Flexure Exercise
November 6, 2008
" Universal solution caused by flexure (Figure 4.8)
That portion of the lithosphere that is laterally strong (not like the Airy hypothesis) is
known as a area in regional isostasy versus a region in local isostasy (Airy or Pratt)
The maximum depression increases with the size of the maximum load and as the
effective elastic plate becomes weaker. The width of the basin decreases as the
lithosphere becomes weak. Typical rifted margins may have a 10 km effective elastic
thickness. A passive margin that is thermally about 100 Ma may have an effective elastic
thickness of 25 km typically, whereas an old Paleozoic margin may have a 80-100 km
effective elastic lithosphere.
…The wavelength is greater as the elastic lithosphere is stronger.
…The amplitude of the bulge is lower as the elastic lithosphere is weaker. "
Question:
For the loads shown calculate what the Teffective is?
Two examples:


Appalachians (50 points) (See Fig. 4.9)
Mariana Trench (40 points) (See Fig. 4.3)
Hint: Place an end-load that has the same value as the total distributed load.
Try to match only the width of the basin and overall depth.
What Teff and load do you calculate? Does this make sense with the above notes?
Discuss briefly (10 points)
There is one thing to remember. The program calculates the deflection for a semi-infinite
plate. If the plate is instead a continuous plate you will need to put at least only half the
load to achieve a similar deflection.
Bonus: Use the basement contour data to calculate another Teff and load for the case of
the Appalachians
Due November 13, 2008