LES TERRAINS ACCRÉTÉS DU COSTA RICA. ÉVOLUTION
TECTONOSTRATIGRAPHIQUE DE LA MARGE OCCIDENTALE DE LA
PLAQUE CARAÏBE
DI MARCO Gianni, 1994
PhD adviser: Prof. Peter BAUMGARTNER, Institut de Géologie et Paléontologie
Published in Mémoires de Géologie (Lausanne), 1994, Nº 20
Detailed mapping of the southern Pacific coast of Costa Rica, together with
stratigraphic, biostratigraphic, paleomagnetic studies, and preliminary geochemical
data, allowed to subdivide this region into four distinct tectonostratigraphic units: the
Golfito Terrane, the Burica Terrane, the Rincón Block and The Osa-Caño
Accretionary Complex. The paleomagnetic study extented to whole of Costa Rica
and western Panama, along with a compilation of new and old data , allowed to
define two more units: the Chorotega Terrane and the Nicoya Terrane, subdividing
the Central America isthmus into a total of six distinct units.
The Chorotega Terrane constitutes most of the southern Middle American
Landbridge and represent the western edge of the Caribbean Plate since the Late
Cretaceous. The other terranes have originated outboard in the Paleopacific and
were brought into contact with the Chorotega Terrane by plate convergence. They
are considered as exotic terranes. The Nicoya Terrane comprises the Santa Elena
Peninsula and most of the outer Nicoya Peninsula. The Nicoya Terrane includes the
Nicoya Complex (sensu stricto) and should therefore be regarded as a composite
terrane. The Golfito Terrane forms the Golfito region and extends into Panama to the
Azuero Peninsula. It is composed of a basaltic basement overlain by Upper
Cretaceous volcano-sedimentary series (Golfito Formation), in turn overlain by
volcaniclastic series (Quebrada Achiote Formation) recording the Paleocene
accretion of the terrane. The terrane is thought to have formed a marginal plateau of
the Caribbean plate, transported northward by strike-slip along the rim of the
Caribbean Plate. The Rincón Block forms the Osa peninsula isthmus. It is composed
of a thick pile of Late Cretaceous to Eocene oceanic basalts, and represents a piece
of island arc (Chorotega ?) tectonically incorporated to the collage of exotic terranes.
The Burica Terrane forms the Burica Peninsula. Late Cretaceous oceanic basalts
form the basement of the terrane, unconformably overlain by Paleocene sediments
partly derived from an intraoceanic platform mounted on an oceanic seamount. The
terrane is thought to represent an accreted, structurally high, portion of an
intraoceanic primitive island arc. The outer Osa Peninsula and the Caño Island are
part of the Osa-Caño Accretionary Complex, a mélange-type complex, characterised
by strongly deformed turbidites, hemipelagic and pelagic sediments. The complex
includes blocks of reworked shallow water Eocene limestones, as well as Late
Cretaceous to Miocene exotic blocks (basalts and associated pelagic sediments),
incorporated to the complex by offscraping of the subducting plate. Three units were
defined within the complex, which are the San Pedrillo Unit, the Cabo Matapalo Unit
and the Salsipuedes Unit.
The paleomagnetic data for the Chorotega Terrane indicate an origin close to its
present latitude and no significant rotation relative to South America since Late
Cretaceous time. The paleomagnetic data obtained from the Nicoya Terrane imply a
low southerly Late Cretaceous paleolatitude and almost no rotation relative to the
Chorotega Terrane. The Nicoya Terrane was about 16° of latitude south relative to
the Chorotega Terrane in Late Cretaceous times. The paleomagnetic data from the
Golfito Terrane indicate a Late Cretaceous equatorial paleolatitude and counter
clockwise rotation of about 60° relative to the Chorotega Terrane. Similar
paleomagnetic data were obtained from the Azuero Peninsula in southwestern
Panama. The paleomagnetic data from the Burica Terrane indicate a low northerly
latitude in the Paleocene and a counter clockwise rotation of nearly 90° relative to the
Chorotega Terrane.