Making mountains lecture 9 10.12.18 (Mantle plumes (Mantle plumes create…
Making mountains lecture 9 10.12.18
Mantle plumes underneath continents generate BASALTS and RHYOLITE (where the heat melts the lower crust).
Mantle plumes create topography in 2 ways;
The hotter plume material is less dense than surrounding mantle and it therefore rises.
This topography is TRANSIENT (when the plume cools and the upward flow stops, the topography disappears).
Magmatism may add new crust (under-plating); the crust becomes thicker and topography is created
Mantle plumes impinging on the lithosphere create uplift, but very little deformation (shallow, weak earthquakes), but they may create strong volcanic activity.
Surface uplift may be the only evidence for an impinging mantle plume.
Rifted margins and escarpments
Occurs when a continental lithosphere is extended and thinned to the point where it breaks apart, leading ultimately to sea floor spreading and production of new oceanic lithosphere.
The new continental margins formed in this way are called rifted margins, or passive margins (because there is no active orogenesis (event that leads to deformation and compositional differentiation of the lithosphere at convergent boundaries)).
These passive margins can be either high elevation (mountains running parallel to and not far inland from the coast) e.g. the eastern margin of Brazil or low elevation.
The high topography along the eastern margin is a mountainous remnant from an orogenic mountain building phase (collisional convergent margin) which has subsequently been rifted along one edge when the Tasman Sea (oceanic) opened around 80 Mya.
South Eastern Australia has high elevation passive margin. Again, not high low relief plateau inland and steep escarpment (steep slope that forms as an effect of faulting or erosion and separates 2 relatively leveled areas having different elevations) zone.
Africa- one of the most studied passive margin escarpment zones is the Drakensberg Escarpment in SE South Africa.
The continental lithosphere breaks as a combination of uplift over plumes and extension (pulling apart) as a result of divergent plate tectonic movements.
From plumes to rifting-
The plume causes uplift and stretches over a "dome"- which may be thousands of km wide.
The stretching causes thinning of the continental lithosphere.
The lithosphere under extension may break, forming a series of normal faults with subsidence in the rift axis.
The result is a rift basin with fault scarp (steep bank or slope) escarpments along its margin.
In the shallow crust the breaking of crust occurs by faulting (brittle failure) because at shallow levels and low temperatures rock behaves as brittle solids.
At a larger lithospheric scale the lithosphere behaves in a ductile fashion, because rocks at high temperature and pressure deform plastically, like chewing gum being stretched. Where stretching leads to thinning of the crust the isostatic balance leads to subsidence (mean density of lithosphere increases because there is thinner low density crust). At the flanks of margins of the rift, where crust has not been thinned but the mantle part of the lithosphere has been thinned we get uplift because the mean density has decreased. Thi effect is enhanced by heat flow into the margins from the hotter mantle that rises to fill the space beneath the thinned lithosphere.