HORIZONTAL AND “HORIZONTAL” SUBDUCTION ALONG THE MIDDLE SECTOR OF THE BACKBONE OF THE AMERICAS: PARTITIONED SEISMOGENIC STRESS-STRAIN AND ACTIVE DEFORMATION

The backbone of the Americas region between central Nicaragua, Costa Rica and northwestern Panamá has been and is currently being affected by the interaction of the Cocos, Caribbean and Nazca tectonic plates. These interactions result in a permanent and very active seismotectonic framework whose origin is mainly controlled by forces generated by plate movements. Subhorizontal subduction along the southernmost Costa Rican sector of the Middle America trench has traditionally been interpreted from regional and local seismological data, and related tectonic models have been developed to explain this behaviour. Relocation of seismic events and deployment of high resolution seismological networks along with existing and ongoing bathymetric and tomographic studies reveal a new scenario where the Benioff zone in this region dips at a high angle that is similar to that to the north. The difference is that the down-turn is farther from the trench. The consequence is a modern stress field with a regional maximum horizontal compressive stress SHmax that is consistently oriented with an average trend of N22°E. The stress field is oriented along and perpendicular to the Pacific margin and is consistent with the convergence direction of the Cocos plate. Thrusting, being moved by deep stresses, dominates below the shallow region and contrasts with an upper plate regional strike-slip regime which forms an X shaped pattern of NE and NW striking fault sets. These faults are strongly superimposed on a less well developed extensional regime within and around the volcanic edifices, showing the dominating influence of the shallow Cocos plate stresses and indenter effects in the Caribbean plate. Miocene to Paleocene rotation of the stress field and blocks due to interactions between the North America and South America plates, the formation of the Caribbean plate and the arrival of the Cocos ridge at the trench further complicate the present tectonic architecture. Destructive events (e.g., Limón earthquake, Mw 7.6; 22 April 1991), clearly exemplify the crustal instability within the local cordillera. These seismotectonic realities can assist in the reinterpretation of past behavior and forecast of future ones in other sectors of the backbone.