Abstract:
An oceanic assemblage of alkaline basalts, radiolarites and polymictic breccias forms the tectonic substratum
of the Santa Elena Nappe, which is constituted by extensive outcrops of ultramafic and mafic rocks of the
Santa Elena Peninsula (NW Costa Rica). The undulating basal contact of this nappe defines several half-windows
along the south shores of the Santa Elena Peninsula. Lithologically it is constituted by vesicular pillowed and
massive alkaline basaltic flows, alkaline sills, ribbon-bedded and knobby radiolarites, muddy tuffaceous and
detrital turbidites, debris flows and polymictic breccias and megabreccias. Sediments and basalt flows show
predominant subvertical dips and occur in packages separated by roughly bed-parallel thrust planes. Individual
packages reveal a coherent internal stratigraphy that records younging to the east in all packages and
shows rapid coarsening upwards of the detrital facies. Alkaline basalt flows, pillow breccias and sills within
radiolarite successions are genetically related to a mid-Cretaceous submarine seamount. Detrital sedimentary
facies range form distal turbidites to proximal debris flows and culminate in megabreccias related to collapse
and mass wasting in an accretionary prism. According to radiolarian dating, bedded radiolarites and soft-sediment-deformed
clasts in the megabreccias formed in a short, late Aptian to Cenomanian time interval. Middle
Jurassic to Lower Cretaceous radiolarian ages are found in clasts and blocks reworked from an older
oceanic basement. We conclude that the oceanic assemblage beneath the Santa Elena Nappe does not represent
a continuous stratigraphic succession. It is a pile of individual thrust sheets constituting an accretionary
sequence, where intrusion and extrusion of alkaline basalts, sedimentation of radiolarites, turbidites and
trench fill chaotic sediments occurred during the Aptian-Cenomanian. These thrust sheets formed shortly
before the off-scraping and accretion of the complex. Here we define the Santa Rosa Accretionary Complex
and propose a new hypothesis not considered in former interpretations. This hypothesis would be the basis
for further research.
