Climatically Steered Tectonics

Bridgeland and Westaway showed that climatic changes can indirectly through isostasy and subcrustal flow have a considerable impact on tectonic situations, well beyond the extension of the isostatic load.

For the Tibet-Plateau this has been calculated as well: Uplift of the plateau was slowed and later depressed by the progressive increase in ice thickness. Each depression was later reversed by interglacial deglaciation. According to this model the average Pleistocene level of Tibet is unlikely to have changed since the beginning of the late Neogene ice ages (Kuhle 1993).

This theoretical deduction is confirmed by field observations. There are ice margins from the glaciations older than the LGP at eleven localities in High Asia. These lie at approximately 150-300 m lower than the LGP-age moraines in NW and NE Tibet (Kuhle 1988e, 1987a) (Fig.1, Nos.16, 45-49). This equates to the 100-200 m difference between the equilibrium lines of the second but last glaciation (Riß ca. 150 000- 120 000 YBP) and the LGP glaciations (Würm ca. 60 000 - 18 000 YBP), known from tectonically less active areas as in Europe.

Two alternatives are possible to explain the observations:

4.7.1. Neogene and Pleistocene Uplift as in the Holocene

In a region of continuing uplift, as Tibet is suspected to be, these older ice marginal positions would have been uplifted at a rate of about 3 mm/yr or about 390-420 m during the last 130-140 ka prior to the LGP if the boundary conditions that established the respective ELA (moisture availability, temperature, firn accumulation) had been the same in different ice-ages.

The preservation of ice marginal positions from the second but last ice-age shows that either the uplift at that time was slower, or the boundary conditions (such as cold vs. warm glaciers) had been different. If the boundary-conditions would have been the same as during the LGP and if the uplift would also have been the same, the old moraines left over from the second but last glacial would have been overridden and destroyed. The reason is that the uplift of approximal 400 m exceeds the normal vertical distance from moraines of the seond but last glacial to those from the LGP. Even if the equilibrium line had run out at a greater altitude, the younger glaciers would have flowed at least 400 m further down than the glaciers of the second but last glacial.

4.7.2. Pronounced Uplift since the Holocene

The other possibility is that in connection with a high mantle-viscosity the short Pleistocene interglacials did not allow for isostatic adjustion through uplift. In this case the Tibet-Plateau would have stayed in a depressed low position. Different positions of ice-margins would reflect overall different glacial advances. Only since the Holocene long and uninterrupted uplift occurs. This interprets the current high uplift rate as partially or nearly fully isostatic.

Was this article helpful?

0 0

Post a comment