CHARACTERIZATION OF BRITTLE AND DUCTILE DEFORMATION ALONG THE BOUNDARY OF THE NORTH ANDEAN SLIVER IN THE NORTHERN CORDILLERA REAL OF ECUADOR

The oblique convergence of the Nazca and South American plates results in slip partitioning on the upper plate. The North Andean Sliver (NAS) comprises an area from northern Ecuador to western Venezuela, and has been geodetically constrained to have an approximate northward strike-slip motion of 7.5 – 9.5 mm yr-1 in Ecuador. In northern Ecuador, the eastern edge of the NAS lies in the Cordillera Real along the La Sofia fault. Offset Quaternary lava flows suggest that the La Sofia fault has a slip rate of 11.3 ± 6.2 mm yr-1, implying that most of the motion of the NAS is accommodated there.

The La Sofia fault is poorly studied, due to difficult access and rugged terrain. To improve our understanding of this feature, we carry out a study of the geology of the fault zone. The fault longitudinally cuts the roots of a Jurassic volcanic arc, characterized by intercalations of several types of granodiorites and enclaves of metasedimentary rocks. We find a zone of distributed faulting pervasively cut by small fault zones with widths that vary from a few centimeters to 2 meters with fault gauge within. The mean strike of these features is ~230° with a dip of ~70° to the W. Sub-horizontal slickenlines confirm strike-slip motion on these features.

We also carry out measurements of the ductile deformations found in the same area. We find that there is a preferred orientation in the foliations with a strike of ~225° and a dip of 67° to the W. There are pervasive mylonite zones that show grain size reduction and have penetrative linear fabrics, with a mean trend ~220° and a plunge of ~15°, suggesting constrictional strain. Kinematic indicators (S-C structures, sigma clast, and small scale folds) indicate dextral, non-coaxial strain, thus suggesting a transpressive deformation episode. Cross-cutting relations between dikes and these mylonite zones suggest that this ductile deformation occurred while there was ongoing magmatism associated to the arc.

We interpret the agreement of orientation of ductile and brittle deformation features as evidence that the Quaternary deformation has reactivated the inherited structural fabrics resultant from Mesozoic deformation. This is further supported by sub-horizontal slickenlines found on the foliation planes, indicating flexural slip motion on them. Further age constraints are needed to confirm this model.