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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. 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