Page 445 - Global Tectonics

REVIEW QUESTIONS 427

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implied paleomagnetic pole position (in present day y mm a , describe the way in which the situation
coordinates), and the geographic latitude of the sill evolves with time for each of the following fi ve
at the time of its intrusion, approximately 190 Ma. cases:
a. y less than x;
Assuming that the difference between the present
b. y equal to x;
and the paleolatitude of the sill is due to the drift
c. y between x and 2x;
of North America with respect to the geographic
d. y equal to 2x; and
pole, calculate the implied average rate of drift
e. y greater than 2x.
during the past 190 Ma. Is this an absolute,
maximum, or minimum average rate? Explain.
(Present latitude and longitude of sill = 40.5°N,
75°W; 1 degree of latitude = 111 km.)
50 In the plate tectonic situation illustrated below
three plates (A, B, and C) are separated by a
symmetrically spreading ridge at R and a subduction Hint: Consider plate C to be fixed and assign

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zone at T. If the rate of spreading at R is x mm a velocities to A, R, and B, relative to C, in terms of

per ridge flank, and the rate of subduction at T is x and y.