Collaborative Research: Crustal Thickening, Shortening, and Uplift in the Forearc of an Erosive, Convergent Margin, Pacific Coast, Costa Rica

This project involves analysis of the onland geology of the upper plate along an active convergent plate boundary, where the Cocos plate and associated seamounts and ridges are underthrust beneath the Central American volcanic chain. In general, convergent plate boundaries can be characterized by two end-members: accretionary margins and erosive margins. The processes that occur along accretionary margins (e.g., the Aleutian, Nankai, Barbados, and Oregon-Washington margins) are well known from studies of onland and marine geology, but there is no comparable knowledge for erosive margins (e.g.,. the Japan, Peru, Middle America margins). Consequently, the principal investigators are creating new maps and cross sections of bedrock geology and dating marine and fluvial terraces along the Pacific Coast of Costa Rica to place constraints on shortening, uplift rates, and surface erosion in such a tectonic setting. Mapping of faults and determination of fault offsets from bedrock geology provide an estimate of the amount of shortening over the last 2 million years. Surveys of marine terraces along the coast are being combined with radiocarbon and optical stimulated luminescence dating to determine rates of vertical motion and slip rates on active faults. Analysis of river terraces and fission track dating of apatite grains in both volcanics and sedimentary rocks are being used to establish the rate and total amount of erosional unroofing. Moreover, on the Osa and Nicoya Peninsulas, newly discovered deposits of Neogene and Quaternary marine terraces and shallow marine deposits provide an unprecedented record of vertical tectonism. Paleomagnetic analysis of these deposits should extend this record back into the late Neogene. Recently, new volcanic rocks have been discovered along the coast that can be dated by Ar40/Ar39 methods to better constrain the age of initiation of active thrust faulting in the coastal deformed belt. The methods that are used in this study are designed to quantify the effects of incoming seamounts and ridges on active faulting and landscape evolution of the upper plate. There is an apparent dichotomy between the offshore response to these bathymetric features (tectonic erosion, subsidence) and the onland response (crustal thickening and uplift). By collecting a dataset that constrains timing, kinematics and rates of deformation, this study is defining the fundamental attributes of erosive convergent plate boundaries and the relationship between onland and offshore regimes.