1. Principles of Global Tectonics
Our Dynamic Earth
The configuration of the continents on our globe has been steadily changing over geological time. Today’s familiar world map of continents and oceans is just a snapshot of a continuous process of movement that has been going on for thousands of millions of years. Rates of movement are typically only a few centimetres per year but these are frequently bought to our attention by the occurrence of the earthquakes and volcanoes that, for the most part, are confined to narrow marginal zones that separate the present ten large tectonic plates.
During much of Phanerozoic time, i.e. from about 540 Ma (million years ago) when the Precambrian came to an end, the present southern continents made up a single stable ‘supercontinent’ known as Gondwana. From about 180 Ma, however, this entity started to rift and fracture and new oceans began to form between the resulting fragments. These oceans – the South Atlantic, Southern and Indian Oceans – now occupy almost half of the world’s surface area. It is only relatively recently (1960s) that earth scientists have realized that it is the growth – and eventual consumption – of such oceans that is central to the prolonged evolution of the earth’s crust through global tectonics. Ocean crust itself is relatively short-lived; no substantial areas of ocean crust exist today that are much older than the start of Gondwana disruption. The lighter, more buoyant continents, by contrast, contain the geological record that goes back to the oldest known rocks, in excess of 4000 million years (4 Ga) in age. It was only even more recently (late 1990s) that technology could map in detail the topography of the ocean floor, from which the precise pattern of its creation with time could be worked out in considerable detail (Smith and Sandwell, 1997).
Ridges, transforms and the growth of oceans
New crustal material is generated at mid-ocean ridges, often called accretionary margins, where the two adjacent plates are pushed apart, making room for new crustal material to be emplaced between them. A ridge is made up of rift sections, where the sides move apart normal to the length of the rift, and transforms where rift-offsets are joined by faults perpendicular to the rift sections across which relative movement is strike-slip. Remarkably, the pattern of rift sections and transform offsets making up a complete ridge is seen to be almost perfectly self-perpetuating. As a result, even those parts of transforms that are no longer seismically active are often continuous for thousands of kilometers either side of the present mid-ocean ridge, sometimes even extending as far as the conjugate shore lines. Maintaining the two parts of such features coincident and collinear during their development is a powerful constraint on modeling the relative movements of any pair of continental fragments, working backwards in time from the present situation.
The geomagnetic time scale
The earth’s magnetic field as a whole has reversed its polarity at irregular – and often quite frequent – intervals over geological time. Magnetisations of opposite polarity in the newly-created ocean crust therefore follow each other in the ocean crust, a distinctive pattern of stripes, something like a bar-code, appearing mirror-imaged either side of the mid ocean ridge. These oceanic magnetic anomalies allow the whole process of ocean creation to be time-calibrated. Unfortunately, the Cretaceous Quiet Zone (or Normal Superchron, KNS) that is devoid of such reversals occupies about 20 per cent of the time period of interest for Gondwana dispersal and the Indian Ocean, particularly, is only sparsely covered with the marine magnetometer tracks that can map these features.
Economy of Hypothesis
Incomplete datasets are commonplace in earth science. Where no evidence to the contrary exists, it has been assumed here that continental movements remain steady over long periods. In building animations, this means that interpolation across gaps in the data can be made. The smoothness of the modeled continental movements is then, in itself, a constraint on unnecessary or unjustified hypothesis. A further constraint is that the destruction of oceanic crust, once formed, appears today to be confined to a limited number of subduction zones globally. This implies that there is a considerable threshold to be overcome before ocean crust, once created, can be consumed back into the earth’s mantle. This is an additional constraint when trying to model the relative movements of the fragments of Gondwana since activities at the plate margins predicted by the model should then always be either constructive (ridges) or conservative (transforms), never consumptive (subduction zones). Moving progressively backwards in time from the present day situation (the only certainty!) and removing older and older ocean has led to a model of Gondwana dispersal that agrees with all data presently to hand and is subject to refinement (within the limits of error!) as new data become available. The main features of the model as at mid 2009 are described by Reeves (2009).
Role of Mantle Plumes
The movements of continents recorded in the ocean crust are relative movements that may, of course, be chained from one continent to the next. Relating this whole system to a fixed rotational axis for the planet has been achieved using what is known as the Hotspot Reference Frame. A number of plumes of warmer material from within the earth’s mantle – probably even arising at first from the surface of the molten core – are recognised to have erupted over the time interval that covers Gondwana dispersal. They may even be responsible for the instigation of this dispersal. All the evidence suggests that, within a few tens of kilometers, these plumes have been fixed in their relative position and with respect to the earth’s rotational axis. When a mantle plume ‘strikes’, its initial impact tends to be quite abrupt and often gives rise to copious amounts of basalt and dyke injection in continental areas (such as the Deccan Traps in India and the Parana basalts in Brazil). But by far the larger part of the output is generally piled onto the ocean floor at later stages, however. This is relatively easy to achieve once two continents have been split apart and large (now) submarine plateaus are created over time (e.g. Kerguelen). The activity of the plume declines over many tens of millions of years, but most are still in evidence in the form of small oceanic island volcanoes such as Reunion and Bouvet. Watch the animation and see if you can find any relation between the break-up of Gondwana and the impact of the plumes…
Reeves, C.V., 2009. Re-examining the evidence from plate-tectonics for the initiation of Africa’s passive margins. Geological Society of Houston/Petroleum Exploration Society of Great Britain, London, 2009 September 9-10
Smith, W.H.F. and Sandwell, D., 1997. Measured and estimated seafloor topography (version 4.2), World Data Center A for Marine Geology and Geophysics research publication RP-1, poster 34" x 53".
2010 April 28