3D/4D mapping of the San Andreas Fault Zone

GMEG - Geology and Geophysics

3D/4D mapping of the San Andreas Fault Zone

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TASK 2 - Earth surface geologic studies

Statement of Problem: Surface geologic mapping is one of the primary inputs into a 3-D geologic map, but modern digital geologic maps do not exist for much of the study area. Furthermore, a section of the study area along the Peninsula San Andreas section has been identified as requiring additional geologic map studies beyond those integrated into the presently available digital geologic map (Brabb and others, 2000, USGS MF-2332). The tectonic events that transformed the coastal California plate boundary from a convergent margin to a transpressive margin, and the effects of the San Andreas Fault system offsets, are recorded in the sediments and basins along the San Andreas Fault system. A detailed paleontologic framework can reveal both time and position data required for the 4-D reconstruction. However, the long-standing problem of mixing lithostratigraphic and time-stratigraphic units in California has influenced nearly every geologic study done in the past 150 years. Furthermore, the microfossil correlations which were developed in California are time transgressive and tend to have a local or regional focus. Focused studies of faunal assemblages can reveal details in timing of deposition, including the detection of time-transgressive units that may be related to deposition across a moving fault. Paleoenvironmental data revealed by faunal assemblages can provide information about latitude at time of deposition, as well as proximity to the coastline and depth below sea level for marine sediments. And matching faunal assemblages in now offset rock bodies may provide piercing "points" that control timing and amount of strike-slip offset.

Objectives: New digital geologic maps and map databases are required for four areas in the study region, broken down by 30' X 60' quadrangle:

1. The Point Arena 30'X60' quadrangle, especially that region north of Sonoma County (Sonoma County is presently available at 1:62,500 scale in Blake and others, 2002, USGS MF-2402)
2. The Point Sur and Coalinga 30'X60' quadrangles, especially the area including and surrounding the San Andreas Fault zone. This mapping would bridge the gap between the existing map of the Monterey 30'X60' quadrangle (Wagner and others, 2002, CGS CD 2002-04) and the work of the Salinas Valley Task of the 3-D Geologic Mapping and Visualization Project presently finishing up.
3. The San Luis Obispo and Taft 30'X60' quadrangles, especially the area including and surrounding the San Andreas Fault zone. This mapping would extend the coverage of modern digital geologic mapping to the southern extent of the study area.
4. The Healdsburg 30'X60' quadrangle (see subtask below).

We also recognize the need for additional geologic mapping along the Peninsula San Andreas Fault in the Santa Cruz Mountains. We have been granted permission to solicit a Mendenhall Postdoctoral Fellow to pursue these studies. The objectives of the paleontological investigations will be focused on expanding the paleontological database, focusing on rocks deposited during the development of the San Andreas Fault system (<30 Ma) in the following 3 areas:

1. Age control. Improve the available age control by reexamining existing fossil collections and recollecting existing localities as required, and developing new fossil localities in collaboration with geologic mappers. The age-transgressive nature of California microfossil zonation demands tie-ins with the global microfossil zonation via an interpretation that separates local, regional, global, and ecological influences.
2. Cross fault correlation. Analyze existing and new fossil collections in comparison to known faunal assemblages in cross-fault units.
3. Paleoenvironmental control for 4-D reconstruction. Analyze existing and new fossil collections for faunal evidence of latitude and depth of deposition.

Methodology: For the production of new geologic maps, we will follow the standard procedures for geologic field data gathering and interpretation. We will especially emphasize collecting data on structural orientation that is important for projecting surface geology into the 3-D geologic map and collecting any fossils/tephra that could yield improved time-stratigraphic information. We will also work with our potential field geophysics colleagues to gather data that could enhance their interpretation of the regional gravity and magnetics. Past experience has suggested that previously erected stratigraphic frameworks will prove to be overly simplified, so we will need to develop new stratigraphic interpretations that take into account the juxtaposition of originally widely separated depositional basins by large fault offsets and the potential for localized erosion/nondeposition.

We will compile the field data into digital geologic map databases in Arc/Info following standard USGS procedures. Paleontologic studies will build on existing fossil collections via a three step process: 1) review existing descriptions of faunal assemblages and fossil localities in academic/industry literature; 2) examine museum/industry collections (for example, molluscan fauna at California Academy of Sciences; Los Angeles County Museum of Natural History; and Museum of Paleontology, University of California at Berkeley) for fossil localities in the study region, and 3) selected field excursions to collect new localities in collaboration with the geologic mappers or recollect existing localities that might yield important new information. New collections will be processed and analyzed using standard molluscan and micropaleontological techniques, including, for microfossils, chemical preparation at the lab in Flagstaff, Arizona.

TASK 2 - Surface geologic studies

Sub Task 1: Healdsburg 30'X60' quadrangle with CGS (Robert McLaughlin)

Statement of Problem: Although modern USGS digital geologic maps are largely confined to within the bounds of highly populated San Francisco Bay region counties, the landscape of northern California beyond these bounds has been changing rapidly as the population, jobs and technology have migrated away from major cities. With this expansion has come significant landscape modification and changes in infrastructure. Based on present trends, expanding development in northern California in the next 30 years will likely extend to Ukiah, eastward to Clear Lake and also northward along the western Sacramento Valley I-5 corridor.

Northeastward expansion of the greater San Francisco Bay region is occurring over the northward and eastward
extensions of the active right-lateral faults of the San Andreas Fault System, including the Hayward-Rodgers Creek-
Maacama Fault, the Green Valley Fault, and the Bartlett Springs Fault. The eastern part of the Healdsburg quad includes northeastward extensions of several of the important but poorly understood faults of the eastern part of the San Andreas transform boundary, including the Collayomi and Konocti Bay, as well as the Hunting Creek, and other faults that may connect the Green Valley Fault to the Bartlett Springs Fault. New development in these areas typically focuses over the thick fills of structural basins that, in turn, are generally bounded by active faults. At present our understanding of how specific faults of this region interact with the San Andreas Fault and with various blocks of the North American Plate is inadequate. Important parameters for determining maximum credible earthquake magnitudes and rupture scenarios such as fault surface and subsurface continuity, segmentation and geometry, are largely unknown.

In addition to earthquake activity along northeastern faults of the transform margin, long period seismic events beneath areas of Quaternary volcanism and active geothermal activity in the Clear Lake region (including the famous Geysers Steam Field) point to the potential for future volcanic eruptions at the fringe of the greater San Francisco Bay region. The area encompasses a fundamental structural boundary marked by ophiolitic rocks, that separates crystalline basement of the Great Valley on the east, from Mesozoic Franciscan rocks of the Coast Ranges on the west. It also includes the largest geothermal field exploited for production of electrical energy in the world (The Geysers steam field).

Digital geologic maps that highlight areas of fault and landslide hazards in this region are of interest to the geothermal industry, PG&E, the California Division of Safety of Dams, oil and minerals exploration companies, municipal and county water agencies, and land trusts affiliated with academic institutions The Healdsburg quad is immediately north of the Napa 100k quadrangle, a nearly completed STATEMAP project of the CGS. In the 1970's and 1980's the USGS mapped much of the western Healdsburg and Lakeport 100k quadrangles at 24k scale. The mapping was open-filed in B/W format for the use of industry and federal decision makers and only a small percentage of the 24k mapping was published in color. None of this mapping was digitally released. Maps that were open-filed in B/W format are difficult to read, and are now largely unavailable or difficult to access. Other mapping in this region resides only on field sheets. This "legacy" geologic mapping along with supportive paleontologic and geochemical data are likely to be lost unless placed into a coherent digital geologic map database.

The USGS and the CGS have had a productive cooperative working relationship in northern California for some time, often with parallel geologic framework mapping projects having very different, but complimentary goals and objectives. This cooperation has been particularly successful in the northern San Francisco Bay region.

Objectives: The ultimate objective of this subtask will be to contribute, in cooperation with the California Geological Survey, to a digital geologic map database for the Healdsburg 100k quadrangle. Due to limitations on funding, availability of geologic and GIS resources and the timing of this subtask, compilation of the geology and publication of Healdsburg 100k quadrangle would probably extend to beyond the life of the San Andreas 3D/4D project. We envision this subtask as providing an initial component in the form of a 24k and 100k digital geologic map database that could then be the leverage for a proposal for future work at the expanding fringe of the San Francisco Bay region.

The role of USGS in a cooperative mapping effort with CGS will initially be the upgrading and release of our 24k legacy geologic mapping in the western half of the Healdsburg 100k quadrangle as digital coverages, as GIS resources and funding become available. Legacy 24k quadrangles that are complete and already have digital files (The Geysers, Whispering Pines, Kelseyville, 24k quads) will have highest publication priority, followed by digitized nearly complete quadrangle coverages (Jimtown and Healdsburg 24k quads) that may require incorporation of geology in flatland areas from existing digital USGS Quaternary geology databases by Knudsen and others (2000) and Witter and others (2006) from recent uncompiled bedrock mapping by McLaughlin, and from the Quaternary Fault map database of Graymer and others, 2006. Digital coverages of incompletely mapped quadrangles within and adjacent to the Healdsburg 100k quadrangle will have lower priority. McLaughlin will compile his unpublished geology from field sheets onto a base that can be scanned and digitized. Published USGS 24K-scale non-digital colored I-maps will also be incorporated into the database.

Another objective will be to integrate the geology and aeromagnetic signatures of Mesozoic and early Tertiary basement rocks east of the northern San Andreas Fault in the Point Arena and Bodega Bay 100k quadrangles and establish links with the western part of the Healdsburg and Napa 100k quadrangles (work of McLaughlin, Phelps, Jachens, Wentworth and Langenheim discussed above). This will provide important insights into the geometry of some deformed fundamental pre-San Andreas structural boundaries prior to their disruption by younger strike-slip faulting.

Methodology: A cooperative geologic mapping effort with CGS in the Healdsburg 100k quadrangle has the goal of publishing USGS digital 24k geologic mapping for use in the 100k map database. Publication of all available geologic mapping will likely not be completed under this subtask, but the maps which are published and digitally compiled will provide the initial component from which a new project to extend beyond 2011.

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