TECTONIC RECONSTRUCTION OF THE NORTHERN ANDEAN BLOCKS: OBLIQUE CONVERGENCE AND ROTATIONS DERIVED FROM THE KINEMATICS OF THE TRANSPRESSIONAL PIEDRAS-GIRARDOT FOLDBELT, COLOMBIA

The tectonic configuration of the northern Andean blocks is reconstructed by defining three continental blocks in the northern Andes: the rigid Maracaibo (MB) block, the rigid Cordillera Central block (CCB), and the weak Cordillera Oriental (COB) block. Rigid blocks accomodate the oblique convergence imposed by the relative eastward motion of the Caribbean Plate as rotations and translations, whereas weak blocks, product of attenuated crust by rifting, accomodate oblique deformation by distortion and dilation. The weaker COB records large values of shear strain and shortening consistent with measurements made in the Piedras-Girardot foldbelt (PG). Published paleomagnetic data indicates that the stronger MB records large clockwise tectonic rotations. The CCB should record smaller values shear strain and convergence, but larger values of northward translation according to this model. These strain values, timing of deformation, and structural styles were derived from detailed analyses made in the PG and are compatible with published studies of kinematics in the northern Andes. Finite deformation in the PG is defined by a shear strain of 1.53 and a convergence factor of 2.03 within a N45E trending shear zone that records tectonic escape due to the ENE insertion of a rigid block. This deformation began during early Campanian times with incipient propagation of NE-trending faults that uplifted gentle domes where the accumulation of the sands did not take place. Unroofing during Maastrichtian times of a metamorphic terrane west of the Piedras-Girardot foldbelt is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodate between 1 and 2 percent extension in a general NE-SW direction. This extensional fabric, distortion of fossil molds, and a moderate cleavage accomodating less than 5 percent contraction in a general NW-SE direction, developed concurrently, during and after the incipient folding, but before large scale faulting and folding took place during Paleogene times. Folding and thrust sheet propagation during Paleogene times is recorded by syntectonic strata and records westward to southwestward propagation of the faults. Neogene deformation took place only in the western flank of this foldbelt.