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STRUCTURAL STYLE AND TIMING OF DEFORMATION IN THE PIEDRAS- GIRARDOT FOLD BELT, COLOMBIA: EARLY TRANSPRESSIONAL DEFORMATION IN THE NORTHERN ANDES Camilo MONTES Corporación Geológica ARES, [email protected] KEYWORDS : Strain, Syntectonic sedimentation, Transpression, Andes, Cordillera Oriental, Three dimensions. INTRODUCTION Reconstruction of deformed strata in two and three dimensions is a kinematic exercise commonly constrained by surface geology and/or subsurface seismic reflectors. Additional constraints derived from detailed geologic mapping such as strain markers, kinematic indicators, and timing of deformation, greatly improve the quality of these reconstructions. This short paper summarizes data (Montes, 2001) regarding the strain state and timing of deformation of the Piedras-Girardot foldbelt (Fig. 1). Two manifestations of deformation in the Piedras-Girardot foldbelt are briefly discussed in this paper: first, the stratigraphic signature of deformation is analyzed to establish the timing, and approximate locus of deformation along mapped faults or folds. Second, the structural signature of deformation is analyzed to approximately define the orientation of finite strain axes, and the magnitude of strain recorded by mesoscopic fabric elements. Together, these stratigraphic and structural criteria were used to propose a history of deformation and sedimentation that was geometrically and kinematically tested using two- and three- dimensional models (Montes et al., in press). The Piedras-Girardot foldbelt This foldbelt is located in the only segment of the northern Andes where the Cordilleras Central and Oriental overlap, thus opening a unique window into an area key to understanding the interaction between the Caribbean, South American, and Nazca plates (Fig. 1). This anomalous geomorphic province exposes critical relationships between the Cordilleras Oriental and Central covered elsewhere by Neogene deposits. Two very distinctive stratigraphic packages are exposed in the Piedras-Girardot foldbelt: a Late Cretaceous marine carbonate and siliciclastic sequence, and a Tertiary nonmarine, coarse grained, and mostly red siliciclastic sequence. These sequences rest on a volcanoclastic, and plutonic basement of Triassic-Jurassic age. Syntectonic sedimentation, Late Cretaceous Stratigraphic pinchouts occur in two sandstone units of Campanian age: the sandy member of the Nivel de Lutitas y Arenas, and the sandy member of the Nivel Intermedio. Pinchouts on two sandstone units of Campanian age were interpreted to record mild uplift during the early Campanian along the northern, northeast-trending segments of two of the major faults of the Piedras-Girardot foldbelt, and along the easternmost (also northeast-trending) structure of 425
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this foldbelt. These coarse-grained marine clastic units represent gentle deformation within the Piedras-Girardot
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