IN THE WAKE OF FLAT SUBDUCTION: UPPER-PLATE TECTONICS ACROSS A STEEP TO FLAT SLAB TRANSITION, PACIFIC MARGIN, COSTA RICA, CENTRAL AMERICA

The late Cenozoic tectonics of southern Central America are profoundly impacted by flat subduction of hot-spot thickened oceanic crust of the southern Cocos plate (Cocos Ridge and seamount domain). A sharp transition from steep subduction of smooth crust (East Pacific Rise origin) to flat subduction of rough and thickened crust (Galapagos Spreading Center origin) occurs along the Middle America Trench (MAT) offshore of central Costa Rica. This location coincides with abrupt changes in upper plate topography, structure, seismicity, and volcanism.

Ongoing geologic and geomorphic investigations along the central Costa Rican margin reveal an unfolding sequence of tectonic events affecting the overriding plate in the wake of flat subduction. Impacts include the propagation of a shallow deformation front into the volcanic arc (Central Costa Rica Deformed Belt - CCRDB), retreat of the magmatic front away from the MAT (Cordillera de Aguacate to Cordillera Central), and structural segmentation of the fore arc (subduction erosion offshore, and uplift and fault-block translation onshore).

Field mapping, geochronology (14C, 40Ar/39Ar), and structural analysis of active faults, offset stratigraphy, and marine and fluvial terraces provide temporal and kinematic constraints on upper plate tectonism across the steep to flat slab transition. Late Neogene-Quaternary slab flattening generated shallow transpressional faulting across the upper plate (CCRDB) and led to retreat of the magmatic front away from the MAT. Arc expansion shifted the Pacific-Caribbean drainage divide to the NE, forming the intra-arc Valle Central basin. While arc migration occurred onshore of the seamount domain, volcanism shut off inboard of the Cocos Ridge to the south.

Marine and fluvial terrace correlation along the Costa Rica fore arc shows variations in Quaternary uplift patterns across the steep to flat slab transition. Sharp changes in uplift rate (1.0-6.5 m/ky) along the central coast reflect differential uplift of fault-bounded blocks above subducting seamounts on the thickened slab. More uniform and lower uplift rates (0.1-0.5 m/ky) occur on the northern coast above the steep slab. Margin-parallel faults on the Nicoya Peninsula in the north may accommodate lateral motion of a fore arc sliver away from ridge collision in the south.