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Effects of subducting seafloor roughness on upper plate vertical tectonism across
the Osa Peninsula
Peter Sak
Donald Fisher
Thomas Gardner
Objective
• To demonstrate that subaerially exposed marine lowstand deposits along the rapidly uplifting Osa Peninsula record an interval of subsidence.
Outline
• Describe previously unrecognized deposits of marine sands - the “Marenco fm”.
• Present radiocarbon dating results.• Evaluate the Marenco fm in the context of
eustatic sea level fluctuations.• Propose a mechanism for the reported
complex history of vertical tectonism.
Marenco formationBasal Unit: (Mean Sea Level)
• Medium to coarse-grained lithic ss with well-rounded, bored, barnacle-encrusted cobbles.
• Deposited on sub-horizontal surface cut into subjacent melange
• Bimodal m-scale cross-beds• Few fossils
Marenco formationMiddle Unit: Above Wave Base (9 ± 6 m)
• Poorly sorted, buff colored lithic sands with angular rock fragments
• Abundant disarticulated fossil shards
• Locally bioturbated
Marenco formationUpper Unit: Below Wave Base (> 15 m)
• Very fine grained gray silty ss
• Bioturbated• Fossiliferous
(articulated bi-valves, gastropods, leaf impressions, woody debris)
• Thin planar bedding
Fault-bend fold model
WhereV = displacement rate (mm yr-1) h = elevation of asperity above
adjacent seafloor (m)= slope of the incoming
feature in the convergence direction.
W = straining distance.
A
A’
SUMMARY
• Thick (> 40 m) accumulations of fining-upward late Pleistocene marine sands require subsidence coincident with deposition.
• Patterns of outer fore arc uplift/subsidence may reflect variations in the shape of the subducting plate.
• Rough crust subduction may result in erosion of the base of the overriding plate.