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Structure and Composition of the Aleutian Island Arc and Implications for Continental Crustal Growth

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Structure and Composition of the Aleutian Island Arc and Implications for Continental Crustal Growth Structure and composition of the Aleutian island arc and implications for continental crustal growth W. Steven Holbrook Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA D. Lizarralde Danish Lithosphere Centre, Øster Vøldgade 10, Copenhagen DK-1350, Denmark S. McGeary Department of Geology, University of Delaware, Newark, Delaware 19716, USA N. Bangs University of Texas Institute for Geophysics, Austin, Texas 78759, USA J. Diebold Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA ABSTRACT We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of conti- nental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a conti- nental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two. INTRODUCTION ing of the composition and rates of magmatic Eocene lower sequence of volcanic rocks, an Understanding the origin of continental crust is production of island arcs. Geophysical data pro- Oligocene to Miocene middle sequence of hampered by our limited knowledge of the com- vide key constraints by providing estimates of marine sedimentary rocks, and a Pliocene and position and structure of island arcs, which have magma volumes and a basis for comparing prop- Quaternary upper sequence of sedimentary and been proposed as a principal site of crustal genesis erties of island arcs and continental crust. In this igneous rocks (Scholl et al., 1987). Magmatism at least throughout the Phanerozoic, and perhaps paper we present a new seismic velocity model of waxed and waned over time (Fournelle et al., longer. Two decades ago, Taylor and McLennan the Aleutian island arc, based on seismic data 1994), with a major arc-building episode in the (McLennan and Taylor, 1982; Taylor, 1977; Taylor acquired in 1994. Our results show that, although Eocene and a concentration of magmatic activity and McLennan, 1981) proposed the “andesite the volume of crust created in the Aleutian island to form summit volcanoes ca. 40 Ma (Scholl model” of continental crustal growth, which holds arc is greater than previously supposed, the seis- et al., 1987). The arc is structurally segmented that arcs produce crust of bulk andesitic composi- mically inferred composition and reflectivity of into blocks that have undergone clockwise rota- tion, in accord with the andesitic bulk composition that crust are unlike those of mature continental tion (Geist et al., 1988). Aleutian arc lavas range of continental crust (Christensen and Mooney, crust, implying that, if island-arc crust forms a in composition from basalt to dacite, with rare 1995; Rudnick and Fountain, 1995). Growing significant portion of continental crust, it must be rhyolite (Fournelle et al., 1994; Kay et al., 1982), evidence, however, indicates that the bulk compo- substantially modified during or after accretion to but the dominant lava is basaltic (Myers, 1988). sition of island arcs is closer to basalt than to a continental margin. A geophysical study by Grow (1973) found a andesite, thus posing an apparent paradox in the crust of maximum 25 km thickness and a volume “island arc” model of continental crustal growth GEOLOGIC SETTING AND SEISMIC of about 2300 km3 per kilometer of arc. (e.g., Kay and Kay, 1986; Smithson et al., 1981). EXPERIMENT The data reported in this paper were acquired in Delamination of mafic and ultramafic lower crust The Aleutian island arc, which is the result of 1994 during a two-ship seismic reflection and has been proposed as a possible solution to this northwestward subduction of the Pacific plate refraction survey. Shots fired by the 20-element paradox. Alternatively, Kelemen (1995) has pro- beneath the North American plate, formed in the airgun array of the R/V Maurice Ewing were re- posed that island-arc crust may contain a substan- early Eocene (55–50 Ma), probably in response corded at near-vertical incidence on a 4-km multi- tial proportion of andesites with high Mg/(Mg + to buckling of the Kula plate (Scholl et al., 1987). channel hydrophone streamer on the Maurice Fe) composition. Reflection seismic and mapping data indicate Ewing and at farther offsets on ocean-bottom In order to test such models of continental that basement rocks of the Aleutian morphologic instruments (Woods Hole Oceanographic Institu- crustal formation, we need improved understand- ridge comprise three stratigraphic units, an tion hydrophones and U.S. Geological Survey Data Repository item 9905 contains additional material related to this article. Geology; January 1999; v. 27; no. 1; p. 31–34; 2 figures; 1 table. 31 Figure 1. P-wave velocity model of litho- 0 4.5 sphere along Aleutian island-arc profile 5.0 4.3 5 4.5 5.2 5.4 5.2 4.4 A1 (white line, lower left inset).Velocities 2.5 5.4 10 6.4 5.0 6.0 6.5 6.6 6.6 (white numbers) are contoured every 5.0 6.8 0.2 km/s from 5.0 to 7.2 km/s. White cir- 15 7.8 7.2 cles are earthquake hypocenters, from 20 7.0 database provided by R. Engdahl, U.S. 25 Geological Survey; only those hypocen- 30 8.0 ? ters are plotted that lie within 125 km to 35 west of line A1 and have hypocentral 190˚ 195û 200˚ ? depth-determination errors <4 km. Black 40 60 circles show locations of ocean-bottom 45 ˚ seismic instruments on which wide- 50 58û Depth (km) angle data were recorded. Lower right 55 56û inset shows part of stacked, multichan- 60 nel seismic (MCS) reflection data on 54û profile A1, extending 25 km horizontally 65 and from 9 to 17 s two-way traveltime. 70 52û Red box shows approximate region cov- 75 50 ered by MCS data in inset; bold white 80 ˚ lines show migrated positions of sev- 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 eral prominent events in MCS data. Distance (km) seismometers) deployed from the R/V Alpha that continues 100 km behind the arc, and a sub- by (1) the rough similarity in thickness and Helix and on portable seismometers deployed on ducting slab that is only 50–60 km beneath the P-wave velocity of this layer and the Pacific Aleutian islands (Fliedner and Klemperer, 1998). present-day arc platform. In addition, one feature oceanic crust south of model km 100 and (2) the One along-arc and two arc-crossing profiles were is notably absent: there is virtually no material continuation of the layer to the north end of the recorded. In this paper we present the P-wave within the arc with velocities of 6.0 ± 0.4 km/s. model, at a depth at which oceanic crust of the velocity structure of the island arc along profile This finding is in marked contrast to recent results backarc must eventually appear. Although our A1, which crosses the island arc in Seguam Pass, from the Isu-Ogasawara arc (Suyehiro et al., survey did not extend far enough to confirm the between Seguam and Amlia Islands (Fig. 1). 1996) and to the velocity structure of continental continuity of the mid-crustal layer farther into the crust, as we discuss in the following. backarc, we tentatively interpret this layer as a VELOCITY MODEL AND CRUSTAL The interpretation of composition from seismic thinned, intruded remnant of Kula plate oceanic COMPOSITION P-wave velocity is nonunique and affected by crust. In this case, the bulk composition of the The P-wave velocity model shown in Figure 1 numerous factors such as pressure, temperature, layer would presumably be that of mid-ocean was derived by traveltime inversion (Zelt and and porosity. Nonetheless, given constraints on ridge basalt (MORB). Smith, 1992) of wide-angle reflections and re- pressure and temperature and some reasonable Velocities of 6.9–7.3 km/s in the thick lower fractions recorded on ocean-bottom instruments assumptions, P-wave velocities can provide in- crust are indicative of a mafic bulk composition on line A1.1 Velocities in the arc are constrained sight into crustal composition. In the upper 7 km (Christensen and Mooney, 1995; Holbrook et al., by intracrustal refractions and reflections ob- of the crust, velocities of 4.3–5.4 km/s are too low 1992). Two candidate compositions for the lower served on 10 instruments. In addition, the struc- to correspond to crystalline rocks and thus indi- crust (Kay and Kay, 1985) are the mafic residua of tures of the forearc and backarc basins were cate fractured, porous, or altered rock. On the either calc-alkalic fractionation (43.2 wt% SiO2, constrained by reflections on the stacked multi- basis of exposed geology, we interpret the upper 13.2% MgO) or tholeiitic fractionation (47.9% channel seismic (MCS) section (Bangs et al., crust to consist largely of extrusive and intrusive SiO2, 12.5% MgO).
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