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Barbara Moths Sheffels
Department of Earth, Atmospheric,
and Planetary Sciences
Massachusetts Institute
of Technology
Cambridge, Massachusetts,
U.S.A.
Abstract
Structural data
from the region of the bend in the Bolivian Andes at 17-19° S lat are
integrated with paleogeographic data, paleomagnetic data, and data on the
along-strike variation in crustal shortening. These data show that the
bend in the mountain belt is due to a combination of (1) primary paleogeographic
control on the eastern margin and within the mountain belt, (2) an inherited
change in strike of the western margin, and (3) a variation in the amount
of crustal shortening along strike, which leads to a synorogenic rotation
of the western margin. The bend is not an orocline, but is rotational.
The Cochabamba region is interpreted as an active, thin-skinned pull-apart
basin, developed along tear faults that accommodate differential movement
between thrust faults. Plunging folds, other tear faults, and arcuate fault
traces suggest footwall morphology due to lateral variations in sedimentary
thickness. These structures are interpreted as resulting from paleogeographic
elements related to the structure of the Paleozoic basin. Previous models
of the formation of the bend are summarized and considered in light of
this synthesis. An implication is that the variation in shortening along
strike may be accounted for by the paleogeography, that is, by the width
of the basin in which décollement units were deposited, and may
not require variations in slab dip or plate convergence. |
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