Subduction and Accretion in Trenches DANIEL E. KARIG Department of Geological Sciences, Cornell University, Ithaca, New York 14850 GEORGE F. SHARMAN III Scripps Institution of Oceanography, P.O. Box 109, La Jolla, California 92037 ABSTRACT INTRODUCTION the associated trench, and plotted at similar horizontal and vertical scales. These Although the reality of subduction has One of the more controversial aspects of profiles, integrated with available seismic been greatly strengthened by recent inves- plate tectonics has been the assumption that reflection profiles and other geophysical tigations, there is little information dealing large-scale subduction of oceanic litho- data, provide insights to the variations in with the mechanisms by which material is sphère takes place along trenches. Early in- subduction-zone structures which were not subducted or accreted to the upper plate. terpretations of gravity data (Worzel and obvious from analysis of scattered separate An attempt to determine the gross evolu- Shurbet, 1955) and of seismic reflection data sets presented at different scales and tion of subduction zones has been made, as- profiles (Scholl and others, 1968; von vertical exaggerations. The basic assump- suming that geographic variations in mor- Huene and Shor, 1969) have been cited as tion utilized in our analysis is that mor- phologic and geophysical characteristics of evidence precluding underthrusting in phologic variations among the arc systems trenches can be transformed into temporal trenches (Beloussov, 1970; Carey, 1970; can be interpreted in terms of evolutionary trends. Deformation associated with sub- Meyerhoff and Meyerhoff, 1972). How- trends. duction extends across the lower trench ever, these interpretations have been shown slope, from the trench axis to the trench- to be erroneous by more recent results using TECTONIC FRAMEWORK OF slope break. This region is a rising tectonic these same techniques (Grow, 1973a; SUBDUCTION ZONES element, but the upper slope is a subsiding Holmes and others, 1972; Beck, 1972) and region of sediment accumulation. An upper by data from the Deep Sea Drilling Project The uniformity of geological and slope discontinuity separates this zone of (Kulm and others, 1973b; Ingle and others, geophysical features among the various subsidence from the rising frontal-arc 1973, 1975), as well as by the results of island-arc systems has often been noted block. Examination of very young trenches earthquake seismological studies (Isacks (Hess, 1948; Umbgrove, 1947) and has indicates that the upper-slope discontinuity and Molnar, 1971; Barazangi and others, since been amplified (Karig, 1971a; Karig marks the upper section of the continental 1972). and Mammerickx, 1972; Grow, 1973a, or insular slope that existed before a sub- It is now apparent that the oceanic 1973b; Dickinson, 1973b). A generalized duction pulse began. As material is fed to lithosphere is subjected to large-scale un- terminology (Karig, 1970, 1974b) based on the subduction zone, the distance between derthrusting at trench axis and that some of this uniformity is utilized, for the most part, the upper slope discontinuity and the trench the uppermost material is transferred from in this paper (Fig. 2). Modification is re- increases, and an accretionary prism de- the lower to the upper plate in the process. quired chiefly in the slope area between the velops, but its shape depends on the relative In some trenches, tectonic erosion of the crest of the frontal arc and the trench, rates of sediment feed from the arc and upper plate during subduction has been where the greatest variability within the arc from the offshore basin. suggested (Scholl and others, 1970); but system is observed. The lower boundary of the accretionary this process is neither well documented nor The slope between the trench and frontal prism is the upper section of the seismic necessary (Karig, 1974a). arc, in all but very young trenches, consists zone, which apparently widens and flattens Accretion of oceanic sediment and deeper of two sections (Fig. 3). The upper section is as one or more accretionary prisms ac- crustal material onto the upper plate during relatively smooth, reflecting a little- cumulate. The sediment cover on the subduction has been postulated using sev- deformed sediment cover, and contrasts downgoing plate and some of the igneous eral different approaches (Dewey and Bird, with a steeper, less regular lower section crust appears to be stripped off the plate be- 1970; Gilluly, 1972; Moore, 1973; Burk, where sediments are either deformed, fore it reaches a point beneath the volcanic 1965; Hamilton, 1969; von Huene, 1972), acoustically unresolvable, or absent. The chain. Turbiditic sediments deposited in the but as yet the variations in the accretionary two slope sections are separated by a ridge, trench axis are preferentially sheared off the process and in the resultant geologic struc- bench, or slope break, which, because of its underlying pelagic sediments and are ac- tures remain almost completely unknown. variable aspect, has received a number of creted to the lower trench wall. The pelagic In large part, this void results from a lack of designations. In the Indonesian arc system, sediments and crustal material are probably data from the inner trench wall where where it forms a ridge that occasionally accreted at deeper structural levels. accretion is suspected to occur. Acoustic breaks sea level, this boundary has been Where turbidites overlie pelagic sedi- methods cannot adequately resolve the termed the tectonic arc (Vening-Meinesz, ments in the trench axis, the turbidites are geomorphic detail or the internal structure 1964), outer arc (Umbgrove, 1947), or stripped off in fold packets with axial sur- of the constituent rocks, and the conven- nonvolcanic outer arc (Van Bemmelen, faces having very low dips. These dewa- tional bottom sampling too often collects 1949). The basin thus formed in the upper tered and rigidified structural units move up only recent sediment overlying the accreted slope area has been referred to as the inter- the lower slope, as subsequent packets are material. deep (Van Bemmelen, 1949). In some areas, accreted. In trenches that subduct litho- This paper reports the results of a sufficient sediment has been deposited be- sphere carrying very thin pelagic sediment broader scale approach to the problem of hind the boundary to fill the upper slope covers, accretion and uplift of crustal slabs accretion at subduction zones. Digitized basin and to produce topographic benches, seem to occur as topographic irregularities bathymétrie profiles across almost all which have been called terraces (Gates and enter the trench. Key words: marine tec- Pacific and Indonesian trenches (Fig. 1) Gibson, 1956) or deep-sea terraces tonics, island arcs, subduction, trenches. were computer-reoriented, perpendicular to (Hoshino, 1969; Tayama, 1950). Recogni- Geological Society of America Bulletin, v. 86, p. 377-389, 8 figs., March 1975, Doc. no. 50314. 377 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/377/3433679/i0016-7606-86-3-377.pdf by guest on 25 September 2021 Figure 1. Index map of the Indo-Pacific region, showing profiles used in this study. Those used and Navy digitized sounding lines deposited in the S.I.O. data bank, along with a few hand re- in other figures in this paper are shown by heavy lines. These are oriented perpendicular to the oriented profiles from references cited in the text. Sediment thicknesses are from the same sources trench and at 20x vertical exaggeration, and are located on Figure 1 by small letters. Other sym- and from Initial Reports of the Deep Sea Drilling Project. bols are shown on the explanation. Profiles are from Scripps Institution of Oceanography (S.I.O.) i Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/377/3433679/i0016-7606-86-3-377.pdf by guest on 25 September 2021 SUBDUCTION AND ACCRETION IN TRENCHES 379 Inactive Remnant Marginal Basin Arc x Outer Swel Z Trench Wedge Upper Slope -REAR Discontinuity FRONT V. E.= 5x -500 -400 -300 -200 -100 -50 • 50 f 100 • 200 KILOMETERS Figure 2. Cross section of a typical island-arc system, showing tectonic units and terminology used in this paper (revised after Karig, 1970, 1971a). tion of the variability of the morphology in and others, 1973b; Creager and others, Japan and eastern Aleutian arc systems, the this two-part slope has led to the more 1973). This zone of deformation between upper-slope discontinuity is a fault zone generalized definitions of midslope base- the trench-slope break and the trench axis (von Huene and others, 1971) or a steep ment high (Karig, 1971a) and "slope limits and probably defines the surface ex- contact without significant morphologic break" (Dickinson, 1971). A nongenetic pression of the subduction zone. expression (Fig. 3). Arc systems having a term, "trench-slope break," has been gen- The rocks dredged or cored from beneath deeper upper-slope trough or apron (such erally agreed upon to cover all aspects of the surficial sediments of the inner trench as the central Aleutians, Marianas, and the boundary between slope sections (Dick- wall are identical or similar in lithology to Luzon systems) display the upper-slope dis- inson, 1973 a) and is used here. those at the trench axis or on the oceanic continuity as the downward extension of a crust (Kulm and others, 1973b; von Huene, steeper upper-slope section, along which Trench Slope Break and Lower Slope 1972; Fisher and Engel, 1969; Hawkins the sediments are faulted, strongly flexed, and others, 1972; Ingle and others, 1973). or lapped against the frontal arc (Fig. 3; Direct observations of the trench-slope Basement rocks on the Mentawai Islands Grow, 1973a; Ludwig and others, 1967). break are restricted to the few areas where and on Barbados can also best be inter- The upper-slope discontinuity in the New the feature emerges as islands. The best ex- preted as having originated in those areas.