Sedimentology, structural geology, and tectonics of the Shikoku subduction zone, southwestern Japan

J. CASEY MOORE Earth Sciences, University of California, Santa Cruz, Santa Cruz, California 95064 DANIEL E. KARIG Department of Geological Sciences, Cornell University, Ithaca, New York 14850

ABSTRACT deep-sea sediment. Studies of uplifted de- (Kanamori and Tsumura, 1971). Kanamori posits provide much detail but fail to reveal (1972) used data from high-magnitude The Shikoku subduction zone is de- deformation and diagenetic-metamorphic shallow earthquakes to postulate recent veloped along the Nankai Trough where processes while they are occurring. The subduction rates of about 3 cm/yr and initi- the Philippine plate is underthrust beneath Deep Sea Drilling Project has recovered the ation of subduction 2 m.y. B.P. The pattern the Asian plate. The landward wall of the best samples from modern subduction of deformation associated with the 1946 Nankai Trough consists of horizontal zones, including cores from the Eastern Nakaido earthquake, as well as the uplift of parallel ridges and basins that trend north- Aleutian Trench (von Huene and Kulm, terraces along the Pacific coast off Shikoku, eastward. A Deep Sea Drilling Project site 1973), the Hellenic Trench (Hsu and Ryan, led Fitch and Scholz (1971) to estimate the (Leg 31, site 298) on the landward flank of 1972), and the Nankai Trough off south- subduction rate as 8 ± 4 cm/yr over a the deepest ridge penetrated 525 m of beds western Japan (Karig, Ingle, and others, period of 1 m.y. or less. in normal stratigraphic position and 86 m 1975). The inner wall of the Nankai of overturned beds (all of Quaternary age), Trough, penetrated to a depth of more than indicating an overturned anticline. The 600 m, reveals a major overturned anti- tight, overturned anticline, which trends cline. These cores represent the deepest and parallel to the Nankai Trough, has an inter- most complete sampling of a modern sub- limb angle of 9°, an axial surface inclined 9° duction zone and are a sequence transi- to 14° landward, and a convergently fan- tional between unconsolidated trench sed- ning axial plane fracture cleavage. A iments and highly deformed lithified mate- coarsening-upward turbidite sequence rial. We discuss the sedimentology, struc- defines a trench facies and demonstrates di- tural geology, and physical properties of rect accretion of deposits from this envi- deposits cored from the inner wall of the ronment. Nankai Trough. We interpret the site of dep- The convergence rate in the Shikoku osition, depth of burial, and structure; subduction zone is estimated to be from 1 further, we provide an overview of the tec- to 2 cm/yr, with a strain rate of about tonic evolution of this subduction zone. 10~13/sec. Tectonic consolidation has re- We informally use the term "Shikoku duced the volume of the subducted and ac- subduction zone" for the dipping seismic creted rocks at least one-third. Olistro- zone and deformed, accreted deposits, and stromes form as a direct consequence of we retain the designation "Nankai Trough" fold evolution in the submarine environ- for the adjacent oceanic deep. ment and can be immediately underthrust, thereby developing a structural fabric. GEOLOGICAL AND GEOPHYSICAL SETTING INTRODUCTION Southwestern Japan has undergone Figure 1. Schematic diagram showing plates A major problem during the formative periodic subduction since the Paleozoic of northwestern Pacific. period of plate tectonic theory was reconcil- (Minato and others, 1965). This margin ing the apparent lack of deformation of sed- forms part of the Asian-Philippine plate A small accretionary prism and the high iments in zones of probable underthrust- boundary (Fig. 1), but prior to the opening incidence of large, shallow earthquakes in- ing in some modern oceanic trenches of the Shikoku Basin in early Miocene time, dicate a short subduction history. A transi- (Scholl and others, 1968, 1970; von Huene it appears that the Pacific plate was in con- tion from turbidite deposits to hemipelagic and Shor, 1969). However, subsequent tact with the Asian plate in this region sediments in the Shikoku Basin at Deep Sea seismic-reflection data from other subduc- (Karig, 1975). During, and probably be- Drilling Project hole 297 marks the de- tion zones indicate that deep-sea deposits cause of, this arc migration, there was a lull velopment in middle Pliocene time (3 m.y. are deformed and accreted in this environ- in volcanic and tectonic activity along this B.P.) of the subduction zone into a trough ment (Chase and Bunce, 1969; Hilde and section of margin. and sediment trap. Moreover, the Nankai others, 1969; Silver, 1969; Holmes and Late Tertiary displacement along this Trough is traceable to a Miocene and others, 1972; Carson and others, 1974; subduction zone has been estimated from younger thrust zone along the southern Beck, 1972; Seely and others, 1974; Kulm both geological and geophysical data. The Fossa Magna (Matsuda, 1962; Kimura, and Fowler, 1974). seismic zone beneath the Nankai Trough 1966; Tsuchi and others, undated), suggest- Marine geophysical techniques delineate reaches a depth of about 70 km and defines ing that slow subduction began approxi- the gross structural framework of accreted a lithospheric slab with a very gentle dip mately 10 m.y. B.P. and that an increasing

Geological Society of America Bulletin, v. 87, p. 1259-1268, 12 figs., September 1976, Doc. no. 60907.

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130 135 140 SEDIMENTOLOGY 40° 40° The sedimentology of the sequence cored at site 298 allows interpretation of the site of deposition and the position of the rocks before tectonic transport. Seventeen cores sampling 154 m (25 percent) of the 611 m drilled are described and interpreted. The recovery in all cores averaged 43 percent; recovery was below average in the upper portion of the hole. 35° 35° Lithology

The cored sequence was divided into two units (Fig. 4). Unit I comprises mud (clayey silt) and silty sand of late Pleistocene—Holocene age. Both sediment types include irregular clasts of mud, mud- stone, and sandstone as much as 10 cm across. The deepest occurrence of the 30° 30° lithified clasts (mudstone and sandstone) defines the base of Unit I, at 194.20 m. Sand beds as much as 1 m thick are both massive and graded, and they generally show basal layers of the Bouma A interval. Unit II extends from 194.2 m to at least Figure 2. Location map of southwestern Japan, showing position of sites 297, 298, and 299. Line the base of hole 298 at 611 m (minimum through site 298 indicates location of Figure 11. thickness 416.8 m). It is composed of mud-mudstone, silt-siltstone, and sand of subduction rate allowed a trough to form at As neither the basement nor the underlying early and late Pleistocene age. Most of the 3 m.y. B.P. Shikoku Basin section seem to be deformed mud and mudstone includes 50 to 60 per- The Nankai Trough is a shallow (Fig. 2) or uplifted on the profile of Hilde and cent silt-sized particles and 40 to 50 percent trench with a small but well-developed ac- others, there must be thickening and de- clay-sized particles. Sand and silt-siltstone cretionary prism (Ludwig and others, 1973; formation, primarily in the basal part of beds represent only 4 percent of Unit II, Karig and Sharman, 1975). The lower the turbidite unit. The contact between the whereas sand beds comprise 48 percent of trench slope is acoustically opaque, and on step area and the lower slope, on the profile Unit I (Fig. 5). In Unit II, these sand and reflection profiles (Hilde and others, 1969; of Hilde and others, appears to be a silt-siltstone beds compose the lower layer Ludwig and others, 1973), it appears as a shallow-dipping thrust, but acoustic- of graded beds, and locally show cross- series of hyperbolic reflectors. A detailed diffraction effects from point reflectors off lamination and basal scour marks. Bouma survey over the lowermost slope section by the ship track might also be responsible. intervals of the C-D-E and D-E type are Karig before the drilling of DSDP hole 298 most common. (Leg 31) demonstrates that these hyper- Muddy deposits show fissility at about boles defined nearly horizontal ridges, 275 m and form mudstone below 300 m. oriented subparallel to the trench for dis- Silt is generally consolidated to siltstone in tances of at least 10 km (Fig. 3). Sediment Unit II, whereas sand is usually unconsoli- ponded between the ridges increases in dated. thickness upslope. The sediment in the inter-ridge ponds is deposited as flat lying Depositional Processes and or slightly tilted seaward and shows in- Site of Accumulation creasing tilt landward with depth. In the inter-ridge area of hole 298, there is no evi- The sand beds of Unit I are broadly simi- dence of significant ponding of sediment. lar to density current deposits reported Toward the upper part of the slope, opaque from submarine fan channels (Nelson and basement is covered with a continuous Kulm, 1973). Therefore, it is likely that layer of sediment. these massive sands are the product of a Near hole 298, flat-lying sediment of the channelized or somewhat laterally re- trench floor is separated from the steep stricted flow path. lower slope by a step, about 3 km wide and The mudstone clasts which define Unit I 50 m high. The Glomar Challenger are enigmatic in that they are hard lithified seismic-reflection profile (Karig, Ingle, and rock and occur in mud layers as well as sand layers. Since they are lithified, the others, 1975) and the original copy of the O Km 10 profile in Hilde and others (1969) show the mudstone fragments cannot be simply ex- step as a zone where the trench turbidite Figure 3. Bathymetry of area around site plained as rip-up clasts. The mudstone deposits are uplifted, mildly flexed at shal- 298. Crest of ridges indicated by heavy solid clasts in the mud layers must have been em- low depth, and sufficiently deformed at lines. Note subhorizontal and collinear orien- placed by mass flow; however, the mud- depth so as to become acoustically opaque. tation of ridges and basins. stone clasts in the massive sands could have

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0-m ow suggests that both Units I and II were de- the seaward-accreting trench inner wall OÜJ posited in a basinal environment. The basin (Fig. 6) (Piper and others, 1973). IO in which site 298 is located has no detect- able horizontal surface, indicating less than a DESCRIPTIVE few tens of metres of undeformed turbidite STRUCTURAL GEOLOGY fill. Therefore only the uppermost portion of Unit I could have been accumulated since The deformation of the deposits cored rrrrr.-r-.m formation of this depression. The turbidite from the Shikoku subduction zone is 200 sequence cored at site 298 could be made reflected by high consolidation, inclined up of a massive slump deposit which glided beds, a strong bedding fissility, and a sec- down from a large slope basin. However, ondary s-surface (cleavage) at shallow slope basins along the lower portion of the depths. However, overturned beds in the a. trench inner wall show only minor sedi- basal two cores present the compelling evi- UJ n a ment fill, rendering slump origin for the dence for folding. The overturning of these cored deposits unlikely. two cores was recognized on the basis of 400 " The Nankai Trough is the nearest large graded beds, scour marks, and cross- turbidite basin, and it is the probable site of bedding, all indicating tops down section. deposition of the sequence cored at site In addition, the overturned cores show sys- 298. The turbidity currents would tend to tematically steeper bedding inclinations hug the inner wall of the trench because of a (Karig, Ingle, and others, 1975) and lower landward tilt of the floor during downbend- porosity than the upright section im- A/WA^A EYWTAAA^; ing and subduction (Hsu and Ryan, 1972) mediately above. The cleavage-bedding re- of the Philippine Sea plate. Moreover, this lationships (Fig. 7) confirm the top direc- portion of the Nankai Trough is supplied tions determined on the basis of sedimen- 600 primarily from the north (Hilde and others, tary structures. Figure 4. Generalized stratigraphie column of 1969), which (due to coriolis effect) would sequence cored at site 298 (holes 298 and 298A). tend to focus density current flow along the Defining the Fold Unit I is composed predominantly of massive and inner wall. The concentration of turbidity graded sand beds, slide deposits, and mud; Unit currents along the steep topographic slope II is made up of fine-grained graded beds and We constructed the fold penetrated at site mud. Break near base of lithologie column is lo- explains why proximal turbidites are inter- 298, following the simplest interpretation cation of transition from upright to overturned bedded with slide accumulations in Unit I. of all available information. The orienta- beds. Cored intervals with recovery indicated by The superposition of Unit I (a proximal tion data were collected by Moore during solid bars in depth column. Absolute biostrati- axial facies) over Unit II (a distal marginal shipboard examination of the cores, and graphic ages indicated between geologic age and facies) defines a coarsening-upward se- they are published in tabular form else- lithologie columns. quence and apparently results from the un- where (Karig, Ingle, and others, 1975) derthrusting of the trench floor relative to along with the bathymetric and seismic profiles in the vicinity of site 298. The cores from site 298 are unoriented. We assume been transported by mass movement or % SAND AND SILT turbulent flow. Moore prefers to interpret that they were cored vertically because the the mud layers with large mudstone clasts maximum deviation of the drill string from as olistrostromes (literally, slide accumula- the vertical at this depth averages less than tions) (Abbate and others, 1970; Elter and 2° (John Shore, 1973, personal commun.). Trevisan, 1972) and thinks that they may If the horizontal collinear ridges and troughs be a primary product of tectonic scarps de- noted in the site survey (Fig. 3) are really veloped by deformation in the subduction the external geomorphic representation of zone. Alternatively, Karig wishes to em- anticlines and synclines, then the symmetry phasize the possible diverse origins of the of folding must be statistically cylindrical whole suite of lithified and unlithified around a horizontal axis. This critical piece clasts. Clasts recovered from other trench of information allows us to reconstruct the floors (Anikouchine and Ling, 1967; Lar- geometry of folding from unoriented cores. sen, 1968) include both soft mud lumps and We have used orientation of bedding and highly lithified pebbles which were derived incipient cleavage from all cores to con- from both the trench slope and a range of struct a simple fold, assuming no later fault- shallower water depths. Karig feels that the ing. We have no evidence that major thrust higher percentage of clasts occurring in faults intersect the cored sequence, al- Unit I is attributable to poor recovery. though minor thrust faults probably occur Since the graded beds of Unit II are com- and were not resolved biostratigraphically. prised of C-D-E and D-E Bouma intervals, We assume that all recovered beds on each they are distal in a purely descriptive sense limb dip in the same direction (we recorded (see definition by Bouma and Hollister, no dip reversals due to subsidiary folds) and 1973, p. 89). These lower-flow regime se- that the folding is cylindrical around a hori- quences could be interpreted as levee or zontal axis. overbank deposits of a channelized turbi- The limiting factors in defining the dite flow (Nelson and Kulm, 1973) or as geometry of the fold are (1) the direction of accumulations at the margin or end of an 298A). Horizontal bars indicate percentage of tops and attitude of bedding, (2) the orien- unconfined turbidity current. each core. Line connecting bars is extrapolated tation of the axial surface, and (3) the ex- The ubiquity of density current deposits frequency curve. ternal geomorphic surface of the fold. By

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Figure 6. Inferred facies rela- seaward face of the ridge may have cut tionships during deposition of across original bedding surfaces, and minor sedimentary sequence cored at infilling of the basin may have occurred at site 298 depicting origin of the site of drilling. coarsening-upward sequence (in Using the preceding data and assump- part after Piper and others, 1973; Budnik, 1974). Axial facies and tions, we present two possible geometries marginal facies, respectively, cor- for the cored anticline. The first (Fig. 8A) is respond to Units I and II in Fig- constructed with an axial surface dipping at ure 4. Layer beneath marginal 9° (from symmetry of bedding), and the facies represents abyssal-basin second (Fig. 8B) utilizes an axial surface sequence, which lies over oceanic dipping at 13.5° (from symmetry of cleav- crust. Erosion of nose of fold by age). The resulting fold would be classified slumping shows interpretation of (see Chap. 7 in Ramsay, 1967) as tight in Moore; Karig favors less- both cases and as either recumbent (first pronounced development of slid- ing phenomena. case) or overturned (second case). Although the cleavage orientations are less numerous and accurate than those of bedding, we feel definition, an axial surface contains the another approximation of the axial surface. that the real fold is more like that portrayed hinge lines of all folded layers. For a sym- The plane of symmetry to cleavage in cores in Figure 8B, because the overall geometry metrical fold, this surface bisects the inter- 13 and 14 versus cores 15 and 16 is inclined is likely to be asymmetrical. Both of the limb angle and splits the fold into mirror at 13.5°. The profiles presented by Hilde preceding reconstructions are very straight- images. The plane symmetrical to the bed- and others (1969) indicate southeastward forward interpretations of the available ding of cores 13 and 14 (upright), and 15 vergence of the deformed reflecting layers. data. If the coring had been continuous and and 16 (overturned) is inclined at about 9° Thus, a combination of seismic-reflection the biostratigraphy more precise, we might and provides one approximation to the and core-orientation data indicates that the have been able to develop a more complete axial surface. However, if the fold is asym- axial surface of the cored anticline dips be- representation of the major fold, including metrical, the axial surface does not bisect tween 9° and 13.5° toward the northwest. parasitic folds and minor thrusts. Our in- the interlimb angle, and it could be steeper We use the bathymetry (with hyperbolic terpretation of the actual fold (Fig. 8C) is than 9°. An observed cleavage is probably distortions) of the basin and ridge at site based on the preferred simple fold geometry of the axial plane variety (see below). If so, 298 as a constraint on the form of the (Fig. 8B) and the Glomar Challenger the average inclination of cleavage provides folded surface. However, slumping off the seismic-reflection profile (Karig, Ingle, and others, 1975).

Incipient Fracture Cleavage

At depths greather than 317 m at site 298, close inspection of the cores revealed two populations of fractures, one parallel to bedding and a second crosscutting bed- ding. The secondary s-surface is nowhere strongly developed. Nevertheless, it is de- scribed here in some detail, because it is a fundamental characteristic of the cored fold. Macroscopically, the secondary s-surface is characterized by discontinuous hairline fractures (< 1 cm long) inclined at low an- gles to bedding (Fig. 7). The fractures are typically separated from 1 to 5 mm in the areas where they occur. Microscopic obser- vations show no obvious reorientation of platy or elongate minerals nor measurable displacements along cleavage planes. It is possible to confuse both bedding- plane fissility and discontinuities induced by drilling with the fractures described above. To circumvent one aspect of the problem, we only made measurements where both bedding and hairline fractures B were visible. In contrast to the hairline frac- 6cm tures of probable natural origin, the drilling fractures are characteristically steeply in- clined (avg 52° from horizontal) and ex- Figure 7. X-radiographs and complementary line drawings illustrating orientation of fracture cleavage. A. (Core 12, section 3, 56 to 61 cm; 424.56 to 61 m below sea floor.) Shallowly inclined tend for several centimetres or tens of cen- fracture patterns parallel to upright bedding; steeply inclined fractures define fracture cleavage. B. timetres. These artificially induced fractures (Core 16, section 1, 68 to 73 cm; 602.18 to 23 m below sea floor.) Steeply inclined fractures parallel to may occur in conjugate sets, show slicken- overturned bedding; shallowly inclined fractures represent fracture cleavage. sides, grade into kink bands, and locally

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NW ture cleavage. Presently, it is impossible to clearly choose between the two possible origins of cleavage on an observational basis, although we have assumed fold origin for one of our geometric construc- tions.

RATE OF DEFORMATION I Km Rate of Subduction

SITE Our long-term estimates of the rate of subduction are based on a steady-state trench model and length of subduction of the down-going slab. Although both methods require substantial assumptions, the results are consistent and indicate a sub- stantially slower rate of subduction than that based on studies of historic earth- quakes. C Figure 8. Structural cross sections, no vertical exaggeration. A. Simple reconstruction of over- Steady-State Trench turned fold with axial surface symmetrical to bedding. B. Same, with axial surface symmetrical to cleavage. C. Preferred reconstruction of area between site 298 and undeformed turbidites in trench, The results of DSDP hole 298, integrated based on fold reconstruction B and interpretation of Glomar Challenger seismic reflection profile with those of hole 297, the pre-site survey, (Karig, Ingle, and others, 1975). The total shortening recorded by the fold-thrust complex is approxi- and other available seismic-reflection mately 2 km. Dashed lines through inflections at step represent probable hinges of presently forming profiles (Hilde and others, 1969; Ludwig fold. Dashed line converging at surface northwest of site 298 is estimated location of thrust upslope from site 298. Slide accumulations below and southeast of anticlinal nose; extent not well controlled. and others, 1973 and unpub. SIO profiles) permit the construction of an internally consistent geometric and kinematic model crosscut the fracture cleavage. In general, Moore and Geigle (1974) show mineral for the Nankai Trough. If steady-state con- the planarity and steep orientation allowed reorientation only in the axial zones of ditions are assumed to exist in the trench the separation of drilling-induced fractures folds. In the Shikoku subduction zone, we for periods of more than twice the age of from hairline fractures of probable natural did not recognize any comparable meso- the oldest wedge turbidites, the input via origin. We interpret the hairline fractures as scopic or microscopic folds, and we cannot sedimentation can be equated with the out- an incipient fracture cleavage. be certain of sampling the axial zone of the put via subduction (Helwig and Hall, 1974; Several aspects regarding the orientation major overturned fold. Material from the Karig and Sharman, 1975). Below, we pre- of the hairline fractures tend to confirm Aleutian Trench studied by Moore and sent a number of methods for determining their probable natural origin. The incipient Geigle (1974) has been subjected to higher the sedimentation rate in the Nankai fracture cleavage dips more steeply than confining pressures than the Shikoku cores, Trough and assuming constant geometry to bedding on the upright limb of the anticline which possibly facilitated mineral reorien- calculate the rate of underthrusting. Al- and more shallowly than bedding on the tation in the former. Transitions between though Helwig and Hall (1974) have chal- overturned limb. The incipient cleavage slaty cleavage or schistosity and fracture lenged the steady-state assumption for the also shows a convergently fanning pattern. cleavage have been documented in some Nankai Trough, new data presented here Both the observed bedding-cleavage rela- ancient sequences (White, 1949; Wilson, and by G. F. Moore and others (1975) sug- tionships and the convergent fanning are 1961, p. 464). A similar transition may gest that this trench has approximated a common in subaerial examples of fracture occur at depth in the Shikoku subduction steady state during its recent history. cleavage (Spencer, 1969, p. 242). zone. The sediment cover entering the Nankai If the cleavage is in fact natural, two pos- The early cementation of sediments by Trough includes two units: the general sibilities exist for its origin. The hairline gas hydrates may be responsible for the de- cover of the Shikoku Basin and a wedge of fractures could be (1) an axial-plane cleav- velopment of fracture cleavage during de- turbidites deposited along the trench axis. age directly related to folding, (2) a fracture formation in the deep-marine environment In a manner similar to that described in the cleavage sympathetic to thrusting, or (3) a (Barnes and Ross, 1975). This attractive Aleutian Trench (Kulm, von Huene, and combination of these two. It has long been hypothesis might explain the presence of others, 1973), the uppermost part of the recognized that slaty cleavage may form by fracture cleavage in the site 298 cores ex- underlying basin sediments thin beneath the the rotation of platy and elongate particles cept that no solid hydrates were observed wedge in the direction of the inner trench during deformation (Wood, 1974). Max- nor were the cores obviously gassy when slope. This thinning, which represents a well (1962) suggested that this process first opened. trenchward decrease in the duration of might occur during dewatering at shallow The possibility of a thrust fault just be- basin sedimentation beneath the trench structural levels; his hypothesis has been neath the cored anticline is consistent with turbidites (Piper and others, 1973, Fig. 2), applied to deformation in subduction zones the development of cleavage sympathetic to can be used to estimate the rate of sedimen- (Moore, 1973; Moore and Geigle, 1974). thrusting. As reviewed by Wilson (1961, p. tation in the trench wedge. A lack of The presence of fracture cleavage and ab- 477—480), fracture cleavages associated reflectors in the uppermost hemipelagic sence of slaty cleavage in the cores from with thrust faults are reported to occur at basin sediments beneath the Nankai Shikoku subduction zone may be a function least 150 m above and below the fault sur- Trough makes this determination less accu- of scale and confining pressure. The meso- face. Thus, effects of thrust faulting may rate than in the Aleutian Trench. scopic and microscopic folds studied by account for all or part of the observed frac- On the Seifu Maru (unpub. data) and

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Glomar Challenger reflection profiles at least 600 to 650 m are suggested. If the quence is thickened and possibly undergoes (Karig, Ingle, and others, 1975), which run dip of the downgoing plate beneath the small-scale folding or thrusting (not visible along similar tracks, the thinning of the wedge did not change during the last 1.5 on available seismic profiles). The longitud- hemipelagic deposits beneath the trench m.y., a calculated steady-state subduction inal strain occurring in the step region is turbidite wedge totals 0.15 sec or more rate of 1.2 cm/m.y. is obtained. mild compared to the large shear strain from the outer edge of the trough to the which occurs when the turbidite wedge is edge of the structural step. A similar value Length of Down-going Slab tucked beneath the trench inner wall. Ap- is suggested on the profile of Hilde and parently, this major deformation of the others (1969). If it is assumed that an aver- An additional approximation of the sub- trench turbidite wedge begins immediately age acoustic velocity of 1.8 km/sec and rela- duction rate has been made by Moore by beneath the previously formed fold. Here tively low compaction of 10 percent due to dividing the length of the underthrust slab the deformation can be initially approxi- rapid sedimentation can be applied to the by the total time elapsed since the inception mated by simple shear. If the thickness of hemipelagic unit beneath the apron, a dif- of subduction. This simple method provides the shear zone and the rate of convergence ference in thickness of between 110 and an estimate of the average rate of under- are known, the strain rate can be calculated 125 m is calculated. The sedimentation rate thrusting during the entire history of the by in the late Pleistocene-Holocene section in Shikoku subduction zone, whereas the é =4>-/sec DSDP hole 297 is approximately 100 steady-state estimate applies to the past 1 'o m/m.y., but because there is a trenchward m.y., and seismically determined rates are where e = strain rate, A1 = displacement of

increase in thickness of these hemipelagic instantaneous. the shear couple, and 10 = the thickness of sediments, attributed to nepheloid deposi- The spatial extent of the Philippine Sea shear couple. tion related to turbidite activity, a rate of plate extending beneath southwest Japan To a first approximation, the thickness of deposition near 150 m/m.y. for the upper- has been determined by Shiono (1974), the simple shear couple is limited by the dis- most section is a more likely estimate. From using travel-time analysis and the distribu- tance between the lower limb of the previ- these estimates, an age of 0.8 ± .15 m.y. for tion of seismic activity. The length of the ously formed fold and the top of oceanic the oldest turbidites in the trench wedge is inclined slab measured along the present crust. In the modern Nankai Trough, this deduced. azimuth of underthrusting (310°; distance is 1.3 km as determined from the At the base of the step where the sedi- Kanamori, 1972) is approximately 290 km. Glomar Challenger seismic-reflection ments are not acoustically nor presumably Since this figure is measured from the tip of profile. We assume a homogenous distribu- structurally disturbed, the travel time the slab to the Nankai Trough, it must be tion of strain over this interval but realize through the trench wedge is 0.69 sec, which reduced by the distance that the slab ex- more complex patterns would occur during is assumed to represent an original, uncom- tended beneath southwest Japan at the be- the development of a substantial décolle- pacted thickness of between 675 and 700 ginning of underthrusting and the extent of ment within sedimentary sequence (Karig, m. The average sedimentation rate through seaward migration of the Nankai Trough 1974; Moore, 1975). For convergence rate, the wedge section is thus between 0.85 and due to accretion. Seismic-reflection profiles we use estimates based on the steady-state 0.9 km/m.y. For comparison, von Huene in the vicinity of site 298 (Karig, Ingle, and trench (1.2 to 2 cm/yr), because this rate is a and Kulm (1973) determined a sedimenta- others, 1975; Karig, 1975) indicate that 85 direct measure of underthrusting beneath tion rate of 2 km/m.y. in the eastern Aleu- km is a conservative estimate of the dis- the trench inner wall, and it applies directly tian Trench. This greater rate is predictable tance of seaward migration of the Nankai the time of development of the cored fold. because of the glacially enhanced erosion of Trough. At the inception of subduction, the For a shear couple 1.3 km thick and a con- the much larger and more rugged drainage initial rupture probably occurred along an vergence rate of 1.2 to 2 cm/yr, the strain area feeding the Aleutian Trench. inclined plane of high shear stress which ex- rate during folding is 3 to 5 x 10~':!/sec. The previously estimated average tended through the lithosphere 50 to 100 sedimentation rate and wedge thickness km beneath southwest Japan. For purposes THRUST FAULTING and the wedge width of 17 km define a sub- of this paper, the tip of the mapped slab is duction rate of 2.0 cm/m.y., or far less than assumed to have been located 75 km land- The deposits cored at site 298 provide no the 8 cm/yr estimated by Fitch and Scholz ward of the continental margin at the be- positive evidence of large-scale thrust fault- (1971). That higher subduction rate with a ginning of underthrusting. After making the ing, as no stratigraphie repetitions were ob- constant influx of sediment would cause the above reductions, the actual distance of un- served downhole. However, the seismic- wedge to shrink rapidly toward a stable derthrusting is thought to have been 130 reflection profiles presented by Hilde and width of 5 km and a maximum thickness of km. Studies of on-land geology suggest that others (1969) show landward-dipping dis- 200 m. convergence began in the Nankai Trough at continuities that emerge in the basins The sediments cored in hole 298 repre- least 10 m.y. B.P. Thus, the long-term esti- separating the anticlinal ridges. Each ridge sent an earlier state of the trench wedge and mate of average convergence is 1.3 cm/yr. is upthrown relative to its seaward coun- constrain the average subduction rate dur- terpart; thus, if the discontinuities are really ing the sampled interval. Very poor core re- Strain Rate faults, they must be of reverse or thrust na- covery and preservation in the upper por- ture. tion of the hole preclude determination of The seismic profiles made on a cruise of The cored fold cannot have taken more the time when the drilled section left the the Glomar Challenger (Karig, Ingle, and than .2 m.y. to form because it contains trench floor and rose onto the step. This others, 1975) and those of Hilde and others well-bedded turbidites of that age. The ac- event must have occurred during the last (1969) indicate that deformation occurs at tual folding my have occurred during a time 0.2 m.y., because well-defined wedge turbi- a stepping up (mild inflection) of the trench period of .1 to .15 m.y. Thus, at a con- dites of that age were recovered. turbidite wedge. The step area comprises vergence rate of 1.2 to 2.0 cm/yr, the hori- After corrections are made for consolida- the seaward extension of the zone of active zontal shortening in the subduction zone tion in the section at hole 298, an average deformation. This zone extends beneath the during folding would have ranged from 1.2 sedimentation rate of at least 500 m/m.y., a trench inner wall and down along the major to 3.0 km. Our preferred reconstruction of maximum age of 1.5 m.y. (Karig, Ingle, and thrust contact between the converging the overturned fold (Fig. 8C) might permit others, 1975), and an original thickness of plates. In the step area, the sedimentary se- shortening of about 1.5 km before thrust-

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ing. Therefore, at the lower convergence were derived and therefore could acquire a rates, little or no thrust faulting would tectonic fabric. This scenario of sliding and occur on the lower limb of the overturned subsequent underthrusting accounts for the anticline. For the maximum estimate of presence of olistrostromes with evidence for horizontal convergence, it is likely that gravity sliding (Kleist, 1974; Maxwell, significant thrusting would take place. Any 1974) transitional to broken formations thrust probably would pass a short distance with systematically oriented boudin long beneath the base of the hole at site 298. axes (Gilbert, 1973; Moore, 1973). More- If the thrust exists, it probably roots near over, the probable intense deformation of the contact between the trench turbidites the slide accumulation makes it likely that and the underlying hemipelagic deposits the evidence of the primary origin would be where sediment shear strength should be at destroyed. a minimum (Karig, 1974). Data supporting As the folding reaches its final stages, a this conclusion include: (1) a lack of thrust may form on the lower limb of the hemipelagic sediments in the fold axis anticline. The partial or complete frictional where drilling and (2) the minor available locking of this thrust then would again volume for such sediments as well as addi- transfer the deformation to the adjacent = D. tional turbidites in the more landward sec- trench floor and precipitate the develop- tion of the axial region of this fold (Fig. ment of yet another fold. 8C). In such a fold, sediments stratigraphi- cally beneath those in the axis at hole 298 Figure 9. Evolution of the overturned fold, PHYSICAL PROPERTIES probably would be less than 200 m thick at probable minor thrust faulting ignored for the next landward syncline-thrust. geometric simplicity. A. Initial inflections as ob- The physical properties of the accreted served on trench floor in Glomar Challenger deposits of site 298 provide a measure of (Karig, Ingle, and others, 1975) profiles and the accelerated consolidation and volume EVOLUTION OF THE profiles by Hilde and others (1969). B. Slightly OVERTURNED FOLD overturned fold modified by mass movement. C. reduction which occurred during their de- Overturned fold. D. Strongly overturned fold as formation. Here we compare depth- The structural evolution of the fold cored reconstructed from site 298 results. porosity plots for site 298 versus sites 297 at site 298 can be best understood by recon- and 299. The lithologic similarity and geo- structing a transition from an initial state (a the development of a steep scarp during the graphic proximity make these sites espe- step on the trench floor) to its present over- monoclinal stage of deformation (Fig. 9). cially attractive for comparison. The poros- turned geometry. The step on the trench The steep inclination of this scarp along ity measurements of sites 297 and 299 are floor is defined by two low-angle (4° to 5°) with the possibility of excess pore pressure averaged to give a generalized curve for flexures which propagate into the turbidite in the deforming mass undoubtedly would comparison to similar data from site 298. sequence below. These initial flexures result in substantial slumping from the nose The porosity analyses we use were com- would be likely to evolve as fold hinges dur- of the fold. Part of the material slumped pleted on shipboard. Of all available data ing continuing deformation. from the nose of the fold would be liquified (Bouma and Moore, 1975), we have The Glomar Challenger reflection profile and would generate turbidity currents, and selected only analyses made by gravimetric (Karig and others, 1975) indicates that part might move as a mass flow. Moore be- methods of muds and mudstones in order reflectors defining the step become more lieves that the slide accumulations of Unit I to minimize effects of lithologic variation. disturbed at depth; moreover, the reflection formed during the development of a prob- At depths greater than 279 m, the mud- record is acoustically opaque beneath this able anticlinal structure upslope from site stones of site 298 were too consolidated for disturbed zone. If the evolving fold has a 298. During continuing folding or thrust accurate volume measurement by ship- tendency toward concentric style, then its faulting, the slide accumulations would be board techniques. Therefore, we have used core region below the step would likely be sheared beneath the fold from which they water-content data to calculate porosity, the location of relatively small-scale folding assuming 100 percent saturation and a POROSITY and minor thrusting (Ramsay, 1967). Al- grain density of 2.74 gms/cc determined though we cannot resolve any details of from the overlying 200 m of sediment. small-scale deformation in the core of the step, the complicated reflector geometry Volume Reduction During Deformation and acoustic opacity are consistent with rel- atively intense deformation and accelerated The initial porosity of muddy sediments dewatering with respect to the simple buried 50 m at sites 297, 298, and 299 is geometry of the step at the surface. approximately 70 percent. The projected The sequential picture of fold evolution average porosity at sites 297 and 299 de- (Fig. 9) records the transition from the creases to 48 percent at about 525 m due to flexure on the trench floor to the overturned normal consolidation (Fig. 10). The fold. The actual geometric development of depth-porosity curve for site 298 shows a the fold observed at site 298 probably sharp decrease from 67 to 48 percent poros- would include considerable minor thrust- ity from 90 to 190 m. Below this interval, ing. Although these probable thrusts are the porosity gradually decreases to 41 per- shown on our preferred interpretation (Fig. cent at a depth of 520 m. A minimum 8C), they are omitted from the scenario of porosity of 37 percent is reached between fold evolution (Fig. 9) for geometric sim- 601 and 611 m at the base of the hole. plicity. Figure 10. Depth-porosity data for sites By comparison of the porosities of sites The transition from flexed trench de- 297-299 (avg) and site 298. Note marked de- 297 and 299 to those of site 298, it is pos- posits to an overturned fold would include watering of deformed deposits from site 298. sible to estimate the water loss and hence

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volume reduction that occurred during de- inner wall. Tectonic dewatering probably duction rates, convergence may be accom- formation. The average difference between begins in the step area. One probable effect modated primarily by folding. However, a the average porosity of sites 297 and 299 of this dewatering is development of acous- more general case would probably involve and that of site 298 is 15 percent between tic opacity in the step at relatively shallow the attenuation of fold limbs and the de- 200 and 500 m, and 19 percent between depths compared to the undeformed trench velopment of thrust faults. If the accretion- 200 and 300 m. The 15 percent figure com- sediments (Karig, Ingle, and others, 1975). ary prism is to grow by the successive un- pares uniformly tectonically dewatered Tectonic dewatering probably continues as derthrusting of successive sediment wedges, rocks on the upper limb of the fold with the fold evolves and is thrust beneath the the locus of faulting must migrate seaward. undeformed deposits of equivalent depth of trench inner wall. A relatively open Thus the thrust at the base of an accreting burial. The same condition holds for the sediment-particle framework may persist at wedge must ultimately lock because of de- comparison between 200 and 300 m (19 shallow depths of burial. However, the watering and compaction, which in turn percent porosity difference), and, in addi- shearing stress may tend to subtly rotate or precipitate the development of another tion, the age difference between deformed laterally translate sediment particles such fold. and undeformed deposits is minimized. that more efficient grain packing can be Glomar Challenger bathymetric and Even so, the sediments in the 200- to 300-m achieved. Also the development of the frac- seismic profiles (Karig, Ingle, and others, interval of sites 297 and 299 are a factor of ture cleavage probably increased the overall 1975; see Fig. 11) show that the inner 7 or 8 older than the site 298 deposits when permeability of the deforming sediments trench slope becomes reorganized into they were deformed. We therefore conserv- which would directly facilitate tectonic de- larger steps from the base upward, with atively estimate that the rocks at site 298 watering. greater relief between the highs and lows. underwent at least a 19 percent porosity re- This might be accomplished by the localiza- duction. TECTONIC EVOLUTION tion of shear into fewer, more discrete The porosity measures volume of voids zones coupled with uplift and rearward tilt- relative to volume of sediment plus voids. This paper is primarily concerned with ing. Thus a 19 percent reduction in porosity is the development of a single fold within a equivalent to a considerably larger reduc- subduction zone. In addition, it is worth- CONCLUSIONS AND IMPLICATIONS tion in total sediment volume. The calcu- while to consider how successive fold- lated reduction in volume over the 200- to thrust packages or accretionary wedges Structural variety in subduction zones is 300-m interval is 36 percent (average of would evolve into an accretionary prism. certainly a function of several variables, in- sites 297 and 299 versus site 298). Thus, we The upslope increase of the thickness of cluding rate of consumption and the nature estimate that the total volume of sediment slope sediments and the size of inter-ridge and thickness of the cover on the down- was reduced more than one-third and pos- sediment ponds provide evidence of the se- going plate. The Shikoku subduction zone sibly as much as one-half during deforma- quential addition of younger material along is a good example of a slowly converging tion. the base of the trench inner wall. The up- plate boundary with a relatively thick sedi- permost inter-ridge sediment pond shows a ment cover. As such, the study of this sub- Tectonic Consolidation distinct landward tilt (Fig. 11). Rather than duction zone provides insights on the representing rotated slump masses (von stratigraphy, deformation, and tectonics of The anomalous consolidation of the site Huene, 1972), we feel that the inclination modern and ancient equivalents. 298 rocks could have been produced by a of the sediment layers indicates landward tectonic stress or by excess overburden tilting of the underlying accreted sedimen- Coarsening-Upward Sequence which has been subsequently stripped off. tary package (Karig, 1974). This landward Slumping has occurred from the nose of the tilting has been recognized in studies of an- The coarsening-upward sequence from cored fold; however, the actual drill site is cient accreted trench deposits (Moore, site 298 represents the first complete sam- located in a synformal depression landward 1973) and may be caused by the successive pling of this facies from a modern trench. of and removed from the zone of mass underthrusting of wedge-shaped structural The existence of the coarsening-upward se- wasting. By equating the porosities of the packages (Karig, 1974; Seeley and others, quence has been inferred in the Aleutian deformed and gravitationally consolidated 1974; Karig and Sharman, 1975). Trench (Piper and others, 1973) but not re- deposits, it can be shown that as much as We have pointed out that at slow sub- covered in a single vertical section. This se- 400 m of excess overburden would be re- quired to achieve the degree of compaction observed at site 298. The sedimentation rate curve for site 298 (Karig, Ingle, and others, 1975) provides no evidence for re- moval of surface sediment nor for a hiatus between the consolidated and unconsoli- dated deposits. Moreover, the preservation I 4 of the external geomorphic form of the fold shows that any slumping has not been of sufficient magnitude to breach the ridge to- g » pography (Fig. 3). While it is simple to measure the tectoni- 6 • cally induced volumetric strain, it is much more difficult to clarify the mechanism whereby this consolidation was achieved. With reference to Figure 9, the trench se- Figure 11. Northwest-southeast cross section of the Shikoku subduction zone, based on bathymet- quence initially was thickened in the step ric, seismic-reflection, and drilling data (modified after Karig, Ingle, and others, 1975). Solid lines area, possibly by minor folding and thrust- through lower limb of anticline and step area indicate location of possible thrust. Dashed lines north- ing, and subsequently entrained in a shear west of site 298 indicate possible boundaries between accretionary packages. Arrows indicate relative zone as it was thrust beneath the trench sense of tectonic transport whether by folding or thrust faulting.

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Tectonic consolidation would occur most markedly during the deformation of slightly dewatered sediments. The process should be less important when deformation begins at high levels of gravitational consolidation. Low strain rates occurring in subduction zones are probably instrumental in allow- ing orderly tectonic dewatering and hence tectonic consolidation of deposits at shal- low depths of burial.

ACKNOWLEDGMENTS

This paper is based almost entirely on Figure 12. Cross section showing hypothetical recurrence of coarsening-upward sequence (axial data and samples collected by the Deep Sea sand and slump facies overlying marginal graded bed facies) in each accretionary wedge. Stippled pat- Drilling Project, which is supported by the tern on diagram shows coarse (axial) facies sequence. Preservation of repeated occurrence of National Science Foundation. We are grate- coarsening-upward sequence would be dependent upon its being underthrust to a depth exceeding ful to the shipboard scientific and technical that of subsequent erosion. staff for their assistance during Leg 31 of DSDP and the staff of the La Jolla core re- quence may provide a more readily deter- mediately underthrust beneath the over- pository for facilitating later study of the minable indicator of trench sedimentation turned fold, giving rise to stratigraphically cores. Our thanks to Gene Gonzales for the than do schemes based on paleocur- discontinuous sheared units which would skillful preparation of thin sections from rents, turbidite continuity, and regional be indistinguishable from tectonically the deformed mudstones. John Maxwell, geology (Moore, 1972; Hesse, 1974). The broken formations. If broken formations or Othmar Tobisch, Eli Silver, W.S.F. Kidd, coarsening-upward sequence has been rec- melanges of true tectonic origin are being and John Dewey provided helpful reviews. ognized in ancient trench deposits (Budnik, developed in the Shikoku subduction zone, Discussions with Albert Smith clarified 1974) in which paleocurrent markers are they must be occurring in regions of deeper many aspects of the seismology of south- either absent or deformed beyond recogni- underthrusting or larger finite strain than west Japan.Acknowledgment is made to the tion. that sampled at site 298. donors of the Petroleum Research Fund, The coarsening-upward sequence is a administered by the American Chemical seaward-transgressive facies which should Tectonic Consolidation Society, for support of the laboratory work cap every vertical sequence thrust beneath and manuscript preparation involved in this or folded against the trench inner wall. Tectonic consolidation accounts for at research. Karig acknowledges support from Therefore, in complex accretionary prisms, least 36 percent and possibly up to 50 per- National Science Foundation Grant the scale of repetition of the coarsening- cent reduction in volume in the upper por- GA-38107. upward sequence may provide a maximum tion of the deformed sequence at site 298. dimension of the structural thickness of the This volume reduction during deformation REFERENCES CITED thrust-bounded accretionary wedges (Fig. is much greater than that normally assumed 12). by structural geologists studying ancient Abbate, E., Bortolotti, V., and Passerini, P., rocks (Ramsay, 1967, p. 186) and would 1970, Olistrostromes and olistoliths: Sed. Gravity Sliding have to be considered in quantitative strain Geology, v. 4, p. 521-557. measurements in these sequences. Anikouchine, W. A., and Ling, Hsin-yi, 1967, Evidence for turbidite accumulation in We find evidence for moderate slide ac- Tectonic consolidation is a strengthening trenches in the Indo-Pacific region: Marine cumulations which probably occurred as a process which facilitates the development Geology, v. 5, p. 141-154. direct consequence of the folding process. of macroscopic folds in subduction zones Barnes, W. C., and Ross, J. V., 1975, Some struc- Nevertheless, the linear ridge-and-basin to- (Moore, 1973; Carson and others, 1974). tural implications of gas hydrates in deep- pography shown by the site survey (Fig. 3) In the cored fold, the slightly consolidated ocean sediments: Geol. Soc. America, Abs. indicates a lack of individual downslope sediments have a low vane shear strength with Programs (Ann. Mtg.), v. 7, no. 7, movements by massive slide blocks. Thus, (low cohesion) (Bouma and Moore, 1975), p. 989-990. Beck, R. H., 1972, The oceans, the new frontier the intensity of gravity sliding along the and the consolidated deposits show no mi- in exploration: Australian Petroleum Expl. inner wall of Nankai Trough is considera- croscopic evidence of cementation. The low Assoc. Jour., v. 12, pt. 2, p. 5-28. bly less than that inferred for the inner wall initial cohesion and lack of cementation Bouma, A. H., and Hollister, C. D., 1973, Deep of the Eastern Aleutian Trench (von Huene, suggest that strength developed during de- ocean basin sedimentation: Soc. Econ. 1972). formation was due to increased friction Paleontologists and Mineralogists Short along grain contacts and possibly the in- Course: Turbidites and deep water Style of Deformation, Olistrostromes creased electrostatic attraction between sedimentation (lecture notes), p. 79-118 (available from SEPM). Versus Broken Formations clay particles. The stress-strain behavior of Bouma, A. H., and Moore, J. C., 1975, Physical deep-sea muds during tectonic consolida- properties of deep-sea sediments from the The fabric of the recovered cores and the tion and subsequent deformation may re- Philippine Sea and Sea of Japan, in Karig, fold reconstruction indicate that shallow semble some drained triaxial tests on D. E., Ingle, J. C., Jr., and others, Initial re- deformation in the Shikoku subduction slightly consolidated soils (Sowers and ports of the Deep Sea Drilling Project, Vol. zone is characterized by orderly folding Sowers, 1970, p. 142). The stress-strain 31: Washington, D.C., U.S. Govt. Printing with possibly some minor thrusting but not curve probably would show an initial Office, p. 535-568. pervasive irregular minor folds nor wide- concave-upward shape, resulting from Budnik, R., 1974, The geologic history of the spread structurally chaotic zones. However, Valdez Group, Kenai Peninsula, Alaska: strengthening during tectonic consolidation Deposition and deformation at a Late Cre- olistrostromes (slide accumulations) de- with a transition to truly ductile behavior at taceous consumptive plate margin [Ph.D. rived from the fold scarp may be im- higher stress levels.

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thesis]: Los Angeles, Univ. California, Los Deep Sea Drilling Project, Vol. 31: sedimentation in the active Eastern Aleutian Angeles, 139 p. Washington, D.C., U.S. Govt. Printing Trench: Geology, v. 1, p. 19-22. Carson, B., Yuan, }., Myers, P. B., Jr., and Bar- Office, p. 317-350. Ramsay, John G., 1967, Folding and fracturing nard, W. D., 1974, Initial deep-sea sedi- Kimura, Toshio, 1966, Tectonic movements in of rocks: San Francisco, McGraw-Hill ment deformation at the base of the the southern Fossa Magna, central Japan, Book Co., 568 p. Washington continental slope: A response analyzed by the minor structures in its Scholl, D. W., von Huene, R., and Ridlon, J. B., to subduction: Geology, v. 2, p. 561-564. southwestern area: Japanese Jour. Geology 1968, Undeformed sediments in the Peru- Chase, R. L., and Bunce, E. T., 1969, Under- and Geography, v. 37, p. 63-85. Chile Trench: Science, v. 159, p. 869-871. thrusting of the eastern margin of the Antil- Kleist, J. R., 1974, Deformation by soft-sediment Scholl, D. W., Christensen, M. N., von Huene, les by the floor of the western North Atlan- extension in the coastal belt, Franciscan R., and Marlow, M. S., 1970, Peru-Chile tic Ocean, and origin of the Barbados Complex: Geology, v. 2, p. 501-504. trench sediments and sea floor spreading: Ridge: Jour. Geophys. Research, v. 74, Kulm, L. D., and Fowler, G. A., 1974, Oregon Geol. Soc. America Bull., v. 81, p. 1413-1420. continental margin structure and stratig- p. 1339-1360. Elter, P., and Trevisan, L., 1972, Olistrostromes raphy: A test of the imbricate thrust model, in Seeley, D. R., Vail, P. R., and Walton, G. G., in the tectonic evolution of the northern Burk, C. A., and Drake, C. L., eds., The 1974, Trench slope model, in Burk, C. A., Apennines, in De Jong, K. A., and Scholten, geology of continental margins: New York, and Drake, C. L., eds., The geology of con- R., eds., Gravity and tectonics: New York, Springer-Verlag, p. 261—283. tinental margins: New York, Springer- John Wiley & Sons, Inc., 502 p. Kulm, L. D., von Huene, R., and others, 1973, Verlag, p. 249-260. Fitch, T. J., and Scholz, C. H., 1971, Mechanism Site 181, in Musich, L. F., and Weser, O. E., Shiono, K., 1974, Travel time analysis of rela- of underthrusting in southwest Japan: A eds., Initial reports of the Deep Sea Drilling tively deep earthquakes in southwest Japan model of convergent plate interactions: Project, Vol. 18: Washington, D.C., U.S. with special reference to the underthrusting Jour. Geophys. Research, v. 76, p. Govt. Printing Office, p. 407-448. of the Philippine Sea plate: Osaka City 7260-7292. Larsen, B., 1968, Sediments from the central Univ. Jour. Geosciences, v. 18, p. 37-59. Gilbert, W. G., 1973, Franciscan rocks near Sur Philippine Trench: Galathea Repts., v. 9, Silver, E. A., 1969, Late Cenozoic underthrusting Fault Zone, northern Santa Lucia Range, p. 7-21. of the continental margin off northernmost California: Geol. Soc. America Bull., v. 84, Ludwig, W. J., Den, S., and Murauchi, S., 1973, California: Science, v. 166, p. 1265-1266. p. 3317-3328. Seismic reflection measurements of the Sowers, G. B., and Sowers, G. F., 1970, Introduc- Helwig, J., and Hall, G. A., 1974, Steady-state southwest Japan margin: Jour. Geophys. tory soil mechanics and foundations: New trenches?: Geology, v. 2, p. 309-316. Research, v. 78, p. 2508-2516. York, The Macmillian Co., 556 p. Hesse, Reinhard, 1974, Long-distance continuity Matsuda, T., 1962, Crustal deformation and Spencer, E. W., 1969, Introduction to the struc- of turbidites: Possible evidence for an Early igneous activity in the South Fossa Magna, ture of the Earth: San Francisco, Cretaceous trench-abyssal plain in the East Japan, in Khopoff, L., Drake, C., and McGraw-Hill Inc., 597 p. Alps: Geol. Soc. America Bull., v. 85, Kuno, H., eds., The crust of the Pacific Tsuchi, R., Sameshima, T., Iwahashi, T., To- p. 859-870. Basin: Am. Geophys. Union Geophys. kuyama, A., Ito, M., Kuroda, N., Fujiyoshi, Hilde, T.W.C., Wageman, J. H., and Hammond, Mon. 6, p. 140-150. A., and Ikeya, N., undated, Geological map W. T., 1969, The structure of Tosa terrace Maxwell, J. C., 1962, Origin of slaty and frac- of Shizuoka Prefecture: Shizuoka City, and Nankai trough off southeastern Japan: ture cleavage in the Delaware Water Gap Japan, Shizuoka Univ. Geol. Inst., Faculty Deep-Sea Research, v. 16, p. 67-76. area, New Jersey and Pennsylvania, in Science. Holmes, M.L.R., von Huene, R., and McManus, Engel, A.E.J., James, H. L., and Leonard, B. von Huene, R., 1972, Structure of the continen- D. C., 1972, Seismic reflection evidence F., eds., Petrologic studies (Buddington vol- tal margin and tectonism at the eastern supporting underthrusting beneath the ume): New York, Geol. Soc. America, Aleutian trench: Geol. Soc. America Bull., Aleutian arc near Amchitka Island: Jour. p. 281-311. v. 83, p. 3613-3626. Geophys. Research, v. 77, p. 959-964. 1974, Anatomy of an orogen: Geol. Soc. von Huene, R., and Kulm, L. D., 1973, Tectonic Hsu, K. J., and Ryan, W.B.F., 1972, Summary of America Bull., v. 85, p. 1195-1204. summary of Leg 18, in Kulm, L. D., von the evidence for extensional and compres- Minato, M., Gorai, M., and Hunahashi, M., Huene, R., and others, 1973, Initial reports sional tectonics in the Mediterranean, in eds., 1965, The geologic development of the of the Deep Sea Drilling Project, Vol. 18: Initial reports of the Deep Sea Drilling Proj- Japanese Islands: Tokyo, Tsukiji Shokan, Washington, D.C., U.S. Govt. Printing ect, Vol. 13: Washington, D.C., U.S. Govt. 442 p. Office, p. 961-976. Printing Office, p. 1011-1019. Moore, G. F., Caldwell, J. G., and Karig, D. E., von Huene, R., and Shor, G. G., Jr., 1969, The Kanamori, H., 1972, Tectonic implications of 1975, Comment on 'Steady-state structure and tectonic history of the eastern the 1944 Tonankai and the 1946 Nankaido trenches?': Geology, v. 3, p. 221-222. Aleutian Trench: Geol. Soc. America Bull., earthquakes: Physics Earth and Planetary Moore, J. Casey, 1972, Uplifted trench sedi- v. 80, p. 1889-1902. Interiors, v. 5, p. 129-139. ments: Southwestern Alaska-Bering shelf White, W. S., 1949, Cleavage in east-central Kanamori, H., and Tsumura, K., 1971, Spatial edge: Science, v. 175, p. 1103-1105. Vermont: Am. Geophys. Union Trans., v. distribution of earthquakes in the Kii Penin- 1973, Complex deformation of Cretaceous 30, p. 587-594. sula, Japan, south of the Median Tectonic trench deposits, southwestern Alaska: Geol. Wilson, G., 1961, The tectonic significance of Line: Tectonophysics, v. 12, p. 327—342. Soc. America Bull., v. 84, p. 2005-2020. small scale structures and their importance Karig, D. E., 1974, Evolution of arc systems in 1975, Selective subduction: Geology, v. 3, to the geologist in the field: Soc. Geol. Bel- the western Pacific: Ann. Rev. Earth and p. 530-532. gique Annales, v. 84, p. 423-548. Planetary Sci., v. 2, p. 51—76. Moore, J. Casey, and Geigle, J. E., 1974, Slaty Wood, D. S., 1974, Current views on the de- 1975, Basin genesis in the Philippine Sea, in cleavage: Incipient occurrences in the deep velopment of slaty cleavage: Ann. Rev. Karig, D. E., and others, Initial reports of sea: Science, v. 183, p. 509-510. Earth and Planetary Sciences, v. 2, the Deep Sea Drilling Project, Vol. 31: Nelson, C. Hans, and Kulm, L. D., 1973, Sub- p. 369-402. Washington, D.C., U.S. Govt. Printing marine fans and deep-sea channels: Soc. Office, p. 857-879. Econ. Paleontologists and Mineralogists Karig, D. E., and Sharman, G., 1975, Accretion Short Course: Turbidites and deep water MANUSCRIPT RECEIVED BY THE SOCIETY and subduction in Pacific trenches: Geol. sedimentation (lecture notes), p. 39—79 FEBRUARY 24, 1975 Soc. America Bull., v. 86, p. 377-389. (available from SEPM). REVISED MANUSCRIPT RECEIVED SEPTEMBER 2, Karig, D. E., Ingle, J. C., and others, 1975, Site Piper, David J. W., von Huene, Roland, and 1975 report, site 298, in Initial reports of the Duncan, John R., 1973, Late Quaternary MANUSCRIPT ACCEPTED NOVEMBER 11, 1975

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