Abstract: Kinematics and Deformation Modes in the Subandean Ranges of Southern Bolivia

Colletta, B.; J. Letouzey - IFP; J. Soares - Braspetro; and M. Specht - Total

The Subandean fold and thrust belt of southern Bolivia is characterized by a thin-skinned style with a complete detachment of the sedimentary cover over a gently west-dipping basement. The main detachment is in the Silurian shales at the top of the Cambro-Ordovician. Compressional deformation started in Oligocene times at about 27 Ma in the inner part of the cordillera. At that time the Subandean zone was the foreland basin of the growing cordillera with continental clastic deposits. Deformation in the Subandean zone occurred later on, during Miocene times at about 10 Ma, and is still active at Present. Restored cross-sections based on fieldwork and the interpretation of new seismic lines across Aguarague and San Alberto-San Antonio anticlines allow us to propose kinematic reconstruction of the front of the Subandean ranges.

The overall continuity and regularity of the N-S trending structures suggests a continuous almost planar deep detachment in that part of the Subandean zone. The San Antonio - San Alberto anticline corresponds with a lift-off fold detached over the basal Silurian shales and characterized by steeply dipping flanks (dips generally higher than 60°). In addition to the major Silurian basal detachment, high contrasts in mechanical behavior between thick ductile shales of the Los Monos Fm. and stiffer sandstones of the Huamampampa Fm (Devonian) and Escarpment Fm.(Carboniferous) control the tectonic style. Ductile shales of the Los Monos Fm. induce the decoupling of the deep (Santa Rosa + Huamampampa Devonian Fm.) and shallow (Carboniferous + post Carboniferous Fm.) structures and constitute an intermediate detachment horizon. Structural style of both San Antonio - San Alberto and Aguarague anticlines is controlled by "fish tail structures," passive roof duplexes and imbricate horses in the Huamampampa / Santa Rosa Devonian sandstones. Thrust faults occurring in the axes of the anticlines are interpreted as late emerging crestal thrusts or backthrusts soling out in the Los Monos.

The simple fault bend fold model cannot be applied for the whole evolution of the structures. The alignment between the uppermost faults and the deeper ones in the restored sections (Fig. 1) suggests the San Alberto and Aguarague folds could have been similar to the present Madrejones-Ipaguazu structure during Mio-Pliocene times (Fig. 1 and 2). We propose that both the San Alberto and the Aguarague anticlines were born as large wavelength fault propagation anticlines (comparable to the Madrejones anticline) and were subsequently tightened when shortening increased. In such a case, location of the main fold axis (anticlines and syncline) remained fixed with respect to the rock material. Axial planes are not active kinks through which strata could be folded and then unfolded. Kinematic reconstruction suggests that the maximum of deformation and secondary fractures due to bending will be concentrated at the apex of the anticline.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil