Am s Sediments

Magnetic fabrics of soft-sediment folded strata within aneogene accretionary complex, the Miura group,

central Japan

Toshiya Kanamatsu a, Emilio Herrero-Bervera b;*, Asahiko Taira c

a Japan Marine Science and Technology Center, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka 237, Japanb Hawaii Institute of Geophysics and Planetology, School of Ocean Earth Science and Technology,

Paleomagnetic and Petrofabrics Laboratory, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822 USAc Ocean Research Institute, University of Tokyo, 1-15-1 Minami-dai Nakano-ku, Tokyo 164, Japan

Received 8 November 2000; accepted 22 February 2001

Abstract

Anisotropy of magnetic susceptibility (AMS) on the middle Miocene^Pleistocene sedimentary sequence in the Bosoand Miura Peninsulas of central Japan was used to study 18 sites in the northern tectonic setting and 37 sites in thesouthern setting. This sequence is associated with abundant synsedimentary deformation structures of folding andfaulting generated in accretionary tectonics. AMS results in different deformation settings such as the forearc, theaccretionary prism and the trench were analyzed. The shapes of the dissimilar magnetic fabrics are compared using theshape parameter (T) and the corrected anisotropy degree (PP) in the so-called T^PP diagrams. Our results have impliedthat the oblate fabric of the trench sediments can be regarded as the result of depositional and compactional processesalone. The AMS shape parameters obtained from the northern sequence (forearc) closely resemble an indication ofundeformed trench sediments. In contrast, a different pattern is observed in the highly prolate-shaped AMS results ofthe southern sequence. The difference apparently reflects the degree of deformation in the three tectonic provinces. Inorder to understand the deformation mechanism of the sedimentary fabric, a detailed AMS study was made on oneanticline system. An AMS evolution from an oblate fabric to a prolate fabric in the anticline system was observed. Wealso found that an AMS tectonic fabric occurred in the center of the anticline. Thickness correlations of the strata understudy indicate that strained sediments formed in the central portion of the fold. As a consequence, one can say that thismechanism can modify the magnetic fabric from the sedimentary form to the tectonic form in a compressionalregime. 2001 Elsevier Science B.V. All rights reserved.

Keywords: magnetic susceptibility; petrofabrics; soft sediment deformation; accretionary wedges; Japan

1. Introduction

The anisotropy of magnetic susceptibility(AMS) measurement is a powerful method in pet-rofabric analysis because of the non-destructive-ness of the sample and its rapid operation as com-

0012-821X / 01 / $ ^ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 2 9 2 - 8

* Corresponding author. Tel. : +1-808-956-6192;Fax: +1-808-956-3188; E-mail: [email protected]

EPSL 5808 26-4-01 Cyaan Magenta Geel Zwart

Earth and Planetary Science Letters 187 (2001) 333^343

www.elsevier.com/locate/epsl

pared with other types of procedures includingthin-section analysis [1,2]. It provides the shapecharacteristics and orientations of the magneticsusceptibility ellipsoid, which have been provento be suitable for the study of rock formationand subsequent deformation (e.g. [3^6]).

Several recent studies have examined the sen-sitivity of AMS in weakly deformed sedimentsfrom environments in dierent parts of the world(e.g. [7^13]). In those studies it has been estab-lished that it is possible to distinguish importantfeatures of their magnetic fabrics correspondingto certain tectonic processes which record the sys-tematic reorientation of the original sedimentaryfabric. In those cases the magnetic lineations areoften perpendicular to the compressional direc-tion.

The AMS method is now considered to be auseful marker in structural analysis of syn-to-post depositional weakly deformed sediments inwhich conventional strain markers cannot beeasily applied, although the mechanism makingsuch a magnetic fabric is still unclear and undercurrent debate. Various mechanisms of fabric de-velopment have been proposed at various stagesof rock deformation. The magnetic fabric can bephysically modied by internal changes and thereorientation of the magnetic grains. The internalchanges arise from magnetostriction, growth ordissolution, and brittle or plastic deformation ofthe magnetic grains (e.g. [14]). Grain rotation canoccur in various modes of shear or ductile defor-mation. Wet sediment deformation accompaniedby dehydration may cause grain rotation ratherthan physical changes of the grains.

In order to understand the deformation mech-anism of the sedimentary fabrics, a useful ap-proach is to make a comparison between the de-formed and undeformed fabrics. The sedimentarysequence distributed in the Boso and Miura Pen-insulas of central Japan provides an ideal oppor-tunity for this purpose. The sequence is generallysimilar in lithology (characterized by hemipelagicsiltstones), and has experienced contrasting syn-chronous deformation in a convergent tectonicsetting. It has been demonstrated that there is aclear distinction in magnetic fabric characteristicsin the tectonic settings of the deformed accretion-

ary prism sequences versus those of the unde-formed forearc basin sequences [15].

This study focuses on the results of an AMSstudy on folded strata within an accretionaryprism sequence, because it is possible to considerthe fabric evolution with information of the spe-cic evolution of a folded system.

2. Geological setting

The middle Miocene^Pleistocene sedimentarysequence in Boso and Miura Peninsulas of centralJapan is characterized by a thick accumulation ofalternating clastic and hemipelagic rocks withabundant synsedimentary deformation structuresassociated with tight folding and faulting. Theselithologies are regarded to be formed by a con-vergent tectonic setting [16,17].

The distribution of the sedimentary sequence isseparated by the Mineoka^Hayama uplift zone(HUMZ) into the southern and the northernareas (Fig. 1). The geology of the HMUZ is com-posed of the Mineoka (Eocene^Oligocene; ophio-litic rocks) and the Hayama^Hota groups (lower^middle Miocene; siliclastics) and has been inter-preted as an ancient trench-slope break since themiddle Miocene. The sedimentary sequence in thesouthern and the northern areas is chronologicallysubdivided into the Miura group (middle Mio-cene^Pliocene) and the post-Miura group (Plio^Pleistocene Kazusa and Chikura^Toyofusagroups). The sedimentation continued up to 0.5Ma, and was uplifted during the Pleistocene.

The Miura group was focused upon in thisstudy because of its wider distribution and varietyof tectonic settings. The sedimentation of theMiura group had occurred in the lower bathyalzone to the lower part of the middle bathyal zoneinterpreted to be 1000^2000 m to 2000^3000 m inwater depth judging from the assemblages ofbenthic foraminifera [18,17]. Two distinct clasticcomponents are observed: volcaniclastics derivedfrom submarine volcanoes of the Izu^Ogasawaraarc, and terrigenous clastics derived from theHonshu arc transported by turbidity currents.The HMUZ has restricted the supply of terrige-nous clastics from the northern area to the south-


T. Kanamatsu et al. / Earth and Planetary Science Letters 187 (2001) 333^343334

ern area, making the southern sequence more en-riched in volcaniclastics. There is also a lateralvariation superimposed on the north^south varia-tion. The strata of the western peninsula (theMiura Peninsula) includes more coarse-grainedvolcaniclastics than the eastern peninsula (theBoso Peninsula) due to its proximity to the Izu^Ogasawara volcanic arc [19].

The structure of the southern sequence is char-acterized by a pervasive synsedimentary deforma-tion including dehydration veins, clastic dikes,healed thrust faults and ductile folds, whereasthe northern sequence shows a more gentle struc-ture. The southern sequence is subdivided intoseveral discrete tectonic units by east^west trend-ing thrust faults. The sequence within these tec-tonic units shows an upward coarsening sedimen-tary cycle. This sequence is interpreted to be theresult of sedimentation of the southern sequenceoccurring in a trough-to-slope basin environmentsubsequently forming an accretionary complex[17]. On the other hand, the sedimentation ofthe northern sequence was continuous from the

middle Miocene to the middle Pliocene, and thesedimentary environment was interpreted to be aforearc basin.

3. Sample preparation

Sampling was conducted mainly in the coastalareas of both peninsulas. For the Miura groupthere were 18 sites drilled in the northern tectonicsetting and 37 sites in the southern setting (Fig.1). Hemipelagic layers were chosen for this studyand no coarse-grained layers were sampled. Thehemipelagite was considered to be uniform in lith-ology, more susceptible to tectonic deformation,and more suitable for AMS and paleomagneticstudies. The layer containing deformation struc-tures such as synsedimentary veins [20,21] andfaults were avoided in this study because theytended to show localized eects [22]. Sampleswere collected using a portable gasoline-powereddrill and oriented by means of a magnetic com-pass and also by hand sampling methods from

Fig. 1. The distribution of the sampling sites in the Miura and Boso peninsulas and position of DSDP site 582B. Rectangularbox in the Miura Peninsula denotes an area of more details in Fig. 4.


T. Kanamatsu et al. / Earth and Planetary Science Letters 187 (2001) 333^343 335

Table 1Summary of AMS results

Site Formation Age N K Mean L Mean F mean PP mean TSlope basin and accretionary prism settingBO01 Amatsu late Miocene 36 1 558.1 1.01 (0.003) 1.02 (0.006) 1.031 (0.007) 0.338 (0.207)BO02 Amatsu late Miocene 36 1 963.8 1.008 (0.002) 1.016 (0.005) 1.025 (0.007) 0.302 (0.177)BO03 Amatsu late Miocene 26 2 007 1.009 (0.003) 1.015 (0.008) 1.025 (0.010) 0.157 (0.337)BO04 Amatsu late Miocene 27 2 422 1.008 (0.004) 1.011 (0.005) 1.02 (0.005) 0.156 (0.297)BO05 Amatsu late Miocene 33 3 310.3 1.007 (0.003) 1.016 (0.009) 1.023 (0.012) 0.423 (0.271)BO10 Amatsu late Miocene 34 2 607.8 1.007 (0.005) 1.03 (0.027) 1.039 (0.033) 0.606 (0.171)MI20 Misaki late Miocene 30 5 406.2 1.01 (0.008) 1.046 (0.011) 1.064 (0.012) 0.317 (0.490)MI21 Misaki late Miocene 41 7 914.6 1.031 (0.003) 1.033 (0.003) 1.055 (0.004) 0.06 (0.263)MI22 Misaki late Miocene 45 3 875.3 1.006 (0.004) 1.027 (0.009) 1.036 (0.012) 0.618 (0.179)MI23 Misaki late Miocene 34 5 089.1 1.01 (0.008) 1.051 (0.017) 1.067 (0.019) 0.666 (0.078)MI24 Misaki late Miocene 33 1 479.6 1.018 (0.004) 1.043 (0.006) 1.064 (0.006) 0.405 (0.147)MI31 Miaski late Miocene 27 2 569.3 1.008 (0.002) 1.005 (0.003) 1.013 (0.003) 30.284 (0.289)MI32 Miaski late Miocene 28 1 140.8 1.003 (0.001) 1.002 (0.001) 1.05 (0.001) 30.109 (0.385)MI29 Hatsuse early Pliocene 32 5 436.2 1.012 (0.005) 1.011 (0.007) 1.023 (0.008) 30.092 (0.382)MI30 Hatsuse early Pliocene 24 13 007 1.01 (0.006) 1.02 (0.012) 1.032 (0.014) 0.25 (0.445)RB04 Amatsu late Miocene 26 5 558.3 1.006 (0.002) 1.011 (0.007) 1.018 (0.008) 0.164 (0.374)RB05 Amatsu late Miocene 13 2 371.1 1.011 (0.003) 1.016 (0.019) 1.022 (0.005) 30.038 (0.266)RB12 Amatsu late Miocene 15 3 455.1 1.006 (0.002) 1.007 (0.003) 1.013 (0.005) 0.077 (0.280)BA01 Amatsu late Miocene 19 2 854.4 1.006 (0.002) 1.022 (0.004) 1.03 (0.004) 0.57 (0.149)BA02 Amatsu late Miocene 16 297.9 1.009 (0.001) 1.015 (0.002) 1.024 (0.003) 0.238 (0.086)BA03 Amatsu late Miocene 13 4 054.1 1.011 (0.004) 1.009 (0.005) 1.021 (0.008) 30.147 (0.264)BA04 Amatsu late Miocene 18 2 648.4 1.016 (0.003) 1.007 (0.003) 1.024 (0.003) 30.429 (0.238)BA05 Amatsu late Miocene 13 3 429.8 1.006 (0.003) 1.015 (0.009) 1.022 (0.000) 0.392 (0.271)BA06 Amatsu late Miocene 12 564.9 1.006 (0.002) 1.025 (0.002) 1.034 (0.002) 0.591 (0.100)RM01 Miaski late Miocene 90 8 727.8 1.01 (0.002) 1.012 (0.004) 1.022 (0.004) 0.093 (0.179)RM02 Miaski late Miocene 15 3 182.9 1.004 (0.002) 1.016 (0.003) 1.021 (0.004) 0.571 (0.186)RM03 Miaski late Miocene 17 3 654.5 1.01 (0.005) 1.036 (0.012) 0.012 (0.010) 0.548 (0.239)RM04 Miaski late Miocene 7 2 667.6 1.014 (0.004) 1.031 (0.006) 1.047 (0.007) 0.354 (0.146)RM05 Miaski late Miocene 7 4 537.6 1.006 (0.002) 1.013 (0.008) 1.019 (0.009) 0.281 (0.273)RM07 Miaski late Miocene 13 2 477.5 1.009 (0.005) 1.027 (0.066) 1.038 (0.009) 0.524 (0.219)RM08 Miaski late Miocene 8 3 626.2 1.003 (0.002) 1.016 (0.004) 1.021 (0.005) 0.682 (0.132)RM09 Miaski late Miocene 12 2 833.4 1.005 (0.002) 1.019 (0.008) 1.026 (0.009) 0.552 (0.223)RM10 Miaski late Miocene 8 6 085.3 1.005 (0.002) 1.003 (0.001) 1.007 (0.002) 30.325 (0.159)RM12 Miaski late Miocene 27 3 351.9 1.003 (0.001) 1.011 (0.004) 1.015 (0.004) 0.491 (0.191)RM13 Miaski late Miocene 16 1 172.8 1.008 (0.001) 1.014 (0.002) 1.023 (0.002) 0.249 (0.104)RM14 Miaski late Miocene 15 1 307.9 1.006 (0.003) 1.011 (0.004) 1.017 (0.006) 0.302 (0.102)RM15 Miaski late Miocene 15 3 599 1.009 (0.003) 1.01 (0.005) 1.02 (0.004) 30.012 (0.331)Forearc basin settingBO14 Amatsu late Miocene 34 812 1.004 (0.002) 1.008 (0.003) 1.012 (0.004) 0.34 (0.031)BO15 Amatsu late Miocene 34 762 1.007 (0.003) 1.016 (0.005) 1.024 (0.007) 0.399 (0.024)BO17 Kiyosumi latest Miocene 29 271.3 1.01 (0.006) 1.06 (0.016) 1.079 (0.017) 0.721 (0.145)CB06 Koyosumi latest Miocene 3 5 370 1.034 (0.004) 1.09 (0.009) 1.127 (0.013) 0.409 (0.044)CB09 Inagozawa latest Miocene 15 262.6 1.021 (0.003) 1.021 (0.007) 1.043 (0.007) 30.037 (0.034)CB10 Inagozawa latest Miocene 14 2 532.2 1.015 (0.007) 1.041 (0.009) 1.06 (0.007) 0.44 (0.274)CB11 Inagozawa latest Miocene 10 2 015.2 1.024 (0.004) 1.093 (0.007) 1.126 (0.010) 0.566 (0.060)CB12 Kiyosumi latest Miocene 14 382.3 1.009 (0.003) 1.044 (0.008) 1.057 (0.007) 0.644 (0.142)CB14 Kiyosumi latest Miocene 14 3 761.1 1.01 (0.005) 1.014 (0.004) 1.025 (0.005) 0.179 (0.327)CB16 Amatsu late Miocene 13 5 883.3 1.003 (0.001) 1.013 (0.006) 1.017 (0.006) 0.529 (0.260)CB17 Amatsu late Miocene 13 1 502.8 1.011 (0.007) 1.021 (0.006) 1.033 (0.008) 0.335 (0.353)RB13 Kiyosumi latest Miocene 16 5 182.4 1.006 (0.003) 1.015 (0.007) 1.022 (0.008) 0.389 (0.255)RB14 Kiyosumi latest Miocene 16 1 634 1.01 (0.002) 1.025 (0.004) 1.037 (0.003) 0.404 (0.154)RB17 Amatsu late Miocene 11 3 691 1.011 (0.003) 1.036 (0.007) 1.05 (0.008) 0.523 (0.119)



closely spaced (2 or 3 m) layers in each site.A total of 1157 specimens were measured forAMS studies. The AMS was measured using aKLY-3 Kappabridge susceptibility instrument lo-cated at the Paleomagnetic and Petrofabrics Lab-oratory of the Hawaii Institute of Geophysics andPlanetology (HIGP) of the University of Hawaiiat Manoa, HI, USA, and also at the Ocean Re-search Institute, University of Tokyo, Tokyo, Ja-pan.

3.1. Magnetic mineralogy studies

The bulk susceptibilities of the Miura groupranges from 1032 to 1034 SI units (Table 1).This indicates a very high magnetic susceptibilityrange for these sedimentary rocks. This maximumsusceptibility reaches the same range as that ofvolcanic rocks (in the 1032 order). AMS of highmagnetic susceptibility (greater than 1033 SI) arisefrom the ferrimagnetic fraction alone. Magneticsusceptibilities less than 3U1034 SI involve eectsof the paramagnetic fabrics [6,12]. Most sites ofthe Miura group depict magnetic susceptibilitiesgreater than 1033 SI (see Table 1), but the AMSof several sites in which magnetic susceptibilitiesare less than 3U1034 SI probably indicates thepresence of paramagnetic minerals. Further mag-netic mineralogic studies were performed on atleast one sample per site. The Curie points ofthe samples from dierent localities were deter-mined using low-eld susceptibility measurements[23,24]. Most of the samples exhibited a simplethermomagnetic behavior characterized by nearlyreversible k^T curves with a single Hopkinsonpeak and a sharp drop in susceptibility indicatingthe reaching of a Curie point in the range of 560^580C, which is characteristic of nearly pure mag-

netite and also of ne grains of single domainmaterial (Fig. 2A).

The second rock magnetic experiment per-formed was the determination of the magnetichysteresis parameters on small rock chips withan alternating gradient force magnetometer (theMicromag 2900). The slope at high magneticelds, which represents the paramagnetic contri-bution, is calculated after removal of this compo-nent. The saturation remanent magnetization(Mr), saturation magnetization (Ms), and coerciveforce (Hc) were calculated after removal of thiscomponent. By applying progressively increasingback elds after saturation, we determined thecoercivity of the remanence (Hcr). Relatively lowvalues of Hc and Hcr have indicated that a verylarge fraction of the NRM was carried by low-coercivity minerals, which is in agreement withthe presence of a high proportion of magnetite.The ratios of the hysteresis parameters plotted asa Day diagram [25] in Fig. 2B show that the grainsize of magnetite is characterized by signicantdispersion in the pseudosingle domain range forthe Boso and Miura Peninsulas samples. Theseexperiments indicate that magnetite is the carrierof the magnetic susceptibility of the Miura group.

4. Results

4.1. The general trend of fabric shape

The shape of magnetic fabric from the forearcbasin (northern area) and the slope basin and ac-cretionary complex (southern area) is representedby the shape parameter (T) and the degree ofanisotropy parameter (PP) in the so-called T^PPplots [26]. Fig. 3 is a combination of earlier results

Table 1 (continued)

Site Formation Age N K Mean L Mean F mean PP mean TMI25 Zushi early Pliocene 29 358 1.016 (0.010) 1.032 (0.015) 1.05 (0.019) 0.302 (0.286)MI26 Zushi early Pliocene 34 2 738.8 1.03 (0.006) 1.048 (0.011) 1.081 (0.011) 0.217 (0.165)MI28 Zushi early Pliocene 33 370.4 1.005 (0.003) 1.018 (0.006) 1.027 (0.008) 0.532 (0.197)MI27 Ikego middle Pliocene 25 1 096.4 1.003 (0.001) 1.004 (0.002) 1.007 (0.002) 0.234 (0.346)

N : number of specimens, K : mean bulk magnetic susceptibility of site, L : Kmax/Kint ; (standard deviation), F : Kint/Kmin ; (standarddeviation), PP : exp{sqr[2U((R13Rm)2+(R23Rm)2)+(R33Rm)2)]}; (standard deviation), T : (2R23R13R3)/(R13R3) ; (standard devia-tion) where R1 = ln Kmax, R2 = ln Kint, R3 = ln Kmin, Rm =

R 1UR 2UR 33p

.



[15] together with the remeasured data fromDSDP site 582B characterized by undeformedsediment used as a reference. This site was drilledfrom the bottom oor of the Nankai Trough dur-ing DSDP Leg 87 [27]. The fabric of Site 582B(Fig. 3A) is oblate and is characterized by aslightly scattered degree of anisotropy (PP). Thisreference can be regarded as constructed by dep-ositional and compactional processes alone.

The shape parameters obtained from the north-ern sequence (forearc basin, Fig. 3B) show scat-tering in the area corresponding to the oblate fab-rics similar to that of the Site 582B (Fig. 3B),although it shows a biased behavior towards thelow-PP areas, especially around the 1.02 values.

In contrast, a dierent pattern is observed inthe southern sequence (slope basin and accretion-ary prism) showing a clear concentration of thedata around the same 1.02 area in PP and a largevariation in the values of T (Fig. 3C).

Overall, the dierence apparently reects thedegree of deformation in three tectonic provinces.Based on these results we chose the southern se-quence for a detailed AMS investigation, where amore severely modied fabric was obtained. Atthis point it is important to consider the relation-ship between the deformation structure of thesouthern sequence and the evolution pattern ofthe fabric in order to understand the fabric defor-mation mechanism.

Fig. 2. (A) Typical examples of low-eld magnetic susceptibility versus temperature dependencies. (B) Plot of the hysteresis pa-rameters, Mrs/Ms (ratio of remanent saturation moment Mrs, to saturation moment Ms) against Hcr/Hc (ratio of remanent coer-cive force, Hcr, to coercive force Hc). Single domain (SD), multi-domain (MD), after [25].



Fig. 3. The correlation of shape susceptibility ellipsoid using the T^PP diagram. (A) Shape of sediments of trench ll and hemipe-lagite (undeformed setting). (B) Shape of forearc sediments (gently deformed setting). (C) Shape of sediments of slope basin andACCRETIONARY prism (highly deformed setting).



Fig

.4.

The

prin

cipa

lsu

scep

tibi

lity

axes

orie

ntat

ions

afte

rbe

ddin

gco

rrec

tion

onth

est

ereo

net

(low

erhe

mis

pher

e)in

the

Tsu

rugi

zaki

site

s.T

hepl

ots

are

bund

led

inre

gion

alst

ruct

ural

sett

ings

(nor

thw

ing,

sout

hw

ing

and

hing

e)re

spec

tive

ly.

Sam

plin

gsi

tes

inth

eT

suru

guza

kisi

tes

and

sim

pli

edco

lum

nar

corr

elat

ion

inth

ean

ti-

clin

esy

stem

[28]

.



4.2. Fabric changes in a fold

A study was made on the Tsurugizaki anticline,within the Miura Peninsula, which has a north-west to southeast trend fold axis. The bedding inthe northern wing gently dips (ca. 25), whereas inthe southern wing it dips steeply (ca. 80) (see Fig.4). A microfault analysis [28] reveals that the an-ticline system was formed by northeast-southwesthorizontal compression. A detailed stratigraphicstudy of the anticline showed that the thicknessof the strata varies from the wings to the center ofthe anticline. The bed thickness of each of thecorrelated layers around the axis is about 10^20% thicker than those around the wings (Fig.4). This thickness change is accommodated mostlyby hemipelagic silty layers (these layers have beendened as incompetent layers) and not by coarse-grained volcaniclastic layers (dened as the com-petent layer). This indicates that the vertical ex-tension of the strata around the axis occurred bythe lateral shortening of the horizontal compres-sion accompanied with the fold formation.

Foliated fabrics are recognized in the northernwing sites (Sites RM 07, RM 08, RM 09 and RM12) with characteristic T values around 0.7^0.5.The Kmax and Kint axes form a girdle planearound the Kmin axis. The foliation closer to thebedding plane suggests that the alignment wasdue to compaction after deposition. Therefore,slightly oblique southwestward (Kmin) inclinationsof the northern wing of 60^70 to the southwestsuggest a smaller degree of alteration of the fabricafter the tilting of the strata.

Sites RM 13 and RM 14 are distinguished bythe fact that the Kmin axis directions are groupedperpendicular to the bedding plane, and Kint andKmax axes cluster parallel to the bedding planeforming a triaxial distribution. T values showless oblate fabric than sedimentary fabric.

The fabrics in the sites closer to the axis of theanticline (sites MI 31, RM 10 and RM 15) show aclear tectonic fabric, whereby Kint and Kmin axesform a girdle around clustered Kmax axes. Theshape parameters, T, of these sites appear moreprolate (negative value). The Kmax directions areperpendicular to the NE^SW direction of the di-rection as suggested by microfault analysis [28].

The fabric is considered to be tectonically con-structed.

The Kmax orientations of sites in the anticlineare mostly normal to the compressional directionand mostly parallel, but slightly clockwise in ro-tation with respect to the axis direction of theanticline. The shape factor T shows the highlymodied fabric around the center. On the otherhand, the T values in the wings show more oblatepatterns (see Fig. 5).

5. Discussion and conclusions

The striking result of the AMS study of this

Fig. 5. The changes of parameters of ellipsoidal shapethrough the anticline and inclination of Kmin axes of eachsite. In the hinge portion the ellipsoid shows a prolate shapewhereas on the wings there is an oblate fabric.



area is the clear contrast of fabric styles with re-spect to specic positions in the fold system. As aresult of our investigation we have obtained threetypes of fabrics in three specic settings in theanticline.

A fabric evolution is observed within the Tsu-rugizaki anticline for a length of several hundredmeters. This evolution starts from a foliated fab-ric, with no magnetic lineation, to an intermediatefabric, with a lineation normal to compression,and ends with a tectonic fabric. This pattern offabric evolution has been also recognized in otherstudies [29,30]. In this study, dierent types offabrics are obtained in a variety of settings in aone-fold system. Thickness correlations within theanticline suggest that much lateral shortening andvertical extension had occurred in the central por-tion of the fold axes during anticline formation.The bed thickness of each of the correlated layersin the center is thicker than in the wings. Thisstrain potentially can be the possible mechanismthat modied the magnetic fabric from a sedimen-tary to a tectonic fabric in the center of the anti-cline (Fig. 6). The obliqueness of the magneticfoliation perpendicular to the bedding plane inthe northern wing suggests that there is an eectof horizontal shortening after tilting. The Kmininclinations of the northern wing and aroundthe axis are very similar in the in situ attitude.Kmin axes in the northern wing are towards thehorizontal direction as well as towards the central

axis. But such oblique characteristics of the mag-netic foliation are not present in the southernwing. This asymmetric pattern of magnetic fabricsuggests that inhomogeneous shortening formedthis anticline system.

If one assumes that such an internal changeoccurred in the sediment due to lateral shortening,the model of fabric changes of unconsolidatedsediment and the relationship between grain align-ment and shortening must be considered. At theearly stage and during shortening, when the com-paction is superior to lateral shortening in thesedimentary pile, the maximum axis of the grainswill tend to align perpendicular to the shorteningdirection due to horizontal constriction. Subse-quently, after the maximum axis alignment iscompleted, and if shortening still proceeded, theminimum axis has to align parallel to the short-ening direction because the minimum axis of ro-tation is an eective way of accommodating lat-eral shortening. This model, however, has to beconrmed by an experimental approach. Never-theless, the systematic trend of fabric change rec-ognized in the shape of the ellipsoid suggests thepresence of a dominant mechanism of magneticfabric development within a highly strained soft-sediment deformation environment such as thetoe of an accretionary prism.

Acknowledgements

This work beneted from discussions withmany colleagues : Drs. Juichiro Ashi, SaneatsuSaito and Hidekazu Tokuyama of the Universityof Tokyo. We also would like to thank Mr. JamesLau for his laboratory work during the length ofthis project. We would like to acknowledge thecareful reviews of Professors C. Aubourg and L.Sagnotti as well as the very constructive com-ments of Professor D.H. Tarling and ProfessorJ.-P. Valet that helped us greatly to improve ourmanuscript with their expertise and criticisms. Fi-nancial support for this study was provided bySOEST-HIGP to E.H.-B. and also from a Na-tional Science Foundation-IF EAR-9706997Grant to E.H.-B. This is SOEST contribution5350 and HIGP contribution 1138.[AC]

Fig. 6. Progressive deformation of fabric of sediments andchanging of thickness during a fold formation.



References

[1] J.W. Graham, Signicance of magnetic anisotropy in Ap-palachian sedimentary rocks, Am. Geophys. UnionMonogr. 10 (1966) 627^648.

[2] A. Taira, B.R. Lienert, The comparative reliability ofmagnetic, photometric and microscopic methods of deter-mining the orientations of sedimentary grains, J. Sedi-ment. Petrol. 40 (1979) 759^772.

[3] A.I. Rees, W.A. Woodall, The magnetic fabric of somelaboratory deposited sediments, Earth Planet. Sci. Lett. 25(1975) 121^130.

[4] R. Kligeld, W.H. Owens, W. Lowrie, Magnetic suscep-tibility anisotropy, strain and progressive deformation inPermian sediments from the Maritime Alps (France),Earth Planet. Sci. Lett. 55 (1981) 181^189.

[5] A. Taira, Magnetic fabrics and depositional processes, in:A. Taira, F. Masuda (Eds.), Sedimentary Facies in theActive Plate Margin, Terrapub, Tokyo, 1989, pp. 43^77.

[6] D.H. Tarling, F. Hrouda, The magnetic anisotropy ofrock, Chapman and Hall, London, 1993, 217 pp.

[7] G.J. Borradaile, D.H. Tarling, The inuence of deforma-tion mechanisms on magnetic fabrics in weakly deformedrocks, Tectonophysics 77 (1981) 151^168.

[8] C. Kissel, E. Barrier, C. Laj, T.-Q. Lee, Magnetic fabric inundeformed marine clays from compressional ows, Tec-tonics 5 (1986) 769^781.

[9] T.Q. Lee, C. Kissel, C. Laj, C.S. Horng, Y.T. Lue, Mag-netic fabric analysis of the Plio^Pleistocene sedimentaryformations of the Coastal Range of Taiwan, Earth Planet.Sci. Lett. 98 (1990) 23^32.

[10] B.A. Housen, C. Richter, B.A. van der Pluijm, Compositemagnetic anisotropy fabrics experiments, numerical mod-els, and implications for the quantication of rock fabrics,Tectonophysics 220 (1983) 1^12.

[11] O. Averbuch, D. Frizon de Lamotte, C. Kissel, Magneticfabric as a structural indicator of the deformation pathwithin a fold thrust structure: a test case from the Cor-bie'res (NE Pyrenees, France), J. Struct. Geol. 14 (1992)461^474.

[12] L. Sagnotti, F. Speranza, A. Winkler, M. Mattei, R. Fu-niciello, Magnetic fabric of clay sediments from the exter-nal northern Apennines (Italy), Phys. Earth Planet. Inter.105 (1998) 73^93.

[13] C. Aubourg, P. Rochette, J.F. Stepham, M. Popo, C.Chabert-Pelline, The magnetic fabric of weakly deformedlate Jurassic shales from the Southern Subalpine Chains(French Alps). Evidence for SW-directed transport direc-tion, Tectonophysics 307 (1999) 15^31.

[14] A. Kapica, Anisotropy of magnetic susceptibility in aweak magnetic eld induced by stress, Phys. Earth Planet.Inter. 51 (1987) 349^354.

[15] T. Kanamatsu, E. Herrero-Bervera, A. Taira, S. Saito, J.Ashi, A. Furumoto, Magnetic fabric development in theTertiary accretionary complex of the Miura Group, in the

Boso and Miura peninsulas of central Japan, Geophys.Res. Lett. 23 (1996) 471^474.

[16] Y. Ogawa, K. Horiuchi, H. Taniguchi, J. Naka, Collisionof the Izu arc with Honshu and the eects of obliquesubduction in the Miura^Boso Peninsulas, Tectonophysics119 (1985) 349^379.

[17] S. Saito, Stratigraphy of Cenozoic strata in the southernterminous area of Boso Peninsula, Central Japan, Con-trib. Inst. Geol. Pal. Tohoku Univ. 93 (1992) 1^37.

[18] K. Akimoto, E. Uchida, M. Oda, Paleoenvironment usingbenthic foraminiferal faunas of middle^late Miocene, Mi-saki Formation, southern Miura peninsula, Mon. Chikyu1 (1991) 24^30.

[19] W. Soh, K.T. Pickering, A. Taira, H. Tokuyama, Basinevolution in the arc^arc Collision Zone, Mio^PlioceneMiura Group, central Japan, J. Geol. Soc. Lond. 148(1991) 317^330.

[20] Y. Ogawa, Beard-like veinlet structure as fracture cleav-age in the Neogene siltstone in the Miura and Boso Pen-insulas, central Japan, Sci. Rep. Kyushyu Univ. 13 (1980)321^327.

[21] J. Ashi, Soft-sediment deformation and development ofvein structure. ^ Examples from the Boso and Miura Pen-insulas, central Japan (M.S. thesis), Kobe University,Kobe, Japan, 1987.

[22] K. Kihara, Anisotropy of magnetic susceptibility for anal-yses on minor faults (M.S. thesis), University of Tokyo,Tokyo, Japan, 1995, 33 pp.

[23] F. Hrouda, A technique for the measurement of thermalchanges of magnetic susceptibility of weakly magneticrocks by the CS-2 apparatus and KLY-2 Kappabridge,Geophys. J. Int. 118 (1994) 604^612.

[24] F. Hrouda, V. Jelinek, K. Zapletal, Rened technique forsusceptibility resolution into ferromagnetic and paramag-netic components based on susceptibility temperature-var-iation measurement, Geophys. J. Int. 129 (1997) 715^719.

[25] R. Day, M.D. Fuller, V. Schmidt, Hysteresis properties oftitanomagnetites: grain size and compositional depen-dence, Phys. Earth Planet. Inter. 13 (1977) 260^267.

[26] V. Jelinek, Characterization of the magnetic fabric ofrocks, Tectonophysics 79 (1981) 63^67.

[27] A. Taira, N. Niitsuma, Turbidite sedimentation in theNankai Trough as interpreted from magnetic fabric, grainsize, and detrital model analysis, Initial Rep. Deep SeaDrill. Proj. 87 (1985) 611^632.

[28] K. Kodama, S. Oka, T. Mitsunashi, Geology of the Mis-ank district, Quadrangle Series, Scale 1:50 000., Geol.Surv. Japan, 1980, 38 pp.

[29] H.D. Bakhtari, C. Frizon de Lamotte, C. Aubourg, J.Hassanzadeh, Magnetic fabric of Tertiary sandstonesfrom the Arc of Fars (Eastern Zagros, Iran), Tectono-physics 284 (1998) 299^316.

[30] J.P. Pares, B.A. van der Pluijm, J. Dinares-Turrell, Evo-lution of magnetic fabric during incipient deformation ofmudrocks (Pyrennees, Northern Spain), Tectonophysics307 (1999) 1^14.



Documents

Am s Sediments