Problems
The following are examples of some simple numerical problems in Geophysical Processes in
Sedimentary Basin Formation. Most can be solved using a hand calculator. However, you might
find it easier to solve others using MathCad.
Unless otherwise stated, assume that the densities of water, sediment, crust and mantle are 1030,
2400, 2800 and 3300 kg m-3 respectively.
1. The melting of an ice sheet results in an increase of water depth on a continental shelf of
100m. What is the maximum thickness of sediments that could accumulate on the shelf?
2. The total tectonic subsidence determined from backstripping in a deep well is 5 km.
Assuming Airy isostasy, estimate a) the thickness of crust, and b) the depth to Moho beneath
the well.
3. During the late Pleistocene, a 300 m deep lake (Lake Bonneville) occupied what is now Salt
Lake Valley in Nevada. Assuming Airy isostasy, calculate the uplift of the crust that would
be expected following the draining of the lake. The observed uplift based on shoreline data is
35 m. Explain.
4. Use the " slab formula" to estimate the the gravity anomaly that you would expect to see over
a 5 km deep sedimentary basin that formed by a) thinning of the crust due to extension at the
time of rifting, b) flexure of the crust due to migrating thrust and fold loads, and c) lateral
displacements of the crust due to strike-slip movements ?
5. The gravity anomaly, g, associated with a rectangular load of half-width m and its
compensation is given by: g = 2GhL(e-m cosm) where  = flexural parameter, h = load
height, G = gravitational constant, L = density of load. Determine the width that a load
emplaced on an elastic plate with Te = 30 km needs to be in order for it to appear fully
compensated.
Click here for a MathCad answer sheet
6. Oceanic Te is given approximately by the depth to the 450 oC isotherm based on the cooling
plate model. Assuming a thermal thickness, thermal diffusivity and, mantle temperature of
125 km, 0.8  10-6 m2 s-1 and 1333 oC respectively, estimate the Te that would be expected for
a) a 5 Myr old load emplaced on 15 Ma old oceanic crust, and b) a 80 My old load emplaced
on 160 Ma old oceanic crust.
The maximum flexure, ymax, beneath a load of density L, height, h and half-width, m, is given
by: ymax = hL(1 - e-m cosm)/(m - L) where  = flexural parameter and m = density of
mantle. Assuming m = 75 km, h = 5 km, L = 2700 kg m-3 and, m = 3300 kg m-3 calculate the
maximum flexure beneath the two loads in a) and b) above.
Click here for a Mathcad answer sheet