Active structure

Abstract: When a region is tectonically deformed, its geometry changes. Some of these changes produce easily identified and often readily datable morphological features such as regions of rapid uplift and subsidence, sediment ponds, or river terraces. These features are usually secondary to the main active structures and consequently do not provide information about them directly. We show, however, that simple models using boundary-element methods can be adapted to relate the evolution of these minor features to motion on major structures. This adds substantially to the information available to determine motion on such features. In this paper, we apply the technique to northwest Greece. We show that, in addition to the long recognized compressional component of motion in Epirus, a substantial left-lateral strike-slip component of motion must be present. The modeling allows us to identify a regional slip vector of N275°E. This slip vector suggests that the plate configuration assumed by previous workers must be modified. A new configuration is proposed that remains consistent with the data used for earlier interpretations and our new data. It is proposed that the Medina Wrench is a significant active structure and that a period of quiescence explains the low seismic activity at present. The plate configuration and angular rotation rates that we suggest require that the Gulf of Arta is associated with a triple junction. Using the slip vectors we have defined for the plates meeting at this junction, we model the regions of uplift and subsidence in the region. The ability of this model to explain many features associated with the Gulf provides assurance that our overall tectonic model is correct.