Tectonics

tectonic1
tectonic1

The figure on the right presents a standard reconstruction of Gondwana, adapted from DeWit et al. (1988), Golonka et al. (1994) and Lawver et al. (1999). The truncated icosahedron comprises a semi-regular polyhedron made up of 12 pentagonal and 20 hexagonal faces. The pentagonal faces are centered on the vertices of an icosahedron. The tessellation is geometrically intolerant, i.e., a single pentagon fixes the entire configuration. Projected onto the Earth’s surface, each tile-edge of a truncated icosahedron subtends 23.28° of arc, or approximately 2600 km.

The Gondwanan tessellation included all or parts of three pentagons and six hexagons of the exact scale and arrangement of a truncated icosahedron at the Earth’s surface. One measure of this congruence is that the distance between the centers of the Antarctic and Central South American pentagons on the reconstruction exactly equals that between icosahedral vertices (60° or about 6600 km at Earth-scale).

The Gondwanan fractures were tensional cracks. Each fracture thus propagated orthogonally to the local horizontal principal tensile stress. Construction of the principal tensile stress direction at the midpoint of each fracture generates the icosadeltahedron, the dual polyhedron of the truncated icosahedron .

The edges of Gondwana appear to have acted as free surfaces that organized the stress tessellation. Six edges of the stress tessellation coincided with the northern rim of Gondwana and parts of the Gondwanides trench margin.On the figure, heavy dashed lines are major fractures that define truncated icosahedral tessellation. Black pentagons at centers of central South America, Antarctica, and Arabia occupy vertices of exact icosahedral triangle (purple) at Earth-scale. Yellow stars are major hot spot volcanoes in modern co-ordinates (note lines of latitude and longitude). Gondwana is restored so that fracture tessellation best fits hot spot tessellation. Red areas are LIPs (Large Igneous Provinces) with eruption dates shown.