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Crustal Structure and Evolution of the Mariana Intra-Oceanic Island Arc

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Crustal Structure and Evolution of the Mariana Intra-Oceanic Island Arc Crustal structure and evolution of the Mariana intra-oceanic island arc Narumi Takahashi Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Shuichi Kodaira Yokohama 236-0001, Japan Simon L. Klemperer Department of Geophysics, Stanford University, Stanford, California 94305, USA Yoshiyuki Tatsumi Yoshiyuki Kaneda Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan Kiyoshi Suyehiro ABSTRACT P-WAVE VELOCITY MODELING TECHNIQUE AND RESULTS A new high-resolution velocity model of the Mariana arc-backarc The construction of a velocity model with high reliability and system obtained from active-source seismic profi ling demonstrates high resolution depends on the signal-to-noise ratio of OBS records. velocity variations within the arc middle and lower crusts of inter- Because almost all our OBS records show clear fi rst phases through not mediate to felsic and mafi c compositions. The characteristics of the only the crust but also the mantle to maximum offsets in some cases of oceanic-island-arc crust are a middle crust with velocity of ~6 km/s, over 150 km (Takahashi et al., 2003), our data have suffi cient quality laterally heterogeneous lower crust with velocities of ~7 km/s, and and depth penetration to image the full crustal thickness of the Mariana unusually low mantle velocities. Petrologic modeling suggests that the arc-backarc system. Using these OBS records and our collinear refl ec- volume of the lower crust, composed of restites and olivine cumulates tion section, we constructed a P-wave velocity model from the Parece after the extraction of the middle crust, should be signifi cantly larger Vela Basin across the West Mariana Ridge and Mariana Trough to the than is observed, suggesting that a part of the lower crust, especially Mariana arc by standard tomographic inversion and two-dimensional ray the cumulates, is seismically a part of the mantle. tracing techniques (Zelt and Barton, 1998; Zelt and Ellis, 1988; Zelt and Smith, 1992) (Fig. 2A). The interface locations and velocity contrasts Keywords: seismic structure, intra-oceanic island arc, crustal growth, were confi rmed by minimizing traveltime residuals and by comparing Izu-Bonin-Mariana arc. waveforms between observed and synthetic data. Figure 2B illustrates the good resolution of the fi nal velocity model. INTRODUCTION The interface and velocity nodes are spaced at 5 km intervals, equiva- Oceanic island arcs are regarded as the prime location for the growth lent to the closest OBS spacing. The resolution values are a measure of of the andesitic continental crust. However, this conjecture has largely model reliability and vary from 0 to 1, where values exceeding 0.5 are been based on petrologic and geologic studies (e.g., Rudnick, 1995), and conventionally regarded as reliable (Zelt and Smith, 1992). Based on direct seismological detection of the andesitic continental crust beneath the this standard, almost all velocity nodes within the crust (except for the arcs has remained elusive. Although a thick middle crust with a tonalitic deepest parts of the middle and lower crusts, and the extreme ends of the (intermediate to felsic) velocity of 6 km/s (Christensen and Mooney, 1995; profi le) are reliable. Similarly, the interface nodes for the crucial bound- Kitamura et al., 2003) has been detected in the northern Izu arc (Suyehiro aries between the middle and lower crusts and the Moho exceed 0.5, and et al., 1996), the Aleutian arc has little or no middle crust with a P-wave thus they meet our reliability criterion and justify our interpretation of velocity of 6 km/s (Holbrook et al., 1999; Shillington et al., 2004). Under- the thickness of the middle and lower crusts and the shape of the Moho. standing crustal growth requires not only knowledge of the distribution of the tonalitic middle crust but also imaging of the velocity variations and N 141° 142° 143° 144° 145° 146° 147° 148° heterogeneity within each crustal layer. In particular, the velocities of the 19° 19° West Mariana Mariana middle and lower crusts and the upper mantle should vary progressively arc Parece Vela Ridge during crustal growth. During growth, the initial basaltic crust differen- 18° Basin Mariana 18° tiates to produce a crust with intermediate to felsic components having 1 Trough 10 20 higher SiO content, and the residual dense mafi c component of the arc 30 40 2 50 60 70 crust should be removed either during arc growth, or during subsequent N 80 90 17° N 100 106 PAAN arc-accretion events (e.g., Kay and Kay, 1993; Jull and Kelemen, 2001). JAJA NIA The nature of the crustal differentiation and the return of mafi c compo- 30° 16° nents into the mantle are keys to understanding crustal growth. Shikoku Basin The Izu-Bonin (Ogasawara)-Mariana oceanic island arc developed as ench an oceanic arc within a purely oceanic crust, without continental collision 15° 20° (e.g., Karig and Moore, 1975; Hall et al., 1995). Subduction initiated at 50– PVB iana tr Mar 40 Ma, and an early initial arc with no backarc rifting formed the nucleus of KPR 14° MT MA WMR 143° 144° 145° 146° 147° 148°E what has since become the Kyushu-Palau Ridge, the present Mariana arc- 10° backarc system, and northern Izu arc (Stern et al., 2003) (Fig. 1, inset). The 130° 140° 150°-10000 -8000-6000-5000-4000-3000-2000-1000 0 E backarc spreading rifted off a part of this early arc to form the Kyushu-Palau Bathymetry (m) Ridge, to the west of the now-inactive Shikoku and Parece Vela Basins at 30 or 29 Ma, and rifting ceased at 15 or 16 Ma (Okino et al., 1994, 1998). Figure 1. Bathymetry of Mariana arc-backarc system around our wide-angle seismic profi le. Thick black line and open circles indicate The Miocene Mariana arc split into the currently active Mariana arc as a locations of air gun shooting line and ocean-bottom seismographs non volcanic margin and the inactive West Mariana Ridge by the Mariana (OBSs), respectively. Numerals denote site numbers of OBSs. Inset Trough and backarc spreading center since 7 Ma (e.g., Stern et al., 2003), shows our profi le superimposed on bathymetry of entire Izu-Bonin- while the northern Izu arc continues to grow as an unrifted arc. For our Mariana arc, and locates Parece Vela Basin (PVB), West Mariana Ridge (WMR), Mariana Trough (MT), Northern Izu arc (NIA), Kyushu seismic study of crustal growth, we conducted a deep seismic profi le, with Palau ridge (KPR), Mariana arc (MA), and Mariana Trench. Open 106 ocean-bottom seismographs (OBSs) and an air gun array with a total arrows indicate boundaries between the arc and backarc areas used capacity of 196.6 L (Takahashi et al., 2003) (Fig. 1). to calculate crustal volumes. © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2007; 2007 v. 35; no. 3; p. 203–206; doi: 10.1130/G23212A.1; 3 fi gures; 1 table; Data Repository item 2007046. 203 W E Mariana arc serpentinite Parece Vela Basin West Mariana Ridge Mariana Trough diapir A 0 current arc frontal arc 4 trough axis forearc 4.0-5.2 UC 4 UC 5 4 OL2 5.0-6.0 5.9 OL2 5 6.0-6.5 5 5 6 6.9-7.2 OL3 6.7-7.17.1-7.4 6.1-6.5 6.3-6.5 10 OL3 6.6-6.9 6 Middle crust 7.2-7.4 7 Middle crust Moho 7.9 7.6 7.9 8.0 6.7-7.2 7.1 6.7-6.9 7.8 6.7 7 15 7.3 8 Lower crust Lower crust Moho Upperp mantle 7.3 20 7.6 8 7.7 Depth (km bsl) 7.7 25 Upper mantle 8 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Vp (km/s) (km/s) 30 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 B 0 5 0.5 0.5 0.5 0.5 10 0.5 Moho 0.5 15 0.5 0.5 Moho 20 Interface node Depth (km bsl) 0.00-0.25 25 0.25-0.50 0.5 0.50-0.75 0.5 0.0 0.2 0.4 0.6 0.8 1.0 30 0.75-1.00 Resolution value for velocity nodes -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 Distance (km) Figure 2. Final velocity model of Mariana arc-backarc system and its resolution. A: Final model. Solid circles indicate ocean-bottom seismo- graph (OBS) locations. Numerals denote P-wave velocity (Vp) (km/s). Open arrows indicate boundaries between arc and backarc areas used to calculate crustal volumes. UC, OL2, and OL3 are upper crust and oceanic layers 2 and 3, respectively. Dark shading shows region of seismic-ray penetration; bsl—below sea level. B: Resolution of fi nal model. Resolutions of velocity nodes are indicated by color scale with contour spacing of 0.1, and those of interface nodes are shown by size of open circles. Red circles indicate OBS locations. Another measure of the quality of the fi t is that almost all residuals are DISCUSSION below 150 ms, which is less than one period of the dominant frequencies Common Structural Characteristics of the Northern Izu Arc of 5 Hz in the OBS data. and the Mariana Arc The Mariana arc consists of the active arc initiated in the Pliocene The crusts of the Mariana arc, West Mariana Ridge, and northern Izu ~ 30 km west of the Eocene-to-Miocene frontal arc and its forearc basin arc share important commonalities (Fig.
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