USING ORTHOGRAPHIC PROJECTION TO CONSTRAIN THE STRAIN AND KINEMATICS OF A TRANSPRESSIONAL FAULT ZONE, CENTRAL RANGE, TRINIDAD

The relationship between fold shape and fold hinge orientation has been used to constrain the kinematics of deformation and the amount of strain in the borderlands adjacent to modern, active transpressional boundaries. This approach is based on orientation data – i.e. strike and dip of bedding – that is typically collected in the field. In some regions, however, outcrop is not sufficient to provide the numerous bedding measurements needed for this approach. We develop a method for using orthographic projection (i.e. the three-point-problem) to extract kinematic and strain data from contact orientations. Orientation data are extracted from georeferenced geologic map and a digital elevation model. Contact position data are transformed into orientation data using a MATLAB script. The script allows for uncertainties in the both the horizontal position of the contact and the elevation of the contact to be propagated into the final kinematic and strain estimates determined for the fault zone. We apply this approach to a modern, active transpressional boundary along the Caribbean-South American plate boundary in Trinidad: the Central Range fault zone. Folds analyzed in the borderlands of the Central Range fault zone give results that suggest that the angle of oblique convergence in transpression (α) varies from almost pure strike-slip kinematics (α<10˚) to near pure convergence (α>80˚). A small group of folds consistent with lower angles of oblique convergence (α<50˚) are confined to a narrow portion of the borderlands immediately adjacent to the Central Range fault. Further away from the fault folds are most consistent with a larger contractional components (α>50˚). Partitioning of the strike-slip component of deformation towards the center of a shear zone is consistent with observations from ductile transpressional shear zones. Strain analysis suggests that folding in the borderlands records approximately 3-10 km of contraction, 2-4 km of strike-slip motion, and 4-11 km of total plate motion. The range of values presented are the result of propagation of uncertainties in the position of the contact into the kinematic and strain calculations. Ongoing research is attempting to apply the same methodology more broadly across southern basin of Trinidad.