MICROSTRUCTURAL STUDY OF MECHANISMS OF DEFORMATION IN DOLOSTONE ALONG THE RONDOUT DECOLLEMENT, HUDSON VALLEY FOLD-THRUST BELT

The deformation mechanisms associated with decollements within fold-thrust belts are poorly understood because they are rarely exposed in outcrop. In this study, the deformation mechanisms and deformational history of the Rondout decollement within the Hudson Valley fold-thrust belt in eastern New York was investigated along NY Route 23 near Leeds, NY. Detailed field study using meter-grid mapping and photomosaic mapping, along with microstructural and sub-microstructural analyses of the decollement, yield insights into the mechanisms of deformation and the relative timing of events within the decollement zone. This decollement, which formed within the Rondout Formation dolostones, deformed the sedimentary rocks primarily through duplexing at the mesoscopic scale. An ESE-WNW (transport-parallel) transect of the decollement revealed that the deformation zone thickened in the transport direction (E to W). This thickening of the deformation zone is a result of strike-perpendicular variations in stratigraphy as well as tectonic thickening. Field and laboratory studies were also used to investigate causes for the decollement forming within the dolostone formation. These causes include thin bedding and interbedding of clays, rock composition, and grain size. Optical microscopy and Scanning Electron Microscopy of samples from within and outside of the decollement deformation zone indicate that diffusion (pressure solution) creep was the dominant grain-scale deformation mechanism, and was followed by low temperature recrystallization and frictional sliding and cataclasis. Both mesoscopic and microscopic criteria were used to establish a deformation history within the decollement zone.