Effects of subducting seafloor roughness on upper plate vertical tectonism across the Osa Peninsula Peter Sak Donald Fisher Thomas Gardner.

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.