Complex Patterns of £Uid and Melt Transport in the Central Andean Subduction Zone Revealed by Attenuation Tomography
Total Page：16
File Type：pdf, Size：1020Kb
Available online at www.sciencedirect.com R Earth and Planetary Science Letters 215 (2003) 105^119 www.elsevier.com/locate/epsl Complex patterns of £uid and melt transport in the central Andean subduction zone revealed by attenuation tomography B. Schurr a;Ã, G. Asch a, A. Rietbrock b, R. Trumbull a, C. Haberland a a GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany b Universita«t Potsdam, Institut fu«r Geowissenschaften, P.O.Box 601553, 14415 Potsdam, Germany Received 18 December 2002; received in revised form 25 July 2003; accepted 28 July 2003 Abstract 31 We present a high resolution 3-D model of P-wave attenuation (Qp ) for the central Andean subduction zone. Data from 1500, mostly intermediate depth (60^250 km) earthquakes recorded at three temporary seismic networks covering the forearc, arc, and backarc around 23‡S were used for tomographic inversion. The forearc is characterised by uniformly high Qp values, indicating low temperature rocks, in accordance with low surface heat flow values. Prominent low Qp anomalies are found beneath the magmatic arc and the backarc in the crust and mantle. Continuous regions of low Qp connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc. Fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge, where they induce melting, explain our observations. We propose that low Qp regions indicate source and ascent pathways of metamorphic fluids and partial melts. Ascent of fluids and melts, as imaged by seismic Qp, are not vertical, as is often implicitely assumed. Instead, sources of fluids are located at different depth levels, and ascent paths are complex and exhibit significant variation within the study area. The largest Quaternary backarc volcano Cerro Tuzgle is fed by mantle melts which are imaged as a plume of low Qp material that reaches to the strong earthquake cluster at 200 km depth. ß 2003 Elsevier B.V. All rights reserved. Keywords: seismology; attenuation tomography; subduction; £uids and melts; central Andes 1. Introduction Widely accepted models for subduction zone PACS classi¢cation codes: 91.30.-f; 91.45.Qv; 91.45.Cg; metamorphism and arc volcanism suggest that de- 91.35.Gf volatisation reactions in subducting oceanic litho- sphere release water and other volatile elements * Corresponding author. Present address: CTBTO, P.O. into the overlying mantle wedge. Water greatly Box 1200, Wagramerstrasse 5, A-1500 Vienna, Austria. reduces the solidus temperature of peridotite and E-mail addresses: [email protected] (B. Schurr), [email protected] (G. Asch), [email protected] triggers partial melting of the mantle wedge, pro- (A. Rietbrock), [email protected] (R. Trumbull), viding the primary magma source for volcanic [email protected] (C. Haberland). arcs [1,2]. A number of studies have addressed 0012-821X / 03 / $ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00441-2 EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart 106 B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 details of the mineral reactions involved in the cesses in the Andes’ (SFB 267). Together, these release of volatiles via subduction metamorphism three networks cover the forearc, arc, and backarc and a voluminous literature also exists on the na- of the Andean continental margin. The overlap- ture and origin of subduction-related magmas. ping distribution of stations and the use of con- Clearly, £uids are generated in the slab and they sistent instrumentation in all three deployments £ux partial melting in the mantle wedge. But has allowed the data sets to be merged, producing questions of where and how £uids leave the a self-consistent model of the entire subduction slab, which pathways they follow through the zone. This study builds on the workof Haberland mantle wedge and by what mechanism, are still and Rietbrock [8] who combined PISCO and AN- open, largely because of the lackof observational CORP data for Qp tomography. The resolution of data. Based on the observation that Benio¡ zones their model was restricted to the forearc and west- are found at a near-constant depth of about 110 ern part of the magmatic arc. Large parts of the km beneath volcanic fronts and on the assump- mantle wedge beneath the arc and the entire back- tion that £uids migrate more or less vertically arc region were not imaged due to lackof ray through the mantle, the source depth of arc mag- coverage. By adding data from the PUNA de- mas is also commonly considered to be approxi- ployment in the arc and backarc regions of Ar- mately constant [1,3]. gentina, and from a re-occupation of PISCO sites Geochemical and isotopic studies of arc vol- in the Chilean arc and forearc [9], the new tomo- canic rocks provide important insights into the graphic models presented here permit a compre- composition and history of magma sources, but hensive and detailed view of the central Andean are generally insensitive as to the location of the subduction factory. source and the path taken during ascent. How- ever, seismic tomography has the potential to pro- vide such information since £uids (aqueous or 2. Tectonic setting melts) signi¢cantly alter the elastic properties of rocks. New seismological data sets from dense The magmatic arc at the western South Amer- networks now allow imaging of subduction zone ica margin is segmented into northern, central, structure with su⁄cient detail to address these and southern zones with magmatic gaps between questions of subduction processes. One of the them. Our study concerns the Central Volcanic most useful properties for imaging the e¡ects of Zone (CVZ, 16^28‡S), where the present conver- £uids is seismic e⁄ciency (Q or attenuation31). In gence rate is 65 mm/yr based on GPS data [10] the chemically relatively homogeneous mantle, and the subducted Nazca plate dips moderately temperature and £uids are the dominant factors at an angle of W35‡ [9]. The CVZ is of special in£uencing intrinsic seismic Q. It was shown in interest because covergence of the Nazca and con- laboratory experiments that intrinsic Q increases tinental South American plates led to intense monotonously with homologous temperature, shortening in the late Oligocene and Miocene. that is, the temperature of a material relative to During this period, the world’s second-largest pla- its solidus [4]. Water dramatically reduces the sol- teau, the Altiplano^Puna plateau, with crustal idus temperature of rocks [5] and also enhances thicknesses as great as 70 km [11] was formed. anelastic relaxation [6]. This makes seismic at- Neogene arc volcanism in the CVZ comprises tenuation not only a good gauge of temperature, three associations: (1) andesitic to dacitic strato- but also for the presence of aqueous £uids. volcanoes of the frontal arc, which exhibit a high In this paper we present a tomographic model degree of crustal contamination, (2) small ma¢c of seismic attenuation for the southern central centres and basaltic ¢ssure £ows typical of the Andes based on combined datasets from the three backarc region (an exception is Cerro Tuzgle, temporary seismic networks: PISCO’94 [7], AN- the only Quaternary stratovolcano in the backarc CORP’96 [8], and PUNA’97 [9] which were active [12]), and (3) large-volume (ca. 104 km3) dacitic to under the research programme ‘Deformation Pro- rhyodacitic ignimbrites which erupted from huge EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 107 Fig. 1. (a) Left: topography and seismograph locations for the three temporary networks. Horizontal lines mark the cross sec- tions shown in Fig. 4. Area above 3000 m elevation is grey shaded. The two event-station paths for seismograms in Fig. 2. are also marked. Right: epicentres of the 1500 earthquakes used in the inversions. Grey area indicates extent of Neogene^Holocene volcanic rocks. Young volcanoes and collapse calderas are also marked. (b) Depth maps of Qp through the crust and uppermost mantle. Low Qp anomalies correlate with the trend of the volcanic front, including its eastern de£ection at 23‡S. Very low values of Qp are also found at the base of the crust (85 km layer). caldera complexes in the arc and backarc regions Puna plateau at 20^24‡S indicate anomalously [13] and are thought to represent large-scale crus- high temperatures and the presence of partial tal melting. melts in the mid-crust today. These include high Geophysical anomalies under the Altiplano^ surface heat £ow [14], low P- and S-velocities, EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart 108 B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 high vp/vs ratios [15], and high electrical conduc- ing the P-onset for waveforms with a single-to- tivity [16,17]. Yuan at al. [11] reported a mid-crus- noise ratio greater than 3, in a frequency band tal zone of low seismic velocity (Andean Low Ve- with lower limit of 1 Hz and an upper limit be- locity Zone or ALVZ) revealed by the receiver tween 7 and 30 Hz depending on the signal function method which extends across the entire power. plateau at ca. 20^25 km depth. The strongest The spectrum of a body-wave from the ith anomalies in the ALVZ occur in the area of Neo- earthquake recorded as the jth station can be ex- gene caldera complexes and their magnitude is pressed as such (conversion coe⁄cients greater than 0.3^ tij 0.4) that only large degrees of partial melting Aijðf Þ¼Siðf ÞI jðf ÞGðsÞ exp 3Zf ð1Þ Qij can explain them [18,11].