HR: 1400h
AN: G43B-14    [Abstracts]
TI: Geodetic And Geological Analysis Of The Tandilia Crust
AU: * Del Cogliano, D
EM: daniel@fcaglp.unlp.edu.ar
AF: Facultad de Ciencias Astronómicas y Geofísicas (UNLP)-CONICET, Paseo del Bosque s/Nr, La Plata, BA 1900, Argentina
AU: DallaSalda, L
EM: dallasalda@cig.museo.unlp.edu.ar
AF: Centro de Investigaciones Geológicas (UNLP)-CONICET, calle 1 Nr 644, La Plata, BA 1900, Argentina
AB: Keywords: Tandilia-Geoid-Anomaly-Collision-Transamazonic The oldest Precambrian rocks of the south-western Gondwana in South America are cropping out in the Río de la Plata craton, it encompasses the western region of Uruguay, the Martín Garcia island and the Tandilia Ranges in the Buenos Aires Province, Argentina. The Tandil Ranges are the oldest region in Argentina (1.8- 2.2Ga); however, some features of the crust still remain unknown. These rocks evolved during two main events:Transamazonian and Brasilian tectonic cycles. The local and regional gravitational effects were analyzed on gravity and height anomalies. The studied are extended on 400 km x 400 km area which includes three geological units: the Tandilia ranges, and the Claromecó and Salado basins. Due the dependence of gravity and height anomalies with the distance, the seconds are more suitable to analyze the crust interior. For this reason a very precise cuasi geoid model was calculated using the point masses method, from gravity and GPS/leveling data. Taking into account the topography (less than 500 m high hills) and the Bouguer anomalies values (| AB | < 35 mGal), the differences between the cuasi geoid and the geoid will are less than 1 cm. In consequence the geoid undulations (N) were used. The EIGEN-CG01C geopotential model allowed to remove the wavelengths of more than 1000 km of the local geoid (N_obs=N-N_eigen). This made it possible to analyze the signals attributed to the structure of Tandilia, and the Claromecó and Salado basins influence on it. In Tandilia geophysical, geodetic and geologic analysis allowed to postulate an isostatic compensated Airy model (N_isost) with a 42 km thick crust (2.84 gr/cm³) resting on a 3.24 gr/cm³ density mantel. Residuals geoid distribution (N_obs-N_isost) shows a tendency to the eastern edge of the ranges. This anomaly is interpreted as the presence of an upper crust excess of mass, which, from Olavarría and Azul hills (north of Tandilia) it extends to the southwest running mainly along the eastern edge of the ranges until the continental platform. This anomaly is attributed to a basic-ultrabasic tectonic slab, a relict of a suture (oceanic bottom and astenosphere) from a continent-continent collision (Transamazonian orogeny). This collisional model was previously based on the presence of wide areas of gneisses, migmatites and granitoids (leucogranites), of sub volcanic and lava flows, of a polifase metamorphic-deformational style, of swarms of pre-metamorphic belts, and of a strong piling up of the crust, associated to thrusting and transcurrence; as well as minor lenses of ultrabasic rocks, that seem to also be part of the suture as a result of a "mantel pinching" during collision.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
SC: Geodesy [G]
MN: 2007 Joint Assembly