Electromagnetic study of the active continental margin in northern Chile

PHYSICS OF THI~ EARTH AND PLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 102 (1997) 69-87

Electromagnetic study of the active continental margin in northern Chile Friedrich Echternacht a, Sebastian Tauber a Markus Eisel Gerhard Schwarz a, Volker Haak b

b Heinrich Brasse a,*,

a Freie Universitiit Berlin, Fachrichtung Geophysik, Malteserstr. 74-100, 12249 Berlin, Germany b GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

Received 6 February 1996;revised 24 October 1996;accepted26 November1996

Abstract Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5 ° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10 -4 s to approx. 2 X 104 s and thus enabled a resolution of deep as well as shallow structures. In this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas. © 1997 Published by Elsevier Science B.V.

1. Introduction The crust of the Central Andes is characterized by the convergence of the subducting Nazca plate and the overriding South American plate and may be subdivided into different morphostructural units (Fig. 1, cf. Reutter et al., 1988). Under changing convergence regimes four magmatic arcs have developed since the Jurassic: the Coastal Cordillera constituting the oldest and the Western Cordillera (with active

* Corresponding author.

volcanism) the most recent one (Scheuber, 1994). In between lie the Longitudinal Valley and the Chilean Precordillera. Due to oblique subduction of the former Farallon plate, several prominent (mainly strike slip) faults have evolved, e.g. the Atacama fault system in the Coastal Range and the West Fissure in the Precordillera. It should be mentioned that the largest porphyric copper deposits in the world (e.g. Chuquicamata) are located at the West Fissure. In the 1980s several magnetotelluric (MT) and geomagnetic deep sounding (GDS) profiles were measured in northern Chile, northwestern Argentina and southern Bolivia together with seismic and

0031-9201/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PH S0031-9201 (96)0326 1-X

70

F. Echternacht et al. / Ph)'sic,s o[ the Earth and t'/am'mr~ lnterior~ 102 (1997) 69 $7

gravimetric investigations by the research group "Mobility of Active Continental Margins" at the Free University of Berlin (cf. Schwarz et al., 1994). Fig. 1 shows the location of the MT sites. In the framework of this paper, we will concentrate mainly on the present magmatic arc (the Western Cordillera) and the forearc (Coastal Cordillera, Longitudinal Valley and Precordillera) in northern Chile. The main geophysical features observed in this region of the Andes are: a crustal thickness of up to 70 km, a low velocity zone and high attenuation of seismic waves below the magmatic arc, where the Moho may not be

71°W

70°W

69°W

clearly resolved (Ocala and Meyer, 1972, Chinn el al., 1980. Wigger et al., 1994), high seismicity at a depth range of approx. 60-100 km below the Precordillera (Barazangi and lsacks, 1976, Comte and Suarez, 1995), distinct negative anomalies of the Bouguer ( = - 4 5 0 reGal) and the residual gravity field (James, 1971. GiStze et al., 1994), following the general trend of the magmatic arc, high heat flow values of 80 mW m 2 and more in the region of the volcanic arc (Henry and Pollack, 1988, Giese, 1994, Hamza and Mufioz. 1996),

68°W

67°W

66°W

20 ° S

20 ° S

21°S

21 ° S

22 ° S

22 ° S

23" S

23 ° S

24 ° S

24 ° S

71" W

70 ° W

69 ° W

68" W

67" W

66" W

Fig. l. Location of former (diamonds) and recent (stars) electromagnetic sites in the Central Andes, with outlines of main morphostructural units: CC, Coastal Cordillera; LV, Longitudinal Valley; PC, Precordillera: PD, Preandean Depression: WC, Western Cordillera: AP. Altiplano. AF and WF refer to Atacama Fault and West Fissure, respectively. MT stations mentioned in the text are also indicated. Two-dimensional modeling mentioned in this text was carried out for data on profiles A C. Profile C is named alter the oasis of Pica in the Longitudinal Valley.

F. Echternacht et aL / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

high electrical conductivities below the Western Cordillera (Schwarz et al., 1994). Fig. 2 shows the results of 2-D modeling of the early electromagnetic data along EW profiles in northern Chile according to Kriiger (1994) and Massow (1994). The most striking feature of both models is a high conductivity zone (HCZ), with resistivities as low as 0 . 5 - 2 l ) m below the Western Cordillera, commencing at a depth of approx. 20 km. It must be pointed out that the depth extent of this HCZ is not clearly resolved due to lack of appropriate signal strength at long periods (recalling an overall conductance of more than 20 000 S!). In the model covering the whole forearc (Fig. 2(b)) anisotropic electrical signatures were modeled by well conducting vertical dykes, hinting at the influence of the Atacama fault system and extending, unexpectedly, below large

b~

0

71

parts of the Longitudinal Valley. All the models were achieved by not only fitting apparent resistivities and phases, but also the magnetic transfer functions (induction vectors), parameters which are less affected by static shift effects. A large conductive anomaly below the volcanic arc in southern Peru, where it lies much closer to the coast and extends further below the Eastern Cordillera in Bolivia, was already detected by Schmucker et al. (1966) during early investigations in the adjacent areas to the north. The authors described it as the "Andean anomaly" or "Andean conductor". Partial melting of deep crustal rocks may explain the high conductivities below the volcanic arc, although a certain influence of fluids may not be excluded. Laboratory experiments and model calculations (Bucher and Frey, 1994) require a tempera-

Coastal Ranae

100 km

Western Cordillera

0 20 40 60 80 km

a)

Preandean Depression

W. Cordillera

20 40

60 8O

10( km Fig. 2. (a) Two-dimensional model of magnetotelluric and geomagnetic data (redrawn from Kfiiger, 1994) on profile A (Fig. 1) from the coast of the Pacific Ocean to the Western Cordillera in northern Chile at approx. 22°S latitude. Resistivity values in f~m. (b) Two-dimensional model of magnetotelluric and geomagnetic data (redrawn from Massow, 1994) on profile B (Fig. l) across the Pre- and Western Cordillera at approx. 23°S latitude. Resistivity values in 12m.

72

F. Echternacht et al. / Physics qf' flw Earth and Phuwtarv lnterioJw 102 (1997) 6 t) $7

ture of at least 650°C for partially molten, watersaturated silicic rocks, while this value is much higher for basaltic melts (ll00°C, depending on water content). To account for resistivities in the range of 1 f~m, a melt fraction of about 20 vol.% is necessary. A more thorough discussion about details concerning the cause of high conductivities below the magmatic arc will be presented in a subsequent paper (Schilling et al., 1997). It is quite interesting that the subducting slab is not seen as a good conductor in the electromagnetic data, as for instance modeled at the North American plate boundary (Kurtz et al., 1986). This might be due to the very low, essentially almost vanishing, erosion rates at the continental margin (the Atacama desert has been one of the driest environments in the world). Thus missing sediments on top of the subducted slab, which would account for a conductive layer, may explain this unexpected behavior. On the other hand, the average resistivity of the upper and middle crust appears relatively low (200 r i m in both models) compared with 'normal' values of more than 1000 l~m. This may hint at a 'wet' crust in large parts of the forearc, although this needs further investigation. It will be shown later, that the crustal resistivities of the forearc are much higher, and thus more normal, in the north. A possible explanation may be found in the extensional regime of the Salar de Atacama Basin in the Preandean Depression between Pre- and Western Cordillera (see Fig. 1), enabling the rise of fluids.

real-time AMT system (METRONIX). It was used at all sites to cover the short period range from 0.001 s to 100 s. Time series for very long periods were recorded with systems based on a fluxgate ring-core magnetometer (MAGSON) and a data-logger storing data in a static RAM (University of G&tingen). The sampling rate of these systems was 0.5 Hz allowing to analyze data in a period range from 10 s to more than 10 000 s, depending on the total recording time. These instruments, herein called LMT. were deployed at every second site on the profiles. At the remaining sites instruments based on PDAS dataloggers (GEOTECH TELEDYNE) in combination with induction coil magnetometers (METRONIX) were deployed, recording at a sampling rate of 20 Hz. This type of equipment is called KMT in this paper. The time series recorded with the KMT yield MT transfer functions in a period range from 0.2 s to 1000 s. The data are stored on an external hard-disk with a capacity of 4 0 0 - 5 0 0 MByte. The aim to employ these additional systems was also to improve the data quality at periods around 1 s where the amplitudes of the natural EM fields are usually low. Time synchronization of the instruments was achieved by GPS receivers. To avoid a possible drift of sensors and instruments due to the very large temperature variations, especially in the high Cordillera, all the equipment was buried deep into the ground. A g / A g C 1 electrodes were used as telluric field sensors. Bentonite served to reduce the contact resistance and to avert long period potential drifts.

2. Instrumentation 3. Processing and data presentation During two field campaigns in 1993 and 1995 data were collected along three profiles. A northsouth profile extends from 20°S to 22°S in the Precordillera, two east-west striking profiles run from the Bolivian border in the Western Cordillera to the Pacific coast at about 21.5 ° and 21°S. The site spacing varies from 20 km on the north-south profile to 7 km on the east-west profiles. Altogether 40 MT soundings were carried out. Three different types of recording systems were used to cover a broad period range and therefore provide a resolution of structures from very shallow to very large depths. The first type of instrument is a

For the processing of the time series recorded with the LMT and KMT systems a program package from Egbert (Egbert and Booker, 1986, Egbert, 1989) was used. The main steps of the processing procedure are as follows: First, the time series were visualized and bad data segments, i.e. very noisy data segments where instruments did not work properly or where one or more channels ran out of the amplifier range, were cut out. Subsequently, spikes were filtered out using a median filter. The remaining continuous time series, which can be up to several days long, are Fourier-transformed using a

F. Echternacht et al, / Physics of the Earth and Planetary. Interiors 102 (1997) 69-87

cascade-decimation like approach with 128 samples long time segments and a decimation factor of 4. Prior to fast-Fourier-transformation the data segments were prewhitened and windowed using a time-bandwidth 1 prolate data taper. The transfer functions are estimated by band- and section-averaging using a regression M-estimator as described by Egbert and Booker (1986). This method is compared with other approaches in Jones et al. (1989). The resulting transfer functions are the 2 X 2 MT impedance tensor Z and the magnetic transfer functions zH and zo, all complex and functions of period. The impedance tensor elements are presented as apparent resistivity and phase curves. The magnetic transfer functions are plotted as induction vectors (IV) defined as

~14)~x + Re(~'o) ~).

(1)

lm(IV) = I m ( z n ) e~ + I m ( z o ) e~.

(2)

Re(IV) = Re(

73

Combining the transfer functions from the different instruments, AMT and KMT or LMT, respectively, yields information over a wide period range as displayed in Fig. 3. Here apparent resistivity and phase are shown for an AMT-LMT combination. The data of the two systems overlap between 10 s and 100 s. Because of the high accuracy of time synchronization using GPS, the remote reference method could be applied even to high frequency data. This method uses the magnetic field components of a remote site to calculate the transfer functions at the local site. It may be useful in the case of noisy magnetic field data. How poor data quality can be improved by this method is displayed in Fig. 4. The left hand part of this graph shows the apparent resistivity and phase curves of the KMT site COP estimated by a single site processing. Clearly visible is the drop-off of the transfer functions in the period range from 1 to 10 s due to low natural signal level and therefore low signal-to-noise ratio. Using the magnetic fields from the simultaneously recorded site ALC (at a distance

where ~ and ~. are unit vectors in north and east direction, respectively.

104~

_ E

1 0 3.

C

._> .= ® n.,

10 ~'101

JJiJjJJJJjj

,

~1~ .!

Q. 13.

<

10 0

10-1 9 ¢"

el

Period I s ] Fig, 3. Apparent resistivity and phase as function of period for the site ARC. located on the Areas fan in the Longitudinal Valley. Combined transfer functions of AMT and LMT soundings.

74

F. Echternacht et al. / Physics t7/ the Earth atul Pla#letarv Interiors 102 (1997) 69-,~77

103

10 3

E

E

102

.~_

...... <3

101

t

....

U}

A

~

A~!~JA

._.

A A

< i

90 i

i

'

101

©

d. Q. 100

!

=Co,./

;'~

i

::Zyx

i

i

I ©

O

10-1

102

i

:!

o OOoO

~72t3-"

-Oco

~O

100 10-1

i

,-~90 ]

o° ° I

.-.

'

60 /

I ' o~%OOo% ~

'

i

o!

L

oo

~ ;:iu ] co-,-~ -

" 10 0 /"1

10 °

, . 101

10 2 Period [sec]

10 3

/104

%-1 10o 101 102 103 lO' Period

[sec]

Fig. 4. Improvement of estimated apparent resistivity and phase by use of the remote reference technique, displayed for site COP. Left: single site robust processing; right: robust processing with station ALC (placed at a distance of 35 km) as a remote reference (cf. Fig. 1).

in more detail. An example is given in Fig. 5, showing real and imaginary parts of the induction vectors as a function of period for the site ARC. This site is located on the alluvial Areas fan at the west-

of 35 km) as a reference a strong improvement of the transfer functions can be achieved as can be seen in the right hand part of Fig. 4. The observed induction vectors shall be discussed

1.0 "6

i

i

i

i

i

i

i

i

0 .5

o.o ~: -o.s

.......

'

7t /tl ll

,

. . . . . . . . . . . . . . . . . . . . . . . . . . .

\

-1.0

1.0 O

I

I

I

i

I

I

I

I

0.5

> 0.0 ~

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

lil/lliAii-cb...iiiiltlll/l!

\\\\

-0.5-

"1"?0-4

10-3

10-2

10-1

100 101 Period Is]

102

103

104

105

Fig. 5. Real and imaginary induction vectors as function of period for site ARC (Areas fan, Longitudinal Valley). The length and direction of the vectors hint at a large conductivity anomaly in the north.

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

ern border of the Precordillera within the Longitudinal Valley (see Fig. 1). At short periods (AMT range) the induction vectors almost vanish completely indicating a one-dimensional resistivity structure which reflects the layering of sediments of the

71 *W

70°W

69"W

75

fan. From periods longer than about 1 s the magnetic transfer functions increase rapidly probably caused by a large vertical or subvertical resistivity contrast. The direction of the real induction vectors changes from southwest at intermediate periods to south at

68"W

67°W

66*W

20"S

20"S

21"S

21 *S

22*S

22°S

23 *S

23°S

24"S

24*S

25*S

25 *S

71 °W

70"W

69"W

68"W

67"W

66°W

Fig. 6. Real induction vectors (T = 1000 s) in the Central Andes drawn in plan view. Strong deviations from two-dimensional behavior may be observed on the northern transversal profile near the coast and in the Precordillera.

76

b: Echternaclll et al. / I'ln'sic~ o f the Earth and Plamqarv Interi(nw 102 (1997) 69 ,~'7

1000 to 10000 s. This directional change of the IVs with increasing period points to a changing strike direction of lateral resistivity contrasts with increasing depth. In Fig. 6 the induction vectors at 1000 s period are presented in a map for all sites of the 1993 and 1995 field experiments together with the data from the previous field campaigns. The expectations from the known conductivity structures (the N - S running coastline and the almost parallel striking 'Andean conductor') would imply a generally two-dimensional image with mostly E - W orientated induction vectors. But it is clearly visible, that more complex, three-dimensional features must be included. For example the induction vectors at the coastal sites (especially at site PER about 10 km from the coast) do not point eastward, as would be expected from the large conductivity contrast ocean/continent where the 8 km deep trench yields a conductance of 32 000 S. A second pattern which does not match the expected two-dimensionality is observed in the Western Cordillera in the northern part of the measuring area, where the induction vectors point southward, indicating a N - S segmentation of the Andean conductor a n d / o r the superposition of several anomalies. The above discussion shows that, even if the decomposition of the MT impedance tensors of the Pica profile suggests two-dimensional structures, as shown in the following section, the data of the investigated region cannot be explained by a single two-dimensional model of approximately northsouth striking structures. First 3-D model studies for the region further south (Miiller, 1995) have shown, that an E - W striking conductor near 22°S at 20 km depth together with a N N W - S S E trending conductor below the Western Cordillera and the N - S striking coastline can explain the observed rotation of the induction vectors from a southwest direction in the Precordillera over southward in the Longitudinal Valley to southeast in coastal region.

4. Decomposition of data In the following we will focus on the interpretation of the northernmost profile (C in Fig. I), named after the oasis of Pica in the Longitudinal Valley, of

120 km length and a direction of approximately N70°E. It extends from the Coastal Cordillera through the Longitudinal Valley up to the Western Cordillera. The setup of the total of 20 sites was mainly restricted by the generally limited accessibility. They were installed along a track running east from the Panamerican Highway towards the high Cordillera at the Chilean-Bolivian border and west towards the coast. At periods longer than 1 s the separation of apparent resistivity and phase curves of the two off-diagonal components indicates higher dimensionality of the underlying resistivity structure. At shorter periods the curves of both components are almost identical, indicating one-dimensional structures. Near-surface resistivities vary strongly along the profile with a general trend of values from around 10 ~ m in the Coastal Cordillera and the Longitudinal Valley to 50-500 ~Qm in the Pre- and Western Cordillera. Local structures, like salars (salt lakes), appear as good surficial conductors. Before starting to model the resistivity structures the impedance tensors were investigated to account for distortion. The conventional parameterization assumes only inductive effects. The skewness parameter S = (Z~,. + - z,.,) gives a measure for the deviation of the observed structures from two-dimensionality. A general trend of increasing skewness with increasing period is observed for all sites on the PICA profile. Assuming a two-dimensional resistivity structure, the minimization of the sum of the main diagonal elements of the impedance tensor gives the strike direction of the structure or, due to the 90 ° ambiguity of the arctan-function, the perpendicular direction (Swift, 1967). Plotting this angle lbr all sites and all periods one can observe a variation between + 20 ° and - 20 ° at periods longer than 10 s. This would imply that it is not possible to find a common coordinate fi'ame in which the data of all sites could be modeled two-dimensionally. To try and estimate a common rotation angle we applied the Groom-Bailey (GB) decomposition scheme (Groom and Bailey, 1989, 1991) to all sites of the profile. This method tries to fit the observed data, i.e. the eight real numbers of the complex 2 × 2-impedance tensor at each period, to seven parameters of a model consisting of a three-dimensional local structure superimposed over a regional two-dimensional structure. The seven parameters re-

Z,..,)/(Z,,

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

90

i i!i:i i ~iili

!iii~ iiiiii! ~ i!iii¸

90

!~i!

~...... i~i~i~'~',i ~iii~i!ii ~ ~!~!iii~iiiiiiiiiii~ ii~i~ii~i !~!iiiiii ~ i~ ~, 60~ O i .......... i ¸¸!! ! i! i li i~ ~ i~ ii:i ~ ~

o

~0

~ 30i il

~£

~!

;i

i

~ii~

77

60~

.....

00~ !

ili!i

i

:

i

~o

0-

~ -30 m.60.

-60 ~

c)

.90

-90

d)!

'

i! !

,,

~ [ ~ ~ ~

Or~

....

;;

D

' ~

'

'

30=.-. .O~

~

,

"'~0-3 10-2 10-1 100 101 102 103 10410 .3 10-2 10-1 100 101 102 103 Period [see] Period [sec]

104

Fig. 7. (a) Swift's rotation angle, (b) conventional skewness, (c) twist and shear from an unconstrained GB analysis and (d) regional strike direction from an unconstrained GB analysis for the site PER located in the Coastal Cordillera.

distorting effect is frequency-independent one expects to get a frequency-independent regional strike angle from the decomposition. And, as the regional structure is assumed to be two-dimensional, the re-

solved are the regional strike, the two regional i m p e d a n c e s (real and imaginary part) and twist and shear, two parameters describing the phase mixing effects of the distorting structure. A s s u m i n g that the

90

90 ¸ E 60

60~

~ 3o

30

~e-

0

........~ ~ -3O • ! : ....

-6o

:!~! ~ ~i ~ ~ii i i: !il i!ii!~

::ai:

O ' oi -

~ ~

-30 ~

~

:

-60 ~"

o)iiii ,

,90

-9~

t =: 10-3

10-2

,==! i 10-1

: 10 0

! 101

Period [sec]

p,

10 2

;=;, 10 3 10-3

i

~!i~!

10-2

¸''

! i:

i ¸¸

10-1

10 o

~!~

101

•

!

t30

10 2

i

10 3

Period [sec]

Fig. 8. The parameters twist, shear and regional strike of a GB analysis with (a, b) the regional strike fixed to - 9 °, (c, d) the regional strike fixed to - 9 ° and shear fixed to - 15° (site PER).

F. Echterma'ht et al. / Phy,~i('~ ~/ flw Earttz and Plam,/ar\ In/erior.~ 102 (/9971 h9 A'7

78

10 4

10 4

E 103 .~.

CE103

~ ~,~.

>

10 2

>

A

102 n-

~101

101 d. <

<

10 °

,

10 °

,

90,1

'

,0

~6ot~

~

10 m

10-1 100

10 -2

101

~

~ 103 10-3 10-2 10-1 100

102

Period [s]

101

7-6o~"

103

102

Period [s]

Fig. 9. Left: apparent resistivity and phase curves of the regional impedances estimated by a GB analysis with fixed values for all three parameters (reg. strike: - 9 ° : shear: 15°: twist: 30°). Right: apparent resistivity and phase curves after rotating the impedance tensor into a - 9 ° coordinate fi'ame (site PER).

gional strike angle for the different sites should be equal as well. As examples we show the results of the distortion 90 60

-= O) P

i

i

'

i

i

i

. . . . . . . .

.

a n a l y s i s f o r t w o sites, P E R , l o c a t e d in t h e C o a s t a l C o r d i l l e r a , a n d S I L A , t h e e a s t e r n m o s t site o f t h e p r o f i l e in t h e W e s t e r n C o r d i l l e r a ( s e e Fig. 1). A t site

90

1

~

60.~.

30

30

~o

~

;~1-3o _~

-30 -60

-.l-60e

c)

al

-90

1.oj

,

~0:4 02 0"0

,

'

: .

~,

' '

o ~°°

[

ld)~'

:~ '

:'

I

. . . o~ . ~~

J

'

'

I--~

0~•

~~-- ~ ~ 1

: ;[-30 "'

.

"10-3 10-2 10-, 100 10, 102 103 10,16-3 'i'0-2"i'0-i '100 10, 102 1'03 i;4 Period [sec]

Period [sec]

Fig. 10. (a) Swift's rotation angle, (b) conventional skewness, (c) twist and shear fi-onl an unconstrained GB analysis and (d) regional strike direction from an unconstrained GB analysis for the site SILA located in the Western Cordillera.

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

90

coincident with the already mentioned anomalous behavior of the induction vectors. The conventional strike angle estimates for these periods are about - 1 5 °, while for shorter periods they vary between + 15 ° and + 4 5 ° as shown in Fig. 7(a). An unconstrained GB analysis yields a quite stable regional strike direction of - 9 ° for periods 0.1 s < T < 10000 s (Fig. 7(d)). Twist and shear of the unconstrained analysis are shown in Fig. 7(c). In the following steps the decomposition parameters, which ought to be frequency independent according to the assumed model, are fixed successively to finally yield the two regional impedances. First the regional strike was fixed to - 9 ° yielding a more stable estimate of the shear ( - 1 5 °) while the twist is still varying with period. This is displayed in Fig. 8(a) and (b). Fixing the shear to - 15 ° results in a constant twist value of - 3 0 ° only at the longest periods ( T > 50 s) as shown in Fig. 8(c) and (d). Finally, all three parameters (regional strike, shear and twist) were fixed to estimate the regional impedances. The resulting apparent resistivity and phase curves are shown in Fig. 9 together with the curves obtained after simply rotating all impedances into the - 9 ° frame. The following figures show the different steps of

60.~

i i+++!+~ ~+i~ i :~+ i~+~ix~ii!+!i !~!~~30 ~ i iiii~ii

i~ilii i ili!Y ~ii!ii i ii!iiiii~ i i~:

0"o -30 "~

'~~i ili~i:i~ix ii:ii??i?i :!~iiii!ii:i~ ill ;!+,~!+

-60 ~ .90

=m

0~, 10-3 10-2

10° 101 Period [sec]

102

1 0 "1

-30 103

Fig. l 1. Twist and shear from a GB analysis with a regional strike fixed to - 9 ° (site SILA).

PER the conventional skewness (Fig. 7(b)) indicates a deviation from two-dimensionality with values of around 0.3 for periods longer than 50 s, which is

104.

104

E 3

.

.

.

.

.

.

.

.

.

•-~ 10 2. Q)

d.

79

.

103

.

~

, E

>

:i

102~

1 ..............................................................

o n-

101.

101 d. Q.

<

<

100. ,..,9°1

100

. . . . .

==

I

. . . . . ....

e"°'l, ++++++++++++++++ +++ + ++ ~ ~ + + + ?o-+ 11o.2 1o-1 +o0 1 0 1 1 0 + P e r i o d [s]

190 0 ,,,,,,,,,~ "

....I, ++++++++ "-~++++'~"+~ '_ ...._ '+ __..++++++++ ,_ lOllO 10- 10- 1o lO lO

"+"'1o

P e r i o d [s]

Fig. 12. Left: apparent resistivity and phase curves of the regional impedances estimated by a GB analysis with fixed values for all three parameters (reg. strike: - 9 ° ; shear: - 3 0 ° ; twist: -15°). Right: apparent resistivity and phase curves after rotating the impedance tensor into a - 9 ° coordinate frame (site SILA).

F. Echternacht et al.

80

)

0 '

v

O

b 0

OoOo O

e°8°

30

/ Physics q f the Earth and Phmelarv Interiors 102 ~1997)

o

o o°~

0

O0

0

oo

o

o

o

i

0 oo

i

87

i

O

008 0 Oo O0 O0 O0

0 008 ~8°°oo o o

~e °

i

69

o o

.m

0

00 0

o@°0°Oo o

O" 0 O~

0

0

0

0

~ 0

0

O

~ 0"0

~J 0 ~0 O00 0 0 0 ,Q 0 0 0

~% o

o 0

n-

J~tin@0 -

Oo

~0

O

0

O o0

-30

0

o

O0 0

O

0

O

9

0

ooo 80

9

10 "2

0o

o

o oo

o

~o o

oo

[

O o 0o

0

%0

O0

~

0 0

0

000

°8c

Oo o

O

10 "1

~o~

o

00( 0

0

O

o 0

o

o8

O0

o o 0

10 "3

0

0

-~^~:~ ~ o

0

oq~,

10 ° 101 Period [s]

0

n©

102

103

104

Fig. 13. Regional strike direction as a function of period for all sites of the PICA profile estimated from an unconstrainedGB analysis.

the GB analysis for the site SILA. A comparison of the strike direction at periods greater than 5 s derived from the Swift (1967) criterion ( = 5 °, Fig. 10(a)) and from an unconstrained GB analysis ( = - 5 °, Fig. 10(d)) shows a slight discrepancy. The conventional skewness displayed in Fig. 10(b) is very small at short periods and increases to long periods but not to such high values as at the site PER. Taking the estimates of the regional strike direction from adjacent sites into consideration we fixed the regional strike to - 9 ° in the second step of the analysis. This results in very stable values for twist and shear at periods longer than 10 s shown in Fig. 1 1. In a final step we fixed all three parameters (regional strike: - 9 ° ; twist: - 1 5 ° ; shear: - 3 0 ° ) . The resulting apparent resistivity and phase curves of the regional impedances are shown in Fig. 12 together with the curves obtained after a conventional rotation into the - 9 ° coordinate frame. The mixing of Z~, and Z,~ in the apparent resistivity curves is observable in the conventional analysis, especially at high frequencies. The phase curves of the GB inversion show more structure than the conventionally rotated ones.

In Fig. 13 the strike angles from an unconstrained GB decomposition for all sites are displayed as a function of period. At short periods, i.e. T < 0.1 s, the regional strike direction varies from site to site. At these periods the data are either one-dimensional or the assumed model of three-dimensional distortion over regional two-dimensional structure is not valid; there might be local two- or even three-dimensional inductive effects. At longer periods, T > 10 s. a mean value of - 9 ° is visible for most sites. This direction is in good agreement with the strike of the magmatic arc represented by the volcanic chain of the Western Cordillera.

5. Interpretation by 2-D modeling After rotating the impedance tensors of all periods and all sites into this coordinate frame two-dimensional forward modeling was carried out to explain the data. A finite element code (Wannamaker et al., 1987) was used. The topography of the Andean mountain chain and the Chilean-Peruvian trench

Fig. 14. Two-dimensionalresistivity model for the PICA profile. Resistivities in O,m. (a) Upper cross-section shows the uppermost structures and the bathymetry of the trench. The ocean is modeled with a uniform resistivityof 0.25 ~Qm. Good near-surface conductors reflect mostly salars, e.g. the Salar de Pintados in the LongitudinalValley. (b) Final (best fitting) model of deep structures. Broken line: schematic run of Wadati-Benioffzone (WBZ) (after lsacks, 1988). Note the vertical exaggerationof 1:6. (c) Alternativemodel which yields a similar fit of B-polarizationdata, but a less satisfactory result for E-polarization.Further discussion:see text.

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

81

a)

-5

km -I0 .72 °

-7] °

I

.69 °

-7o °

J

-68 o

West Fissure

W

.67 o

IE

b)

70°00 '

69°00 '

68°00 '

E

W

e) 1 O0

200

300

400 km

70°00 '

68°00 '

69°00 ' 0

50

lOOkm

82

F. Echternacht et al. / Physics r~t'the Earth and Planetary htteriors 102 (1997) 6 9 - 8 7

bathymetry (Isacks, 1988) were incorporated into the models of profile C (see Fig. 1), leading to large grids of 300 × 100 elements. The initial models were derived from the structures appearing in the pseudosections of apparent resistivities and phases (see Fig. 15). In the first steps simplified block structures of these sections were calculated. In subsequent steps more and more detailed structures were investigated. The significance of the most prominent features is discussed below. Fig. 14 shows the final model split into a shallow and a deep section. In Fig. 15 pseudosections of apparent resistivity and phase of both polarizations are presented to compare the model responses with the data. Special emphasis was put on fitting the B-polarization data, since they are usually less sensitive to three-dimensional effects (cf. Wannamaker et al., 1984, 1989). The data pseudosections (upper plots in Fig. 15(a)-(d)) exhibit several distinct conductive features along the profile: The central part of the Coastal Cordillera (sites PER and BLA) is characterized by low resistivities only at the shortest periods (up to 1 s) reflecting a thin sedimentary cover and a possible deep-reaching weathering of basement layers. This is less pronounced at site FOR. • Sediments in the Longitudinal Valley appear to be thicker indicated by low resistivities up to 100 s period. The Precordillera is dominated by very low apparent resistivities over the whole period range. • Approaching the Western Cordillera (sites SIL and SILA) the higher values of apparent resistivities at long periods of the B-polarization, which is the more sensitive one to lateral contrasts, indicate a change to high resistivities to the east. • The region of extremely high apparent resistivity below the Coastal Cordillera and the Longitudinal Valley visible in the B-polarization section result from the strong lateral contrast between the conductive ocean and the Precordillerean conductor

on the one hand and the resistive basement of the Andean crust between both on the other. The model presented in Fig. 14 reproduces all these features quite well, with the B-polarization data in particular show a good fit. The E-polarization data may be influenced by more distant structures which are out of the scope of the profile and are more sensitive to three-dimensionality which cannot be excluded for this region. The most prominent structure in this model is the dyke-like vertical conductor below the Precordillera. Compared with the models derived by Kfiiger (1994) and Massow (1994) for profiles further south (see Fig. 2) this HCZ is clearly restricted to the Precordillera. In the models of Fig. 2 the conductor reaches much further to the east under the Western Cordillera. A number of alternative models were tested for profile C. For this profile, the hypothesis that the HCZ reaches below the actual volcanic arc can be rejected. Especially the B-polarization data at all the eastern stations require a poor conductor in the east. However, a less extensive conductive zone below the eastern margin of the Western Cordillera (50 km to the east, not shown in Fig. 14) would not contradict the data. A prolongation of the profile into Bolivia would be desirable to further clarify this point. The HCZ consists of two clearly separated structures, one with resistivities of 5 and 2 l~m from shallow depths to about 35 km, the other commencing at a depth of approximately 100 km with a resistivity as low as 0.5 ~ m . However, the resistivity of the separating intermediate layer may not be too high, i.e. not higher than 500 1~ m. The 0.5 l~l m zone must extend into the upper mantle; in Fig. 14 it is bounded at a depth of approximately 180 km with an additional connection to the underlying good conductor at z = 300 km, which may be interpreted as an image of the electrical asthenosphere. The lower part of Fig. 14 shows an alternative to the final model. The region of high conductivity commences at a depth of already 60 km and does not

Fig. 15. Comparison of observed and modeled apparent resistivities and phases, drawn as pseudosectionsfor both polarizations for the PICA profile (top: data; bottom: model response). The data were rotated by an angle of - 9 ° according to the result of the GB decomposition. Apparent resistivities and phases for B-polarization (a, b) and E-polarization (c, d).

F. Echternacht ez al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

,,a+,p..++,,,+

............ ......................................................

~

. . . .

~'im-M to ti 4o 10o

iOOo IQW

I W

;: :+. AHOYX

0

20

..................

:

40

60

80

B-P~ Model

.....................

KM

E

....................

)hm4A lO

16

lOO ltlo

lm

W

0

~ ii

(b)

~ +.

~) iiiiiiiiiiiii

~

KM

E

................ i

PHS~

o

6 t2 1!

24 JA t~ 16 ?I

N

B-Pol ilodel PHSYX

~rm

:::~

:~S_ =~_-:.~+~ST/~:;~: ¸ • ; ::~;~:;~:;~::::~++:--+~7;~:~ +M

~

~::~:::::: :

83

84

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 1¢92 (19971 69 87

( c ):---~-

+-

. . . . . . . .

-~-+

"~--

-

+

/-++

............

P

+i i

i+

+! 4

w

o

r:+~

....... _:+

.....

~o

4o

+o

80 KM

i

i

:t

+

Mod~ + : : : ~ E-Pal ...... ...........................

E : :::::+ A

I

I i +

(d)

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87

reach as deep (120 kin). The connection to the mantle is broader with a resistivity of 200 t i m . This model yields an almost similar fit to the data as far as the B-polarization is concerned, whereas the Epolarization shows small differences for longer periods. Keeping in mind that E-polarization is more sensitive to regional lateral 3-D contrasts, this model may be considered as equivalent. Another problem concerns the large unstructured block of high resistivity below the ocean and parts of the forearc ( p = 10000 rim). It is assumed that the conductivity contrast ocean-continent screens any deeper structures at least within the achieved data quality (the electric field parallel to the coast is orders of magnitudes smaller than the perpendicular one). The upper bound of the underlying conductor (asthenosphere?) seems to be very deep (300 km), but is covered by many stations. The pseudosections show that the model response does not fit the data sufficiently in the area below the Coastal Cordillera. 3-D effects (also anisotropy?) may exist at long periods and impede a clear resolution (observe the N - S pointing real induction vectors near the coast, see Fig. 6). A conductive body as an electrical image of the down-going slab below the Coastal Cordillera cannot be excluded. Ocean bottom measurements are probably necessary to clarify these two points. The upper anomaly of the split HCZ below the Precordillera may be explained by the influence of the deep reaching West Fissure (WF) with an increased hydraulic routing for deep fluids. However, the lateral extent of the fault zone is not well determined. Additionally the high conductivities reflect deeper structures below the Salar de Huasco, situated immediately to the west, which is regarded as a pull-apart-basin (G. Wtirner, personal communication, 1997). Schilling et al. (1997) carried out laboratory experiments and model calculations and compared them with MT results for the region further south. They considered partial melting in the deep crust as the most plausible cause for the enhanced conductivities and deduced melt fractions of 14-28%. A similar explanation could be chosen here for the deep anomaly in Fig. 14. The implications of the achieved model concerning the deep anomaly are far-reaching: the volcanic

85

arc itself in this area is obviously not connected with a deep HCZ in contrast to all previous models in other regions of the central volcanic zone of the Andes. Geological and geochemical investigations may hint at a possible explanation: the volcanism in this part of the Andes is older than in the adjacent areas to the north and south. The only recent ( < 3 Ma) volcanic center in this 'Pica gap' is Cerro Porquesa approximately 20 km to the north of the profile (Wtirner et al., 1994). It may be speculated, that the magma below the Cordillera has already cooled out below the solidus and thus displays much higher resistivities than would be encountered for phases of partial melts. This coincides with a recent steepening of the subducting slab, i.e. a westward movement of the overriding South American plate (P.G. Giese, personal communication, 1997). It would follow, that the conductor below the Precordillera is a first sign of evolving magmatism. In this sense the second model in Fig. 14 may be more attractive: it is more restricted to the crust (although the deeper parts reach the upper mantle) than the 'final' model (b). As mentioned before, the induction vectors at several sites are contradictory to a 2-D approach. In the eastern section of the profile and at periods of 1000 s they point southwards instead of westwards, as would be assumed if the Andean conductor existed below this stretch of the Western Cordillera, and thus indicate a conductive structure to the north (Fig. 6). Consequently, several additional stations were set up to the north of the Pica profile. Surprisingly, the induction vectors at these sites still point southwards with the same order of magnitude. A structure with increasing conductance towards the north could explain this behavior, a hypothesis which has to be proved by three-dimensional modeling.

6. Conclusions and implications for further work The presented interpretation of magnetotelluric data conducted along a profile across the forearc in northern Chile enables us to clarify the resistivity structure of this region. A decomposition of the impedance tensors yields a strike direction of the resistivity structures which is coincident with the strike of the Andean mountain chain.

86

F. Echternacht et al. / Physics of the Earth and Planeta O, Interiors 102 (1997) 69 87

The prominent features of the final two-dimensional model are: 1. The Precordillera is dominated by a highly conductive structure which extends from great depth almost to the surface. 2. It can be ruled out that this HCZ reaches further to the east below the recent volcanic a r c - - i n contrast with models derived for regions further south. This coincides with the so called Pica gap which is characterized by a lower recent volcanic activity. 3. A conductive zone below the Coastal Cordillera, and thus an image of the down going slab, may exist but is not clearly resolved. More detailed three-dimensional models have to be developed to satisfy the whole dataset. An extension of the profiles further to the east is planned to complete the data across this conductive anomaly.

Acknowledgements The authors want to express their gratitude to the colleagues from the Universidad Catolica del Norte (Antofagasta), the Universidad de Chile (Santiago) and CODELCO Chile, for supporting the field campaigns. We also thank P. Giese and P. Denny for carefully reading the manuscript. Two anonymous reviewers helped to improve the paper. This study was carried out within the framework of the Special Research Project "Deformation Processes in the Andes", funded by the Deutsche Forschungsgemeinschaft.

References Barazangi, M., Isacks, B.L., 1976. Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America. Geology 4, 686-692. Bucher, K., Frey, M., 1994. Petrogenesis of Metamorphic Rocks, 6th ed. Springer, Berlin. Chinn, D.S., Isacks, B.L., Barazangi, M., 1980. High frequency seismic wave propagation in western South America along the continental margin, in the Nazca plate and across the Altipiano. Geophys. J. R. Astron. Soc. 60, 209-244. Comte, D., Suarez, G., 1995. Stress distribution and geometry of the subducting Nazca plate in northern Chile using teleseismically recorded earthquakes. Geophys. J. Int. 122 (2), 419-440, Egbert, G.D., 1989. Multivariate analysis of geomagnetic array

data II, random source models. J. Geophys. Res. 94 (BI(tl, 14249-14265. Egbert, G.D., Booker, J.R., 1986. Robust estimation of geomagnetic transfer functions. Geophys. J. R. Astron. Soc. 87, 173-194. Giese, P., 1994. Geothermal structure of the central Andean crust implications for heat transport and rheology. In: Reutter, K.-J.~ Scheuber, E,, Wigger, P. (Eds.), Tectonics of the Southern Central Andes. Springer, Berlin. G~3tze. H.J., Lahmeyer, B., Schmidt, S., Strunk, S., 1994. The lithospheric structure of the central Andes (200-26 ° S) its inferred from quantitative interpretation of regional gravity. In: Reutter, K.-J., Scheuber, E., Wigger, P. (Eds.), Tectonics of the Southern Central Andes. Springer, Berlin. Groom, R.W., Bailey, R.C., 1989. Decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion. J. Geophys. Res. 94 (B2), 1913-

1925.

Groom, R.W., Bailey, R.C., 1991. Analytic investigations of the effects of near-surface three-dimensional galvanic scatterers on MT tensor decompositions. Geophysics 56 (4), 496-518. Hamza. V.M., Mufioz, M., 1996. Heat flow map of South America. Geothermics 25 (6). Henry, S.G., Pollack, H.N., 1988. Terrestrial heat flow above the Andean subduction zone in Bolivia and Peru. J. Geophys. Res. 93 (B12), 15 153-15 162. lsacks, B.L., 1988. Uplift of the central Andean plateau and bending of the Bolivian orocline. J. Geophys. Res. 93 (B4), 3211-3231. James, D.E., 1971. Andean crustal and upper mantle structure. J. Geophys. Res. 76 (B 14), 3246-3271. Jones, A.G., Chave, A.D., Egbert, G., Auld, D., Bahr, K., 1989. A comparison of techniques for magnetotelluric response function estimation. J. Geophys. Res. 94 (BI0), 14201-14213. Kriiger, D.. 1994. Modellierungen zur Struktur elektrisch leitf~ihiger Zonen in den stidlichen zentralen Anden, Berliner geowiss. Abh. (B), 21, Selbstverlag Fachbereich Geowiss., Free Univ. Berlin. Kurtz, R.D., Delaurier, J.M., Gupta, J.C., 1986. A magnetotelluric sounding across Vancouver Island detects the subducting Juan de Fuca plate. Nature 321, 596-599. Massow, W., 1994. Magnetotellurik in der Westkordillere Nordchiles. Unpublished Diploma Thesis, Fachrichtung Geophysik, Free Univ. Berlin. Miiller, M., 1995. 3D-Modellierung von Leitf~ihigkeitsanomalien in Nordchile. Unpublished Diploma Thesis, Institute of Geophysics, Univ. Geophysics, Univ. Berlin. Ocala, L.C., Meyer, R.P., 1972. Crustal low-velocity zones under the Peru-Bolivia Altiplano. Geophys. J. R. Astron. Soc. 30, 199- 209. Reutter, K.-J., Giese, P., G6tze, H.J., Scheuber, E., Schwab, K., Schwarz, G., Wigger, P., 1988. Structures and crustal development of the central Andes between 21 o and 25 ° S. In: Bahlburg, H., Breitkreuz, C., Giese, P. (Eds), The Southern Central Andes, Lecture Notes on Earth Sciences 17. Springer, Berlin, pp. 231-261. Scheuber, E., 1994. Tektonische Entwicklung des nordchilenis-

F. Echternacht et al. / Physics of the Earth and Planetary Interiors 102 (1997) 69-87 chen aktiven Kontinentalrandes: Der Einflul~ von Plattenkonvergenz und Rheologie. In: Weber, K. (Ed.), Geotektonische Forschungen, 81. E. Schweizerbart'sche Buchhandlung, Stuttgart. Schilling, F.R., Partzsch, G.M., Brasse, H., Schwarz, G., 1997. Partial melting below the magnetic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data. Phys. Earth Planet. Inter. in press. Schmucker, U., Hartmann, O., Giesecke, A.A., Casaverde, M., Forbush, S.E., 1966. Electrical conductivity anomaly in the earth's crust in Peru. Carnegie Inst. Washington Yearb. 65, 11-28. Schwarz, G., Chong-Diaz, G., Kriiger, D., Martinez, M., Massow, W., Rath, V., Viramonte, J., 1994. Crustal high conductivity zones in the southern Central Andes In: Reutter, K.-J., Scheuber, E., Wigger, P. (Eds.), Tectonics of the Southern Central Andes. Springer, Berlin. Swift, C.M., 1967. A magnetotelluric investigation of an electrical conductivity anomaly in the south-western United States. Technical Report M.I.T. Wannamaker, P.E., Hohmann, G.W., San Filipo, W.A., 1984. Electromagnetic modelling of three-dimensional bodies in layered earths using integral equations. Geophysics 49 (1), 60-74.

87

Wannamaker, P.E., Stodt, J.A., Rijo, L., 1987. A stable finite element solution for two-dimensional modeling. Geophys. J. R. Astron. Soc. 88, 277-296. Wannamaker, P.E., Booker, J.R., Jones, A.G., Chave, A.D., Filloux, J.H., Waff, H.S., Law, L.K., 1989. Resistivity cross section through the Juan de Fuca subduction system and its tectonic implications. Geophys. J. R. Astron. Soc. 94, 1412714144. Wigger, P., Schmitz, M., Araneda, M., Asch, G., Baldzuhn, S., Giese, P., Heinsohn, W.-D., Martinez, E., Ricaldi, E., R~wer, P., Viramonte, J., 1994. Variation in the crustal structure of the Southern Central Andes deduced from seismic refraction investigations. In: Reutter, K.-J., Scheuber, E., Wigger, P. (Eds.), Tectonics of the Southern Central Andes. Springer, Berlin. WiSrner, G., Moorbath, S., Horn, S., Entenmann, J., Harmon, R.S., Davidson, J.P,, Lopezes-Escobar, L., 1994. Large- and finescale geochemical variations along the Andean arc of northern Chile (17.5°-22 ° S). In: Reutter, K.-J., Scheuber, E., Wigger, P. (Eds.), Tectonics of the Southern Central Andes. Springer, Berlin.