NEOGENE TO QUATERNARY EXTENSIONAL REACTIVATION OF A FOLD AND THRUST BELT IN THE SOUTHERN CENTRAL ANDES DETERMINED FROM GRAVITY MODELS CONSTRAINED BY SEISMIC, BORE-HOLE AND FIELD DATA (38°- 39ºS)

ROJAS VERA, EMILIO; FOLGUERA, ANDRES; GIMENEZ, MARIO E; ZAMORA VALVARCE, GONZALO; RAMOS, VICTOR

Viena

Congreso; EGU General Assembly 2010; 2010

EGU

The Andean orogenic front between 38 and 39ºS is formed by extensional structures delimiting the Loncopué trough. These structures are superimposed to Late Cretaceous-Late Miocene  compressive 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. This residual reflects short wave-lengths gravimetric components, mostly corresponding to upper crustal density contrasts, many of them associated with basement structure. Description of Bouguer anomaly and Residual maps allows inferring general aspects of basin geometry at depth. An inversion model was calculated from the gravimetric residuals. For this purpose the upper crust was divided into two layers with different mean densities. This inversion model shows, over the western boundary of the Loncopué trough, four depocenters whose sedimentary infill was calculated in the order of 8-10 km. Those are aligned in a N direction following the main western boundary trough. Thicknesses of 8-10 kilometers for depocenters identified beneath the Loncopué trough imply that locally at least 4-5 km of the Neuquén basin is present at depth. Moreover, depocenters seem to be systematically aligned with faults recognized at surface affecting less than 5 Ma rocks. In order to highlightbasement discontinuities and therefore main sedimentary depocenters, we have also used the Tilt Gradient which enhances maximum horizontal gradients in low wavelength anomalies. 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. At the Loncopué trough we used the inversion gravity model as a proxy for basement structure refined by structural cross sections developed at surface. Densities were taken from averages corresponding to values obtained at borehole data in the western Agrio fold and thrust belt. 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 at the western Agrio fold and thrust belt implies that a common basement discontinuity is recurrentlybeing reactivated through time, probably in coincidence with the eastern main Loncopué fault boundary.