Download Mountains and basins Isostasy

Mountains and basins
Isostasy
• Bouyancy -- the principle that an object
immersed in a fluid experiences an upward
force equal to the weight of the fluid
displaced
• If the object is less dense than the fluid,
then the object will float in bouyant
equilibrium
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• Lithospheric plates float in the underlying
asthenosphere
– Thick and less dense continental crust stands
higher isostatically; continental surface is
several hundred meters above sealevel
– Thin and more dense basaltic ocean crust stands
lower; top of ocean crust (the sea floor) lies
several kilometers below sealevel
• Orogenic (mountain-building) processes
tend to thicken the crust, causing it to reach
a new equilibrium, with the surface at a
higher level
– Heat -- thermal expansion inflates crust
– Thrust faulting -- multiplication of slabs of
crustal rock
– Intrusion and extrusion (volcanism) of magma
of granitic composition
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Erosion
• Erosion of mountain belts thins the crust,
disturbing isostatic equilibrium
• Plate rises to new equilibrium point, lower
than the original; this process of erosion and
isostatic uplift can continue for hundreds of
millions of years
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Collage tectonics
• Mountain belts found to contain suspect
terranes, with distinctive:
– Fossil contents
– Paleomagnetic signatures
• Direction of magnetic pole
• Inclination of paleomagnetic field
– Rock types
• Modern oceans found to contain areas of
anomalously thick crust, which might
accrete to a continental margin rather than
be subducted
– Hot spot basalt plateaus
– Extinct volcanic arcs
– Fragments of rifted continental crust
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Continental collision
• Passive margins of continents are sites of
great thickness of sediments (10-15 km)
• Closure of ocean by subduction can deform
and uplift the passive margin strata,
thrusting them onto the adjacent continent
to form a mountain belt
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Sedimentary basins
• If the continental crust is stretched
(extensional strain), it tends to thin by
faulting near the surface and by ductile flow
at depth
• Thinner crust stands lower isostatically;
with sufficient stretching it will subside
below sealevel and provide space for
accumulation of sediments
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