Kinematic Analysis – Taking the First Step
Kinematic analysis is the study of the motions behind geological change, at scales from mineral grain to continental block or tectonic plate. Although traditionally associated with the oceans, it can be applied successfully to areas of continental crust and transitional margins of exploration interest. The ways in which plates move relative to each other are governed by a firm set of predictive geometric rules, giving us the power to open and close oceans and evolve plate circuits quantitatively. However, all too often, these kinematic rules are not applied or are misapplied, and the ‘Plate Tectonic Interpretation’ sections of many articles are correctly seen as unreliable ‘arm-waving’. Industry decision-makers grew suspicious of such interpretations, often ignoring or discounting them, unaware of potential benefits of robust kinematic analysis.
Simple principles provide the basis for exploring the power of kinematic analysis. Figure 1a shows a two- plate system in which block A moves north-north-east relative to B. Relative displacement is shown by the red vector between dots representing the plates. We can use the blue vector to retract the measured offset. In a three-plate system, we consider the motions between three pairs of plates. As an analogy, consider two runners, A and B, running from home plate to first and third base on a baseball diamond (see Figure 1b). The displacement between home plate and runners A and B is north-east and north-west, but relative motion between the runners is east-west. A plate boundary separating plates represented by the two runners would be extending east-west. In a three-plate example (see Figure 1c), we restore two known offsets (A–C) and (A–B) to determine the unknown offset between the third plate pair (B–C). Tieline B–C shows the net direction (north-east) and displacement (76km) of the B–C plate boundary. If this is a thrust belt with the orientation as shown, then the dextral strike-slip (blue, 30km) and convergent (red, 70km) components of net motion can be calculated easily, providing vital information about a hybrid structural style. Finally, in the larger two-plate example of Figure 1d, 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 plate separation, and transforms lie on small circles. The rate of plate separation and of transform displacement increases with distance from the pole of rotation and transforms become straighter. The mathematics of plate motion on the surface of a sphere is not much more complex than the ‘flat earth’ examples shown.
Figure 1
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