1.
Gravity survey
The new gravity survey covers an area of 5900 km
2
in the Altiplano-Puna Volcanic
Complex of the Central Volcanic Zones of the Andes, in the Sur Lípez region in
Southern Bolivia (Fig. 1). It is entirely within the thus far constrained extent of the
APMB [ Yuan et al.
, 2000; Zandt et al.
, 2003], and fills in a gap in the regional dataset
[
Götze et al.
, 1996]. Our surveys were carried out during March-April 2010 and
October 2011.
1.1. Survey setup
A SCINTREX CG5 Autograv spring gravimeter (serial # 572) has been used for all surveys. The linear instrument drift was ~1.3 mGal/day and the uncorrected drift was kept to a minimum through fortnightly drift recalibrations and by re-occupying reference stations a minimum of three times per day. The absolute scale of the gravimeter has been calibrated to 1.00032, on a well-established 80 mGal calibration line in Madrid, Spain [ Camacho et al.
, 2002].
The new survey comprises 160 new benchmarks, with average spacing of ~5 km (Fig.
1); 21 benchmarks were used as bases for repeat measurements. Benchmark locations were chosen taking into account survey areal coverage, minimal near-field terrain effect and good accessibility. CG5 6 Hz gravity data at each benchmark were averaged over two to five 60-second periods, with final measurement uncertainty ranging between 0 and 3 μGal. Benchmark elevation differences were controlled with a static survey of dual frequency GPS, using 3-second epochs for measurement times of approximately one minute per baseline kilometre distance, to achieve ~5 cm repeatability in elevation.
1.2. Data Reduction
Standard methodologies in gravimetric data reduction for the effect of tides, terrain, latitude, free-air and Bouguer effects were employed: a) The theoretical Earth model by Wahr, Dehant and Defraigne [1999] was used to de-tide the data, whereas ocean loading and diurnal pressure variation effects were studied through continuous gravity observation, performed in 2010 by an automated
Burris Basic gravimeter (serial # B-28) over a 7-day period at Quetena Chico village
(22.192ºS, 67.34ºW), near the centre of the survey network. The time series revealed a ±3 µGal and ±2 µGal uncertainty for ocean loading over semidiurnal frequencies and for pressure admittance over diurnal frequencies, respectively. b) De-tided data in the network has been adjusted using a least-squares global compensation of uncorrected drifts, tares and optimal value for repeated stations. The data is adjusted with a root mean square residual of 16 µGal. c) Topographic effects are corrected for using an automated algorithm based on dense circular zones (34 zones from 1 m to 60 km radius, 894 compartments in total), similar to Hammer zones [ Hammer , 1939], around the individual benchmarks. Three

arc-second (~90 m) SRTM data was used for up to 18 km away from survey benchmarks, and a ten-time-subsampled SRTM was used for the far field. Initial correction is done for an arbitrary density of 1000 kg m
-3
, which is then scaled up to the chosen terrain density. d) Latitude is corrected for using the standard gravity formula from 1980 [ Moritz ,
1980]. e) The free-air anomaly is corrected using the theoretical value of -308 µGal m
-1
, following confirmation from free-air gravity gradient measurements performed at six locations in the survey area. The linear correlation between measured free-air gradients and terrain correction shows that, in the absence of topography, the free-air gradient for this area is statistically indistinguishable from the theoretical value. f) In addition to terrain density, the Bouguer slab density is amongst the most critical parameters for gravimetric data reduction. During the calculation of these values the aperture of our network (hereafter GravUt) is increased by introducing 62 measurements from the AnGrav database (hereafter AnGrav) of the SFB267 project
(Fig.1). The extensive SFB267 dataset provides, amongst other parameters, geographical coordinates, orthometric elevations and raw gravity measurements referenced to IGSN71. AnGrav orthometric and GravUt ellipsoidal elevations were standardised by means of the BolGeo geoid [ Corchete et al.
, 2006]. Overlap of both surveys along the periphery of the GravUt data allows calculation of an offset value that provides an optimal overlapping between 12 “nearly mutual” stations from the two surveys. The fitting criterion is obtained by maximum correlation between
Bouguer Anomaly values, and each dataset is reduced independently beforehand, using the same parameters. Overall, the reduced GravUt data is placed within the reduced AnGrav reference with a <2 mGal uncertainty.
Initial data reduction and merging of GravUt and AnGrav data is done by exploring the density range 1800-2670 kg m
-3
. The mean values and standard deviations of the corresponding Bouguer anomalies increase linearly with increasing reduction density.
For the lower reduction densities the absolute minima is located on the abandoned mining town of Dulce Nombre on the southern limit of the Vilama Caldera. However, for the higher reduction densities the minima migrates south to the Dulce Nombre mine on the Piedras Grandes complex at 5000 m a.s.l. The Bouguer anomaly for 2670 kg m
-3
matches the anomaly data presented by Götze and Kirchner [1997]
.
A correction of the reduction density used is applied during the inversion. An additional terrain density unknown is added and solved for by minimising shallow, short wavelength anomalies, which are attributed to excess or defect of the terrain density value used. The result is similar to that of Nettleton’s Method where the Bouguer anomaly is not correlated to the topography, as given by the elevation of gravity benchmarks.
Results indicated that 2270 kg m
-3
is the optimum reduction density, and this value is used here for the topography and Bouguer slab.
Moreover, a close look at the local geology reveals that within the elevation range of the survey (3600–6000 m a.s.l.) material is mostly Tertiary and Quaternary volcanic and sedimentary rocks with a volumetrically inferior Ordovicium low-grade

metamorphic unit and minor Tertiary plutons [ Reutter et al.
, 1994]. Measurements on selected samples from local outcrops and published values of similar materials suggest a density value of ~2300 kg m
-3
.
Normalised cross-sections of the Bouguer anomalies using 1800 and 2670 kg m
-3 reduction densities are shown in Fig. 1c. By using a Bouguer slab density of 2270 instead of 2670 kg m
-3
the amplitude of the Bouguer anomaly lows is significantly decreased, most notably over higher elevations. This leaves us with smaller residuals for the inversion. However, we note here that regardless of the selected slab density all inversion results show the presence of low-density bodies discussed in detail in the main text (see results in supfig1). This consistency in the results demonstrates that the strong localised negative anomalies are not an artefact of the reduction density, but rather the effect of the presence of low-density material below ground.
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