Anyone who has watched a watchmaker at work may well have felt a tinge of panic at the sheer tininess of the screws, sprockets and gears, and awe at the near-superhuman patience and concentration involved in such micro-engineering. Developments in geodesy based on the Global Positioning System of navigational satellites push towards such aching precision. The fixed stations of the International GPS Service (IGS) measure geographic position and topographic elevation to within less than a millimetre. Corrections for known plate motions and Earth tides reveal motions that must be due to other forces.
Ultimately, the forces shaping the Earth’s surface are gravitational, and thus reduce to shifts of mass within and upon our planet. By far the most rapid movements of matter are those involving the atmosphere and the water vapour that it carries. Through variations in atmospheric density and the mass of water residing in soil moisture and snow cover, which arises from varying precipitation, surface load changes on an annual cycle. Meteorological and remote sensing estimates of these loads allow geophysicists to model the elastic response of the surface to the seasons. Why they have done this is not abundantly clear to me, but starting position is essential to astronavigation, hence similar attention to the Chandler Wobble (see Atmosphere linked to Earth’s rotation, Earth Pages, September 2000). Anyhow, the records suggest an annual mass transfer from hemisphere to hemisphere of around 1013 tonnes, which is sufficient to cause elastic deformation within the scope of GPS measurements. Geomaticians from the universities of Nevada and Newcastle upon Tyne (Blewitt, G. et al. 2001. A new global mode of Earth deformation: seasonal cycle detected. Science, v. 294, p. 2342-2345) have been able to chart the actual motions over the period from 1996 to mid-2001.
Performing the necessary computations on the weekly data from 66 IGS stations, and fitting curves to the results, Blewitt et al. present convincingly repetitive cycles in the motions towards the intersection of the Greenwich meridian and the Equator, towards the North Pole, and perpendicular to the surface. These tie very well to the theoretical model. Interestingly, they were able to model the shifts of displacement globally, and in series of maps show that the positions of maximum displacement shift along a path linking the continents. That is not surprising in itself, for the oceans respond by changes in water level, and only exposed continental lithosphere is likely to flex. The poles sink by around 3 mm each winter, and the Equator swings towards the winter pole by 1.5 mm. Results tally extremely well with estimates of seasonal mass shifts and theory. The surprises include an anomaly in vertical displacement in 1996-7 preceding the 1997-8 El Niño event, probably due to changes in Pacific sea level driven by winds and anomalous monsoon precipitation.
Yet more on tectonics of the Tibetan Plateau
In the previous issue of Earth Pages was a resumé of a paper in Science that discusses the lateral tectonic motions that result from India’s collision with Eurasia (Continental tectonics of eastern Eurasia December 2001 Earth Pages)). The compliment to that appeared in the 22 November 2001 issue of Science (Tapponnier et al. 2001. Oblique stepwise rise and growth of the Tibet Plateau. Science, v. 294, p. 1671-1677). How and when the India-Eurasia collision zone achieved its pattern of huge elevated masses is partly an issue of tectonics, but they bear on any climatic effect that changed elevations might have had on climate, both regionally in the case of the South Asian monsoon circulation, and globally (one view is that Tibet’s deflection of atmospheric circulation may have been an important trigger for the onset of northern hemisphere glacial conditions).
Many geologists have considered the whole lithosphere of the region to have behaved in a ductile manner during collision, so that shortening and thickening were distributed more or less evenly. They ascribe the uniform height (>5 000 metres) to gravitational rebound when part of the thickened lithosphere detached and fell into the mantle, around the mid-Miocene. Erosion being unable to keep pace with uplift, the Tibetan Plateau is then thought to have become unstable and started to collapse laterally. That is seen by many as an explanation for clear evidence of E-W extension from both numerous N-S rift systems and extensional first motions on Tibetan earthquakes. However, this vast area is clearly subdivided into several major blocks by large strike-slip systems. The prevailing notion is that these faults are effects of “soft” collisional tectonics. Tapponier et al. assemble detailed evidence in relation to these faults and the blocks that they bound. They support tectonic evolution which has been controlled by coherent blocks of lithosphere, a process which was episodic rather than continuous, and accompanied by decoupling of crust and mantle lithosphere.
The linchpin of their model is the diachronous calc-alkaline magmatism of the region during the Tertiary, which becomes younger towards the north. As well as the principal site of northward subduction of Indian lithosphere beneath the Zangbo Suture, they propose that this magmatism was related to southward subduction that migrated northwards, and is hidden by thickened crust. The huge strike-slip systems are, to Tapponier et al., nothing less that oblique suture zones. The crustal blocks that they separate are, according to their model, large thrust wedges founded on major crustal detachments that accomplished most of the shortening. The process did not involve destruction of oceanic basins, but subduction of sub-continental mantle lithosphere, when crust and mantle became detached. Each successive subduction-accretion episode added its own increment to surface uplift, there probably having been three major steps in creating the highest average topography on Earth.