Laboratory Manual for Introductory Geology 4e

EXERCISE 2.10
Estimating the Amount and Rate of Motion in a Continental Transform Fault
Name:
Course:
Section:
Date:
The more we know about the history of a continental transform fault that lies close to heavily populated regions, the
better we can prepare for its next pulse of activity. Geologists try to find out how long a continental transform fault has
been active, how much it has offset features on the plates it separates, and how fast it has moved in the past.
Geologists estimate that the San Andreas fault system, mapped below, has been active for about 20 million years. This exercise
shows how geologic markers cut by transform faults enable us to measure the amount and rate of motion along a fault.
Simplified geologic setting of the San Andreas fault system.
Juan
de Fuca
Plate
Mendocino
transform
San Francisco
Pacific
Plate
~400 km
~250 mi
Ridge
segment
Transform
Trench
Cascade
Trench
Triple
junction
Los Angeles
San Andreas fault
Gulf of
California
North American
Plate
Salton
Sea
N
At its northern end, the
San Andreas links to the
Cascade Trench and an
oceanic transform fault.
At its southern end, the
San Andreas links to a
mid-ocean ridge in the
Gulf of California.
Triple
junction
Amount of fault movement indicated by offset bodies of
identical rock.
124º 120º
42º
Sedimentary rocks
Point Reyes
0
San Francisco
km
200
Los Angeles
38º
Volcanic igneous rocks
Metamorphic rocks
Intrusive igneous rocks
116º
34º
Before tackling the San Andreas system, let’s practice on a much simpler area. Field geologists have mapped an active
continental transform fault (the red line in the figure on the next page) for several hundred kilometers. A 50-million-year-old
(50-Ma) body of granite and a 30-Ma vertical dike have been offset by the fault as shown.
(a) Draw arrows on opposite sides of the fault to show the direction in which the two plates moved relative to each
other.
(b) Measure the amount of offset of the 50-Ma granite body.
(c) The geologists have proved that the faulting began almost immediately after the granite formed and continues
today. Assuming that the plates moved at a constant rate for the past 50 million years, calculate the rate of offset.
km
(d) Now do the same for the 30-Ma dike. Offset:
movement for the past 30 million years, calculate the rate of offset.
km. Assuming a constant rate of
(e) Compare the two rates. Has faulting taken place at a constant rate, or has the rate increased or decreased over
time? Explain in as much detail as possible.
(continued )
44 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS