Antonio Introcaso

A crustal model based mainly on gravity data in the area between the Bermejo Basin and the Sierras de Valle Fértil, Argentina

Abstract The Sierras Pampeanas ranges of central-western Argentina (26°00′S to 33°15′S, 63°30′W to 68°30′W; an area about 450xa0km wide and 800xa0km long) consists of a series of uplifted basement blocks bounded by deep sedimentary basins of relatively low relief and covered by modern sediments. The basement blocks of the Sierras Pampeanas are bounded by longitudinal faults that originated during or were reactivated by the Andean Orogeny, then uplifted by reverse faults during the late Tertiary and the Quaternary. Utilizing primarily gravity data, interpreted with the aid of geological information and seismological and seismic data, we developed models consistent with: (1) seismic data on the Moho, and (2) a significant regional gradient of “g” from west to east. In order to be consistent with (1), we took into account the gravimetric effects of the subhorizontal Nazca plate, whereas to be consistent with (2), it was necessary to wedge the Bermejo Basin sediments under very dense positive masses within the upper crust underlying the Sierra de Valle Fertil. Two crustal models were developed by inversion of the Bouguer anomaly. These fully fit the Bouguer anomaly and the geological data and reveal two zones of high density that are interpreted as paleosutures.

Curie Point Depths Beneath Precordillera Cuyana and Sierras Pampeanas Obtained from Spectral Analysis of Magnetic Anomalies

Abstract A gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western Argentina: Cuyo Precordillera, the Sierras Pampeanas of San Juan and La Rioja, and Famatina System, the results of which are displayed in three maps: Bouguer anomaly, total field magnetic anomaly and total field reduced to the pole. The top and bottom boundaries of the magnetized crust were calculated from power-density spectra of the total-field anomalies from our terrestrial database in 90 2D windows. The depths obtained for the bottom of magnetized crust are assumed to correspond to Curie point depths. The values thus obtained for the Precordillera range between 29 and 40 km, whereas for the Sierras Pampeanas, in the Sierra de Pie de Palo, and other mountain chains along the Bermejo-Desaguadero lineament or Valle Fertil lineament, such depth ranges between 20 and 35 km. These results are consistent with Curie point depths determined on different continental regions in the world. A map of regional heat flow has been prepared in the present work based on the depth of the Curie point isotherm, which shows that heat flow patterns in Precordillera are different from those found in Sierras Pampeanas. A significant heat flow extending in Northeast-Southwest direction seems to be directly related to Juan Fernandez Ridge trace. The results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime.

Geophysical indicators of Neogene strike-slip faulting in the Desaguadero–Bermejo tectonic lineament (northwestern Argentina)

Abstract The depocenters of the Bermejo and Ischigualasto-Villa Union Cenozoic sedimentary basins lie on opposite sides of the Desaguadero–Bermejo fault and show a left-lateral offset of about 70xa0km. We postulate that this offset in fact reflects strike-slip displacement along the Desaguadero–Bermejo fault related to late Andean tectonics. This conclusion is supported by the residual Bouguer anomaly map and by the total magnetic map reduced to the pole, as well as by refraction and reflection seismic data and earthquake focal mechanisms. We put forward a gravity model that explains the structural history of these two basins. The model proposes that an originally single foreland basin was divided into two basins by strike slip along the Desaguadero–Bermejo fault. The eastern portion of this original basin moved northward, uplifted, and rotated clockwise.

Geophysical Study of the Valle Fértil Lineament Between 28°45′S and 31°30′S: Boundary Between the Cuyania and Pampia Terranes

Abstract A gravimetric and magnetometric study was carried out in the north-eastern portion of the Cuyania terrane and adjacent Pampia terrane. Gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes. Magnetometric models led to propose the existence of different susceptibilities on either side of the suture. The Curie temperature point depth, representing the lower boundary of the magnetised crust, was found to be located at 25 km, consistent with the lower limit of the brittle crust delineated by seismic data; this unusually thick portion of the crust is thought to release stress producing significant seismicity. Moho depths determined from seismic studies near western Sierras Pampeanas are significantly greater than those obtained from gravimetric crustal models. Considering mass and gravity changes originated by the flat-slab Nazca plate along Cuyania and western Pampia terranes, it is possible to reconcile Moho thickness obtained either by seismic or by gravity data. Thus, topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by: (a) the oceanic subducted Nazca plate with “normal slope” also including asthenospheric materials between both continental and oceanic lithospheres; (b) flat-slab subducted Nazca plate (as shown in this work) without significant asthenospheric materials between both lithospheres. These changes influence the relationship between topographic altitudes and crustal thickness in different ways, differing from the simple Airy system relationship and modifying the crustal scale shortening calculation. These changes are significantly enlarged in the study area. Future changes in Nazca Plate slope will produce changes in the isostatic balance.