Tectonophysics 472 (2009) 51–61

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Tectonophysics

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Reflection Image Spectroscopy across the Andean subduction zone

M. Yoon a,⁎, S. Buske b, S.A. Shapiro b, P. Wigger b

a Institut für Geophysik, Zentrum für Marine und Atmosphärische Wissenschaften, Universität Hamburg, Bundesstr. 55, D-20146 Hamburg, Germanyb Fachrichtung Geophysik, Freie Universität Berlin, Malteserstrasse 74-100 (Haus D), D-12249 Berlin, Germany

⁎ Corresponding author.E-mail address: mi-kyung.yoon@zmaw.de (M. Yoon)

0040-1951/$ – see front matter. Crown Copyright © 20doi:10.1016/j.tecto.2008.03.014

A B S T R A C T

A R T I C L E I N F O

Article history:

This paper presents new ins Received 23 February 2007Accepted 27 March 2008Available online 11 April 2008

Keywords:Deep seismic imagingHeterogeneitiesScatteringAndean subduction zoneFluid migration

ights into the South American subduction zone from reprocessed seismic images.We applied a 3DKirchhoff prestack depthmigration scheme to data sets containing different narrow frequencyranges in order to extract additional details from seismic reflection images. This approach accounts for theeffects of scattering on the seismic image, especially for structures below a heterogeneous overburden. Thereflection image in such environmentswill differ significantlywhen focusing on different frequency ranges dueto the frequency dependence of scattering that is likely to be present. The narrow frequency range imagesuncover reflectors in one frequency range that are masked in another range. Furthermore, the images enablefor instance the characterization of the medium in terms of scatterer concentration and thus improve thestructural interpretation. The analysis of these imagesmight help to distinguish between small-scale structuresin the high-frequency band and large-scale structures in the low-frequency band. We call this imagingapproach Reflection Image Spectroscopy (RIS). We applied the RIS approach to the ANCORP'96 data set, anonshore deep seismic reflection profile across the South American Central Andes. The narrow frequency rangeimages revealed additional details that we interpret as features directly linked to fluid migration processes inthe subduction zone. Furthermore, structural details of the oceanic crust and the overlyingmantle and crust arerevealed. From the narrow-frequency range images we interpret that the top of the so called Nazca reflector at70 km depth marks the upper limit of the hydrated mantle wedge, whereas the bottom of the reflectorrepresents the top of the subducted oceanic crust. The compilation of our results with local earthquake dataconfirms this interpretation. Similar features observed in another deep seismic profile (PRECORP'95) supportthis interpretation, too. Furthermore, the RIS images show a highly reflective heterogeneous zone between theNazca reflector at 70 kmdepth and a prominentmid-crustal Bright Spot (Quebrada Blanca Bright Spot) at about30 km depth. We associate this zone with a complex network of ascending fluids or partial melts, initiated byascending fluids released from the subducted oceanic plate. This observation links the Quebrada Bright Spotdirectly to the dehydrating oceanic plate.

Crown Copyright © 2008 Published by Elsevier B.V.All rights reserved.

1. Introduction

Intensive geological and geophysical studies were carried out toinvestigate the South American convergent margin and its relatedsubduction processes (e.g. Oncken et al, 2006; Reutter et al, 1996;Buske et al., 2002; ANCORP Working Group, 2003; Sick et al., 2006).The long-term subduction of the oceanic Nazca Plate below the SouthAmerican Plate formed the Andes, the largest Cordilleran type orogen(e.g. Scheuber and Giese, 1999). The Altiplano-Puna Plateau betweenthe Western and the Eastern Cordilleras has an average height of4000 m, with a crustal thickness reaching 70 km. Previous studies onearthquakes and focal mechanism in the Central Andes between 20°Sand 23°S indicated a complex internal structure of the Wadati-Benioff-Zone (e.g. Rietbrock andWaldhauser, 2004). Its relation to theplate interface could not fully be explained yet, e.g., earlier investiga-

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08 Published by Elsevier B.V.All righ

tions revealed an apparent vertical offset between seismic reflectionsinterpreted as the top of the subducted plate and the subduction zoneseismicity (e.g. ANCORP Working Group, 2003).

Two onshore deep seismic reflection profiles – PRECORP'95 andANCORP'96 – acquired along 21°S and 22.5°S (Fig. 1), were processed inorder toobtain additional details of the crust andmantle along theCentralAndean subduction zone. Both data sets were processed using 2D and 3DKirchhoff prestack depth migration (KPSDM) (Yoon et al., 2003). TheKPSDM schemewas implemented especially accounting for the irregularsurvey geometry and the topography along the profile. It yielded detailedstructural images of the subduction zone. The ANCORP depth sectionprovidedanalmost complete imageof the subductedoceanicplate aswellas enhanced visibility of reflectors in the overlying continental crust andmantle along 21°S (Fig. 2, from Yoon et al., 2003). A strong, east dippingreflector is visible for almost 160 km along of the whole profile. Inaddition, the section shows a prominent west dipping Bright Spot in themiddle crust. The east dipping reflector is called the Nazca reflector,mainly representing the oceanic crust. Strikingly the reflector changes its

ts reserved.

Fig. 1. Overview on the study area. The location of the ANCORP and PRECORP profiles (red) and the local coordinate system used for the 3D depth migration (blue). The black linesmark the Precordilleran Fault System (PCFS) which splits up into the West Fissure (WFS) and the Sierra-de-Moreno Fault System (SMFS) south of 21.5°S.

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appearancewith depth, i.e. the shape and strength: Along the first 20 kmthe section shows two parallel, clearly distinguishable east dippingreflectors at 40–50 km depth. These reflectors are interpreted as the topand bottomof the oceanic crust (Buske et al., 2002). The upper reflector isvisible down to depths greater than 80 km. Below depths of 70 km itbecomes one thick and blurred reflector with no noticeable internalstructure. At x=110 km and at depths larger than 80 km the reflectorbecomesweak and diffuse and is hardly visible. The variation and suddendecrease of the reflector strength is not only an indication of thecomplexityof the reflector andpossible changes of petrologic parameters,but can be also caused by the influence of heterogeneous overburden onthe reflector image. Especially, it is assumed that the abrupt disappear-ance of the Nazca reflector at x=115 km is linked to the highly reflectiveQuebrada Blanca Bright Spot (QBBS) which is located in the crust at

Fig. 2. Top-2D prestack depth section of ANCORP section with local earthquake hypocentersoffset between the Nazca reflector and the hypocenter locations between 100 and 160 km.

depths between 15 and 30 km (Fig. 2). The seismic image of the Nazcareflector is likely to be affected by energy loss and fluctuation due toscattering that occurs in the area of the QBBS. Another importantobservation is the apparent offset between the top of the localearthquakes (Rietbrock and Waldhauser, 2004) and the Nazca reflectorbetweenx=110–160kmatdepths larger than80km. Itwasproposed thatbelow this depth the Nazca reflector does not represent the oceanic crustitself, but rather trapped fluids at the serpentinization front in thehydrated mantle wedge above the slab (Yuan et al., 2000; ANCORPWorking Group, 2003). However, features supporting this interpretationare hardly observed in the reflection data itself.

In order to extract additional structural details that might providea reasonable explanation for the latter issues we reprocessed theANCORP'96 data set. The reprocessing was performed using only

indicated by black dots (Rietbrock and Waldhauser, 2004). Note the increasing apparent

Fig. 3. Synthetic depth sections. The reflectivity in the heterogeneous layer increases for strong velocity fluctuations. The deep reflector appears weak and discontinuous for largevelocity fluctuation and small horizontal scale length. It nearly completely disappears for 20% variance in all images.

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narrow-frequency ranges of the data. This was done to account for thepossible effects of frequency dependent scattering due to heterogeneitiesin the overburden.

We call this approach Reflection Image Spectroscopy (RIS). Anintroduction to theRIS approach is given in the followingpart. In Section3,the application of theRISmethod to theANCORPdata set is described. The

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results are shownand interpreted in Section4. Adetaileddiscussionof theresults is given in Section 5. A conclusion closes the paper.

2. Reflection Image Spectroscopy

Seismic imaging in the presence of strong heterogeneities is still achallenging task as scattering and travel time fluctuations maysignificantly affect wave propagation.

Scattering is regarded as the redistribution of seismic energy intoreflected (backscattered) and transmitted (forward scattered) waves.The amount of scattered energy and of wave field fluctuations varies independence on the magnitude of the velocity fluctuation as well as onthe ratio between the wavelength and the spatial size of theheterogeneities. Therefore, seismic images will significantly differwhen migration is performed focusing on different frequency ranges.To provide an analytical description of scattering phenomenamultiplescattering effects have to be considered which are difficult to handle.Numerical modeling of seismic attenuation due to scattering showedthat the amount of scattering attenuation depends on the correlationproperties of the medium, the P- and S-wave velocities, and thefrequency contents of the incident wave (Hong and Kennett, 2003).Other modeling studies show that scattering and travel timefluctuation caused by a heterogeneous overburden affect the imageof a reflector below (Yoon, 2005). Fig. 3 shows synthetic depthsections generated for a subsurface model with a heterogeneous layerbetween 15–35 km depth and a reflector at 70 km depth. The velocityfluctuations are distributed according to a 2D exponential autocorre-lation function. The horizontal scale length and the velocity fluctua-tions of the heterogeneous layer vary between 1000 m, 4000 m and6000m and 1%, 5%,10% and 20%, respectively. The vertical scale lengthof the heterogeneities was 200 m for all models. Five synthetic shotgathers were calculated using a finite-difference forward modeling

Fig. 4.Narrow-frequency range depth sections. In the 15–20 Hz image the deep reflector is sein the 0–5 Hz image.

scheme for each velocity model. The shot gathers were prestack depthmigrated using a constant migration velocity and stacked afterwards.The images show that the distortion of the reflector image depends onthe strength of the velocity fluctuation and the scale length of theheterogeneities in the overburden. The image distortion increaseswith increasing velocity fluctuation and decreasing scale lengths. Themigrated sections containing narrow-frequency ranges of the data areshown in Fig. 4. The data set was bandpass filtered before migrationsuch that four narrow-frequency range data sets were obtained. Thefrequency ranges were 0–5Hz, 5–10Hz,10–15 Hz and 15–20 Hz. Thesedata sets were migrated separately. The images reveal that the imagefluctuations differ in different frequency ranges and that strongscattering leads to severe amplitude loss and phase fluctuations in acertain frequency range of the data while it appears less severe inanother range (compare the 15–20 Hz and 0–5 Hz image in Fig. 4). Theimage of a deep reflector below a strongly heterogeneous overburdenis affected by the loss of coherency and reflection strength. Thereflector shape is biased by wave field fluctuations. In the case ofextremely strong velocity fluctuations in the overburden, the lowerreflector is hardly visible at all. Thereby, scattering in anotherfrequency band might be less severe, such that reflectors are imagedproperly with respect to their shape and amplitude (e.g. Fig.10, 0–5 Hzimage). In the broadband image these fluctuations are superimposed.Besides the extraction of the true shape of the reflectors, in principlethe method can be used to gain additional information on the spatialparameters of the heterogeneities in the medium. The ReflectionImage Spectroscopy (RIS) method is designed to extract structuraldetails from seismic reflection images in strongly heterogeneousmedia. The method accounts for the frequency dependence ofscattering and its influence on the reflection image.

The principle scheme of the RIS approach is displayed in Fig. 5. Theconventional imaging scheme consists of preprocessing of the data

verely affected and suffers from coherency loss, whereas it appears clear and undistorted

Fig. 5. Schematic RIS flow. The conventional imaging scheme consists of preprocessing of the data and prestack depth migration (upper flow). In the RIS approach only narrow-frequency ranges – selected by additional bandpass filtering after the preprocessing – are imaged separately instead of using the full-frequency range of the data.

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and prestack depth migration. In the RIS approach bandpass filters areapplied to separate different frequency ranges of the data set after thepreprocessing and prior to migration. Afterwards, instead of using thefull-frequency range the data subsets containing only the selectednarrow-frequency range are imaged separately. The frequency rangesand the bandwidths of the bandpass filters are dependent on the dataand adjusted to the frequency content of the data. The bandpass filterparameters are determined by analysis of the frequency spectra of thedata and testing of different bandpass parameters.

3. Application of the RIS method to the ANCORP'96 data set

The ANCORP profile was acquired in 1996 across the Central Andes.A total of 131 explosion shot gathers were recorded along a total of385 km profile length. The nominal maximum offset was 25.1 km.Strong ground roll, occurrence of seismological events during therecordings, bad as well as non-uniform receiver coupling, changingdata coverage and altitude variations of more than 4000 m along theprofile required a non-conventional processing scheme. This includeda 3D prestack depth migration scheme (Buske, 1999) implementedfrom topography and an additional offline stacking to improve thesignal-to-noise ratio (Yoon et al., 2003).

The application of the RIS method to the datawas performed usingsimilar preprocessing steps (see Table 1) and migration parametersused for imaging the depth section as presented in Fig. 2 (Yoon et al.,

able 1erformed processing steps.

reprocessing Trace editingSpike mutingBandpass filterf–k domain filterTop and bottom mute

IS Bandpass filter parameter testingSeparation of frequency ranges using bandpass filter ⇒ 5–10 Hz,10–15 Hz and 15–20 Hz

aging 3D Prestack depth migration of data sets containing limitedfrequency rangesCalculation of envelope sections after migrationFinal offline stacking

TP

P

R

Im

2003). The preprocessing of the data consisted mainly of trace editing,muting and noise filtering in the frequency and the frequency-wavenumber domain. After preprocessing the frequency spectrum of thedata ranged between 5 and 20 Hz. To separate the different narrow-frequency ranges the preprocessed data were then bandpass filtered.After testing of different bandpass filters with different bandwidthsand frequency ranges bandpass filters with a fixed frequencybandwidth of 5 Hz were applied to the data. The frequency bandswere 5–10 Hz, 10–15 Hz, and 15–20 Hz. It provided most adequateseparation of reflections visible in three different narrow-frequencybands. The bandpass filtered sub data sets were then migratedseparately. The resulting depth migration volume was 50 km wide(N–S), 250 km long (W–E) and 130 km deep each. To enhance thevisibility of reflectors, the envelope sections of the migrated shotgathers were calculated, i.e. the envelopes for each trace of themigrated sections which enhanced apparently weaker reflections inareas with poor receiver coupling prior to the final stacking. Similarto the previous processing finally parallel W–E slices that are locatednear the profile line were stacked and provided the final 2D depthsections. The final stacking further improved the signal-to-noise ratioand thus the visibility of reflectors. These sections are referred to asthe low (5–10 Hz)-, the middle (10–15 Hz)- and the high-frequency(15–20 Hz) image in the following. Lateral amplitude balancing wasnot applied to the data.

4. The results

The application of the RIS method to the ANCORP'96 data yieldedthree narrow-frequency range depth images. Each of the images revealsdifferent features of the crustal structure and of the Nazca reflector. Theinterpreted narrow-frequency range images compiled with the localhypocenters (Rietbrock andWaldhauser, 2004) are displayed below thecorresponding images in the following figures. The analysis of the faultplane solutions of the relocated seismicity indicated fluid related eventsthat are locally restricted to the oceanic crust and the uppermostmantle.Thereby, the top of the seismicity lies just beneath the interface of thecontinental mantle and the oceanic crust.

Fig. 6 shows the 5–10 Hz image. At the beginning of the profile(x=0–20 km) two thin parallel east dipping reflectors (Nazca reflector)are clearly visible. Between x=20–50 km the shape of the reflectors

Fig. 6. a) The low-frequency image (5–10 Hz). b)With interpretation and hypocenter locations (Rietbrock andWaldhauser, 2004). The Nazca reflector at the beginning of the profile isidentified as two parallel east dipping reflectors, changing its shape at 70 km (see arrow) and becomes one thick wedge-shaped reflection. The dashed lines mark the reflectionsinterpreted as the top and bottom of the oceanic crust.

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become less clear and discontinuous, but between 50 and 70 km bothparallel reflectors are clearly visible again. At x=70 km at depthslarger than 70 km instead of two reflectors only one dipping thickreflector appears. This reflector becomes blurred and thicker withincreasing depth towards the middle of the profile. The apparentthickness of the reflector increases from 6 km (x=~70 km) to almost14 km (x=~110 km). At x=115 km the appearance changes abruptlyagain and only diffuse and weak reflectors segments are visible whichtotally disappear at x=160 km. A vertical, apparently highly hetero-geneous zone can be observed between x=100–110 km, at depthsbetween 15–45 km, where the Nazca reflector appears with itsmaximum thickness of about 14 km. Further to the east the QBBS isimaged between x=120–160 km at depths between 20–45 km. Itappears as a zone of diffuse reflector elements showing higherreflectivity at the western part and less reflectivity at its eastern end.The top of the QBBS shows a west dipping component, the lowerboundary appears rather horizontal.

The 10–15 Hz image is displayed in Fig. 7. The two sharp paralleleast dipping reflectors are only visible over the first 20 km of theprofile. Between x=20–50 km the image of the Nazca reflector isslightly weaker and diffuse compared to the reflector visible in thelow-frequency image. Between x=50–70 km only one reflector isvisible. This reflector lies in the prolongation of the upper of the twoparallel reflectors. Similar to the 5–10 Hz image the Nazca reflectorbecomes thicker and blurred again at 70 km. It appears as a thickwedge-shaped, very strong reflector between x=80–115 km. Theimage does not reveal any internal structure. At x=115 km thereflector apparently disappears. In contrast to the low-frequencyimage the continental upper crust appears almost transparent abovethe Nazca reflector for the first 80 km of the profile. However, a largevertical zone with significant reflectivity can be observed betweenx=80–115 km reaching down to depths of about 70 km. Thisheterogeneous zone shows a large number of short and diffusereflectors without preferred orientation. It apparently connects the

Fig. 7. a) The intermediate-frequency image (10–15 Hz). b) With interpretation and hypocenter locations (Rietbrock and Waldhauser, 2004). The image reveals a vertically extendedheterogeneous zone which is associated with ascent path of fluids released from by the subducted plate, migrating through the overlaying mantle and crust. Thin, weak reflection atdepths between 50 and 60 km are observed that indicated the continental Moho.

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deeper part of the Nazca reflector directly to the western edge of theQBBS. The QBBS itself appears as a relatively thin and distinct bendedreflector with a clear west dipping component. In addition, thin weakeast dipping reflectors are visible between x=65–90 at depthsbetween 50–60 km. These reflectors are interpreted as the continentalMoho.

In the high-frequency image (Fig. 8) the reflectors generally appearweaker and more diffuse compared to the other narrow-frequencyimages. Contrary to the low- and intermediate-frequency image thetwo parallel dipping reflectors are hardly visible at the beginning ofthe profile. At x=60 km a thin east dipping reflector is observed at60 km depth, that can be traced down to almost 90 km depth. Thewedge-shaped Nazca reflector between x=70–110 km reveals itsinternal structure. The strong reflector can be differentiated into twomajor parts: the upper wedge-shaped part with mainly horizontalreflectors at depths between 70 and 85 km. This is interpreted as partof the hydrated mantle wedge. The lower part shows thin parallel eastdipping reflectors. These events marking the lower boundary of theNazca reflector are interpreted as the top of the oceanic crust as well

as the oceanic Moho. The Nazca reflector abruptly disappears atx=115 km. Again, a heterogeneous zonewith high reflectivity is visiblebetween x=90–110 km at depths of 20–50 km, similar to the otherimages. The western edge of the QBBS appears with less clear shapecompared to the intermediate-frequency image. However, the easternpart of the bended reflector shows a clear west dipping component.

The comparison of all three images shows that the 5–10 Hz imageprovides the most pronounced and clear reflectors from the oceaniccrust at the beginning of the profile (x=0–50 km). In both othersections these reflectors appear less coherent. The distortion of thesereflectors is strongest in the 15–20 Hz image. The 5–10 Hz imagereveals sharp reflectors from the oceanic crust between x=50–70 kmof the profile. These reflectors are interpreted as the continuation ofthe oceanic crust visible between x=0–20 km. Both other images onlyprovide pronounced reflectors from the top of the oceanic crust, butnot from the oceanic Moho. In the 5–10 Hz and in the 10–15 Hz imagethe Nazca reflector appears as a large strongly reflective zone with amaximum height of 15 km between x=70–110 km. The images do notreveal internal structural details of the Nazca reflector.

Fig. 8. a) The high-frequency image (15–20 Hz). b) With interpretation and hypocenter locations (Rietbrock and Waldhauser, 2004). The image reveals the internal structure of theNazca reflector at depths larger than 70 km: strong horizontal reflections indicate a wedge-shaped zone of trapped fluids above the slab. Weaker east dipping reflections below areinterpreted as the top of the oceanic crust. The prolongation (red dashed) shows a good correlation with the local Wadati–Benioff zone.

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The RIS images indicate that the amount of image distortion is notonly a function of the wave length, but rather dependent on therelative size of the heterogeneities in the overburden compared to thedominant wave length. Thus, depending on the scale lengths of theheterogeneities, scattering might be more severe in the lowerfrequency band than in the higher frequency band range and viceversa.

The RIS images of the ANCORP'96 data reveal additional internalstructural features of the Nazca reflector and the continental crust andmantle that will be discussed in detail in the next section.

5. Discussion

The narrow-frequency range images of the ANCORP'96 profilerevealed important structural details of the subduction zone. Some ofthem might help to understand and explain two major issues that arenot fully clarified yet. Firstly, the apparent offset between the Nazcareflector and the hypocenter locations. Secondly, the role of fluids in

the subduction zone with respect to local seismicity and reflectivity.Both issues are discussed in the following.

A schematic sketch with the most important results and theirinterpretation is shown in Fig. 9. The numbers (no.1–5)mark importantfeatures recovered by the narrow-frequency range images, the lettersA–D mark prominent way points along the interpreted fluid path.

5.1. Apparent offset between the Nazca reflector and the hypocenterlocations

As mentioned before an apparent offset is observed between theNazca reflector and the top of the hypocenter locations. One possibleexplanation considers a systematic deviation in reflector andhypocenterdepths due to the usage of different velocity models for imaging andlocalization. The hypocenter locations were calculated using a velocitymodel derived from local earthquake tomography. A comparisonshowed that the latter model contained relatively higher velocitiesthan themodel from Lueth (2000), which was used for migration of the

Fig. 9. Combined results from RIS, receiver function studies (Yuan et al., 2000), local earthquake analysis (Rietbrock and Waldhauser, 2004), geothermal studies (Springer, 1999) andinterpreted fluid migration scenario for the ANCORP profile at 21°S. PCFS: Precordilleran fault system. Black solid: verified reflectors from RIS analysis; red magenta: Moho derivedfrom receiver function study; dashed black: top of oceanic crust; grey solid: isotherms; blue arrows: possible fluid and melt migration paths. The numbers 1–5 mark the mainimportant features obtained from RIS, the letters A–D mark way points along the fluid paths. For detailed description of A–D and 1–5 see text.

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reflection data. This difference might explain the apparent offset.However, a comparison of depth sections calculated with differentvelocity models indicated that the observed offset cannot be explainedby the difference of the velocity models only (Yoon, 2005). The studyshowed that the maximum deviation of reflector depths due touncertainties in the migration velocity model is in the range of about3–5 km. The localization error of the hypocenter locations is about 3 to5 km in depth (Rietbrock and Waldhauser, 2004). Thus, if estimatederrors are reliable, even a superposition of both errors cannot fullyexplain the depth differences between the Nazca reflector and thehypocenter locations up to 15–20 km at x=160 km. The ANCORPWorking Group (2003) proposed an alternative explanation. Basedon seismic and petrophysical observations they interpreted that theNazca reflector at depths larger than 70 km depth is the image of thehydrated serpentinized mantle wedge and not the oceanic plateitself. Thereby, the reflector strength and the abrupt disappearanceare attributed only to serpentinization and its breakdown due totemperature increase with depth. Our results provide indications fora more detailed structural interpretation. The high-frequency imagerevealed that the Nazca reflector at depths larger than 70 kmrepresents two structural units: the subducted oceanic crust on onehand and the hydratedmantle wedge in the overlayingmantle on theother hand (Fig. 9, no. 2+3). The prolongation of the reflectorinterpreted as the top of the oceanic crust (black dashed line)correlates well with the top of the hypocenter locations. The shapeand the location of the reflector are also in good correlation with thegeometry of the oceanic plate derived from receiver functionsanalysis (Yuan et al., 2000). As a result the hypocenters are mainlylocated in the oceanic crust and in the upper oceanic mantle.

5.2. The role of fluids in the subduction zone with respect to localseismicity and reflectivity

Fluids and fluid migration play an important role in subductionzones. A large number of fluid migration models and transportationmechanism in subduction zones were proposed, modeled anddiscussed (e.g. Tatsumi, 1989; Peacock, 1990; Iwamori, 1998). In thefollowing, we will try to link some of the key features observed in thenarrow-frequency images to fluid migration (e.g. aqueous fluids andwater-rich melts) and related phenomena.

It is commonly accepted that the oceanic lithosphere dehydratesduring subduction. Depending on the dip angle of subduction and thecomposition of the down going plate the depth interval and the amountof fluid released from hydrated sediments, amphibolized basalts andserpentinized peridotites varies (Ruepke et al., 2004). The geothermalgradient controls the depths where aqueous fluids are released andmigrate into the overlaying mantle (Iwamori, 1998).

Based on our results we propose the following fluid migrationscenario for theAndean subduction zoneat 21°S: At the beginningof theprofile the Nazca reflector appears as two sharp parallel east dippingreflectors, which are clearly interpreted as the top of the oceanic crustand the oceanic Moho. The parallel reflectors can be traced untilx=75 km, down to depths of 70–75 km. There, the low-and theintermediate-frequency images indicate a clear break in the reflectionpattern (Fig. 9, no. 1). The Nazca reflector changes its appearance fromtwo parallel thin reflectors to one thick and strong reflector, which has amore undefined shape. The reflector becomes blurred at larger depth.This change in its appearance is attributed to the fact that the oceaniclithosphere starts to releasewater continuously on theway frompoint Adown to point B (Iwamori, 1998). The released fluids ascend verticallyinto the overlayingmantle and formhydrous phases, e.g. serpentine andchlorite. In the beginning a thin layer of serpentinite is formed. Withongoing subduction chemically bound water, mainly in the form ofchlorite, lawsonite, amphibole and serpentine, is transported down togreater depths. Simultaneously, dehydration of the oceanic platecontinues inducing local seismicity by dehydration embrittlement andlocal reactivation of features in the slab, e.g. fracture zones (Dobson et al.,2002; Kirby et al.,1996; Hacker et al., 2003). TheWadati–Benioff zone iswell imaged by intermediate-depth seismicity linked to these dehydra-tion processes (Rietbrock and Waldhauser, 2004). With the increasingamount of released and ascending fluids, a larger water volume is beingtrapped in the overlaying mantle wedge, forming a thick serpentinizedmantle wedge, which was observed in teleseismic studies in Cascadia(Bostock et al., 2002). This contributes to the thick and blurredappearance of the strong Nazca reflector as observed at depths largerthan 70 km (Fig. 9, no. 3). Above and east of the hydratedmantle wedgethe temperatures in the fore arc region become too high (higher than500 °C) and serpentinization stability breaks down which possiblyexplains the suddendecrease of reflectivity. However,we link the abruptlateral apparent decrease of reflectivity not only to the breakdown of

Fig. 10. a) Prestack depth section of the PRECORP profile. b) The PRECORP profile withlocal hypocenters (Graeber and Asch, 1999) and interpretation with respect to thefindings in the RIS images of the ANCORP data set. Black dashed: Reflections interpretedas top of oceanic and top of hydrated mantle wedge. Blue dashed: interpolatedextrapolation of top oceanic crust.

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serpentinization, but also to the influence of the QBBS on the seismicimage (e.g. see Fig. 4). TheQBBS is interpreted as a highly heterogeneouszone where strong scattering occurs thus affecting the image of theNazca reflector below.

Due to the breakdown of serpentine stability fluids are releasedagain and migrate towards higher temperature regions by aqueoussolution (dashed blue arrow pointing to C in Fig. 9). Thereby, the fluidpaths, eventually along pre-existing faults, are assumed to be not onlyvertical, but also horizontal and sub-horizontal, building a complexnetwork of fluid and melt migration paths. During the vertical ascendthe released water reacts with peridotites and form hydrous phases,e.g. amphibole again (Davies and Stevenson, 1992). The transport ofhydrous phases by the slab-induced mantle wedge flow then leads tothe net transport of water being horizontal, across the mantle wedgefrom the slab. Two other commonly cited fluid flow mechanisms are

porous flow along grain boundaries and buoyancy-driven propagationof fluid filled cracks. Thereby, numerical modeling of fluid filledfracture propagation showed that even for a simple model migrationof fluids via fluid filled fractures results in complex flow patterns(Dahm, 2000). Furthermore, vertically ascending fluids (aqueoussolution or water-rich melt) lower the solidus temperature of therock and cause significant melting along fluid ascending paths(Tatsumi, 1989). A complex network of vertical and horizontal fluid(water, water-rich melt and partial melt) filled fractures and pores isbuilt. The narrow-frequency range images reveal a vertically extendedzone of heterogeneous reflections (Fig. 9, no. 4). This zone isinterpreted as the mentioned network of ascending fluid pathsindicating that the Nazca reflector is directly linked to the QBBS(Fig. 9, D). This zone of high reflectivity shows a good spatial corre-lation with a low-velocity zone observed in P-wave tomography(Koulakov et al., 2006). There, this low-velocity anomaly is interpretedas a zone of rising diapires containing partially molten felsic rocksoriginating from the hydrated mantle wedge. Also, results fromattenuation tomography studies in this area show a vertical zone ofincreased attenuation (Schurr et al., 2003). Thereby, the observedstrong attenuation in the crust and mantle was attributed to fluidsand/or partial melts. The QBBS is interpreted to be a region of trappedfluids and partial melts with its fluid source located in the oceanicplate. The strong reflectivity of the QBBS is related to these fluids andpartial melts trapped within either small-scale pockets or larger scalenon-connected fluid traps. The observed strong crustal reflectivitymight also be related to small-scale magmatic traps and/or stronglysheared zones. Other models, e.g. larger scale inter-connected traps,are hardly correlated with the observation from magnetotelluric datainversion as they cannot explain the lack of the required low-resistivity anomaly (ANCORP Working Group, 2003).

The western flank of the QBBS and the eastern flank of the verticalheterogeneous zone can be linked to the location of the sub-verticalPrecordilleran fault system at the surface. Its eastern flank is linked tothe recent volcanic arc at the surface that is active since 13 Ma (Bakerand Francis, 1978). In the area above the QBBS the world’s largestconcentration of porphyry copper mineralization can be found (e.g.Quebrada Blanca mine). Hence, the proposed fluid paths through themantle up to the crust are interpreted as the conduits for mineralizingfluids (ANCORP Working Group, 2003).

5.3. Continental Moho reflectors

The intermediate-frequency image recovered weak reflectorsbetween x=70–90 km at depths between 50–60 km that show asmall east dip component (Figs. 7 and 9, no. 5). These reflectors areinterpreted as the continental Moho. In the full-frequency rangeimage these reflectors were only visible in the western part of theprofile. The Moho reflectors correlate well with the Moho locationderived from receiver function analysis (Yuan et al, 2000).

5.4. The PRECORP'95 profile

With respect to the findings of the narrow-frequency band imagesfor the ANCORP section an advanced structural interpretation of thePRECORP depth section is provided. The west–east heading 50 kmlong deep seismic profile was located approximately 150 km further tothe south of the ANCORP'96 profile at 22.5°S (see Fig. 1). In the 2Ddepth image (Fig. 10) similar features to those that were observed inthe ANCORP section are identified, e.g. a prominent mid-crustal BrightSpot (the so-called Calama Bright Spot) at depths between 15–25 kmand east dipping reflector segments at a depth of about 65 kmbetween x=50–60 km (Yoon et al., 2003). The section further revealsalmost horizontal reflectors at depths of about 80 km (x=90–120 km).Below these reflectors thin east dipping reflectors are visible. Theresults from the RIS analysis indicate that the horizontal reflectors in

61M. Yoon et al. / Tectonophysics 472 (2009) 51–61

the middle of the profile at a depth of about 80 km represent thehighly reflective hydrated mantle wedge as seen in the ANCORPsection. However, the reflectors appear thinner and weaker comparedto the strong Nazca reflector observed in the ANCORP'96 profile. Thethinner east dipping reflectors at depths larger than 80 km areinterpreted as the top of the subducted oceanic crust. The prolonga-tion of these dipping reflectors correlates well with the top of the localhypocenter locations (blue dashed in Fig. 10b). It indicates that thehypocenters are mainly located in the oceanic crust and in the uppermantle. The reported uncertainty of the absolute vertical hypocenterposition is about 3–4 km (Rietbrock and Waldhauser, 2004).

6. Summary and conclusion

The application of the RIS method to the ANCORP'96 datasuccessfully provided seismic images that reveal important additionalstructural details of the Andean subduction zone. The analysis of thenarrow-frequency band images indicates that the Nazca reflector canbe separated into two parts. The upper part at 70 km depth representsthe hydrated mantle wedge, whereas the reflectors in the lower partreveal the top of the subducted slab itself. The prolongation of thesereflectors to greater depths fit with the top of the hypocenter locationsand with the slab geometry derived from receiver function analysis.Hence, the apparent offset between the Nazca reflector and thehypocenter locations can be resolved and reasonably explained.

In addition, the narrow-frequency range images reveal structuralfeatures that can be directly linked to fluid related phenomena insubduction zones. For example the change of shape and strength ofthe Nazca reflector along its appearance at x=70 km in all RIS imagesindicates the beginning of serpentinization of the mantle wedge dueto rising fluids from the oceanic plate. Ongoing dehydration andconsequently larger amounts of released fluid cause an increase of thehydrated zone in the mantle. The hydrated mantle wedge appears asan increasing strong thick reflector until the reflector apparentlybreaks down. This breakdown is not only caused by the breakdown ofserpentine stability, but also is caused by strong scattering in theheterogeneous zone above the Nazca reflector. There, the QBBS islocated, a west dipping highly reflective mid-crustal structure. Theimages further indicate that the QBBS is directly linked to the oceanicplate. The narrow-frequency range images indicate a complexnetwork of vertical and horizontal fluid migration paths in theoverlaying mantle and crust probably initiated by vertically ascendingfluids released from the oceanic plate. We propose the RIS method asan additional tool that enables the extraction of additional informa-tion from reflection data and thus improves the structural interpreta-tion of seismic images, especially in heterogeneous environments.

Acknowledgments

The ANCORP96 project was funded by the Bundesministerium fürBildung, Wissenschaft, Forschung und Technologie within DEKORPand by the Deutsche Forschungsgemeinschaft within the Collabora-tive Research Center 267 (Deformation Processes in the Andes). Wethank the German and Chilenian field crews, the Universidad Catolicadel Norte (Antofagasta), Codelco Chile and numerous other partici-pants and institutions. The field instruments were provided by theFreie Universität Berlin and the Geophysical Instrument Pool (GIP) ofthe GeoForschungsZentrum Potsdam. We like to thank the reviewers(Camelia Knapp and anonymous) for their comments and suggestionsthat helped in improving the manuscript.

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