Hollister: Calavaras Fault and associated features

The picture above shows how man-made structures respond when built directly on an active fault.

Travel on the bus. We will leave the Santa Clara Valley and travel south through the Salinas Valley. The mountain range to our right is the Santa Cruz Mountains, the Mountain Range to our left is the Diablo Range. As we enter the Pinnacles, heading south on HWY 101, we are in the Gabilan Range another distinct mountain range in the Coast Ranges.

Answer these questions before going on the trip:

 The San Andreas fault runs roughly parallel and east of the park. This fault represents a transform boundary. We will examine surface features formed in association of a strike-slip fault.  The website that contains a description of the walk can be found at: http://www.openspace.org/TRANCOS.html or http://pubs.usgs.gov/gip/earthq3/ , a USGS website.

 1.       The San Andreas fault is a __________________ plate boundary. It  separates the ______________ plate and the ______________ plate. This fault is classified as a ________________ lateral strike slip fault which means that the relative movement is to the ______________. The fault is ________ miles or __________ kilometers long.

2.     Draw a diagram of California. Include the San Andreas fault.

3.     Santa Cruz, Monterey, and Half Moon Bay are on the ____________ plate.

4.     San Jose, San Francisco, and Berkeley are on the ____________plate.

5.     There are variations in the straightness of the San Andreas fault. This produces different landforms. Terms are defined in the glossary: http://earthquake.usgs.gov/image_glossary/ .

1. If the fault is straight, the following features are formed:

Offset:

Offset of streams:

The San Andreas fault near Watsonville.

Linear valleys:

Crystal Springs Reservoir

Shutter ridges:

Springs:

2. If the fault has a right bend tensional stress forms depressions (explanation found in book):

Draw this configuration

Sag ponds are often found in this setting:

3. If the fault has a left bend, compressional stress forms uplift (information found in book).

How were the Santa Cruz Mountains formed?

1. Geologists describe the movement on a fault by a rate (distance/time). We use rate when describing how fast we drive, miles per hour. Movement on faults in California averages about as fast as a fingernail grows or is measured in centimeters/years. Later in the course we will complete an exercise showing how rocks formed in the same location were displaced or offset by the San Andreas fault. The rocks found at Pinnacles National Monument can be used as an example. The Pinnacles are the results of volcanic activity occurring in southern California and displacement of the rocks along the San Andreas fault.  The volcanic rocks are offset approximately 300 kilometers and are dated at approximately 21 million years using radiometric dating methods.
   1. Volcanic rocks are classified by_____________________________________.

2. The volcanic rocks of the Pinnacles are rhyolite. This means they have a composition of ________________________.

3. This composition is associated with what type of crust?

2. Rate of movement. Using the map provided, determine the following:

a. Distance between Neenach formation and Pinnacles :   _______km

b. Age of volcanic sequence:                                           _______My

c. Approximate rate:

 ________km x 1my______   x 1000 meters x 100 cm = -______cm/yr

                  my   1,000,000yr            1km              1 m

3. General overview: We are in the Coast Ranges geologic province in California. Pinnacles Monument is part of the Gabilan Range, one of the smaller ranges that makes up the Coast Ranges. The current tectonic setting of central and most of southern California is a transform plate boundary (see figure 1.). The large fault that forms this plate boundary, is named the San Andreas fault. There are many faults that make-up the San Andreas fault zone. Together the faults create the typical parallel to sub-parallel topography of the Coast Ranges. The Calavaras fault is a typical example of one of these faults and is one of the major faults of the Bay Area.
   1. On figure 2 draw small arrows indicating the relative movement of the Bay Area faults. Mark a “U” on uplifted areas or mountainous areas and a “D” on down-dropped areas or valleys.

 Major faults of the Bay Area. All except the Zayante fault are right-lateral strike-slip faults.

4. On the attached map the main trace of the Calavaras fault is indicated in a solid line. Your job is to begin at Locust St. and examine the offset of the sidewalk. Continue examining bent curbs and sidewalks, offset curbs, warped buildings (garages, houses, etc.) offset walls and different effects on concrete and wood.  Mark the locations where you see offset or evidence of creep. Connect the locations that should indicated the main trace of the Calavaras fault. Draw large arrows showing relative direction of movement.

5. Stop #2. Fault Creep at Hollister:

We will measure the displacement of the curb on the north side of Sixth Street in an attempt to estimate the average creep rate of the Calavaras fault since 1929.

Displacement of curb (constructed in 1929): ________________mm

Duration of creep: current year-1929          _________________yrs.

Average creep:    ____________________________mm/yrs

This page was last modified by P. Jefferis on 08/22/05 . She is a lecturer in the Department of Geology at San Jose State University