Coeval hindward- and forward-imbricating thrusting in the south-central Pyrenees, Spain: Timing and rates of shortening and deposition

Clear demonstrations of detailed sequences of thrust development are rare, particularly with respect to hindward-imbricating successions. Similarly, well-constrained chronologies of the timing of multiple thrust motions and associated rates of shortening are also uncommon. Along the eastern oblique ramp of the South-Central Unit (SCU) of the Pyrenees, well-exposed structural geometries and crosscutting stratigraphic packages unambiguously illustrate a sequence of break-back thrusts that developed synchronously with the growth of an adjacent duplex. The breakback sequence includes both the Sierras Marginales and Montsec thrusts: structures defining two of the major east-west partitions within the SCU. Balanced, sequentially restored structural cross sections indicate that a minimum of 7.5 km of shortening occurred within the imbricate stack. An additional 9 km of contraction was accommodated by the duplex, prior to translation of the duplex and imbricate stack to the south along a detachment within upper Eocene evaporites. Magnetic polarity stratigraphies have been used to calibrate these late Eocene to Oligocene episodes of deformation. These temporal data indicate that Sierras Marginales thrusting began in the study area -39.8 Ma and that the Montsec thrust ceased movement -36.2 Ma, yielding a mean rate of shortening of 2.1 mm/yr. Coeval shortening within the duplex was -2 .5 mm/yr, and subsequent translation across the footwall ramp occurred at -2 .5-4 mm/yr. Despite the partitioning of shortening into distinctively different styles of deformation, the mean rate of contraction remained steady over 4 m.y. These new chronologies indicate that the locus of large-scale thrusting did not progress systematically toward the foreland, but shifted episodically within the SCU, such that, during the late Eocene shortening, the Sierras Marginales thrust was active both prior to and south of the Montsec thrust.