HR: 0800h
AN: T51A-0293    [Abstracts]
TI: Active Forearc Response to CO-NZ-CA Triple Junction Migration, Southern Central America
AU: * Morell, K
EM: kmorell@geosc.psu.edu
AF: Penn State University, Department of Geosciences Deike Bldg., University Park, PA 16801, United States
AU: Fisher, D
EM: fisher@geosc.psu.edu
AF: Penn State University, Department of Geosciences Deike Bldg., University Park, PA 16801, United States
AU: Gardner, T W
EM: tgardner@trinity.edu
AF: Trinity Univerity, Department of Geosciences One Trinity Place, San Antonio, TX 78212,
AB: Southeast migration of the CO-NZ-CA triple junction at a rate of ~55 mm/yr results in an abrupt increase in convergence rate, slab thickness and subduction direction within the upper plate of the Central American convergent margin. At the triple junction, an active transform fault (the dextral Panama Fracture Zone) subducts beneath the Caribbean plate at the Middle America Trench, and juxtaposes the thick, orthogonal and shallow subduction of the Cocos plate against the thin, oblique and steeper subduction of the Nazca plate. New bedrock geology, Quaternary mapping and Ar/Ar dates of fluvial and volcanic deposits inboard of the triple junction provide evidence that both the outer and inner forearc of this system is actively responding to the dynamic changes presented by triple junction migration. Our results confirm that the Fila Costeña, a thin-skinned inner forearc thrust belt, is active and likely propagating in concert with triple junction migration. Mapping within the area overriding the Panama Fracture Zone indicates that thrusting develops only in those areas experiencing Cocos subduction; the thrust belt dies out coincident with the on-shore projection of the Panama Fracture Zone, and balanced cross-sections indicate a lateral gradient in the amount of shortening near the termination of the thrust belt. Along-strike variations in drainage basin morphometry suggest that drainage divides of the Fila Costeña are propagating to the southeast with the triple junction, resulting in hook-shaped drainage patterns and asymmetric basin shapes. A survey of a flight of 3-4 fluvial terraces along the Río Chiriquí Viejo indicates recent thrusting along a prominent thrust fault of the Fila Costeña. These terraces are also inset into multiple lahar flows with an upper surface tentatively constrained at ~507 ka based on an Ar/Ar hornblende plateau age. Recent work indicates that this thrust fault displaces surficial lahar deposits, suggesting that it must have become emergent less than 500,000 years ago. At least 4 marine terraces occur along the southern tip of the Burica Peninsula in the outer forearc that straddles the subducting transform approximately 20 km inboard of the triple junction. The oldest (late Pleistocene) and highest (60-70 m) terrace surface appears tilted to the southeast, possibly as a consequence of the change in slab thickness at the Cocos-Nazca plate boundary that migrates to the southeast with the triple junction. These observations indicate that the changing plate tectonic framework associated with triple junction migration has a large impact on forearc structure and landscape evolution through time.
DE: 8099 General or miscellaneous
DE: 8158 Plate motions: present and recent (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8175 Tectonics and landscape evolution
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: 2007 Fall Meeting