Refraktionsseismische Untersuchungen on- und offshore Costa Rica

The following work presents crustal models derived from wide-angle data from the projects TICOSECT and COTCOR. These projects continue extensive geoscientific efforts to illuminate the genesis of the Middle American landbridge and the effects generated by the subduction of different crustal segments of the Cocos plate and the Cocos Ridge under the Caribbean plate, the western border of which is Costa Rica. First, the known facts about tectonics and geology in Costa Rica and the Pacific offshore area are documented, summarising earlier work. Based on the data acquired in 1995 and 1996, two long crustal models and several others were developed for the central and southern part of Costa Rica. They show steep subduction angles in the middle part (normal and seamount dominated oceanic crust) and shallow subduction angles in the southern part (Cocos Ridge), where the slab is believed to be involved in the lack of volcanic activity during the past 5 ma. The extension of the ophiolites detected in central Costa Rica within the wedge is shown by a new 3-dimensional tomographic approach based on refraction data from the Gulf of Nicoya. The velocity models do not support former theories characterising this wedge as an accretionary structure, while an erosive character of the subduction zone is stressed. The influence different types of subducting oceanic crust, as observed off Costa Rica, have on earthquake generation can be described in quality, based on the models and known areas of earthquake generation. The subduction of the seamount dominated segment of the crust generates a frequent occurrence of shallow quakes beneath the wedge in central Costa Rica. The southern Costa Rica transect profile extends from the Middle American Trench on the Pacific side to the Caribbean coast of Costa Rica. It shows the subduction of the more than 12 km thick Cocos Ridge to a depth of 35 km. The Pacific wedge is divided into three segments extending into the magmatic arc of the Talamanca Cordillera. The middle part of the wedge contains an igneous basement showing a tendency of uplift towards the Osa peninsula. The existence of a "slab window" beneath southern Costa Rica is indicated by an absence of earthquakes deeper than 50 km and high densities in the lower part of the Isthmus, while the Caribbean Mohorovicic' discontinuity, identified in northern and central Costa Rica by other researchers, is indicated at a depth of app. 35 km according to gravimetric modelling. The available geophysical data does not indicate a strong change of the properties or structures at the transition from the landbridge to the Caribbean Plate, which leads to the assumption that the main part of the Middle American landbridge is built from thickened Caribbean crust