I"
Journal of South American Earth Sciences 1 i PERGAMON Journal of South American Earth Sciences 13 (2000) 3-19
Local earthquake tomography of the Andes at 20"s: Implications for the structure and building of the mountain range Frédéric Masson a,*, Catherineborbathb, ', Clande/Martinez ', Gabrielt Carlier "CNRS Laborotoire de Géopliysiqiie. Tectotiiqire et Séclitiieritologie, Uiiiversité Moritpellier II, 4 pluce EigPtie B~rtciilloti,34095 MotitpeNier Cedex 5, Frutice bCNRS Itistitiit (le Pliysiqite di Globe de Strcisboiirg, 5 rire Retié Descurtes, 67054 Strusboirrg Cecles, Frurice 'IRD (ORSTOM), 213 rile Lu Fuyelte, 75450 Paris Cedex IO, Fratice Accepted 27 March 2000
Abstract
Arrival-times of local events recorded in northern Chile and southern Bolivia were used to determine the P velocity structure above the subducted Nazca plate. The data were recorded between June and November 1991 by the French "Lithoscope" network 41 vertical and 14 three-component short-period seismic stations were installed along a 700 km long profile crossing the main structures of the Andean chain, from the Coastal Cordillera to the Subandean Zone. The inversion method used is a I modified version of Thurber's 3D iterative siniultaneous inversion code. The results were compared with a model obtained from previous German nearby refraction seismic studies and supplemented by field geological observations. The relocated seismicity is consistent with an -30" dipping slab between O and 170 km depth. \Ve found a variation of about 30 kni of the Moho depth along the profile. The crustal thickness is about 47 km under the Coastal Cordillera, 70 kni under the Western Cordillera and the western part of the Eastern Cordillera, and 60-65 km beneath the Altiplano. Close to the surface, a good agreement between the velocity model and the geological structures is observed. Generally. in the upper crust, high velocities coincide with zones where basement is present near the surface. Low velocities are well correlated with the presence of very thick sedimentary basins or volcanic material. At greater depth, the trend of the velocit) model is consistent with the existence of asymmetrical west-dipping imbricated blocks, overthrusting toward the east, which explain the asymmetrical pattern of the sedimentary basins. Beneath the Western Cordillera, the active volcanic arc, a large zone of low velocity is observed and interpreted to be due to partially molten matehal. A clear velocity contrast appears between the a-estern and eastern parts of the upper mantle beneath the Andes; this geometry suggests the existence of a low velocity wedge in the mantle above the slab and the presence of a thick old lithosphere in the eastern part of the Andes. 0 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction crust beneath the Altiplano is well established (e.g., James, 1971; Wigger et al.. 1994). its structure as well The Central Andes, in southern Bolivia and north- as the processes involved in its thickening remain lar- ern Chile, is a classic example of a young continental gely unresolved. Crustal shortening and/or magmatic orogen. In this region an oceanic plate, the Nazca addition (e.g., Isacks, 19%: Dewey and Bird, 1970; plate, is subducting under a major continental litho- Kono et al., 1989) are the processes classically evoked spheric plate, the South America plate. The mountain to explain the crustal thickening and the high elevation building process of the Central Andes is still a subject of the Altiplano. A basic question is the respective of ongoing debate. Although the great thickness of the contribution of each process. In order to get a better understanding of this problem, geophysical investi- gation of the deep structure of the Andes was per- * Corresponding author. Fax: + 33-4-67-52-39-08. formed. E-mail address: [email protected] (F. Masson). Between June and November 1994, a temporary net- i 0895-9811/00/$ - see ffont matter 0 2000 Elsevier Science Ltd. I I l 4 F, Mnsson et nl. 1Journal of South American Earth Sciences 13 (2000) 3-19 work of seismic stations was operated along an east- the Andean chain in the region (Wigger et al., 1994; 1 west profile between 19" and 20"s in northern Chile Beck et al., 1996; Zandt et al., 1996). Wigger et al. i and southern Bolivia, across the main structural fea- (1994) studied the P-wave velocity structure of the i tures of the Andean chain (Figs. 1 and 2). The seismic crust using seismic refraction data along a line at network, extended from the Coastal Range to the Sub- 21.25"S, parallel to our profile. Beck et al. (1996) have andean Zone, recorded distant earthquakes and local determined the thickness of the crust using P to S con- events. P-teleseismic delay times were used to perform verted phases from the Moho. They computed the a tomographic inversion in order to examine the whole lithosphere and the subducted slab (Dorbath et al., depth of the Moho and found depth variations of 1996; Dorbath, 1996). In this paper, we present new nearly 30 km across the Andes. Zandt et al. (1996) i results of the inversion of arrival times of P- and S- modelled broadband waveforms for paths crossing the waves generated by earthquakes located mainly in the Altiplano and found a crustal thickness of 65 km and I Andean Subduction Zone, which provide the velocity a low average P-wave velocity of 6.0-6.1 km/s. Finally, structure above the slab and an improved relocation of we compare our results with geological field obser- I ' the earthauakes. vations in the same area in order to propose an unified These results are compared with recent studies of model for the Central Andes. 70'W 65'W 15's 20"s '4 !Qr" \' ?i t r\ __ -&'------ @II n11 ,p!! p' II II I t II i I 550 600 25"s Fig. 1. Distribution of stations and events used for the local earthquake tomography. The shaded circles show the relocated epicenters after the inversion, the darker the deeper, The small lines indicate the shift between the initial and final locations. The triangles indicate the location of the stations (black, 1 component temporary station; white, 3 component temporary station; grey, permanent station). The shaded area represents the Altiplano (Peru-Bolivia) and Puna (Argentina). The bold dotted line indicates the limit of the Subandean Zone; the thin dotted lines indicate the depth of the subducted slab (Kirby et al., 1995). F. Masson et al. Joitrnal of South Aniericari Earth Sciences 13 (2000) 3-19 5 2. Geological setting The pre-Mesozoic basement of the Precordillera and the western part of the Altiplano is composed of Pre- The building of the Andes occurred during the Cen- cambrian and Palaeozoic metamorphic rocks (Leh- ozoic, following compression that induced crustal mann, 1978; Basei et al., 1996). In the eastern part of thickening of the western edge of South America the Altiplano and in the Eastern Cordillera, the above an oceanic subduction zone dipping to the east. Palaeozoic metamorphic rocks are replaced by a thick The present day structure of the chain, along an east- (> 10 kin) marine sequence of shales and sandstones west transverse, is characterised by different north- (Castaños and Rodrigo, 1978; Martinez, 1980). During south morphological units which are, from west to the Mesozoic (Triassic to Early Cretaceous), a thick east (Fig. 2): the Coastal Cordillera, the Longitudinal (> 10 kin) sequence of calc-alkaline andesite to rhyolite Valley, the Precordillera, the Western Cordillera, the lava-flows intercalated with marine and continental Altiplano, the Eastern Cordillera, and the Subandean sediments was deposited in the Coastal Cordillera and Zone. These morphological units are not always co- the Longitudinal Valley (Muñoz and Charrier, 1993; incident with the geological structures. For example, in Scheuber et al., 1994). This sequence is intruded by a geological sense, the Western Cordillera is a part of large Jurassic and Cretaceous calc-alkaline batholiths the Altiplano crowned by volcanoes. The Andean (Aguirre, 1983). East of the Longitudinal Valley, the chain is widest (about 700 km) from the coast to the Mesozoic cover ($2000 m) is characterised by marine Chaco plain. The mean elevation is near 4000 in in the to continental Jurassic and Cretaceous shales, sand- Altiplano and the Eastern Cordillera and, locally, can stones, and limestones (Pareja et al., 1978; Mapa Geo- reach 5000 m in the Western Cordillera and Meseta de logico de Chile, 1982; Ardil1 et al., 1996). Since 70 Ma, los Frailes, a late Cenozoic volcanic field. continental molassic deposits and widespread mainly ' COASTAL CORDILLERA WESTERN CORDILLERA and ALTIPLANO EASTERN CORDILLERA SUBANDEAN ZONE Plio-quaternary V Cenozoic volcanism v, volcanism :::,:.: Cenozoic Paleozoic Mesozoic Cenozoic gneiss and volcano- Cenozoic Mesozoic magmatism sediments intrusif Meso-Cenozoic granite and Paleozoic Precambrian Mesozoic and continentalMeso-Cenozoic Hanging wall Foot wall Eocambrian marin sediments paleozoic paleozoic Trench ind. fault Strike-slip fault Reverse fault Normal fault the Fig. 2. Geological setting of the region under study. From the Pacific Ocean to the Subandean Zone, the main units are the Coastal Cordillera, ,Iof the Longitudinal Valley, the Precordillera, the Western Cordillera, the Altiplano, the Eastern Cordillera, and the Subandean Ranges. The map is ents based on the geological maps of Chile and Bolivia (see text for references), revised. Legend AFS=Atacama Fault System, CFZ=Coniri Fault cate Zone, CRFZ= Cordillera Real Fault Zone, KUFZ= Khenayani Uyuni Fault Zone, PFS =€'recordilleran Fault System, SAFZ=San Andrés Fault Zone, SIFZ = Sevaruyo-Incapuquio Fault Zone, SSF = San Simón Fault Zone. 6 F. Masson et al. /Journal of South American Earth Sciences 13 (2000) 3-19 i3 calc-alkaline volcanic activity are associated with the progressive uplift of the Central Andes (Martinez, 1980; Benjamin et al., 1987). Molassic deposits accu- mulated mainly within both the Altiplano (5-10 km) and the Subandean Zone (3-4 km) basins (Schlatter and Nederloff, 1966; López, 1974; Martinez, 1980). Magmatic activity that progressively migrated from the Longitudinal Valley to the Western Cordillera during the Paleogene and the Eocene thereafter stabil- ized within two volcanic belts, the Main Volcanic Arc and the Inner Volcanic Arc, respectively. The former, along which large volumes of calc-alkaline basic to acid volcanics erupted during the last 28 million years, is located above the Western Cordillera (Backer and Francis, 1978; Scheuber et al., 1994). The latter is characterised by the Mio-Pliocene peraluminous acidic ash-flow tuffs of the Meseta de los Frailes (Ericksen et al., 1990; Leroy and Jiménez, 1996) and by the Qua- ternary high-K calc-alkaline to shoshonitic edifices of the Altiplano (Davidson and de Silva, 1992, 1995). In addition, Mio-Pliocene and Quaternary volcanic rocks, ultrapotassic rocks (lamproites), have also been observed in the area by one of the authors (GC). The distribution of the pre-Mesozoic basement, its Mesozoic and Cenozoic cover, and the Cenozoic mag- matic activity are controlled by the main Andean crus- tal structures (Fig. 2). The Atacama Fault System is a north-south normal-sinistral fault (Delouis et al., 1998). The Preeordilleran Fault System (PFS, Reutter et al., 1996), a strike-slip system, is a bi-vergent struc- ture ,along which the pre-Mesozoic basement is thrust centers that have a high quality location. We kept only above Cenozoic deposits (Muñoz and Charrier, 1996). those events that satisfy the following criteria: conver- ~ l Eastward, the E-verging San Andrés Fault Zone gence of the hypocentral depth to similar values what- . (SAFZ) and its southern prolongation correspond to the western boundary of the highly subsiding Northern conditioning factor lower than 300 (ratio of the largest and Central Altiplano Basins (Martinez et al., 1995). to the smallest singular values of the matrix of partial I To the east, the southward prolongation of the Cordil- derivatives), and at least two S arrival times. The S lera Real Fault Zone (CRFZ) is an important N-trend- ing crustal discontinuity zone along which erupted the easternmost Cenozoic volcanism of Bolivia. The tran- sition from the Eastern Cordillera to the Subandean regional distances, some seismograms of intermediate- I Zone is characterised by complex crustal imbricated depth earthquakes in the subducted Nazca plate dis- I stuctures (Schmitz, 1994; Kley et al., 1997). The com- play a large-amplitude arrival following the P wave. plexity of the chain is increased by some transverse This arrival appears to be an S wave at first glance but structures. For example, the Sevaruyo-Incapuquio it is in fact a shear-coupled P-wave. Therefore some of ~ Fault Zone (SIFZ) is NW-trending structures which our S picking may be shear-coupled P-waves. Never- localize the magmatic activity of the Inner Volcanic theless, the number of shear-coupled P-waves talcen as I Arc. S waves during the inversion is certainly insignificant. First, the S data were strictly selected during the pick- ing and, after the location, all the data with anomalous 3. Data and inversion residuals were systematically rejected. Second, using the two complementary permanent seismic networks, 3.1. Data we added S data with different azimuths and epicentral distances. Therefore, the probability of observing a This study uses arrival times of P and S waves from cross-over of the S wave and the shear-coupled P-wave local earthquakes recorded by a temporary seismic net- for an event at all the stations decreased greatly. ? I F. Mussoit et al. 1Jolirital of South Anlericuit Earth Sciences 13 (2000) 3-19 I 1 1 e 2 , 1. e e Il i I i 1 I- -2 ll -70 -68 -66 -64 X (in degrees) I- kI âS \ / ,e is 1- / -15 P Jb 2/ o, ic I O 4 +ha t', -16 % et h ts -17 ,le l Il- -68 -66 -64 -1s .1Z X (in degrees) of Il- -19 .nt i lar -20 ! 2 , lie ,O- -21 IlY i 2r- at- -22 -2 s: -70 -68 -66 -64 es1 -23 X (in degrees) tia1 I I I 7 -69 -68 -67 '6 -65 -64 -63 .s ! ind ted \ lear ; 2 Y tte- dis- , nrn ;soE ive. * but , E-c 2 of I ver- 1 -2 I as -7 ant. lick- lous Fig. 3. P residual times (difference between computed and observed traveltimes) plotted along the seismic profile for 6 events. The crosses con- Irks, nected by a dotted line indicate the true value of the residual times. The black line shows the smoothed pattern of the residual times along the profile. The location of the events is indicated on the map. Two events are located far from the profile, two at an intermediate distance, and two just beneath the profile. The chosen events are symmetrically displayed with respect to the profile. A clear consistency appears for the 2 events of each group indicating the north-south symmetrical trend of the structures. mve 8 F. Masson et al. /Journal of Soirth American Earth Sciences 13 (2000) 3-19 Finally, the erroneous S phases were rejected during the location process. We selected 184 events which passed the criteria and had a mean RMS of 0.36 s and a mean conditioning factor of 146. We rejected all events without any data from offline stations, except 28 events located close to the line between 19" and 21"S, those related mainly to the Subduction Zone. The retained events provided 4869 arrival times consisting of 4049 P and 820 S arri- vals. The general pattern of the seismicity recorded during the experiment is similar to that reported in bulletins (i.e., NEIC - National Earthquake Infor- mation Center): the deepest seismicity is located pri- marily in the southernmost part of the study area, and the intermediate seismicity (between 100 and 250 km depth) is located in the central part of the target area from 19" to 22"s. P residual times are the difference between computed travel-times and observed travel-times and are related mainly to differences between a standard earth model and the local velocity model. Figure 3 shows P residual times along the seismic profile plotted for 6 events. Two events are located far from the profile, two at an mean value of the residuals, and the chosen size of the intermediate distance, and two just beneath the profile. cells used during the inversion. There is a scaling fac- The chosen events are displayed symmetrically with tor and the ratio of the residual travel-time to the ray- respect to the profile. The amplitude of variations in tracing error remains constant. the residuals reaches 1 s along the profile, which is For the initial P-wave velocity model, a smoothed similar to the amplitude obtained in northern Bolivia version of the model used for hypocentral locations by Dorbath and Granet (1996). For each event, the was chosen (Table 1). The mean crustal P-wave vel- variation of the residuals is consistent for neighbouring ocity was 6.0 km/s. corresponding to the value stations from one end of the profile to the other. A obtained by Wigger et al. (1994) and Zandt et al. clear consistency appears for the 2 events of each (1996) beneath the Altiplano. The S-wave velocity was group, indicating the north-south symmetrical trend of initially set by using a Vp/Vs ratio of 1.73. The vel- i the structures. The residuals are not clearly correlated ocity model is desiribed-by a grid aligned along á with the structural units (Fig. 2) and a simple in- north-south direction, perpendicular to the direction of terpretation of the residuals is not possible. We used the profile. The horizontal grid spacing is one third of local earthquake tomography to determine the struc- a degree in the central part of the model and larger tures beneath the Andean chain. outside this zone. The starting model was made of layers of grid points from the surface down to the 3.2. Inversion maximum depth of hypocenters. A reasonable choice for the damping value was To perform the tomographic inversion, we used an obtained constructing trade-off curves of resolution updated version of the program adapted by Eberhart- against solution error. The optimal value substantially Philips (1986) from the method developed by Thurber increases the resolution without introducing large vel- (1983). In this program, an iterative process inverts P- ocity errors. We ran several inversions with various and S-wave arrival time data for 3D velocity structure damping values and finally selected the value of 150. and hypocentral parameters. The parametrization of The initial data variance was 0.28 s2 and 0.83 s2 for P the study region was achieved by assigning velocity and S waves, respectively. After 5 iterations, the var- values at fixed points on a non-uniform 3D grid. The iances were reduced to 0.04 s2 and 0.19 s2, respectively. velocity at any point is linearly interpolated between The final reduction of the initial data variance was the nodes surrounding it. In contrast to methods using 86% and la%, respectively. block models, a continuous velocity field is assumed The reliability of the inversion is indicated mainly and a solution with smooth changes in velocity is pro- by the resolution matrix. Later, we considered only the duced. An approximate ray-tracing algorithm (ART, nodes crossed by at least 50 rays and with a resolution Thurber, 1983) is used to calculate travel time between greater than 0.1. For the whole model, the diagonal the station and the receiver. This method combines resolution elements reached 0.72 for the P-wave vel- 1 F. Masson et al. Jorirnal of Soiitli American Earth Sciences 13 (2000) 3-19 9 ocity and 0.64 for the S-wave velocity models. For the on the hypocentral location. Nevertheless, the changes 5 shallower layers. of grid points, the space is well on hypocentral locations appear to better delineate the sampled by rays and the resolution is high. Below seismicity of the Subduction Zone. The results are con- 120 km, the resolution is lower because the nodes are sistent to those of previous studies (i.e. Kirby et al., not well sampled by the rays. The value of the diag- 1995, dotted line on the sections). From the trench to onal elements of the resolution matrix for the 5 shal- a depth of 170 km, the seismicity is consistent with an lower layers is presented on Fig. 4. -30" dipping slab. For the final model, the standard deviation within each layer is about 0.20 km/s for P-waves and 0.08 km/ 4.2. Velocity model (Fig. 6) s for S-waves, whereas the average standard error is about 0.043 km/s for P-waves and and 0.034 km/s for Figure 6 shows the main features of the absolute vel- of I 111- S-waves. Therefore the P-velocity variations are signifi- ocity model obtained by tomography. We present the he cant with respect to the uncertainties. The S-velocity P-wave velocities for 5 levels (O, 30, 60, 90 and 120 km el- variations are smaller. In this paper, we do not present depth), We consider that the Andean crust corresponds to the 3 shallower levels (Fig. 6a). The westernmost ter ~ the S-velocity model which is only used to locate the er- seismic events. and easternmost parts of level 3 correspond to the er- oceanic Nazca plate and to the Subandean upper man- ! tle, respectively. The two deeper levels (4 and 5) yield YO- 4. Results information on the upper mantle (Fig. 6b), except in ar- , the westernmost part where they image the structure the 4.1. Earthquake location (Fig. 1 aiid 5) of the subducted Nazca plate. At this stage, the large the f variations of the Moho depth along the profile are not the The P- and S-wave velocity models obtained from considered in this description (see below). ac- I the local earthquake tomography inversion were used to relocate the events occurring beneath the study 4.2.1. The Andean criist (Fig. 60) ay- f area. A comparison of the original and final event lo- Generally the model for the upper crustal structures lied i cations is shown on Fig. 1. The variations of the epi- (level 1, O km depth) determined in this study can be Ii central positions are, on average, 12 km in the closely correlated with the major morphologic features. vel- horizontal direction and 15 km in depth. The standard The Coastal Range, the Precordillera, the eastern part due ; error calculated for the relocated epicenters is 0.6 kin of the Eastern Cordillera, and the Subandean Zone are east-west, 2.3 km north-south, and 2.3 km in depth. imaged as velocities greater than 5.0 km/s, locally was As expected, the standard error is higher in the direc- reaching 5.5 km/s. The Longitudinal Valley, the Wes- vel- i1 tion perpendicular to the profile (N-S) than in the tern Cordillera, and the Altiplano display velocities ig a direction parallel to the profile (E-W). lower than 5.0 km/s. No clear velocity difference, in Il of An E-W cross-section through the relocated hypo- comparison with the mean velocity (about 5.0 km/s), is d of centers is presented in Fig. 5 and compared with a documented for the western part of the Eastern Cordil- rger cross-section through the initial hypocenters. No sys- lera. The pattern of the middle crust (layer 2, 30 km e of tematic variation is indicated by a vertical comparison depth) is simpler; the western and eastern borders of the Table 1 .was Ition Layer Depth Initial Vp No. of Inverted Cells Average Final Vp Standard Deviation Average Standard Error -ially a. Initial P velocity model and characteristics the final P velocity model vel- of 1 O 5.00 48 5.00 0.198 0.03 rious 2 30 6.00 12 6.50 0.208 0.04 150. 3 60 6.80 85 6.84 0.233 0.05 for P 4 80 8.00 113 8.06 0.219 0.05 var- 5 120 8.11 106 8.18 0.208 0.06 ively. 6I 160 8.22 94 8.24 0.128 0.05 220 8.39 56 8.40 0.096 0.05 - was b. Initial S velocity model and characteristics of the final S velocity model 1 O 2.89 28 2.90 0.067 0.02 iainly 2 30 3.41 44 3.48 0.090 0.03 ly the 3 60 3.93 51 3.94 0.076 0.03 lution 4 80 4.62 66 4.62 0.104 0.04 5 120 4.70 61 4.72 0.081 0.04 lgonal 6 160 4.15 58 4.78 0.085 0.04 e vel- 10 F. Masson el al. 1Journal of South American Earth Sciences 13 (2000) 3-19 Depth O km Depth 30 km 19 .g'CIal -Y 20 21 22 22 23 23 71 70 69 68 67 66 65 64 63 71 70 69 68 67 66 65 64 63 longitude longitude Depth 60 km Depth 80 km 17 17 18 18 19 19 aal 4 .eeY 20 2 20 + 3d 21 21 22 22 23 23 71 69 71 70 69 68 67 66 65 64 63 70 68 67 66 65 64 63 longitude longitude Depth 120 km 18 19 4 .< 20 -Y IIkIII,,, 0.2 0.0 0.4 0.6 0.8 21 resolution 22 23 71 70 69 68 67 66 65 64 63 longitude Fig. 4. Values of the diagonal element of the resolution matrix corresponding to each block in the 5 shallower layers of the tomography. The darkest blocks are the best resolved in the inversion. F. Masson et al. Journal of Soutli Ainericmt Earth Sciences 13 (2000) 3-19 11 crust, low velocities persist beneath the western part of the Eastern Cordillera. 4.2.2. Upper inntitle (Fig. 6b) A clear zone of low velocity that persists down to ocity zones (6.5 km/s) cut by a slow central part 120 km is located below the Altiplano at about 67"W. (5.75 kmls). At 30 kin depth, velocities less than It is characterised by a minimum velocity of 7.5 km/s 5.5 km/s are observed only beneath the Altiplano and at 80 km depth and 7.6 km/s at 120 km depth. High velocities characterise the easternmost part of the model. The western part represents a more complex pattern, with numerous low and high velocity zones. Between 69"W and 70"W, the velocity model at 120 km depth has a north-south high velocity zone in the southern part of the model, which is interpreted as the track of the cold subducted slab. 71" 10" 69" 68" 67" 66" 65" W Azimuth :N90 O '. 50 '. . . L1 INITIAL 100 I .E, 150 a 8 200 I 250 \. 300 350 km Coast 71" 10' 69" 68" 67" 66" 65"W Azimuth: N90 O . '. ' 50 RELOCATED 100 9 150 a & 200 250 \ 300 350 km ~ Fig. 5. TOP:Vertical cross section along the profile through the initial hypocenters. The dotted line indicates the location of the slab determined ~ phy. Th by Kirby et al. (1995). Bottom: Same as top section for the relocated epicenters after the inversion. 12 F. Masson et al. I Jorrrnal of South American Earth Sciences 13 (2000) 3-19 Depth O km 5.50 o .a .eUa 5.00 knds CJ I 4.50 70" 69" 68" 67" 66' 65" 64"W longitude Depth 30 km - 5.50 - 5.00 70" 69" 68" 67" 66" 65" 64" W longitude Fig. 6. (a) Smoothed P-wave velocity in the three upper levels of the tomographic model, corresponding mainly to the Andean crust. Only nodes with a resolution greater than 0.1 are plotted. The grey scale is centered on the velocity of the initial model. TO get the smooth pattern, a bilinear interpolation function was applied within each horizontal layer, preserving the true amplitudes for individual nodes of the original discretization. The horizontal grid spacing is 0.33" in the central part of the model. Higher velocities correspond to darker shading. On the first layer are reported the locations of the morphological units. The model for level 1 is closely correlated with the major morphological units and shows a complex pattern. The pattern of level 2 is simpler: the western and eastern borders of the model show hi,mh velocities while in the central part the velocities are low. Velocities smaller than 5.5 km are observed only beneath the Altiplano and the Western Cordillera. The main feature of level 3 is a clear low velocity structure (6.0 km/s) associated with the Western Cordillera. The westernmost and easternmost parts of layer 3 corre- spond to the oceanic Nazca plate and to the Subandean upper mantle, respectively. (b) Same as Fig. 6a for layers 4 and 5 corresponding mainly to the upper mantle. High velocities characterise the easternmost part of the model. A zone of low velocity which persists down to 120 km is located at about 67"W. The western part represents a more complex pattern with numerous low and high velocity zones. At 80 and 120 km depth, the westernmost part of the velocity model is characterised by a north-south zone of high velocity which is interpreted as the track of the cold subducted Nazca plate. nl. (2000) F. Massoii et 1Journal of South Anierican Earth Scieilces 13 3-19 13 Depth 60 km 18"- 7.80 19' - - 7.30 a, Q .- 20"- - L~/s Y3 6.80 -n 21" - - 6.30 - 5.80 70" 69" 68" 67" 66" 65" 64"W longitude Fig. 6 (coiztinrrrd) 5. Discussion blocks; to the west, the mean velocity of these three blocks decreases in both models. Between 68.5"W and 5.1. Conzparisoiz \vith ci reficictioii model 69.5"W, the RM shows a zone with a velocity of 6.5 km/s, including a small high velocity zone. This A vertical cross-section through the smoothed P- latter feature corresponds to the top of a zone of high I wave velocity model of the tomographic inversion is velocity in the TM. The shapes of the structures and presented in Fig. 7 [Tomographic Model (TM), grey the velocities in the westernmost part of the models scale plot]. This section is compared to a velocity are less easily comparable. Generally, the dips are model obtained by seismic refraction [Refraction greater in the TM and some differences in the vel- Model (RM), contour plot] at 21.25"s across the ocities are noted beneath the Coastal Cordillera: the whole Andean chain (Wigger et al., 1994). Both RM differs from the TM by the presence of alternating models are in good agreement, keeping in mind that layers of low and high velocity, which cannot be the refraction studies are most sensitive to the depth of resolved using local earthquake tomography. the interfaces while the local earthquake tomography The overall consistency between both models sup- provides a smoothed velocity model. These models ports the validity of our results and allows us to com- show similar zones of relatively high velocity in the pare them with geological data. upper crust (see at 65"W and 69"W) on both sides of a i. Only nodes xn, a bilinear large zone of low velocity that extends from 66"W to 5.2. Structure of the cnat (Figs. 6a and 8) discretization. 68.5"W. In the lower crust, the similarities are re- first layer are inforced. The deeper interfaces of the RM in the east- Figure Sa shows a geological cross-section through and shows a ern part of the cross-section (between 66"W and the Andes along the study area. Figure 8b represents a entra1 part the 63.5"W) dip to the west as do the structures obtained vertical cross-section through the smoothed P-wave eature of level layer 3 corre- by the tomography. Large west-dipping structures are veIocity model of the tomographic inversion. A good Ionding mainly observed in both models. The large zones of low vel- agreement between the velocity model and the geologi- $1 to 120 km is ocity defined from the easternmost part of the model cal structures is observed. Generally, in the upper ,O and 120 km to 68.5"W in the RM, with velocities of 6.2 km/s, crust, high velocities coincide with zones where base- he track of the 6.1 kmls, and 5.9 km/s, respectively, are compatible ment is present near the surface. In the Precordillera with the variation in the mean velocity of TM in these (Figs. 6a and 8; stations C103-C102), the presence of I (2000) 14 F. Mosson et al. Joiirnnl of Soiitli American Earth Sciences 13 3-19 Depth SO km 17"- 9.00 18"- 19"- 8.50 ma 5.00 knds .IB 20"- c)m I 21"- 7.50 22" - 97.00 23" - 70' 69" 68" 67" 66" 65" 64"W longitude Depth 120 km 9.10 8.60 - 8.10 kds - 7.60 -7.10 69" 68" 67" 66" 65"W longitude 6 Fig. (continired) Cambrian (or older) rocks is documented by small out- ocity probably corresponds to Precambrian baseme crops ,in northern Chile (Basei et al., 1996). In the Alti- uplifted by imbricated thrusts, as suggested by Kley plano (stations B104-B105), a zone of high velocity al. (1997), in the southernmost part of Bolivia. T1 could be explained by the existence of a west-dipping zone of high velocity of the Coastal Cordillera (Fig. 6 Precambrian block overthrusting the Mesozoic and represents a Mesozoic magmatic arc (Prinz et a Cenozoic series of the Altiplano (stations B105-Bl06) 1994). The lower velocities are clearly associated wi (Martinez et al., 1995). In the Eastern Cordillera, the presence of very thick sedimentary basins and vc between stations BI20 and B125, the zone of high vel- canic material. The Longitudinal Valley (statio -.I- .--..--.- I Coastal Subandean Trench Cordillera Pre-C. West. C. Altiplano Eastern C. Ranges Chaco I O O 20 20 2 40 40 .k 60 60 so so 100 100 120 120 ltm 69" 68" 70" 67" 66" 65" 64" W Lu I 4.0 4v3 5.0 6.0 7.0 8.0 9.0 hldS Fig. 7. Velocity models obtained by local earthquake tomography at 19.5"s (grey scale underlined by dashed lines) and refraction seismic study at 21.2"s (contour plot) (Wigger et al., 1994). Ver- tical exaggeration = 2. Both models are in good agreement, keeping in mind that the refraction studies are most sensitive to the depth of the interfaces whereas local earthquake tomography pro- vides a smoothed velocity model; sce text for a morc detailcd comparison. The Moho obtnined by Wiggcr ct al. (1994) is indicated by a hold line. The Moho obtained in this study (7.0 km/s line) is indicated by a bold dashed line. --i' I 16 F. Masson et al. I Journal of South American Earth Sciences 13 (2000) 3-19 W CHILE BOLIVIA E ' ALTI PLAN O EASTERN CORDILLERA SUBANDEAN ZONE jE60 64" km I , lOOkm , Andean Ea Cenozoic 0cnlst uPaleozoic aMesozoic 0Cenozoic m magmatism volcanism 70" 69" 68" 67" 66" 65" 64"W 5.0 6.0 7.0 8.0 kds Fig. 8. Top: Geological and structural cross-section at 19.59 (from Fig. 2). Legend: AFS =Atacama Fault System, KUFZ= Khenayani Uyuni Fault Zone, CRFZ= Cordillera Real Fault Zone. PFS =Precordilleran Fault System, SAFZ= San Andres Fault Zone, SSF= San SimÓn Fault Zone. Bottom: Corresponding vertical cross-section down to 200 km through the P-wave velocity model. The Moho is indicated by a bold line. Station locations are indicated between the geological and tomographic cross-section (Cxxx = Chilean station, Bxxx: = Bolivian station). No verti- cal exaggeration. In the upper crust, high velocities coincide with zones where basement is present near the surface (see for example the Precordil- lera, the SAFZ, or the eastern part of the Eastern Cordillera). Low velocities are well correlated with the presence of very thick sedimentary basins (Central Altiplano Basin between Salinas and Sevaruyo) and volcanic material (Western Cordillera). At greater depth, the general trend of the velocity model indicates the existence of asymmetrical west-dipping imbricated blocks, overthrusting toward the east along the SAFZ, KUFZ, and CRFZ. Beneath the Western Cordillera, low velocities are observed and interpreted to be due to partiab molten material (see also Fig. 6a, layer 3). Variations of the Moho depth are estimated to be 30 km along the profile. In the mantle, a velocity contrast appears between the western and eastern parts of the upper mantle beneath the Andes (at about 67"W) which Suggests the existence of a IOW velocity wedge in the mantle above the slab. Toward the east, the homogeneous high velocity mantle probably documents the presence of a thick old lithosphere. C105-C104) is covered by Paleogene calc-alkaline vol- (Backer and Francis, 1978). The low velocity of the canic series and thin recent alluvial deposits (Scheuber Central Altiplano Basin (stations B105-BllO) is related a et al., 1994). Beneath the Western Cordillera we see to the presence of very thick M~sozoic(Cherroni, low velocities (stations B101-Bl04) corresponding to 1977) and Cenozoic Sediment sequence. East of the Neogene volcanic material of the main volcanic arc Central Altiplano Basin (stations B110-B118), low vel- b F. Masson et al. /Journal of South Ainericari Earth Sciences 13 (2000) 3-19 17 ocities could be associated with the Los Frailes volca- inverted model can only show smooth changes in vel- nic field overlaying the thick west-dipping Palaeozoic ocity. Nevertheless,. the Moho can be roughly defined deposits. In the east to station B129, the Mesozoic and as the zone where the velocity gradient is the steepest. Cenozoic Subandean foreland basin (Lòpez, 1974; In our model, this zone corresponds to the 7.0 km/s Baby et al., 1989) is not clearly correlated with low contour line. The refraction model (Wigger et al., velocities, confirming the small thickness of the Suban- 1994) shows the Moho as a velocity step from 6.2 to dean Basin and the presence of basement at shallow 8.1 km/s. The 7.0 km/s contour line corresponds to the depth. middle of this step and shows a good fit with the The main fault systems are closely related to the Moho defined by the refraction seismic method limits of the velocity zones. In particular, the San (Fig. 7). Andrés Fault Zone (SAFZ, stations B105-B106) is The Moho discontinuity, referenced as the 7.0 km/s marked by high velocities that separate the Western contour line, shows variations of about 30 km along Cordillera and the Central Altiplano Basin. The Cor- the profile. The crustal thicknesses beneath the Central dillera Real Fault Zone (CRFZ, station B122) corre- Andes vary from about 47 km under the Coastal Cor- sponds to the limit between the low and high velocity dillera and the eastern part of the Eastern Cordillera zones of the Eastern Cordillera. The isovelocity lines to 70 km under the Western Cordillera and the wes- suggest that these two fault systems define three large tern part of the Eastern Cordillera. The crustal thick- domains and extend to greater depth, dipping to the ness beneath the Altiplano is about 60-65 km. west. East of CRFZ, the high velocity of the eastern Since James (1971), the thickness of the crust domain also observed in the lower crust strongly beneath the Altiplano has been roughly known and a suggests the presence of an uplifted lower crust. mean Moho depth of 70 km is generally accepted. Our Between the CRFZ and the SAFZ, a second domain model is in general agreement with the Moho depth corresponds to a large west-dipping block overthrust- defined by Wigger et al. (1994) in the eastern part of ing the eastern domain. This second domain is itself the model (Subandean Ranges and eastern part of the overthrust along the SAFZ by the basement of the Eastern Cordillera). By contrast, toward the west, the western part of the Altiplano. The east vergence of Wigger's model is smooth whereas our model shows these large crustal blocks explains the asymmetrical noticeable variations (Fig. 7). Our results are in very structure of the subsiding sedimentary basins (stations close agreement with those obtained by Beck et al. C102-B104 and B105-B113). The western part of these (1996) who used P to S converted waves at the Moho basins is subsiding whereas the eastern part is marked to estimate a Moho depth of 70-74 kin beneath the by an uplift of the substratum. In summary, the gen- Eastern and Western Cordillera, 60-65 kin beneath the eral trend of our model is the existence of asymmetri- Altiplano, and 43-47 km beneath the Subandean cal west-dipping imbricated blocks, overthrusting Zone. Similar results have been obtained by Zandt et toward the east, in agreement with some other models al. (1996) beneath the Altiplano (65 km) and by Whit- (i.e., Suárez et al., 1983). man (1994) beneath the eastern margin of the Andes Some other features are noticeable in our model. in northwest Argentina (60-65 km beneath the Puna Low velocities are observed in the lower crust beneath and 40-45 km beneath the Subandean Zone). the main volcanic arc (stations B101-Bl04, Figs. 6a and 8). On the cross-section, the 6.5 km/s contour is 5.4. Mantle structure located close to 60 km depth. This low velocity struc- ture, as suggested by Schmitz (1993), could be due to Several velocity contrasts appear in the upper mantle partially molten material in the lower crust. The Pre- (Fig. 8). These contrasts are well documented follow- cordillera appears similar to a symmetrical zone and ing the 8.0 km/s isoline. The extremely high velocity - corresponds to eastern and western reverse faulting, observed in the westernmost part of the model (west bordered by two vertical strike-slip faults. The sym- of 69"W) is certainly related to the subducted Nazca metrical zone generates a double sedimentary deposit, plate. Between stations CI07 and B101, the isoline toward the east and the west (Reutter et al., 1991). dips from 80 to 140 km, showing the expected velocity contrast between the cold subducting slab and the 5.3, Moho depth overlying mantle. From station BlOl to B107, the depth of the isoline fluctuates between 90 and 150 km. The Moho is a first-order discontinuity the depth of At B108, a significant change occurs and the isoline which changes across the Andean chain. Therefore its sharply rises. From B114, the isoline remains flat at location and the velocities below and above it cannot 80 km. Therefore a contrast appears between the wes- be determined accurately by local earthquake tomogra- tern (69"W to 67"W) and the eastern (east of 67"W) phy. Effectively, as pointed out before, Thurber's parts of the model (Fig. 8). method assumes a continuous velocity field and the The two low velocity structures between the sub- 18 F. Masson et al. 1 Journal of South American Earth Sciences I3 (2000) 3-19 ducted slab and the crust from 69"W to 67"W suggest blocks, overthrusting toward the east beneath the East- the existence of a globally low velocity wedge in the ern Cordillera and the Altiplano. The thickening of the mantle above theislab, west of 67"W. This anomalous crust, beneath the Western Cordiller% results from mantle could correspond to a heated mantle (Isacks, another type of mechanism involving addition of mag- 1988) or a hydrated mantle (Tatsumi, 1989; Sudo and matic material. Nevertheless, according to our model, Tatsumi, 1990). East of station B114, the homo- tectonic thickening appears to play the dominant role geneous high velocity mantle probably documents the in crustal shortening in the Central Andes. presence of a thick lithosphere corresponding to the Brazilian shield or to a pericratonic block. The limit zone between these two parts, which coincides with an References important anomalous high heat flow (Henry and Pol- lack, 1988), is characterised by the Quaternary shosho- Ardill, J., Flint, S., Stanistreet, I., Chong, G., 1996. Sequence strati- nitic volcanic activity of the Inner Arc (Davidson and graphy of the Mesozoic Domeyko Basin, northern Chile, Third ISAG St Malo, Frunce, extended abstracts, 269-272, ORSTOM de Silva, 1995). This magmatic activity is generally Eds, Paris, France. considered to result from melting of mantle sources Aguirre, L., 1983. Granitoids in Chile. Geol. Soc. Am. Bull. 159, previously metasomatized by slab-derived fluids (i.e., 293-3 16. Davidson and de Silva, 1995). 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