This month's column, the first of a four-part series, is titled "Regional Plate Kinematics: Arm Waving, or Underutilized Exploration Tool?"
By JAMES PINDELL
The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by M. Ray Thomasson.
Editor's note: Pindell, Kennan and Barrett are with Tectonic Analysis, in Sussex, England, and Houston, Texas. Pindell is also adjunct associate professor at Rice University, Houston.
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 developed.
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 developed.
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 models.
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.
In Figure 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 the plates.
To palinspastically restore the offset back in time, we would use the blue vector to retract the accrued measured offset.
Progressing 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 fault displacements.
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 thrust belt.
Finally, in 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 exploration strategy.