SEMIDUCTILE DEFORMATION IN PELAGIC LIMESTONES AT DIAGENETIC CONDITIONS

Semiductile to ductile deformation phenomena within unmetamorphic, fin e-grained, pelagic limestones of the Northern Calcareous Alps (NCA) in the Eastern Alps were studied. The investigated pelagic limestones in clude the Triassic Hallstatt Limestone, the Liassic Adnet Limestone, t he Middle Jurassic Strubberg MarI and the Upper Jurassic Oberalm Limes tone. These pelagic limestones and marls display structures related to semiductile to ductile deformation, including shear planes, solution seams, stylolites and dynamic recrystallization similar to structures in protomylonites due to strata-parallel simple shear. The structures preferentially developed along clay-rich layers and along boundaries b etween more competent marry limestones showing a protomylonitic appear ence due to disjunctive anastomosing foliation. Along the boundaries a ragonite and fine-grained calcite were dissolved and insoluble clay mi nerals were concentrated. Increasing strain resulted in the developmen t of a penetrative foliation, generating S-C fabrics even within compe tent limestone layers. Furthermore, mesoscale out-of-sequence shear pl anes caused decomposition of competent limestone layers into clasts an d nodules. These clasts acted as rigid objects within a more viscous, argillaceous matrix. When ideally oriented, asymmetric pressure shadow s were generated around these nodules and a-clasts were developed. The transition from massive limestone beds to nodular layers depends on s ilt and clay contents. While limestones with low clay content were str ucturally resistant to deformation, clay-rich limestones were easily d eformed. The Adnet and Hallstatt limestones formed decollement horizon s accomodating high strain during Cretaceous nappe stacking and thrust ing within the NCA, while the Strubberg and Oberalm limestones were in volved during Tertiary transpressive overprint in large strike-slip fa ults and thrusting within an associated triangle structure.