From seamount accretion to tectonic erosion: Formation of Osa Mélange and the effects of Cocos Ridge subduction in southern Costa Rica

Abstract

The Costa Rica portion of the Middle America Trench (MAT) is characterized by active tectonic erosion, a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward. Offshore studies demonstrate accelerated subduction erosion starting at the Miocene‐Pliocene boundary, as the result of subduction of thickened Galapagos related crust, as presently represented by the Cocos Ridge. The subduction of the Cocos Ridge also caused uplift and exposure of the outer forearc on the Osa Peninsula, which offers a window to explore the tectonic evolution of the area. The rocks outcropping on Osa Peninsula are a middle Eocene–middle Miocene mélange dominated by basalt, chert, and limestone resulting from accretion of seamounts. The accretion‐dominated period of the MAT evolution ended at the Miocene‐Pliocene boundary, when thickened crust, a paleo‐Cocos Ridge, produced at the Galapagos hot spot arrived at the trench. The thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the Osa Mélange. The change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small (kilometer to tens of kilometers size) blocks that are going through differential vertical movements in response to the morphology of the subducting ridge. Subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin, so that mass removal can develop on a previously growing margin and completely remove an accretionary prism.

Expedition 344 summary

“…The CLIP is composed of accreted oceanic islands and aseismic ridge terranes (Hauff et al, 1997(Hauff et al, , 2000Sinton et al, 1997;Hoernle et al, 2002). The Quepos and Osa terranes are interpreted as accreted material derived from subducted edifices generated by the Galapagos hotspot (Hauff et al, 1997;Vannucchi et al, 2006). On land and close to the CRISP transect, the seawardmost unit is the Osa Mélange, which is dominated by basalt, radiolarite, and limestone (Vannucchi et al, 2006).…”

“…The Quepos and Osa terranes are interpreted as accreted material derived from subducted edifices generated by the Galapagos hotspot (Hauff et al, 1997;Vannucchi et al, 2006). On land and close to the CRISP transect, the seawardmost unit is the Osa Mélange, which is dominated by basalt, radiolarite, and limestone (Vannucchi et al, 2006). Alternatively, upper plate basement may be composed of paleoaccretionary prism material.…”

Crustal redistribution, crust–mantle recycling and Phanerozoic evolution of the continental crust

“…The seamount chains formed by more steady state hotspot activity are much smaller and mostly subducted. Where these collide with tectonically erosive active margins there is some evidence to suggest a temporary accretion of a modest part the seamount into the outer forearc (Johnson et al, 1991), although this is then typically removed by tectonic erosion with time (Vannucchi et al, 2006). Even in accretionary plate margins volcanic seamounts are generally subducted, with only thin slices of basalts (<1 km thick) being offscraped into the wedge of trench sediment (Taira et al, 1988;Collot et al, 1994;Kusky et al, 1997).…”

Seamount chain–subduction zone interactions: Implications for accretionary and erosive subduction zone behavior

“…The chaotic nature of the mélange prevents us from estimating their offsets 1 GSA Data Repository item 2018109, Item DR1 (detailed geological map of the San Pedrillo Unit), Items DR2-DR4 (photographs, thin sections, cross sections, and lithological descriptions of the Cocolito, Punta Marenco, and Drake packages, respectively), and Item DR5 (comparative Scale of the Osa Mélange and seamount moats/debris aprons), is available online at http://www.geosociety.org/datarepository/2018/ or on request from editing@geosociety.org. (Vannucchi et al, 2006). However, it appears that most deformation is accommodated by distributed shear within the matrix, which appears as a pervasively well-developed lenticular fabric with abundant anastomosing shear bands.…”

“…The isotope geochemical character of the basalt in the Osa mélange has been used to interpret a Galápagos ocean island basalt (OIB) geochemical affinity like that of the Osa Igneous Complex (Hauff et al, 2000;Vannucchi et al, 2006). Rare dacitic blocks, instead, have an ambiguous geochemical signature resembling that of the early stages of a volcanic arc (Buchs et al, 2009).…”