Biology Reference
In-Depth Information
CENTRAL AMERICA
During the Jurassic and Early Cretaceous, a fragment of land located in the
Pacifi c Ocean off the coast of Mexico was moving to the southeast through
plate movement. It is called the Chortis Block, and it became sutured on to
the southern end of the Maya Block in the Late Cretaceous to form present-
day northern Central America (fi gs. 2.17 and 2.30). The contact is the
Polochic-Motagua fault system across northern Guatemala, which also
marks the boundary between the North American and Caribbean plates.
Movement along the fault causes devastating earthquakes in the region,
like the one that hit Guatemala in 1976 (fi g. 2.31), or the 7.3-magnitude
quake off the coast of Honduras on 28 May 2009 (posted as an image of the
day on NASA's Earth Observatory site, http://earthobservatory.nasa.gov/
images). Ophiolites along the Polochic-Motagua fault system have been
dated at 70-66 Ma, which establishes the accretion of the Chortis Block
and the time of origin of northern Central America. The southern margin of
the Chortis Block is the Santa Elena fault system that extends eastward into
the Caribbean Sea as the Hess Escarpment just south of Largo Nicaragua.
Southern Central America (southern Costa Rica and Panama) had a dif-
ferent origin, and the evolution of the landscape there played an important
role in the development of the biota of the New World. After the fi rst line
of volcanic islands was carried eastward to form the Greater Antilles, a sec-
ond arc developed between the southern boundary of the Chortis Block
and what was then northern South America (again, southern Costa Rica
and Panama). Volcanic activity and uplift from subduction of two Pacifi c
subplates called the Cocos and Nazca plates (fi g. 2.20) into the Middle
American Trench at about 8 cm a year, along with compression forces from
the movements of North and South America, eventually established a con-
tinuous land connection. The subsequent geological, oceanographic, and
climatological consequences were extraordinary. There was a reorganiza-
tion of middle Miocene deep water circulation (Burton et al. 1997; Nisan-
cioglu et al. 2003; Schneider and Schmittner 2006), a late/middle Miocene
drop in CaCO
3
deposition in the eastern Pacifi c (Lyle et al. 1995; Roth et al.
2000; Newkirk and Martin 2009), and a regulatory effect on Northern
Hemisphere glaciations via transport of heat and moisture from the trop-
ics (Lunt et al. 2008). The biological consequences were equally profound.
South America had begun separating from Africa in the south about 120 Ma
as new crust was generated along the Mid-Atlantic Ridge. The separation
continued progressively northward until northern South America sepa-
rated from Africa at about 90 Ma. Around 32 Ma the continent parted from
