Preliminary Geophysic Results in the Calingasta Bolson, Province of San Juan, Argentina

MARTINEZ, M PATRICIA; GIMENEZ, MARIO E; INTROCASO, ANTONIO; RUÍZ, FANCISCO

Acapulco

Congreso; Eos Trans. AGU, 88/(23), Jt. Assem. Suppl.; 2007

AGU

The Calingasta bolson is situated between the Frontal Cordillera and the Andean Precordillera from 31º to 32º south latitude and 69º 20’ to 69º 45’ West longitude. Its average elevation is about 1600 m a.s.l. The seismic information indicates that the Eastern margin of this tectonic depression was affected by convergence during the latest Tertiary and the Quaternary. The depositional Mesozoic sequences in this bolson lie on a strong angular unconformity evidenced by a marked lithologic and structural change. These deposits Triassic in age occur in remnants of continental half-grabens formed by rifting that gave rise to NNW – SSE trending basins with rapid subsidence (Kokogian and Mancilla, 1989; Kokogian et al., 1999). These rift basins are coeval to the Triassic depocenters in the oil producing Cuyana basin (Legarreta et al., 1992).   Contemporary marine sedimentation is restricted to the Chilean territory, that is, to the west of the Frontal Cordillera, where narrow troughs were opened to the NW as marine embayments (Suárez and Bell, 1992). In the Calingasta bolson, the outcropping Triassic deposits were laid mainly in fluvial and lacustrine environments and are exposed along the western flank of the Precordillera, as observed at the latitude of the locality of Hilario, among other localities. The bolson can be considered as a ramp basin formed between the Precodillera and the Frontal Cordillera, limited by antithetic overthrusts  controlled by the inversion of the extensional faulting associated to the Triassic fill of the basin (Rossello et al., 1996). A new gravimetric survey carried out in the Calingasta bolson area led to the generation of a recent gravimetric chart, which was processed with different modern analytical and interpretation techniques such as Analytic Signal, Tilt and its Gradient and Euler Deconvolution. The Bouguer anomaly chart was filtered with the upward continuation at H= 40 km. The negative Bouguer isoanomalies in the resulting residual anomaly chart describe the geometry of the Calingasta bolson. Two outstanding depocenters with concentric isoanomalies of -20 mGal were determined, one at the latitude of the Calingasta village and the other situated to the SW of the locality of Barreal. The depths arising from the solutions of the localized Euler deconvolution, in the depocenters, reach 5000 meters. The distribution of the solutions of the Euler Deconvolution, as the responses of the Analytic Signal and Tilt Gradient, indicate the presence of zones with NNW-SSE and NNE-SSW trends which segment the bolson and separates both depocenters. This trends are interpreted as produced by antithetic (or conjugated) faulting, which is in accordance to seismic results.