Analysis of deformation and tectonic history of the Southern Altiplano Plateau (Bolivia) and their importance for plateau formation

Abstract

(TYPE=abstract)The Altiplano represents a key region of the Central Andes, where the interplay between faults and syn-tectonic sediments allow the reconstruction of the kinematic evolution of the Central Andean high plateau. This study aims, by the use of incrementally-balanced crosssections, interpretation of reflection-seismic profiles, 3D strain analysis, gravity data interpretation, isotopic-age dating, and surface observations, to reconstruct the geological and tectonic history of the Southern Altiplano at 21o S between the Eastern and the Western Cordillera. The Southern Altiplano is a complex intramontane basin with 6-8km Cenozoic fill. It can be structurally divided in three domains; the Eastern, Central, and Western Altiplano. 2D balanced cross-sections based on seismic-reflector analysis and field observations show that the Eastern Altiplano is the buried, thin-skinned deformation front of the western part of the Eastern Cordillera's bivergent thrust system. The 20-40° dipping, blind faults merge into a shallow, eastward-dipping detachment at 7-9km depth that continues into the Eastern Cordillera. The Central Altiplano forms a bivergent system with 30-90° dipping, basement-involving thrusts in the east, and fault-propagation folds in the west. The shallow, westward-dipping detachment lies at 9-10km depth and possibly continues into the Western Altiplano, which forms a separate bivergent thrust-system.The computer-aided (GeoSec and 2DMove), incremental restoration of the balanced crosssections of the Eastern and Central Altiplano, and preliminary line-length balancing of the Western Altiplano, yields 38km shortening due to folding and thrusting. 3D strain analysis of sandstone grain shapes reveals that an additional 7.7% of shortening was accumulated as ductile, micro-scale strain. This increases the total shortening in the entire cross-section of the Southern Altiplano at 21° S to 60km or 21%. In addition, I suggest that the contribution of outcrop-scale structures possibly accounts for another 20 km. 3D strain analysis further shows that the 7.7% of microscale strain were accompanied by 13% orogen-parallel extension. These shortening estimates more than double the published shortening values from the Altiplano. Crustal thickening and plateau uplift in the arc-backarc domain of the South American convergent margin took place during the Cenozoic. K-Ar and Ar-Ar age-dating on syn-tectonic sediments, together with seismic-sequence analysis, demonstrates that the Southern Altiplano structure formed during two independent compressional increments (Early Oligocene [>27 Ma] and Middle/Late Miocene [17-8 Ma]), which were preceded by an Eocene/Oligocene extensional event that led to the formation of a half graben in the Central, and possibly a second in the Eastern Altiplano. Horizontal contraction of the Altiplano ended between 11-8 Ma, was indicated by the age of undeformed volcanic rocks. Detailed seismic analysis of single syn-tectonic basins combined with isotopic ages of syntectonic sediments, reveal a complex deformation history characterised by spatially and temporally irregular fault activation, which excludes the existence of large-scale eastward or westward propagating deformation during plateau formation. This diffuse pattern of deformation was characteristic for the entire plateau domain, i.e. from the western flank to the eastern edge of the Eastern Cordillera, during a first stage of plateau formation between 30 and 10 Ma. This possibly indicates that the plateau has remained at since its formation and did not evolve from an initially doubly-vergent orogen. The syn-tectonic stratigraphic units of the Southern Altiplano domain overlie shallow marine, Late Cretaceous sediments that still form a sub-horizontal regional near sea level. This indicates that plateau surface-uplift in this part of the plateau was mainly achieved by sedimentary inll of tectonically-controlled, internally-drained basins, and not by tectonic uplift. The tectonic evolution of the Southern Altiplano was largely accompanied by magmatic activity. An episode of strong volcanic activity affected the entire width of the Altiplano and adjacent parts of the Eastern Cordillera between 25-8 Ma. However, a causal relationship between magmatism and deformation could not be shown for the Southern Altiplano. Strong Oligocene/Miocene volcanic activity, together with the diffuse pattern of deformation, suggests that the formation of the Altiplano Plateau was initiated by magmatically-controlled thermal weakening of the crust, possibly as the result of the removal of the mantle lithosphere. At present, the Altiplano has a flat topography, high heat-flow, and is spatially related to a variety of geophysical anomalies that are interpreted as partial melting of the middle crust (20-40km depth). From this evidence, I propose that the process of plateau formation is still active.