Quaternary Processes of the Monterey Bay Area
Field Trip Notes
A color version of this field trip guide, with directions is available at:
http://www.mpcfaculty.net/alfred_hochstaedter/geology.htm
Click on “Field Trip #2—Directions”
The goal of this field trip is to see examples of geomorphology related to the San Andreas
Fault in the field. The San Andreas Fault (SAF) is an excellent example of a right-lateral
strike-slip fault. Examples of strike-slip fault features are shown in the diagram below.
Some San Andreas Fault
(SAF) Background
The SAF is considered
the main boundary between the
Pacific Plate and the North
American Plate. In the vicinity
of the San Francisco and
Monterey Bay Areas, the plate
boundary is actually distributed
between several faults. These
include, from southwest to
northeast, the San Gregorio, the
San Andreas, the Hayward, and
the Calaveras. We will see the
San Andreas and the Calaveras
today. Slip rate on the SAF is
about 35 mm/yr south of the
Monterey Bay Area. North of
here, however, the rate slows
to about 22 mm/yr. Why
does the rate slow? Where is
the additional 13 mm/yr of
motion? What do you think
happens as the SAF breaks
into several different faults.
The diagram below
shows the SAF, Hayward (H),
and Calaveras ( C ) faults.
Draw on the San Gregorio
yourself. The Calaveras splits
from the SAF just south of
Hollister, where we will visit
today. See if you can draw the
SAF, the Hayward, and
perhaps the Calaveras on the
topographic map on the front
of this field guide.
Stop #1 The SAF at the San Juan Bautista Mission – Directions
Once you’re at the Mission, walk across the big grassy field. The main mission building
should be on your left. Stop when you get to the scarp, or steep slope, leading to the
agricultural fields below. This scarp is the fault.
Stop #1 The SAF at San Juan Bautista
San Juan Bautista represents the transition along the SAF from a quickly moving
creeping section to the south (lots and lots of little EQs all the time) to a more slowly and
intermittently moving section to the north, as explained above (also see the seismicity
pattern on the next page). Note that the mission at San Juan Bautista is on a hill with the
fault along a long scarp towards the northeast. It could be that the mission is rising to the
heavens as it gets compressed between the quickly moving section to the south and the
slowly moving section to the north.
Stop #2 The
Calaveras Fault at
Hollister –
Directions
Note in the image at right
that the Calaveras Fault
crosses 6th, 5th, and 4th
Streets between Powell and
West Streets. Please
investigate the fault in these
areas. See if you can find
evidence for the fault similar
to that shown on the next
page.
When you are done, proceed
to the Pinnacles by driving
east towards San Benito St
and turning right on San
Benito St. Follow the signs
to South on Highway 25.
Stop #2 The
Calaveras Fault at
Hollister
This section
of the Calaveras fault
is creeping (see all
the EQs in the
diagram below). We
will look for
evidence for the
creep as we look at
cultural features
(houses, curbs, and
sidewalks) as we
walk around in
downtown Hollister.
Can you find
evidence of the
Calaveras Fault?
The diagram below
shows a cross section
of seismicity along
the SAF. Note all of
the EQ activity south of San Juan Bautista and how little there is to the north. The line
labeled “1989” represents the 1989 Loma Prieta EQ and its associated aftershocks. Much
of this region would be filled in with seismicity if this diagram had been made within the
last 10 years.
Stop #3 The Pinnacles – Directions
From Hollister,
find your way to
south on highway
25.
Drive south to
Pinnacles National
Monument.
Once you arrive at the Pinnacles, pay the entry fee and drive in to the parking area near
the Visitor Center.
1. Tour the Visitor Center
2. Walk up the trail to Bear Gulch Reservoir, observing the volcaniclastic rocks along the
way.
Stop #3 The Pinnacles
We’ll spend the afternoon at the Pinnacles. The Pinnacles Volcanic Formation (PVF) in
San Benito County is an early Miocene (22Ma to 24Ma) sequence of calc-alkaline
andesite, dacite, and rhyolite flows with interbedded pyroclastic and volcaniclastic rocks.
The volcanic field is located within the Salinian block, east of the San Andreas Fault (see
Fig. 12-26). The volcanic rocks form a west-dipping homocline, truncated on the
northeast by the Chalone Creek Fault, which is an early trace of the San Andreas Fault
(remember that the San Andreas Transform has migrated through time). The Neenach
Volcanic Formation (NVF) in located near Gorman, California in the westernmost
Mojave Desert east of the San Andreas Fault (see Fig. 12-26). The Neenach volcanics are
also composed of the same calc-alkaline andesite, dacite, and rhyolite flows with
interbedded pyroclastic and volcaniclastic rocks as the PVF (see Fig. 14-9). Structurally,
the NVF forms a northwest dipping homocline, broken by many faults and truncated on
the southwest by the San Andreas Fault.
Work done by former UCSC graduate student Vincent Mathews suggests that these two
formations were contiguous across the San Andreas Fault Zone at the time of their
formation 23.5 million years ago and have since been separated and transported via the
San Andreas Fault to their present locations. The Pinnacles-Neenach correlation provides
some of the most conclusive evidence of large-scale, right-lateral displacement on the
San Andreas Fault Zone. The criteria are as follows: (1) A precise distance of separation
can be determined because both areas are directly adjacent to traces of the San Andreas
Transform; (2) The age of the formations can be can be determined accurately because
the rocks are volcanic and suitable for isotopic age dating; (3) The correlation is based on
10 different rock types, each with distinct petrologic features. Lithologic, faunal, and
facies similarites (Hill and Dibblee, 1953) show that younger rocks have been displaced a
shorter distance than older rocks, implying a steady rate of movement.
Displacement
The Pinnalces-Neenach volcanic rocks erupted 23.5 million years ago and since have
been offset 315 km with a mean rate of 1.3 to 1.4 cm/yr. At this rate, the Pinnalces
formation would be 288km from Neenach. Correlation has been found between the
Parkfield volcanic rocks 90 km southeast of the Pinnacles with formations in both the
PVF and NVF to suggest the the Pinnacles went through two periods of displacement.
There are different models that could explain this, but many believe that the Parkfield
rocks were displaced 225 km from the Neenach area since the late Miocene and that the
Pinnalces were displaced 90 km presumably on the Chalone Creek Fault, from the
Neenack-Parkfield volcanic rocks prior to the late Miocene time.
At what kind of plate boundary does rhyolite typically erupt?
Is this the same kind of plate boundary that exits today in this region?
The rock at the Pinnacles is clearly layered. Is the main rock type at the Pinnacles
igneous, sedimentary, or contain aspects of both?
I hope you answered “aspects of both” to the previous question. What are the
sedimentary and igneous features of these rocks?
What about their eruption caused them to contain aspects of both igneous and
sedimentary rocks?
What is special about the rocks at the Pinnacles to allow them to be used to measure slip
rates along the San Andreas Fault?