Bay Area Geology Field Trip in Association with the 2009 Advanced Summer School of Radioactive Waste Disposal

with Social-Scientific Literacy Dr. Mick Apted, Field Trip Leader

Purpose Safe disposal of nuclear waste in deep (300 to 700 meters) geological repositories is based, in part, on maintaining favorable environmental conditions of hydrology, geochemistry, and rock mechanics. The great depth shields a repository from short-term perturbations occurring at the Earth’s surface. Over the long-term (≥10,000 years) isolation period, however, natural events (‘scenarios’) may occur that have the potential for perturbing the environmental conditions and even disrupting the physical integrity of the repository system. Such events include

• climate change, • faulting and folding of the host rock, and • uplift and erosion of the entire repository block.

The likelihood of some scenarios (e.g., faulting and folding, uplift) can depend on the location and the type of geological host rock for the repository, while other scenarios can more properly be considered as global and of long duration, potentially affecting any repository site (e.g., climate change). The purpose of this field trip is to visit locations (which are definitely NOT candidate sites!) where there is evidence of these natural events occurring in the past and even today. Such locations can provide visual, analogue illustrations and insights to the participants, and we will discuss how characteristics of such natural events may relate to potential scenario impacts on a generic repository.1 The “Big Picture”: Plate Tectonics The surface of the Earth is divided into a large number of rigid ‘plates’ (Figure 1). There are two basic types of plates; relatively thin (~10 km) ‘oceanic plates’ made of a volcanic rock called basalt, and more thick (~100 km) ‘continental plates’ composed of more silica (SiO2) rich rock similar to granite. The plates move around the Earth’s surface atop a viscous layer of the upper mantle called the asthenosphere. Convection of the hot asthenosphere causes the plates to move. Where two plates move apart, molten material (magma) from the asthenosphere rises to form a ‘divergent plate boundary’,. The most typical example are the mid-ocean ‘spreading ridges’, although divergent plate boundaries called ‘rift system’, such as the East African rift system, can occur on land.. When two plates slide by each other, this is called a ‘transform-fault boundary’. Where two plates collide, this is called a ‘convergent plate boundary’. When plates collide, one plate is typically thrust (‘subducted’) beneath the over-riding plate, a situation that is discussed in more detailed in the next section. Plate tectonics has played a key role in aiding geologists’ basic understanding of topography (mountains, valleys), structures (faults, folded rock), ore deposits, geothermal systems, and future geological hazards. The geological past is the key to the future.

1 The information used in this field guide is a compilation from several excellent field guides that are referenced throughout the text and in the Reference section. Important contributions from Prof. Mark Cloos, University of Texas at Austin are also gratefully acknowledged.

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California and Japan: Past and Present Convergent Plate Boundaries Today the active geology of the Japanese islands is attributable to a rather complex set of convergent plate margins, in which oceanic crust is being subducted beneath the Eurasian Plate. One hundred million years ago, California was the site of a similar convergent plate margin (Figure 2), in which the Pacific Plate oceanic crust was being subducted beneath the North American Plate. The results of this ancient subduction process in California is largely preserved and well exposed today, which means that Californian geology presents an excellent analogue site to gain insight into modern day geological processes in Japan.

Figure 2: Schematic Cross Section of California Convergent Plate Margin Approximately 100 Million Years Ago (courtesy of Prof. M. Cloos). Figure 2 shows key features of the California convergence (following from ‘west’ to ‘east’ on Figure 2) that are characteristics of essentially all convergent plate margins:

(1) formation of a deep ‘trench’ at the location where the oceanic plate was being subducted beneath North America,

(2) formation of an ‘accretionary prism’ or ‘mélange’, called the Franciscan Complex in California, of oceanic sediments (clay, chert, limestone) that were scraped off from the subducting oceanic plate. Accretionary prisms/ mélanges also can contain rocks that were subducted to great depths and experienced high-pressure but low-temperature metamorphic reactions before being brought up to the surface via a ‘flow-mélange’ process.

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(3) formation of a fore-arc basin of eroded sediments (called the Great Valley Group in California) from the building of the volcanic arc to the east, and

(4) subduction of the oceanic plate into the hot mantle beneath the North American plate causes the colder oceanic plate material to heat up, de-water and eventually undergoes partial melting. These hydrous melts buoyantly rise into the overlying North American crust, to form an arc of volcanoes with granite root-zones that are the present Sierra Nevada Mountains of California.

These four geological features are exactly analogous to geological features, processes and events that characterize both the past 5-million years and present-day Japan. There are numerous trenches east of the Japanese islands where oceanic plates are being subducted. There are thick accretionary arc prisms of oceanic sediments being accumulated between the trenches and the Japanese islands. There are well-developed fore-arc basins (e.g., present-day Shikoku, Pacific coastal regions of Honshu), and of course, there is the volcanic arc that runs down the spine of the Japanese islands. The San Andreas Fault System Beginning about 30 million years ago, the North American Plate began to override a divergent plate boundary (ocean spreading ridge) of the subducting Pacific Plate (Figure 3). While the exact dynamics of this process are beyond the scope of this trip, a simplified view is that when a convergent plate boundary overrides a divergent plate boundary, a transform-fault boundary is formed. Over time, the continued overriding of the spreading ridge caused the formation and extension of the San Andreas Fault (SAF) system in California. Today, the SAF transform-fault system extends from the Mendocino coast of northern California, where the SAF connects to a triple plate junction offshore of the Oregon and Washington coast, southward to where the SAF connects to a triple plate junction in the Gulf of California between Baja and mainland Mexico. In a transform boundary, the adjacent plates slide past each other in a so-called ‘strike-slip’ movement. The slippage can occur via a slow, steady ‘creeping’ movement or via sudden, episodic jumps. Creeping faults are characterized by frequent but extremely low magnitude earthquakes, whereas sudden, large fault displacements lead to infrequent but larger magnitude earthquake events. There are other basic geometries for fault movement. In tension, two rock blocks will pull away from each other, and the block on the lower side (‘hanging wall’) of the dipping fault will drop down, in what is called a ‘normal fault’. In compression, the rock block on the hanging wall moves over the other rock block, forming a ‘reverse fault’, also called a ‘thrust fault’. While the present day SAF system is characterized by mostly strike-slip movement, there are associated faults that locally can display normal or reverse type movement.

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Figure 3. Plan View of Plate Re-construction of the Formation of the San Andreas Transform Fault. From Anderson et al. Climate Change, Glaciation and Sea Level In addition to the convergent plate story, there is also evidence of modern-day Earth processes recorded in Californian geology that match modern-day (and assumedly future) processes in Japan. Of particular concern, because it is a global natural event, is climate change. There are abundant, multiple lines of

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evidence to show cycles in global climate, notably leading to formation of massively thick (up to 3 km) continental sheets of ice during glaciations, occurring repeatedly over the last several millions of years. These glacial cycles of 100,000 to 140,00 years are believed to be driven by orbital and tilt variations in Earth’s movement around the Sun. The Earth is currently in an inter-glacial period (i.e., relatively warm, limited extent of glaciers) that started about 12,000 years ago; inter-glacial period typically last on the order of 20,000 years, followed by 80,000 to 120,000 years of much colder glacial periods before the cycle starts again. As continental glaciers grow during the extended glacial periods, the water forming this ice is derived from the oceans, so that sea level decreases quite drastically. Sea level was approximately 150 meter lower than today’s sea level during the last peak glaciation, which allowed connections among land masses (Asia-North America, British Isles to Europe). Conversely, during warmer inter-glacial periods, sea level naturally increases as glaciers melt. In many places (including California and Japan), the occurrence of such elevated sea level are recorded and preserved in so-called marine terraces (Figure 4). These are areas of the shore in which the wave action during times of elevated sea level formed a wave-cut platform and beach at this ancient shore line. As sea level has dropped over time during glacial cycles, these ancient marine terraces can be preserved, age-dated and used to evaluate past climate conditions, durations and other characteristics. It is cautioned, however, that there is another mechanism for forming raised marine terraces; vertical uplift of the shore attributable to tectonic forces. Much of the coast line of present-day Japan shows evidence of past and continuing uplift arising from plate convergence. Thus, it is important for climatologists using marine terrace data in climate predictions to evaluate whether there is any additional impacts or contributions from tectonics.

Figure 4. Schematic Cross Section of Key Features for a Raised Marine Terrace. ‘WCP’ is wave-cut platform and “SLA’ is shore line angle. From Anderson et al.

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Start of Trip We will meet at Berkeley at 8 AM. There will be a check of personal items and a quick orientation before boarding the bus at 8:30 AM. Please be on time! Lunch will be provided. Figure 5 is an aerial photograph showing key geological and landform features of the San Francisco Bay area, and planned Stops along the trip.

Figure 5. Approximate Route of Field Trip. From Stoeffer.

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STOP 1: Hayward Fault The San Francisco Bay area is characterized by a series of faults related to the San Andreas Fault (SAF) system (Figure 5). In STOP 2 we will view from afar the main trace of the SAF, but STOP 1 allows us to closely inspect the related Hayward Fault that cuts directly through numerous cities in the East Bay. The Hayward Fault (HF) is a so-called ‘right-lateral’ fault, in which an observer on one side of the fault would see that objects on the other side of the fault are displaced to right by fault movement. In this region of the town of Hayward for which the fault is named, the fault motion is actually creeping at a slow but steady rate of about 9 mm/year. Such ‘creeping’ strands of the HF contrast with other strands and other faults in which the movement is ‘locked’, and the tectonic stresses (stored energy) that move the plates are relieved episodically and suddenly, resulting in high magnitude earthquakes. Note that the Hayward Fault also extends through the UC Berkeley campus. One of the most outstanding examples of the creeping nature of the HF was its displacement through the Berkeley football stadium, but this damage is now being repaired. There are, however, other places in the Berkeley area where small displacement (bending) of sidewalks and house foundations can be seen as evidence of the slowly creeping deformation from displacement along the HF (Figure 6). Topics for discussion: First, try to visual the geometry of a fault that might intersect (or come close to) a deep geological repository system. What are your concerns regarding potential impacts of a fault on a repository system? Are these concerns the same whether a fault is ‘creeping’ or ‘locked’? What characterization activities (e.g., mapping and site location?) and countermeasures (e.g., engineering?), if any, might be effective in reducing such concerns?

Figure 6. Example of Right-lateral Displacement by Creeping Fault. From Stoeffer. STOP 2: San Andreas Fault Overlook This overlook provides a view of the main strand of the San Andreas Fault (SAF) system, and associated land form features (Figure 7). The SAF, like the Hayward Fault, is a right-lateral fault. Thus portion of the SAF is mostly locked, with an average rate of movement over time of about 17 mm/year. During the 1906 San Francisco earthquake, the maximum displacement was about 7 meters (Figure 8), with severe associated ground motion (Figure 9), Note, however, that shaking intensity from earthquakes quickly diminishes with depth, so that shaking impacts on a deep repository would likely be negligible.

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The forces generated by the slipping of one plate past another can lead to lowering the strength of the rocks along the fault trace. This fragmentation of rock leads to generally higher erosion (lower topography), formation of ‘sag ponds’, and generation of groundwater springs arising along lower permeability fragmented rock (Figure 10). Depending on available routes, the field trip may later re-cross the SAF at a location closer to downtown San Francisco (Devil’s Slide area), so be ready to contrast what you see here at STOP 2 with other places where we might see the trace of the SAF.

Figure 7. Aerial View of San Andreas Fault Near STOP 2. From Stoeffer.

Figure 8. Displaced Road from the 1906 San Francisco Earthquake, From Stoeffer.

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Figure 9. Estimated Shaking Intensity from the 1906 San Francisco Earthquake.. From Williams.

Figure 10. Typical Land-Surface Features Associated with Faults. From Graymer.

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Topics for discussion: Following on from STOP 1, try and visualize the difference between a single fault and a system of faults (a ‘shear zone’). Are your concerns about the potential impact of faults on a repository the same whether it is a shear zone or single fault? Small faults tend to grow into larger faults, and large faults can grow in to a multiple-fault shear zone; how does this time perspective affect your concerns, and the effectiveness of site characterization or engineering countermeasures? STOP 3: Marine Terraces Around Half Moon Bay Depending on the availability of parking space, we will be able to view marine terraces in and around the Half Moon Bay airport (Figure 11). The geological interpretation of these features has not reached a consensus, with competing hypotheses between sea-level change and tectonic uplift being compared. If the tidal position is favorable and time permits, we may also visit beach areas where outstanding examples of intense folding of sedimentary rocks can be seen (Figure 12). Topics for Discussion: First, assume that the raised marine terraces here were formed solely by a past (warming) climate change leading to a rise in sea level. What are your potential concerns about how a change in sea level might impact a deep geological repository system? How might the cycling of sea level (maybe 5 meters higher changing to 150 meters lower) affect a repository? Are there any site characterization or engineering countermeasures that might be effective in reducing these concerns? There may be other features from climate change, such as change in rainfall and formation of thick glaciers at the surface above a repository; what might be impacts on a repository system from changes? Next, assume that the raised marine terraces here were formed solely by a continuing uplift attributable to tectonic forces along a convergent plate boundary. What are your potential concerns about how uplift (and erosion) might impact a deep geological repository system? Are there any site characterization or engineering countermeasures that might be effective in reducing these concerns?

Figure 11. Exposure of Marine Terraces in the Half Moon Bay Region. From Stoeffer.

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Figure 12. The Folded Sediments in the Half Moon Bay Region Arising from Tectonic Deformations. From Stoeffer. STOP 4: Golden Gate Headlands and Broken Formation of Franciscan Mélange At this stop we will be looking at the structure and rock fabric (‘lithology’) of rocks of the Franciscan mélange complex (Figure 13). This chaotic mixing of rock types, such as volcanic basalt, cherts formed from deep-ocean siliceous oozes, limestone and abyssal clays, are intimately mixed together during the process of subduction (Figure 2). Similar chaotic or broken formations exist along the Japanese coast (e.g., Shikoku, Kii Peninsula), developed during past subduction processes that formed the Japanese islands. The basalt, chert and limestone rocks of this area are probably the remnants of a seamount (an underwater volcano) on the Pacific Plate that collided against the North American Plate (Figure 14). The raised profile of the seamount caused it to fragment and become mixed with the deep abyssal clay sediments being scraped off of the descending Pacific Plate (see Figure 2). Topics for Discussion: First, envision the two-dimensional surface outcrop of rocks we see here as a three-dimension rock volume. In some places different rock lithologies form large blocks (up to 10’s kilometers in other parts of the Franciscan), while in other places the small bits of different lithologies are mixed together at a scale of centimeters. What are your concerns about locating a repository system of engineered barriers into such a chaotic or broken formation? What might be engineering and construction challenges? What might be the disadvantages regarding long-term (≥10,000 years) performance of a repository located in such a formation? What might be the advantages of siting a repository in such a chaotic or broken rock formation? References Anderson, D., A. Sarna-Wojcicki and R. Sedlock. 2001. Field Trip 4: San Andreas Fault and Coastal Geology from Half Moon Bay to Fort Funston: Crustal Motion, Climate Change and Human Activity, in Geology and Natural History of the San Francisco Bay Area, 2001 Fall Field Conference of the National Association of Geoscience Teachers, US Geological Survey Bulletin 2188, pp 87-104. Elder, W. 2001. Field Trip 3: Geology of the Golden Gate Headlands, in Geology and Natural History of the San Francisco Bay Area, 2001 Fall Field Conference of the National Association of Geoscience Teachers, US Geological Survey Bulletin 2188, pp. 61-86. Graymer, R. 2001. Field Trip 2: A Geological Excursion to the East San Francisco Bay Area, in Geology and Natural History of the San Francisco Bay Area, 2001 Fall Field Conference of the National Association of Geoscience Teachers, US Geological Survey Bulletin 2188, pp. 33-60.

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Stoeffer, P. 2005. The San Andreas Fault in the San Francisco Bay Area, California: A Geology Fieldtrip Guidebook to Selected Stops on Public Lands, US Geological Survey Open Field Report 2005-1127. Williams, J. 2001. Field Trip 5: Elements of Engineering Geology on the San Francisco Peninsula-Challenges When Dynamic Geology and Society’s Transportation Web Intersect, in Geology and Natural History of the San Francisco Bay Area, 2001 Fall Field Conference of the National Association of Geoscience Teachers, US Geological Survey Bulletin 2188, pp. 105-144.

Figure 13. A Cross-section of the Geology and Structure Around the Golden Gate Headlands. The bottom figure is not to true scale; From Elder.

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Figure 14. Chert (upper left) Deposited on Basalt in Franciscan Formation of the Golden Gate Headlands. From Elder.