TSK 11 Göttingen 2006
 Structural contacts in the
 Late Paleozoic accretionary
 wedge of central Chile and
 their tectonic significance for
 the evolution of the accre-
 tionary complex Vortrag
 Peter P. Richter1 Uwe Ring2 Arne P.
 Willner3 Bernd Leiss4
 The Chilean accretionary wedge is part
 of a Late Paleozoic subduction complex
 that developed during subduction of the
 Pacific plate underneath South Amer-
 ica. The wedge is commonly subdivided
 into a structurally lower Western Series
 and an upper Eastern Series. Under-
 standing the contact between both se-
 ries has been a long standing problem
 and is fundamental for the understand-
 ing of the evolution of the wedge sys-
 tem. We show the progressive develop-
 ment of structures and finite strain from
 the least-deformed rocks in the eastern
 part of the Eastern Series of the accre-
 tionary wedge to higher grade schist of
 the Western Series at the Pacific coast.
 Upright chevron folds of sedimentary
 layering are associated with a penetra-
 tive axial-plane foliation, S1. As the
 F1 folds became slightly overturned to
 the west, S1 was folded about recum-
 bent open F2 folds and an S2 axial-plane
 foliation developed. Near the contact
 between the Western and Eastern se-
 ries S2 represents a prominent subhor-
 izontal transposition foliation. Towards
 the structural deepest units in the west
 1 Institut für Geowissenschaften, Johannes
 Gutenberg-Universität, 55099 Mainz, Germany
 2 Department of Geological Sciences, Canter-
 bury University, Christchurch, New Zealand
 3 Institut für Geologie, Mineralogie und Geo-
 physik, Ruhr-Universität, 44870 Bochum,
 Germany 4 Zentrum für Geowissenschaften,
 Universität Göttingen, Göttingen, Germany
 the transposition foliation became pro-
 gressively flattened. Finite-strain data
 as obtained by Rf/φ analysis in meta-
 greywacke and X-ray texture goniome-
 try in phyllosilicate-rich rocks show a
 smooth and gradual increase in strain
 magnitude from east to west. There is
 no evidence for normal faulting or signif-
 icant structural breaks across the con-
 tact. We interpret the progressive struc-
 tural and strain evolution between both
 series to reflect a continuous change in
 the mode of accretion in the subduction
 wedge. Before ca. 320-290Ma the rocks
 of the Eastern Series were frontally ac-
 creted to the Andean margin. Frontal
 accretion caused horizontal shortening
 and upright folds and axial-plane foli-
 ations developed. At ca. 320–290Ma
 the mode of accretion changed and the
 rocks of the Western Series were under-
 plated below the Andean margin. This
 basal accretion caused a major change
 in the flow field within the wedge and
 gave rise to vertical shortening and the
 development of the penetrative subhor-
 izontal transposition foliation.
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