Three-Dimensional Geologic Map of the Hayward Fault Zone, San Francisco Bay Region, California By G.A. Phelps, R.W. Graymer, R.C. Jachens, D.A. Ponce, R.W. Simpson, and C.M. Wentworth Pamphlet to accompany Scientific Investigations Map 3045 U.S. Department of the Interior U.S. Geological Survey 1 Contents Abstract..................................................................................................................................................................................3 Scope and Purpose of this report ......................................................................................................................................3 1. Earthquake studies. .....................................................................................................................................................3 2. Ground water studies..................................................................................................................................................3 3. Mineral hazards studies..............................................................................................................................................4 4. Education and scientific inquiry...............................................................................................................................4 Introduction...........................................................................................................................................................................4 Geologic Background .........................................................................................................................................................6 Mesozoic terrane complexes .........................................................................................................................................6 Paleogene unroofing and overlap .................................................................................................................................8 Neogene transpression and development of the San Andreas Fault system .........................................................8 Methods.................................................................................................................................................................................9 Hayward Fault................................................................................................................................................................ 12 East Hills Area ............................................................................................................................................................... 12 Cenozoic sedimentary deposits and tertiary volcanic deposits ............................................................................. 13 San Leandro gabbro of Ponce and others (2003)..................................................................................................... 14 Geology west of the Hayward Fault........................................................................................................................... 14 Description of Map Features ........................................................................................................................................... 15 Description of Map Units................................................................................................................................................. 16 Description of Map Structures ........................................................................................................................................ 17 Hayward Fault................................................................................................................................................................ 17 Calaveras Fault............................................................................................................................................................... 18 Rodgers Creek Fault...................................................................................................................................................... 18 Palomares-Miller Creek-Moraga-Pinole Fault System........................................................................................... 18 Chabot Fault ................................................................................................................................................................... 19 Other San Leandro gabbro-bounding faults.............................................................................................................. 19 Cretaceous/Tertiary (KT) faults east of the Hayward Fault................................................................................... 19 San Pablo Bay Fault...................................................................................................................................................... 19 Cenozoic (Ce) faults east of the Hayward Fault ...................................................................................................... 20 Mesozoic (Mw) faults west of the Hayward Fault .................................................................................................. 20 Data ...................................................................................................................................................................................... 20 Acknowledgments ............................................................................................................................................................. 24 References Cited................................................................................................................................................................ 24 Tables 1. Density-depth function for Cenozoic basin deposits and Cenozoic volcanic rocks ...................................14 2. Description of data and algorithms .............................................................................................................21 Figures 1. Generalized map of the major faults of the Hayward Fault zone ..................................................................5 2. Generalized terrane map of the Hayward Fault zone (modified from ...........................................................7 3 .Isostaticresidualgravityanomalymapofthestudyarea. ...................................................................................10 4. Magneticanomalymapofthestudyarea...........................................................................................................11 Appendixes Appendix I. Data Files ....................................................................................................................................31 Appendix II. Introduction to the Basic EarthVision™ 3D Viewer Tools.......................................................32 2 THREE-DIMENSIONAL GEOLOGIC MAP OF THE HAYWARD FAULT ZONE, SAN FRANCISCO BAY REGION, CALIFORNIA By G.A. Phelps, R.W. Graymer, R.C. Jachens, D.A. Ponce, R.W. Simpson, and C.M. Wentworth Abstract A three-dimensional (3D) geologic map of the Hayward Fault zone was created by integrating the results from geologic mapping, potential field geophysics, and seismology investigations. The map volume is 100 km long, 20 km wide, and extends to a depth of 12 km below sea level. The map volume is oriented northwest and is approximately bisected by the Hayward Fault. The complex geologic structure of the region makes it difficult to trace many geologic units into the subsurface. Therefore, the map units are generalized from 1:24000-scale geologic maps. Descriptions of geologic units and structures are offered, along with a discussion of the methods used to map them and incorporate them into the 3D geologic map. The map spatial database and associated viewing software are provided. Elements of the map, such as individual fault surfaces, are also provided in a non-proprietary format so that the user can access the map via open-source software. The sheet accompanying this manuscript shows views taken from the 3D geologic map for the user to access. The 3D geologic map is designed as a multi-purpose resource for further geologic investigations and process modeling. Scope and Purpose of this report The purpose of this new map is to compile the best available data on: 1) identity and subsurface distribution of bedrock units, 2) sedimentary deposits, and 3) geologic structures at a regional scale, and to integrate the data into a 3D map. The 3D nature of the map allows the modeled geology to be incorporated into physically based process models that are used to predict the behavior of geologic systems. The 3D map also offers interesting new possibilities for visualizing complex geologic relations with the help of increasingly powerful computer software. Potential applications of the 3D map include: 1. Earthquake studies. The intensity of earthquake shaking has been shown to depend strongly on the distribution of geologic units. For example, the intensity increases within sedimentary basins. Simulations of the shaking produced by the great 1906 San Francisco, California earthquake were based on a 3D velocity model made by assigning seismic velocities to the various rock units in a 3D geologic map of the San Francisco Bay region (see, for example, Hartzell and others, 2006; McPhee and others, 2007). Accurate seismic velocity models are required to precisely locate earthquake epicenters, which can be used to map