Figure 2.7. Geologic map of the
southern Central American Region.nic arc associated with the subduction of the Cocos
plate beneath the Caribbean plate (Burke 1988); (c) the
fusion of the southeastern end of the volcanic island
arc (now Panama) with northwestern South America
at the Atrato suture; and (d) the collision of the sub-
marine Cocos Ridge with the southern Pacific Coast
of Costa Rica (Fig. 2.7).
These events had momentous tectonic, biogeo-
graphic, and ecologic consequences for southern Cen-
tral America and the Cordillera de Tilaran. For ex-
ample, the increased rate of subduction of the Cocos
plate along the Middle America Trench further in-
creased island arc volcanic activity, which had been
relatively quiescent in the Costa Rican region since
the early Tertiary (50-60 Ma; Wadge and Burke 1983,
Burbach et al. 1984, Burke 1988). The important bio-
logical impact of these events was the creation of the
land bridge that allowed intercontinental biotic
movement approximately 3.5 Ma (Marshall et al.
1982, Gomez 1986, Coates et al. 1992). The Caribbean
Sea was separated from the Pacific Ocean, which lim-
ited the movement of marine organisms and floating
plant propagules (Coates et al. 1992); warm surface
water currents associated with the trade winds were
also redirected (Keigwin 1982, Burke 1988).2.2.1. Recent Volcanic and Tectonic Events
in the Cordillera de Tilaran
Both volcanism and tectonic uplift have contributed
to the growth of the Cordillera de Tilaran. Renewed
and accelerated subduction of the Cocos plate at the
Middle America Trench resulted in three major epi-
sodes of volcanic activity since the late Miocene.
Compression, fault formation, and tectonic uplift have
resulted in the horst block uplift that built the present-
day Cordillera.
The Cordillera de Tilaran is composed largely of
volcanic rocks of the Aguacate Group (Chaves and
Saenz 1974). The oldest of these have radiometric
dates of 8.5-10.5 Ma (late Miocene; Chaves and Saenz
1974). They were derived from magma produced at
relatively low temperatures and pressures at depths
of 80-100 km along the zone of plate contact, early
during the renewed subduction (Laguna 1985). The
youngest rocks in the Aguacate Group date from 2.6-
4.3 Ma (Pliocene). They were derived from magma
produced at depths of 150-200 km and at higher tem-
peratures and pressures during later phases of sub-
duction (Laguna 1985). These rocks are conspicuous,
forming the dramatic cliffs at 1200-1500 m through-
out the Cordillera. The Aguacate Group as a whole has
been uplifted as a horst between two large trench-
parallel faults: the Las Juntas Fault, which follows
the Pan-American Highway on the Pacific side of the
Cordillera; and the Arenal Fault, which extends
through Volcan Arenal, Cerro los Perdidos, and the
Caribbean side of the range (Dengo and Levy 1969).
Elevations of the cliffs formed by the Aguacate Group
decline to the northwest from a peak on the east flank
of Cerro Ojo de Agua. This suggests that the Cordil-
lera de Tilaran horst was uplifted and tilted by the
same mechanisms that have produced, and are still
producing, similar tilt and rotation of the Nicoya and
Osa peninsular blocks (Gardner et al. 1992, Marshall
and Anderson 1995).
Subsequent volcanic activity in the Quaternary
indicates ongoing maturation of the subduction zone,21 The Physical Environment