STRUCTURAL AND TECTONIC EVIDENCE OF THE ORIGIN OF CRESTONES AT CHIRRIPO NATIONAL PARK, COSTA RICA

Inboard subduction of the aseismic Cocos Ridge under mainland Costa Rica is a topic of great interest in Central American tectonics. Cradled in the Talamanca Cordillera, the study area in Chirripó National Park is inaccessible by road and densely vegetated. This region contains the highest elevations in Central America and among many summits, the iconic Crestones Peak. Cerro Crestones is an elongated structural feature that offers insight into the tectonics and deformation style of the region. Fracture systems and fault zones project inland from the location of the Cocos Ridge subduction into the park and are an evidence of the strain derived from the impinging of this bathymetrical feature. In the National Park, a set of northeast-southwest trending fractures and normal faults align with the direction of tensional strain while northwest-southeast striking closed joints represent compressional deformation. By correlating the direction of elongation of Crestones Peak with the orientation of open fractures and normal fault networks, and detailed petrographic analyses, we believe that extensional deformation allowed for the creation of pathways for hydrothermal fluids to ascend and modify pre-existing lava flows. The combination of the open-fracture networks which match the measured extensional crustal deformation orientation related to the indentation of the Cocos Ridge in the subduction zone, with the pervasive hydrothermal alteration in the Crestones, we have established a connection between the aseismic ridge caused stain and the deformational features observed on land. It is by this connection that the intense silicification observed in the mineralogy, chemistry, and macro and microtexture can be used to explain the jagged and elongated, weathering resistant rocks of Crestones Peaks. The formation of Crestones allows us to further understand the processes involved in aseismic ridge subduction, the deformation styles related to this tectonic process and the role of brittle deformation during the opening of conduits for hydrothermal alteration. Additionally, our research could shed some light into the timing of the Central American Isthmus closure and the arrival of the Cocos Ridge to the Middle America Subduction zone by providing time constraints on deformational events.