Vannucchi, Paola, Ranero, Cesar and Scholl, Dorothee (2001) Forearc Mass Removal and the Effects of Subduction Erosion off the Nicoya Peninsula of Costa Rica. [Talk] In: AGU Fall Meeting 2001. , 10.-12.12.2001, San Francisco, USA .

Abstract

Since early-middle Miocene subduction erosion has been the dominant process controlling the tectonics off northern Costa Rica. Leg 170's Site 1042, located 7 km landward of the Middle America Trench (MAT), reached the acoustically defined Base Of Slope Sediment (BOSS) horizon at a depth of ~ 3900 mbsl and yielded a carbonate-cemented breccia, 16.5 myr-old, formed in a nearshore setting. The overlain Pleistocene to Miocene slope sequence shows a benthic foraminifera record implying the subsidence of the margin from the upper bathyal to the abyssal depth. The breccia rests unconformably above an older breccia composed of Nicoya basement rocks attesting that the coastal exposed basement of the Nicoya Peninsula extends seaward below Miocene and younger slope sediment to near the inner trench wall. Here a ~16.5 myr-old shoreline has subsided ~4 km, and that since this time at least 10-12 km of crustal thinning has occurred beneath the BOSS horizon implying that the trench axis has migrated landward at ~3 km/myr. Seismic images show that normal faulting is widespread across the overriding plate. However, measured extension by normal faulting can only account for a minor amount of the subsidence of the upper plate. Thus, removal of material by basal tectonic erosion has to be invoked to the explain the thinning and long term margin subsidence. Along this segment of the MAT the rate of crust removal approaches 35 km3/myr/km of trench. Basal erosion of older forearc material allows low concentrations of 10Be in the arc. The 10Be output in the arc increase toward Nicaragua where seismic data suggest lower rates of recent subduction erosion.

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