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Tectonic record of convergence changes in a collision area: the Boso and Miura peninsulas, Central Japan
Earth and Planetary Science Letters, 81 (1986/87) 397-408 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
397
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Tectonic record of convergence changes in a collision area: the Boso and Miura peninsulas, Central Japan Jacques Angelier 1 and Philippe Huchon 2 I CNRS, UA 215 & Tectonique Quantitative, Universit8 Pierre et Marie Curie, 4, place Jussieu, 75230 Paris Cedex 05 (France) 2 CNRS, UA 215 & Laboratoire de Gbologie, Ecole Normale Sup$rieure, 24, rue Lhomond, 75005 Paris (France)
Received July 16, 1986; revised version accepted November 5, 1986 Late Cenozoic formations in Boso and Miura have been affected by several tectonic events. Tectonic analysis enables us to reconstruct six different paleostress types: (1) early extension affecting the Oligocene/early Miocene Mineoka Group, (2) and (3), relatively minor compressional and extensional events probably early Pliocene in age, (4) major NNE-SSW compression dominating prior to 2 or 3 Ma ago, and (5) and (6) more recent major NNW-SSE compression to the west and WNW-ESE extension to the east, both types affecting the Pleistocene and prevailing since 1-2 Ma ago. The counterclockwise change from NNE-SSW to NNW-SSE compression is not accurately dated, but very likely occurred between 2 and 3 Ma ago; it is compared to similar evolutions in other areas of the Izu collision zone. We conclude that it corresponds to a major counterclockwise change in the direction of plate convergence (from SSE-NNW to SE-NW). The relationships between the directions of convergence and the distributions of Plio-Quaternary compressional paleostresses in and around the collision zone are described through a simple analogy, for the two stages of Plio-Quaternary collision. This counterclockwise change in stress fields and relative motions, also described in the Taiwan collision zone along the same Philippine Sea plate-Eurasia boundary, is interpreted as a major event at the scale of the plate. The possible significances of the other paleostress types identified in Boso are discussed. We conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution.
1. Introduction T h e Boso and M i u r a peninsulas are located on e a s t e r n a n d western sides of the T o k y o Bay strait respectively, a b o u t 40 k m south of T o k y o (Fig. 1). This area, b o u n d e d b y the large K a n t o basin to the n o r t h a n d b y the Sagami trough system to the south a n d southeast, is a p p r o x i m a t e l y 80 k m long ( E - W ) a n d 50 k m wide (N-S). In this p a p e r , we a i m at d e c i p h e r i n g the app a r e n t l y c o m p l i c a t e d evolution of paleostress a n d d e f o r m a t i o n in this area, b a s e d on qualitative a n d q u a n t i t a t i v e o b s e r v a t i o n s o f L a t e Cenozoic structures. The succession of tectonic events recons t r u c t e d from our field analysis will be c o m p a r e d with i n d e p e n d e n t i n f o r m a t i o n o n the g e o d y n a m i c e v o l u t i o n of the area, which occupies a p a r t i c u l a r p o s i t i o n in the Izu colhsion zone of C e n t r a l J a p a n (Fig. 1).
2. Geological and geophyisical framework 2.1. T h e I z u collision z o n e o f C e n t r a l J a p a n
T h e Sagami t r o u g h system, south off Boso, 0012-821X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
c o r r e s p o n d s to the active b o u n d a r y b e t w e e n the P h i l i p p i n e Sea p l a t e a n d n o r t h e r n J a p a n (Fig. 1). In terms of p l a t e tectonics, n o r t h e r n J a p a n is i n t e r p r e t e d as b e l o n g i n g either to Eurasia or N o r t h A m e r i c a [1], or to an i n t e r m e d i a t e p l a t e [2]. T h e 1923 K a n t o e a r t h q u a k e occurred in this S a g a m i trough system [3]. A l t h o u g h the convergent c o m p o n e n t of m o t i o n has certainly d o m i n a t e d in the p a s t b a s e d on p l a t e tectonic reconstructions [4,5], d e x t r a l strike-slip m o t i o n p l a y s the m a j o r role a l o n g this b o u n d a r y that strikes a p p r o x i m a t e l y W N W - E S E while the vector of relative m o t i o n t r e n d s N W - S E [6-9]. T o the southeast, the deepest p o r t i o n of the S a g a m i trough system is i n t e r r u p t e d b y the N - S trench system that c o m p r i s e s the J a p a n T r e n c h to the n o r t h a n d the I z u - B o n i n T r e n c h to the south, where the Pacific p l a t e is s u b d u c t e d b e n e a t h n o r t h e r n J a p a n a n d b e n e a t h the P h i l i p p i n e Sea plate, respectively (Fig. I). T h e Boso a n d M i u r a p e n i n s u l a s r e p r e s e n t the p o r t i o n of J a p a n closest to the T T I ' - t y p e triple j u n c t i o n b e t w e e n n o r t h e r n J a p a n , Pacific a n d P h i l i p p i n e Sea plates [10] (Fig.
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1). One may consequently expect close relationships to exist between the geodynamic evolution of the area and the overall evolution of convergent plate motions.
(Fig. 3) belong to the Mineoka belt defined originally in Boso. The Mineoka Group (black in Fig. 3) is older than about 20 Ma (Table 1); it contains large blocks of ophiolitic rocks within hemi-pelagic and terrigeneous sediments. In detail, the youngest sediments of this melange are early to middle Miocene in age [11] (Table 1), whereas older ra-
2.2. The stratigraphy of Boso-Miura peninsulas The oldest terranes of Miura (Fig. 2) and Boso
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Fig. 2. Simplified geological m a p of the M i u r a p e n i n s u l a a n d results from our tectonic analysis in terms of d i r e c t i o n s of c o m p r e s s i o n (convergent arrows). 1 = Holocene, 2 = K a z u s a G r o u p ; 3 = u p p e r M i u r a G r o u p (Ikego, Z u s h i and H a t s u s e F o r m a t i o n s ) ; 4 = lower M i u r a G r o u p ( M i s a k i F o r m a t i o n ) ; 5 = Y a b e G r o u p ; 6 = H o t a G r o u p (Monto, A b u z u n , O o y a m a a n d K i n u g a s a F o r m a t i o n s ) ; 7 = faults. Phase 1 a n d p h a s e 2 are the two successive tectonic events identified from fault analyses in 18 sites.
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Fig. 3. Simplified geological map of the Boso peninsula [33,34] and results of the tectonic analysis. 1 = Kazusa Group, 2 = upper Awa Group (Inakosawa, Kiyosumi and Chikura Formations); 3 = lower Awa Group (Nishizaki, Amatsu and Kinone Formations); 4 = Sakuma Formation; 5 = Hota Group; 6 = Mineoka Group; 7 = fault; 8 = anticline; 9 = syncline. Compression (ol) and extension (a3) as convergentand divergent arrows, respectively(size increasing with amplitude); S = folding; indexes 1, 2, 3, 4 refer to local tectonic chronologiesof events.
diometric ages have been determined in ophiolitic rocks (30-40 Ma [12,13]). This Mineoka belt (Fig. 3) could well correspond to an older location of the Philippine Sea plate-northern Japan convergent boundary [11,14,15]. The Neogene sedimentary formations (Table 1) include, from base to top, the Hayama-Hota Groups (older than about 16 Ma), the YabeSakuma Groups (approximately 14-15 Ma), the Miura-Awa Groups (approximately 3-15 Ma) and the Kazusa Group (younger than about 3 Ma). Except for the last group, these couples of names refer to Miura and Boso peninsulas (respectively). These four main groups are separated by major unconformities, such as the Kurotaki unconformity at the base of the Kazusa Group (Table 1). Other unconformities are present, especially within the Miura-Awa Groups. Other names of forma-
tions mentioned in the text and in the captions of Figs. 2 and 3 are given in Table 1. The upper portion of the Miura-Awa Groups has been individualized in Figs. 2 and 3. Most formations dated between 20 and 3 Ma contain volcaniclastic material; they are marine and have deposited at variable, commonly large depths. Lithology, stratigraphy and sedimentology are described by Ogawa and Horiuchi [16] and Ogawa [11,14]; most paleocurrents took place from south to north. In addition, determinations of paleodepths in the Boso peninsula have led Kitazato (personal communication, 1985) to reconstruct a continuous uplift from 6 Ma ago (4 km water depth) to the Present (approximately sea level); most of the uplift (i.e., 3 km) has occurred since 2 Ma ago. All these stratigraphic and sedimentologic data
400
TABLE 1 S o m e m a j o r s t r a t i g r a p h i c units of the M i u r a a n d Boso peninsulas (G. = G r o u p ; F. = F o r m a t i o n , U. = U n c o n f o r m i t y ) . A f t e r [29-32], simplified.
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suggest that the sediments of most Neogene formations have originated in the forearc basin of the Izu-Ogasawara volcanic arc (that is on the Philippine Sea plate) and that they have subsequently been incorporated to the northern Japan margin and progressively uplifted. This history is consistent with a Late Cenozoic southward migration of the convergent margin, suggesting that the Sagami trough system is a recent feature and that the older trough system was located along the present Mineoka zone, approximately 40 km north of the present Sagami trough.
ern flanks. Thrusts are south-vergent. Recent motion on large fault zones that trend W N W - E S E is dextral [17], with thrust component [18]. The vertical component of total motion is large, defining horst and graben structures related to plate bending by Scholtz and Kato [19]. In addition, the easternmost portion of Boso peninsula is affected by numerous N-S normal faults (Fig. 3), which are rare or absent to the west. 3. Analysis of successive tectonic events
3.1. Monophase and polyphase tectonics 2.3. Structure The formations described above, especially those older than about 3 Ma (pre-Kazusa), are folded along W N W - E S E to ENE-WSW axes in the Boso and Miura peninsulas (Figs. 2 and 3). In detail, the sinuous shape of numerous folds may be explained by the contrast between W N W - E S E axes (which dominate in Miura and eastern Boso near Kamogawa) and ENE-WSW axes (such as in southernmost Boso). En 6chelon fold systems are common. Most folds are asymmetric, with steepest south-
As the coasts of the Boso and Miura peninsulas provide numerous excellent outcrops, detailed observations could be made in 79 sites (61 in Boso, 18 in Miura). Tens to hundreds of measurements have been collected in each outcrop, so that the whole data mass represents about 2500 measurements of faults (with or without indicators of motion vector), tension gashes, joints, folds and bedding surfaces. However, these quantitative data have little value without qualitative observations that depict the style of faulting and folding (Fig. 4) and enable one to determine the relative chro-
401
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Fig. 4. Examples of monophase tectonic features observed in outcrops, Boso peninsula. Sections approximately vertical (cliffs, A - G and I) or approximately horizontal (flat rocky shore, H and J). Scales shown using open bars, all 1 m long. Most features (faults and beds) approximately perpendicular to sections. Bedding arbitrarily shown in black and white (lithology: sandstones, siltstones, claystones, turfs). Locations numbered in Fig. 3 as follows: A, Nojimazaki (35); B, Kawaguchi (27); C, Shirahama (31); D, Oobara (3); E. T~to-Zaki (2); F. Ubara (11); G, Uchi-Ura (14); H, Emi (23); I, Tomiura (47); J, Amatsu (16). Stratigraphic formations, from base to top: Hota (H), Amatsu (G, I, J), Kiyosumi (F), Chikura (A, B, C), upper Kazusa (D, E). Types of structures: reverse faults and folds (set I: A, B, C), normal faults (Set II: D, E, F), strike-slip faults (set III: G, H, I, J). Particularities: conjugate faults systems (A, D, H); association with minor folds (A, B, C) or major folds (G, H); extensional block-tilting (D, E, F); apparently opposite offsets (normal and reverse) across parallel strike-slip faults, observed in section approx, perpendicular to motion vectors (G); system of conjugate strike-slip faults that developed after most of folding, in a vertical fold flank (H); system of apparently conjugate normal faults in section, which in fact corresponds to strike-slip faults as revealed by striations (I); Riedels and other small shear features (E, F, G, H, J; the latter within a right-lateral strike-slip fault zone).
W
Fig. 5. Examples of polyphase tectonic features observed in outcrops, Boso peninsula. Sections approximately vertical (A, B) or "approximately horizontal (C-H). General explanations as in Fig. 3. Locations numbered in Fig. 3 as follows: A, Hagyu-Kanaya (56); B, Shirahama (32); C, and D, Oogawa (29); E and G, Sunozaki (42); F and H, Nokogiriyama (53). Stratigraphic formations (from base to top): Inakosawa (A, F, H); Nishizaki (E, G); Chikura (B, C, D). Types of tectonic successions: reverse faults cut by normal ones (A, B); pre-folding reverse fault systems observed in steeply dipping fold flanks (C-F), the compression axis of this first reverse faulting event being approx, perpendicular (C) or oblique (D) to the major fold axis; pre-folding reverse fault systems cut by syn- to post-folding strike-slip faults (E-H) in steeply dipping fold flanks, the compression axis of the late strike-slip faulting event being oblique to fold axis in case H. Disharmonies in A, C, G correspond to faulting occurring prior to complete compaction of sediment. Distinction between pre-folding reverse or normal faults and post-folding strike-slip faults is not easy in sections, but has been made by using three-dimensional geometrical relationships between different features.
n o l o g y o f t e c t o n i c m o v e m e n t s (Fig. 5), r e v e a l e d in detail by fault intersections or superimposed striat i o n s for f a u l t m o v e m e n t s . We have especially studied the relationships
402 between faulting and folding. Because extensive discussion of such criteria cannot take place in this short paper, we limit ourselves to some examples observed in the field. Fig. 4 refers to cases of apparently polyphase deformation which in fact probably corresponds to a single event. Fig. 5 shows examples of actual polyphase deformation, with successive mechanisms that contrast in terms of orientation and type (compressional-extensional). In both figures, examples are illustrated in planar sections, for simplicity; a complete discussion should involve a three-dimensional analysis which has been done but is not described herein. Because the tilt of beds related to folding provides a guide to reconstruct the relative chronology of tectonic events, particular attention has been devoted to steep fold flanks where pre-, synand post-folding events can be distinguished (Fig. 4H and 5 C - H ) . In contrast, areas with shallow bedding dips display simpler successions of major events and limit difficulties related to rotations around non-horizontal axes during folding. Finally, the accumulation of simple individual observations (as in Figs. 4 and 5) led us to separate the major events in terms of relative chronology, style, type, amplitude and orientation. Because sites are numerous, these definitions have statistical value: we could distinguish major tectonic events (widespread and often related to major as well as minor structures) from minor deformation episods (rare, local, and generally related to minor structures only). 3.2. Paleostress reconstructions Using fault slip data sets, one reconstructs the average paleostress axes o 1 (maximum compressional stress), 02 (intermediate stress) and o 3 (minimum stress). Simple geometrical analyses provide robust but incomplete results, whereas more sophisticated stress tensor determinations lead to more complete and accurate information but need computations and error estimates. These methods are not discussed herein (see [20] for bibliography, practical and theoretical aspects, limitations and requirements). Such local determinations and paleostress axes are strengthened by observations and measurements of other structures, such as tension cracks. Comparison with large-scale structures has been done systematically.
Finally, combining qualitative observations and collection of measurements in numerous sites led us to obtain a significant picture of tectonic events with various styles and types (extensional, compressional) on a regional scale, in terms of chronology, amplitude and orientation. We have carried out this analysis in the Miura and Boso peninsulas, obtaining the overall paleostress patterns summarized in Figs. 2 and 3. Because local analyses in Boso are too numerous (61 sites located in Fig. 3) to be systematically referred to in this paper, we have indicated in Fig. 3 the main results obtained in 16 groups of sites (defined based on geographic proximity, common age of formations affected and similar tectonic histories). In contrast, all sites of Miura are shown in Fig. 2 (18 sites). In Figs. 2 and 3, the nature of each event (compressional for maximum compressive stress 01 horizontal, extensional for o 1 vertical) is simply indicated by double arrows (convergent and divergent, respectively). "Extensional" corresponds to dominating normal faulting, whereas "compressional" may refer either to dominating reverse faulting (03 vertical) or to dominating strike-slip faulting (02 vertical). Because reverse and strikeslip types are mixed in a non-significant way and commonly associated to folding, no distinction is made herein. The variable relative amplitudes of events in Boso are illustrated by the size of arrows in Fig. 3, attached indexes refer to local relative chronologies, which commonly imply up to four independent events. Whereas the results obtained in Boso need more discussion (Fig. 3), most structures observed in Miura clearly belong to two main compressional events (Fig. 2). 3.3. Age and geometry of successive events Compressional tectonics dominate. Figs. 2 and 3 reveal a contrast between two main trends of oa: NNE-SSW and NNW-SSE. The relative chronology is the same in all places where it is reliably reconstructed: the NNE-SSW compression (average azimuth of o 1 = 0 3 0 ° ) is older than the N N W - S S E one (average azimuth of 01 = 160°). This is clear in Boso (Fig. 3) and obvious in Miura (Fig. 2). Both compressional events are related to large-scale structures, with some significant regional variations: for example, the older N N E SSW trending compression plays an important
403 role in the southern Boso peninsula, where it is obviously related to several hectometric-kilometric W N W - E S E trending folds (e.g., near Tateyama). The recent, NNW-SSE trending compression is represented by hectometric-kilometric structures (such as the Nokogiriyama syncline) in the western half of the Boso peninsula, whereas to the east (Kamogawa) it is often difficult to identify and principally represented by minor fault sets. Such differences are illustrated in Fig. 3 by variations in arrow sizes. Finally, the Late Cenozoic deformation in the Miura peninsula is almost entirely explained by this succession of two major compressional events (Fig. 2) that induced the formation of various folds, strike-slip and reverse fault systems. The successions of tectonic events reconstructed in the Boso peninsula are more complex (Fig. 3), partly because sites and measurements are more numerous so that minor events are better known. First, several paleostress patterns that cannot belong to any of the two major compressional events have been reconstructed in Boso. E-W to WNW-ESE compression has often been identified as the oldest faulting event in Miocene/early Pliocene sediments (Fig. 3); it was never observed in the late Pliocene/Pleistocene Kazusa Group. Approximately NNE-SSW extension has also been identified in rare places; elements of relative chronology (Fig. 5) suggest that it has occurred between the two major events discussed first (Fig. 3). Other determinations of particular old stress patterns are exceptional and considered doubtful. The identification of the two small events mentioned above (E-W compression and NNE-SSW extension) cannot be explained by local deviations of stress or rotations of faulted material within the framework of the major compressional events. However, because the amplitudes remain very small, we infer that these events are much less significant than the other ones in terms of geodynamic evolution. Second, a major extensional event can be easily identified in eastern Boso, whereas it is absent to the west. Normal fault systems affect the Pleistocene of the upper Kazusa Group (Kiwada formation) as well as older terranes. There is an obvious relation between the identification of this extensional event in outcrops and the eastward develop-
ment of NNE-SSW faults in the geological map (Fig. 3). Extension (03 axis) strikes NW-SE (average azimuth 125°). In all but a few sites, this event is the most recent. However, some observations suggest that the main N N W - S E E compression and this NW-SE extension are partly synchronous: the extension (increasing to the east) and the compression (increasing to the west) have been alternatingly active in some areas. Both types of deformations affect the Kazusa Group. A remarkable feature is the syn-sedimentary or, more commonly, the pre-diagenesis character of many faults and folds. In fact, numerous observations suggest that the first E-W compression that we identified occurred prior to the compaction of sediments of the Nishizaki and Chikura Formations (Table 1). This deformation of soft sediments may occur very late after deposition (depending on sedimentation rates and other factors). In any case, this observation suggests that the compressional tectonic events that we identify have occurred after about 5 - 6 Ma ago, but not very recently. Similar observations have been made in the Inakosawa and Nishizaki Formations of western Boso, where the main NNE-SSW compressional features developed before or during sediment compaction. This additional observation suggests that the change from the WNW-ESE to the main NNE-SSW compression has occurred between approximately 3 and 5 Ma ago, maybe at the time of the Kurotaki unconformity (Table 1). Note also that in eastern Boso, the NW-SE extension was clearly active during the sedimentation of the Kiwada Formation (i.e., the upper Kazusa Group). The oldest extensional tectonic event has been identified in the ophiolitic rocks of the Mineoka belt in three sites (Fig. 3, sites 17, 18, 61) of the Kamogawa region. This NNE-SSW extension (azimuth 035 °) is older than the major NNE-SSW compressional event. Because we did not find such systems of WNW-ESE normal faults in more recent terranes, we suspect that this extension is older than the middle Miocene.
4. Geodynamic implications 4.1. Chronology and significance of tectonic events in the collision zone The single significant tectonic event probably
404 older than the late Miocene, that we could reconstruct based on detailed tectonic analysis, is the N 0 3 5 ° E trending extension that affects the Mineoka Group in southeastern Boso peninsula. We tend to interpret this extensional event as related to the early history of these terranes, possibly to plate bending on the southern side of the former boundary Philippine Sea plate-northern Japan before it jumped southward when the present Sagami trough system developed; the present folded Mineoka belt probably represents the result of the collision stage that has induced this southward migration [11,14,15[. The two next events that we could reliably reconstruct despite of minor role, are the W-E to W N W - E S E compression and the NNE-SSW extension (Fig. 3). The compressional event is obviously older than the compaction of the youngest sediments affected (in the late Miocene/early Pliocene Nishizaki and Chikura Formations); we consequently believe that this compressional event may reflect a state of stress that developed at least during the last stages of formation of the thick pile of late Miocene/early Pliocene sediments, between about 6 and 3 Ma ago. The two major compressional events that clearly dominate the paleostress patterns of Figs. 2 and 3 are more recent. They are the most important in terms of amplitude, and also the most significant within the framework of the Izu collision (Fig. 6). In the Miura-Boso area, the first compressional
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event corresponds to a remarkably homogeneous NNE-SSW trend of paleo-o a (N030°E). Because we did not find evidence of this compression in the Kazusa Group of the late Pliocene/Pleistocene, and also because it predates complete compaction of sediments in the late Miocene/early Pliocene Inakosawa, Chikura and Nishizaki Formations, we believe that most of this large NNESSW compression has occurred prior to the development of the Kurotaki unconformity, approximately 3 Ma ago (Table 1). Although we did not find evidence on the outcrop scale, we tend to relate the development of the Kurotaki unconformity itself to a paroxysm of this NNE-SSW compression. We cannot reject the possibility for this "event" to reflect a state of stress that dominated during a long time, either after the two events mentioned above or even before and after them (or one of them). This hypothesis would enable one to explain the apparently larger amplitude of structures related to N N E compression in the oldest terranes of the Awa Group, but is not supported by any definite observation. The second major compressional event corresponds to a NNW-SSE (N160 ° E) trend of paleo-o 1 in Miura and Boso peninsulas. This event clearly affects the late Pliocene/Pleistocene terranes of the lower Kazusa Group; it also affects, apparently in a less intense way, the upper Kazusa Group. We conclude that this major compressional regime has dominated since no more than 2 - 3 Ma ago.
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Fig. 6. Neotectonic evolution of the Izu collision zone and related directions of compression determined by using tectonic analysis. Lines with triangles: thrusts (triangles on the upper block). Trajectories of 01 dashed. Directions of compression (ol) locally reconstructed: double convergent arrows. Direction of relative motion Philippine Sea plate-Eurasia: black arrows. Domain of extension in eastern Boso hatched (direction of 03 shown as divergent arrows). (a) Collision event older than 2-3 Ma (about 3 Ma ago); (b) collision event younger than 2-3 Ma (about 1 Ma ago).
405 The counterclockwise change in the directions of compression that has been reconstructed based on tectonic analysis in the Miura-Boso area is not exceptional in Central Japan (Fig. 6). On the western edge of the Izu collision zone, near Kakegawa, Huchon [21] has described a counterclockwise change of 30 ° in the trend of paleo-o 1 from N135°E to N105°E; the first tectonic regime has predominated during the Pliocene (Fig. 6a), the second one is still active (Fig. 6b). This trend change locally postdates 2.2 Ma ago and probably corresponds to the late Pliocene/early Pleistocene. North of Kakegawa, the reconstructed directions of compression vary from N140°E to N l l 0 ° E ; no systematic variation with time could be demonstrated, due to large dispersion of local paleostress trends related to intense deformation and complex stress patterns in a N-S trending belt of large folds, strike-slip, faults and thrusts [22]. North of the Izu peninsula, the dominating trend of o I is N135°E (Fig. 6). It corresponds to the folding of the Ashigara trough, where thick sediments accumulated during the early Pleistocene, and to the related southward thrusting of the Tanzawa Mountains (Kannawa thrust, Fig. 6b). A very recent local clockwise change of o I trends has occurred 0.3 Ma ago according to Huchon and Kitazato [23]; it reflects local modifications of stress patterns close to a major thrust during the continuing collision process, rather than regional variations (Fig. 6b). North of this Ashigara area, the Tanzawa Mountains (Fig. 6a) probably represent a volcanic island of the Izu-Bonin arc, accreted to central Japan during the early Pliocene (prior to the folding of the Ashigara trough); an age that corresponds to a paroxysm in NW-SE compression near Kakegawa. Compression related to Tanzawa collision was N-S (Fig. 6a). At least two "micro-collisions" (Tanzawa and Ashigara) occurred successively during the Plio-Pleistocene, resulting in southward jumps of the collision boundary (Figs. 6 and 7), and in progressive incorporation to central Japan of pieces of the IzuBonin arc. Incidentally, this evolution incites one to locate the future collision boundary south of Izu. A particular feature of the recent stress field in Boso (Fig. 3) is the transition from compression (Miura and western Boso peninsulas) to extension (normal faulting in eastern Boso). Based on geo-
graphic proximity and compatible trends, we relate this phenomenon to the transition between the Izu collision zone (to the west) and the westernmost portion of the Japan active margin in front of the Japan trench system (to the east, Fig. 1). This interpretation implies that the subduction process in the Japan Trench induces extensional phenomena in the upper margin in eastern Boso (Fig. 6b).
4.2. Age of geodynamic significance of the counterclockwise change in major compression trends There is a striking homogeneity in the evolution of both sides of the Izu collision zone during the Plio-Quaternary. A major counterclockwise change in the direction of compression has occurred with time. This change ranges from about 30 ° (to the west) to about 50 ° (to the east), as Fig. 6 shows (compare a and b). The age determined for this major change based on stratigraphic data varies between 1-2 Ma (to the west) and 2-3 Ma (to the east). This phenomenon could well be time-transgressive as a function of the distribution of motion and local deformations along an evolving plate boundary, so that there is no reason to expect a rigorous synchronism between west and east. The two major stages of collision are illustrated in Fig. 6, where data from Tanzawa (unpublished) and Ashigara [23] areas have been incorporated. Fig. 7 summarizes the distribution of the o 1 trajectories in and around the collision area, based on the extrapolation of these results of our tectonic analyses. Note that (1) the recent stress pattern of Fig. 7b is in agreement with present-day data obtained from numerous geodetic and seismic studies of Japanese authors, and (2) this stress pattern can be related to the direction of relative plate motion independently determined (see section 2.1i, either through a simple qualitative approach based on mechanical comparisons and approximate symmetries, or through a more rigorous finite-element analysis [24]. In turn, such comparisons between the orientation of the vectors of relative plate motions and the distribution of 01 trajectories around a collision zone (the shape of which is known) lead one to reconstruct the former motion vector, using the older distribution of a 1 trajectories in the same area. This comparison is done in Fig. 7: the counterclockwise change in o 1 trajectories (from a to b)
406 implies that a counterclockwise rotation of the direction of relative motion has also occurred, from N N W to NW. This interpretation, based solely on the results of our tectonic analyses, is supported by independent plate tectonics data that also imply a counterclockwise rotation of the direction of Philippine Sea plate-Eurasia motion [25].
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4. 3. Changes in paleostress trends and hypothetic block rotations Our tectonic analyses do not provide rigorous means to identify actual paleostress rotations and rotations of large blocks (such block rotation results in apparent paleostress changes even within a constant stress field). As a result, we cannot definitely decide whether the apparent counterclockwise 50 ° rotation of a I trends in the Miura-Boso
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Fig. 8. Possible effect of hypothetic clockwise block rotation on the eastern side of the collision zone. Explanations as for Fig. 7, except for dotted lines that indicate the present trend of paleostress o I trajectories related to event (a) in an area that has undergone clockwise rotation between events (a) and (b).
1/ /
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,,/ Fig. 7. Relationship between the direction of plate convergence (arrows), the shape of the collision zone (barbed line) and the distribution of 01 trajectories (compressional stress, dashed). (a) Older event (the Izu region moves to the NNW); (b) younger event (the Izu region moves to the NW). Note the southward jump of the convergent boundary at the tip of the Izu region and the difference in the distribution of o I trajectories, between (a) and (b).
peninsulas is due to a real counterclockwise 50 ° rotation of o I axes (as in Fig. 7), or to any combination of counterclockwise change in o 1 directions and clockwise block rotation. The latter hypothesis is illustrated in Fig. 8, where an arbitrary 25 ° clockwise rotation affects a large area long the eastern side of the collision zone, so that the angle between real successive a I trends is only 25 ° counterclockwise (compare Fig. 7a and 8a). N o apparent clockwise rotation of o 1 trends has occurred on the western edge of the collision zone (where the change, on the contrary, has occurred counterclockwise: Fig. 6). This suggests that block rotation, if it is present, did not play a major role in the Kakegawa and Miura-Boso regions, as far as the late Pliocene/Quaternary deformations are concerned (such block rotation should have occurred counterclockwise west of the collision zone). If, however, any limited clockwise block rotation had actually occurred recently in the Miura-Boso peninsulas, this should be demonstrated by further paleomagnetic studies.
407
5. Conclusions The tectonic analysis provides a simple and reliable way to describe the paleostress evolution of a complex convergent boundary such as the Philippine Sea plate-Eurasia collision zone of Central Japan. This subsequently provides a way to interpret the changes in paleostress fields, in order to reconstruct the variations in the direction of convergence. The first step is described in Figs. 4 and 5 (outcrop scale) and 2 and 3 (area scale), while the second step is illustrated in Figs. 6 and 7 (regional scale). It is worthwhile to observe that very similar counterclockwise changes in the directions of regional o 1 trends occurred during the PlioQuaternary in the collision zone of Taiwan [26]. These paleostress changes in Taiwan have been described and interpreted elsewhere [27]. The distance between Taiwan and Izu is larger than 2000 km, along the same major plate boundary (Philippine Sea plate-Eurasia). Despite of some stratigraphic uncertainties that still affect the dating of this event, especially in Taiwan, these similar changes reflect a" more general phenomenon at the scale of the entire Philippine Sea plate [28]. Independent geophysical data and plate tectonic reconstructions lead to the same conclusion, that is the occurrence of a significant counterclockwise change in the direction of relative motion. These examples indicate first that a change in a direction of convergence and related stress fields, although it is recorded in relatively small collision areas such as central Japan, may be significant at the scale of a major plate; second, that tectonic analysis based on accurate observation in numerous sites provides a key to decipher some major features of geodynamic evolution. Acknowledgments This work has been carried out within the framework of the French-Japanese Kaiko project, and supported by the CNRS, the French Minister of Foreign Affairs and the IFREMER. Discussions with Dr. Y. Ogawa and K. Horiuchi, and Messrs. Koga and Taniguchi, were fruitful. We thank P. Choukroune, J. Letouzey and H. Okada for reviewing this paper.
References 1 K. Nakamura, Possible nascent trench along the eastern Japan Sea as the convergent boundary between Eurasian and North American plates, Bull. Earthq. Res. Inst., Univ. Tokyo 58, 711-722, 1983. 2 T. Seno and S. Maruyama, Is northern Honshu a microplate?, Tectonophysics 115, 117-196, 1985. 3 M. Ando, A fault origin model of the Great Kanto earthquake of 1923 as deduced from geodetic data, Bull. Earthq. Res. Inst., Univ. Tokyo 49, 19-32, 1971. 4 T. Seno and S. Mamyama, Paleogeographic reconstruction and origin of the Philippine Sea, in: Geodynamics of Back-Arc Regions, R.L. Carlson and K. Kobayashi, eds., Tectonophysics 102, 53-84, 1984. 5 Y. Matsubara and T. Seno, Paleogeographic reconstruction of the Philippine Sea plate at 5 m.y.B.P., Earth Planet. Sci. Lett. 51,406-414, 1980. 6 T. Seno, The instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate, Tectonophysics 42, 209-226, 1977. 7 J.B. Minster and T.H. Jordan, Rotation vectors for the Phillipine and Rivera plates, in: American Geophysical Union Fall Meeting Abstr., EOS, Trans. Am. Geophys. Union 60, 958, 1979. 8 B. Ranken, R.K. Cardwell and D.E. Karig, Kinematics of the Philippine Sea plate, Tectonics 3(5), 555-575, 1984. 9 P. Huchon, Comment on "Kinematics of the Philippine Sea plate", by B. Ranken et al., Tectonics 5(1), 165-168, 1986. 10 X. Le Pichon and P. Huchon, Central Japan triple junction revisited, Tectonics, in press, 1986. 11 Y. Ogawa, Mineoka ophiolite belt in the Izu forearc area--Neogene accretion of oceanic and island arc assemblages on the northeastern comer of the Philippine Sea plate, in: Accretion Tectonics in the Circum Pacific Regions, M. Hashimoto and S. Uyeda, eds., pp. 245--260, Terra Scientific Publ. Co., Tokyo, 1983. 12 Y. Yoshida, Finding of Foraminiferas from the Mineoka Mountains, Chiba Prefecture, Chishitsu News (Geological News), Geol. Surv. Jpn., 233, 30-36, 1974 (in Japanese). 13 Y. Takigami, I. Kaneoka and M. Hirano, K-Ar and 4°Ar39Ar age determination of the Mineoka ophiolite, Bull. Volcanol. Soc. Jpn. 23, 308, 1980. 14 Y. Ogawa, Tectonics of some forearc fold belts in and around the arc-arc crossing area in central Japan, in: Trench and Fore Arc Geology, Sedimentation and Tectonics in Modem and Ancient Active Plate Margins, J.K. Leggen, ed., Geol. Soc. London, Spec. Pub. 10, 49-61, 1981. 15 Y. Ogawa and J. Naka, Emplacement of ophiolitic rocks in forearc areas: examples from central Japan and IzuMariana-Yap island arc system, in: Ophiolites and Oceanic Lithosphere, I. Gass et al., eds., Geol. Soc. London, Spec. Publ. 13, 291-301, 1984. 16 Y. Ogawa and K. Horiuchi, Two types of accretionary fold belts in central japan, J. Phys. Earth 26 (Suppl.), $321-$336, 1978. 17 S. Kaneko, Right-lateral faulting in Miura peninsula, south of Tokyo, Japan, J. Geol. Soc. Jpn. 75(4), 199-208, 1969.
408 18 D.N. Williams, Late Quaternary displacement at the Hikihashi and Kitatake faults, Miura peninsula, Daiyonki Kenkyu (Quaternary Research) 21(4), 289-299, 1983. 19 C.H. Scholtz and T. Kato, The behavior of a convergent plate boundary: crustal deformation in the south Kanto district, Japan, J. Geophys. Res. 83 (B2), 783-797, 1978. 20 J. Angelier, Tectonic analysis of fault slip data sets, J. Geophys. Res. 89 (B8), 5835-5848, 1984. 21 P. Huchon, Les contraintes tectoniques h l'arri+re d'une zone de subduction: relation avec la direction de convergence et 6volution dans le temps; l'exemple de la r~gion de Shizuoka (Japon central), C.R. Acad. Sci., Paris, Ser. II 296, 787-790, 1983. 22 P. Huchon and J. Angelier, From subduction to collision; the Suruga-Fujigawa belt, Izu collision zone, Japan, Bull. Soc. G6ol. Fr., in press, 1986. 23 P. Huchon and H. Kitazato, Collision of the Izu block with central japan during the Quaternary and geological evolution of the Ashigara area, Tectonophysics 110, 201-210, 1984. 24 P. Huchon and J.C. de Bremaecker, Influence of Izu subduction-collision on the deformation of central Japan, in: Formation of Ocean Margins, N. Nasu, ed. pp. 701-717. Terra Scientific Publ. Co., Tokyo, 1985. 25 P. Huchon, G6odynamique de la zone de collision d'Izu et de la jonction triple du Japon central--leur place dans l'6vohition de la plaque Philippine, M~m. Sci. Terre, Univ. P. & M. Curie 85-56, 414 pp., 1985. 26 E. Barrier, Tectonique d'une cha]ne de collision active: T~wan, M6m. Sci. Terre, Univ. P. & M. Curie 85-29, 492 pp., 2 vol., 1985.
27 J. Angelier, E. Barrier and P. Huchon, Sur les relations entre trajectoires de contrainte et directions de mouvement le long d'une fronti+re convergente: exemple de la subduction hell6nique (Gr+ce) et de la collision Philippines-Eurasie (Tal/wan et Japon). C.R. Acad. Sci., Paris, Ser. II 294, 745-748, 1982. 28 J. AngeEer, E. Barrier and H.T. Chu, Plate collision and plaeostress trajectories in a fold-thrust belt: the Foothills of Taiwan, Tectonophysics 125, 161-178. 29 N. Niitsuma, Magnetic stratigraphy of the Japanese Neogene and the development of the island arc of Japan, J. Phys. Earth 26 (Suppl.), $367-$378, 1978. 30 K. Sawamura and T. Nakajima, Miocene silicoflagellate zones in the Boso peninsula, Bull. Geol. Surv. Jpn. 31, 333-345, 1980. 31 R. Tsuchi and M. Ibaraki, Correlation of Neogene sequences on the Pacific coast of southern Japan by means of planktonic Foraminifera and Molhisca, Rep. Fac. Sci., Shizuoka Univ. 15, 53-68, 1980. 32 N. Kotake, Stratigraphy and biochronology of the southernmost part of Boso Peninsula, Abstr. Geol. Soc. Jpn., p. 123, 1985. 33 Geological Survey of Japan, Geological map of the Sagami Bay and adjacent area, 1 : 100,000, 1977. 34 Geological Survey of Japan, Geological map, sheet "Kamogawa" 1:50,000, Quadrangle series, NI-54-20-14, 1981.
397
[4]
Tectonic record of convergence changes in a collision area: the Boso and Miura peninsulas, Central Japan Jacques Angelier 1 and Philippe Huchon 2 I CNRS, UA 215 & Tectonique Quantitative, Universit8 Pierre et Marie Curie, 4, place Jussieu, 75230 Paris Cedex 05 (France) 2 CNRS, UA 215 & Laboratoire de Gbologie, Ecole Normale Sup$rieure, 24, rue Lhomond, 75005 Paris (France)
Received July 16, 1986; revised version accepted November 5, 1986 Late Cenozoic formations in Boso and Miura have been affected by several tectonic events. Tectonic analysis enables us to reconstruct six different paleostress types: (1) early extension affecting the Oligocene/early Miocene Mineoka Group, (2) and (3), relatively minor compressional and extensional events probably early Pliocene in age, (4) major NNE-SSW compression dominating prior to 2 or 3 Ma ago, and (5) and (6) more recent major NNW-SSE compression to the west and WNW-ESE extension to the east, both types affecting the Pleistocene and prevailing since 1-2 Ma ago. The counterclockwise change from NNE-SSW to NNW-SSE compression is not accurately dated, but very likely occurred between 2 and 3 Ma ago; it is compared to similar evolutions in other areas of the Izu collision zone. We conclude that it corresponds to a major counterclockwise change in the direction of plate convergence (from SSE-NNW to SE-NW). The relationships between the directions of convergence and the distributions of Plio-Quaternary compressional paleostresses in and around the collision zone are described through a simple analogy, for the two stages of Plio-Quaternary collision. This counterclockwise change in stress fields and relative motions, also described in the Taiwan collision zone along the same Philippine Sea plate-Eurasia boundary, is interpreted as a major event at the scale of the plate. The possible significances of the other paleostress types identified in Boso are discussed. We conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution.
1. Introduction T h e Boso and M i u r a peninsulas are located on e a s t e r n a n d western sides of the T o k y o Bay strait respectively, a b o u t 40 k m south of T o k y o (Fig. 1). This area, b o u n d e d b y the large K a n t o basin to the n o r t h a n d b y the Sagami trough system to the south a n d southeast, is a p p r o x i m a t e l y 80 k m long ( E - W ) a n d 50 k m wide (N-S). In this p a p e r , we a i m at d e c i p h e r i n g the app a r e n t l y c o m p l i c a t e d evolution of paleostress a n d d e f o r m a t i o n in this area, b a s e d on qualitative a n d q u a n t i t a t i v e o b s e r v a t i o n s o f L a t e Cenozoic structures. The succession of tectonic events recons t r u c t e d from our field analysis will be c o m p a r e d with i n d e p e n d e n t i n f o r m a t i o n o n the g e o d y n a m i c e v o l u t i o n of the area, which occupies a p a r t i c u l a r p o s i t i o n in the Izu colhsion zone of C e n t r a l J a p a n (Fig. 1).
2. Geological and geophyisical framework 2.1. T h e I z u collision z o n e o f C e n t r a l J a p a n
T h e Sagami t r o u g h system, south off Boso, 0012-821X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
c o r r e s p o n d s to the active b o u n d a r y b e t w e e n the P h i l i p p i n e Sea p l a t e a n d n o r t h e r n J a p a n (Fig. 1). In terms of p l a t e tectonics, n o r t h e r n J a p a n is i n t e r p r e t e d as b e l o n g i n g either to Eurasia or N o r t h A m e r i c a [1], or to an i n t e r m e d i a t e p l a t e [2]. T h e 1923 K a n t o e a r t h q u a k e occurred in this S a g a m i trough system [3]. A l t h o u g h the convergent c o m p o n e n t of m o t i o n has certainly d o m i n a t e d in the p a s t b a s e d on p l a t e tectonic reconstructions [4,5], d e x t r a l strike-slip m o t i o n p l a y s the m a j o r role a l o n g this b o u n d a r y that strikes a p p r o x i m a t e l y W N W - E S E while the vector of relative m o t i o n t r e n d s N W - S E [6-9]. T o the southeast, the deepest p o r t i o n of the S a g a m i trough system is i n t e r r u p t e d b y the N - S trench system that c o m p r i s e s the J a p a n T r e n c h to the n o r t h a n d the I z u - B o n i n T r e n c h to the south, where the Pacific p l a t e is s u b d u c t e d b e n e a t h n o r t h e r n J a p a n a n d b e n e a t h the P h i l i p p i n e Sea plate, respectively (Fig. I). T h e Boso a n d M i u r a p e n i n s u l a s r e p r e s e n t the p o r t i o n of J a p a n closest to the T T I ' - t y p e triple j u n c t i o n b e t w e e n n o r t h e r n J a p a n , Pacific a n d P h i l i p p i n e Sea plates [10] (Fig.
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1). One may consequently expect close relationships to exist between the geodynamic evolution of the area and the overall evolution of convergent plate motions.
(Fig. 3) belong to the Mineoka belt defined originally in Boso. The Mineoka Group (black in Fig. 3) is older than about 20 Ma (Table 1); it contains large blocks of ophiolitic rocks within hemi-pelagic and terrigeneous sediments. In detail, the youngest sediments of this melange are early to middle Miocene in age [11] (Table 1), whereas older ra-
2.2. The stratigraphy of Boso-Miura peninsulas The oldest terranes of Miura (Fig. 2) and Boso
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Fig. 2. Simplified geological m a p of the M i u r a p e n i n s u l a a n d results from our tectonic analysis in terms of d i r e c t i o n s of c o m p r e s s i o n (convergent arrows). 1 = Holocene, 2 = K a z u s a G r o u p ; 3 = u p p e r M i u r a G r o u p (Ikego, Z u s h i and H a t s u s e F o r m a t i o n s ) ; 4 = lower M i u r a G r o u p ( M i s a k i F o r m a t i o n ) ; 5 = Y a b e G r o u p ; 6 = H o t a G r o u p (Monto, A b u z u n , O o y a m a a n d K i n u g a s a F o r m a t i o n s ) ; 7 = faults. Phase 1 a n d p h a s e 2 are the two successive tectonic events identified from fault analyses in 18 sites.
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Fig. 3. Simplified geological map of the Boso peninsula [33,34] and results of the tectonic analysis. 1 = Kazusa Group, 2 = upper Awa Group (Inakosawa, Kiyosumi and Chikura Formations); 3 = lower Awa Group (Nishizaki, Amatsu and Kinone Formations); 4 = Sakuma Formation; 5 = Hota Group; 6 = Mineoka Group; 7 = fault; 8 = anticline; 9 = syncline. Compression (ol) and extension (a3) as convergentand divergent arrows, respectively(size increasing with amplitude); S = folding; indexes 1, 2, 3, 4 refer to local tectonic chronologiesof events.
diometric ages have been determined in ophiolitic rocks (30-40 Ma [12,13]). This Mineoka belt (Fig. 3) could well correspond to an older location of the Philippine Sea plate-northern Japan convergent boundary [11,14,15]. The Neogene sedimentary formations (Table 1) include, from base to top, the Hayama-Hota Groups (older than about 16 Ma), the YabeSakuma Groups (approximately 14-15 Ma), the Miura-Awa Groups (approximately 3-15 Ma) and the Kazusa Group (younger than about 3 Ma). Except for the last group, these couples of names refer to Miura and Boso peninsulas (respectively). These four main groups are separated by major unconformities, such as the Kurotaki unconformity at the base of the Kazusa Group (Table 1). Other unconformities are present, especially within the Miura-Awa Groups. Other names of forma-
tions mentioned in the text and in the captions of Figs. 2 and 3 are given in Table 1. The upper portion of the Miura-Awa Groups has been individualized in Figs. 2 and 3. Most formations dated between 20 and 3 Ma contain volcaniclastic material; they are marine and have deposited at variable, commonly large depths. Lithology, stratigraphy and sedimentology are described by Ogawa and Horiuchi [16] and Ogawa [11,14]; most paleocurrents took place from south to north. In addition, determinations of paleodepths in the Boso peninsula have led Kitazato (personal communication, 1985) to reconstruct a continuous uplift from 6 Ma ago (4 km water depth) to the Present (approximately sea level); most of the uplift (i.e., 3 km) has occurred since 2 Ma ago. All these stratigraphic and sedimentologic data
400
TABLE 1 S o m e m a j o r s t r a t i g r a p h i c units of the M i u r a a n d Boso peninsulas (G. = G r o u p ; F. = F o r m a t i o n , U. = U n c o n f o r m i t y ) . A f t e r [29-32], simplified.
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EARLY MIOCENE
Hota U. (20 Ma)
Mineoka
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25OLIGOCENE
Ma
suggest that the sediments of most Neogene formations have originated in the forearc basin of the Izu-Ogasawara volcanic arc (that is on the Philippine Sea plate) and that they have subsequently been incorporated to the northern Japan margin and progressively uplifted. This history is consistent with a Late Cenozoic southward migration of the convergent margin, suggesting that the Sagami trough system is a recent feature and that the older trough system was located along the present Mineoka zone, approximately 40 km north of the present Sagami trough.
ern flanks. Thrusts are south-vergent. Recent motion on large fault zones that trend W N W - E S E is dextral [17], with thrust component [18]. The vertical component of total motion is large, defining horst and graben structures related to plate bending by Scholtz and Kato [19]. In addition, the easternmost portion of Boso peninsula is affected by numerous N-S normal faults (Fig. 3), which are rare or absent to the west. 3. Analysis of successive tectonic events
3.1. Monophase and polyphase tectonics 2.3. Structure The formations described above, especially those older than about 3 Ma (pre-Kazusa), are folded along W N W - E S E to ENE-WSW axes in the Boso and Miura peninsulas (Figs. 2 and 3). In detail, the sinuous shape of numerous folds may be explained by the contrast between W N W - E S E axes (which dominate in Miura and eastern Boso near Kamogawa) and ENE-WSW axes (such as in southernmost Boso). En 6chelon fold systems are common. Most folds are asymmetric, with steepest south-
As the coasts of the Boso and Miura peninsulas provide numerous excellent outcrops, detailed observations could be made in 79 sites (61 in Boso, 18 in Miura). Tens to hundreds of measurements have been collected in each outcrop, so that the whole data mass represents about 2500 measurements of faults (with or without indicators of motion vector), tension gashes, joints, folds and bedding surfaces. However, these quantitative data have little value without qualitative observations that depict the style of faulting and folding (Fig. 4) and enable one to determine the relative chro-
401
SW~-4NE SW NE
Fig. 4. Examples of monophase tectonic features observed in outcrops, Boso peninsula. Sections approximately vertical (cliffs, A - G and I) or approximately horizontal (flat rocky shore, H and J). Scales shown using open bars, all 1 m long. Most features (faults and beds) approximately perpendicular to sections. Bedding arbitrarily shown in black and white (lithology: sandstones, siltstones, claystones, turfs). Locations numbered in Fig. 3 as follows: A, Nojimazaki (35); B, Kawaguchi (27); C, Shirahama (31); D, Oobara (3); E. T~to-Zaki (2); F. Ubara (11); G, Uchi-Ura (14); H, Emi (23); I, Tomiura (47); J, Amatsu (16). Stratigraphic formations, from base to top: Hota (H), Amatsu (G, I, J), Kiyosumi (F), Chikura (A, B, C), upper Kazusa (D, E). Types of structures: reverse faults and folds (set I: A, B, C), normal faults (Set II: D, E, F), strike-slip faults (set III: G, H, I, J). Particularities: conjugate faults systems (A, D, H); association with minor folds (A, B, C) or major folds (G, H); extensional block-tilting (D, E, F); apparently opposite offsets (normal and reverse) across parallel strike-slip faults, observed in section approx, perpendicular to motion vectors (G); system of conjugate strike-slip faults that developed after most of folding, in a vertical fold flank (H); system of apparently conjugate normal faults in section, which in fact corresponds to strike-slip faults as revealed by striations (I); Riedels and other small shear features (E, F, G, H, J; the latter within a right-lateral strike-slip fault zone).
W
Fig. 5. Examples of polyphase tectonic features observed in outcrops, Boso peninsula. Sections approximately vertical (A, B) or "approximately horizontal (C-H). General explanations as in Fig. 3. Locations numbered in Fig. 3 as follows: A, Hagyu-Kanaya (56); B, Shirahama (32); C, and D, Oogawa (29); E and G, Sunozaki (42); F and H, Nokogiriyama (53). Stratigraphic formations (from base to top): Inakosawa (A, F, H); Nishizaki (E, G); Chikura (B, C, D). Types of tectonic successions: reverse faults cut by normal ones (A, B); pre-folding reverse fault systems observed in steeply dipping fold flanks (C-F), the compression axis of this first reverse faulting event being approx, perpendicular (C) or oblique (D) to the major fold axis; pre-folding reverse fault systems cut by syn- to post-folding strike-slip faults (E-H) in steeply dipping fold flanks, the compression axis of the late strike-slip faulting event being oblique to fold axis in case H. Disharmonies in A, C, G correspond to faulting occurring prior to complete compaction of sediment. Distinction between pre-folding reverse or normal faults and post-folding strike-slip faults is not easy in sections, but has been made by using three-dimensional geometrical relationships between different features.
n o l o g y o f t e c t o n i c m o v e m e n t s (Fig. 5), r e v e a l e d in detail by fault intersections or superimposed striat i o n s for f a u l t m o v e m e n t s . We have especially studied the relationships
402 between faulting and folding. Because extensive discussion of such criteria cannot take place in this short paper, we limit ourselves to some examples observed in the field. Fig. 4 refers to cases of apparently polyphase deformation which in fact probably corresponds to a single event. Fig. 5 shows examples of actual polyphase deformation, with successive mechanisms that contrast in terms of orientation and type (compressional-extensional). In both figures, examples are illustrated in planar sections, for simplicity; a complete discussion should involve a three-dimensional analysis which has been done but is not described herein. Because the tilt of beds related to folding provides a guide to reconstruct the relative chronology of tectonic events, particular attention has been devoted to steep fold flanks where pre-, synand post-folding events can be distinguished (Fig. 4H and 5 C - H ) . In contrast, areas with shallow bedding dips display simpler successions of major events and limit difficulties related to rotations around non-horizontal axes during folding. Finally, the accumulation of simple individual observations (as in Figs. 4 and 5) led us to separate the major events in terms of relative chronology, style, type, amplitude and orientation. Because sites are numerous, these definitions have statistical value: we could distinguish major tectonic events (widespread and often related to major as well as minor structures) from minor deformation episods (rare, local, and generally related to minor structures only). 3.2. Paleostress reconstructions Using fault slip data sets, one reconstructs the average paleostress axes o 1 (maximum compressional stress), 02 (intermediate stress) and o 3 (minimum stress). Simple geometrical analyses provide robust but incomplete results, whereas more sophisticated stress tensor determinations lead to more complete and accurate information but need computations and error estimates. These methods are not discussed herein (see [20] for bibliography, practical and theoretical aspects, limitations and requirements). Such local determinations and paleostress axes are strengthened by observations and measurements of other structures, such as tension cracks. Comparison with large-scale structures has been done systematically.
Finally, combining qualitative observations and collection of measurements in numerous sites led us to obtain a significant picture of tectonic events with various styles and types (extensional, compressional) on a regional scale, in terms of chronology, amplitude and orientation. We have carried out this analysis in the Miura and Boso peninsulas, obtaining the overall paleostress patterns summarized in Figs. 2 and 3. Because local analyses in Boso are too numerous (61 sites located in Fig. 3) to be systematically referred to in this paper, we have indicated in Fig. 3 the main results obtained in 16 groups of sites (defined based on geographic proximity, common age of formations affected and similar tectonic histories). In contrast, all sites of Miura are shown in Fig. 2 (18 sites). In Figs. 2 and 3, the nature of each event (compressional for maximum compressive stress 01 horizontal, extensional for o 1 vertical) is simply indicated by double arrows (convergent and divergent, respectively). "Extensional" corresponds to dominating normal faulting, whereas "compressional" may refer either to dominating reverse faulting (03 vertical) or to dominating strike-slip faulting (02 vertical). Because reverse and strikeslip types are mixed in a non-significant way and commonly associated to folding, no distinction is made herein. The variable relative amplitudes of events in Boso are illustrated by the size of arrows in Fig. 3, attached indexes refer to local relative chronologies, which commonly imply up to four independent events. Whereas the results obtained in Boso need more discussion (Fig. 3), most structures observed in Miura clearly belong to two main compressional events (Fig. 2). 3.3. Age and geometry of successive events Compressional tectonics dominate. Figs. 2 and 3 reveal a contrast between two main trends of oa: NNE-SSW and NNW-SSE. The relative chronology is the same in all places where it is reliably reconstructed: the NNE-SSW compression (average azimuth of o 1 = 0 3 0 ° ) is older than the N N W - S S E one (average azimuth of 01 = 160°). This is clear in Boso (Fig. 3) and obvious in Miura (Fig. 2). Both compressional events are related to large-scale structures, with some significant regional variations: for example, the older N N E SSW trending compression plays an important
403 role in the southern Boso peninsula, where it is obviously related to several hectometric-kilometric W N W - E S E trending folds (e.g., near Tateyama). The recent, NNW-SSE trending compression is represented by hectometric-kilometric structures (such as the Nokogiriyama syncline) in the western half of the Boso peninsula, whereas to the east (Kamogawa) it is often difficult to identify and principally represented by minor fault sets. Such differences are illustrated in Fig. 3 by variations in arrow sizes. Finally, the Late Cenozoic deformation in the Miura peninsula is almost entirely explained by this succession of two major compressional events (Fig. 2) that induced the formation of various folds, strike-slip and reverse fault systems. The successions of tectonic events reconstructed in the Boso peninsula are more complex (Fig. 3), partly because sites and measurements are more numerous so that minor events are better known. First, several paleostress patterns that cannot belong to any of the two major compressional events have been reconstructed in Boso. E-W to WNW-ESE compression has often been identified as the oldest faulting event in Miocene/early Pliocene sediments (Fig. 3); it was never observed in the late Pliocene/Pleistocene Kazusa Group. Approximately NNE-SSW extension has also been identified in rare places; elements of relative chronology (Fig. 5) suggest that it has occurred between the two major events discussed first (Fig. 3). Other determinations of particular old stress patterns are exceptional and considered doubtful. The identification of the two small events mentioned above (E-W compression and NNE-SSW extension) cannot be explained by local deviations of stress or rotations of faulted material within the framework of the major compressional events. However, because the amplitudes remain very small, we infer that these events are much less significant than the other ones in terms of geodynamic evolution. Second, a major extensional event can be easily identified in eastern Boso, whereas it is absent to the west. Normal fault systems affect the Pleistocene of the upper Kazusa Group (Kiwada formation) as well as older terranes. There is an obvious relation between the identification of this extensional event in outcrops and the eastward develop-
ment of NNE-SSW faults in the geological map (Fig. 3). Extension (03 axis) strikes NW-SE (average azimuth 125°). In all but a few sites, this event is the most recent. However, some observations suggest that the main N N W - S E E compression and this NW-SE extension are partly synchronous: the extension (increasing to the east) and the compression (increasing to the west) have been alternatingly active in some areas. Both types of deformations affect the Kazusa Group. A remarkable feature is the syn-sedimentary or, more commonly, the pre-diagenesis character of many faults and folds. In fact, numerous observations suggest that the first E-W compression that we identified occurred prior to the compaction of sediments of the Nishizaki and Chikura Formations (Table 1). This deformation of soft sediments may occur very late after deposition (depending on sedimentation rates and other factors). In any case, this observation suggests that the compressional tectonic events that we identify have occurred after about 5 - 6 Ma ago, but not very recently. Similar observations have been made in the Inakosawa and Nishizaki Formations of western Boso, where the main NNE-SSW compressional features developed before or during sediment compaction. This additional observation suggests that the change from the WNW-ESE to the main NNE-SSW compression has occurred between approximately 3 and 5 Ma ago, maybe at the time of the Kurotaki unconformity (Table 1). Note also that in eastern Boso, the NW-SE extension was clearly active during the sedimentation of the Kiwada Formation (i.e., the upper Kazusa Group). The oldest extensional tectonic event has been identified in the ophiolitic rocks of the Mineoka belt in three sites (Fig. 3, sites 17, 18, 61) of the Kamogawa region. This NNE-SSW extension (azimuth 035 °) is older than the major NNE-SSW compressional event. Because we did not find such systems of WNW-ESE normal faults in more recent terranes, we suspect that this extension is older than the middle Miocene.
4. Geodynamic implications 4.1. Chronology and significance of tectonic events in the collision zone The single significant tectonic event probably
404 older than the late Miocene, that we could reconstruct based on detailed tectonic analysis, is the N 0 3 5 ° E trending extension that affects the Mineoka Group in southeastern Boso peninsula. We tend to interpret this extensional event as related to the early history of these terranes, possibly to plate bending on the southern side of the former boundary Philippine Sea plate-northern Japan before it jumped southward when the present Sagami trough system developed; the present folded Mineoka belt probably represents the result of the collision stage that has induced this southward migration [11,14,15[. The two next events that we could reliably reconstruct despite of minor role, are the W-E to W N W - E S E compression and the NNE-SSW extension (Fig. 3). The compressional event is obviously older than the compaction of the youngest sediments affected (in the late Miocene/early Pliocene Nishizaki and Chikura Formations); we consequently believe that this compressional event may reflect a state of stress that developed at least during the last stages of formation of the thick pile of late Miocene/early Pliocene sediments, between about 6 and 3 Ma ago. The two major compressional events that clearly dominate the paleostress patterns of Figs. 2 and 3 are more recent. They are the most important in terms of amplitude, and also the most significant within the framework of the Izu collision (Fig. 6). In the Miura-Boso area, the first compressional
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event corresponds to a remarkably homogeneous NNE-SSW trend of paleo-o a (N030°E). Because we did not find evidence of this compression in the Kazusa Group of the late Pliocene/Pleistocene, and also because it predates complete compaction of sediments in the late Miocene/early Pliocene Inakosawa, Chikura and Nishizaki Formations, we believe that most of this large NNESSW compression has occurred prior to the development of the Kurotaki unconformity, approximately 3 Ma ago (Table 1). Although we did not find evidence on the outcrop scale, we tend to relate the development of the Kurotaki unconformity itself to a paroxysm of this NNE-SSW compression. We cannot reject the possibility for this "event" to reflect a state of stress that dominated during a long time, either after the two events mentioned above or even before and after them (or one of them). This hypothesis would enable one to explain the apparently larger amplitude of structures related to N N E compression in the oldest terranes of the Awa Group, but is not supported by any definite observation. The second major compressional event corresponds to a NNW-SSE (N160 ° E) trend of paleo-o 1 in Miura and Boso peninsulas. This event clearly affects the late Pliocene/Pleistocene terranes of the lower Kazusa Group; it also affects, apparently in a less intense way, the upper Kazusa Group. We conclude that this major compressional regime has dominated since no more than 2 - 3 Ma ago.
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Fig. 6. Neotectonic evolution of the Izu collision zone and related directions of compression determined by using tectonic analysis. Lines with triangles: thrusts (triangles on the upper block). Trajectories of 01 dashed. Directions of compression (ol) locally reconstructed: double convergent arrows. Direction of relative motion Philippine Sea plate-Eurasia: black arrows. Domain of extension in eastern Boso hatched (direction of 03 shown as divergent arrows). (a) Collision event older than 2-3 Ma (about 3 Ma ago); (b) collision event younger than 2-3 Ma (about 1 Ma ago).
405 The counterclockwise change in the directions of compression that has been reconstructed based on tectonic analysis in the Miura-Boso area is not exceptional in Central Japan (Fig. 6). On the western edge of the Izu collision zone, near Kakegawa, Huchon [21] has described a counterclockwise change of 30 ° in the trend of paleo-o 1 from N135°E to N105°E; the first tectonic regime has predominated during the Pliocene (Fig. 6a), the second one is still active (Fig. 6b). This trend change locally postdates 2.2 Ma ago and probably corresponds to the late Pliocene/early Pleistocene. North of Kakegawa, the reconstructed directions of compression vary from N140°E to N l l 0 ° E ; no systematic variation with time could be demonstrated, due to large dispersion of local paleostress trends related to intense deformation and complex stress patterns in a N-S trending belt of large folds, strike-slip, faults and thrusts [22]. North of the Izu peninsula, the dominating trend of o I is N135°E (Fig. 6). It corresponds to the folding of the Ashigara trough, where thick sediments accumulated during the early Pleistocene, and to the related southward thrusting of the Tanzawa Mountains (Kannawa thrust, Fig. 6b). A very recent local clockwise change of o I trends has occurred 0.3 Ma ago according to Huchon and Kitazato [23]; it reflects local modifications of stress patterns close to a major thrust during the continuing collision process, rather than regional variations (Fig. 6b). North of this Ashigara area, the Tanzawa Mountains (Fig. 6a) probably represent a volcanic island of the Izu-Bonin arc, accreted to central Japan during the early Pliocene (prior to the folding of the Ashigara trough); an age that corresponds to a paroxysm in NW-SE compression near Kakegawa. Compression related to Tanzawa collision was N-S (Fig. 6a). At least two "micro-collisions" (Tanzawa and Ashigara) occurred successively during the Plio-Pleistocene, resulting in southward jumps of the collision boundary (Figs. 6 and 7), and in progressive incorporation to central Japan of pieces of the IzuBonin arc. Incidentally, this evolution incites one to locate the future collision boundary south of Izu. A particular feature of the recent stress field in Boso (Fig. 3) is the transition from compression (Miura and western Boso peninsulas) to extension (normal faulting in eastern Boso). Based on geo-
graphic proximity and compatible trends, we relate this phenomenon to the transition between the Izu collision zone (to the west) and the westernmost portion of the Japan active margin in front of the Japan trench system (to the east, Fig. 1). This interpretation implies that the subduction process in the Japan Trench induces extensional phenomena in the upper margin in eastern Boso (Fig. 6b).
4.2. Age of geodynamic significance of the counterclockwise change in major compression trends There is a striking homogeneity in the evolution of both sides of the Izu collision zone during the Plio-Quaternary. A major counterclockwise change in the direction of compression has occurred with time. This change ranges from about 30 ° (to the west) to about 50 ° (to the east), as Fig. 6 shows (compare a and b). The age determined for this major change based on stratigraphic data varies between 1-2 Ma (to the west) and 2-3 Ma (to the east). This phenomenon could well be time-transgressive as a function of the distribution of motion and local deformations along an evolving plate boundary, so that there is no reason to expect a rigorous synchronism between west and east. The two major stages of collision are illustrated in Fig. 6, where data from Tanzawa (unpublished) and Ashigara [23] areas have been incorporated. Fig. 7 summarizes the distribution of the o 1 trajectories in and around the collision area, based on the extrapolation of these results of our tectonic analyses. Note that (1) the recent stress pattern of Fig. 7b is in agreement with present-day data obtained from numerous geodetic and seismic studies of Japanese authors, and (2) this stress pattern can be related to the direction of relative plate motion independently determined (see section 2.1i, either through a simple qualitative approach based on mechanical comparisons and approximate symmetries, or through a more rigorous finite-element analysis [24]. In turn, such comparisons between the orientation of the vectors of relative plate motions and the distribution of 01 trajectories around a collision zone (the shape of which is known) lead one to reconstruct the former motion vector, using the older distribution of a 1 trajectories in the same area. This comparison is done in Fig. 7: the counterclockwise change in o 1 trajectories (from a to b)
406 implies that a counterclockwise rotation of the direction of relative motion has also occurred, from N N W to NW. This interpretation, based solely on the results of our tectonic analyses, is supported by independent plate tectonics data that also imply a counterclockwise rotation of the direction of Philippine Sea plate-Eurasia motion [25].
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4. 3. Changes in paleostress trends and hypothetic block rotations Our tectonic analyses do not provide rigorous means to identify actual paleostress rotations and rotations of large blocks (such block rotation results in apparent paleostress changes even within a constant stress field). As a result, we cannot definitely decide whether the apparent counterclockwise 50 ° rotation of a I trends in the Miura-Boso
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peninsulas is due to a real counterclockwise 50 ° rotation of o I axes (as in Fig. 7), or to any combination of counterclockwise change in o 1 directions and clockwise block rotation. The latter hypothesis is illustrated in Fig. 8, where an arbitrary 25 ° clockwise rotation affects a large area long the eastern side of the collision zone, so that the angle between real successive a I trends is only 25 ° counterclockwise (compare Fig. 7a and 8a). N o apparent clockwise rotation of o 1 trends has occurred on the western edge of the collision zone (where the change, on the contrary, has occurred counterclockwise: Fig. 6). This suggests that block rotation, if it is present, did not play a major role in the Kakegawa and Miura-Boso regions, as far as the late Pliocene/Quaternary deformations are concerned (such block rotation should have occurred counterclockwise west of the collision zone). If, however, any limited clockwise block rotation had actually occurred recently in the Miura-Boso peninsulas, this should be demonstrated by further paleomagnetic studies.
407
5. Conclusions The tectonic analysis provides a simple and reliable way to describe the paleostress evolution of a complex convergent boundary such as the Philippine Sea plate-Eurasia collision zone of Central Japan. This subsequently provides a way to interpret the changes in paleostress fields, in order to reconstruct the variations in the direction of convergence. The first step is described in Figs. 4 and 5 (outcrop scale) and 2 and 3 (area scale), while the second step is illustrated in Figs. 6 and 7 (regional scale). It is worthwhile to observe that very similar counterclockwise changes in the directions of regional o 1 trends occurred during the PlioQuaternary in the collision zone of Taiwan [26]. These paleostress changes in Taiwan have been described and interpreted elsewhere [27]. The distance between Taiwan and Izu is larger than 2000 km, along the same major plate boundary (Philippine Sea plate-Eurasia). Despite of some stratigraphic uncertainties that still affect the dating of this event, especially in Taiwan, these similar changes reflect a" more general phenomenon at the scale of the entire Philippine Sea plate [28]. Independent geophysical data and plate tectonic reconstructions lead to the same conclusion, that is the occurrence of a significant counterclockwise change in the direction of relative motion. These examples indicate first that a change in a direction of convergence and related stress fields, although it is recorded in relatively small collision areas such as central Japan, may be significant at the scale of a major plate; second, that tectonic analysis based on accurate observation in numerous sites provides a key to decipher some major features of geodynamic evolution. Acknowledgments This work has been carried out within the framework of the French-Japanese Kaiko project, and supported by the CNRS, the French Minister of Foreign Affairs and the IFREMER. Discussions with Dr. Y. Ogawa and K. Horiuchi, and Messrs. Koga and Taniguchi, were fruitful. We thank P. Choukroune, J. Letouzey and H. Okada for reviewing this paper.
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408 18 D.N. Williams, Late Quaternary displacement at the Hikihashi and Kitatake faults, Miura peninsula, Daiyonki Kenkyu (Quaternary Research) 21(4), 289-299, 1983. 19 C.H. Scholtz and T. Kato, The behavior of a convergent plate boundary: crustal deformation in the south Kanto district, Japan, J. Geophys. Res. 83 (B2), 783-797, 1978. 20 J. Angelier, Tectonic analysis of fault slip data sets, J. Geophys. Res. 89 (B8), 5835-5848, 1984. 21 P. Huchon, Les contraintes tectoniques h l'arri+re d'une zone de subduction: relation avec la direction de convergence et 6volution dans le temps; l'exemple de la r~gion de Shizuoka (Japon central), C.R. Acad. Sci., Paris, Ser. II 296, 787-790, 1983. 22 P. Huchon and J. Angelier, From subduction to collision; the Suruga-Fujigawa belt, Izu collision zone, Japan, Bull. Soc. G6ol. Fr., in press, 1986. 23 P. Huchon and H. Kitazato, Collision of the Izu block with central japan during the Quaternary and geological evolution of the Ashigara area, Tectonophysics 110, 201-210, 1984. 24 P. Huchon and J.C. de Bremaecker, Influence of Izu subduction-collision on the deformation of central Japan, in: Formation of Ocean Margins, N. Nasu, ed. pp. 701-717. Terra Scientific Publ. Co., Tokyo, 1985. 25 P. Huchon, G6odynamique de la zone de collision d'Izu et de la jonction triple du Japon central--leur place dans l'6vohition de la plaque Philippine, M~m. Sci. Terre, Univ. P. & M. Curie 85-56, 414 pp., 1985. 26 E. Barrier, Tectonique d'une cha]ne de collision active: T~wan, M6m. Sci. Terre, Univ. P. & M. Curie 85-29, 492 pp., 2 vol., 1985.
27 J. Angelier, E. Barrier and P. Huchon, Sur les relations entre trajectoires de contrainte et directions de mouvement le long d'une fronti+re convergente: exemple de la subduction hell6nique (Gr+ce) et de la collision Philippines-Eurasie (Tal/wan et Japon). C.R. Acad. Sci., Paris, Ser. II 294, 745-748, 1982. 28 J. AngeEer, E. Barrier and H.T. Chu, Plate collision and plaeostress trajectories in a fold-thrust belt: the Foothills of Taiwan, Tectonophysics 125, 161-178. 29 N. Niitsuma, Magnetic stratigraphy of the Japanese Neogene and the development of the island arc of Japan, J. Phys. Earth 26 (Suppl.), $367-$378, 1978. 30 K. Sawamura and T. Nakajima, Miocene silicoflagellate zones in the Boso peninsula, Bull. Geol. Surv. Jpn. 31, 333-345, 1980. 31 R. Tsuchi and M. Ibaraki, Correlation of Neogene sequences on the Pacific coast of southern Japan by means of planktonic Foraminifera and Molhisca, Rep. Fac. Sci., Shizuoka Univ. 15, 53-68, 1980. 32 N. Kotake, Stratigraphy and biochronology of the southernmost part of Boso Peninsula, Abstr. Geol. Soc. Jpn., p. 123, 1985. 33 Geological Survey of Japan, Geological map of the Sagami Bay and adjacent area, 1 : 100,000, 1977. 34 Geological Survey of Japan, Geological map, sheet "Kamogawa" 1:50,000, Quadrangle series, NI-54-20-14, 1981.