Abstract
Deformation processes in the early deformation stages of thrusting in growing fold-and-thrust belts and the
role of factors controlling and localizing the deformation are still not well understood. There are only few
natural examples or experimental studies to allow the identification of factors controlling initial states of
deformation. In particular, the spatial interactions in curved fold-and-thrust belts or in segments striking
obliquely to the convergence direction are rarely systematically examined.
The Boomerang Hills Region (BHR), located at the eastern flank of the Central Andes close to the axis of
the Bolivian Orocline, is an appropriate area to study such problems. There, oblique contraction is caused by a
south-dipping basement interacting with a WSW-ENE to SW-NE convergence direction. In order to infer the
controlling factors of deformation, the structural pattern associated with the active development of a new
thrust sheet and the structural history of the adjacent foreland was examined.
The detailed spatial-temporal relationships of Andean and pre-Andean structures within the BHR were
investigated using a dense network of 100 reflection-seismic profiles for subsurface interpretation and
structural modeling. In addition, satellite images and drainage pattern were interpreted to infer neotectonic
activity. Modeling techniques included cross section balancing for validation and quantification of geological
processes, horizon unfolding for kinematical interpretation, and critical taper estimates to derive the possible
dynamic status.
Results show that the main factor controlling the evolution of the Boomerang Hills Thrust Unit and the
eastern flank of the Bolivian Orocline are paleogeographic features. The crystalline basement of the BHR
comprises an inherited structure of several syn- and antiforms. Some of these structures were partly reactivated
as minor reverse faults with associated small folds in the sedimentary cover at an early stage of the Tertiary
foreland basin evolution. Basement faulting is probably a result of spatial problems linked to the initiation of a
foreland basin in a heterogeneous crust.
WNW-ESE striking normal faults were active in the foreland of the BHR in the Late Tertiary and show
maximum activity during the intermediate state of foreland basin sedimentation. Further to the north, closer to
the Brazilian Craton, normal faults are presently active. Foreland extension sums to 1.1%-1.5% and results
from minor flexural extension due to bending of the Brazilian Shield lithosphere through Andean orogenic
loading.
The southern part of the BHR is represented by a young, evolving thrust sheet. Two zones can be
differentiated along the Andean deformation front: (1) a W-E to NW-SE-striking frontal segment of
predominantly orthogonal shortening, comprising a thrust / anticline system and accommodating at least 1400-
2000 m horizontal shortening on the basal decollement ; (2) A WSW-ENE-striking lateral zone of oblique
shortening comprised by a complex system of thin-skinned strike-slip faults and minor folds.
The deformation front always follows a pronounced edge in the topography of the top basement surface
close to the boundary of the Paleozoic basin. Usually, faults are located close to asperities in the top basement
surface; fold axes trend parallel to the contours of basement depth.
The orientation of the deformation front as well as the laterally varying structural style is the result of
deformation localization and strain partitioning. Because of the impossibility to accommodate non-orthogonal
contraction by oblique faulting close to the surface, strain partitioning must occur along the deformation front.
Consequently, a N 35°E thrusting direction is divided into orthogonal and tangential components, both with
respect to the orientation of the deformation front and to depth contours of the basement. The two components
are accommodated by convergent and strike-slip structures, respectively, which join via a common detachment
horizon.
The Boomerang Hills Thrust Unit is affected by left-lateral shear as interpreted from the particle
displacement field of an unfolded horizon. This shear is invoked by the spatially distributed accommodation of
thrust movement along the obliquely striking deformation front.
The N 35° thrusting direction of the Boomerang Hills Thrust Unit represents the orthogonal component of
strain partitioning caused by the WNW-ESE orientation of the Subandean Zone with respect to a WSW-ENE
convergence between the Andes and the Brazilian shield. The tangential component is compensated at the
back of the Boomerang Hills Thrust Unit where a positive flower structure is present and strain partitioning is
made possible via the detachment horizon.
Critical taper analysis shows that thrusting on the south-dipping basement is possible without any
substantial internal deformation due to the wedge shape of the sedimentary prism. Therefore, the most
important factor controlling the localization of the deformation front is the loss of critical taper at a
pronounced edge along the Paleozoic basin boundary. Secondary reasons for deformation localization are
small asperities in/close to the top of the basement, e.g. small folds of the early foredeep evolution.
Strain partitioning may also play a major role for the evolution of the eastern border of the Bolivian
Orocline and in other curved fold-and-thrust belts as discussed for natural and experimental examples.
Generally, orthogonal accreting segments evolve faster than obliquely orientated segments due to strain
partitioning at the deformation front, hence leading to an increase in the curvature of the belt.
Strain partitioning itself is caused by the orientation of the deformation front with respect to the shortening
direction. Thus, factors controlling the localization of the deformation front, i.e. paleogeographic features,
dominantly control the evolution of curved fold-and-thrust belts.
For the eastern flank of the Bolivian Orocline it is postulated that an early Paleozoic rift basin exerts
paleogeographic control on Andean deformation, leading to the WNW-ESE orientation of the Subandean
Zone between the Chapare and Santa Cruz. Based on map interpretation it is suggested that (a) the previously
assumed style of deformation using main thrusts is questionable for this segment of the Andes and (b)
deformation involving basally accreting foredeep sediments has to be taken into account. Previously assumed
estimates of shortening for the Chapare area in the northern limb of the Bolivian Orocline may therefore be
significantly low.
