GEOLOGY | December 2012 | www.gsapubs.org 1087

INTRODUCTIONThe Pacifi c-type orogeny has played a key role

in forming juvenile continental crust through-out Earth’s history, resulting in the oceanward accretionary growth of arc crust that includes the extensive exhumation of high-pressure (HP) metamorphic belts (Matsuda and Ueda, 1971; Maruyama, 1997). By compiling geologic set-tings of more than 250 HP belts in the world, Maruyama et al. (1996) concluded that the peak activity of the Pacifi c-type orogeny is marked by the ridge (spreading center) subduction when the orogenic core (i.e., HP belt) is tectonically exhumed. Furthermore, detailed geological stud-ies in southwestern Japan have shown that one cycle of the Pacifi c-type orogeny starts with a ridge subduction and ends with the next ridge-subduction event (e.g., Maruyama, 1997). In a Pacifi c-type orogenic cycle in an arc-trench sys-tem, the following major coeval orogenic com-ponents generally formed an accretionary com-plex (AC), HP metamorphic rocks (HP-AC), forearc basin (FAB) sediments, and a tonalite-trondhjemite-granodiorite (TTG) batholith suite.

Phanerozoic basement rocks of Japan record ~500 m.y. of Pacifi c-type orogeny, i.e., ocean-ward growth of the arc crust since the Cambrian. The tectonic history of Japan has been previously explained in terms of intermittent but unidirec-tionally oceanward growth that formed the sub-horizontal stacking of thick subduction-related elements (i.e., ACs and HP-ACs) in a clear oceanward and tectonically downward younging trend (e.g., Isozaki, 1996; Maruyama, 1997).

Recently, systematic chronological analyses of zircon from Paleozoic to Cenozoic sand-stones and metasandstones in Japan have shown

that tectonic erosion occurred at least four times during the past 500 m.y. (Isozaki et al., 2010). This new aspect considerably revised the tec-tonic history of Japan; however, the major limi-tation in documenting details of tectonic erosion is in the scarceness of age constraints to the tim-ing because tectonic erosion is basically a pro-cess to erase preexisting geologic units rather than to generate/archive new elements.

In this study, in order to constrain the Cre-taceous putative tectonic erosion in Japan, we conducted high-resolution U-Pb dating of detrital zircons of the Cretaceous sandstones and metasandstones from southwestern Japan by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). This article reports new age data and discusses the implica-tion of tectonic erosion in the Cretaceous tecton-ics of the East Asian margin.

GEOLOGIC OUTLINEAs to the Cretaceous orogeny in southwestern

Japan, Aoki et al. (2011) recently discriminated two independent orogenies, one in the Early Cretaceous and the other in the Late Cretaceous, i.e., “Sanbagawa orogeny” and “Shimanto orog-eny” (Fig. 1) (Aoki et al., 2009, 2011; Isozaki et al., 2010).

The Sanbagawa orogen consists of four coeval components: the Early Cretaceous Sanbosan AC, Sanbagawa HP-AC, Ryoseki-Monobegawa FAB sediments, and Ryoke-Sanyo batholith granites (Figs. 1B and 1C). The Sanbosan AC is mainly composed of non- to weakly metamor-phosed sandstone and mudstone (trench-fi ll tur-bidites) with a minor amount of paleoseamount basaltic greenstones, paleoatoll carbonate, and deep-sea chert from the subducted oceanic plate. The Sanbagawa HP-AC is mainly composed of

the same rock types as the Sanbosan AC and was metamorphosed under conditions ranging from the pumpellyite-actinolite to eclogite facies. The Ryoseki-Monobegawa Group consists of nonmetamorphosed sandstones/mudstones of shallow marine to nonmarine facies. The Ryoke-Sanyo batholith is composed of Early Creta-ceous granite with associated low-pressure/high-temperature metamorphosed older ACs.

The Shimanto orogen consists of the North-ern Shimanto AC, Shimanto HP-AC, Izumi FAB sediments, and San-in batholith granites (Figs. 1B and 1D). The Northern Shimanto AC is composed mainly of sandstone/mudstone with an extremely small amount of oceanic rocks. The Shimanto HP-AC is mainly composed of blue-schists formed in a progressive metamorphism of the pumpellyite-actinolite to epidote-amphib-olite facies. The Izumi Group is thick nonmeta-morphosed turbidites. The San-in batholith is composed of Late Cretaceous granites.

SAMPLES, ANALYTICAL METHOD, AND RESULTS

In order to separate detrital zircons, we col-lected sandstones from the Sanbosan and North-ern Shimanto ACs, Ryoseki-Monobegawa and Izumi FAB sediments, and metasandstones in the Sanbagawa and Shimanto HP-ACs in Shi-koku and the Kii peninsula (Figs. 1C and 1D). Detrital zircons of igneous origin were selected for dating by visualizing microdomains with oscillatory zoning structure (Corfu et al., 2003) with cathodoluminescence (CL) imaging. The U-Pb isotope analyses of zircons were per-formed by LA-ICP-MS at Kyoto University, Japan. Ablation was done using a pulsed 193 nm Ar excimer laser with fl uence of ~2.72 J/cm2 and irradiance of ~0.54 GW/cm2 at a repetition rate of 6 or 8 Hz and pit size of ~15 µm. See Iizuka and Hirata (2004) for more detailed ana-lytical procedures.

The U-Pb ages of 505 analyzed zircon grains are summarized in Tables DR1–DR8 and Fig-ures DR1 and DR2 in the GSA Data Repository1. Figure 2 summarizes the 206Pb/238U age popula-tion of the detrital zircons as probability age

*E-mail: kazumasa@ea.c.u-tokyo.ac.jp.

Tectonic erosion in a Pacifi c-type orogen: Detrital zircon response to Cretaceous tectonics in JapanKazumasa Aoki1*, Yukio Isozaki1,2, Shinji Yamamoto1, Kenshi Maki3, Takaomi Yokoyama3, and Takafumi Hirata3

1Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan2 Département des Sciences de la Terre, Université Lille 1 (UMR 8217 Géosystèmes CNRS–Lille 1), 59655 Villeneuve d’Ascq, France3Department of Geology and Mineralogy, Kyoto University, Kyoto 606-8502, Japan

ABSTRACTU-Pb dating of detrital zircons from the Lower Cretaceous Sanbagawa and the recently rec-

ognized Upper Cretaceous Shimanto high-pressure (HP) metamorphic rocks in southwestern Japan has revealed the presence of abundant Proterozoic (ca. 1500–2000 Ma) detrital grains. In contrast, coeval non- to weakly metamorphosed accretionary complex (AC) and forearc basin sediments in southwestern Japan lack these older signatures. The only possible source of the Proterozoic detrital grains is the Jurassic AC in southwestern Japan, which structurally overlies the Cretaceous HP units. The Proterozoic grains were incorporated into the protoliths of HP-ACs, without polluting coeval forearc basin to trench sediments, likely by tectonic ero-sion in the forearc domain. Along the Cretaceous Wadati-Benioff plane, the tectonic erosion peeled off the sole part of the pre-existing forearc crust and mixed it with the subducting trench sediments prior to the peak HP metamorphism. In the Cretaceous subduction-related margin around Japan, the tectonic erosion likely occurred twice.

GEOLOGY, December 2012; v. 40; no. 12; p. 1087–1090; Data Repository item 2012315 | doi:10.1130/G33414.1 | Published online 18 September 2012

© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

1GSA Data Repository item 2012315, Tables DR1–DR8 (isotopic analytical data) and supplemen-tal fi gures, is available online at www.geosociety.org/pubs/ft2012.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

1088 www.gsapubs.org | December 2012 | GEOLOGY

frequency curves, according to Isoplot/Ex 3.0 (Ludwig, 2003). In the Sanbagawa orogen, the youngest U-Pb age for each component is given as follows: the Sanbosan AC (JC6) 158 ± 5 Ma, Ryoseki-Monobegawa FAB sediments (JC8) 125 ± 2 Ma, and Sanbagawa HP-AC (BK11) 156 ± 7 Ma (this study; Nakama et al., 2010). The deposition age of each unit is constrained to the Early Cretaceous by the youngest age of detrital zircon, conventional K-Ar mica ages, and microfossil age (Isozaki and Itaya, 1990). All these units belong to the Early Cretaceous Sanbagawa orogen (ca. 160–90 Ma; Aoki et al., 2011). Concordia diagrams and age frequency curves for the Sanbosan AC and the Ryoseki-Monobegawa clastics also show an older age cluster of 150–300 Ma. The Sanbagawa HP-AC, Sanbosan AC, and Ryoseki-Monobegawa clas-tics commonly have numerous detrital zircons of ca. 150–300 Ma; however, there is a notable difference between the fi rst and the latter two; the fi rst has abundant Proterozoic (ca. 1500–2000 Ma) detrital zircons that are absent in the latter two.

In the Shimanto orogen, the age spectra of detrital zircons, including the youngest grain, are totally different from those of the San-bagawa orogen, i.e., Northern Shimanto AC (09405-4) 100 ± 3 Ma, Izumi FAB strata (IZ01) 82 ± 3 Ma, Shimanto HP-AC (BK12, BK7, and NK1) 85 ± 7, 91 ± 6, and 82 ± 14 Ma respec-tively (this study; Otoh et al., 2010). Their dep-

Biwa Lake

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DSanbagawa HP-AC(high-P metamorphic belt)

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Ryoseki-Monobegawa FAB(forearc basin)

Sanbagawa orogen

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San-in belt (batholith belt)

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38° N 136° E

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ceneEarly Eocene5070110120 (Ma)130146

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Shimanto HP-AC

Northern Shimanto AC

San-in batholith belt

Izumi FAB

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exhumation and hydration

exhumation and hydration

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Shimanto HP-AC (BK12: 35 grains)

Northern Shimanto AC (09405-4: 36 grains)

Rs-Mn FAB (JC8: 112 grains)

Izumi FAB (IZ01: 44 grains)

Shimanto HP-AC (BK7: 49 grains)

Shimanto HP-AC (NK1: 76 grains)

ESanbagawa HP-AC (BK11: 56 grains)

Sanbagawa orogen Shimanto orogen

Figure 1. A: Spatial distribution of Cretaceous orogens in southwestern Japan (modifi ed from Aoki et al., 2011). B: Chronology of the San-bagawa and the Shimanto orogenies. C and D: Distribution of major orogenic components of the Sanbagawa and Shimanto orogens. Circles show sampling points. 1—BK11 (Sanbagawa HP-AC); 2—JC8 (Ryoseki-Monobegawa Group); 3—JC6 (Sanbosan AC); 4—09405-4 (Northern Shimanto AC); 5, 6, and 7—BK12, BK7, and NK1 (Shimanto HP-AC); 8—IZ01 (Izumi Group). AC—accretionary complex; FAB—forearc basin; HP-AC—high-pressure metamorphic rocks.

Figure 2. Probability age frequency curves of analyzed samples from the Sanbagawa (A, C, E) and Shimanto (B, D, F, G, H) orogens. Rs-Mn—Ryoseki-Monobegawa.

GEOLOGY | December 2012 | www.gsapubs.org 1089

ositional ages are constrained by the young-est U-Pb zircon age and fossil ages. All these sediments were accumulated during the Late Cretaceous Shimanto orogeny (ca. 90–70 Ma; Aoki et al., 2011). Concordia diagrams and probability density curves show that the North-ern Shimanto AC and the Izumi FAB clastics have U-Pb age clusters of ca. 100–300 and 80–110 Ma, respectively. In contrast, the Shi-manto HP rocks BK7 and BK12 have a distinct age spectrum with two prominent age clus-ters, i.e., 90–250 Ma and ca. 1500–2000 Ma. These results clearly show that the Late Cre-taceous (ca. 90–85 Ma) FAB and trench sedi-ments of southwestern Japan were depleted in the Proterozoic zircons, whereas the nearly coeval protoliths of the Shimanto HP-AC were enriched in these older zircons.

DISCUSSIONSandstones in the forearc usually contain

abundant surface-eroded material from the exposed crusts of the active continental margin, i.e., older continental basement and younger arc crust. All contemporary sandstones in the same forearc (i.e., FAB and trench sediments) are generally expected to share an identical age population of detrital zircons in front of the same provenance. In the case of Cretaceous Japan, this study indeed confi rmed that the Lower and Upper Cretaceous sandstones of the Sanbagawa and Shimanto orogens contain abundant detrital zircons of late Paleozoic to mid-Mesozoic age, proving that the late Paleo-zoic to mid-Mesozoic arc batholith belts were exposed extensively in the Cretaceous arc-trench system around Japan. In particular, the dominance of the late Paleozoic–Mesozoic zir-cons also indicates that the Mesozoic batholith belts likely have built a large barrier (mountain range) to suppress the terrigenous fl ux from the older continental crust of China.

Nonetheless, we have identifi ed the occur-rence of Proterozoic zircons in the metasand-stones of both the Sanbagawa and Shimanto HP-ACs. This signal has never been detected in the nonmetamorphosed sediments of the coeval AC and FAB sediments; thus this highlights a remarkable contrast to the protoliths of the HP-ACs. Because the Japanese geology is essen-tially characterized by the parallel arrangement of most of the Phanerozoic orogenic elements (Isozaki, 1996), along-arc difference in prov-enance composition is unlikely to explain the above phenomena. The conventional surface transport of terrigenous clastics across the forearc cannot explain this conundrum; thus another possible supply mechanism is needed for delivering the older detrital zircons into the protoliths of HP-ACs.

In East Asia around Japan, Paleo-Meso-proterozoic crust occurs solely in north China (e.g., Maruyama et al., 1989). Most of Japan

was separated from north China until the Middle Triassic because the Paleozoic evolu-tion of Japan occurred mostly along the Pacifi c margin of south China (Isozaki et al., 2010). The Middle Triassic continental collision between north and south China generated an ultrahigh-pressure metamorphic belt along the collisional suture. After the Late Triassic uplift of the suture zone, likely forming a mountain range, the Japan margin started to receive ter-rigenous clastics from the uplifted domain, in particular from the hanging wall of the suture, i.e., north China. The Jurassic ACs of the Mino-Tanba (Chichibu) belt in Japan contain abundant Mesoproterozoic clasts, i.e., from boulders of gneiss/granite to detrital grains of zircon/monazite (e.g., Shibata and Adachi, 1974; Suzuki et al., 1991). During the Creta-ceous, therefore, possible sources of the Paleo-Mesoproterozoic zircons in Japan were either the North China Craton in the interior of the margin or the Jurassic AC in the forearc.

Sanbagawa CaseThe extreme rareness of Proterozoic zir-

cons in the Lower Cretaceous forearc and trench sandstones (Figs. 2A and 2C) indicates that the Jurassic–Cretaceous batholith belts likely have blocked the across-arc surface transportation of Proterozoic zircons from north China to the forearc. As to the Protero-zoic zircons in the Sanbagawa HP-AC, the one and only possible source in front of the batholith belts was the Jurassic AC; however, this unit was not yet exposed on surface then to feed the FAB and trench.

In order to explain this apparent discrepancy, we speculate that the Proterozoic zircons in the Sanbagawa HP rocks were supplied from the structurally overlying Jurassic AC within the forearc crust through tectonic erosion, i.e., a mechanical material transferring process beneath the forearc along the Wadati-Benioff plane (e.g., von Huene and Scholl, 1991; Yamamoto et al., 2009; see also Stern, 2011, for a longer refer-ence list). Tectonic erosion was classifi ed by von Huene and Lallemand (1990) and Vannucchi et al. (2008) into two distinct processes, i.e., fron-tal erosion and basal erosion. The former process occurs adjacent to the trench to tectonically erode pre-existing accretionary wedge/slope sediments. On the other hand, the latter works at much deeper levels of the Wadati-Benioff plane where mate-rial from the hanging wall is tectonically peeled off and mixed with subducted trench sediments within the subduction channel (e.g., Clift and Vannucchi, 2004; Clift et al., 2009). The basal erosion appears more effective in mechanical mixing of protoliths of HP-ACs, and in fact, this is the only possible mechanism that can explain the unique abundance of ca. 1500–2000 Ma zircons solely in the Cretaceous HP metasand-stones in Japan. The Proterozoic zircons were likely reworked by basal erosion from the over-lying Jurassic AC above the Cretaceous Wadati-Benioff plane (Fig. 3A). The depositional age of the trench sediments was estimated at ca. 150 Ma on the basis of the youngest detrital zircon age (this study) and microfossil ages (Isozaki and Itaya, 1990). On the other hand, the peak San-bagawa metamorphism occurred in 120–110 Ma (mid to Early Cretaceous) (Itaya et al., 2011).

Jurassic ACSanbosan AC(frontal erosion)oceanic crust + sediments

not to scale

Shimanto HP-AC(deposition age: ca. 90–85 Ma)

N. Shimanto AC

Shimanto HP-AC(deposition age: after ca. 80 Ma)

Sanbagawa HP-AC (basal erosion)Izumi FAB sediments (with ca. 100–80 Ma zircons) B

normalfault

sediments

Rs-Mn FAB sediments (with ca. 300–150 Ma zircons)

Sanbosan AC (with ca. 300–150 Ma zircons)

supplyJurassic AC (previously formed)

oceanic crust

Sanbagawa HP-AC(with ca. 300–150 Ma + 1500–2000 Ma zircons)

tectonic erosion(basal erosion)

not to scale

Décollement

AFigure 3. A: Schematic il-lustration of the tectonic erosion in the subduc-tion zone at the time of the Sanbagawa orogeny. Basal erosion likely oc-curred by infi ltration of fl uid from subducting ma-terial (modifi ed from von Huene et al., 2004) into the previously formed Ju-rassic accretionary com-plex. The fractured frag-ments were then mixed with subducted materials on the incoming plate. B: Schematic illustration of the tectonic erosion of the Shimanto orogeny. Tectonic erosion likely occurred by the subduc-tion of the Izanagi-Kula ridge. Transferred mate-rial was mixed with trench sediments.

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Therefore, the timing of tectonic erosion and material mixing was constrained to sometime in the Early Cretaceous (150–120 Ma).

Shimanto CaseAmong the three units of the Shimanto orogen,

the protolith of HP-AC alone is enriched with the Proterozoic zircons. This likewise indicates that the protoliths of the Shimanto HP rocks (90–85 Ma) incorporated older zircons from pieces of the Sanbagawa HP-AC and/or Lower Creta-ceous Sanbosan AC through the tectonic erosion of the hanging wall. On the basis of the above discussion, we conclude that extensive tectonic erosion has occurred at least twice in Cretaceous Japan, and that the trench possibly retreated toward the continental side signifi cantly. The latter speculation can be checked by the relative position of coeval arc batholith belts. In gen-eral, when a subduction-related orogen grows oceanward, the positions of trench and volcanic front naturally shift in parallel to the ocean side. From this conventional viewpoint, the relative position of the Late Cretaceous San-in batholith was a long-term mystery in the Japanese geol-ogy, because it occurs on the continent side of the older mid to Late Cretaceous Ryoke-Sanyo batholith (Fig. 1). This apparent contradiction in the distribution of the Cretaceous batholith belts is readily explained by the proposed tectonic erosion in the forearc domain; i.e., the entire arc-trench system likely retreated continentward in the Late Cretaceous, forming the San-in belt on the continent side of the older Ryoke-Sanyo belt.

At present, the trigger of the Cretaceous tectonic erosion in Japan is still unknown. The subduction of topographical features, such as seamounts, oceanic plateaus, or ridges, might induce pervasive erosion of the overriding plate by the frictional mechanical coupling between the two converging plates (e.g., Dominguez et al., 2000). Maruyama and Seno (1986) once suggested the northward passage of the Izanagi-Kula ridge along the Cretaceous (ca. 90–85 Ma) Japan margin. If this was the case, the putative tectonic erosion during the Shimanto orogeny might have been driven by the active ridge sub-duction at ca. 90–85 Ma (Fig. 3B).

The comparison of zircon age spectra of major units within the same Cretaceous orogen in Japan has shown the utility of this approach in identifying ancient tectonic erosion and offers further application to other orogens of different time-space framework.

ACKNOWLEDGMENTSK. Collerson corrected the English language. T.

Sato and R.S. Hori helped in sample preparation. P.A. Cowie, P. Vannucchi, and one anonymous reviewer provided constructive review comments. This study was fi nancially supported by a Research Fellowship of the Japan Society for the Promotion of Science (JSPS)

for Young Scientists (23-6135 to K.A.) and a Grant-in-Aid of the JSPS (20224012 to Y.I.).

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Manuscript received 15 March 2012Revised manuscript received 10 May 2012Manuscript accepted 20 May 2012

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