HR: 17:00h
AN: T24A-05    [Abstracts]
TI: Forearc Deformation, Arc Volcanism, and Landscape Evolution near the Cocos-Nazca-Caribbean Triple Junction
AU: * Morell, K D
EM: kmorell@geosc.psu.edu
AF: The Pennsylvania State University, Deike Bldg., University Park, PA 16802
AU: Fisher, D
EM: fisher@geosc.psu.edu
AF: The Pennsylvania State University, Deike Bldg., University Park, PA 16802
AU: Gardner, T
EM: tgardner@trinity.edu
AF: Trinity University, One Trinity Place, San Antonio, TX 78212
AU: Protti, M
EM: jprotti@una.ac.cr
AF: OVSICORI, Universidad Nacional, Heredia, 2346-3000 Costa Rica
AB: New geologic mapping in SE Costa Rica and SW Panama reveals a system of structures and landscape features that are actively propagating with the Cocos-Nazca-Caribbean Triple Junction. The triple junction migrates to the SE at ~50 km/my, so the upper plate inboard of the Nazca plate experiences a rapid change from steep, slow subduction of the Nazca plate to shallow, rapid subduction of the Cocos plate. High plate boundary coupling for ~100 km NW of the triple junction has led to the development of the Fila Costena Thrust Belt. Balanced cross-sections indicate that shortening rates are highest near the center of the thrust belt, and decrease to the SE nearest the triple junction, where the thrust belt abruptly terminates. Right lateral tear faults cut the thrusts of the Fila Costena and allow for a sharp gradient in upper plate shortening above the subducted projection of the Panama Fracture Zone (PFZ), or the Cocos-Nazca boundary. East of the triple junction, a ridge exposes a fault-related anticline that may represent the incipient propagation of the Fila Costena into Panama. The volcanic arc is active just to the east of the onland projection of the subducting PFZ (e.g., Volcan Baru), with the extinct Talamanca arc lying to the west of this projection. Lahar fans on the slopes of the active Volcan Baru are analogous to the backtilted lahars now restricted to the rear of the Fila Costena. The spatial and temporal distribution of arc volcanism is consistent with a mantle wedge restricted to the east of the PFZ that migrates eastward with the triple junction. The Rio Chiriqui drainage system is currently the only river that carries arc volcanics to the eastern thrust front. The river skirts the southeast edge of the thrust belt and is inset into lahar fans on the slopes of Volcan Baru. Uplifted Quaternary fluvial terraces, located several kilometers west from the current drainage system, are offset at the thrust front by about 100-150 m. Andesite clasts in these terraces suggest that these gravels were probably deposited by the Rio Chiriqui prior to the uplift of the Fila Costena, when the river channel was west of its current location. Thus, the Rio Chiriqui drainage network and its associated landscape are migrating eastward through time. These observations indicate major changes in the arc-forearc system associated with the changing subduction parameters in the wake of the migrating triple junction.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8104 Continental margins: convergent
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005