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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 zone revealed by attenuation tomography

B. Schurr a;, 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 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 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

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 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 and . 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 ^Puna , 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 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 which erupted from huge

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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^ volcanic rocks. Young volcanoes and collapse 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). 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]. Finally, very low Qp anomalies were found in large parts of the mag- where S(f) is the original source spectrum, I(f) the matic arc crust [8,15], where they were attributed instrument response, G(s) geometrical spreading, to an extensive region of partially molten crust [8]. and the exponential term describes the high-fre- Most studies in the south-central Andes have quency fall-o¡ due to attenuation. Q is the quality focused on processes in the crust related to heat factor and describes the fractional energy loss per and £uid/melt transport in the arc and the plateau cycle Q = 32ZE/vE. In the inhomogeneous me- region. Similar information about the underlying dium, the whole path attenuation operator 31 mantle has been lacking. Our study of Q-tomog- tijQij = tij* can be expressed as a path integral raphy provides the ideal complement to previous through ¢eld of (Q(s)v(s))31 : studies because our primary focus is on properties Z and processes in the mantle wedge. ds tij ¼ ð2Þ pathQðsÞvðsÞ

3. Data and estimation of path attenuation t* where v(s) is the velocity along the ray path. The path integral (Eq. 2) closely resembles the basic The data used in this study are P-wave spectra equation for travel time tomography, and if the calculated from seismograms of 1500 local earth- velocity ¢eld v is known, it can be treated equiv- quakes. We also scrutinised S-waves, but strong alently, with the t* operator as basic datum. attenuation causes such a severe lackof signal We determine t* by modeling the spectrum and bandwidth at many stations that retrieval of A(f). Because only the pass band of the recording t* parameters is not possible, preventing a com- system is used, Ij(f) can be neglected. Assuming a prehensive study of this data type. Data were ac- Brune-type source [20,21], the spectrum can now quired during about 100 days, respectively in the be rewritten as: years of 1994 (PISCO), 1996 (ANCORP), and the 3Z second half of 1997 (PUNA). Most earthquakes 6 0ij expð ftijÞ Aijðf Þ¼ ð3Þ originated in the Benio¡ zone. Data acquisition f 2 1 þ employed three-component short-period sensors 2 f ci (MARK L4-3D) and PDAS-100 data loggers re- cording continuously at 100 samples per second where 60 is the signal moment that contains all (Fig. 2). As shown in Fig. 1a, the three networks frequency independent parts, including G(s)=1/s; covered the entire continental margin from fore- fc is the source corner frequency. arc to backarc with an average station spacing of In a non-linear spectral inversion scheme [22] about 40 km. whole path attenuation t* and signal moment Seismic attenuation of the medium between 60 are determined for each ray, accounting for source and receiver can be estimated from the heterogeneous attenuation structure and variable spectra of body-waves. Smoothed spectra [19] radiation from the source. A single corner fre- were determined for 2.56-s time windows follow- quency is sought for each source, neglecting direc-

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Fig. 2. Velocity seismograms and P-wave spectra for two intermediate depth events recorded at the PUNA network. Vertical (upper trace) and horizontal component seismograms are shown for an event beneath the forearc (104 km depth) recorded at the forearc station JER and a backarc event (218 km depth) recorded at the station in the magmatic arc (LAC). Events, stations, and paths are marked in Fig. 1a. For the 2.56-s windows before and after the P arrival, noise and signal spectra, as well as the modeled spectra (thickgrey line) are shown. Whole path Qp is high for the forearc station JER and low for station LAC in the magmatic arc. Note the complete lackof S energy and low-frequency content in seismograms for station LAC. Attenuation clearly in£uences the spectral shape, where it is exploited to determine the t* parameter. tivity e¡ects, which appears justi¢ed for the small arc (LAC). These seismograms are characterised magnitude events (Ml 1.0^4.5) used. Correct sep- by low frequencies, little coda, and an almost aration of t* and fc from the spectrum has been complete lackof S energy. This is typical of checked by analysis of neighbouring ray paths many stations in the arc and backarc. The e¡ect from di¡erent sources. The model assumptions of attenuation is also seen in the shape of the have also been validated for the PISCO and AN- P-wave spectra, which is exploited to determine CORP data by comparing the spectral inversion the t* operator (Fig. 2). results to a spectral ratio technique, which makes no assumptions about the source model. No sig- ni¢cant deviations beyond statistical errors could 4. Attenuation tomography be established [8]. Fig. 2 shows two seismogram examples with From Eq. 2, for a starting model of Q with L their associated P-wave spectra. The two corre- model nodes, known hypocentres, ray paths, and sponding event-station paths are marked in Fig. velocity ¢eld, theoretical t* values can be calcu- 1a. The ray towards station JER travels entirely lated, compared to the observed ones, and related through forearc rocks. The associated seismo- to variations in Q31 : grams show high frequency content, normal S-waves, and high Q . In contrast, the second p XL D t set of seismograms comes from an event beneath tobs3tcal ¼ ij v Q31 ð4Þ ij ij D Q31 l the backarc recorded at a station in the magmatic l¼1 l

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Fig. 3. Synthetic input and recovered model for two exemplary cross sections. Realistic noise was added to the synthetic t* pa- rameters before inversion. Generally, anomalies are recovered well, albeit with blurred geometry and underestimated amplitudes where formal resolution de¢ned by the spread value is low.

For 3-D inversion of Qp structure we use a model variance. Data variance after the 3-D in- modi¢ed version of Thurber’s [23] local earth- version was reduced by 85%. quake tomography code [24]. Hypocentres and the velocity ¢eld were taken from velocity tomog- raphy [25] and held ¢xed in the inversion process. 5. Resolution In this code the Qp model is de¢ned on nodes at the intersections of a rectangular grid. Qp values The appraisal of resolution is vital for mean- between neighbouring nodes are determined by ingful interpretation of seismic tomographic im- linear B-spline interpolation. The spacing of grid ages. We approach this problem by using the nodes was chosen based on ray density and ranges model resolution matrix (MRM) and testing a from 25 to 40 km in the horizontal direction (Fig. synthetic model with the same ray geometry as 1) and 20 to 25 km vertically (Fig. 4). This model the real one to assess solution quality. Each row parameterisation was found to be a good compro- of the MRM (averaging vector) describes the de- mise between robustness of the inversion and de- pendence of a particular model parameter (grid sired resolution. As starting model a homogene- node) on all other parameters. The spread func- ous half-space with the average value of Qp for all tion [26] condenses each averaging vector to a rays (Qp = 600) was chosen. The system of equa- single number that accounts for both the absolute tions (Eq. 4) is solved by damped least squares. value of the diagonal elements (DE), and the From a series of test inversions with varying peakedness of the averaging vector. Large DEs damping parameters, a value of 0.05 was found and small o¡-DEs result in small spread values to optimally weight between data variance and and vice versa. If the spread value for a particular

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Fig. 4. Left: cross sections through the 3-D Qp model. Earthquakes, volcanoes, and stations within 20 km of the section are also plotted. The thickwhite contour indicates regions of good resolution as measured by the spread value. Regions of poor resolu- tion are drawn in slightly faded colour. Right: spread values as a measure of resolution. Low spread means good resolution and little smearing. The thickcontour line of spread = 3 encloses areas of acceptable resolution. Thin white lines contour the averag- ing vector of selected nodes along 40%, 60%, and 80% of the size of the diagonal element, visualising spatial smearing.

EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart 112 B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 node is large, that node is not well resolved and verted Qp model, indicating that the solution is information is smeared to adjacent nodes. To vis- robust against errors in the velocity model used. ualise the direction of this smearing, we contour averaging vectors along fractions of the size of the DE [27]. Spread values and isolines of DE-frac- 6. Results: distribution of seismic Qp tion for some key nodes are plotted in the right- hand panels of Fig. 4. Examination of the ¢gure Fig. 1 shows horizontal sections through the shows that the forearc and western part of the arc tomographic Qp model at 40 and 85 km levels. are well resolved for the entire model, whereas The crustal thickness in this region is 60^70 km, good resolution of the backarc, as measured by so the depth levels represent the mid-crust and the the spread function, is con¢ned to the region uppermost mantle regions, respectively. Fig. 4 south of 22.5‡S where data from the PUNA array shows ¢ve cross sections through the tomographic were available. Minimal smearing occurs in the model over a range of about 300 km EW and 400 forearc, and in that region spatial resolution is km NS. The spacing between the sections is 80 km given by the grid spacing. In the backarc, smear- and the location of the sections is marked on Fig. ing is mostly vertical as indicated by the elongated 1a. The colour scale in the ¢gures is equidistant in 31 averaging contours. The absolute values of the Qp , which re£ects the actual sensitivity of the spread function depend strongly on damping inversion. Also shown in Fig. 4 are the spread and grid geometry, but from synthetic tests and values in the same sections. Within the examination of individual averaging vectors, we spread = 3.0 contour (white outline), the spatial consider parameters with spread values 6 3.0 to resolution of the tomographic sections is deter- be well resolved and have identi¢ed regions with mined mostly by the grid spacing, i.e. 25^40 km larger spread values by faded colours in Figs. 1 horizontally and 20^25 km vertically, and within and 4. those limits the geometry of Qp anomalies are We also performed tests using synthetic data by considered reliable. Some vertical smearing of tracing rays through a 3-D velocity and Qp model nodes in the backarc region is indicated by the to calculate synthetic t* parameters. The 3-D Qp elongate contours of averaging vectors (Fig. 4). model was endowed with anomalies comparable Although the location and shape of anomalies in size and amplitude to those observed in the real are well-resolved, their absolute amplitudes are data, but with di¡erent geometry and sign in or- a¡ected by inversion parameters (damping, sta- der to avoid local mathematical minima in regions tion corrections), making quantitative interpreta- which are not well resolved. Normally distributed tions in terms of temperature or £uid/melt con- noise with a standard deviation of 0.008 s was tents di⁄cult. then added to the t* parameters. The test showed The subduction zone in all sections shows high that anomalies are generally well recovered, Qp regions in the forearc and the subducted slab although geometries may be blurred due to smear- (W600^2000) and prominent low Qp (W80^150) ing and the amplitudes are underestimated be- anomalies beneath the volcanic front and in the cause of damping (Fig. 3). Even in regions that backarc region. The highest Qp values are found are characterised as not well resolved based on in the crust and upper mantle below the Salar de spread values 9 3, the geometry and sign of the Atacama basin. The seismic properties suggest a anomalies are reproduced. The average error of relatively cold and rigid block, which, in the to- 31 the recovered amplitudes is 0.0006 for Qp and mographic sections from 22.1 to 23.5S (Fig. 4), errors on this order of magnitude are considered abruptly limits the western extent of the low Qp representative for the real data set as well. Finally, regions (LQR)s under the magmatic arc (see also we also inverted the synthetic data set using a Fig. 1). simple 1-D velocity model instead of the 3-D vp Since Whitman et al.’s [28] ground-breaking model that was used in the forward calculation. workit is knownthat the southern part of the This caused only a subtle di¡erence in the in- central Andean plateau is underlaid by rocks of

EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 113 low seismic Q. Now the LQRs are imaged in de- low the backarc volcano Cerro Tuzgle. The sec- tail and appear as the most striking and variable ond ‘branch’ overlaps with the ¢rst at 150^200 km features of the tomographic images in Figs. 1 and depth, then diverges up to the west, parallel to the 4. The lowest Qp values (Qp 9 90) in the entire slab and extends into the crust below the volcanic tomographic model are located in the mid-crust front. The latter resembles an inclined low-veloc- (40 km layer) and at the base of the crust and ity channel in the mantle above the subduction uppermost mantle (85 km layer) below the vol- zone in NE Japan reported by Wyss et al. [30]. canic arc (Fig. 1b). Large station corrections in- dicate that the LQRs also penetrate the very shal- low layers of the model, which are not resolved in 7. Low-Q anomalies, £uids, and intermediate-depth Fig. 4. In the crust, the strong LQRs coincide seismicity closely with the belt of Neogene volcanoes includ- ing its de£ection to the east around the Salar de Three physical mechanisms contribute domi- Atacama basin (see 40-km depth section; Fig. 1). nantly to the loss of seismic energy during wave In addition to this, a more di¡use and broader propagation. For dry rocks, dissipation due to LQR in the mid-crust covers a large area of the relative movements along grain boundaries is the backarc beneath the Altiplano^Puna plateau. This main mechanism [31]. This intrinsic attenuation is a crustal feature since it is absent in the 85-km depends strongly on state variables such as pres- depth section of Fig. 1. The broad mid-crustal sure and homologous temperature. If £uids (aque- LQR correlates with the Andean Low Velocity ous and melts) are present, so-called viscous re- Zone of Yuan et al. [11] and the low resistivity laxation and £uid £ow in pores and along grain anomaly of Partzsch et al. [29], both of which boundaries also become important [32]. A further were interpreted to re£ect high temperatures and important factor is scattering at inhomogeneities partial melting in the mid-crust. Note also the [33]. local maximum in the backarc LQR present at Several reasons lead us to attribute the ob- 24‡S and 66.5‡W, the location of the largest back- served LQRs mainly to intrinsic attenuation arc volcanic centre, Cerro Tuzgle. rather than scattering. First, LQRs correlate well In the mantle wedge, the position and geometry with low vp and high vp/vs regions [25], as is ex- of the LQRs are highly variable from north to pected for intrinsic Q controlled mainly by ho- south (Fig. 4). The features of particular interest mologous temperature and the presence of £uids. in terms of subduction processes are the relation- LQRs also coincide with regions of high conduc- ships between the LQRs and intermediate-depth tivity where magneto-telluric measurement exist, earthquake clusters in the subducting slab. In the i.e. in the magmatic arc [34] and in the backarc two northernmost tomographic sections the low- beneath Cerro Tuzgle [35]. Second, seismograms Qp anomaly in the mantle extends from the prom- from stations in the highly-attenuating volcanic inent earthquake cluster at 100 km depth [8] to arc and Puna plateau show simple low-frequency the base of the . In the 21.3‡S section P-pulses with very little coda and S-waves that (top panel of Fig. 4) the LQR is rather well con- are extremely atttenuated or even absent (Fig. ¢ned to the region below the arc, but in sections 2). If scattering was the dominant cause of wave farther south, the low-Q anomalies spread consid- attenuation, energy would be displaced into the P erably to the east. In sections 23.5‡S and 24.2‡S, and S coda, which is not observed. In the forearc, the LQRs lose connection to the 100-km cluster where seismograms show higher complexity (Fig. beneath the forearc and extend towards the zone 2) but attenuation is low, analysis of coda-waves of deeper seismicity at about 200 km depth. Fi- [36] showed that scattering attenuation is compa- nally, the LQR distribution in the 24.2‡S section rable to or smaller than intrinsic attenuation. appears to be branched, with the most prominent Thus, we attribute the low-Qp anomalies in the anomaly extending from the 200-km-depth earth- crust and mantle mainly to the e¡ect of high tem- quake cluster vertically upward into the crust be- peratures, £uids and/or partial melts and interpret

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Fig. 5. Interpretations of the cross sections for Qp at 22.1 and 24.2‡S (Fig. 4). H2O is released from the oceanic lithosphere at the discrete earthquake cluster. H2O causes £ux melting in the overlying hot mantle wedge. Regions of Qp 9 200 are orange. Here, mantle and crustal rocks probably contain signi¢cant amounts of partial melt. Melt ascent ways are not straight up as is often implicitly assumed, but have a signi¢cantly horizontal component. the distribution of LQRs as markers of heat and of magmas, particularly in the backarc where mass transport in the subduction zone (Fig. 5). In thickened cold lithosphere would otherwise choke the crust, the correlation of LQRs with volcanism magmatism. in the arc and backarc, and agreement with other As described above, the distribution of LQRs in geophysical evidence for partial melting [16^18,11] the mantle is complex and variable from north to supports this interpretation. In the mantle, varia- south. However, in general there is a spatial cor- tions in absolute temperature due to complex as- relation of LQRs with the volcanic arc and back- thenospheric structure may in£uence location and arc on the one hand, and with seismicity clusters patterns of the LQRs. A commonly cited geody- in the Benio¡ zone on the other. The clusters of namic model for magma genesis in the backarc of intermediate-depth seismicity in the downgoing the Puna Plateau suggests a comparatively sudden slab and the relationship of LQRs with those clus- in£ux of hot asthenospheric mantle following ters (Fig. 4) are both likely to relate to £uid con- gravitational instability and detachment of litho- trolled processes; their interpretation is the main spheric blocks [37]. These hot mantle rocks may object of the following sections. serve as substrate that allows genesis and ascent The cause of intermediate-depth slab seismicity

EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 115 is generally attributed to metamorphic reactions and regions of low Qp at much greater depths, in the oceanic crust. The most important mecha- implying additional sources of £uids. There is nism may be an increase in pore pressure due to seismic evidence from dispersed body-wave phases dehydration reactions, which reduces e¡ective that hydrated oceanic crust persists to depths stress and allows brittle failure at p-T conditions greater than 200 km in many subduction zones where rocks would otherwise deform in a ductile [40]. Recent studies showed that the - manner (dehydration embrittlement) [38]. A con- out reaction does not completely dehydrate the tributing mechanism are stress changes caused by slab and an estimated 2 wt% of H2O are released the transition from blueschist or amphibolite to in continuous reactions to depths greater than 200 eclogite. Kirby et al. [38] suggested that the km [41,42]. This could explain the continued seis- main cause of intermediate slab seismicity is de- micity at deeper levels in the Benio¡ zone (Fig. 4). hydration embrittlement of pre-existing, strongly It should also be noted that subducted oceanic hydrated faults in the oceanic lithosphere. They crust is not the only potential source of £uids. also noted a peakin the depth distribution of Tatsumi [43] suggested that a hydrated periodite slab seismicity in global subduction zones at ap- above the slab may be dragged down with it by proximately 110 km, which coincides with the viscous coupling, then undergoing dehydration. average depth of Benio¡ zones beneath active Amphibole and chlorite in this layer can be ex- arcs. Wiemer and Benoit [39] investigated the spa- pected to release water at W110 km whereas tial variations of frequency-magnitude relations phlogopite would be more stable, undergoing de- (b-values) of earthquakes in the Alaska and hydration at W200 km depth. Tatsumi [43] ap- New Zealand subduction zones. They found plied this concept to explain the existence in anomalously high b-values (i.e. very frequent, some arcs of a volcanic front 110 km above the small-magnitude events) at around 100 km depth, Benio¡ zone and a second volcanic chain some roughly beneath the volcanic fronts, and attrib- 200 km above the slab. The Tatsumi model prob- uted these anomalies to high pore pressure caused ably is not valid for the central Andes since the by dehydration reactions. A slightly di¡erent dis- seismicity seen in Fig. 4 has been shown [44] to be tribution of seismicity and arc volcanism was re- located within the oceanic crust or the underlying ported from northeastern Japan [30]. There, oceanic mantle. anomalously high b-values were reported at To summarise the points discussed above, inter- 140^150 km depth within the slab, well beyond mediate-depth slab seismicity in subduction zones the position of the frontal arc. An inclined low is most likely related to £uids released by dehy- velocity channel in the mantle wedge above the dration reactions in the oceanic lithosphere and slab was also observed. These features led the such reactions can be expected to take place authors to suggest that magma was generated over the 100^200-km range of depths observed from £uid-£uxed melting at the 140^150 km depth in the present data set (Fig. 4). How metamorphic and that melt ascent was parallel to the slab as £uids are transported from reactions sites into the marked by the low-density channel. overlying mantle wedge is not known but earth- In our study area (Fig. 4), distinct seismicity quakes may play an important role. Davies [45] clusters in the Benio¡ zone occur at 90^110 km proposed that earthquakes in the slab will tempo- depth and 190^250 km depth. The vertical sepa- rarily interconnect pore space, and once a su⁄- ration of the volcanic front from the Benio¡ zone cient water column is interconnected, hydro-frac- varies between 120 (north of 22.5‡S) and 140 km turing at the upper end of the cracks can (23^24‡S). The earthquake clusters at ca. 100 km transport water into the mantle. However, this depth conform to the global average and can be transfer takes place in detail, the association of accounted for by processes related to the break- strong low-Q anomalies in the mantle wedge down of amphibole in the subducting oceanic with seismicity in the slab provides support to crust at pressures of 2.3^2.5 Gpa [1,3]. However, the general model linking slab dehydration with we observe strong intermediate depth seismicity seismicity and mantle melting (Fig. 5).

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8. Fluid and melt pathways through the mantle able geometries we observe in the Q-tomography wedge images also argue against such simple models. However, mantle £ow can in£uence porous £ow Water in the mantle wedge will cause partial of £uids by exerting deviatoric stresses and de- melting in regions where temperatures exceed forming pore space, which can result in very com- the wet solidus of peridotite, and in the sub-sol- plex £ow ¢elds [49]. idus region water will raise the homologous tem- An alternative e⁄cient transport mechanism perature of the mantle. Both processes have the which overcomes the limits of slow mantle £ow e¡ect of lowering seismic Qp and therefore the is fracture propagation [50]. Evidence from Alpine LQRs can be interpreted as rough traces of £uid peridotites suggests that fracturing and £uid £ow or melt pathways in the mantle (Fig. 5). can occur in mantle rocks at depths of at least 70 The Q-tomography images demonstrate that km [51]. Dahm [50] modeled the propagation of the pathways of £uid and melt transport through £uid-¢lled fractures in a subduction environment the mantle wedge can be complex and variable, under varying conditions of slab dip and subduc- and that large lateral distances are covered. A tion velocity using a regional stress ¢eld calcu- detailed interpretation of the LQR patterns in lated for stationary corner £ow. The results terms of transport mechanisms and the dynamics show that even for this simple situation the pat- of mantle £ow in the Andean subduction zone is terns of fracture propagation can be complex. De- beyond the scope of this paper. However, the viatoric stresses cause fractures to propagate sig- scale of variations in the LQRs suggests some ni¢cant horizontal distances away from the wedge broad constraints on possible transport mecha- corner. Models for subduction dips steeper than nisms. The two most commonly cited mechanisms 45‡ show that ascent paths can be vertical upward for £uid transport in the mantle wedge are porous and can also diverge backalong the slab, similarly £ow along grain boundaries and buoyancy-driven to the LQR patterns we observe in our data set propagation of £uid-¢lled cracks. It has been (Fig. 4). Yet, the models assume a uniform elastic shown that grain boundary wetting by £uid in medium, which is unlikely to apply for the real periodite might be complete (dihedral angles asthenospheric mantle undergoing partial melt- 9 60‡) at the pressures and temperatures of the ing. It is also not clear if arrays of isolated £uid- mantle wedge [46], thus allowing porous £ow of ¢lled cracks would cause the large continuous H2O in the mantle. Iwamori [47] developed a nu- Q-anomalies we observe. merical model for permeable £ow and melt gen- The relationship between the distribution of eration in the mantle wedge which envisioned ver- slab seismicity, mantle LQRs and surface volca- tical ascent of £uids into the high-temperature nism in the 24.2‡C section of Fig. 4 are particu- region of partial melting, after which melts are larly interesting. Seismicity is clustered at about transported trenchward by corner £ow. A simple 200 km depth and the mantle above the earth- vector addition of vertical ascent with de£ection quake cluster exhibits a vertical Qp anomaly by corner-£ow was also used by Wyss et al. [30] to which extends into the crust precisely below Cerro explain the inclined channel of low S velocity ex- Tuzgle, the largest Quaternary volcanic centre in tending from the slab to the base of arc volcanoes the backarc. Cerro Tuzgle [12] is a composite stra- in the Japanese subduction zone. U-series dating tovolcano made up of an older ^ of young suggests rapid transport from the series and a younger series dominated by ande- slab to the magma source region in the mantle sites. More ma¢c, shoshonitic lavas occur in wedge ( 9 40 000 yr), and rapid transport of melts smaller monogenetic centres nearby. Coira and from there to the surface ( 9 8000 yr) [48]. These Kay [12] concluded that both Cerro Tuzgle and time scales are too short to allow transport of the shoshonites were derived from mantle mag- £uids and melts through movement of the rock mas and that the Tuzgle magmas were also con- matrix, and make direct de£ection of melt ascent taminated by mixing with crustal melts in the low- by mantle £ow unlikely. The complex and vari- er crust. This is consistent with our observations

EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119 117 of a pronounced LQR extending straight through 9. Conclusions the mantle wedge from beneath Tuzgle to the earthquake cluster in the slab at 200 km depth. This study of seismic attenuation in the central For this region lithospheric delamination that Andes subduction zone demonstrates the power would provide hot mantle rocks has been pro- of Qp-tomography to image pathways of heat posed [37]. Fluids £uxed from the earthquake and £uid (or melt) transport on a regional scale. cluster into the heated mantle may trigger mantle Analysis of some 1500 local earthquakes, most melting responsible for the most recently observed located in the subducting slab, were used for andesitic magmatism. Qp-tomography. The results revealed distinct Note from Fig. 4 that there is no earthquake and highly varied distribution of LQRs in the cluster associated with the slab below the frontal crust and mantle. Crustal LQRs are strongest arc as in the sections farther north, yet the Qua- under the active volcanic front and also cover a ternary arc volcanism in the region is well devel- broad zone below the Altiplano^Puna plateau. oped, with seven potentially active volcanoes from Together with independent geophysical anomalies 24 to 25‡S [52]. It is intriguing that the LQR in (seismic, magneto-telluric, heat£ow) and geologic this section appears to be branched, with the low- evidence (large-volume sheets), these Q anomaly beneath the frontal arc extending to- crustal LQRs are interpreted as zones of high ward the earthquake cluster at 200 km depth temperature and partial melting in the mid-crust. where it overlaps with the vertical LQR branch The most striking characteristics of the LQRs below Cerro Tuzgle. At ¢rst glance, this branch- in the mantle wedge are their complexity and var- ing of the LQRs suggests that the frontal arc iation from north to south in the study area. We magmas may originate from the same deep source interpret LQRs as rough traces of £uid and melt region as the backarc centres. However, geochem- ascent through the mantle wedge. The pathways ical di¡erences between the arc and backarc vol- in the mantle wedge are not always vertical as is canoes at this latitude show that they must have often implicitly assumed in subduction models, di¡erent mantle sources. Compared with the fron- nor can a simple combination of vertical ascent tal arc, Cerro Tuzgle and other backarc centres as with corner £ow in the mantle explain the ob- a group have consistently lower contents of trace served LQR variations. The correlation of earth- elements (e.g. B, Ba, La, Sr, U), which are gen- quake clusters in the slab with low Qp anomalies erally regarded as being derived from the subduct- in the overlying mantle wedge support the concept ing slab via £uid transport. This slab signature, that £uid processes are responsible for both. Ei- typical of volcanic arcs worldwide, is best ex- ther the earthquake clusters mark the sites of de- pressed by a ratio of £uid-mobile elements to an- hydration reactions and £uid production in the other element with similar incompatible behav- slab (£uid embrittlement), or earthquakes are im- iour in magmas but much lower concentration portant for the transfer of £uids from the slab in slab £uids. Using the ratio of incompatible el- into the mantle wedge. A strong low-Qp anomaly ements minimises the e¡ect of subsequent magma extends from a strong earthquake cluster at 200 di¡erentiation. For example, the Ba/Ta and La/ km depth through the mantle wedge and into the Nb ratios in Cerro Tuzgle lavas range from 160 to crust below the largest Quaternary volcano in the 260 and 1.3 to 1.6, respectively [12], whereas Qua- backarc, Cerro Tuzgle. Recent, dominantly andes- ternary lavas of similar composition from the itic, volcanism might have been caused by £uids frontal arc at 24^26‡S have Ba/Ta = 400^600 and £uxed into mantle rocks that were heated in lith- La/Nb = 3^5 [53]. ospheric detachment processes.[BARD] An alternative interpretation for the LQR dis- tribution which is consistent with geochemical ar- References guments, is that separate magma source regions exist for the arc and backarc centres and that [1] J.B. Gill, Orogenic and Plate Tectonics, Spring- their associated low-Qp anomalies overlap. er, Berlin, 1981.

EPSL 6807 29-9-03 Cyaan Magenta Geel Zwart 118 B. Schurr et al. / Earth and Planetary Science Letters 215 (2003) 105^119

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