An unfortunate fact of
geology is that most datasets, including seismic, rarely allow for a unique
interpretation of a geological problem.
Having to wrestle with multiple
working hypotheses is perhaps especially common in the structural arena,
where one or another of many theoretical models of "ideal" crustal deformation
can be made to fit a given structural pattern. This can be frustrating
and potentially costly if the optimum exploration strategy is dependent
upon the interpretation finally chosen.
Integration of multiple and diverse
data sets is one popular approach to reducing the range of possible interpretations,
the goal being to minimize exploration risk.
But on too many occasions, if
your "best" data set can't give you a clear solution, then mixing in diverse
secondary data sets can muddle the picture even further.
Worse, this multifaceted picture
may not be fully understood by anyone on the work team, and the full implications
of the "integrated solution," which will provide the basis of the exploration
model, might never be recognized.
It is widely recognized that broadening
the scale of geological assessment to beyond the limits of the block or
field can help to constrain a unique solution to a given problem. Indeed,
many plate tectonic and structural processes evolve over scales far larger
than most blocks, and to ignore the larger scale can lead to serious misinterpretations.
But broadening the scale of examination
to beyond the block remains, in many cases throughout industry, little
more than a matter of describing what is out there. In other words, mapping.
As geologists, we all know that
mapping is a key part of geology, but it is very important to take the
next step and understand how and why a given set of mapped structures
Can this help to resolve our interpretation
of geological problems?
Can it tell us anything more about
an exploration play?
Can it trigger the identification
of new plays altogether?
We believe it can.
When we shift from trying to address
the "what" questions of structural analysis into the "how" questions,
we move from static description into time-progressive kinematic analysis.
Kinematic analysis can be performed
at all scales in geology -- from mineral grains to tectonic plates --
and it embraces the motions of material undergoing geological change.
Defining the motions of the plates and crustal blocks, where possible,
can tremendously facilitate understanding how certain types of structures
Plate kinematics addresses the
history of motion of the plates and blocks that comprise or have comprised
the earth's surface. Although plate kinematics is traditionally associated
with the oceans, it also can be applied successfully to areas of continental
crust and margins of real exploration interest.
In the late 1960s, one of the
most exciting early realizations of the plate tectonic revolution was
that the ways in which plates move relative to each other, both past and
present, are governed by a firm set of predictive, or retrodictive, geometric
rules. Plate kinematics gave us the power to quantitatively open and close
oceans, collide continents and evolve plate circuits in area-balanced
Earth's geological history became
an intellectual playground for "plate pushers" who began to decipher Earth's
global tectonic evolution.
However, all too often, these
kinematic rules were either not applied, misapplied or applied to inappropriate
places, such that by 1980 many journal articles, no matter what the discipline,
ended with "bandwagon" Plate Tectonic Interpretation sections, which correctly
came to be viewed as mere arm waving.
Similarly, industry decision-makers
grew to be suspicious of such tectonic scenarios -- with good reason --
and often ignored or discounted them. Thus, the potential of kinematic
analysis often was never reached.
Sadly, these very powerful rules
are no longer even taught in many universities, and quantitative plate
kinematic analysis is becoming something of a lost art.
Very powerful plate kinematic
rules, however, do still exist.
Here, in this first in a series
of three articles, we review some of these principles to provide the basis
for exploring the power of kinematic analysis.
1a, we show a simple two-plate system in which block A moves NNE relative
to B with time. Displacement during the particular time interval of concern
can be drawn as shown by the red vector between the dots representing
To palinspastically restore the
offset back in time, we would use the blue vector to retract the accrued
to a three-plate system, we must consider the motions between the three
pairs of plates. A simple analogy of this situation is to consider, in
Figure 1b, two runners, A and B, running from
home plate to first and third base on a baseball diamond.
The displacement between home
plate and runners A and B, respectively, is NE and NW, but the relative
motion between the two runners is east-west. A plate boundary separating
plates represented by the two runners would be extensional, with net E-W
In the three-plate
example of Figure 1c, we can restore, moving
back in time, two known offsets (A-C) and (A-B) to determine the unknown
offset between the third plate pair (B-C). The measured directions and
displacements of plates B and C are drawn relative to Plate A. Tieline
B-C will then approximate the net direction (NE) and displacement (76km)
of the common B-C fault zone.
If this happens to be a thrust
belt with the orientation as shown, then the strike-slip (blue, 30km)
and convergent (red, 70km) components of net motion can be inferred by
construction of the right-triangle, thereby providing vital information
about overall structural style, with the expectation of dextral transpressive
(combination of strike-slip plus compression) strain partitioning at that
the larger two-plate example of Figure 2,
plates A and B diverge by seafloor spreading at the ridge (red) and transcurrent
motions at the transform faults (green). The continuations of the transforms
into adjacent oceanic crust are fracture zones where differential thermal
subsidence occurs, but without active strike-slip faulting. Ridge segments
lie on great circles to the pole defining the plate separation, whereas
the transforms lie on small circles.
The rate of plate separation and
also of transcurrent displacement at the transforms increases with distance
from the pole. Transforms also become straighter as distance increases
from the pole of rotation.
Subsequent articles in this series
will apply these principles to two well-known oil provinces, Colombia/western
Venezuela and the Gulf of Mexico, showing how formal kinematic analysis
can offer some of the most sound constraints available to guide and to
favor certain geological interpretations over others.
Further, it can provide the basis
for defining or rejecting play concepts, therefore strongly influencing