Free-air and simple Bouguer anomaly maps of the Venezuelan
continental margin (from 60°W to 72°W and from 7°N to 13°N)
are presented. The major features of the free-air map are:
the large lows associated with the deep sedimentary basins,
-200 mgal in the Eastern Venezuela basin and -164 mgal in the
Maracaibo basin; the high of greater than 300 mgal over the
Venezuelan Andes; and a belt of highs associated with the offshore
islands extending from Blanquilla to Curacao and then
over the Guajira peninsula, where they terminate. The Bouguer
anomaly map shows a large low (-196 mgal) over the Eastern
Venezuela basin and relative minimums over the coastal mountains.
A minimum associated with the Venezuelan Andes is
shifted to the northwest of the topographic axis and lies
over the flank of the Andes and part of the Maracaibo basin.
Using the gravity data, structural sections were constructed
for a series of profiles across the Venezuelan Andes and
Caribbean mountains. They show that there is no light crustal
root under the Andes, the relative mass excess is as much as
600 kg/cm2, and that there is an excess of low density material
under the Maracaibo basin. This appears to be caused by a
combination of a southeastward dipping shear zone in the
lithosphere under the basin-mountain boundary and a component
of compressive stress perpendicular to this zone, both of
which have resulted in the uplift of the crust under the Andes,
and downwarp under the basin. The apparent flexural rigidity
of the lithosphere under the Maracaibo basin is 0.6 x 1023
newton-m, a normal value for lithosphere deformations of
The Caribbean mountains have a light crustal root which
has been formed by the sliding of blocks of crustal material
from the north over the rocks to the south, and perhaps by the
underthrusting of oceanic crust under the continental crust.
This underthrusting may have been a result of the formation of
a downgoing slab of lithosphere along the Venezuelan continental
margin during the late Cretaceous. The downgoing slab may have
existed until mid-Eocene time. The gravity minimum over the
Eastern Venezuela basin is due to the downwarping of lighter
crustal material into the higher density mantle. This may be
a result of compression from the north along a north-south
direction causing plastic downbuckling of the lithosphere.
The present deformation along the northern boundary appears to
be due to differences in relative motion between the North
and South American plates.
Because the Caribbean mountains are partially isostatically
compensated, while the Venezuelan Andes are above isostatic
equilibrium, this suggests that the relative motion
of the Caribbean plate with respect to the South American
plate is eastward. The compressive stress across the boundary
in the region of the Venezuelan Andes is probably greater than
the compressive stress across the Caribbean mountains.