University, Kochi, Japan Y. Ogawa Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan Received 25 March 2005; revised 17 June 2005; accepted 5 July 2005; published 12 October [1] The upper Miocene Misaki and Nishizaki formations on the Miura and Boso peninsulas in central Japan preserve the deformation features of an off-scraped accretionary prism. The spatial distribution, geometry, and style of accretion-related deformation with paleotemperature and burial depth estimation are elucidated in this study. The deformation structures and textures are similar to those of modern accretionary prisms. The low maximum paleotemperature (<50 C) and high preserved porosity of the sediments (30 50%) imply a maximum burial depth of less than 1000 m. On the basis of the mode of deformation, this off-scraped body is divided into an imbricate thrust, a thrust unit, and an upper coherent unit in ascending order. The imbricate thrust corresponds to a branch from the basal décollement zone and is subdivided into a brecciated zone, a main gouge zone, and a shear band zone. The thrust unit hosts a concentration of thrust systems that form various orders of duplex structures, while the upper coherent unit is characterized by gravitational instability- and earthquake-induced deformation without thrust faulting. The duplex distribution, shear strain, and fluid migration associated with the offscraping processes are clearly localized within the imbricate thrust and thrust unit. This accretion-related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation, thickening of the sedimentary sequence, and earthquake-induced liquefaction. These processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism. Citation: Yamamoto, Y., H. Mukoyoshi, and Y. Ogawa (2005), Structural characteristics of shallowly buried accretionary prism: Copyright 2005 by the American Geophysical Union /05/2005TC Rapidly uplifted Neogene accreted sediments on the Miura-Boso Peninsula, central Japan, Tectonics, 24,, doi: / 2005TC Introduction [2] The study of modern accretionary prisms provides a rare opportunity to develop a better understanding of primary deformation and the physical changes in semilithified muddy sediments [e.g., Karig, 1986; Moore, 1989; Maltman et al., 1993; Moore and ODP 171A Shipboard Scientific Party, 1998]. However, research on modern examples has so far been restricted to core-scale analyses, in which it is difficult to trace the successive variations in physical and deformation properties. The examination of on land examples of accretionary prisms can provide important data on kinematics, related deformation styles, geometries, fluid-deformation interactions, and local tectonics in four dimensions and on various scales, and many detailed structural analyses have been conducted for the Shimanto, Kodiak, and Franciscan accretionary prisms [e.g., Sample and Moore, 1987; Ujiie, 1997, 2002; Hashimoto and Kimura, 1999; Kusky and Bradley, 1999]. However, the original structural features that ought to be preserved in muddy layers are often no longer present in on land examples of accretionary prisms, and the successive changes in the physical properties of shallow sediments (less than m below seafloor) are often overprinted by strong deformation and metamorphism during incorporation and uplift. The study of young, nonmetamorphosed examples of accretionary prisms on land would therefore be a great complement to existing research. However, such examples are very rare, as uplift is inevitably accompanied by continuing on land and tectonic processes such as erosion, structural and thermal overprinting associated with successive underplating, and rearrangement of the accretionary prism in association with out-of-sequence thrust (OST) movement. [3] The upper Miocene Miura-Boso accretionary prism is a unique example of an on land accretionary prism 1of17

2 Figure 1. (top) Plate configuration of the Izu-Bonin Arc collision zone, central Japan. (bottom) Geologic map of the (left) Miura and (right) Boso peninsulas. The key tuff beds So, Ns, and Mk are shown. Enlarged cross section of the Miura Peninsula is described in Figure 3a. (Figure 1), representing a shallow burial structure composed of alternating beds of diatomaceous siltstone and volcaniclastic sand/pebblestone. The siltstone in this accretionary prism retains a porosity of 35 50% [Yamamoto, 2003], and has avoided strong metamorphism. The volcaniclastic layers have characteristic features that identify individual beds, and the well-established chronology makes it possible to study the deformation styles on individual horizons in three dimensions. While a wide range of analyses examining the shallow part of the accretionary prism are possible in extremely high resolution, the present paper focuses on accretion-related deformation in terms of the spatial distribution, geometry, and style of deformation. There are many previous studies on sedimentology and stratigraphy [e.g., Kotake, 1988; Saito, 1992; Kotake et al., 1995; Lee and Ogawa, 1998; Stow et al., 1998], but structural analyses are rare. We also present quantitative analyses on the paleotemperature and burial depth. 2. Geologic Setting [4] The Miura-Boso accretionary prism consists of the Misaki and Nishizaki formations, which are exposed in the southern parts of the Miura and Boso peninsulas, respectively (Figures 1 and 2). These formations are unconformably overlain by trench slope cover sediments corresponding to the Hatsuse and Kagamigaura formations, respectively. Younger cover sediments of the Chikura Group are also distributed in the southern Boso Peninsula (Figure 1). [5] The Misaki and Nishizaki formations have depositional ages of 9 6 Ma (based on calcareous nannofossils and K-Ar dating [Kanie et al., 1991; Okada et al., 1991]) 2of17

3 Figure 2. Geologic map and cross section of the Nishizaki and Kagamigaura formations on the Boso Peninsula (area delineated in Figure 1). and Ma (based on radiolarian biostratigraphy [Kawakami, 2001; Yamamoto and Kawakami, 2005]), respectively. The depositional age of the Miura-Boso accretionary prism can therefore be set roughly as 10 6 Ma. The Hatsuse Formation is assigned as CN10c (5 4.8 Ma [Berggren et al., 1995]) based calcareous nannofossils [Kanie et al., 1991], while the Kagamigaura Formation contains the CN11-12 boundary [Saito, 1992] (3.75 Ma [Berggren et al., 1995]) and the C2Ar-C2An.3n geomagnetic boundary [Yamamoto and Kawakami, 2005] (3.58 Ma [Cande and Kent, 1995]). The Chikura Group is estimated to be Ma in age based on geomagnetic chronology [Kotake et al., 1995]. [6] The Misaki and Nishizaki formations are exposed along the present-day plate boundary, the Sagami Trough that is still subjected to oblique plate convergence [Ogawa et al., 1989; Seno et al., 1989]. Therefore subduction zone that the Misaki and Nishizaki formations were accreted on formed along the Sagami Trough. The evidences that they consist of accretionary prism include the inferred origin of clastic sediments and paleoenvironmental analysis. These formations only contain hemipelagic diatomaceous siltstone and volcaniclastic material derived from the Izu-Bonin arc. The Izu-Bonin origin is supported by the observation that the volcaniclastic layers become thinner and the grain size decreases from west to east [Taniguchi et al., 1991a; Lee and Ogawa, 1998], suggesting that the volcaniclastic material was derived from west of the Miura Peninsula, corresponding to the Izu-Bonin arc [Taniguchi et al., 1991a]. In contrast, the Kagamigaura Formation and the lowest Chikura Group contain red recrystallized chert derived from the Honshu island arc [Yamamoto and Kawakami, 2005]. Furthermore, while the volcaniclastic material in the Misaki and Nishizaki formations is composed of low-alkaline tholeitic basalt, that in the Hatsuse Formation has a calc-alkaline composition [Taniguchi et al., 1991a, 1991b]. [7] In addition to the sediment origin, the paleowater depth during deposition of the Misaki and Nishizaki formations is estimated to be m based on benthic foraminifera fossils [Akimoto et al., 1991; Saito, 1992], which is much deeper than any sedimentary basin in the present Honshu island arc but similar to the depths prevalent in the present Izu-Bonin forearc. This differs considerably from the estimated depth of m for deposition of the Chikura Group [Saito, 1992]. Therefore there are considerable differences in the origin of clastic material 3of17

4 Figure 3 4of17

5 Figure 3. (continued) and the sedimentary environment between the Misaki- Nishizaki formations and the overlying cover sediments (Kagamigaura and Hatsuse formations and Chikura Group). [8] Finally, the Misaki and Nishizaki formations are highly deformed, exhibiting several stages of faulting as well as evidence of large layer-parallel shortening such as duplex structures and imbricate thrusts [Hanamura and Ogawa, 1993; Yamamoto et al., 2000]. This mode of deformation differs markedly from that exhibited by the Hatsuse and Kagamigaura formations, which do not host large numbers of faults [Yamamoto and Kawakami, 2005]. This suggests that the Misaki and Nishizaki formations were deformed before deposition of the Hatsuse and Kagamigaura formations. The nature of deformation is indicative of accretion of the Misaki and Nishizaki formations, which were originally deposited on the Izu arc side [Soh et al., 1989, 1991; Hanamura and Ogawa, 1993; Lee and Ogawa, 1998; Yamamoto and Kawakami, 2005], whereas the Hatsuse and Kagamigaura formations were deposited on the Honshu forearc [Soh et al., 1991; Yamamoto and Kawakami, 2005]. [9] The clockwise rotation of the Misaki and Nishizaki formations has been verified by paleomagnetic study [Kanamatsu, 1995; Yamamoto and Kawakami, 2005]. The rotation occurred in two stages of the Izu-Bonin arc collision, in the period Ma probably in association with the Tanzawa block collision, and around 1 Ma during the Izu block collision [Yamamoto and Kawakami, 2005]. The original WSW-ENE trending plate boundary has thus been bent convex northward (Figure 1), requiring paleomagnetic correction in order to identify the original direction of the bedding plane and structural data. 3. Structural Units of the Upper Accretionary Prism 3.1. Identification of Structural Units [10] The deformation structures and structural vergence in the upper part of the Misaki Formation have been reported to differ considerably from those in this lower section [Yamamoto et al., 2000], exemplified by NW verging gravity-driven slump deposits in the upper part and SE verging thrust systems in the lower part. Similar differences in deformation regime between the upper and lower sections, specifically a concentration of thrust systems in the lower part and slumping- and liquefactioninduced chaotic sediments in the upper part, have also been recognized in the Nishizaki Formation. [11] In this paper, the thrust system-dominated lower part in the Misaki and Nishizaki formations is defined as a thrust unit, and the nontectonic upper part is defined as an upper coherent unit. In the Misaki Formation, the thrust unit contains the key tuff bed Mk, while the upper coherent unit is marked by the tuff bed So (Figure 1). In the southernmost Miura Peninsula, the pair of the thrust unit (containing Mk ) and the coherent unit (So) is overlapped by the Jogashima Thrust (Figures 1 and 3a). This distribution indicates that the sedimentary sequence of the Misaki Formation was cut, duplicated, and thickened by imbricate thrusts. The Miura-Boso accretionary prism is therefore considered to be composed Figure 3. Photographs of the Sengen imbricate thrust on the southernmost Miura Peninsula. (a) Schematic cross section of the Sengen Thrust and adjacent area. Dashed line denotes the boundary between eroded and exposed parts. (b) Photograph of the Sengen Thrust (looking east). (c) Photograph of the area delineated in Figure 3b (plan view). (d) Cross section of the Sengen Thrust. Figures 3e 3j are photomicrographs of the imbricate thrust. Split arrows indicate the shear sense. (e and f) Plain light observations of the gouge zone (Figure 3e) and breccia zone (Figure 3f). (g) Reflected microscopy of the breccia zone. (h) Undeformed radiolarian fossil (white arrow) identified in the breccia zone. (i) Crossed nicols observation of the gouge zone in Figure 3e. (j) Crossed nicols observation of the breccia zone in Figure 3f using an interference plate. 5of17

6 Figure 4. Structural data for the Sengen Thrust (imbricate thrust). Equal-area projection to the lower hemisphere. of two structural units and an imbricate thrust. Although the imbricate thrust is not recognized on land in the Nishizaki Formation, the exposure to the SE of trench fill conglomerates (Shirahama Formation, Chikura Group; Figure 1) deposited contemporaneously with the end of accretion suggests that the imbricate thrust may have developed to the south [Yamamoto and Kawakami, 2005] Imbricate Thrust [12] The imbricate thrust (Jogashima Thrust) and the thrust-related folds extend WNW-ESE parallel to the major geologic trend in the southernmost Miura Peninsula (Figure 1). An overturned anticline (Jogashima Anticline) and an open syncline (Jogashima Syncline) associated with the thrust movement are present on the northern hanging wall side and the southern footwall side, respectively (Figures 1 and 3a). On the hanging wall side, the key tuff layer Mk (circa 9 Ma in age) forms the anticline. On the footwall side, the exposed sediments are composed of the uppermost Misaki Formation (circa 6 Ma; So) and the lowermost Hatsuse Formation in unconformable contact. On a macroscopic scale, this Jogashima Thrust branches off from the basal décollement zone, cutting and duplicating almost the entire Misaki Formation. Another large-scale thrust (Sengen Thrust), a branch from the Jogashima Thrust, is recognized in the Sengen area, a few kilometers east of the Jogashima area (Figure 1). [13] We estimate the original compacted thickness between Mk and So to be approximately 300 m, equivalent to the amount of uplift associated with the thrust faulting. Considering the thrust related Jogashima anticline and Sengen Thrust, the total amount of uplift associated with the imbricate-thrust system apparently reaches 1000 m or more. [14] The Sengen Thrust strikes almost E-W, and dips northward (Figure 4), paralleling the axis of the Jogashima Anticline and cutting the interlimb to the north. The Sengen Thrust is composed of three distinct parts: a black gouge zone of a few centimeters thick as a main slip surface, a breccia zone of a few meters thick on the hanging wall side, and a shear band zone on the footwall side (Figures 3c and 3d). The boundaries between the gouge zone and the breccia or shear band zones are undulatory (Figure 3d). The gouge injects in places into the breccia zone and the shear band zone, indicative of high syntectonic fluid pressure in the gouge zone. Blocks of breccia are entrained in the gouge zone in association with these injections (Figure 3d). The gouge zone and breccia zone contain arrays of asymmetric fish (or S-C)-like structures with a dip-slip thrust sense (Figures 3d, 3g, and 4), and exhibit strong foliation parallel to the thrust plane on an outcrop scale (Figure 3c). The foliation in the gouge zone is more intense than that in the breccia zone, displaying strong clay mineral alignment (compare Figures 3e and 3i) oriented parallel to the thrust plane (Y shear plane) with oblique P foliation. [15] The clay mineral alignment in the breccia zone is weaker, with development subparallel to the thrust plane. The alignment never cuts through the brecciated clasts (compare Figures 3f and 3j). Clay minerals are oriented systematically in the matrix of the breccia zone with preferred orientation parallel to the thrust, whereas the orientation of fabric within clasts is random, apparently preserving the texture of the original host rock without overprinting by subsequent shear deformation. In contrast, the preferred clay mineral orientation in the gouge zone and outer rim of each brecciated clast reflects the realignment associated with shear deformation. The brecciated clasts in the breccia zone are clearly smaller within a few centimeters from the gouge zone (Figure 3d), accompanied by a higher matrix-to-clast ratio. These features indicate that the shear deformation and associated clay mineral orientation are localized in the gouge zone and become weaker with distance on the hanging wall side. [16] The coarser sand and silt grains scattered in the gouge, breccia, and shear band zones are angular to subangular in shape and are oriented subparallel to the thrust plane. The grain size of the coarser grains is relatively consistent throughout the gouge, breccia, shear band, and coherent layers, and the silt and fine sand do not appear to have been destroyed by the shear. Furthermore, the gouge and breccia zone contains undeformed radiolarian fossils (Figure 3h). These features suggest that this thrust was characterized by independent particulate flow, representing soft sediment deformation. [17] Calcite veins are commonly recognized on the hanging wall side of the thrust. Although the zone of calcite veining extends across stratigraphic horizons along the thrust, it is limited to within 100 m of the thrust itself, 6of17

7 Figure 5. Photographs of the thrust unit exposed in the Nishikawana area on the southernmost Boso Peninsula. White or light gray colors denote siltstone. The positions of Figures 5a and 5e are shown in Figure 8. Split arrows indicate the positions of the thrust and the shear sense, and black arrows denote the key bed. (a and b) Examples of duplex structures. (c) Slickenlines on the surface of the thrust fault (from duplex, Figure 5a). (d and e) Examples of drag fold syncline (Figure 5d) and anticline (Figure 5e). (f and g) Adjacent photomicrographs of thrust faults forming the duplex structure. White arrows indicate gouge injections into the coherent hanging wall layers. indicating that fluid flowed only along the imbricate thrust or in the immediate vicinity Thrust Unit [18] The thrust unit is defined as the thrust-dominated zone in the lower structural unit of the Misaki and Nishizaki formations immediately above the imbricate thrusts (Jogashima and Sengen thrusts). The upper limit of this unit is transitional in the Misaki Formation, probably occurring between the key tuff beds of Mk (9 Ma) and Ns, but is better recognized in the Nishizaki Formation (Figure 2). [19] Outcrop-scale duplications (dominantly a duplex structure; Figures 5a and 5b) associated with thrusts and 7of17

8 Figure 6. Key bed and unit distribution based on 1/250 mapping of the thrust unit in the Nishikawana area (area delineated in Figure 2). Numbered black arrows denote the thrust faults forming the duplex arrays. 8of17

9 Figure 7. Structural data for the thrust unit in the Nishizaki Formation (equal-area projection to the lower hemisphere). related folds (Figures 5d and 5e) are dominant in the thrust unit. One outcrop-scale duplication in the Nishizaki Formation, defined by a single key bed, involves more than three duplications, as indicated by circles in Figure 2. Over 100 duplications were recognized in the Nishizaki Formation, with an extreme concentration of duplication in the stratigraphic zone around the southern coastal line (Nishikawana area, Figure 6, box in Figure 2). This area hosts the lowest part of the Nishizaki Formation (approximately Ma [Kawakami, 2001]). [20] The duplicated strata are concentrated in a zone of approximately 200 m in thickness and can be traced laterally for at least 1500 m. This zone is predominantly located above the northern rim of the Ito anticline, but extends in part along the axis and to the southern rim (Figure 2). The lithofacies changes dramatically from the lower to upper horizons. The lower horizon is composed of pumiceous sandstone-dominated alternating beds of sandstone and siltstone to an estimated thickness of approximately 60 m, whereas the upper horizon is similar to other parts of the Nishizaki Formation in that it consists mainly of siltstone-dominated alternating beds of siltstone and scoriaceous sandstone with tuff beds of various colors. The duplications are apparent on both sides of this lithologic boundary. [21] The vergence of these thrust duplications was determined based on the attitude of the slickenlines, the directions of axes of the thrust anticlines and drag folds, and the array of the thrust faults. These structural data indicate that the duplications verge SW to WSW before correction (Figure 7), and S to SSE after correction for the clockwise rotation associated with Izu-Bonin island arc collision following accretion (40 60 clockwise rotation) [Yamamoto and Kawakami, 2005]. The original vergence is therefore similar to that of the Misaki Formation (SE vergence) [Yamamoto et al., 2000]. [22] At least 20 major thrusts have been recognized in 1/250-scale surveys (Figure 6). The duplications form wellregulated arrays along the major thrusts, duplicating structurally lower strata in the east and upper strata in the west. The duplications are arranged on planes extending from SE to NW, from lower strata duplication to upper strata duplication. The thrust faults cut and duplicate the lower key bed or unit, and converge to a single thrust above the duplication. The thrust then branches off again and forms another duplication in the upper strata (Figure 8). The displacement of each major thrust ranges from 4 to 75 m, and the total displacement in the Nishikawana area reaches approximately 594 m. The original compacted thickness of the beds that compose the Nishikawana area is estimated to be approximately 150 m (key bed A to F in Figure 6). They were duplicated and thickened by thrust movement, and apparent thickness reached approximately 240 m. The thickening factor is therefore 1.6. [23] The thrust faults responsible for the duplications have thin gouges of approximately 1 2 mm in thickness (at most 5 mm). The gouge has scaly foliations with P and Y systems identifiable by preferred mineral orientation (Figures 5f and 5g). The gouge contains tuffaceous clasts with characteristic R1 brittle shear planes, and injects into the hanging wall side where it displays abundant hostsiltstone clasts. These gouge injections indicate that these 9of17

10 Figure 8. Key bed distribution in the Nishikawana area corresponding to the areas delineated in Figure 6. The younging direction is to the north (upward in this figure). The contour of the topographic map is 20 cm. 10 of 17

11 Figure 9. Photographs of chaotic sediments in the upper coherent unit. Black denotes volcaniclastic sandstone or conglomerate, and white denotes siltstone. (a) Layered body of liquefied chaotic sediments containing siltstone blocks in black sand-pebble matrices. (b) Massive chaotic sediments associated with injection of liquefied sediments into coherent layers. (c and d) Examples of chaotic sediments associated with gravity sliding, showing flow folds composed of sandstone layers. (e and f) Mudflow structures associated with gravitational instability. Split arrows indicate the positions of the sticking faults and the shear sense, and the white arrow indicates an undeformed foraminifera fossil. thrusts occurred before the sediments had lithified and while the fluid pressure was high. Pumice clasts are stretched and display S-C structures in thrusts cutting the pumiceous sandstone or conglomerate. All of these asymmetric fabrics match the direction of thrust movement well. Calcite veins and calcite-supported breccia are also present in the thrusts, suggesting fluid migration along these thrust faults. [24] Liquefied chaotic sediments are distributed in the lower pumiceous sandstone/conglomerate-dominant horizon of the Nishikawana area, where the sediments are both injected upward through the thrust faults and cut by the thrust faults. The interrelations between the liquefaction and thrust faulting in the thrust unit indicate that the thrust systems formed before lithification of the sandstone and conglomerates Upper Coherent Unit [25] Thrusting and related deformation are extremely rare in the upper part of the Misaki and Nishizaki formations, but chaotic sediments are widespread (Figure 9). This coherent unit, which lies immediately above the thrust unit, contains the key tuff beds So (6 Ma) and Ns in the Misaki Formation, and strata displaying Ma radiolarian biostratigraphy [Yamamoto and Kawakami, 2005] in the Nishizaki Formation. The coherent unit is partly eroded by the Hatsuse and Kagamigaura formations, both shallow marine sedimentary sequences derived from the other island arc (Honshu Arc). [26] Detailed descriptions and analyses of the origin of the chaotic sediments have been reported for the Misaki Formation [Yamamoto et al., 2000; Yamamoto, 2003]. The chaotic sediments are considered to have formed by earthquake-induced subsurface liquefaction and injection, and by gravity-driven slumping on the seafloor. Most of the chaotic sediments exhibit a layered distribution parallel to the bedding plane, and the layered bodies of liquefied chaotic sediments contain light gray siltstone blocks in black sandpebble matrices (Figure 9a). These sediments are injected into adjacent coherent layers and form a massive distribution of huge blocks (Figure 9b). The layered bodies of the liquefied chaotic sediments have homogeneous matrices free of sedimentary or tectonic structure, whereas the injected sediments exhibit flow structures and laminae. [27] The chaotic sediments associated with gravity sliding have layered bodies with siltstone-dominated matrices, slip plane below the chaotic sediments, and flow folds consist of sandstone layers (Figures 9c and 9d). The liquefied chaotic sediment bodies always cut the gravity slide structures, indicating that liquefaction occurred after gravitational sliding. Mudflow structures associated with gravitational instability are also well developed in the upper coherent unit (Figure 9e). The foliation associated with the mudflow has an oblique attitude to the bedding plane and is cut by layer-parallel deformation bands, indicative of independent particulate flow deformation (Figures 9e and 9f). [28] The same classification holds for the Nishizaki Formation. The limit of chaotic sediment development can be recognized on the coastal line of the Nishizaki Formation (Figure 2). Chaotic sediments are dominant north of this boundary, but are extremely rare to the south. Approximately 100 m south of this boundary, a major thrust forms 11 of 17

12 Figure 10. Histogram showing vitrinite reflectance for the Misaki and Nishizaki formations (sampling sites are indicated in Figures 1 and 2). another boundary (bold line in B B 0 cross section). The region north of the latter boundary corresponds to the upper coherent unit of the Nishizaki Formation, and region south of the boundary corresponds to the thrust unit. 4. Maximum Temperature [29] The Miura-Boso accretionary prism consists of highly porous and nonmetamorphosed volcaniclastic and hemipelagic sediments. The maximum paleotemperature of these sediments was estimated based on vitrinite reflectance data. [30] Five samples of organic matter were collected from siltstone of the Misaki (MSV01, MSV02) and Nishizaki formations (NZV03-1, NZV03-2, NZV06) (Figures 1 and 2). The reflectance data were measured using a silicone-diode microphotometer with polarized 546 nm light following the International Organization for Standardization (ISO) standard , the American Society for Testing and Materials (ASTMD) standard , and the Japan Industrial Standard (JIS) M [31] The representative equations for calculation of maximum temperature from the reflectance R O presented by Sweeney and Burnham [1990] are used in this study. The heating time is defined as the total time that a sample stays between the maximum temperature it ever attains and 14.4 C below this maximum temperature [Sekiguchi and Hirai, 1980]. As this time is typically 1 10 m.y. on active margins [Laughland and Underwood, 1993; Ohmori et al., 1997], two values are calculated to provide lower and upper limits corresponding to 1 and 10 m.y. The equations are as follows [Sweeney and Burnham, 1990]: Tð CÞ ¼ 174 þ ð93 ½ln percentage R O ŠÞ Tð CÞ ¼ 158 þ ð90 ½ln percentage R O ŠÞ [32] Most samples have very low and relatively constant values of R O, independent of the rock facies or geologic structure. The R O values range from 0.2 to 0.3%, with a high concentration of values near 0.25% (Figure 10 and Table 1). Several grains had a high R O ( %), and are considered to represent reworked vitrinite that had been entrained form deeper levels of thermal maturity [Bostick, 1979; Dow and O Connor, 1982]. Such data were therefore excluded. The standard calibration deviation was 0.06% or less for each sample, corresponding to an error range of ±15 C. Table 1. Vitrinite Reflectance, Paleotemperature, and Lithofacies a Sample R O,% R O II, % Paleotemperature, C 10 m.y. b 1 m.y. b Host Rock NZV (±0.05) 0.25 (±0.05) 33 (±15) 45 (±15) siltstone NZV (±0.05) 0.27 (±0.05) 40 (±15) 52 (±15) siltstone NZV (±0.05) 0.22 (±0.05) 20 (±15) 32 (±15) siltstone MZV (±0.05) 0.27 (±0.05) 40 (±15) 52 (±15) siltstone MZV (±0.05) 0.25 (±0.05) 33 (±15) 45 (±15) siltstone a R O II is R O data after elimination of the reworked vitrinite data. Paleotemperatures are estimated using the equations reported by Sweeney and Burnham [1990]. b Heating time. 12 of 17

13 [33] It has been reported that scattered R O values, indicating heterogeneity of the thermal maturity of vitrinite in the samples, are unreliable and do not reflect the interrelations with depth for extremely shallow burial (R O < 0.5%) [Hirai, 1979], in which cases the low R O values may not indicate reasonable maximum heating temperatures. However, as the R O values in the Misaki and Nishizaki formations are relatively constant (Figure 10), the level of thermal maturity in the Miura-Boso accretionary prism appears to be homogeneous and R O can be used to estimate the maximum temperature. The R O value of the Misaki and Nishizaki formations suggests a predominant maximum temperature of 40 C, with a range from approximately 20(±15) to 52(±15) C (Table 1). 5. Summary and Discussion 5.1. Paleotemperature and Burial Depth [34] The Miura-Boso accretionary prism exposed in the south of the Miura and Boso peninsulas is young and nonmetamorphosed, exhibiting a maximum paleotemperature of only 52(±15) C. This temperature suggests a maximum burial depth of approximately 1000 m assuming a paleogeothermal gradient similar to that in the present Nankai Trough (40 50 C/km) [Ashi and Taira, 1993]. The apparent total thickness of the Misaki and Nishizaki formations is estimated to be about 700 and 400 m, respectively [Kodama et al., 1980; Kawakami, 2001], overlain by approximately 400 and 300 m of sediments of the Hatsuse and Kagamigaura formations [Saito, 1992]. The burial depth of the deepest part of the Misaki and Nishizaki formations is therefore estimated to be approximately 1000 m below seafloor (mbsf) based on these stratigraphic analyses. The porosity of the siltstone layers in the Miura- Boso accretionary prism, 35 50% [Yamamoto, 2003], is comparable to that at depths of mbsf in the present Nankai accretionary prism [e.g., Taira et al., 1992; Ujiie et al., 2004]. The maximum burial depth of the Miura-Boso accretionary prism can therefore be concluded to be less than 1000 m. This is further supported by the ductile flow of siltstone of the Miura Group under 100 MPa confining pressure in triaxial tests, which demonstrates that burial compaction must have been shallower than 3 km [Hoshino et al., 1972]. Although numerous small-scale structures are present in the Misaki [Ogawa and Horiuchi, 1978] and Nishizaki formations [Yamamoto and Kawakami, 2005], all have developed under shallow burial conditions in a semilithified state. This is supported by the observation of drag folds due to ductile deformation associated with formation of the thrust system, crosscutting relationships between the thrust systems and liquefaction, and gouge injection Deformation and Comparison With Modern Accretionary Prisms [35] Modern accretionary prisms developing at similar shallow levels characteristically occur by off scraping rather than underplating [e.g., Taira et al., 1992]. In the Misaki and Nishizaki formations, the shear strain is localized only in the lower part (thrust unit and imbricate thrust), while the upper part (coherent unit) exhibits no evidence of shear strain associated with accretion and the sequence is capped by trench slope sediments. These features all support off scraping as the accretion style for the Miura-Boso accretionary prism. [36] According to paleowater depth analyses [Akimoto et al., 1991; Saito, 1992; Kitazato, 1997], the top surface of the off-scraped sediments was uplifted by m prior to deposition of the trench slope cover sediments, accompanied by rotation and thickening of the sedimentary sequences due to accretion. Given this uplift and total thickness, and the original thickness of the accreted sediments ( m), the plate boundary basal décollement is likely to have extended to approximately m below the top of the off-scraped body of the Misaki and Nishizaki formations. This type of setting corresponds to the imbricate thrust zone (ITZ) in the present Nankai Trough [e.g., Moore et al., 2001]. Furthermore, given the burial depth of the Miura-Boso accretionary prism, approximately 1000 mbsf based on vitrinite reflectance and porosity data, the present exposure of the Miura-Boso accretionary prism appears to reflect the middle to upper part of the off-scraped body. [37] A schematic cross section of the Miura-Boso accretionary prism and the structural column showing properties of these structural units are shown in Figures 11 and 12, respectively. Structural analyses of the Misaki and Nishizaki formations suggest that an off-scraped slice in the toe of the accretionary prism was divided into two units, the upper coherent unit, and the thrust unit. These units would have been cut and duplicated by the imbricate thrust that branched off from the décollement zone and developed immediately below the thrust unit. The cover sediments (Hatsuse and Kagamigaura formations and the Chikura Group) unconformably overlay these off-scraped sediments. [38] The upper coherent unit corresponds to the uppermost part of the off-scraped body (Figure 11). The unit contains only rare thrust faulting, but is rich in chaotic sediment associated with gravity flow and liquefaction (Figure 12). These gravity-driven chaotic sediments are rare in the thrust unit, indicating that gravitational instability was induced by bedding tilt near the convergent boundary [Yamamoto et al., 2000]. Liquefaction of the underlying sediment occurred in an unlithified condition [e.g., Tsuji and Miyata, 1987], and the liquefaction features cut the gravity flow. These features indicate that liquefaction occurred just after or during accretion. If the chaotic phenomena occurred at an arc-ward distance from the convergent boundary, the sediments should have been more compacted and thus not sufficiently soft to allow liquefaction. This is consistent with the observations of liquefaction sediments cutting the accretion-related thrusts and vice versa. [39] The thrust unit, above the imbricate thrust and below the upper coherent unit, corresponds to the lower part of the off-scraped body (Figure 11). This unit contains a dense distribution of thrust systems (including duplex structures), which are rare in the upper coherent unit. These thrust systems verge to the SE in the Misaki Formation 13 of 17

14 Figure 11. Schematic showing the cross section and geometry of the off-scraping accretionary prism. (a) Schematic cross section showing the shallow part of the accretionary prism. Off-scraped sediments (consisting of the imbricate thrust, thrust unit, and upper coherent unit) correspond to the Misaki and Nishizaki formations, and cover sediments correspond to the Hatsuse and Kagamigaura formations and Chikura Group. (b) Geometry of thrust unit. (c) Cross section of the upper coherent unit. [Yamamoto et al., 2000], and to the S or SSE in the Nishizaki Formation after paleomagnetic restoration. This vergence is subperpendicular to the orientation of the proto- Nankai Trough [Seno et al., 1989]. [40] The distributions of gravity-driven chaotic sediments and thrust systems are consistent with the modern examples of off-scraped accretionary prisms. For example, Site 808 in the Nankai Trough penetrated the frontal thrust and décollement zone of the Nankai subduction zone and recovered slump deposits that were only present in the uppermost part [e.g. Taira et al., 1992]. On the other hand, fault distribution in the borehole showed marked concentration in the lower part, but scarce in the upper part. [41] The imbricate thrust accompanied by a thrust-related anticline, lying below the thrust unit in the lowermost Misaki Formation (Figures 1 and 3), is primarily composed of a black gouge of a few centimeters thick as a main slip surface, a breccia zone of a few meters thick on the hanging wall side of the gouge, and a shear band on the footwall side. The arrays of fish-like structures in the breccia zone are consistent with the vergence of the thrust systems in the thrust unit. [42] The deformation features of the imbricate thrust are similar modern examples, such as the décollement zones of the Nankai and Barbados accretionary prisms, and the Costa Figure 12. A schematic structural column showing properties of the structural units of the Miura-Boso accretionary prism. The relative frequencies of slumps liquefied sand layers, accretion-related thrusts, and calcite veins are indicated by line thickness. 14 of 17

15 Rica convergent margin. The clay mineral orientations in the breccia zone, preferred alignment along the outer rims of the brecciated clasts and random internal fabrics (Figures 3j and 12), are also similar to the breccia texture in the upper part of the décollement zone in the Costa Rica [Vannucchi and Tobin, 2000] and Nankai [Morgan and Karig, 1995; Ujiie et al., 2003] examples. The decreasing clast size distribution and increasing intensity of shear deformation and clay mineral alignment downward toward the gouge zone are consistent with the features of the décollement zone in the Costa Rica margin and the Nankai accretionary prism. [43] The deformation features exhibited by the Miura- Boso accretionary prism are consistent with those of modern shallow accretionary prisms and convergent margins in many respects. However, the deformation bands reported for modern accretionary prisms are indistinct in the Miura-Boso accretionary prism. These deformation bands are observed as planar zones in which clay minerals are aligned subparallel to the edge of the zone, and are dominantly bedding oblique in attitude with small displacement. Although these features are abundant in the Miura-Boso accretionary prism, most exhibit layer-parallel attitudes with respect to the bedding plane [Hanamura and Ogawa, 1993; Yamamoto et al., 2000], and no kink-like [Maltman et al., 1993; Vannucchi and Tobin, 2000] or shear-induced S-C [Ujiie et al., 2004] internal textures have been identified. [44] There is a possibility that subsequent deformation and/or lithification processes overwrote the deformation bands. However, this is improbable because sediment-filled vein structures are exceptionally well developed in the Miura-Boso accretionary prism [Pickering et al., 1990; Hanamura and Ogawa, 1993; Maltman, 1998]. Vein structures have also been reported for modern convergent plate margins [e.g., Lundberg and Moore, 1986; Ogawa et al., 1992]. In either model of the origin of the vein structures, whether by gravity-driven down-slope movement [Pickering et al., 1990] or earthquake waves [Hanamura and Ogawa, 1993; Brothers et al., 1996], the sediments in which the veins formed are unlithified, indicating that the events occurred soon after sedimentation. Therefore the vein structures appear to have formed in an earlier stage or even contemporaneously with deformation band formation. Considering the high preserved porosity of siltstone exposed in this area, these observations preclude any interpretation that while the initial deformation were preserved the deformation bands were not. It is plausible that the deformation bands failed to develop because of specific mechanical properties of the sediment or complete strain localization within the imbricate thrust. It is also possible that the deformation bands may have developed in a restricted, unexamined area Duplex Arrays in the Thrust Unit [45] The thrust unit consists of thrust systems that form outcrop-scale duplex structures. These thrust faults duplicate the same unit and converge upward, then branch off again to form the next duplication. These outcrop-scale duplex structures therefore develop into well-regulated arrays, which can be seen along the step-up thrust associated with the repetition of branching and convergence. The duplexing of tectonically lower horizons occurs in the east, while the upper horizons are duplicated in the west (Figures 6 and 8). At least 20 thrust array domains can be recognized in this area associated with the step-up thrust from the SE-NW. These domains appear to converge to the imbricate thrust at the limit of the thrust unit. [46] Considering the concentration of these duplications within a limited zone, with only rare examples in the upper coherent unit, it is inferred that the domains converge upward in this area to form a large-scale (kilometer order) duplex structure (Figure 11b). It can therefore be concluded that the thrust unit consists of large-scale duplex structures formed by smaller outcrop-scale duplex arrays. Duplex arrays have also been reported for the Miocene Emi Group in the mid Boso Peninsula, and such structures may play an important role in the thickening of the entire shallow part of the accretionary prism [Hirono and Ogawa, 1998]. However, in the Misaki and Nishizaki formations, the duplex arrays develop only in the thrust unit, not in other zones of the off-scraped sediments, and the amount of thickening (approximately 90 m) and thickening factor (1.6) is not large. The duplex array is therefore restricted to the thrust unit in the off-scraped accretionary prism, where the resultant thickening of the off-scraped body is reflected by macroscale and mesoscale duplications: overlapping and rotation of each off-scraped slice by the imbricate thrust (Figure 11a); and thickening of each slice by duplex arrays in the thrust unit (Figure 11b), respectively. The former is the main contribution to thickening off-scraped body, while the latter contributes less than 10% of the expected total amount of thickening calculated from stratigraphic and paleowater depth analyses Deformation Style and Fluid Migration [47] The faults gouges of the imbricate thrust and thrust faults in the thrust unit are injected into the hanging wall breccia zone and coherent layers (Figures 3d, 5f, 5g, and 12). These injections produced clasts containing both host and hanging wall sediments. These features are suggestive of high fluid pressure along these thrusts. The gouge and breccia zones in the imbricate thrust contain undeformed radiolarian fossils (Figure 3h), as well as large angularsubangular clastic grains of uniform grain size (compare Figures 3e and 3f). In the thrust faults, the clastic grains in the gouge are larger than those in the coherent layers of the hanging wall (Figures 5f and 5g). These features indicate that cataclastic deformation did not occur during the off-scraping deformation of the Miura-Boso accretionary prism. That is, deformation occurred by independent particulate flow. [48] In the Nankai accretionary prism, the décollement zone exhibits high bulk density, low porosity and elevated fluid pressure [e.g., Taira et al., 1992; Morgan and Karig, 1995; Ujiie et al., 2003], apparently resulting from a twostage process involving destruction of cementation by fluid pressure fluctuations and pore collapse due to clay mineral rotation along the slip surface [Morgan and Karig, 1995; 15 of 17

16 Ujiie et al., 2003]. The first stage of deformation is preserved as a random fabric in the brecciated clasts, while the latter is evidenced by a preferred orientation at the outer rim of each clast. Similar processes are also expected to have occurred in the imbricate thrust in the Miura-Boso accretionary prism (and Costa Rica margin) due to the similarity in deformation style and texture. The detailed physical properties and clay mineral chemistry will be investigated in a future study. [49] Calcite veining is clearly localized on the hanging wall side of the imbricate thrust and in the thrust unit (Figure 12). In the Misaki Formation, the zone of calcite veining can be traced both eastward and westward along the imbricate thrust. In the Nishizaki Formation, calcite veins have developed in association with the thrust systems and succeeding conjugate transversal fault systems in the Nishikawana area (including the eastern succession). These calcite veins are probably evidence of methane-rich fluid migration considering the many reports on methane-rich fluid escape via the décollement zone and imbricate thrusts in modern accretionary prisms [e.g., Kobayashi, 2002]. [50] In the Nishizaki Formation, pumiceous sandstone and conglomerate dominate the sedimentary sequence exposed in the lower part of the thrust unit. These coarse sand and conglomerate layers were liquefied to form chaotic sediments by earthquakes associated with accretion. These liquefaction features are cut by thrust faults and vice versa. The liquefied chaotic sediments are in part developed along the thrust fault. It is known that liquefaction releases fluid at a rate of approximately 2% of the liquefied volume associated with a volume reduction [Tsuji and Miyata, 1987]. This liquefaction-derived fluid would therefore have elevated the fluid pressure under undrained conditions, and may have been a trigger of thrusting. The concentration of duplications in this stratigraphic zone is thus likely to have been controlled by these lithologic changes and the specific conditions of fluid flow. Pore collapse associated with shear strain and consolidation, and liquefaction of the coarsergrained sediment layer, would therefore have produced fluid as a control on the style and spatial distribution of deformation in the off-scraped accretionary prism. [51] Acknowledgments. Part of this study was supported by a grant-in-aid from the Japan National Oil Corporation. We are grateful to Kiyofumi Suzuki, Tatsuro Chiba, and Hisanori Wakamatsu for their assistance and for providing a 1/250-scale topographic map. We wish express our special thanks to the students who belong to the Press, University of Tsukuba, for their numerous and useful discussions. Gregory F. Moore and J. Casey Moore are appreciated for their detailed and constructive reviews, which improved the manuscript. Finally, we sincerely thank Ryo Murakami and his family for their kind hospitality during our field work. References Akimoto, K., E. Uchida, and M. Oda (1991), Paleoenvironmental analyses by using benthic foraminifera fossils from middle to upper Miocene Misaki Formation, southernmost of the Miura Peninsula, central Japan (in Japanese), Chikyu, 139, Ashi, J., and A. Taira (1993), Thermal structure of the Nankai accretionary prism as inferred from the distribution of gas hydrate BSRs, Mem. Geol. Soc. Am., 273, Berggren, W. A., D. V. Kent, C. C. Swisher, and M. P. Aubry (1995), A revised Cenozoic geochronology and chronostratigraphy, in Geochronology, Time Scales and Global Stratigraphic Correlation, edited by W. A. Berggren et al., SEPM Spec. Publ., 54, Bostick, N. H. (1979), Microscopic measurement of the level of catagenesis of solid organic matter in sedimentary rocks to aid exploration for petroleum and to determine former burial temperatures-a review, in Aspects of Diagenesis, edited by P. A. Scholle and P. R. Schluger, SEPM Spec. Publ., 26, Brothers, R. J., A. E. S. Kemp, and A. J. Maltman (1996), Mechanical development of vein structures due to the passage of earthquake waves through poorly consolidated sediments, Tectonophysics, 260, Cande, S. C., and D. V. Kent (1995), Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic, J. Geophys. Res., 100, Dow, W. G., and D. E. O Connor (1982), Kerogen maturity and type by reflected light microscopy applied to petroleum generation, in How to Assess Maturation Paleotemperatures, SEPM Short Course, 7, Hanamura, Y., and Y. Ogawa (1993), Layer-parallel faults, duplexes, imbricate thrust and vein structures of the Miura Group: Key to understanding the Izu fore-arc sediment accretion to the Honshu forearc, Island Arc, 2, Hashimoto, Y., and G. Kimura (1999), Underplating process from mélange formation to duplexing: Example from the Cretaceous Shimanto Belt, Kii Peninsula, southwest Japan, Tectonics, 18, Hirai, A. (1979), Vitrinite reflectance, J. Jpn. Assoc. Pet. Technol., 44, Hirono, T., and Y. Ogawa (1998), Duplex arrays and thickening of accretionary prisms: An example from Boso Peninsula, Japan, Geology, 26, Hoshino, K., H. Koide, K. Inami, S. Iwamura, and S. Mitsui (1972), Mechanical properties of Japanese Tertiary sedimentary rocks under high confining pressures, Geol. Surv. Jpn. Rep., 244, 200 pp. Kanamatsu, T. (1995), A study on the magnetic fabric of the sediment in the accretionary complex, Boso and Miura peninsulas, central Japan, Ph.D. thesis, 136 pp., Univ. of Tokyo, Tokyo. Kanie, Y., H. Okada, Y. Sasahara, and H. Tanaka (1991), Calcareous nannoplankton age and correlation of the Neogene Miura Group between the Miura and Boso peninsulas, southern-central Japan, J. Geol. Soc. Jpn., 97, Karig, D. E. (1986), Physical properties and mechanical state of accreted sediments in the Nankai Trough, Southwest Japan Arc, Mem. Geol. Soc. Am., 166, Kawakami, S. (2001), Upper Miocene radiolarians from the Nishizaki Formation and Ishido Group in the southern part of Boso Peninsula, Japan and their geological significance, News Osaka Micropaleontol., 12, Kitazato, H. (1997), Paleogeographic changes in central Honshu, Japan, during the late Cenozoic in relation to the collision of the Izu-Ogasawara Arc with the Honshu Arc, Island Arc, 6, Kobayashi, K. (2002), Tectonic significance of the cold seepage zones in the eastern Nankai accretionary wedge: An outcome of the 15 years KAIKO projects, Mar. Geol., 187, Kodama, K., S. Oka, and T. Mitsunashi (1980), Geology of the Misaki District, Quad. Ser. 8, 93 pp., Geol. Surv. Jpn., Tokyo. Kotake, N. (1988), Upper Cenozoic marine sediments in southern part of the Boso Peninsula, central Japan, J. Geol. Soc. Jpn., 94, Kotake, N., M. Koyama, and K. Kameo (1995), Magnetostratigraphy and biostratigraphy of the Plio-Pleistocene Chikura and Toyofusa groups, southernmost part of the Boso Peninsula, central Japan, J. Geol. Soc. Jpn, 101, Kusky, T. M., and D. C. Bradley (1999), Kinematics analysis of mélange fabrics: Examples and applications from the McHugh Complex, Kenai Peninsula, Alaska, J. Struct. Geol., 21, Laughland, M. M., and M. B. Underwood (1993), Vitrinite reflectance and estimates of paleotemperature within the Upper Shimanto Group, Muroto Peninsula, Shikoku, Japan, Mem. Geol. Soc. Am., 273, Lee, I. T., and Y. Ogawa (1998), Bottom-current deposits in the Miocene-Pliocene Misaki Formation, Izu forearc area, Japan, Island Arc, 7, Lundberg, N., and J. C. Moore (1986), Macroscopic structural features in Deep Sea Drilling Project cores from forearcs, Mem. Geol. Soc. Am., 166, Maltman, A. J. (1998), Deformation structures from the toes of active accretionary prisms, J. Geol. Soc. London, 155, Maltman, A. J., T. Byrne, D. E. Karig, and S. Lallement (1993), Deformation at the toe of an active accretionary prism synopsis of result from ODP Leg 131, Nankai, SW Japan, J. Struct. Geol., 15, Moore, G., et al. (2001), Proceedings of the Ocean Drilling Program, Initial Reports [CD-ROM], vol. 190, Ocean Drill. Program, College Station, Tex. Moore, J. C. (1989), Tectonics and hydrogeology of accretionary prisms: Role of the décollement zone, J. Struct. Geol., 11, of 17