STRAIN PARTITIONING AND ACTIVE FAULTING DURING OBLIQUE CONVERGENCE, NORTHERN ANDES


The Andes are the type example of ocean-continent collision. The Nazca plate subducts beneath the South American continent, giving rise to the high mountains along the coast and the great plateaus of the Central Andes. In Ecuador, the imposing Andean arc divides into two volcanic chains that define the eastern and western margins of the Inter Andean Valley, a long, north south oriented topographic low at high elevation. Quito, Ecuador's capital city, sits within the Interandean valley between the two chains. Its 1.5 million citizens live amidst some of the Earth's more active geologic processes and hazards, including active, complex faulting, explosive volcanism, and rapid and extreme erosion and sedimentation driven by the high elevations in the area.

The Interandean Valley of Ecuador is an excellent place to study strain partitioning and fault dynamics during oblique convergence. The geology is well mapped and the region is extremely active tectonically making it easy to observe and quantify deformation. A major fault system, the Delores-Guayaquil Megashear, a transpressive dextral strike-slip system, trends onshore at Guayaquil and passes into the Inter Andean Valley. The Delores-Guayaquil Megashear is similar in scale and seismic activity to San Andreas Fault system in California. Like the San Andreas, the Delores-Guayaquil Megashear is a composite fault system, composed of a number of individual fault strands. While portions of the fault system have been identified using offset geomorphic features, sag ponds, and fault scarps, the fault system has not been systematically mapped at the surface. In many places the location of the fault is masked by active erosional and depositional processes. Nowhere has the fault or associated deformation been imaged in the subsurface. There is no coherent framework for integrating the numerous faults and the development of fault controlled structures into a kinematic model.


We are conducting a pilot project to acquire high-resolution seismic reflection profiles in three strategic locations in the Inter Andean Valley to determine subsurface fault/fold geometry and deformation history within the Delores-Guayaquil Megashear. The results of this study will improve our understanding of strain partitioning in transpressional settings and are immediately relevant to assessing the seismic hazards for Quito, Ecuador. This project is a collaborative effort between Lehigh University and the Institutor Geophysics at the Secular Polytechnic National, Quito, Ecuador.