H.-J. Götze, and the MIGRA Group (M. Alvers, G. Goltz, A. Kirchner, A. Müller, U. Schäfer, S. Schmidt, M. Araneda, H. Ugalde, G. Chong D., L. Barrio, N. Lopez, and R. Omarini)
H.-J. Götze, Institut für Geologie, Geophysik und Geoinformatik, FU Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany, and the MIGRA Group (M. Alvers, G. Goltz, A. Kirchner, A. Müller, U. Schäfer, S. Schmidt, M. Araneda, H. Ugalde, G. Chong D., L. Barrio, N. Lopez, and R. Omarini)
Between 1993 and 1995 a group of scientists from Chile, Argentina, and Germany incorporated some 2000 new gravity observations into a database that covers a remote region of the Central Andes in northern Chile and northwestern Argentina (between 64° - 71° W and 20° -29° S). The database can be used to study the structure and evolution of the Andes. About 14,000 gravity values are included in the database, including older, reprocessed data. Researchers at universities or governmental agencies are welcome to use the data for noncommercial purposes.
The group that compiled the data is called MIGRA, a Spanish acronym for International Gravity Survey/Research in the Central Andes. For all gravity data the group calculated gravity reductions within a radius of 200 km. Therefore, anomalies calculated are "complete Bouguer anomalies". The database includes point data, and 10 km x 10 km data grids of several types of anomalies, including free-air, different types of Bouguer, and isostatic-residual. It was exchanged with both the Defense Mapping Agency and the National Oceanic and Atmospheric Administration, who will check the data and use them to compile gravity maps of South America.
Reprocessed data were compiled by the Freie Universität Berlin and the Institut für Angewandte Geodäsie, Aussenstelle Potsdam (Germany); Universidad de Chile en Santiago, Universidad Cat˘lica del Norte en Antofagasta (Chile); and the Universidades Nacionales de La Plata, Tucuman, y Salta (Argentina), and the oil and mining industry. For additional information, contact H.-J. Götze by e-mail at hajo@geophysik.fu-berlin.de or refer to the Gravity Research Group's WWW home page at http://userpage.fu-berlin.de/~wwwgravi/.
The previous network of gravity data in the Central Andes [e.g., Dragicevic, 1970] was coarsly meshed and is only suitable for regional gravity field interpretations. In 1982, the departments of Earth Sciences at Freie and Technische Universität Berlin launched the "Mobility of Active Continental Margins" research unit to provide more insight into lithospheric structures and the evolution of the Central Andes in a segment between 20° to 26° S with an interdisciplinary approach that made use of a variety of geophysical, geological, and petrological studies [Reutter et al., 1994]. Both gravity field research and reprocessing of older data were done under this project, and then from 1982 to 1986, more than 2500 new gravity observations were recorded in the traverse. Including some 2100 reprocessed observations, the data base contained approximately 4600 point gravity data [e.g., Götze et al., 1990].
Figure 1. Topography and the geophysical data base of the "SFB 267" in the Central Andes. Topography grid is from Isacks [1988]. Volcanoes represent the magmatic arc. IQU = Iquique; TOC = Tocopilla; CAL = Calama; ANT = Antofagasta; TAL = Taltal; UYU = Uyuni; TAR = Tarija; ORA = Orán; SOC = Socompa; JUJ = Jujuy; SAL = Salta; TUC = Tucumán; CAT = Catamarca.
The original research unit was the "pathfinder" of a second scientific project, the "Collaborative Research Center 267, (SFB 267)" called "Deformation Processes in the Central Andes" which was established at the Freie and Technische Universität Berlin and GeoForschungsZentrum Potsdam. Under the umbrella of this long-term project, 17 task groups are using both laboratory and field campaigns to gain a better understanding of the crustal processes acting at this convergent plate margin. Figure 1 shows the location of geophysical investigations (refraction seismics, gravity and magnetotellurics). Two task groups work to acquire and process data, and to interpret potential fields, including the geoid, from an interdisciplinary perspective. In particular, the gravity group focuses on constructing a modern gravity data base tied to international standard networks by reprocessing existing data and complementing them with new field surveys and on interdisciplinary interpretation of gravity.
MIGRA is a child of both the former research unit and the more recent SFB 267. As in the case of other foreign research programs, successful work in this inaccessible region is possible only through the assistance of South American colleagues. Toward this end, MIGRA benefits from the combined efforts of experts from both the Europe and South America.
The region investigated covers a 900 km x 1000 km area in the central part of the Andean orogenic system. It is an arid/semiarid zone, where elevations vary from sea level to heights greater than 6.000 m at the Andean volcano summits (Figure 1). The young Andean orogen between 20°-29° S is composed of different structures that evolved on a Precambrian-Paleozoic basement. This belt of ancient rocks is also described as the border of the "Faja Eruptiva Occi- dental." Two of the gravity surveys obtained structural information with new stations covering the northern and southern edges of this Precambriam-Paleozoic rock belt, near Calama (Chile) and in the Southern Argentinean Puna respectively. The area investigated has a sprawling topography, and it is remote and arid. Its population density is low and there is little infrastruc- ture. Field work was also limited by the lack of topographic maps and geodetic networks in some regions.
Figure 2. Residual isostatic gravity map of the Central Andes. A Vening-Meinesz model (density contrast 0.35 g/cm3, normal crustal thickness: 35 km, and flexural rigidity of 1023 Nm) was eliminated from the Bouguer gravity field. IQU = Iquique; TOC = Tocopilla; CAL = Calama; ANT = Antofagasta; TAL = Taltal; UYU = Uyuni; TAR = Tarija; ORA = Orán; SOC = Socompa; JUJ = Jujuy; SAL = Salta; TUC = Tucumán; CAT = Catamarca.
Gravity stations are spaced approximately every 5 km along all passable tracks; in some places, stations are spaced more closely (Figure 1 and 2). The database includes gravity observations from different sources (Table 1), and with the exception of some inaccessible regions in the "eastern" and "western cordillera," gravity coverage is fairly uniform. All measurements are tied to the 1971 International Gravity Standardization Net (IGSN71) of the International Union of Geodesy and Geophysics gravity datum at base stations in Oran, Argentina; Iquique, Chile; and Tucumán, Argentina. The size of the area and logistical problems sometimes prevented determination of the drift of the gravity meters, which is done by repeating the measurements at each station. However, even when bad tracks were used, the drift of the instruments rarely exceeded 0.1 mGal per day. The highest gravity reading was made (observation by A. Kirchner, A. Müller, and U. Shäfer) at an elevation of 6.200 m south of Paso Socompa (see SOC in Figure1). The gravity stations with the lowest readings are located near the shoreline of the Pacific Ocean in Chile.
Table 1. Data sources of the "Central Andes" gravity database.
| Number | Source | State |
| 344 | BGI (Bureau Gravimétrique International, Toulouse) | France |
| 462 | IGM (Instituto Geográfico Militar, La Paz) | Bolivia |
| 311 | YPF (Buenos Aires, National Oil Company) | Argentina |
| 873 | YPFB (Santa Cruz, National Oil Company) | Bolivia |
| 2120 | CODELCO (Santiago); National Copper Mining Company; high station density | Chile |
| 3427 | ENAP (Santiago); National Oil Company; high station density | Chile |
| 53 | Geodetic Institute, University of Buenos Aires | Argentina |
In contrast to research performed in the 1980s, a satisfying determination of geographic coordinates of sites (in x and y direction) was made using GPS hand receivers. However, the z-component (height) could not be determined to better than about 100 m (in absolute GPS mode) due to short stays of only 10-15 minutes at gravity stations. Only about 35% of the gravity sites could be tied directly to benchmarks, such as levelling lines or trigonometric heights. Altimeters were used to determine height. To improve the quality of the barometric measurements, time-dependent drift corrections were calculated with the method usually used for gravity measure- ments, using as many benchmarks and repeated measurements as possible. Moreover, several days worth of profiles were complied to eliminate systematic errors. The scales of the barometers were calibrated on levelling lines with an altitude difference of about 2000 m. Error estimations showed that even in the worst case, the accuracy was better than 20 m, giving an error in the Bouguer anomaly of about 4 mGal, which is less than 1% of the overall magnitude of more than 450 mGal.
For processing purposes, S. Schmidt wrote a Fortran-based computer program that stores reference number, coordinates, and heights for each new gravity station. It calculates tidal corrections, a three-dimensional topographic correction on the base of the 3' x 3' digital elevation model by Isacks [1988], the drift corrections of both gravity and barometric readings, and the final "complete" Bouguer- and free-air anomalies and even isostatic residual fields. A linked plot program contoured the new anomalies as soon as new data were processed. This allows us to check the new field data against the old database and recognize "bugs" immediately.
The basis of the calculation of gravity anomaly values are the following equations:
CBA = gabs -with:h +
gtop +
FA = gabs -
h:
gtop:
gbou:
For the terrain correction (up to 167 km around all stations), a method developed for gravity investigations in the Alps that uses calculations of the Earth s curvature was used, after it was adapted for the special situation in the Central Andes. Reduction density was 2.67 g/cm3. For different morphological units, the following typical values of topographic reduction were obtained: longitudinal valley and Chaco region: 0.5-1 mGal; coastal-, pre-, and western cordillera, Altiplano/Puna, and subandean belt: 1-10 mGal; and steep coast and eastern cordillera: 10-25 mGal.
Onshore the Bouguer anomaly drops down to a regional minimum of about - 450 mGal in the
area of the recent volcanic arc, because of crustal thickening by isostatic compensation. The
effect of isostatic compensation of topography was calculated assuming the model of Vening-Meinesz with the following parameters: density contrast of the Earth s mantle and crust 
= 0.35 g/cm3, normal crustal thickness: 35 km and a flexural rigidity of 1023 Nm. The gravity effect
of the isostatic compensation root was eliminated from the Bouguer gravity and the resulting
anomaly serves as a residual field (Figure 2).
This field has several interesting features. First, there are positive values in the area of the forearc with isolated complexes parallel to the coastline. They are regionally caused by the presence of the dense subducting plate (gravity effect of about 50 mGal; density contrast: 0.05 g/cm3) and locally by uplifted jurassic batholiths intruded into the "Formación La Negra." The second interesting feature is the north-northwest/south-southeast-striking positive anomaly from Calama (CAL) by the Salar de Atacama to southern Puna. This gravity maximum can be explained by the presence of a highly metamorphic and high-density Paleozoic/Precambrian structure, the "Faja Eruptiva Occidental", which is oblique to the recent north-south-oriented volcanic belt. Third, there is a local minima along the recent volcanic arc point to reservoirs of partly molten material at depths of 15-20 km. Fourth, minima following a line from Ollagüe (OLL) to Calama (CAL) along 69 W, are caused by the Eocene volcanic arc with low-density volcanic material in the upper crust. Finally, alternating gravity highs and lows in the backarc region east of 67° W are observed in wide areas of the Argentine Puna and the Eastern Cordillera with a general northeast/southwest trend. The minima point to the position of Mesozoic basins, which are located in the Argentine Puna and extend northward to the territory of Bolivia. Gravity highs correlate with outcrops of Precambrian/Paleozoic basement in the Puna and eastern cordillera.
The updated gravity data will play an important role in both local investigations of applied geophysics and regional interdisciplinary interpretations of pure geophysics. The Andean gravity field is a sensitive indicator that is linked to many processes that contribute to the tectonic framework of the Nazca subduction zone. In the summer of 1995 MIGRA participated in the "CINCA" offshore experiment of the German research vessel "Sonne;" it was performed between 20°S to 24° S. The offshore gravity data will be linked with the on land survey to draw a complete gravity/geoid picture of this ocean-continent transition zone. In close cooperation with the Geological Survey of Chile (Santiago), MIGRA expects to complete the survey in the southwestern and southern part of the traverse (Figure 1) in early 1996 and fill the gaps that still exist because of logistical problems. In 1997 MIGRA plans to join participants from North and South Americas and Germany for an international seismic reflection program in the Altiplano of Bolivia.
Acknowledgments: We thank P. R. Kress (Buenos Aires) and R. Riquilme (Santiago), who joined the field measurements in 1993. The support with maps and geodetic information by the following institutions is gratefully acknowledged: Institutos Geográficos Militares in Santiago, Buenos Aires and La Paz, Yacimientos Petrolˇferos Fiscales of Argentina and Bolivia, Empresa Nacional de Petróleo, and CODELCO (G. Behn R.), both from Chile. The research unit that compiled the database is financed by the Deutsche Forschungsgemeinschaft, both Universities of Berlin, and the GeoResearchCenter in Potsdam. This paper presents results of the gravity group D3 of the SFB 267 which is funded by the Deutsche Forschungsgemeinschaft (DFG) and Freie Universität Berlin.
Dragicevic, M., Carta gravimétrica de los Andes meridionales e interpretación de las anomalías
de gravedad de Chile central, Publ. 93, Dep. de Geofis. y Geod., Univ. de Chile, Santiago, 1970.
Götze, H. -J., B. Lahmeyer, S. Schmidt, S. Strunk, and M. Araneda, A new gravity data base in
the central Andes (20° - 26°S), EOS Trans., AGU, 71, 16, 401, 1990.
Isacks, B.L., Uplift of the central Andean plateau and bending of the Bolivian orocline, J. Geophys. Res., 93, 3211, 1988.
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