subsidence and uplift

subsidence and uplift The terms ‘subsidence’ and ‘uplift’ refer, respectively, to descent and elevation of the Earth's surface relative to the geoid. Rates of vertical movement across the Earth's surface vary from 100 mm per year to 10 m per Ma, the greatest rates being recorded from the plate margins, where the driving forces are at their strongest. Three main processes are essential to the understanding of mechanisms of large-scale subsidence and uplift: thermal perturbations of the lithosphere, isostasy (mass balancing), and flexure.

Perturbations of the normal temperature field of the crust (about 30 °C temperature increase per kilometre depth) cause changes in its buoyancy, and uplift or subsidence at the surface is the direct result. Imagine that a volume of crust is stretched, and therefore thinned, resulting in the bunching together of the isotherms and a consequent increase in geothermal gradient. The immediate effect of an extensional phase will be for the thinner, and therefore hotter and more buoyant, crust to undergo doming, the centre of the dome being sited over the most attenuated crust. Thermal uplift occurs today in the Afar region of Ethiopia, where extension at the junction of the Red Sea and the East Africa rift system has produced a domed region approximately 1100 km in diameter.

Conduction of heat between the heated lithosphere and its adjacent thermally normal lithosphere will gradually return the lithosphere to thermal equilibrium. This will cause the uplifted region to subside thermally, resulting in a sedimentary basin located above the locus of thermal uplift. The phase of heating during initial thermal uplift, and subsequent subsidence and burial, is responsible for creating the necessary conditions of heat and temperature that led to the expulsion, and migration into suitable reservoir rocks, of most North Sea oil.

Lithospheric extension generates thinned crust which is lighter than the mantle it replaces. Consequently, the mass of a column through part of the lithosphere which under-went stretching will be greater than the mass of a similar column through an adjacent region whose thickness remains unchanged. The thinned crust will subside isostatically until the mass of the two columns overlying a notional equilibrium depth is equal. Conversely, the central parts of most mountain ranges comprise rocks which, although now exposed at the surface, formed at great depths, often as deep as the base of the crust itself. An appreciation of the concept of isostasy allows us to see how an initial episode of uplift and erosion will lead to more isostatic uplift, and hence more erosion. Continuation of this process over several millions of years will bring deep-seated rocks to the surface in a process known as exhumation. For example, at the foot of Nanga Parbat, in the Pakistan Karakoram, rocks which were formed about 2Ma ago at a depth of about 35 km are today exposed at the surface, thereby giving a time-averaged exhumation rate of nearly 20 mm per year.

The stiffness of the lithosphere means that it can partially support the extra mass represented by a load such as a mountain belt or volcano. Volcanos are a special case because they constitute a point load, rather than the more common line loads represented by linear features such as mountain belts. Studies of ocean-floor bathymetry around Hawaii, a major active volcanic complex, reveal an oceanic ‘moat’ recording flexural subsidence in response to the extra mass represented by the weight of the volcano.

Jonathan P. Turner