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Tectonic erosion along the Japan and
Peru convergent margins
R. VON HUENE U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025, and GEOMAR, Kiel, West Germany
S. LALLEM AND Université Pierre et Marie Curie, Paris, France
The volume of material removed by subduction erosion can be estimated quantitatively if the position of the volcanic arc, the
position of the paleotrench axis, and a paieodepth reference surface are known. Estimates
based on these parameters along the Japan
and Peru Trenches indicate rates of erosion
comparable to well-known rates of accretion.
Proposed erosional mechanisms along the
plate boundary, where horsts on the lower
plate abrade the upper one, appear insufficient to handle the minimum volumes of
eroded material. Some mechanisms of tectonic erosion at the base of the trench slope
can be observed at colliding seamounts and
ridges where structures are large enough to
be seismically imaged. Local tectonic erosion
of the lower slope of the Japan Trench resulted when seamounts entered the subduction zone, uplifted the slope, and oversteepened it. The oversteepened slope failed, debris
slumped into the trench axis, and much of it
was then subducted. Where a seamount was
subducted, a large re-entrant was left in the
slope, which filled rapidly by local accretion
of abundant sediment. Subduction of the
oblique-trending Nazca Ridge off Peru produced many similar structures. Erosion is
dominated by uplift and breakup of the lower
slope, with subduction of the debris rather
than abrasion under high-stress conditions.
Modern convergent margins are commonly
associated with the accretion of sediment from
the subducting lower plate to the upper plate,
whereas erosion of the upper plate has received
much less attention. The difference in attention
may result from the difficulty in resolving erosional structures in seismic records. Tectonic
foreshortening of the lower slope of a trench has
been reported and is explained by lateral translation of terranes or by erosion of the upper plate
through piecemeal subduction. The latter has
been called "subduction erosion" (Scholl and
others, 1980). In this paper, we examine subduction erosion but begin with a review of evidence
for the subsidence of the Japan and Peru margins and discuss why it probably originates from
the tectonic erosion of the upper plate. We then
estimate quantities of materials removed along
these margins and find rates of erosion that are
similar to rates of accretion; these rates exceed
those that can be accommodated by the horst
and graben chain-saw model (Hilde, 1983). To
explain these high rates of erosion, we reexamine some mechanisms of subduction erosion and illustrate morphologies and structures
indicative of erosional processes.
Another form of tectonic erosion occurs
along the base of the upper plate. Its magnitude is indicated by massive subsidence along
the margin; however, because of deep burial,
the structure resulting from basal erosion is
rarely imaged in seismic records. The volume
of material eroded along the base of the upper
plate exceeds that eroded from the front of
the lower slope.
Tectonic erosion along modern convergent
margins was first demonstrated convincingly
when Deep Sea Drilling Project (DSDP) drilling penetrated ancient rocks near the trench axis.
The study of drill cores from the Japan (Legs 56
and 57), Middle America (Legs 66,67, and 84),
and Peru (Leg 112) Trenches showed nonaccretion or Neogene accretionary complexes from
10 to 15 km wide stacked against a buttress of
Mesozoic and Paleozoic consolidated or metamorphosed rocks. The older rocks observed in
DSDP cores required tectonic erosion of the
missing rock that once covered or extended
seaward from them.
Geological Society of America Bulletin, v. 102, p. 704-720, 11 figs., 1 table, June 1990.
Both tectonic accretion and tectonic erosion
occur along the lower slope of convergent margins and along the underside of the upper plate.
Erosion along the lower slope of a convergent
margin as indicated by landward retreat of the
trench slope is herein called "frontal erosion."
Erosion farther landward along the base of the
upper plate inferred from general margin subsidence during convergence is herein called "basal
Regional subsidence during the latest period
of plate convergence was established during the
DSDP program along the Japan Trench margin
(von Huene, Nasu, and others, 1978) by drilling
through a subaerial erosion surface many kilometers below sea level. That erosion surface
corresponds to an angular unconformity that
cuts across tilted beds and is buried beneath
subhorizontal strata of the outer shelf and slope
(Fig. 1). The unconformity extends throughout a
150-km-long area (Nasu and others, 1980; von
Huene and others, 1982) and shows no signs of
ending beyond, the published seismic coverage.
A similar unconformity is recorded from the adjacent Joban Basin area to the south (Mitsui,
1971; Kato, 1980), from the southern end of the
Japan Trench (Lallemand and others, 1989),
and from north of the Japan Trench along the
Kuril Trench (S. Lallemand and R. von Huene,
unpub. data). Across the unconformity, seismic
velocities increase abruptly from —1.9 to 4.2
km/s (Murauchi and Ludwig, 1980), consistent
with the contact between unconsolidated Oligocene to Quaternary strata and well-consolidated
Cretaceous rock as drilled at DSDP Site 439.
The sedimentary strata above the unconformity
consist of a 48-m-thick breccia and conglomer-
ate of dacite and rhyolite boulders, covered by
50 m of medium-grained sand containing
abundant little-transported macrofossils, which
was in turn buried by silt and sand turbidites
(Scientific Party, 1980) with a probable seaward
source (von Huene and others, 1982). The upper
800 m of the section consists of Miocene diatomaceous mud. The regional extent of rock types
and erosion was explained by subsidence of a
landmass during the past 22 m.y. (Scientific
Party, 1980; von Huene and others, 1982).
Benthic microfossils from the sediments indicate a succession of water depths (Fig. 2) con-
sistent with such a history (Arthur and others,
1980; Keller, 1980). Despite questioning the regional extent of such subsidence from drilling
results on DSDP Leg 87 (Karig, Kagami, and
others, 1983), the depth-versus-age relations observed in samples from DSDP Site 584 (Lagoe,
1986) are consistent with those from DSDP
Sites 438/439, located about 50 km landward
(G. Keller, 1988, personal commun.).
Geophysical and drilling studies along the
Peruvian margin have recorded a similar geology. A continuous unconformity from the shelf
across the upper and middle slopes is revealed in
multichannel seismic reflection records covering
an -600-km-long stretch of the margin (Hussong and Wipperman, 1981; Ballesteros and
others, 1988; von Huene and Miller, 1988;
Moore and Taylor, 1988). At the edge of the
shelf, drilling and sampling have produced crystalline basement below the unconformity and
sandy Eocene strata containing shallow-water
megafossils above it (Kulm and others, 1988).
Beneath the continental slope, the Eocene strata
above the unconformity also contain shallowwater microfauna (Sites 682, 683, and 688;
Suess, von Huene, and others, 1988a; Resig,
P r a f a r ú n n c
slump debris
top of
E 8>
top of
ocean crust
normal faults
trench fill and
slump debris
20 km
VE = 2
Figure 1. Line drawings of seismic sections across the Japan Trench (ORI78-4) after von Huene and Culotta, 1989) and Peru Trench (CDP-1
after von Huene, Suess and others, 1988), illustrating major tectonic features. Japan Trench stratigraphy was followed from DSDP sites
landward of the seismic image shown here. The average subsidence over 22 m.y. is shown by arrow, and the dashed line represents the
minimum seaward extent of the 22-m.y. continental slope. The Peru Trench stratigraphy is known from ODP sites (numbered), and the rates of
subsidence and paleotopography of the margin at 5 Ma are indicated. Note three unconformities.
I » S u b a e r i a l exposure
Figure 2. Subsidence history at Sites 438/439, based on benthic
foraminiferal stratigraphy (after Keller, 1980, and Arthur and others,
1980). Vertical bars indicate the depth ranges of benthic foraminiferal
assemblages from Site S84; arrows indicate depths greater than 2,000
m (Lagoe, 1986).
Considerable normal faulting is observed in
seismic records, SeaBeam bathymetry, and cores
across both the Japan and Peruvian margins.1
Vertical displacement rarely exceeds 100 m, and
the structural pattern is commonly like the
stacked domino model where blocks about 0.5
to 1.5 km wide are separated by faults that offset
the entire sediment section and the underlying
unconformity (Fig. 3). DSDP Sites 438/439 on
the Japan margin are in a faulted area, and
below the 400 m level, the cores contain many
microfaults; however, thinning of individual
beds by pervasive microfaulting is not noticeable
in seismic records when the faulted segments are
compared with adjacent unfaulted segments.
1989). Rocks below the unconformity have
a velocity of 5 km/s, indicating the same
crystalline basement sampled at the edge of the
shelf (Hussong and Wipperman, 1981), and
above are stratified rocks with velocities around
2 km/s.
Conventional sampling of the middle slope
yelded samples containing late Miocene microfauna from shallow environments indicating
subsidence (Kulm and others, 1984). At Ocean
Drilling Program (ODP) drill Sites 683 and
688 on the lower slope, an upper Miocene upwelling facies was recovered (Suess, von Huene,
and others, 1988b). This depth-sensitive facies
presently accumulates in water shallower than
500 m.
The litho- and biostratigraphy of the Peru
margin indicate subaerial erosion of the crystalline basement, followed in Eocene time by deposition in shelf and upper-slope environments.
The Eocene sedimentary sequence was again
eroded in Oligocene and early Miocene time,
followed by regional subsidence since early and
middle Miocene time. Part of that regional subsided surface was uplifted before and during
subduction of the Nazca Ridge, and beneath the
Lima Basin, it has subsided since late Miocene
time (von Huene, Suess, and others, 1988).
A subaerial rather than submarine origin for
these unconformities is supported by differences
_ l
from disconformities commonly mapped in
seismic stratigraphic studies. Along the Japan
and Peru Trench margins, the erosional unconformity is a regional feature extending thousands
of kilometers beneath the shelf and lower slope.
The rock truncated along the unconformity is
hard and resistant to erosion; truncated beds
have an apparent thickness of at least 1 km, and
the metamorphosed and crystalline rock sampled requires the removal of a thick overburden.
The older rock below the unconformity is separated from the younger rock above it by a hiatus
spanning many cycles of change in sea level.
Post-erosional sediment on the unconformity is
conglomerate and sandstone of a near-shore
shallow-water lithofacies, consistent with the
enclosed biofacies upon which mudstone from
successively deeper-water environments was
deposited. These observations are most easily
explained by subaerial and surf-zone erosion followed by regional subsidence to present levels.
On the slope, where the sediment section is
flexed downward toward the trench, the faults
become more numerous. Normal faults between rotated blocks 300-500 m wide also
occur in the stacked domino configuration
(Leggett and others, 1987). At DSDP Site 584,
the stratigraphy in 3 holes drilled 0.5 to 0.75
km apart corresponds with general seismic
structure only if many normal faults between
the holes are inferred (Karig, Kagami, and
others, 1983). The structure imaged with
further processing of the seismic record since
DSDP drilling supports that interpretation
(R. von Huene and J. Miller, unpub. data).
The number of small faults increases toward
the mid-slope until coherent reflections are no
longer resolved with the seismic techniques
used. Much of the normal faulting on the
slopes is thought to represent slope failure
driven by gravity tectonics (von Huene and
Culotta, 1989). The structural pattern seen in
seismic records off Japan is consistent with
structures at the smaller scale of drill cores
from the Peruvian margin. Extensional structures on the Peruvian shelf are associated with
abundant dewatering features, subsidence, and
local extension, whereas on the slope, struc-
Subsidence of a continental margin could
result from a change in the configuration of the
Benioff zone or from crustal thinning. The
mechanisms commonly invoked as causes for
crustal thinning are (1) listric normal faulting
and (2) subcrustal erosion.
'For Japan, compare Nasu and others, 1980; von
Huene and others; 1982; Kagami, Karig, and others,
1986; Leggett and others, 1987; Cadet and others,
1987; von Huene and Culotta, 1989; for Peru, compare Hussong and Wipperman, 1981; von Huene and
others, 1985; Thornburg, 1985; Bourgois and others,
1986, Ballesteros and others, 1988; Moore and Taylor,
1988; von Huene and Miller, 1988.
Figure 3. Seismic record Shell P-849 (Lehner and others, 1983), reprocessed at the U.S. Geological Survey, showing the unconformities cut
across landward-dipping Cretaceous strata and the overlying upper Oligocene to Quaternary section cut by normal faults. Vertical exaggeration
at sea floor, 2.5x.
ture is commonly related to gravity sliding
(Kemp and Lindsley-Griffin, 1989).
Crust thinned by normal faults is exemplified in the Basin and Range province of western North America and along some passive
margins of the Atlantic (compare Wernicke
and Burchfiel, 1982; Beach and others, 1987;
Sibuet and others, 1987). Boillot and others
(1987) have advanced a simple shear interpretation of the seismic reflection data and results
from deep ocean drilling along the Galicia
margin where half-graben blocks are 13 to 18
km wide in the upper slope and 9 to 13 km
beneath the lower slope. Faults displace the
ocean crust from 4 to 7 km, and the possible
plane of horizontal detachment is 2 km above
the Moho (Sibuet and others, 1987). By comparison, the greatest normal fault displacement imaged along the Peruvian margin
offsets an 11-km-wide block 880 m along a
steep fault that is not imaged through the
upper plate (Bourgois and others, 1988; von
Huene and others, 1989). Some faults not so
clearly imaged on the Japan margin could involve displacements of several hundred meters
(von Huene and Culotta, 1989).
Extensional structures along the Japanese
and Peruvian margins are much smaller than
the extensional structures associated with
crustal thinning in the Basin and Range or on
passive margins. The Japanese and Peruvian
margins have 150-km-wide areas without
major extensional structure that have subsided
3 and 4 km (Fig. 3). In such areas, crustal
thinning by listric faulting in a simple shear
system is not possible. In areas of steeper topography on the landward slope of the trench,
the abundant small normal faults may thin the
sediment section; however, they are not sufficient to explain the 4 to 6 km of subsidence
observed there.
Changes in the configuration of a subduction zone may be caused by loading as the
accretionary prism grows (Karig and others,
1976) or as the subducted oceanic crust
changes density (Langseth and others, 1981).
Because a relative negative buoyancy of the
descending cold dense oceanic lithosphere is
a main force configuring the subduction zone,
changes in the age (temperature), the rate
and direction of convergence, and the thickness of the subducting lithosphere are the
most likely causes of change in subduction
zone configuration.
The plate-tectonic history of the Japanese
margin contains little change in age of the
subducting crust, the relative rate of plate convergence, or the subduction of major features
on the oceanic lithosphere during the past 22
m.y., as discussed below.2 On northern Honshu, the position of the volcanic front has re-
The Japan triple junction remained south of the
area of observed subsidence (Jolivet and others,
mained within a belt the width of which is
12% of the arc-trench distance since 22 Ma
except for a single extrusion at the southern
end of the trench near the triple junction
(Tsunakawa, 1986). Furthermore, the retreat
of the Japan Trench margin would locally unload the lower plate, causing it to rise rather
than subside.
A major plate-tectonic change observed along
the Peruvian margin is the subduction of the
Nazca Ridge. Subduction of the ridge was concurrent with the change from erosional to accretionary tectonics along the northern transect of
ODP Leg 112 studies (Suess, von Huene, and
others, 1988a). The uplift and subsidence of the
Lima Basin correlates approximately with the
subduction of the leading and trailing ridge
flanks, respectively, based on plate reconstruction (Cande, 1985) and drilling results (Suess,
von Huene, and others, 1988a). The adjacent
Trujillo and Salaverry Basins show little stratigraphy in seismic records (Thornburg, 1985) or
cores to indicate a period of uplift and subsidence during ridge subduction. Only Lima Basin
shows the subsidence. Morphology in the present area of ridge subduction shows topographic
expression of uplift or subsidence only along
the lower continental slope (Prince and others,
1980). The depth of the Benioff zone just
landward of Lima Basin decreased after subduction of the ridge (Boyd and others, 1984) in
a vertical sense opposite to subsidence of
the basin.
Although the 8 Ma to Quaternary subsidence
of Lima Basin was local, subsidence of the 21- to
22-m.y. surface (Suess, von Huene, and others,
1988a) cut on Eocene and Oligocene strata extends through all forearc basins off Peru. From
the latitude of the Nazca Ridge to the Ecuadorian border, plutonic activity from middle Miocene to early Pliocene time appears to have
stayed in a single belt (Sillitoe, 1988). The lack
of major plate changes, shifts in the volcanic-arc
position, and decrease rather than an increase in
depth of the Benioff zone suggest little direct
influence on subsidence from changes in the
configuration of the descending slab.
Frontal Erosion
Frontal erosion is illustrated in a seismic record across the Japan Trench (Fig. 1). A pervasive 1-km-high scarp in the lower slope (Cadet
and others, 1987) produces debris that has accumulated at its base. This debris is accreted like
the sediment transported into the trench (von
Huene and Culotta, 1989), but most of the de-
bris and trench sediment must be subducted because accretion has not piled the sediment
sufficiently high to stabilize the slope. An indication of the entrainment of debris from mass
wasting into the subducted sediment is the 300to 400-m thickening of the underthrust sediment
layer as it passes landward of the trench axis
(Fig- 1).
Frontal erosion is also required to explain the
structure seaward of the Lima Basin. The lithoand biostratigraphy established during ODP Leg
112 along the Peruvian margin (Suess, von
Huene, and others, 1988a) indicate a subaerial
erosion surface beneath the Eocene and Oligocene shelves, and because the Eocene shelf is
now near the trench axis, erosion of the missing
Eocene trench slope is required. That erosion
occurred during the period of plate convergence
when the present Andes were formed. The end
of frontal erosion along the lower slope north of
Lima Basin was dated during Leg 112 at Site
685, and it corresponds in time to the subduction
of the Nazca Ridge.
Basal Erosion
An erosional origin is proposed for truncated
bedding seen 15 to 30 km down the Japan
Trench subduction zone in a reprocessed seismic
record (von Huene and Culotta, 1989). Above
the layer of subducting sediment are landwarddipping beds truncated at the décollement surface (Fig. 1). These beds crop out at the sea
floor, where dredging produced samples of rocks
lithologically equivalent to the Cretaceous silicified mudstones drilled at DSDP Site 439. Positive age equivalence was not established because
the dredge samples lacked age-diagnostic fossils,
but radiometric analysis of micas indicates an
age in excess of 28 m.y. (Takigami and Fujioka,
1989). Because the small accreted wedge along
the Japan Trench is 20 m.y. younger than this
minimum age where drilled (Scientific Party,
1980) and is lithologically different from the
dredged rock, the best correlation is with the
Cretaceous or perhaps an unsampled Paleogene
basement rock of this margin.
Another indication of basal erosion is seen
between the Peru Trench and the Lima Basin
(Fig. 1). Eocene strata of shallow-water bio- and
lithofacies drilled at Site 688 cover rock that has
seismic velocities of the crystalline basement.
These Eocene rocks can be followed down a
seaward-dipping incline and are now truncated
at the décollement along the plate boundary.
Such a structure requires the removal of considerable underlying crystalline basement (Fig. 1).
100 km
The amount of material removed during erosion of the Japan and Peru margins was estimated by reconstructing their former configuration and comparing it with their present one.
The effects of large thermal changes were circumvented by selecting a former configuration
when the volcanic arc was already in its present
position. The parameters involved in the reconstruction are shown in Figure 4, where dashed
lines indicate the past, and solid lines the present,
configurations of the margin.
Figure 4. Diagram illustrating parameters from which a quantitative estimate of subduction
erosion and retreat of the slope was made. S indicates the depth of subsidence across a margin
as indicated by biostratigraphy and other measures of past water depth. Pb is the paleobathymetric profile reconstructed from stratigraphic geometry, such as the downlap on an unconformity observed in seismic records. Rt is the distance that the trench slope retreated landward
during erosion. The paleoslope is reconstructed from the seawardmost point (point A) at which
a paleodepth can be established. A paleoslope is projected to the paleotrench depth from point
A, using the present slope angle under the assumption that the ancient critical-wedge angle was
similar to that along the modern trench landward slope. Ps is the paleoslab, and its profile is
approximated by joining the paleotrench axis to a 100-km-deep point beneath the paleoarc.
The Magnitude of Frontal Erosion along the
Japan Trench Margin
We estimated erosion of the Japan Trench
margin in the period since opening of the Sea of
Japan. DSDP drill cores indicate that the beginning of explosive arc volcanism was in early
Miocene time (20 Ma) (Cadet and Fujioka,
1980). Togashi (1983) reported a shift to islandarc composition of the volcanic rocks on northern Honshu after 20 Ma. A sudden change in
Figure 5. Cretaceous and Jurassic magnetic lineations of the northwest Pacific west and north of Shatsky Rise. For simplicity, only each fifth
isochron is shown. Sources of data are Sager and others (1989), dark numbers and solid lines; Hilde and others (1976), light numbers and dotted
lines. C.Q.M.Z., Cretaceous quiet magnetic zone; K.K.T., Kuril-Kamtchatka Trench; J.T., Japan Trench; I.B.T., Izu-Bonin Trench; N.T.,
Nankai Trough; PT, paleotrench; M.T., Mariana Trench; E. smt, Erimo Seamount; D.K. smt, Daiichi Kashima Seamount. Thin-shafted arrow
indicates the amount and direction of convergence since 20 Ma.
rate of rotation of the Oga Peninsula after 20
Ma is observed in studies of paleomagnetism
(Tosha and Hamano, 1988). From these tectonic and volcanic events, we assume that the
present arc-trench system began about 20 Ma.
Oceanic magnetic lineations along the Japan
Trench (Fig. 5) trend N65°E; the youngest
is M-8 (129 Ma; Kent and Gradstein, 1985),
and the oldest is probably anomaly M-15
(140 Ma) (Sager and others, 1989). Thus, the
average age of the present subducting lithosphère is 135 ± 5 m.y.
From 0 to 5 Ma, the convergence of the Pacific and Eurasian plates off Japan was 114
km/m.y. at 288°, and from 5 to 28 Ma, it was
Figure 6. The present and the reconstructed landward slope of the Japan Trench, based on parameters developed from observations to show
the trench retreat for different values of paleotrench depths. From point A, a paleoslope (Rt) is projected to depth Pt (see Fig. 4). The present
slope angle was used for the projection, assuming that the missing materials and wedge angle were similar to those along the present trench
landward slope. VE = 2x.
94 km/m.y. at 281°-282° (Engebretson and
others, 1985). We inferred a paleotrench 100
km oceanward of the present trench axis (developed below).
To establish the depth of the paleotrench, it is
necessary to backtrack the subducted lithosphere and reconstruct its age 2,400 km west of
the 135 m.y. isochron. As the left-lateral transform offset along the southeast-trending fracture
zones is subducted, the magnetic lineations west
of the Japan Trench axis become younger (Fig.
5). The lack of the history of transform motion
on the subducted oceanic crust leaves few constraints on the age of the oceanic crust much
beyond the 83 m.y. isochron. Thus, this isochron
yields a minimum backtracked crustal age of 61
m.y. at 20 Ma.
Hilde and Uyeda (1982) showed an empirical
relation between the age of the subducting lithosphere and trench depth. The present Japan
Trench is 3.5 km shallower than predicted, despite the conformity of the adjacent ocean
lithosphere to the subsidence curve of the Pacific
basin off Japan (Parsons and Sclater, 1977;
Heestand and Crough, 1981). We assume that at
20 Ma, the trench depth was consistent with a
range of 8 ± 1 km in the depths where 61-m.y.
or older crust is being subducted. Thus, the paleo-Japan Trench depth minimum is 7 km.
With this information, we estimated a retreat
of the Japan Trench slope since 20 Ma. The
subaerially eroded Paleogene unconformity was
followed in seismic records to point A (Fig. 6).
The present depth of point A along seismic line
ORI 87-4 and Shell P-849 is approximately
6,400 and 5,625 m below sea level, respectively
(von Huene and others, 1982; von Huene and
Culotta, 1989) and is located only 15 km from
the present trench axis in both lines. Along seismic line JNOC-2, point A is imaged 30 km
from the trench axis at a depth of 5 km (Nasu
and others, 1980) and was subaerial until about
16 Ma3 (Nasu and others, 1980). If an average
continental slope is assumed, dipping 5° seaward to the trench axis from point A on seismic
line ORI 78-4, the result is a retreat (Rt minimum) of 75 km in 20 m.y. or about 3 km/m.y.
(Fig. 6). The same calculation using the parameters from JNOC-2 gives a minimum retreat
of 50 km in 16 m.y. or also about 3 km/m.y.
This rate of retreat is nearly twice as great as the
conservative estimate made previously (von
Huene and others, 1982).
The Peru Trench Margin
The same approach to estimate retreat was
applied to the Peru Trench margin in the area of
Lima Basin for two time periods. The paleoconfiguration of the Peru margin was first constructed from the regional unconformity that
subsided since 20 Ma as shown by ODP Leg 112
drill cores. Seismic data indicate that the unconformity on top of the Eocene continues to its
termination near the trench axis (Figs. 1 and 7).
Sediment overlying that unconformity was
again eroded during the subduction of the Nazca
Ridge. The younger unconformity was dated at
site 679 by a hiatus between 8 and 11 Ma. The
subduction of the Nazca Ridge at this latitude is
The age of final inundation of the paleolandmass
was estimated at 10 m.y. (von Huene and others,
1982), but this age was revised to 16 m.y. based on
improved seismic imaging across the normal faults in
the west part of the area shown in Figure 3.
seen in the subsidence at Sites 682 and 688 after
5 Ma, which is consistent with the plate reconstruction of Cande (1985). From these two
dates, the reaction of the margin to subduction
of the Nazca Ridge appears more complex than
simple uplift and subsidence of a passive prism
over the subducting ridge, and we consider the
estimate of an erosional rate from the Sites 679,
682, and 688 paleobathymetry a possible maximum in the Lima Basin area.
Oceanic magnetic lineations along the Peru
Trench were backtracked to 20 Ma by Cande
(1985). Anomalies 30 and those older than 33
were in the trench axis opposite Lima Basin at 8
and 20 Ma, respectively (Fig. 8), resulting in a
trench depth of ~8 km (Table 1), in accord with
the trench depth/age relationship (Hilde and
Uyeda, 1982). In the Lima Basin area, point A'
was established from the seawardmost seismic
image of the Oligocene-Miocene unconformity
(Fig. 1). Point A was established from cores
from ODP Sites 683 and 688, located about 30
km landward of the present trench axis. At Sites
683 and 688, cores containing primary coastal
upwelling facies with 5-m.y.-old shallow-water
microfauna were recovered (Suess, von Huene,
and others, 1988b). Similar coastal upwelling
facies are presently deposited off Peru in water
no deeper than 350 m. Rates derived by inferring subsidence of the unconformity from sea
level during the past 5 m.y. are similar to those
required by the microfaunal assemblages in
dredged samples from Lima Basin (Kulm and
others, 1984). Point A was placed along the
seawardmost recovery location of primary upwelling deposits that were drilled at Site 688
(Fig. 7). The paleoslope was projected at the
average angle of the present lower slope to a
Lima Basin area
VE = 5.4
Figure 7. Cross sections of the Japan and Peru Trench
subduction zones, showing configurations used to calculate
amounts of material eroded. The volume of ocean crust that
compensates for removal of surface material is calculated
using the method of Karig and others (1976). The observations that constrain subsidence are from DSDP/ODP drilling
results and reprocessed seismic records showing extent of
subaerial erosion.
paleotrench depth of ~ 8 km (Table 1). An estimated 28-km retreat of the lower slope of the
trench in 8 m.y. gives an average rate of retreat
of about 3.5 km/m.y.; the 50-km retreat in 20
m.y. gives a rate of about 2.5 km/m.y. The latter
rate is a minimum, because it was estimated
without regard to the recent episode of accretion
and advance of the trench slope (Fig. 1) that
began about 3 Ma (von Huene, Suess, and others, 1988).
Basal erosion is a principal explanation for the
kilometers of Neogene subsidence along the
Japan and Peru margins. Subsurface erosion can
be computed from the amount of subsidence;
however, it requires a knowledge of isostatic
compensation, which has clear maximum and
minimum limits. If the lower plate in a subduction zone behaves rigidly, then the mass removed along the base of the upper plate is not
much more than subsidence of the sea floor. If,
on the other hand, the lower plate is compliant,
then high-density lower-plate material will replace and compensate for the lower-density
eroded mass. Therefore, at the one extreme, the
subsidence at the surface approximates the volume of material eroded, and at the other
extreme, it represents only a fraction of the
eroded material because the lower plate has isostatically risen to fill some of the space left by
subsurface erosion.
Isostatic compensation of lithosphère within
an oceanic plate has been modeled as a thin
elastic layer over a fluid (Watts and Steckler,
1979; Steckler and Watts, 1982; Karner and
Watts, 1982). Compensation is largely a function of lithospheric rigidity, which depends on
its age; the increased flexural rigidity of the
oceanic plate with increasing age is known
(Watts and Cochran, 1974; Watts, 1978; Watts
and others, 1980). The load of the island of
Oahu was compensated in less than 0.5 m.y.,
despite the Mesozoic oceanic crust on which it
was built (ten Brink and Watts, 1985). Thus,
isostatic compensation of oceanic lithosphere
appears to be rapid relative to the millions of
years considered here.
Along a convergent margin, the rate of isostatic adjustment appears to be similar. The
subducted part of Daiichi Kashima Seamount is
compensated by about 1,200 m of crustal depression in 0.2 m.y. of subduction (Lallemand
and others, 1989). The rate of compensation is
similar to compensation of continental litho-
Figure 8. Nazca-South America plate interactions since 20 Ma, after Cande (1985). Arrow shows the backtracked path of a point in Lima
Basin along the seismic line used for Figure 7. The locations of the ridge relative to South America at 5 and 10 Ma are plotted, and subducted
magnetic lineations are outlined, on the basis of the mirror image of the oceanic crust near Tuamotu Ridge.
Time of reconstruction: T
Trench depth
Subducting lithosphere age
Landward trench retreat: Rt
Distance trench-point A
Depth of point A
Mean dip, landward slope
Cross-sectional area of
subsidence = minimum
Peru (Lima Basin)
20 m.y.
8 ± 1 km
>61 m.y.
7.5 km
135 ± 5 m.y.
50 km
15 km
6.4 km
8 m.y.
8 km
66 m.y.
6 km
45 m.y.
8 km
35 km
4 km
0-0.35 m
20 m.y.
>8 km
>60 m.y.
6 km
45 m.y.
50 km
18 km
6.3 km
0 km
800 km 2
290 km 2
480 km 2
Assuming complete
isoslatic compensation
1,100 km 2
370 km 2
610 km 2
Estimated erosion rate
(40)-55 km 2 /m.y.
(361-46 km 2 /m.y.
(24)-31 km 2 /m.y.
Note: the rates of erosion in parentheses are derived from the area of subsidence alone without any compensation by the lower plate; thus, they are limiting
normal faults
partially filled
Figure 9. Perspective diagram of Daiichi Kashima and Katori Seamounts at a vertical exaggeration of 1.5*. Katori Seamount illustrates the
first stage of subduction, where the trench axis becomes constricted and normal faults begin to break up the seamount. Note filled re-entrant
from collision with a seamount at a prior time just opposite Katori Seamount. Daiichi Kashima was at the stage equivalent to that of Katori 0.1
to 0.2 Ma, and about half of the leading flank has been subducted. Above the subducted leading flank, the upper plate has been pushed up as
much as 1,000 m, and beneath the seamount, the ocean crust has been depressed about 1,000 to 1,400 m.
sphere during déglaciation; however, calculations of erosion that assume complete compensation yield high volumes of eroded material.
We use the model of a one-dimensional bending
of a thin elastic plate subjected to a hydrostatic
restoring force, developed by Karig and others
(1976), to estimate the deflection of the lower
Basal Erosion along the Japan
Trench Margin
The time stratigraphy of the DSDP drill holes
was extended into seismic records that show the
transgression of Neogene slope deposits over the
subaerial erosion surface. At Sites 438/439, the
subaerial erosion ceased and marine deposition
began in latest Oligocene time (Nasu and others,
1980). The shoreline transgressed eastward of
Sites 438/439 across a gentle slope (Fig. 3, right)
until about 16 Ma, when the last insular topographic highs at the edge of the shelf were overwhelmed (Fig. 3, left). The history of inundation, clearly observed across the landward flank
of the insular topography, is obscured in seismic
records on the seaward flank by tectonism, seafloor erosion, and great water depth. Therefore,
point A was placed on the landward flank and is
a minimum extent of the erosion surface. From
16 Ma to the present, only the average rate of
subsidence can be determined except at Sites
438, 439, and 548, where benthic foraminiferal
assemblages were recovered.
Erosion along the Japan margin is estimated
from a generalized crustal cross section (Fig. 7).
Due to isostatic adjustment, the configuration of
the descending slab adjacent to the trench axis
probably changed as the slope subsided and retreated, but the generally stationary volcanic arc
on northern Honshu during the past 20 m.y.
(Tsunakawa, 1986) suggests general stability of
the plate boundary. Subsidence of the Japan
margin during the past 20 m.y. has substituted
water for a cross-sectional area of 800 km 2 of
rock (Table 1). Isostatic compensation (Fig. 7)
increases the cross-sectional area of eroded
material to 1,110 km 2 . The average rate of erosion per million years is 55 km 2 (Table 1).
Transport of this volume of eroded material
down the subduction zone at the 100 km/m.y.
average rate of convergence requires that on average, a 550-m-thick layer of eroded material be
incorporated into the subducting sediment.
Basal Erosion along the Peru Trench
Regional subsidence of the Peru margin since
late Oligocene and early Miocene time is shown
by shelf and upper-slope biostratigraphic indicators (Suess, von Huene, and others, 1988a). Su-
perposed on the regional subsidence is the local
uplift and subsidence of Lima Basin over the
past 8 m.y., which is known with greater precision than is the regional subsidence. Subsidence
of the middle of Lima Basin occurred at a rate
of more than 0.5 km/m.y. over 8 m.y.; however, subsidence began after 5 Ma at Site 688.
We calculated the rate of erosion local to Lima
Basin over the past 8 m.y. and also erosion over
20 m.y.
The estimate of local erosion across Lima
Basin was made in the same manner as for the
Japan Trench margin. Restoration of the margin
at 8 Ma (Fig. 7 and Table 1) was controlled by
microfossil assemblages in drill holes (Suess, von
Huene, and others, 1988a) and by conventional
sampling (Kulm and others, 1984). The eroded
cross-sectional area is 370 km 2 , assuming compensation (Karig and others, 1976) for the
eroded mass. The estimated erosion rate is 46
km 2 /m.y., which requires subduction of a layer
of continental material 460 m thick.
The estimate of erosion in the past 20 m.y. is
calculated using the regional unconformity as a
sea-level reference surface. Point A' is the seaward end of the eroded middle Eocene strata
(Fig. 7). The eroded cross-sectional area is 610
km 2 , assuming compensation (Karig and others,
1976) for the eroded mass (Table 1). The 31
km 2 /m.y. erosional rates estimated for the 20m.y. period are about 65% of those estimated for
the local erosion of Lima Basin since 8 Ma, but
the latter are closer to those estimated along the
Japan Trench.
Our estimates of the amount of eroded material are surprisingly large, even those indicated
by the limiting minima (Table 1). These minima
are equivalent to the area of subsidence and unrealistic, because they fail to account for any
isostatic compensation. Retreat of the Japan and
Peru Trench margins occurred at similar or
greater rates than the advance from tectonic accretion at some other margins.4 Such rates of
subduction erosion have not been previously reported, raising the issue of an adequate tectonic
mechanism. The most commonly inferred mech-
The 3-km/m.y. retreat along the Japan margin
exceeds the 2-km/m.y. rate of lower-slope advance
from accretion along the Middle America Trench off
southern Mexico (20-km-wide accretionary complex
that is about 10 m.y. old; Watkins, Moore, and others,
1982). It is about equivalent to the rate of slope advance along the Nankai Trough (Karig and Angevine,
1986) and Barbados lower slope (Mascle, Moore, and
others, 1987). Thus, the rate of erosion can be as rapid
as that of accretion.
anism of subduction erosion is abrasion by the
horst and graben on the subducting ocean crust
where the horsts scrape away part of the upper
plate, and the graben transport this material
down the subduction zone (Schweller and
Kulm, 1978; Hilde and Sharman, 1978). Such a
mechanism is inadequate here, because (1) the
observed graben have but half of the volume of
the eroded material5 and (2) the subducted
graben are 500 to 800 m beneath the décollement (von Huene and Culotta, 1989, and Fig.
1), preventing any contact of the abrasional
teeth (horsts) with the upper plate. Furthermore,
the overall coherent appearance of the subducted sediment, even - 3 0 km landward of the
trench axis and as much as 15 km deep (von
Huene and Culotta, 1989), observed overpressure (Carson and others, 1982; Cadet and others, 1987), and the lack of larger subductionzone earthquakes beneath the lower and middle
slope (Yoshii, 1979) indicate conditions of reduced, rather than high, friction along the plate
Mechanisms of Frontal Erosion
The erosional action of ocean floor topography in the Japan and Peru subduction zones
appears to involve wedging of positive topographic features into the subduction zone, with
uplift and breakup of the upper plate. The subduction of seamounts and ridges produces structural and morphological effects sufficiently large
to detect by swathmapping and seismic techniques. Two of four reported observations (von
Huene, 1986; Lallemand and Le Pichon, 1987;
Collot and Fisher, 1988, 1989; Lallemand and
others, 1989; Yamazaki and Okamura, 1989;
Ballance and others, 1989) are illustrated here
with perspective diagrams (Figs. 8 and 9) to
form the basis for a model of frontal erosion.
Two subducting seamounts along the Japan
Trench are used to show stages of frontal erosion. Daiichi Kashima Seamount (Kobayashi
and others, 1986) at the west end of a chain of
seamounts (Fig. 9) is the latest to subduct beneath the Japan Trench. Daiichi Kashima is on
the seaward slope of the trench where horst and
graben on the flexed oceanic crust continue
across the seamount. The subducted leading
flank of the seamount, imaged in seismic records
(Lallemand and others, 1989), dips less than
15°, and underthrusting of the flank has elevated
the upper plate by wedging it upward. In front
of and over the subducted flank of the seamount,
The present average graben depth ( - 4 0 0 m) and
spacing (~ 10 km) with a constant rate of 100 km/m.y.
of convergence are assumed as past conditions also.
Figure 10. Two perspective diagrams of the Erimo Seamount area. Upper: looking northeast toward Hokkaido Island. Smoothed bathymetry
is from conventional soundings; area inside light dashed line is from SeaBeam data (Cadet and others, 1987), at 3x vertical exaggeration. Lower:
looking north-northwest toward Hokkaido Island, same data as above, with adjacent SeaBeam coverage to the south but only 1.5x vertical
contractile strain increases, so that uplift also
involves thickening by thrust faulting. Where
the landward slope of the trench has been observed and sampled from a submersible, it consists of highly fractured mudstone and includes
clasts of seamount material (Cadet and others,
1987). Some of the rubble from the seamount
appears to be transferred to the landward slope,
but the bulk of the seamount remains with the
subducting oceanic plate.
At the northern end of the Japan Trench, at
its juncture with the Kuril Trench, is Erimo
Seamount (Fig. 10). Erimo Seamount is just entering the axis of the Japan and Kuril Trenches,
and the normal faults paralleling the Kuril
Trench cut the seamount. Adjacent to the seamount on the landward slope of the trench is a
large re-entrant where another seamount has just
been subducted (Lallemand and Chamot-Rooke,
1986; Yamazaki and Okamura, 1989). Evidence for the buried seamount is a strong magnetic anomaly from a source beneath the
bathymetric high reflecting the subducted seamount. Modeling of the anomaly indicates the
presence of a buried seamount about 2 km high
and about 30 km in diameter.
Lallemand and Le Pichon (1987) modeled a
subducting seamount like Daiichi Kashima from
its present stage of subduction backward in time
to its pre-collision configuration. The subducted
leading flank of the seamount increased the critical taper of the margin front and steepened its
slope, causing collapse. The critical taper is a
function of the internal friction in the accreted
sediment (Davis and others, 1983), which becomes overcritical as the seamount begins to
subduct, thereby causing collapse, and undercritical after the seamount has subducted, causing
accretion. The lower slope re-entrants along the
Japan Trench from previous seamount subduction are largely filled and difficult to detect even
with SeaBeam bathymetry (Fig. 9). This filling
appears to be a rapid response to a change in the
critical taper as the slope adjusts to the absence
of a subducting seamount. The early stages of
such filling have recently been described in the
New Hebrides subduction zone, where an accretionary ridge along the trench axis has closed a
large re-entrant like the one near Erimo Seamount (Collot and Fisher, 1989).
trench axis
lower slope at critical
angle of failure
normal fault
uplift above leading
flank of seamount
overcritical, slope in failure
From these snapshots, we show in four diagrams a general model of seamount subduction
(Figs. 11A-11D). In the initial diagram (Fig.
Figure 11. Four stages in a general model
of the subduction of a seamount. Part A
shows initial stage where only trench sediment has been wedged up. B is a stage comparable to Daiichi Kashima Seamount and is
patterned after it. Along trenches without
strong faulting on the seaward slope, seamounts are not so severely broken by faults.
C illustrates collapse of the lower slope as the
seamount crest has subducted and the thickened welt that pushed up in front of the seamount slides down its trailing flank. D
illustrates healing of the re-entrant by accretion along the deformation front. This stage
occurs after that shown with the re-entrant in
Figure 9.
undercritical area
of sediment ponding
i fracturing
Figure 11. (Continued).
thickened welt
of fractured rock
sediment ponding
critical slope
11 A), the landward flank of a seamount has
wedged beneath the sediment filling the trench
The second diagram (Fig. 1 IB) shows a stage
comparable to that of Daiichi Kashima Seamount (Fig. 9). The front of the upper plate has
ridden up the incline of the seamount's leading
flank. Horizontal shortening and greater critical
taper cause imbrication and thickening of the
upper plate. The front of the margin has retreated a small amount due to contractile deformation and failure at the base of the slope.
In the third diagram, the trailing flank of the
seamount is subducting (Fig. 11C). The thickened welt of faulted and fractured rock that
developed above the leading flank is now above
the descending slope of the trailing flank. The
thickened rock mass, already weakened from
fracturing, is further oversteepened by the subduction of a trailing flank and is increasingly
prone to gravity failure. This results in accelerated mass wasting and a retreat of the slope. The
re-entrant at the southern end of the Kuril
Trench (Fig. 9) illustrates a later interval than in
Figure 11C, when the slope has further retreated
as the crest of the seamount subducted. Mass
wasting produces debris avalanches that create a
sediment apron in the axis of the trench. Loading of sediment in the trench axis from repeated
debris avalanches should cause elevation of
pore-fluid pressure in the underlying sediment
section. Sediment subduction proceeds efficiently here to remove the debris, otherwise it
would accrete against the slope and stabilize it.
The fourth diagram (Fig. 11D) shows how
the re-entrant forms a backstop for accreting
sediment. In our example, the margin is in an
accretionary configuration except where the
seamount is subducted. Therefore, once the
seamount is subducted, the front of the margin
will return to its initial state. The debris from
mass wasting is left in the wake of the subducting seamount, and along with sediment deposited in the trench axis, it accretes along the
deformation front and builds a low ridge. That
ridge becomes the seaward flank of a lowerslope basin in which sediment ponds. This process is observed along the New Hebrides Trench
where accretion is closing a re-entrant from a
subducted seamount (Collot and Fisher, 1988,
Filling of the re-entrant by accretion in the
area where the Kashima seamounts are being
subducted appears to be completed between the
subduction of one seamount and the arrival of
the next. The seamounts are spaced about 60 km
apart, and the rate of convergence is 100
km/m.y. Thus, the margin is healed in approximately 0.6 m.y. or less. The model indicates the
importance of wedge configuration as a factor to
determine the accretionary or nonaccretionary
condition of the Japan and Peru margins despite
the probable less than true Coulomb behavior of
the material involved.
Our proposed mechanism for frontal erosion
of the Japan and Peru margins underscores the
role of wedging and breakup of the upper plate
to produce debris that is transported away by
sediment subduction. The difference with the
chain-saw model is that erosion results from
subduction of a step or disturbance in the basement topography without requiring sharp hard
protrusions to abrade and tear up the lower
The Response of Continental Margins to the
Subduction of Ridges and Scarps
Along the Peru Trench, the last phase of erosion was simultaneous with subduction of the
Nazca Ridge; once the ridge was subducted, the
margin became accretionary (von Huene, Suess,
and others, 1988). The southward-traveling
trailing flank of the Nazca Ridge appears to have
left collapse structure and debris from mass
wasting, as indicated by Leg 112 drilling at Site
685 (Suess, von Huene, and others, 1988a), and
buried jumbled material that is difficult to image
seismically (Kulm and others, 1986). The scarp
from subduction erosion formed a backstop of
crystalline and Eocene to middle Miocene sedimentary rock against which upper Miocene and
Quaternary sediment accreted. Thus, many
processes illustrated in our seamount model
appear to have operated during the subduction
of Nazca Ridge.
Similar tectonic mechanisms may operate
during subduction of the 250- to 500-m-high
scarps of horst and graben on the ocean floor.
Along the Japan Trench, some scarps continue
locally as a low ridge in the accreted sediment
(Lallemand and others, 1986). These could form
low waves of tectonism traveling obliquely
along the foot of the slope. Fault scarps are subducted frequently, because the average spacing
between horsts off South America and Japan
commonly averages less than 10 km (Aubouin
and others, 1981; Bourgois and others, 1986;
Cadet and others, 1987). Off northern Japan,
where the rate of convergence is 100 km/m.y.,
more than 20 scarps along horsts and graben are
subducted every million years. Although it was
shown before that the graben off Japan are insufficient to transport all the eroded debris, they
may constantly erode at a rate that is less than
the rate of accretion. Once the configuration of
the margin becomes less accretionary, erosion
may dominate.
Mechanisms of Basal Erosion
The truncation at the base of the upper plate
of beds that predate present accretion along the
Japan Trench (von Huene and Culotta, 1989,
and Fig. 1) suggests erosion. The time of the
truncation is unknown, but because the sea floor
above the truncated beds subsided in Pliocene
time, it was inferred that the basal erosion occurred at the same time. The material detached
from the upper plate may be in smaller pieces
than resolved in the seismic images. Plucking of
many small clasts by traction along the base of
the upper plate could occur in an environment
of overpressured, circulating pore fluids. Upward migration of the water released during
subduction has been suggested as a mechanism
to "soften" the base of the upper plate (Murauchi and Ludwig, 1980). Fragmentation by hydrofracturing may produce a slurry of clasts that
becomes part of the subducting sediment mass
(von Huene and Lee, 1983; Piatt, 1989). Rock
at the base of the upper plate may disaggregate
as pressured pore fluid invades and permeates
through fractures. Highly fractured, consolidated
sediment that is broken into pebble- or gravelsized clasts is well known from cores recovered
on the Japan margin (Carson and others, 1982;
vori Huene and others, 1982; Leggett and others,
1987) and from submersible observations
(Cadet and others, 1987). Although the previously discussed estimates of eroded material
from the Japan and Peru margins are huge,
about 100 km of ocean crust is subducted beneath these margins every million years. A layer
of gravelly breccia formed of clasts from the
base of the upper plate needs to average from
—300 to 550 m thick in the sequence of subducted sediment to transport subcrustally eroded
material of the volume estimated (Table 1).
Such thicknesses are consistent with the beds
paralleling the lower plate (that is, subducted
sediment) imaged in seismic records of the
Japan and Peru margins. It is difficult, however,
to differentiate clearly between underplated and
subducting strata, using the seismic data alone.
The significance of the huge quantities of material eroded from the overriding plate in a con-
vergent margin is the ultimate repository of the
material and how this affects lithospheric processes. Where does the subducted erosional
material go, to magmatic products of the associated arc, to the base of the crust where it could
be magmatically underplated, or to deeper parts
of the subduction zone that enter the mantle?
The underplating beneath the parts of the upper
plate and beneath the volcanic arc should be
reflected at the surface as a topographic uplift;
however, crustal thickening by overthrusting has
been proposed in recent studies of Andean uplift
because of the rapid vertical tectonics (Isacks,
1988). Inclusion in the volcanic and plutonic
rocks of the arc should be detected by isotopic
signatures. Many geochemists, however, find
that their data are better explained by a model
that employs the mixing of subcrustal and deepcrustal magmas, rather than contamination by
continental materials in the source region. The
products of the tectonic erosion beneath Lima
Basin are presently not reaching the surface because of the lack of volcanic activity in this part
of the Peruvian Andes. Subduction beyond the
zone of partial melting seems difficult because of
the low density and melting temperatures of
continental material. Thus, the fate of subducted
materials remains a significant problem.
Subduction erosion, like accretion, can occur
at the base of the slope or along the underside of
the upper plate. Erosion along the base of the
slope, or frontal erosion, has caused the landward slope of the Japan and Peru Trenches to
retreat at average rates of 2 to 3 km/m.y. One
probable agent of such erosion is the subduction
of many topographic features, from 200-m fault
scarps to 3,000-m seamounts and ridges.
Subducting seamounts along the Japan
Trench illustrate some stages in this process. As
these seamounts subduct, the leading flank of the
seamount wedges up the overriding plate, and
the trailing flank lets it fall; the slope material is
pushed up and weakened by fracturing, and the
wedge angle becomes overcritical, causing slope
failure, which sends debris avalanching into the
trench axis. Gravity failure and efficient subduction of the increased volume of trench sediment
local to the seamount leave a large re-entrant in
the slope after the seamount has subducted. The
re-entrant later fills by accretion of sediment
from the trench axis when the dip of the subducting plate returns to its former condition.
This sequence of processes also appears to operate where ridges are subducted.
The subduction of seamounts, ridges, and
horst and graben probably accelerates frontal
erosion for a short time. Sustained frontal erosion is more likely caused by the fundamental
configuration of the margin, but this is not yet
well understood. One indication of this sensitivity to configuration is seen in the sequence of
changes during seamount and ridge subduction
along Japan and Peru. The Japan and the Peruvian margins are presently accretionary although
not strongly so. After a seamount or ridge subducts and causes temporary local erosion, the
margins return to the accretionary state. The rate
of plate convergence has remained constant, and
the wedge angle and sediment load have varied,
indicating the control of tectonic processes by
these parameters. We speculate that the constant
subduction of small topographic features (including horst and graben not buried by sediment
in the trench axis) is erosive but is here superposed on a stronger accretionary tectonics governed by plate configuration.
Erosion along the underside of the upper
plate, or basal erosion, removes more material
than does frontal erosion from the Japan and
Peru margins. It affects these margins by thinning the crust, thereby causing subsidence beneath the continental terrace and the slope. The
mechanisms of subsurface erosion are inferential
because they have not been observed. One inferred mechanism involves the invasion of overpressured water into fractures, which disaggregates the rock along the underside of the upper
plate and makes a profusion of small fragments
vulnerable to plucking by traction along the
plate boundary.
The estimated rates of erosion through a cross
section of the northern Japan Trench and the
Peru Trench along Lima Basin in the past 20
m.y. is from 31-55 km 2 /m.y. Such rates require
subduction of a layer of material about 0.5 km
thick in addition to the subducted ocean material. This amount is possible, given the thickness
of reflections from subducted material in seismic
We are grateful for the extensive efforts of
Mark Brandon, Dan Karig, and David Scholl,
who through their thoughtful and thorough reviews have helped us greatly improve earlier
versions of manuscripts describing our study.
The seismic data of Figure 3 were provided by
Shell International Petroleum Mij. B.V. in the
Hague and were reprocessed by Rutt and John
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