Last month's Geophysical Corner
outlined some of the principles and methods of kinematic analysis
as a means of better deciphering the structural history of basins.
In this second of our four-article series, we will apply some of the
principles of kinematic analysis to the first of our example areas:
the northern Andes of Colombia and western Venezuela.
We also will illustrate some
of the uses and benefits of this analysis to petroleum geology and
exploration in continental settings.
When applied to continental
areas, kinematic analysis can provide map-view palinspastic reconstructions
of deformed regions prior to the deformation(s), analogous to balancing
cross sections in the vertical plane.
Two very useful applications
of continental block kinematics for exploration are:
To allow more accurate
plotting of paleofacies for times prior to deformations.
To allow more rigorous
reassembly of continental blocks that have become separated during
rifting, thereby enhancing the understanding of the development
of hydrocarbon-bearing continental margins.
Here we show a set of simple
steps for restoring the northern Andean ranges and basins for Early
Oligocene and earlier time, prior to the majority of "Andean" deformation.
Note that variations in the reconstruction will derive from applying
different numbers of steps (accuracy can be increased by accounting
for more fault motions between more blocks), and also from adjusting
various input parameters, such as magnitudes of strike-slip offset
on certain fault zones.
A reference frame is needed
to begin: In this example, Andean motions are assessed relative to
the Guyana Shield.
First, we address the relative
motion of the Maracaibo Block by assessing displacement in the Mérida
Andes, which separate the Maracaibo Block and the Shield.
1 shows the dextral offset across the Mérida Andes of the "Eocene
thrustbelt," which came to rest in the Early Oligocene, measured by
many as about 150 kilometers. In addition, shortening in the Mérida
Andes has been estimated as about 40 kilometers. Thus, in the Early
Oligocene, the Maracaibo Block lay significantly farther southwest
relative to the Shield than it does today.
2, we construct a tie line between the Shield and Maracaibo Block
by performing vector addition of the strike-slip (150 kilometers)
and thrust (40 kilometers) components. Because we wish to restore
the accrued offset (155 kilometers), we draw the tie lines opposite
to the real-life sense of fault displacements, i.e., moving back in
Having defined the Oligocene
paleoposition of Maracaibo relative to the Shield, our next concern
is the Perijá Range, deformation of which accounts for movements between
the Maracaibo Block and the Santa Marta Massif Block.
of post-Early Oligocene Perijá shortening are roughly 25 kilometers
along an azimuth of east-southeast/west-northwest, as shown by the
Perijá vector in figure 2. Thus, displacing
Santa Marta Massif to the west-northwest of Maracaibo by 25 kilometers
gives the Early Oligocene position of Santa Marta relative to both
Maracaibo Block and Guyana Shield.
Santa Marta strike-slip fault displaces the Santa Marta Massif Block
from the northern part of Colombia's Central Cordillera. Left-lateral
offset of about 110 kilometers (figures 1,
2) is believed to have occurred on this
fault zone since the Late Oligocene.
This strain is transferred
into the Eastern Cordillera along the south-southeast continuation
of the fault, where it is called the Bucaramanga Fault. Interestingly,
the Bucaramanga Fault is flanked by the high, compressive topography
of Santander Massif; this is because the Bucaramanga Fault defines
the boundary between the Central Cordillera and the Maracaibo Block,
not the Santa Marta Block.
in figure 2, the trend shown for the Bucaramanga
Fault (in orange) defines only the total strain between those blocks,
i.e. the sum of the strike-slip and orthogonal components of relative
Finally, we restore Colombia's
Guajira Block, also relative to the Santa Marta Block, by removing
about 125 kilometers of dextral shear on the Oca Fault in order to
realign the western flanks of continental basement in the two blocks
prior to fault displacement.
these simple considerations, and assuming that only minor vertical
axis rotation of these blocks has occurred during their relative motions,
we can now fill out other tie lines in the vector "nest" of figure
2 to define offsets between other pairs of blocks in the system.
For example, the total strain
in the Eastern Cordillera since the Oligocene is seen to be roughly
200 kilometers toward the east-southeast (red tie line). This can
then be broken down into components of orthogonal and strike-parallel
strain of 180 kilometers (blue line) and 100 kilometers (green line),
respectively, which translates geologically into shortening (180 kilometers)
and dextral shear (100 kilometers), moving forward in time.
that this value of shortening (180 kilometers) falls in the middle
of the range of published values of estimated shortening in Eastern
Cordillera. Thus, vector nests such as figure
2 can be used to help choose between alternative balanced cross
section models assessing shortening, because different assumptions
of depths to detachments or degrees of basement involvement produce
very different modeled shortening values.
In addition, it also allows
detection and estimation of the strike-slip component, which usually
cannot be seen in cross sections. Our inferred dextral shear in the
Eastern Cordillera is supported by seismicity, GPS data and field
(i.e., pre-Late Oligocene) palinspastic reconstruction of the northern
Andes continental region (figure 3) now
can be made by restoring the motions of the blocks defined in figure
limit of pre-Mesozoic continental crust has been identified in figure
3 to show the pre-Andean geometry of the northern Andes "autochthon,"
to which a number of oceanic terranes have been accreted in the Cenozoic.
Additional information can now be added to better focus the picture.
We can, for example, draw
the occurrence of Eocene formations, sedimentary facies and paleoenvironments
on our reconstruction in order to build palinspastically accurate
models of regional Eocene depositional systems. This practice also
allows better sequence stratigraphic interpretation and correlation
at the regional scale, which is helpful to determining migration pathways
through the strata.
Also, the depositional models
can be compared more meaningfully to modern analogues and analyzed
for implications concerning reservoir potential, such as sand body
orientation, sinuosity, flow direction, sand grain provenance and
Finally, the reconstruction
also allows a better interpretation of Cretaceous source rock character,
quality and original areal extent.
Using the same block/plate
restoration technique, we can depict Eocene-aged structures and the
Eocene position of the Caribbean Plate relative to South America,
to better understand the driving forces of Eocene sedimentation patterns
4 thus shows the Caribbean Plate driving an Eocene foredeep basin
in the northern Maracaibo area -- much like today's Persian Gulf --
which caused an important early hydrocarbon maturation event in western
Venezuela and Colombia's Cesar Basin.
4 also shows depositional systems with important reservoir facies
belts at the Middle to Late Eocene boundary, as well as the existence,
continuity and origin of an Eocene "Maracaibo Tar Belt" in western
Venezuela (also recognized in Middle to Late Eocene field sections).
The concept of this "textbook"
foredeep basin for the Eocene of Maracaibo Basin had remained darkly
veiled for decades by today's grossly different geography.
Next month, we will use plate
kinematics to reconstruct Africa and South America, and to progressively
close the Atlantic Ocean during Mesozoic times, in order to set the
stage for tracing the evolution of the Gulf of Mexico and the Florida/Bahamas
region in our fourth article of the series.