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Phys. Chem. Earth, Vol. 21, No. 4, pp. 289-293, 19960 1997 Elsevier Science LtdAll rights reserved. Printed in Great Britain0079- 1946/96 $15.00 + 0.00PII: SOO79- 1946(97)00050-5

3D-Density Mo delling with Seismic Constraints in the CentralAndesA. Kirchner, H.-J. G6tze and M. SchmitzInstitut fi_irGeologie, G eophysik und Geoinforrnatik, FU Berlin, Maltes erstrasse 74-100,12249 Berlin, Germany _ _

Received 12 June 1996; accept ed 1 Oct ober 1996

A b s t r a c t . A new 3D-density model of the Central An-dean lithosphere and the downgoing Nazca plate hasbeen designed based on more recent constraints givenby refraction seismics and seismology. Offset-free mo-delling is achieve d by fitting both gravity and geoid withone density structure and by modelling relative to a re-ference density structure. Contributions to the CentralAndean geoid by some principal parts of the lithosphereare calculated and displayed along a WE trending profileat 20s. T he distribution of the model masses down tomantle regions is used to investigate the isostatic state ofthe lithosphere model in comparison to Airy and Vening-Meinesz isostatic models.0 1997 Elsevier Science Ltd

1 I n t r o d u c t i o nWe present the results of intensive wide angle seismicrefraction and gravity research in the southern CentralAndes (20 - 29S) during the 80s and early 9 0s. Thegross crustal structures could be determined coveringthe principal morphostructural units of the Andes bet-ween 2 1 and 25 S (e.g. Wigger et al., 1994). In 1994(PISCO 94 - Proyecto de Investigacibn Sismol6gica dela Cordillera Occidental) and 1995 (CINC A 95 - CrustalInvestigations Off- and On-shore Nazca Central Andes)seismic investigations focussed on the arc and forearc re-gions in order to provide more detailed insight into thecrustal structure of this area. Th e structural and velo-city information obtained by seismic refraction experi-ments is available along N-S and W-E trending profilescovering the central region of the 3D-gravity model.

From 1993 to 1996 the MIGR A group (MedicibnesInternacionales de Gravedad en 10s Andes) with partici-pants from Chile, Argentina and Germany has surveyedsome 3.500 new gravity observations in an Andean Geo-traverse covering N-Chile and NW -Argentina betweenCorrespondence to: A. Kirchner

64-71W and 20-290S. Including reprocessed data ofFreie Universit& Berlin and older data of South Ame-rican Universities and oil and mining industry, there isnow a data base of about 15.00 0 gravity values avail-able, which can be used together with other geophysicaland geological information for an interdisciplinary inter-pretation of the structure and evolution of the CentralAndes (GGtze et al., 1994). The data base which in-cludes point data and data grids of free-air-, completeBouguer anomalies (i.e. including topographic reduc-tion), and isostatic residual anomalies, has frequentlybeen described by Giitze et al. (1995 ) and GGtze andthe MIGRA Group (1996).

2 T h e M o d e l li n g M e t h o dIn density foreward modelling we calculate the gravityof plausibly assumed density distributions in order tocompare it with a given gravity anomaly. It is our aimto adapt the calculated field to the measured one. Un-less we model the entire Earth, there is usually an offsetbetween the measured and the calculated signals andthe two fields are often shifted into each other.

Various reasons could be held responsible for suchan offset, e.g. ed ge effects of the model, calcu lation inthe plain where model dimensions suggest a sphericalcalculation, a constant error in used densities or longwavelength gravity anomalies.

In order to achieve shift-free modelling, model mas-ses were constructed in an area much larger than thearea of interest with a plausible density distribution inthe entire model area, not only in the area of interest.The calculation was done on the sphere and the assu-med densities are relative to a layered density referencemodel (2670 kg/m3 from 0 - 15 km, 2900 kg/m3 from15 - 35 km and 3370 kg/m3 from 35 - 220 km), which isa simplyfied version of the Preliminary Reference EarthModel (Dziewonski and Anderson, 1981).

28 9

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290 A. Kirchner et al.W PacificOcean

trench0

Coastal Cordilh PrecOrd. Preandean Depr. Western C.Anti- PelfI

Puna E

40 0 50 0 60 0UTM (km) (500 km = 69W)

Fig. 1. Cross section with seismic discontinuities in northern Chile at lat. 23305. Crustal structures can be traced from the CoastalCordillera with increasing thicknes s towards the Western Cordillera. The shape of the subducted Nazca Plate can be inferred from Mohorefractions in the Coastal Cordillera and from the distribution of the earthquake hypocenters derived from the PISCO 94-seismologicalcatalogue (small dots, p. Asch, A. Rudloff, F. Graeber, pers. comm.). Crosses indicate the position of N-S trending refraction profi les;black triangles give the position of the magmatic arc.

Additionally, we calculated the geoid undulations of in the Pacific to a 50 m high between 16 and 19S pas-the density model used to fit the gravity field and com- sing a relative minimum above the trench and decliningpared it with the observed geoid (Rapp et al., 1992, to 20 m in the Chaco plain. There is an isolated maxi-OSUS lA, ) in order to interprete long wavelength gra- mum of 45 m in the volcanic arc at 27S (Rapp et al.,vity anomalies in the Central Andes. 1992).

3 Gravity Anomalies and t h e G e o i dOnshore the Bouguer anomaly drops down to a regionalminimum of about -450 mGa1 in the area of the recentvolcanic arc, related to crustal thickening. The gravityeffect of an isostatic compensation root was eliminatedfrom the Bouguer gravity and the resulting anomaly ser-ves as a residual field.

The most interesting features of this field are: (1)Positive values in the area of the Jurassic forearc withisolated complexes parallel to the coastline. They are re-gionally caused by the presence of the dense subductingplate and locally by uplifted jurassic batholiths intru-ded into the volcanic La Negra formation. (2) TheNNW -SSE striking positive anomaly from Calam a bythe Salar de Atacama to southern Puna which can beexplained by a highly metamorphic and high-density Pa-leozoic/Precambrian structure, which is oblique to thegenerally N-S trending recent volcanic belt. (3) Localminima along the recent volcanic arc point to partlymolten material at depths of 15-20 km. (4) Minima fol-lowing a line from Ollague to Calam a along 69 W arecaused by the Eocene volcanic arc with low-density vol-canic material in the upper crust. Maps are providedby Giitze and the MIGRA Group (1996).

The maximum-typegeoid in the Central Andes mainlyreflects the topographic edifice. It rises from about 20 m

4 T h e 3 D D e n s i t y M o d e l a n d i t s C o n s t r a i n t s4 .1 Forearc Crustal Structures from Seismic Refaction

DataIn the Coastal Cordillera between 21 and 24S, uppercrustal P-wave velocities vary between 6.0 and 6.7 km/s.The top of the lower crust is located between 7 and12 km depth, reaching down to 20-22 km, proven byprograde phases with velocities up to 7.2 km/s. The pro-minent discontinuity at 40 km depth (V, = 8.2 km/s)is interpreted as a Moho surface of the subducted NazcaPlate with an average velocity of 6.6-6.7 km/s (Wiggeret al., 1994).

Upper and lower crustal structures can be followedinto the Precordillera with the upper crust increasing to20 km thickness and the lower crust extending down to40-45 km depth. Three distinct discontinuities with ve-locities from 7.3-8.0 km/s, seperated by low velocity lay-ers, are located between 50 and 70 km depth. The aver-age velocity down to 65-70 km decreases to 6.3 km/s.

Towards the Western Cordillera the average crustalvelocity once again decreases (5.9-6.0 km/s), whereasthe upper crust and deeper crustal parts continue. Themost striking feature in the record sections of the We-stern C ordillera, a discontinuity at 20 km depth (V, =6.4 km/s) beneath Ollague, is interpreted from a struc-tural point of view as the top of the lower crust, although

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3D-Density Modelling with Seismic Constraints 29 1

.____Solid line Measured Bauguer AnomalyDashed line, Calculated Gravity

Antotagasta Peine 2450

56 0 66 0UTM (km) (500 km = 69OW)

Fig. 2. Cross section of the 3D density model at lat. 233 05 including subducting Nazca Plate with earthquake hypocenters, Andeancrust, continental lithosphere and an asthenospheric wedge. Black triangles indicate the magm atic arc, black dashes a Vening-M eineszisostatic Moho (D=35 km, 6p = 350 kg/m 3, R=1 0z3 Nm) . According to 2D raytracing m odels there is material interpreted as ser-pentinized old mantle with densities lower as the high density matrial in the Coastal Cordillera located above. Average crustal densitydecreases towards t he Western Cordillera including two low density zones beneath the recent arc. The transition of oceanic crust basaltsto eclogite with a density of 3500kg/ m3 is completed at a depth of 60-70 km.

the crustal units below are characterised by partly re-duced velocities. There are locally limited high velocitydiscontinuities observed in this crustal unit down to 45-50 km depth. Assuming this depth range as base of thelower crust, a 25-30 km thick lower crust results withlow average velocities of only 6.0 to 6.2 km/s. Fan ob-servations between the Precordillera and the WesternCordillera give indications for a deep reflector (V, =8.2 km/s) interpreted as the eclogitized top of the sub-ducted Nazca Plate (see Fig. 1).4.2 3D ModellingA large scaled 3D density mode1 was constructed to in-vestigate the regional structure and density distributionof the Andean lithosphere. The modelled area reachesfrom 1 2 to 355 and from 57 to 79W comprising largeparts of both the Nazc a Plate in the West and the Bra-silian Shield east of the Andean orogen as a referencelithosphere. The principal parts of the mode1 are the do-wngoing Nazc a Plate, the Andean crust, the continentallithosphere with Brasilian shield crust and an astenos-pheric wedge.

The mode 1 resembles the new results of refractionseismics based on two WE a nd two NS trending raytracing models at 24 and 23.5S and 68 and 69W,respectively, as described above ( 4.1 and Fig. 1). Ad-

ditionally, a balanced cross section at 21S according toearlier refraction seismics (Schmitz, 1994) was also in-cluded. As a preliminary study the velocities of seismicmodels were directly converted into densities by usinge.g. the Nafe & Drake relationship or similar velocity-density relations. Initial mode1 geometry was slightlymodified by the use of interactive computer graphics (S.Schmidt, pers. comm.) to verify regional trends im-plied by the Bouguer gravity field and the topography-reduced geoid. Fig. 2 shows the major crustal parts ofthe density model at 2330 s.

In the forearc a 10 km thick upper crust with a den-sity of 2850 kg/m3 tops a 20 km thick high-density lo-wer crust material (2950- 2930 kg/m3), followed by partswith reduced density (29 30-290 0 kg/m3) which are in-terpreted as serpentinized Pre-Andean mantle material.In the Precordillera the upper crust thickens to about20 km with a decrease of density to 2770 kg/m3. It canbe followed to the recent arc with the same thicknessand further decreasing density (273 0 kg/m3). Towardsthe back arc the upper crust base rises to 15 km andits density increases to Precordilleran values. The Co-astal Cordilleran high-density material can also be pur-sued in WE direction with decreasing densities down to2860 kg/m3 and its base reaching 40 km as it passesbeneath the recent ,arc. There is a low-density zone of2700 kg/m3 underneath the volcanic chain between 20

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29 2 A. Kirchner et al .

T r e nc h

-250.0 L ,-5Cil.0 0. 0 500.0 looo.o 1500.0uTM(km)(sc0kJn =69w)

- observedA- A Top/eata )D -aAi ry M oho (b )* * L.ow cr rust (c)o. * Slab(d)*r * Wedge(e)

W E3 0 0 . 0

4 0 0 . 0l( 1 6 0 0 . 0 11cQ.oUTM (km ) (500 km = 69W)

Fig. 3 . Es t ima t ion s of poss ible cont r ibu t ions to the geoid and the gravi ty fi e ld v i sua l i zed a long a WE t r en ding prof i l e a t 205 of (1) thetopo gra ph y (2670kg/m3) an d bat hym etry ( -1640 kg/m3), (2 a an Airy isosta t ic root (D=40 km , 6p = 300 kg/m3), (2b) the mod elslower cru st (6~ var ying from -470 to -370 kolm31. (3) the subd uc t ing Nszca Pl a t e w i t h a m e a n de ns i t y su r p l u s o f 20 kg/m3 and (4)th e as th eno~p her icwe~ge wit h a lack of -10 cg/mj ..

and 35 km depth resembling the low-velocity zone de-tected by refraction seismics.

Lower crustal bodies are modelled with a density of2980- 3000 kg/m3 below 40 km placing a gravimetricmoho below the arc and Altiplano/Puna to a depth ran-ging from 56 km at 2 4S to 65 km a t 21S accordingto 2D ray tracing models. A seismically implied secondlow-density zone is modelled between 44 and 50 km witha density of 2920 kg/m3.

The Brasilian shield crust consists of a 4-6 km thicksedimentary cover (2450 kg/m) followed by a mid cru-stal layer with 2880 kg/m3 and a lower crust downto 40 km with a thickness of 10 km and a density of3000 kg/m3. The shield crust lies on top of the continen-tal lithosphere modelled with a density of 3370 kg/m3.

The shape of the subducting Nazc a Plate is infer-red from results of Cahill et al. (1992) and the earth-quake hypocenters of the PISCO 94-seismic catalogue(G. Asch, A. Rudloff, F. Graeber, pers. comm.)-. Fordefinition of the descending Moho angle in the forearcwe have observations by refraction seismics between 21and 24s. The transition of oceanic crust to a 3500 kg/m3eclogite layer on top of the downgoing slab, which startsat about 50 km and is completed at 70 km depth, is si-mulated by a model body with a density of 3150 kg/m3of oceanic crustal thickness within this depth range. Inthe model we present the subducting Nazc a Plate to bemodelled down to 670 km with a mean density contrastto the surrounding asthenosphere of 20 kg/m3.

The model also contains an asthenospheric wedgewith a density of 3360 kg/m3 filling the space betweenthe slab and the shield lithosphere. According to studieson Andean magmatism (Kay et al., 1994 ) and P-waveattenuation (Whitman et al., 1992 ) it reaches the crustalroot beneath the recent arc with an alongstrike changeof geometry.

5 P o s s ib l e C o n t r i b u t i o n s t o t h e G e o i d a t 20 SThe SD-density model with fitted geoid and Bouguergravity w as used to estimate possible contributions tothe geoid and the gravity field of the topography andbathymetry, an Airy isostatic root, the models lowercrust, the slab and the asthenospheric wedge. To visua-lize these contributions (see Fig. 3) we chose an exem-plary W E trending profile at 20S where the geoid andthe Bouguer Anomaly reach 46 m and -365 mGa1 res-pectively.

In order to obtain a geoid signal comparable to theBouguer anomaly we calculated within the entire mo-del area the contribution of the continents topographicmasses above sea level with a density of 2670 kg/m3 an dof the oceans missing masses between sea level andocean floor with a density of -1640 kg/m3. The cal-culated contribution which accounts for a maximum of148 geoid meters at 20S was subtracted from OSUSlA.

An Airy isostatic root was determined with a normalcrustal thickness of 40 km and a crust mantle densitycontrast of 300 kg/m3 within the entire model area. Thecalculated geoid contribution has a minimumof -124 mand mirrors the signal due to topography and bathym-etry. The gravity effect drops to -365 mG a1.

Alternatively we regarded the effects on geoid andgravity field of the models lower crustal parts. The den-sities again are taken relative to the reference value of3370 kg/m3 and vary from -370 kg/m3 to -470 kg/m3.Fig. 3 shows a geoid curve (minimum of -212 m) beingconsiderably lower than the Airy Mohos effect, whereasthe gravity contribution of a -295 mG a1 low is a littlehigher than that of the Airy Moho. It should be notedthat surplus masses due to the negative root in oceanareas are not taken into account here.

The subducting Nazca Plate is modelled between thetrench and a depth of 670 km with a mean positive den-

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3D-Density Modelling with Seismic Constraints 29 3sity contrast of 20 kg/m3 apart from a 5-6 km thickeclogite layer with a density difference of +130 kg/m3between 70 km and 220 km and a metastable olivinewedge reaching into the spine1 zone with -100 kg/m3from 400 km to 600 km. This slab structure contributes47 m to the geoid and 60 mG a1 to the Bougue r anomaly.

The asthenospheric wedge described in section 4.2,modelled with a density contrast of -10 kg/m3, ac-counts for -7 geoid meters and -13 m Ga1.

Discussion of the geoid is carried out at 20S becausethe contribution from the Andean topography and theNazc a plate are largest at this latitude.

6 ConclusionsA three dimensional density model in the Central Andesexplains the observed geoid and gravity without offsetbetween the calculated and measured signals. Exceptthe downgoing slab the model does not take into ac-count other density inhomogeneities below 220 km. Thedensity distribution in forearc and arc regions w as ca-librated to the generalized geometry of 2D ray tracingmodels. Crustal densities were preliminarily determi-ned according to common velocity-density relations andslightly changed under the need to fit curves w ith fixedgeometry.

The models crust-mantle boundary, deepest belowthe recent arc and the Altiplano-Puna zone, shows analongstrike change in depth ranging from 56 km at 24Sto 65 km at 20-21%. Density contrasts at the gravi-metric Moho vary between 360-37 0 kg/m3 in arc andbackarc areas and 450 kg/m3 in the Coastal Cordilleradue to low density bodies directly above the subductingslab. A Vening-Meinesz isostatic Moho derived fromtopographic loads with a normal crustal thickness of35 km, a crust-mantle density contrast of 350 kg/m3and a lithospheric rigidity of 1O2 3 Nm is situated wellbelow the model crust-mantle boundary in the south atabout 2330 s (compare Fig. 2), but coincides well withit at 21 s. This possibly reflects concepts of lithosphericdelamination south of 23S (e.g. Kay et al., 1994).

The isostatic state of the model, derived by locallycomparing the density distribution relative to the refe-rence model with the topographic load, shows a signifi-cant overcompensation (mass deficit below sea level) inthe forearc between WS and 23 s. This coincides witha region w here m odelled gravity and geoid trend a little

lower than the observed signals. This could be explainedby the fact that the top of the modelled slab is situatedtoo low thus leaving too much room for light weightedcrustal material. A slab subduction at a lower angle issuggested in this zone.Acknowledgements. T h a n k s a r e d u e t o K . L e ss e l f or p r o v i d i n gd i s c u s s i o n a n d t o G . J e n t z s c h a n d U . C a s t e n a n d f o r r e v i e w i n gt h e m a n u s c r ip t .

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