Field geologists have mapped an active continental transform fault (red line in

Question

Field geologists have mapped an active continental transform fault (red line in the figure below) for several hundred kilometers. A 50-million-year-old (50-Ma) body of granite and a 30-Ma vertical layer of marble have been offset by the fault as shown. (a) Draw arrow to indicate the direction in which the plate on the northeast side of the fault has moved relative to the plate on the southwest side. (b) Measure the amount of offside of the 50-Ma granite body. ___ km. (c) The geologists have proved that faulting began almost immediately after the granite formed and continues today. If the fault blocks moved at a constant rate for the past 50 million years, calculate the rate of offset. ___ km/million years: mm/year ___ (d) Was the rate constant for 50 million years? Measure the offset of the 30-Ma marble. ___ km (e) Assume that the fault blocks moves at a constant rate for the past 30 million years. Offset rate: ___ km/30 million years: ___ km/million years: ___ mm/year

Explanation / Answer

A) To the southeast,The plate on the northeast side of the fault has moved southeast relative to the plate on the southwest side.

B) The granite body has been offset by about 1,100 km. Be sure that you're measuring the distance between the same edges of the offset bodies (e.g., from one western edge to the other western edge) and to convert the measured distance to kilometers using the scale provided. For example, I measured the distance between the offset granite bodies as 77 mm.

I measured 100 km on the scale as 7 mm. Set up a ratio to find the distance between the granite bodies: (77 mm / 7 mm) = (d / 100 km).
Cross-multiply to solve for d: (77 mm)(100 km) = (7 mm)d
d = 7,700 km / 7 = 1,100 km

c) 22 km/Myr = 22 mm/yr

D = rt
r = D/t
r = 1,100 km / 50 Myr = 22 km/Myr = 22 km/106 yr
r = 22 km/106 yr x 1,000 m/km x 1,000 mm/m = 22 mm/yr

D) 310 km

The marble layer has been offset by about 310 km. You can probably tell by eyeballing the figure that the distance between the offset marble layers is less than the distance between the offset granite bodies.

Be sure that you're measuring the distance from the same edges of the offset bodies (e.g., from one western edge to the other western edge) and to convert the measured distance to kilometers using the scale provided. For example, I measured the distance between the offset marble layers as 22 mm.

I measured 100 km on the scale as 7 mm. Set up a ratio to find the distance between the marble layers: (22 mm / 7 mm) = (d / 100 km).
Cross-multiply to solve for d: (22 mm)(100 km) = (7 mm)d
d = 2,200 km / 7 310 km

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