Question

Wallace Creek is an ephemeral stream that is dry most of the time but flows during the wet seasons. Flow is from the North American side toward the Pacific side. Points a, b, and c on the Pacific Plate side mark points where Wallace Creek has flowed across the fault at various times in the past from point d on the North American side. Geologists interpreted the approximate ages of initial development of the Wallace Creek channels as follows: (see Figure above).

a = 13,000 yrs
b = 10,000 yrs
c = 3,700 yrs
1. What is the general direction of displacement (movement) of the Pacific Plate side of the fault?
N
NE
E
SE
S
SW
W
NW
2. What is the average velocity (cm/yr) of displacement on this fault in the last 13,000 yrs? Provide your answer in cm/yr.

Answer 1

The Pacific Plate side of the San Andreas Fault is moving northwest. Without specific displacement data, we cannot calculate the exact average velocity of this movement over 13,000 years. The Juan de Fuca Plate is subducting beneath the North American Plate, influencing the Cascade Range's formation, and will ultimately be subducted completely.

Step-by-step explanation:

The general direction of displacement of the Pacific Plate side of the San Andreas Fault is to the northwest. This is inferred by tracking the historical movement across the fault where points a, b, and c were once connected to point d, with flow typically occurring from the North American side toward the Pacific side.

To calculate the average velocity of displacement of the Pacific Plate over the last 13,000 years, we need a measure of total displacement during that time. However, this information is not provided in the question. To find the average velocity (in cm/yr), you would typically divide the total displacement (in cm) by the time (in years). For instance, if we knew that the Pacific Plate had displaced 390 cm in 13,000 years, the calculation would be 390 cm / 13,000 years, giving an average velocity of 0.03 cm/yr. Without the actual displacement measurement, we cannot provide a specific numerical answer here.

Regarding the West Coast tectonics and the fate of the Juan de Fuca Plate, it is bounded by various types of plate boundaries. It is being subducted beneath the North American Plate off the coast of Washington and Oregon, contributing to the tectonic processes that formed the Cascade Range, including Mt. St. Helens. Eventually, the Juan de Fuca Plate is expected to be fully subducted, leading to its disappearance.