Petrological model of the northern Izu–Bonin–Mariana arc crust: constraints from high-pressure measurements of elastic wave velocities of the Tanzawa plutonic rocks, central Japan

doi:10.1016/S0040-1951(03)00229-4

Tectonophysics

Volume 371, Issues 1–4, 14 August 2003, Pages 213-221

https://doi.org/10.1016/S0040-1951(03)00229-4 Get rights and content

Abstract

Ultrasonic compressional wave velocities (Vp) and shear wave velocities (Vs) were measured with varying pressure up to 1.0 GPa in a temperature range from 25 to 400 °C for a suite of tonalitic–gabbroic rocks of the Miocene Tanzawa plutonic complex, central Japan, which has been interpreted as uplifted and exposed deep crust of the northern Izu–Bonin–Mariana (IBM) arc. The Vp values of the tonalitic–gabbroic rocks increase rapidly at low pressures from 0.1 to 0.4 GPa, and then become nearly constant at higher pressures above 0.4 GPa. The Vp values at 1.0 GPa and 25 °C are 6.3–6.6 km/s for tonalites (56.4–71.1 wt.% SiO₂), 6.8 km/s for a quartz gabbro (53.8 wt.% SiO₂), and 7.1–7.3 km/s for a hornblende gabbro (43.2–47.7 wt.% SiO₂). Combining the present data with the P wave velocity profile of the northern IBM arc, we infer that 6-km-thick tonalitic crust exists at mid-crustal depth (6.1–6.3 km/s Vp) overlying 2-km-thick hornblende gabbroic crust (6.8 km/s Vp). Our model shows large differences in acoustic impedance between the tonalite and hornblende gabbro layers, being consistent with the strong reflector observed at 12-km-depth in the IBM arc. The measured Vp of Tanzawa hornblende-bearing gabbroic rocks (7.1–7.3 km/s) is significantly lower than that Vp modeled for the lowermost crustal layer of the northern IBM arc (7.3–7.7 km/s at 15–22 km depth). We propose that the IBM arc consists of a thick tonalitic middle crust and a mafic lower crust.

Introduction

Intra-oceanic arc crust is a fundamental building block of continental crust, and the continents are widely believed to have been formed by coalescence of arcs and other oceanic terranes along convergent margins (e.g., Taira et al., 1992, Condie, 1997). The continental crust has an andesitic bulk composition (cf. Rudnick and Fountain, 1995, Wedepohl, 1995, Christensen and Mooney, 1995) and the simple arc accretion model, in which total arc materials are added to continent, implies that the bulk chemical composition of these accreted arc fragments is comparable to the composition of continental crust. However, modern petrological studies indicate that the parental magmas in intra-oceanic arcs are not andesitic but basaltic and these studies argue strongly against the simple arc-accretion model (Arculus, 1981). To resolve this discrepancy between a mantle–crust basaltic flux at convergent margins and an andesitic bulk composition of continental crust, various models have been proposed. Most of them require multistage melting processes of a mantle of ultramafic composition to derive the evolved features of continental crust (for a more detailed discussion, see, e.g., Rudnick, 1995).

Recent seismic experiments carried out at the northern Izu–Bonin–Mariana (IBM) arc Suyehiro et al., 1996, Takahashi et al., 1998 and at the eastern Aleutian arc Fliedner and Klempere, 1999, Holbrook et al., 1999 provide important constraints to understand the structure and composition of intra-oceanic arc crusts. The seismic studies of the northern IBM arc revealed that the middle crust has seismic characteristics typical of felsic plutonic rocks (∼6.0 km/s Vp) at a depth ranging from 7 to 12 km (about 30% of the arc crust) overlying a mafic lower crustal layer (Fig. 1a, 7.1–7.3 km/s Vp; Suyehiro et al., 1996, Takahashi et al., 1998). Based on these velocity structures, Taira et al. (1998) suggest a mean crustal composition of 56% SiO₂ for the northern IBM arc. These results clearly distinguish the northern IBM arc from the Aleutian arc in which such a 6-km-thick “continental crust-like” layer is missing Holbrook et al., 1999, Fliedner and Klempere, 1999, Fliedner and Klempere, 2000. This suggests that the Aleutian arc crust has a mafic bulk composition, in contrast to the andesitic composition of the northern IBM arc (Holbrook et al., 1999).

In this study, we measured P wave velocities of Tanzawa plutonic rocks, which may represent the middle–lower crust of the northern IBM arc (Kawate and Arima, 1998), and compare these measurements with the seismic velocity profile of the northern IBM arc Suyehiro et al., 1996, Takahashi et al., 1998 to develop a petrological model of the northern IBM arc crust.

Section snippets

Geology and sample description

The Tanzawa Mountains, central Japan, provide a unique opportunity to examine the felsic middle crust of the northern IBM arc. Collision of the northern Izu–Bonin–Mariana (IBM) arc system with the Honshu arc occurred after the middle Miocene Itoh, 1986, Soh et al., 1991. One manifestation of this collision is uplift and exposure of a Miocene (7 Ma; K–Ar ages; Saito et al., 1991) plutonic complex covering ∼140 km² in the Tanzawa Mountains (Fig. 1b,c). The Tanzawa plutonic complex intruded into

Experimental procedures

Core specimens of 14-mm diameter were extracted from each rock sample and were cut to 12-mm length. Both ends of the core samples were diamond-polished. These cores were then oven-dried at 120 °C for 24 h to minimize the effect of pore fluid on elastic velocity measurements (cf. Wulff and Burkhardt, 1997). We consider that these cores provide representative samples because these cores contain over 2000 grains (average grain size: 0.7 mm in diameter, volume of core=1850 mm³).

Vp and Vs in the

Results

The ultrasonic compressional and shear wave velocities were measured at 0.1 GPa intervals during pressurization up to 1.0 GPa (Table 2). We also measured Vp and Vs during subsequent depressurization to 0.1 GPa for selected samples (Fig. 2). The Vs values measured below 0.5 GPa during pressurization were significantly lower than those measured during decompression, while comparable values were obtained above 0.6 GPa in all runs. Similarly slower Vp values were measured in pressurization runs but

Crustal structure of IBM arc

Our Vp data measured at 25 °C are compared with the seismic velocity profile of the northern IBM arc Suyehiro et al., 1996, Takahashi et al., 1998 assuming a geothermal gradient of 40 °C/km as the maximum value of the northern IBM arc crust (cf. Nakajima and Arima, 1998). The temperature derivative (−2.7 to −5.3×10⁻⁵ km/s °C) obtained in the present study is considerably lower than those reported by Rudnick and Fountain (1995) for an average of crustal rocks (−4×10⁻⁴ km/s °C). Kern and Richter

Acknowledgments

We are indebted to Sinichi Kawate for providing us with rock samples used in this study. Yoshiyuki Tatsumi and Kazuhiko Ito kindly provided valuable technical support for the high-pressure measurement of elastic velocity. The manuscript was greatly improved by the perceptive comments of Moritz Fliedner and Shaocheng Ji. We thank Narumi Takahashi for valuable comments during study. This study was supported by the Grant-in-Aid for Scientific Research provided by the Japanese Society for the

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Petrological model of the northern Izu–Bonin–Mariana arc crust: constraints from high-pressure measurements of elastic wave velocities of the Tanzawa plutonic rocks, central Japan

Abstract

Introduction

Section snippets

Geology and sample description

Experimental procedures

Results

Crustal structure of IBM arc

Acknowledgments

Tectonophysics

Earth Planet. Sci. Lett.

Tectonophysics

Tectonics of Tanzawa Mountains: constraints from metamorphic petrology

J. Geol. Soc. Jpn.

The velocity of compressional wave in rocks to 10 kilobars: Part 1

J. Geophys. Res.

The velocity of compressional wave in rocks to 10 kilobars: Part 2

J. Geophys. Res.

Seismic velocity structure and composition of the continental crust: a global view

J. Geophys. Res.

Structure of an island-arc: wide-angle seismeic studied in the eastern Aleutian Islands, Alaska

J. Geophys. Res.

Magmatic processes under arc and formation of the volcanic front

J. Geophys. Res.

Structure and composition of the Aleutian island arc and implications for continental crustal growth

Geology

Differential rotation of Northern part of southwest Japan: paleomagnetism of early to late Miocene rocks from Yatsuo Area in Chichibu District

J. Geomagn. Geoelectr.

Petrogenesis of the Tanzawa plutonic complex, central Japan: exposed felsic middle crust of the Izu–Bonin–Mariana arc

Island Arc

Temperature derivatives of compressional and shear wave velocities in crustal and mantle rocks at 6 kbar confining pressure

J. Geophys.

Rock velocities at atmospheric pressure and room temperature in Tanzawa plutonic rocks from central Japan