lownal of Snvth American Emth Scicncrs. Vol. 10. No. 2. pp. 17Y-MB, WY7 0 IY97 F’uhliskd by Elsevicr S&ace W Pergamon Interpretation PII: so895-9811(97)ooo14-x All righu reserved. Pkted in Great Britain 08Y5-YRlvY7 s17m+ Iwo of Gravity and Geoid in the Central Andes 20” and 29” S HANS-JiiRGEN G@IZE and ANDREAS KIRCHNER between Institut fiir Geologie, Geophysik und Geoinformatik, FU Berlin, Haus N, Malteserstrabe 74-100, D-12249 Berlin, Germany (Received 1 July 1996; accepted 17 January 1997) Abstruct - Most recently the MIGRAgroup with participants from Chile, Argentina and Germany has surveyed some 3.500 new gravity observations in an Andean &traverse covering N-Chile and NW-Argentina between 64”-71”W and 20”-29”s. MIGRA is an international gravity research group which was established under the umbrella of the “Collaborative Research Center’ “Deformation Processes in the Andes” at Freie Universitit Berlin (Germany). Oil and mining industry gravity data were reproc- essed and included into the MIGRA data base which contains more than 15.000 gravity data at present. Due to big logistical prob- lems (terrain access, no maps, no levelling lines etc.) the overall error of the calculated gravity anomalies results of approximately 4-5 mGal which is about 1% compared with the magnitude of about 450 mGal’ of the Bouguer anomaly. Under the framework of the interdisciplinary research project CINCA (Cmstai Investigations Off- and Onshore NazcaKentral Andes) gravity surveys of the MIGRA group has been extended to the Pacific ocean. In summer 1995 MIGRA took part in the “CINCA” offshore experiment SO- 104 of the German Research Vessel “Sonne” between the latitudes 20”s and 24”s. The data base which includes point data and 10 km * 10 km data grids of free-air-, different types of Bouguer- and isostatic-residual anomalies are presented here in maps of the isostatic residual field along with an interpretation by 3D density modelling. Con- straints were mainly taken from results of 2D my tracing refraction seismics and tectonics. Offset-free modelling is achieved by fitting both gravity field and geoid with one density stmcture of the lithosphere-system and by modelling relative to a reference density structure derived from the Preliminary Referende Earth Model (PREM, Dziewonski and Anderson, 1981). 0 1997 Pub- lished by Elsevier Science Ltd Resumen - Elhltimo grupo MIGRA con participantes de Chile, Argentina y Alemania ha me&do alrededor de 3.500 nuevas esta- ciones de gravedad dentro de una geotraversa que cubre el N de Chile y el NW de Argentina entre 10s64” y 7 1”Wy 10s20” y 29”s. MIGRA es un grupo intemacional de investigaciones de gravedad, el cual se ha establecido bajo la tutela del “Centro de colabora- ci6n para las investigaciones” “Procesos de deformaci6n en 10sAndes” de la Universidad Libre de Berlin (Alemania). Datos de gravedad procedentes de la industria petrolera y minera fueron reprocesados e inclufdos dentro de la base de datos de MIGRA, la cual contiene actualmente mL de 15.000 datos de gravedad. Debido a grandes problemas logfsticos (terrenos de diffcil acceso, falta de mapas o de curvas de nivel etc.) el error general en el c~culo de anomalias de gravedad resulta de aproximadamente 4-5 mGal el que se estima de 1% comparado con la magnitud de alrededor de 450 mGal de la anomalia de Bouguer. Dentro de1 esquema de1 proyecto de investigaciones interdisciplinarias CINCA (Investigaciones corticales en tierra y en mar de la placa de nazca en 10sAndes centrales) mediciones de gravedad del grupo MIGRA han sido extendidas al oc&anopacffico. En el verano de 1995 MIGRA tom6 parte en el experiment0 marino SO-104 en el barco de investigaciones “Sonne” entre las latitudes 20” y 24”s. La base de datos, la cual incluye una malla de 10 km x IOkm de datos de anomalias de aire libre, de Bouguer (diferentes tipos) e isostitica-residual, es presentada aqui en 10smapas de camp0 isostiico residual junto con una interpretaci6n de un modelo de den- sidad 3D. Las restricciones fueron principalmente hechas de 10sresultados de la refmcci6n sismica y tect6nica de un ray tracing 2D. Modelado sin compensaci6n es logrado por medio de una adaptaci6n entre el camp0 de gravedad y el geoide con una estruc- tura de densidad de1 sistema litosferico y por medio de modelado relativo a una estructura de densidad de referencia derivada de1 Preliminary Reference. Earth Model (PREM, Dziewonski and Anderson, 1981). INTRODUCTION The previous network of gravity data in the Central Andes (e.g. Dragicevic, 1970) was only coarse-meshed and suit- able for regional gravity field interpretations or does not cover the area under consideration (Introcaso et al., 1992). Shorter wavelengths indicating anomalous structures in the crust were not visible due to the wide spacing of gravity stations used in this map. In 1982, the departments of Earth Sciences at Freie and Technische Universitsit Berlin launched a Research Unit entitled “Mobility of Active Continental Margins”. It was the project’s intention to pro- vide more insight into lithospheric structures and the evo- lution of the Central Andes in a segment between 20” and 26” latitude south (Reutter et al., 1994; Gatze et zyxwvutsrqponml al., 1994). This Research Unit was the “pathfinder” of a second scientific project, the “Collaborative Research Center 267, (SFB 267)” called “Deformation Processes in the Andes” which was established at the Freie and Technische Univer- sitit Berlin, GeoForschungsZentrum Potsdam and the Uni- versittit Potdam. It is financed by the Deutsche Forschungsgemeinschaft, the Universities, and the Geo- ForschungsZentrum and concentrates interdisciplinary geoscientific research on the Central Andes crossing the fields of geology, geophysics, geodesy, remote sensing, Correspondence to H.-J. Giitze, Tel.: 0049 30 7792874; FAX: 0049 30 775 3078; Email: hajo@zedat.fu-berlin.de ‘mGal is used throughout this paper: 1 mGa1 = lo-’ m s-l 179