Plate Motion and the San Andreas Fault = Activity 2.2 Name: Course/Section: Date: Learni ng GOAL You will examine two points that were originally in the same place along the San Andreas Fault and measure how far they have moved apart. Using GPS velocity data, you will estimate how long it might have taken for this displacement to occur. A Displacement Along the San Andreas. The San Andreas Fault in California is an important part of the boundary between the Pacific Plate to the west and the North American Plate to the east. Between about 19.0 and 24.1 million years ago, a volcano erupted near the western margin of the North American Plate, and later that same volcano was split by a fault in the San Andreas Fault system. The displaced halves of that volcano now form the Pinnacles and Neenach Volcanics (Fig. A2.2.1). Pinnacles now moves with the Pacific Plate and Neenach remains on the deforming edge of the North American Plate. ® San'Francisco r " :;::t,' ,;“v‘.'s; ' Bakersfield /? ;P a2 { ( ~ ' 4 |, 12l 3 g =% Neenach i #5"Volcanics 4North TP comas - ’ 0 100 km A e i - J - //._’1'14 _‘(‘Q’j N ! 1:4;000,000 Los"Angeles & / : ,) #? st - ’ - B - ,_'- Bi22 121° 120 119° JPPL18 LELE - g hAais L e ~ = Figure A2.2.1a 1. Measure the distance between Neenach and Pinnacles along the San Andreas Fault. Because the fault is not a straight line on the map, you might want to use a string in the measuring process. Carefully position the string along the curved trace of the fault. Mark the average positions of Neenach and Pinnacles on the stnng. Straighten the string along a ruler and measure the distance between the two points on the string in millimeters. Each millimeter on the map represents 4 km on Earth’s surface. 300 usekscale and figure out how many time it fit into the distance 2. The age of the Neenach-Pinnacles Volcanics is between about 19.0 and 24.1 Myr. Based on your answer to the previous question and making the first-order assumption that faulting began soon after volcanism ceased (around 19.0 Myr), estimate the average rate at which Neenach and Pinnacles have moved apart since 19 Myr. Rate = distance/time Approximate distance from Neenach to Pinnacles: Estimated average rate of displacement since 19 Myr: 15.78 km/Myr
