Download High rates of arc consumption by subduction processes: Some

High rates of arc consumption by subduction
processes: Some consequences
Serge Lallemand
Laboratoire de Géophysique et Tectonique, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier, Case 60,
place Bataillon, 34095 Montpellier, France
ABSTRACT
New estimates of long-term consumption of Japanese and Peruvian continental forearc margins together with recent drilling data (Ocean Drilling Program) on some other
erosional island arcs such as Izu-Bonin, Mariana, or Tonga allow the calculation of a mean
rate of 7 6 3 km/m.y. landward migration of the arc-trench system on both east and west
sides of the Pacific basin during Cenozoic time. Such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the Atlantic Ocean.
Retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper
mantle resistance to contraction of the Pacific basin. Mass transfers in subduction zones
must be reconsidered in the light of these new estimates, especially the balance between net
growth and loss of continental crust in recent geologic time. A 50 m.y. period is long enough
to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone,
assuming a mean trench–volcanic-arc landward migration of 5 km/m.y. and a mean trench–
volcanic-arc distance of 250 km. This circumstance thus raises the question of whether the
Izu-Bonin-Mariana and Tonga arcs are older than their reported Eocene age.
GLOBAL ESTIMATES OF SUBDUCTED
SOLID ROCK
According to global estimates, ;21 km3
of oceanic crust is subducted each year beneath active margins (Crisp, 1984; Parsons,
1981). In addition to the oceanic crust, pelagic and terrigenous sediment as well as arc
crustal material are dragged with the subducting plate (e.g., Scholl et al., 1977; Cloos
and Shreve, 1988a, 1988b; von Huene and
Lallemand, 1990; Tao and O’Connell,
1992). On the basis of a global compilation
of convergent margins, von Huene and
Scholl (1991, 1993) estimated that half of
the margins (23 000 km) are currently growing by sediment accretion and half (21 000
km) are either stable or shrink by the removal of arc material. In the first group of
accreting margins, 70% to 80% of incoming
sediment is underthrusted and the rest is accreted, whereas in the second group of nonaccreting margins, 100% of incoming sediment is underthrusted in addition to the arc
material that is tectonically eroded. The total amount of long-term global sediment
subducted beneath the margin-rock framework is estimated at ;0.7 km3/yr, whereas
the plausible global erosional rate is at least
0.6 km3/yr (range of 0.6 to 1.1 km3/yr), according to von Huene and Scholl (1993),
thus providing a long-term total rate of 1.3
km3/yr. This last component of tectonically
eroded material from the margin framework
was probably underestimated by von Huene
and Scholl because they used a very conservative model to reconstruct paleomargins
(von Huene and Lallemand, 1990). Lallemand et al. (1992) proposed another model
of reconstruction that satisfied the equilibrium between interacting convergent plates
and explained the migration of the volcanic
arc through time. Lallemand et al. concluded that the long-term subcrustally
eroded sediment quantities were at least
twice those proposed by von Huene and
Scholl (1991). Lallemand (1992) concluded
that the total global rate of subducted terrestrial material was at least 2 km3/yr.
Typical orders of magnitudes (Fig. 1),
concerning the thickness of incoming sediment section, convergence rate, and ratio of
offscraping or underplating along two endmember types of subduction zones, are
taken from von Huene and Scholl (1991,
1993) and Lallemand et al. (1994). To simplify, we name these two extreme types accretionary and erosional margins, even
though there is some evidence for simultaneous frontal accretion and subcrustal erosion or frontal erosion associated with underplating (Lallemand et al., 1994). It would
thus be an oversimplification to assume that
one-half of the modern convergent margins
is accretionary and the other half is erosional. We observe that, in both examples,
the thickness of sediment bypassing the
margin core is about the same (1 km in our
sketch), but the convergence rate is 2.5
times higher along erosional margins; therefore, the subducted sedimentary flux is also
2.5 times higher. Along most convergent
margins, the entire ocean-basin layer of sed-
Geology; June 1995; v. 23; no. 6; p. 551–554; 2 figures; 1 table.
imentary deposits (pelagic and hemipelagic
sediment) is subducted and corresponds
approximately to the lower 50% of each subducting section. The upper 50% is composed of material derived from the overriding plate along both types of margins. The
only difference is the locus of erosion from
the surface of the upper plate along accretionary margins (e.g., drainage by rivers and
canyons) to mainly subcrustal locations
along erosional margins (hydrofracturing of
the base of the upper plate).
ARC CONSUMPTION
We assume that significant concomitant
margin subsidence and landward volcanicarc migration (retreat) along any subduction
zone are necessarily reflections of tectonic
removal of the margin framework in the
subduction zone (Lallemand, 1992). The
amplitude of the subsidence of erosional
margins always increases from near the
present coastline toward the present trench
(Fig. 2). Reconstructions of the paleogeometry of these margins (before subsidence)
require a downward deflection of the subducting slab to satisfy either isostatic or elastic adjustment; thus, considerable quantities
of arc material have been removed (von
Huene and Lallemand, 1990; Lallemand et
al., 1992). If the volcanic front has retreated
landward simultaneously with the trench,
this implies that the entire slab has migrated
laterally with respect to the original position
of the volcanic arc (Lallemand, 1992).
Margin subsidence in concert with vol551
Figure 1. Typical orders of magnitudes of sedimentary and terrestrial fluxes along two extreme types of subduction zones. Values are averaged
among several subduction zones studied in von Huene and Scholl (1991) and Lallemand et al. (1994). Large variations are encountered among
modern subduction zones.
Figure 2. Five bathymetric profiles across erosional margins simplified after Lallemand
(1992) showing amplitude of vertical movements and volcanic-arc landward migrations
during erosional period. Vertical exaggeration
is ;10. Numbers represent datings (in Ma) of
volcanic-arc material, either outcrops or
drilled. Inset: Schematic diagram illustrating
two stages of arc geometry before (dotted
lines) and after (solid lines) tectonic erosion.
Overriding plate is supposed fixed, and both
trench and volcanic-arc retreat so that only
slab lateral migration can account for these
retreats.
552
canic-arc retreat has been documented
along five transects (Fig. 2, Table 1; Lallemand, 1992). Data about subsidence are derived from deep-sea drilling on respective
margins. The amplitude of the subsidence is
either known from drilling results (Mariana,
Izu-Bonin, Tonga) or from a trenchward-depressed seismic horizon of known paleobathymetry that has been traced seaward to
near the trench axis (northern Japan, Peru,
Sunda). This circumstance means that the
Mariana, Izu-Bonin, and Tonga subsidence
listed in Table 1 reflects the minimum subsidence of the outer edge of the margin.
Data on volcanic-arc retreat are deduced
from deep-sea drilling results and onland
surveys (the occurrence of island-arc volcanic rocks attests to landward migration of volcanic fronts through time). The reference
periods are, for most areas, the longest periods for the studied retreat and subsidence
events; thus, the proposed rates are
smoothed long-term measurements. According to these limitations, we observe that
the studied margins subside at a minimum
average rate of 0.25 6 0.15 km/m.y., whereas
the volcanic front retreats at a rate of 7 6 3
km/m.y. The rate of trench retreat estimated
off Japan and Peru by Lallemand et al.
(1992) is higher than 6 and 5 km/m.y., respectively. Thus, the rate of volcanic-arc and
trench retreat is of the same order of magnitude. When looking at shorter time intervals of intense tectonic erosion, rates of retreat of the volcanic-arc may have reached
60 km/m.y. in the Mariana Trench during
the past 2 m.y. (Lallemand, 1992), and rates
of subsidence may have reached 0.7 km/m.y.
along the Peru margin during the past 5 m.y.
(Sosson et al., 1994). For example, Stern and
Bloomer (1992) documented a mean rate of
volcanic-arc retreat of 15 km/m.y. since the
development of the Izu-Bonin-Mariana arc
from the middle Eocene (48 – 42 Ma) to the
latest Oligocene (24 Ma). Unfortunately, we
have no information about margin subsidence during this time interval.
TECTONIC CONSIDERATIONS
Possible Origin of Arc Consumption
This study shows that the order of magnitude of the landward migration of both
volcanic-arc and trench is 7 km/m.y. along
active margins on both east and west sides
of the Pacific Ocean, from the Kurile
(Schnürle et al., 1995) to the Tonga islands
(Pelletier and Dupont, 1990), and from
Central to South America (von Huene and
Aubouin, 1982; von Huene and Lallemand,
1990). Garfunkel et al. (1986) suggested that
the Pacific Ocean closed at a mean rate of 20
km/m.y. (10 km/m.y. on each side) during
the past 40 m.y., in response to the opening
of the Atlantic Ocean. We propose that
trench rollback occurs at a lower rate than
required by the Garfunkel et al. (1986)
model, because of the sub-Pacific upper
mantle resistance to contraction. The subPacific upper mantle can only flow outward
across limited asthenospheric windows, such
as between the South America and Antarctica plates (Shemenda and Grokhol9skiy,
1986). Furthermore, Russo and Silver
(1994), on the basis of shear-wave splitting
of S and SKS phases revealing the anisotropy and strain field of the mantle, showed
that there is evidence for horizontal trenchparallel flow in the mantle beneath the
Nazca plate along much of the Andean subduction zone. Consequently, part of the
GEOLOGY, June 1995
Eocene Izu-Bonin-Mariana arcs (Tracey et
al., 1964), for example, where tectonic erosion was especially active. Tao and O’Connell (1992) mentioned in their model of ablative subduction that episodes of vigorous
ablation would remove records of ancient
subduction, biasing arc ages toward anomalously young values.
ACKNOWLEDGMENTS
I thank Xavier Le Pichon for helpful discussions on the coincidence between the rate of volcanic arc retreat and the required rate of closure
of the Pacific Ocean, and David Scholl and Mark
Cloos for their constructive reviews.
opening of the Atlantic Ocean is accommodated by shrinking of the Pacific Ocean, as
attested to by the trench-parallel mantle
flow reflecting the trench rollback, whereas
the rest could be accommodated through
tectonic erosion, i.e., consumption of the
overriding plate.
Mass Balance of Transfers in Subduction
Zones Must Be Reconsidered
The high rates of arc consumption along
some active margins must influence regional
tectonism and magmatism as well as the
global mass balance of material transfers to
the mantle. Depending on the fate of subducted terrestrial material, we can compare
the destruction-subduction and the production rates of continental crust. Reymer and
Schubert (1984, 1986) proposed a rate of
continental crust production of 1.65 km3/yr
during Mesozoic-Cenozoic time to be compared with 0.6 km3/yr of subducting material: they concluded that continents grew at
a mean rate of 1 km3/yr during that period.
Albarède (1989) subtracted 0.8 km3/yr of
subducted material to 2.5 km3/yr of additional juvenile igneous rock. The rate of
continental growth then reached 1.7 km3/yr.
Von Huene and Scholl (1991) concluded
that the global volume of terrestrial crust
may not have changed greatly during the
past several hundred million years. Uncertainties about both net growth or the loss of
crustal material are such that it is not possible to conclude in favor of a continental or
GEOLOGY, June 1995
oceanic growth. We know that the amount
of subducted material must have increased,
but we do not know the volume of material
restored to the upper plate by underplating
and magmatism. Several experimental studies, performed at high pressure and/or high
temperature, have shown that continental
crust and terrigenous and pelagic sediments
can be subducted to depths greater than 200
km under certain conditions, such as fast
subduction rates or entrainment by subducting slabs (e.g., Staudigel and King, 1992; Irifune et al., 1994). Arc-magma chemistry
does not support more than a few percent of
sediment recycling (e.g., Gill, 1981). Therefore, if the arc crust is partially recycled in
the arc lavas, it becomes impossible to discriminate between crustal contamination
during ascent of the magma or source contamination from the descending slab.
Arc Ages Deduced from the Oldest Rock
Outcrops are Minimum Ages
The ages of the oldest subduction-related
igneous rocks are minimum ages inasmuch
as the initial igneous products may have
been removed. This is particularly true
along long-lived subduction zones, i.e., older
than 20 –100 Ma. Typical trench-arc distances are on the order of 200 –300 km, so
40 – 60 m.y. is enough to remove the entire
initial arc–fore-arc massif, assuming a mean
trench–volcanic-arc retreat of 5 km/m.y.
Some paleogeographic reconstructions may
thus be reconsidered along the presumably
REFERENCES CITED
Albarède, F., 1989, Sm/Nd constraints on the
growth rate of continental crust: Tectonophysics, v. 161, p. 299–305.
Cloos, M., and Shreve, R. L., 1988a, Subductionchannel model of prism accretion, melange
formation, sediment subduction, and subduction erosion at convergent plate margins:
1. Background and description: Pageoph,
v. 128, no. 3/4, p. 455–500.
Cloos, M., and Shreve, R. L., 1988b, Subductionchannel model of prism accretion, melange
formation, sediment subduction, and subduction erosion at convergent plate margins:
2. Implications and discussion: Pageoph,
v. 128, no. 3/4, p. 501–545.
Crisp, J. A., 1984, Rates of magma emplacement
and volcanic output: Journal of Volcanology
and Geothermal Research, v. 20, p. 177–211.
Diament, M., and eight others, 1992, Mentawai
fault zone off Sumatra: A new key to the geodynamics of western Indonesia: Geology,
v. 20, p. 259–262.
Garfunkel, Z., Anderson, C. A., and Schubert, G.,
1986, Mantle circulation and the lateral migration of subducted slabs: Journal of Geophysical Research, v. 91, p. 7205–7223.
Gill, J. B., 1981, Orogenic andesites and plate tectonics: Berlin, Heidelberg, and New York,
Springer-Verlag, 390 p.
Herzer, R. H., and Exon, N. F., 1985, Structure
and basin analysis of the southern Tonga
forearc, in Scholl, D. W., and Vallier, T. L.,
eds., Geology and offshore resources of Pacific island arcs, Tonga region: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 2, p. 55–73.
Hussong, D. M., and Uyeda, S., 1981, Tectonic
processes and the history of the Mariana arc:
A synthesis of the results of DSDP Leg 60, in
Initial reports of the Deep Sea Drilling
Project, Volume 60: Washington, D.C., U.S.
Government Printing Office, p. 909–929.
Irifune, T., Ringwood, A. E., and Hibberson,
W. O., 1994, Subduction of continental crust
and terrigenous and pelagic sediments: An
experimental study: Earth and Planetary Science Letters, v. 126, p. 351–368.
James, D. E., 1971, Plate tectonic model for the
evolution of the Central Andes: Geological
Society of America Bulletin, v. 82,
p. 3325–3346.
Jolivet, L., and Tamaki, K., 1992, Neogene kinematics in the Japan Sea region and volcanic
activity of the northeast Japan arc, in Tamaki, K., Suyehiro, K., Allan, J., McWilliams,
M., et al., Proceedings of the Ocean Drilling
Program, scientific results, Volume 127–128,
553
Part 2: College Station, Texas, Ocean Drilling Program, p. 1311–1331.
Kaiho, K., 1992, Eocene to Quaternary benthic
foraminifers and paleobathymetry of the IzuBonin arc, Legs 125 and 126, in Taylor, B.,
Fujioka, K., et al., Proceedings of the Ocean
Drilling Program, scientific results, Volume
126: College Station, Texas, Ocean Drilling
Program, p. 285–310.
Keller, G., 1980, Benthic foraminifers and paleobathymetry of the Japan trench area, Leg 57,
in Initial reports of the Deep Sea Drilling
Project, Volume 56 –57, Part 2: Washington,
D.C., U.S. Government Printing Office,
p. 835– 866.
Kroenke, L. W., and Tongilava, S. L., 1975, A
structural interpretation of two reflection
profiles across the Tonga arc: South Pacific
Marine Geological Notes, v. 1, no. 2, p. 9–15.
Lagabrielle, Y., Sizun, J.-P., and Arculus, R. J.,
1992, The constructional and deformational
history of the igneous basement penetrated
at Site 786, in Fryer, P., Pearce, J. A., Stokking, L. B., et al., Proceedings of the Ocean
Drilling Program, scientific results, Volume
125: College Station, Texas, Ocean Drilling
Program, p. 263–276.
Lallemand, S. E., 1992, Transferts de matière en
zone de subduction, Volume 1: Travaux publiés depuis 1990, 189 p.; Volume 2: Réflexions sur les conséquences de l’érosion tectonique: Paris, Université Pierre et Marie
Curie, Mémoire des Sciences de la Terre, no.
92–27, 96 p. (in French with English
abstract).
Lallemand, S. E., Schnurle, P., and Manoussis, S.,
1992, Reconstruction of subduction zone paleogeometries and quantification of upper
plate material losses caused by tectonic erosion: Journal of Geophysical Research, v. 97,
p. 217–239.
Lallemand, S. E., Schnurle, P., and Malavieille, J.,
1994, Coulomb theory applied to accretionary and nonaccretionary wedges—Possible
causes for tectonic erosion and/or frontal accretion: Journal of Geophysical Research,
v. 99, p. 12,033–12,055.
Letouzey, J., and Pertamina, L., 1982, Geological
map of western Indonesia: Beicip, scale
1:2,000,000.
Malod, J.-A., and 11 others, 1993, Déformations
du bassin d’avant arc au Nord-Ouest de
Sumatra: une réponse à la subduction
oblique: Paris, Académie des Sciences
Comptes Rendus, v. 316, no. II, p. 791–797
(in French with English abstract).
Marlow, M. S., Johnson, L. E., Pearce, J. A.,
Fryer, P. B., Pickthorn, L. G., and Murton,
B. J., 1992, Pleistocene volcanic rocks in the
Mariana forearc revealed by drilling at Site
781, in Fryer, P., Pearce, J. A., Stokking,
L. B., et al., Proceedings of the Ocean Drilling Program, scientific results, Volume 125:
College Station, Texas, Ocean Drilling Program, p. 292–310.
Meijer, A., Anthony, E., and Reagan, M., 1981,
Petrology of volcanic rocks from the forearc
sites, in Hussong, D. M., Uyeda, S., et al.,
Initial reports of the Deep Sea Drilling
Project, Volume 60: Washington, D.C., U.S.
Government Printing Office, p. 709–729.
Mitchell, J. G., Peate, D. W., Murton, B. J.,
Pearce, J. A., Arculus, R. J., and van der
Laan, S. R., 1992, K-Ar dating of samples
from Sites 782 and 786 (Leg 125): The IzuBonin region, in Fryer, P., Pearce, J. A., Stok554
king, L. B., et al., Proceedings of the Ocean
Drilling Program, scientific results, Volume
125: College Station, Texas, Ocean Drilling
Program, p. 203–210.
Ohguchi, T., Yoshida, T., and Okami, K., 1989,
Historical change of the Neogene and Quaternary volcanic field in the northeast Honshu arc, in Kitamura, N., et al., eds., Cenozoic
geotectonics of northeast Honshu arc: Geological Society of Japan Memoir 32,
p. 431– 455 (in Japanese with English
abstract).
Parsons, B., 1981, The rate of plate creation and
consumption: Royal Astronomical Society
Geophysical Journal, v. 67, p. 437– 448.
Pelletier, B., and Dupont, J., 1990, Effets de la
subduction de la ride de Louisville sur l’arc
des Tonga-Kermadec: Oceanologica Acta,
Special Volume 10, p. 57–76 (in French with
English abstract).
Pelletier, B., and Louat, R., 1989, Mouvements
relatifs des plaques dans le Sud-Ouest
Pacifique: Paris, Académie des Sciences
Comptes Rendus, v. 308, no. II, p. 123–130
(in French with English abstract).
Pubellier, M., Rangin, C., Cadet, J.-P., Tjashuri,
I., Butterlin, J., and Müller, C., 1992, L’ı̂le de
Nias, un édifice polyphasé sur la bordure interne de la fosse de la Sonde (Archipel de
Mentawai, Indonésie): Paris, Académie des
Sciences Comptes Rendus, v. 315, no. II,
p. 1019–1026.
Reymer, A., and Schubert, G., 1984, Phanerozoic
addition rates to the continental crust and
crustal growth: Tectonics, v. 3, p. 63–77.
Reymer, A., and Schubert, G., 1986, Rapid
growth of some major segments of continental crust: Geology, v. 14, p. 299–302.
Russo, R. M., and Silver, P. G., 1994, Trenchparallel flow beneath the Nazca plate from
seismic anisotropy: Science, v. 263,
p. 1105–1111.
Schnürle, P., Lallemand, S. E., von Huene, R.,
and Klaeschen, D., 1995, Tectonic regime of
southern Kurile Trench as revealed by multichannel seismic lines: Tectonophysics (in
press).
Scholl, D. W., Marlow, M. S., and Cooper, A. K.,
1977, Sediment subduction and offscraping
at Pacific margins, in Talwani, M., et al., eds.,
Island arcs, deep-sea trenches and back-arc
basins: American Geophysical Union Maurice Ewing Series, v. 1, p. 199–210.
Shemenda, A. I., and Grokhol9skiy, A. L., 1986,
Geodynamics of the South Antilles region:
Geotectonics, v. 20, no. 1, p. 58– 66.
Shipboard Scientific Party, 1981, Site 460: Inner
wall of the Mariana Trench, in Hussong,
D. M., Uyeda, S., et al., Initial reports of the
Deep Sea Drilling Project, Volume 60:
Washington, D.C., U.S. Government Printing Office, p. 371–398.
Shipboard Scientific Party, 1992, Introduction,
background and principal results of Leg 135,
Lau Basin, in Parson, L., Hawkins, J., Allan,
J., et al., Proceedings of the Ocean Drilling
Program, scientific results, Volume 135: College Station, Texas, Ocean Drilling Program,
p. 5– 47.
Situmorang, B., and Soepraptono, 1975, Note on
the petroleum prospect of the interdeep
basin off west Sumatra–Indonesia: Tokyo,
Committee for Coordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas, 19 p.
Printed in U.S.A.
Soeria-Atmadja, R., Maury, R. C., Belon, H.,
Pringgoprawiro, H., Polve, M., and Priadi,
B., 1991, The Tertiary magmatic belts in
Java—Proceedings, Silver Jubilee on the Dynamics of Subduction and Its Products: Yogyakarta, Java, Research and Development
Center for Geotechnology–Indonesian Institute of Science, p. 98–121.
Sosson, M., Bourgois, J., and Mercier de Lépinay,
B., 1994, SeaBeam and deep-sea submersible
Nautile surveys in the Chiclayo canyon off
Peru (78S): Subsidence and subduction-erosion of an Andean-type convergent margin
since Pliocene times: Marine Geology, v. 118,
p. 237–256.
Staudigel, H., and King, S. D., 1992, Ultrafast subduction: The key to slab recycling efficiency
and mantle differentiation?: Earth and Planetary Science Letters, v. 109, p. 517–530.
Stern, R. J., and Bloomer, S. H., 1992, Subduction
zone infancy: Examples from the Eocene
Izu-Bonin-Mariana and California arcs: Geological Society of America Bulletin, v. 104,
p. 1621–1636.
Suess, E., von Huene, R., and Leg 113 Shipboard
Scientists, 1988, Ocean Drilling Program Leg
112, Peru continental margin: Part 2, Sedimentary history and diagenesis in a coastal
upwelling environment: Geology, v. 16,
p. 939–943.
Sugi, N., Chinzei, K., and Uyeda, S., 1983, Vertical crustal movements of northeast Japan
since middle Miocene, in Hilde, T. W. C., ed.,
Geodynamics of the western Pacific-Indonesian region: American Geophysical Union
Geodynamic Series, v. 11, p. 317–329.
Tao, W. C., and O’Connell, R. J., 1992, Ablative
subduction: A two-sided alternative to the
conventional subduction model: Journal of
Geophysical Research, v. 97, p. 8877– 8904.
Tracey, J. I., Jr., Schlanger, S. O., Stark, J. T.,
Doan, D. B., and May, H. G., 1964, Geology
and hydrogeology of Guam, Mariana Islands:
U.S. Geological Survey Professional Paper
403A, p. A1–A104.
von Huene, R., and Aubouin, J., 1982, Summary—Leg 67, Middle America Trench transect
off Guatemala, in Aubouin, J., von Huene,
R., et al., Initial reports of the Deep Sea
Drilling Project, Volume 67: Washington,
D.C., U.S. Government Printing Office,
p. 775–793.
von Huene, R., and Lallemand, S. E., 1990, Tectonic erosion along the Japan and Peru convergent margins: Geological Society of
America Bulletin, v. 102, p. 704–720.
von Huene, R., and Scholl, D. W., 1991, Observations at convergent margins concerning
sediment subduction, subduction erosion,
and the growth of continental crust: Reviews
of Geophysics, v. 29, p. 279–316.
von Huene, R., and Scholl, D. W., 1993, The return of sialic material to the mantle indicated
by terrigenous material subducted at convergent margins: Tectonophysics, v. 219,
p. 163–175.
Manuscript received October 7, 1994
Revised manuscript received February 28, 1995
Manuscript accepted March 15, 1995
GEOLOGY, June 1995