International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

SEMIDUCTILE DEFORMATION IN PELAGIC LIMESTONES AT DIAGENETIC CONDITIONS

Pages: 361 - 370

Authors: JOACHIM SCHWEIGL, FRANZ NEUBAUER

Abstract: Semiductile to ductile deformation phenomena within unmetamorphic, fine-grained, pelagic limestones of the Northern Calcareous Alps (NCA) in the Eastern Alps were studied. The investigated pelagic limestones include the Triassic Hallstatt Limestone, the Liassic Adnet Limestone, the Middle Jurassic Strubberg Marl and the Upper Jurassic Oberalm Limestone. These pelagic limestones and marls display structures related to semiductile to ductile deformation, including shear planes, solution seams, stylolites and dynamic recrystallisation similar to structures in protomylonites due to strata-parallel simple shear. The structures preferentially developed along clay-rich layers and along boundaries between more competent marly limestones showing a protomylonitic appearence due to disjunctive anastomosing foliation. Along the boundaries aragonite and fine-grained calcite were dissolved and insoluble clay minerals were concentrated. Increasing strain resulted in the development of a penetrative foliation, generating S-C fabrics even within competent limestone layers. Furthermore, mesoscale out-of-sequence shear planes caused decomposition of competent limestone layers into clasts and nodules. These clasts acted as rigid objects within a more viscous, argillaceous matrix. When ideally oriented, asymmetric pressure shadows were generated around these nodules and σ-clasts were developed. The transition from massive limestone beds to nodular layers depends on silt and clay contents. While limestones with low clay content were structurally resistant to deformation, clay-rich limestones were easily deformed. The Adnet and Hallstatt limestones formed decollement horizons accomodating high strain during Cretaceous nappe stacking and thrusting within the NCA, while the Strubberg and Oberalm limestones were involved during Tertiary transpressive overprint in large strike-slip faults and thrusting within an associated triangle structure.

Keywords: Eastern Alps, pelagic limestone, plasticity, deformation mechanisms, deformation partitioning

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