Geoscience Reference
In-Depth Information
6. What are the criteria for deciding whether the Earth's crust at any location is oceanic
or continental in origin?
7. The structure of the oceanic crust is very much the same irrespective of whether it
was created at a slow- or a fast-spreading ridge. Discuss why. Should we also expect
to find such worldwide lack of variation in the continental crust?
8. Calculate the lithospheric thicknesses at the ridge-transform-fault intersection shown
in Fig. 9.32. Use the lithospheric thickness-age model
z
(
t
)
11
t
1
/
2
=
with
z
in kilo-
metres and
t
in Ma.
9. (a) Draw a bathymetric profile along the north-south 10-Ma isochron of Fig. 9.32.
(b) Draw a similar profile along a north-south line crossing the continuation of the
fracture zone 500 km to the east.
10. Draw fault-plane solutions for earthquakes occurring within the active zone of the
transform fault shown in Fig. 9.32. Discuss the relative frequency of earthquakes
occurring between the two ridge segments and outside this zone. Draw a fault-plane
solution for an earthquake occurring on the fault at the 50-Ma isochron on (a) the
west and (b) the east side of the fault.
11. Calculate the topographic relief across a transform fault if the age offset is (a) 10 Ma,
(b) 20 Ma and (c) 50 Ma. Plot the relief against distance (age) from the ridge axis for
these three faults. What can you deduce about earthquakes on the inactive portions of
these faults?
12. The boundary between the African and Eurasian plates between the Azores Triple
Junction and Gibraltar runs approximately east-west. What is the nature of the bound-
ary? Draw fault-plane solutions for earthquakes occurring along it.
13. What would happen to the plates shown in Fig. 9.32 if the rotation pole were suddenly
to move so that the spreading rate remained unchanged in magnitude but altered 20
◦
in azimuth? Illustrate your answer with diagrams.
14. It is possible to make a first approximation to the temperatures along a transform fault
by assuming that the temperature at any point along the transform is the average of
the temperatures on either side. Using the cooling half-space model (Section 7.5.2),
plot 400, 600 and 800-
◦
C isotherms beneath a 20-Ma-offset transform fault. What
assumptions have you made? If the base of the plate is assumed to be the 1000-
◦
C
isotherm, how does this model agree with the depth to which faulting occurs on
transform faults?
References and bibliography
Abers, G. A. 1992. Relationship between shallow- and intermediate-depth seismicity in the
eastern Aleutian subduction zone.
Geophys. Res. Lett.
,
19
, 2019-22.
Abrams, L. J., Detrick, R. S. and Fox, P. J. 1988. Morphology and crustal structure of the Kane
fracture zone transverse ridge.
J. Geophys. Res.
,
93
, 3195-210.
Anderson, H. and Jackson, J. A. 1987. Active tectonics of the Adriatic region.
Geophys. J.
Roy. Astr. Soc.
,
91
, 937-84.
Anderson, R. N., DeLong, S. E. and Schwarz, W. M. 1978. Thermal model for subduction
with dehydration in the downgoing slab.
J. Geol.
,
86
, 731-9.
