Download Variations in the structure and rheology of the lithosphere.

Variations in the structure and rheology of the lithosphere.
J. Jackson (1*)
(1) University of Cambridge, Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, UK
*Corresponding authorː [email protected]
Differences in the structure, composition and rheology of the lithosphere between the oceans,
young orogenic belts and the ancient Precambrian shields are responsible for first-order variations in
tectonic history seen at the Earth’s surface over geological time. The last decade has seen a number
of developments in the understanding of the lithosphere, some of which have challenged previously
accepted views, and opened up many new directions in research. A coherent picture is now
emerging that reconciles observations from fields as diverse as seismology, gravity, heat flow, rock
mechanics, metamorphic petrology and geochemistry.
The principal points of this new view are as follows:
1) Earthquakes in the mantle are confined to regions colder than about 600oC.
2) With very few exceptions, earthquakes everywhere are confined to a single seismogenic
layer which, in the oceans is limited by the 600oC isotherm, in young orogenic belts is
typically limited to the upper crust (~350oC), and in ancient shields may include the whole
crust (in material as hot as 600oC. An apparent exception is in the Himalaya, where the
seismogenic lower crust of India underthrusts the seismogenic upper crust of Tibet, giving a
bimodal depth distribution, but one that is not in steady-state and has no generic
significance for continental rheology.
3) Where it is well resolved, the elastic thickness is everywhere less than the seismogenic
thickness, and nowhere do the data require it to exceed the seismogenic thickness. This
observation is consistent with long-term strength residing in the seismogenic layer, and
regions of active deformation, the mantle generally plays no role in the long-term support of
loads on the continents.
4) Lateral strength changes in the continents between ancient shields and young orogenic
regions are important and cannot be represented by a laterally uniform continental
rheology. They allow mountains to be supported by their adjacent forelands without
requiring the mantle beneath the forelands to be strong.
5)
The great strength of the ancient shields, responsible for lower crustal earthquakes and
larger elastic thickness than in younger continental lithosphere, is related to the composition
of the lower continental crust, which is probably dominated by a dry granulite facies mineral
assemblage.
6) The cause of the granulite metamorphism is likely to be continental collision, resulting in
extreme crustal thickening and internal heat production, causing melting and the removal of
water by the extraction of granite. This was probably common in the Archean, when
radiogenic production was greater, but may be happening today in Tibet.
7) Many of the ancient shields of the continents are characterized by extreme lithosphere
thicknesses, which may reach 250 or more km. Surface-wave seismic tomography now
allows us to map the distribution of such regions. The stability of the thick lithosphere is
related to its chemical depletion during melting, which occurred prior to its subsequent
thickening.
8) The stability and survival of the ancient shields and cratons over geological time is thus
related to both their strength and buoyancy, neither of which can easily be changed.