Summary: Transverse fold evolution in the External Sierra, southern Pyrenees, Spain
David J. Anastasio*, James E. Holl1
Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015-3188, USA
Received 25 January 2000; accepted 29 June 2000
Fault-slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern
margins of the Jaca basin, southern Pyrenees, Spain. The Spanish Pyrenean foreland consists of thrust sheets and leading-edge de┬collement
folds which developed within piggyback basins. Guara Formation limestones on the margins of the Jaca basin were deposited synchronously
with deformation and are exposed in the External Sierra. Within the transverse folds, principal shortening axes determined from P and T
dihedra plots of fault-slip data show a shift from steep shortening in stratigraphically older beds to NNE▒SSW horizontal shortening in
younger beds. Older strata are characterized by extensional faults interpreted to result from halotectonic (salt tectonics) deformation, whereas
younger strata are characterized by contraction and strike-slip faults interpreted to result from thrust sheet emplacement. The interpretation of
the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes
determined from «nite strain analysis, calcite twins, and regional thrusting directions determined from fault-related folds and slickenlines.
This study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny.
q 2001 Elsevier Science Ltd. All rights reserved.
The External Sierra along the southern margin of the Jaca
piggyback basin, southern Pyrenees, Spain, are characterized
by large-scale transverse folds (Fig. 1). The folds were