Discussion and Conclusions
The reconstructions reviewed here are a representative sample of the historical literature on the development of the fit of South America and Africa. The ideas of Wegener and Du Toit have evolved beyond simple “coastline matching,” and today the best model is provided by using gravity data to locate the continent-ocean boundary, marine magnetic anomalies and transforms as constraints on ages and magnitudes of rotation, and incorporating intra-plate deformation and rifting. Royer et al. (1992) provided the most detailed model.
Previous rigid-plate reconstructions were presented and compared with deformable-mesh reconstructions using Pplates. The areas of deformation were tightly restricted to a few areas of known extensional zones, though the user per his knowledge or hypotheses can change this. Applying crustal volume conservation and isostasy result in elevation changes in the deformed zones. Although the information displayed is generally qualitative, we could expect an elevation increase to correspond to the depth to the basement of the sediment in the modern-day extensional basins when realistic deformation regions are defined.
A special note must be taken with deformable plates: representing the movement of an entire plate with a single Euler rotation is inadequate. To continue to use Euler rotations, it is necessary to specify the two regions of the earth linked by the measurement of the rotation. These regions should be as small as possibly allowed by the geology and geophysics of the measurement itself. For example, if we wished to use an Euler rotation to represent, say, the movement of a craton in southern South America relative to a craton in NW Africa, we would not pressume that it represents the movement of regions larger than the cratons involved without further evidence. As more movement data accumulates for different areas of each continent/plate (relative to one another or to a common reference frame), then deforming the continent as done by Pplates becomes more attractive than creating ever-smaller sub-plates. There is an added advantage in that the deformed regions depicted by Pplates can be made to conform to regions of known geological deformation, if desired, thus creating more geologically-constrained reconstruction scenarios.
Further development of Pplates will refine and extend the use of meshes as a device for simulating the three-dimensional movements of both surface and subducted lithosphere in tectonic reconstructions. With the formalism discussed in this paper, Pplates is able to calculate the crustal/lithospheric strain induced by the deformations. Also presently being incorporated into this software is a model of mesh faces that includes stress (forces) between mesh nodes. The model incorporates elasticity (Hooke’s Law) and can also include viscosity. Thus it is possible to introduce stresses in the mesh via deformations (strain) and distribute stress and strain throughout the mesh over time.
Since this pseudo rheology is only the first instance of a more general introduction of constraints, Pplates will increasingly become a simulator rather than a kinematic visual reconstruction aid. Simulation will include constraining movements using both geological data and the adaptation of rules based on the geophysics of lithospheric deformation and tectonic processes on a global scale. The latter will be drawn both from observations and the insight gained from forward modelling using high-performance computer (HPC) codes. Observations now include meshes built from seismic tomography images of subducted slabs which, when included in reconstructions with appropriate pseudo rheology, will constrain hypothetical reversals of subduction zone processes. Further to this, we plan to use Pplates as an interface to such HPC forward-modelling capability that can model the subduction in the time-forward direction and provide plausibility to a hypothetical reconstruction of an initial subduction zone of the ancient earth.
By developing and applying such technology, we hope to provide valuable contributions to the modelling and reconstruction of Gondwana, in particular, and especially the Arica Bend of South America including the development of sub-Andean basins and the evolution of the East African Rift system. This will give a more comprehensive reconstruction of the break up of Gondwana.