The Santa Elena Peninsula : an ophiolitic nappe and a sedimentary volcanic relative autochthonous

Abstract

The Santa Elena peninsula, northern Pacific coast of Costa Rica, is formed by three structural units: a relative autochthonous unit, an allochthonous peridotitic unit and a sedimentary cover (Campano/Maastrichtian to Paleogene in age). The autochthonous unit crops out as tectonic windows in the Center and the South of the peninsula. It consists of strongly folded layered radiolarites and cherts, Lias-lower Dogger to Cenomanian in age. Various sequences of dykes and pillowed basaltic flows display alkaline compositions. The allochthonous unit is a 35 km long peridotitic massif. Most peridotites are diopside bearing harzburgites and Iherzolithes and correspond to relatively poorly depleted mantle rocks. Plagioclase peridotites and scarce dunites also occur. Pyroxenitic layers parallel the foliation of the enclosing peridotites. The mantle sequence is cut by dykes composed of ultramafic cumulates, pegmatitic gabbros and very abundant dolerites. A layered sequence made of cumulates, gabbros and scarce pla- giogranites is overthrust by the peridotites. The allochthonous unit does not present the stratigraphy of a classical ophiolitic complex and the various sequences of mafic rocks are not cogenetic. Similar mantle peridotites occur in the Rio San Juan area. The possible occurence of a 150 km long East-West peridotitic suture is discussed. Such a structure could be the result of the Senonian closure of an "oceanic Basin" by convergence between the Southern Central American block and the Chortis block.