2D Flexural modelling along an E-W profile at 21.4°S Central Andes: Evolution of the Chaco foreland basin

CLAUDIA PREZZI; CAROLA HULKA; CORNELIUS UBA; HANS-JÜRGEN GÖTZE; C. HEUBECK

Potsdam, Alemania

Simposio; 19th Colloquium on Latin American Geosciences; 2005

A 2D flexural modelling was carried out along an E-W profile at approximately 21.4°S Central Andes, trying to reproduce the Chaco basin evolution and the Cordillera Oriental and Subandean Ranges uplift history through time. The tAo software was used in the calculation of the flexural deflection of the lithosphere, applying the finite-difference technique. Varying values of elastic thickness along the profile were considered. The deformation by shortening was kinematically introduced in the model, each fault having its own geometry, position, shortening and timing of activity. The topography produced by thrusting and modified by regional isostatic response of the lithosphere was used at each time step to calculate the erosion and sedimentation. Constant sedimentation rate under water, additional diffusive erosion/sedimentation and continental background erosion were considered. Two different rheological behaviours for the lithospheric plate were applied: pure elastic and depth and temperature dependent elastic-plastic rheology.Several balanced cross sections constructed by various authors were taken into account to define the thrust loading process though time along the profile, deformation occurring since 40 Ma ago. Published values of the diffusive erosion constant, and of sedimentation rates were used. The model is constrained by new stratigraphic data obtained from a wealth of 2D industry seismic lines and well logs and collected in the field through the measurement of type sections of Tertiary-age rocks in the foothills of the eastern Suandean belt. An updated paleoelevation curve, the actual topography, Bouguer anomaly, available Moho depths and structural cross sections were also used as constraining data. We compared the model predicted and the measured thicknesses along the profile 14 Ma ago (Petaca Fm.), 6 Ma ago (Petaca, Yecua and Tariquia Fms.) and 0 Ma ago (Petaca, Yecua, Tariquia, Guandacay and Emborozú Fms.). The model showed that the Petaca Fm. corresponds to forebulge deposits. To fit its measured thicknesses along the profile we had to use a lower sedimentation rate than for the other Fms., and elastic thicknesses of approximately 60 - 70 km between 40 -14 Ma ago. This type of modelling would provide valuable new insight on the geodynamic evolution through time of the Central Andes and particularly on the variations of the elastic thickness and the geotherms of the lithosphere through time.