LATE MIOCENE TO RECENT TECTONIC EVOLUTION OF THE UPPER PLATE OF THE MIDDLE AMERICA TRENCH, COSTA RICA AND WESTERN PANAMA

The late Miocene-Recent evolution of the upper (Caribbean-Panama) plate of the Middle America subduction system in Costa Rica and Western Panama is dominated by three factors: 1) collision of incoming seamounts and plateaus, 2) arrival and underthrusting of the aseismic Cocos Ridge, and 3) lateral migration of the triple junction associated with the subducting Panama Fracture zone. Seamount and plateau collision is mainly reflected in the segmentation of the forearc into fault-bounded blocks with variable rates of uplift. Vertical deformation along the length of the forearc correlates spatially with the incoming distribution of bathymetric features on the incoming plate. Underthrusting of the Cocos Ridge impacts all aspects of the subduction system from the trench to the back arc. The trench shows a major embayment in the area of Cocos Ridge subduction, and marine terraces record short wavelength variations in uplift rate that correspond to variations in bathymetry across the top of the ridge. The outer forearc is thus thin, weak and responsive to the curvature associated with roughness on the incoming plate. In the inner forearc, an Eocene-late Miocene forearc basin was telescoped into an active coastal fold-and-thrust belt that allows underthrusting of the outer forearc and accounts for much of the trench embayment. This thrust belt consists of five thrust slices that record more than 40 km of shortening, with the greatest shortening directly inboard of the ridge axis. The detachment of this system steepens along the front of the Talamanca Range, the extinct volcanic arc in the region of Cocos Ridge subduction. The Talamanca Range is a popup bound on both sides by thrust belts with opposing vergence. Thermochronometry and landscape evolution are consistent with rapid uplift related to recent arrival of the ridge. At the point where the Panama Fracture zone intersects the MAT, there is a sharp northwest to southeast change from rapid, orthogonal subduction of anomalously thick crust to slow, oblique subduction of more normal thickness crust. This subducting tear migrates east-southeast relative to the upper plate, so the upper plate along the MAT experiences shortening, crustal thickening, uplift, and mountain-building in the wake of the passage of the triple junction.