Neogene to Quaternary Extensional Reactivation of a Fold and Thrust Belt in The Southern Central Andes Determined from Gravity Models (38- 39S)

ROJAS VERA E. A.; FOLGUERA, A.; GÍMENEZ, M.; ZAMORA VALCARCE, G.; BOTTESI, G.; MARTÍNEZ, P.; RUIZ, F.; RAMOS, V. A.

Foz do Iguazú

Congreso; The Meeting of the Americas; 2010

American Geofisical Union (AGU)

structures that formed the Agrio fold and thrust belt. Detailed structural mapping together with gravity and limited borehole and seismic data were used to construct structural cross sections across the area of Neogene extensional deformation and remnants of the previous compressive structure. The axial zone of the Loncopué trough is completely covered by less than 5 Ma volcanic and sedimentary rocks, reason by which gravity and seismic data have been of particular importance to map buried structure. In order to isolate the short wave-lengths anomalies (mostly corresponding to upper crustal gravimetric effects) and particularly those related to the structure of the Neuquén basin and Loncopué trough, we have discounted long wave-length anomalies using upward continuation analyses. An inversion model was calculated from the gravimetric residuals, from which four depocenters whose sedimentary infill was calculated in the order of 8-10 km were distinguished. Moreover, depocenters seem to be systematically aligned with faults recognized at surface affecting less than 5 Ma rocks. Moreover, a series of rhomboedric shaped structures appears aligned along the axial part of the Loncopué trough. Most of these structures are coincident with the ones mapped at surface affecting Quaternary strata. At the Agrio fold and thrust belt, we have used borehole data as well as seismic lines getting a rather accurate 2D geometry in our models. Densities were taken from averages corresponding to values obtained at borehole data. An acceptable match has been obtained between calculated anomalies for two transects across the Loncopué trough, and the observed residual gravity anomalies. The obtainned structural cross sections were restituted for the two stages when the Loncopué trough was developed, Late Oligocene-Early Miocene to Pliocene to Quaternary related to extensional structures. Finally, our model suggests that a series of deep and large depocenters, where Late Oligocene to Early Miocene sequences are buried, were located particularly at the Present trough boundaries; while smaller and probably shallower depocenters were concentrated at the axial-eastern trough following structures at surface with young morphological expression. The former are interpreted as representative of the initial extensional stage when Late Cretaceous compressional basement structures were relaxated, while the latter could be the product of incipient-young collapse of the Agrio fold and thrust belt. General coincidence between both areas of extensional relaxation