small faults and kink bands in the nankai accretionary complex: textural observations from site 808...

The Zsland Arc (1997) 6, 183-196

Research Article Small faults and kink bands in the Nankai accretionary

complex: Textural observations from Site 808 of ODP Leg 131

JONATHAN C. LEWIS,~ TIM BYRNE~ AND DAVID J. PRIOR2

‘Department of Geology and Geophysics, University of Connecticut, 354 Mansfield Road, Box U-45, Storrs, Connecticut, 06269 USA, 2Department of Earth Sciences, The Jane Herdman Laboratories, Liverpool University,

Brownlow Street, PO Box 147, Liverpool L69 3BX, United Kingdom

Abstract We present backscattered scanning electron microscope and petrographic microscope observations of deformed sediments from Ocean Drilling Program (ODP) Site 808 in order to better understand the dewatering and deformation history of the Nankai accretionary complex. This synthesis of deformation textures has three implications. First, the early structures that dominate the Nankai prism, small faults and kink bands, have very different electron microscope versus optical microscopic expressions. This observation is important to investigations of fine-grained sediment in both stable and active tectonic settings, in part, because these materials have often been studied almost exclusively by electron microscope methods. In sediments of this type, investigators often forego petrographic analysis because of the relative opacity of samples at normal (i.e. 30 pm) thin section thicknesses. Sec- ond, the textural observations we have compiled suggest that these deformation structures acted as ‘single-event’ pathways that contributed to diffusive dewatering of the prism. Third, our observations serve as a reference frame for the early tectonic structures that are important to the dewatering history of a ‘sandy’ accretionary prism.

Key words: accretionary prism, dewatering, faults, kink bands.

INTRODUCTION

Convergent margins are some of the most tectonically active regions on Earth. In submarine envi- ronments, these plate boundaries are sites where sediments undergo progressive lithification accom- panied by progressive deformation. During the past few years it has become widely recognized that the fluids expelled during lithification and deformation play an important role in influencing the morphologies and dynamics of submarine accretionary prisms. The detailed relations between dewatering, fluid flow and deformation, however, remain poorly understood. To this end, substantial effort has focused on the structures expected t o be important in transmitting fluids through accretionary prism sediments (e.g. scaly fabrics, veins, faults). For example, Taylor et al. (1991) used a combination of electron microscope techniques to describe structures inferred t o be dewatering channels in the Barbados forearc.

Accepted for publication July 1996

Similarly, Byrne et al. (199313) suggested that the pervasive deformation structures present in the Nankai accretionary prism, primarily small faults and kink bands, acted as pathways for dewatering the sediments above the decollement. Based on an estimated 17% volume loss attributed to com- paction and dewatering, they concluded that these structures probably acted to dewater immediately adjacent sediments, contributing to diffusive dewatering at the scale of the prism. However, on the basis of laboratory analyses they were not able to resolve whether the structures acted as relatively high permeability conduits or as zones of fabric collapse associated with a transient single phase of fluid flow (i.e. an apparent permeability increase). In addition, the relative role of porous- media flow in the sediment domains between these structures has not been established.

Because of the fine-grained nature of much of the sediment in accretionary prisms, many studies have relied on techniques that allow ‘submicro-

184 J. C. Lewis et al.

scopic’ observations. These techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM) and backscattered electron (BSE, or alternatively, ‘backscattered’) microscopy. The value of these techniques is well- established in studies of fine-grained sediments from a myriad of tectonic settings (Reynolds & Gorsline 1991). In this paper we use one of these techniques, specifically backscattered imagery, to build on a previous study of thin-section-scale observations of deformation structures from the toe region of the Nankai accretionary complex in Southwest Japan. We focus on core-scale deformation structures that are recognized to be important in dewatering the prism, namely small faults and kink bands.

This work has implications for geologists inves- tigating convergent margins as well as more stable tectonic settings. First, the microscopic (i.e. petrographic) and electron microscopic observations suggest substantially different textures for the same structures. It is therefore important that deformation studies in sediments with similar characteristics incorporate both scales of observation. Second, the compiled observations suggest that the small faults and kink bands acted as ‘single-event’ pathways that contributed to diffusive dewatering of the prism. Finally, the results taken as a whole provide an overview of dewatering structures in a ‘sandy’ accretionary prism.

PREVIOUS STUDIES

To date, the most extensively studied deformation feature in accretionary prism sediments is probably scaly fabric (Moore et al. 1986; Agar et al. 1989; Prior & Behrmann 1990a), with core-scale faults and kink bands having received less attention. Prior & Behrmann (1990a) used backscattered imagery to characterize scaly fabrics in prism sediments and decollement samples from the Barba- dos accretionary complex and found that the core- scale scaly fabrics could not be attributed to specific microstructure. On the basis of the lack of evidence for cataclastic, diffusive mass transfer and/or crystal plastic processes they concluded that the dominant deformation mechanism in both locations was particulate flow (Borradaile 1981). In contrast, BSE imagery on fine-grained sediments from the Argille Scagliose (Italy) and the Okitsu melange (Japan) suggests that cataclasis, diffusive mass transfer and/or crystal plastic processes are responsible for the scaly fabrics and mineral preferred orientations observed in these rocks (Agar et al. 1989).

In this paper we focus on the textural aspects of small faults and kink bands in the Nankai accretionary complex using cores collected from Site 808 during Ocean Drilling Program (ODP) Leg 131 (Fig. 1). These structures are the dominant deformation features of the Nankai prism. The small

Fig. 1 Byrne e ta / . 1993b).

Location of ODP site 808 (from

Faults and kinks in the Nankai prism 185

banddm 10 20

- - - - -

- F, E c

-

, , l , ~

Fig. 2

(Degrees) 20 40 60 80

. .

. . ... .

.... . . . .... i;.

p 3: Ac.. -" ' '

-mmmn RP'

'3 '

..$' . .

mmmD mmnm %" :, , 2,. . . .. ....

m:.. . . . . Ezza .., .$.? . . ,. .. .

. .. :. 4.. .. I I I , I I I I

Epoch Facies association , LOwm;;oPE ,

UPPER AXIAL TRENCH WEDGE

LOWER AXIAL rRENCH WEDGE

'ROYTAL THRl85r .-?Lf ",hi,"

H " k R"XC

IUTER MARGlNAl RENCHWEDGE

TRENCH-loBASIN TRANSITION

UPPER SHIKOKU

BASIN

Porosity (%)

, . . . . . . . . . . .. . . . .,>; . - . .. , .. . . . . . . ... . _ . . . . _

1

-llllllnrmllllmnlllln , \

Facies

: J boundaries mmmmmmmmm

:T '

DCcollement

Stratiqraphic, structural and phvsical property data from ODP Site 808 (from Bvrne eta/ 1993a) Note that the kink bands are SDatiallv associated with the frontal , I

thrust whereas the small faults are more evenly distributed between the frontal thrust and the decollemen't, mbsf, meters below sea level.

faults are the most abundant structures recognized at Site 808 and are evenly distributed throughout the interval between the frontal thrust and the decollement (Fig. 2). In contrast, the kink bands are less abundant and occur almost exclusively within sediments of the outer marginal trench wedge, and just beneath the frontal thrust (Fig. 2). Cross-cutting relations also consistently show that the kink bands predate the small faults (Byrne et al. 1993a). The observation that the majority of the kink bands strike northeast and dip either northwest or southeast with dihedral angles of 3045" centered on bedding (Maltman et al. 1993a), led Byrne et al. (1993b) to conclude that the kink bands formed as conjugate sets in response to northwest-directed shortening.

The core- and thin-section scale characteristics of the small faults and kink bands were originally described by Byrne et al. (1993b) and Maltman et al. (1993a,b). Here we briefly review these characteristics, and present new data and observations based on backscattered images from two represen- tative samples collected from Site 808; sample 808C 19R(A)-3 133-141 from 476.54m below the sea floor (mbsf) and sample 808C 20R-5 012-016 from 488.00 mbsf (Fig. 3). The hydrologic and physical properties of samples from similar depths

were investigated in the laboratory by Byrne et al. (1993b). They performed static and dynamic permeability tests on whole round core samples in order t o address the role that the deformation structures played in dewatering. In addition, they used computed tomography (CT) to compare the relative density of these structures to that of the less deformed matrix. In total, their results suggest that the sediment within the deformation zone is overconsolidated; that is, all of the deformation structures record densification. The deformation structures appear to act as barriers to fluid flow, and there is a general decrease in permeability as sediment deformation occurs. An exception t o this decrease in permeability is noted during sediment failure at high strain values when pulses of transient permeability increases occur.

DESCRIPTION OF STRUCTURES

To understand the deformation fabric developed within the small faults and kink bafids it is important to have an adequate frame of reference. For this purpose we use the primary sedimentary fabric (PSF or 'sedimentary fabric' for brevity) which is essentially a bedding fabric. This fabric is


Fiaures 7f & 79

Figures 7a -

Figures 7h

d Fig. 3 Photographs and line drawings of core samples showing locations of chips used for backscattered electron microscopy (BSE) and thin section ( ts ) analyses of small faults (a,b) and kink bands (c,d). (a,b) Sample 80% 20R-5 012-016. (b,c) Sample 808C 19(A)-3 133-141. (a$) modified from Byrne eta/. (1993b) Each increment on the scale visible at the bottom of (a) and (c) is 1 mm.

defined by a moderate- t o well-developed bedding-parallel preferred orientation of clays and phyllosilicates, and is most clearly observed at the microscopic scale using polarized light and an accessory plate (1 wavelength gypsum plate). In other cases the bedding is preserved as irregular laminae andfor weak graded bedding (Fig. 3b). At many depths at Site 808, the sediments of the Nankai prism show extensive bioturbation. This

reworking gives the samples a highly mottled appearance in both hand sample (Fig. 3) and thin section. In thin section, the mottling is characterized by irregular, brown domains distributed in a dusty brown matrix. In spite of reworking however, the phyllosilicate grains within these zones generally display a bedding-parallel preferred orientation.

All microscopic observations were made on thin-


of core typically range from several millimeters to tens of millimeters (limited by the size of the core). The small faults generally dip moderately to gently and display a wide variety of strikes. The result of the varying strikes is a low-dipping fabric that forms a 3-D web of intersecting faults, some of which occur as conjugates. Changes in the dihedral angles of these conjugate faults with depth have been inferred by Lallemant et al. (1993) t o reflect a decrease in the coefficient of friction angle with depth.

At the thin-section scale, the small faults are characterized by reduced grain size and millimeter-scale thick zones with well-developed phyllosilicate fabrics (both relative t o the PSF in the same sample) that display significant substructure. In general, the phyllosilicate fabrics are subparallel, or occur at low angles t o the fault zone boundaries. This fabric is defined by phyllosilicate grains with crystallographic preferred orientations and is very clear when viewed using a gypsum plate under crossed polars. The phyllosilicate fabrics display geometric similarities with three common shear zone substructures, S planes (i.e. of S-C mylonite terminology), and Riedel R- and Y-type shears (Fig. 4a). The expressions of these preferred orientations are often subtle and they vary with the type of substructure. For example, the 'interiors' of many of the small fault zones are characterized by a very well-developed crystallographic preferred orientation oriented 15-20' from the fault zone boundaries and inclined in a manner analogous to S planes. This fabric is so well-developed in

sections oriented perpendicular to the plane of the core-scale structure of interest and parallel to the transport direction of the structure.

SMALL FAULTS

The small faults display significant substructure, the expression of which is dependent upon the scale of observation. Viewed with the naked eye they are typically narrow (< 1 mm), sharply bound- ed zones of displacement that appear much darker than either the kink bands or the surrounding sediments (Byrne et al. 1993b). On the basis of the observation that dried samples do not show this light-dark contrast, the darkness of the fault zones and the kink bands (described here) is probably related to the preferential presence of pore fluids. The apparent retention of fluids within these structures is consistent with the interpretation that they are zones of reduced porosity relative to the host rock (Byrne et al. 1993b).

At the scale of the core, the faults are commonly polished and lineated, and they record displacements of variable magnitude and direction. Fault displacements at Site 808 generally range from a few millimeters t o tens of meters, and record normal, strike-slip, or reverse motion (Byrne et al. 1993a). Moderate t o low-angle thrust displacements, however are most common (Byrne et al. 1993a). The maximum fault displacement recorded at Site 808 is 259m, specifically along the frontal thrust (Byrne et al. 1993a). Apparent displacements of the small faults within individual sections

'Y' shea plane

R shear plane

Fig. 4 Details of small fault zones. (a) Line drawing of a typical small fault in thin section of sample 808C 20R-5 012- 016. The penetrative crystallographic preferred orientation within the fault zone is indicated with very fine lines. The width of view is - 2 mm. (b) Photograph mosaic and line drawing of a typical small fault from the interval of the frontal thrust (from Byrne eta/. 1993b).


Fig. 5 Backscattered electron microscopy (BSE) and transmitted light microscopy images of a typical small fault zone from sample 80% 20R-5 012-016. (a,e) Low magnification BSE transects. (b-d, f-h) High-magnification BSE images. The width of (h ) is - 160 pm. (i) Transmitted light microscope image with crossed polars. Note that (a-h) are rotated - 20" counterclockwise and (i) is rotated 90" clockwise from orientations shown in Fig. 3.

Faults and kinks in the Nunkai prism 189

Fig. 5 (Continued)


some cases that when viewed with an accessory plate, the entire fault zone simultaneously displays the same interference colors during rotation of the microscope stage (Fig. 4a). As such, this fabric can be described as penetrative at the scale of the small faults. In contrast, the phyllosilicates that display Riedel R shear geometries tend to occur in discrete clusters that are irregularly spaced (Fig. 4a). Within the fault ‘interiors’ this fabric is closely spaced (i.e. millimeter scale), whereas within the matrix immediately adjacent to the fault zones it is more irregularly distributed and more widely spaced (millimeter to tens of millimeters scale). Finally, the grains that display the Riedel Y shear geometries tend to be restricted to the margins of the fault zones. These grains occur either individually or in clusters oriented with their long axes parallel to the fault zone boundaries (Fig. 4a).

Some fault zones contain distributed lens- shaped remnants of less-deformed (i.e. moderately deformed) material that appear similar t o the sediment in the wall rock (Fig.4b). These remnants are often separated by faults that are synthetic to the overall shear sense on the fault zone (i.e. Riedel R-like shear zones) and result in local extension of the fault zones. This extensional strain typically gives the faults a pinch-and-swell geometry in which the remnants of less-deformed material have a ‘shingled’ appearance (Fig. 4b).

In backscattered images, the small faults appear as bands that are slightly brighter than the surrounding sediments (Figs 5a,e). Similar bands have been observed in sediments from other forearcs (Pickering et al. 1990; Rochford et al. 1995). Con- trast in BSE is related to the mean atomic number of the material being imaged (Lloyd 1985). In some of the Nankai samples, the bright bands can be attributed to minerals that are heavier than those in the surrounding sediment, particularly sulfides. However, in the majority of bands in this study, and in other studies (Rochford et al. 1995), the brighter backscatter bands cannot be attributed to chemical or mineralogical differences between the bands and the host sediment. The brighter BSE signal must therefore be related t o the denser packing of sediment and concomitant reduction of porosity within the bands.

The BSE images reveal two dominant characteristics; dark, anastomosing seams throughout the fault zones (Figs 5d,g) and subtle preferred orientations of elongate grains that are locally developed along the fault zone margins (Figs 5b,c). The seams are individual, epoxy-filled cracks that

tend to be discontinuous but that collectively form a web-like, anastomosing fabric. These cracks are attributed to desiccation andlor stress relaxation after core retrieval and thus are not considered to be related to in situ deformation. The grains along the fault zone margins display a weakly developed, spaced shingling arrangement with grains inclined at low angles to or subparallel to the fault zone margins (Figs 5b,c and the bottom of 5e). We inter- pret this weak preferred orientation to be an expression of the grains in the Riedel Y shear posi- tion that are apparent with microscopic investigation. Phyllosilicate grains with similar orientations occur within the ‘interiors’ of the fault zones, although these are not common.

KINK BANDS

As with the small faults, the characteristics of the kink bands depend on the scale of observation. Macroscopically, the kink bands are slightly darker than the surrounding material (probably due t o the retention of water within small pores), are 1-2 mm in cross-sectional width, vary from roughly planar to anastomosing and curviplanar, and typically displace markers (e.g. bedding) several millimeters or less (Byrne et al. 1993a). The kink bands are generally oriented at moderate angles to bedding and all documented displacements record contractional strain relative to bedding. At both the microscopic and electron microscopic scales, the kinks display a spatial association with finer-grained and more fissile (phyllosilicate-rich) sediments (Figs 7a,b). In fact, in some cases the kinks terminate at contacts with more coarse-grained (phyllosilicate-poor) material.

At the microscopic scale the kink bands vary from relatively simple, parallel-sided bands with consistently oriented internal fabrics to irregular zones that display substantial substructure (Figs 6,7). The margins of the kink bands typically appear diffuse in plane light or with crossed polars. With the insertion of a gypsum plate, however, the kink bands are easy t o discern from the host sediment (Fig. 7j) because the PSF is rotated - 90” relative to the PSF in the matrix (Fig. 6). These rotated phyllosilicate grains typically display a strong crystallographic preferred orientation (relative to the local PSF) that is evident with the insertion of a gypsum plate. In addition, the margins of the kink bands are sometimes marked by relatively widely distributed phyllosilicate grains that are oriented approximately parallel to the kink band boundaries. The interior areas of the


within a zone of deformation that is wider than the individual kink bands typical of the Nankai samples. The degree of rotation of the thin bedding laminae within this relatively wide deformation zone increases with increasing distance from its margins (i.e. toward the ‘interior’ of the zone). In spite of this variation in the degree of rotation, when viewed with an accessory plate, the zone displays the same interference colors nearly simultaneously with stage rotation and thus resembles a poorly developed small fualt. The thin, fault-like substructures display a strong crystallographic preferred orientation and a rotation of mineral grains that is consistently greater (relative to the sedimentary fabric) than that displayed by the broad zone of deformation. The strong crystallographic preferred orientation and rotation of the sedimentary fabric suggest that these substructures are akin to the small faults commonly observed in the Nankai sediments. The major difference between these substructures and the more typical small faults is that the latter occur within undeformed (or more generally minimally deformed) ‘host’ matrix, whereas the former occur within a weakly developed zone of kinking. Fur- thermore, these fault-like substructures are oriented nearly parallel t o the margins of the ‘host’ deformation zone and the mineral grains within them are oriented subparallel t o their margins. In some cases, these substructures appear similar t o the small faults. As a consequence of the complex substructure that these deformation zones display, they are regarded as gradational between the small faults and the kink bands in the Nankai sediments. In BSE images these structures appear as diffuse bands that are slighly brighter than the host sediment. However, the thin kink bands that constitute the dominant substructures are nearly indistinguishable from the material outside of the kink-like bands (Figs 7h,i). Where these bands can be distinguished, they sometimes display a deflection of the sedimentary fabric that is morphologi- cally similar to that seen under the petrographic microscope (Figs 7d,e). Importantly, this fabric is very poorly expressed in comparison to the concentrated phyllosilicate fabric that is evident with microscopic observation.

Fig. 6 Line drawing of typical kink bands (from Byrne et al. 199313)

kink bands appear to lack similarly oriented mineral grains.

The substructure of many of these kink bands is most clearly expressed as changes in their down- dip thicknesses. The thinned areas appear to be sites where the kink bands have been extended along minor, intra-kink faults. These faults are oriented - 15” from the kink band boundaries such that the faults are inclined with the same sense as the ‘rotation’ displayed by the sedimentary fabric within the kink. Additionally, the phyllosilicate fabrics within these intra-kink faults are more concentrated and well developed than those within and external to the kink bands. As a result of extension along these structures, the kink bands tend t o have weakly developed pinch-and-swell morphologies. The geometries and kinematics of these faults are consistent with Riedel R,-like shear zones and they are considered to have formed with the kink bands (Byrne et al. 1993b).

The sample investigated with the backscattered electron microscope contains kink-like structures that display more substructure than the kink bands most commonly encountered at Site 808. These structures, however, are more clearly expressed in thin section than in backscattered images. In this sample, the kink-like bands occur

INTERPRETATIONS

Microscopic analyses show that the small fault zones are typified by very well-developed mineral fabrics. The intensity of these phyllosilicate fabrics

192 J . C. Lewis et al.

Fig. 7 Backscattered electron microscopy (BSE) and transmitted light microscopy images of typical kink bands from sample 808C 19(A)-3 133-141. (a,f,h) Low-magnification BSE images. (b-e, g,i) High-magnification BSE images. ( j ) Transmitted light microscope image with crossed polars and a gypsum plate inserted. Note that (j) is rotated -20" clockwise from orientation shown in Fig. 3.

suggests that these zones record consolidation (i.e. In contrast, the backscattered images of the porosity collapse) relative to the surrounding small faults suggest a very weak preferred orien- matrix (Byrne et al. 1993b). This conclusion is sup- tation of elongate mineral grains that is generally ported by CT results for the same samples which restricted to the fault zone margins. These grains show that increases in density correlate with tend to be oriented subparallel to the fault zone increases in CT attenuation (Plate 2 in Byrne et al. boundaries and lie in an orientation analogous to (1993b). Riedel Y-type shear zones (Figs 5b,c), However,


Fig. 7 (Continued)

this fault-zone parallel texture is not well- expressed in the backscattered images. The fault zone interiors appear to lack the strong preferred orientations revealed in thin section (Figs 5d,g).

Taken alone, the backscattered observations suggest that the small faults are sites of minor grain-scale fabrics. Similar observations led Prior and Behrmann (1990a) t o suggest that particulate flow operated on nearly equant phyllosilicate aggregates within the decollement of the Barbados forearc and as a consequence, strong preferred ori-

entations were not developed. Importantly, the microscopic observations of the small faults in the Nankai sediments suggest significant grain-scale fabrics. The fabrics suggest collapse of the sediment framework, consistent with the fault zones representing sites of fabric consolidation during a pulse of fluid flow.

Kink band geometries (internal and external angles) and CT data both consistently indicate a decrease in volume during formation (Byrne et aL. 1993b). On the basis of these observations Byrne


et ul. (1993b), suggested that these structures probably acted as dewatering pathways. In this view, the concentrated phyllosilicate fabric evident at the microscopic scale developed in response t o the collapse of the sediment framework during dewatering.

Rotation of the PSF is readily observable at the microscopic scale and is preserved as a well-developed mineral preferred orientation with grain long axes oriented at - 90" from the PSF. These observations suggest the operation of particulate flow during development of the kink bands.

The most apparent expression of the kink bands in backscattered images is as diffuse, bright areas relative to the host sediment. Rotated or deflected mineral fabrics are not well expressed in these images (Figs 7e,i). Clear deflection of elongate minerals is only apparent in Figs 7d and 7e, which show a preferred orientation subparallel to the boundaries of the kink-like bands. As in the observations of the small faults, the expression of the phyllosilicate fabrics in the kink-like bands is much weaker in the backscattered mode than at the microscopic scale. As a consequence, the evidence for particulate flow is very weak in the BSE images.

The spatial association of kink bands with finer- grained materials evident at both microscopic and electron microscopic scales suggests that particulate flow was favored in these materials. The grain rotation and fabric consolidation associated with kinking may have been favored in clay-rich sediments with a mineralogically controlled planar anisotropy such as a bedding fissility. In contrast, more silt-rich zones that lacked such anisotropies appear not to have been favorable sites for kink band formation.

The disparate expression of the same structure, depending on which technique is used, emphasize the need t o use a combination of electron microscopic and microscopic (petrographic) methods in order t o adequately characterize deformation. For example, a weak alignment of grains is the only evidence from the backscattered images that the sedimentary fabric within the small fault zones might reflect an episode of collapse (i.e. consolidation). Yet, petrographic examination reveals clear evidence of consolidation. We conclude that the differences between the microscopic and electron microscopic expressions of the structures in the Nankai samples may be, in part, related t o a fun- damental difference in the techniques. The petrographic techique allows inspection of structures over a thickness of - 30 pm normal to the plane of

the thin section (i.e. essentially 3-D examination). In contrast, BSE imagery allows only 2-D examination. In sediments such as those from the Nankai prism, the electron beam penetration under most electron microscope operating condi- tions is unlikely t o exceed 1 pm (Prior & Behr- mann 1999b). Three-dimensional complexity of the mineral grains with preferred orientations and/or the continuity of these fabrics in 3-D appears to contribute to the relatively strong expression of the deformation structures at the microscopic scale. That is, thin sections allow the observer to take advantage of the additive or multiplicative effect of the oriented grain fabrics. This interpretation is particularly relevant to the textures within the small fault zones that we consider to be analogous to S planes. This texture is commonly so well developed that it appears penetrative in thin section, yet virtually absent in the backscattered images. In this view, the limitation imposed by 2-D examination via backscattered methods results in subdued expressions of the grain fabrics.

DISCUSSION

The most striking outcome of our work is how different similar structures appear when viewed with transmitted light as compared to BSE microscopy. For example, the strong phyllosilicate fabric that is apparent within the small faults at the microscopic scale is largely absent in the backscattered images (compare Figs 5d and 5i). The backscattered images reveal only a weak grain preferred orientation that is restricted to the fault zone margins. Similarly, the relatively dramatic rotation of the sedimentary fabric and the well-developed crystallographic preferred orientation observed at the microscopic scale within the kink bands are poorly expressed in the backscattered images of the kink bands. At the microscopic scale, the kink bands are characterized by a rotation of the sedimentary fabric of - 90", and by a crystallographic preferred orientation that is well expressed when viewed with a gypsum plate under crossed polars. In contrast, in backscattered mode the kink-like bands appear slightly brighter than the host sediment with very poorly developed grain preferred orientations. Although we did not examine 'end- member' kink bands with the backscattered technique, the weak expression of the more complex kink bands suggests that the 'end-member' kink bands would display similarly subtle if not more subtle characteristics.

F a u l t s and kinks in the N a n k a i prism 195

The observations presented in the previous sections suggest that the Nankai accretionary prism has deformed differently than more clay-rich prisms such as Barbados or Cascadia. In contrast to these more clay-rich prisms, deformation structures a t the Nankai prism appear to be pervasive. This observation in combination with geometric and CT data suggest that the Nankai structures acted collectively as dewatering pathways so that the prism experienced diffuse dewatering (Malt- man e t a l . 1992; Taira e t al . 1992; Byrne e t a l . 1993b). This interpretation is consistent with the poor development of structures attributable to focused fluid flow (e.g. mineralized veins and clas- tic dikes) and the preponderance of structures that record volume decrease as opposed to dilation in the Nankai prism. We therefore conclude that the small faults and kink bands in the Nankai prism probably acted as ‘single-event’ dewatering structures as opposed to conduits during multiple episodes of flow. The Nankai prism is generally regarded as relatively ‘sandy’ and this may account for some of the differences in structural character between i t and more clay-rich prisms. This observation suggests that lithology (i.e. sediment type) exerts an important control on the style of deformation and dewatering in accretionary prisms.

Finally, our observations of backscattered images from the Nankai prism have implications for investigators studying dewatering of sediments from tectonically more ‘stable’ settings that are typically considered to be minimally deformed. For example, dewatering of abyssal plain sediments may be accomplished, in part, by flow through deformation structures such as faults (Buckley & Grant 1985). Such settings are often characterized by fine-grained sediments and thus have long been subject to investigation using SEM, TEM and BSE techniques (Bryant e t al. 1991; Dadey et a l . 1991; Reynolds & Gorsline 1991). The very subtle backscattered expression of the structures from the Nankai prism emphasizes the importance of examining fine-grained sediments using both microscopic and electron microscopic methods in order to ensure adequate characteriza- tion of deformation structures.

ACKNOWLEDGEMENTS

This work was supported by National Science Foundation grant EAR-9418344 to Tim Byrne. Tim would like to acknowledge the shipboard crew

of ODP Leg 131 for stimulating discussions regarding the structures described in this paper. Backscattered electron microscopy work was com- pleted at the electron optic suite of the depart- ments of Earth Sciences and Material Sciences a t the University of Leeds. Geoff Lloyd, Tony Nichells and Eric Condliffe are thanked for their help in Leeds.

REFERENCES

AGAR S. M., PRIOR D. J. & BEHRMANN J. H. 1989. Back- scattered electron imagery of the tectonic fabrics of some fine-grained sediments: Implications for fabric nomenclature and deformation processes. Geology 17, 9104.

BORRADAILE G. J. 1981. Particulate flow of rock and the formation of cleavage. Teetonophysics 71, 305-21.

BRYANT W. R., BENNETT R. H., BURKETT P. J. & RACK F. R. 1991. Microfabric and physical property characteristics of a conslidated clay section: ODP Site 697, Weddell Sea. In Bennett R. H., Bryant W. R. & Hulbert M. H. eds. Microstructure of Fine-Gained Sediments: Frontiers in Sedimentology, pp. 73-92. Springer-Verlag, New York.

BUCKLEY D. E. & GRANT A. C. 1985. Fault-like features in abyssal plain sediments; possible dewatering structures. Journal of Geophysical Research 90,

BYRNE T., BRUCKMANN W., OWENS W., LALLEMANT S. & MALTMAN A. 1993a. Structural synthesis: Correla- tion of structural fabrics, velocity anisotropy, and magnetic susceptibility data. In Hill I. A., Taira A., Firth J. V. & Vrolijk F’. J. eds. Proceedings of the Ocean Drilling Program, Scient8c Results, 131, pp. 365-78. College Station, TX, USA.

BPKNE T., MALTMAN A., STEPHENSON E., SOH W. & KNIPE R. 1993b. Deformation structures and fluid flow in the toe region of the Nankai accretionary prism. I n Hill I. A., Taira A., Firth J. V. & Vrolijk P. J. eds. Proceedings of the Ocean Drilling Program, Scientific Results, 131, pp. 83-101. College Station, TX, USA.

DADEP K., LEINEN M. & SILVA A. J. 1991. Anomalous stress history of sediments of the Northwest Pacific: The role of microstructure. In Bennett R. H., Bryant W. R. & Hulbert M. H. eds. Microstructwre of Fine- G a i n e d Sediments: Frontiers in Sedimentology, pp. 229-36. Springer-Verlag, New York.

LALLEMANT S. J., BYRNE T., MALTMAN A., KARIG D. & HENRY P. 1993. Stress tensors at the toe of the Nankai accretionary complex: An application of inverse methods to slikenlined faults. In Hill I. A., Taira A., Firth J. V. & Vrolijk P. J. eds. Proceedings of the Ocean Drilling Program, Scientijic Results, 131, pp. 103-22. College Station, TX, USA.

91 73-80.


LLOYD G. E. 1985. Review of instrumentation, tee- niques and applications of SEM in mineralogy. I n White. J.C. ed. Applications of Electron Microscopy in Earth Sczences, vol. 11, pp. 155-88. Short Course- Mineralogical Association of Canada.

PARTY 1992. Structural geological evidence from ODP Leg 131 regarding fluid flow in the Nankai prism, Japan. Earth and Planetary Science Letters

MALTMAN A. J., BYRNE T., KARIG D. E. & LALLEMANT S. 1993a. Deformation at the toe of an active accretionary prism: Synopsis of results from ODP Leg 131, Nankai, SW Japan. Joui-nal of Structural Geol-

MALTMAN A. J., BYRNE T., KARIG D. E., LALLEMANT S., KNIPE R. & PRIOR D. 1993b. Deformation structures at Site 808, Nankai accretionary prism, Japan. In Hill I. A., Taira A., Firth J. V. & Vrolijk P. J. eds. Proceedings of the Ocean Drilling Program, Scien- tific Residts, 131, pp. 123-33. College Station, TX, USA.

MOORE J. C., ROESKE S., LUNDBERG N. et al. 1986. Scaly fabrics from Deep Sea Drilling Project Cores from forearm. I n Moore J.C. ed. Structural Fabrics from Deep Sea Drilling Project Cores from Forearcs. Geo- logical Society of America Memoir 166, 55-73.

PICKERING K. T., AGAR S. M. & PRIOR D. J. 1990. Vein structure and the role of pore fluids in early wet sediment deformation, late Miocene volcanoclastic rocks, Nuira Group, S E Japan. I n Knipe R. J. & Rut- te r E. H. eds. Deformation Mechanisms, Rheology and Tectonics. Geological Society Special Publica- tion, London 54,417-30.

PRIOR D. J. & BEHRMANN J . H. 1990a. Thrust-related

MALTMAN A., BYRNE T., LALLEMANT s. & SHIPBOARD

109, 463-8.

ogy 15,949-64.

mudstone fabrics from the Barbados Forearc: A backscattered scanning electron microscope study. Journal of Geophysical Research 95, 9055-67.

PRIOR D. J. & BEHRMANN J. H. 1990b. Backscatter SEM imagery of fine-grained sediments from Site 671, Leg 110 - preliminary results. In Moore J . C. & Mascle A. eds. Proceedings of the Ocean Drilling Program, Scientific Resul ts , 110, pp. 245-55. College Station, TX, USA.

REYNOLDS S. & GORSLINE D. S. 1991. Silt microfabric of detrital, deep sea mud(stone)s (California Continen- tal Borderland) as shown by backscattered electron microscopy. I n Bennett R. H. Bryant W. R. & Hul- bert M. H. eds. Microstructure of Fine-Gained Sed- iments: Frontiers in Sedimentology, pp. 203-110. Springer-Verlag, New York.

ROCHFORD E. I,., PRIOR D. J., AGAR S. M. & MALTMAN A. 1995. Microstructural analysis of deformation bands from site 860, Chile margin. In Lewis S. D., Behrmann J. H., Musgrave R. J. & Cande S. C. eds. Proceedings of the Ocean Drilling Program, Scien- tific Resul ts , 141, pp. 13-26. College Station, TX, USA.

TAIRA A., HILL I., FIRTH J. et al. 1992. Sediment deformation and hydrogeology of the Nankai accretionary prism: Synthesis of shipboard results of ODP Leg 131. Earth and Planetary Science Letters, 109,

TAYLOR E., BURKETT P. J., WACKLER J. D. & LEONARD J. N. 1991. Physical properties and microstructural response of sediments to accretion-subduction: Bar- bados forearc. I n Bennett R. H., Bryant W. R. & Hulbert M. H. eds. Microst.r.ucture of F ine-Gained Sediments: Frontiers in Sedimentology, pp. 213-28. Springer-Verlag, New York.

431-50.

small faults and kink bands in the nankai accretionary complex: textural observations from site 808...

Documents