Quaternary Processes of the Monterey Bay Area Field Trip Notes

Transcription

1 Quaternary Processes of the Monterey Bay Area Field Trip Notes A color version of this field trip guide, with directions is available at: 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. 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. 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

2 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.

3 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 6 th, 5 th, and 4 th 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.

4 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.

5 Stop #3 The Pinnacles Directions From Hollister, find your way to south on highway 25. Drive south to Pinnacles National Monument.

6 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.

7 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 ). 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 ). 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.

8 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?