HR: 14:30h
AN: T22D-04
TI: Flexural Faulting and Mantle Serpentinization at the Middle America Trench
AU: * Ranero, C R
EM: cranero@geomar.de
AF: GEOMAR, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: Morgan, J P
AF: GEOMAR, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: McIntosh, K D
AF: UTIG, 4412 Spicewood Springs Rd., Austin, TX 78759-8500 United States
AU: Reichert, C
AF: BGR, Stilleweg 2, Hannover, 30655 Germany
AB: Dehydration of serpentinized mantle in the slab has been proposed to cause both intermediate depth earthquakes and arc volcanism at subduction zones. It has been suggested that serpentinization occurs at the outer rise; here earthquakes due to flexural normal faulting cut $>$ 20 km deep, potentially allowing seawater to percolate into the mantle. However, little is known about flexural faulting at convergent margins: about how many normal faults cut across the crust and how deeply penetrate into the mantle; about the true potential of faults as conduits for fluids. Multibeam bathymetry along the Pacific margin of Costa Rica and Nicaragua shows large changes in flexural faulting along the incoming plate. Several parameters may control the variability. Off south Costa Rica thick crust of Cocos Ridge flexes little, and little faulting develops near the trench. Off central Costa Rica, normal thickness crust has magnetic anomalies striking oblique to the trench. Here pervasive faulting of small offset (~200 m) and strike similar to the magnetic anomalies develops within ~25 km of the trench. Off north Costa Rica, magnetic anomalies are normal to the trench, and a few ~100m-offset faults develop parallel to the trench. Further north, off Nicaragua, magnetic anomalies are parallel to the trench and pervasive normal faulting develops. The faulted region widens to the north from ~25 to ~60-km, and fault offsets increase from ~100 to ~500 m. Seismic reflection images in this area show a pervasive trenchward dipping reflections across the crust extending into the mantle to depths $>$ 20 km. Some reflections project updip to offsets in top basement and seafloor, suggesting that they are fault plane reflections. This deeply penetrating fabric could not develop during crustal creation at the spreading center - at fast spreading axes the brittle layer is a few km thick. Thus they must be created during flexure of the plate entering the trench. Three implications are: (1) deep and widespread serpentinization of the incoming plate can occur when the lithosphere is strongly faulted as it enters the trench; (2) the extent of deep faulting is related to the crustal structure of the plate; (3) the amount of deep faulting can change dramatically within a hundred km distance along a trench.
DE: 3025 Marine seismics (0935)
DE: 3045 Seafloor morphology and bottom photography
DE: 8010 Fractures and faults
SC: T
MN: 2001 AGU Fall Meeting