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

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TASK 1 - 3D/4D geologic map construction and assembly

New geologic mapping is needed in three areas, and this new mapping will be supported by concurrent gravity and magnetic interpretations in cases of ambiguity or where critical relationships are concealed. New subsurface geologic mapping is needed along most of the fault, and will follow the philosophy of starting with the surface geologic map and progressively carrying critical contacts into the subsurface. This work will depend heavily on modeling of potential field anomalies, detailed analysis of seismic tomography models, analysis of micro-seismicity, and integration of drill hole data. Construction of the 4D geologic map will require integration of the results of tasks 1) and 2) within a framework that explicitly incorporates time-histories of along-fault, cross-fault and vertical deformation. This task will be sequenced geographically, with the Bay Region section (between the San Andreas/Calaveras junction and the San Andreas/San Gregorio junction) being the focus of the initial effort.

In this task we will be constructing the 3-D geologic map of the San Andreas Fault Zone north of the Transverse Ranges and a 4-D reconstruction of the deformation that occurred to this crustal volume over the lifetime of the San Andreas system (described further in Objectives). The underlying scientific problems are twofold:

1. How does the 3-D geology of a fault zone affect fault behavior?
2. What is the deformational history of the western continental margin since about 30 Ma?
Map of the San Andreas Fault System modified from Shultz and Wallace, 1997, showing three sectional subdivision: Northern, Central, and Southern fault zones in California

The objectives of this Task are:

1. The creation of the 3-D geologic map of the San Andreas Fault zone,
2. The 4-D reconstruction of deformation associated with the fault zone, and
3. The study of relations between fault behavior and the 3-D geology of the fault zone.

HOW …is the task being conducted (examples of methods, software, laboratory, etc.)

The 3D geologic map will be assembled in EarthVision (TM), a rules-based geologic structure building software system which has served us well in constructing past models. The system is designed for 3D geologic maps so the explicit incorporation of temporal evolution will require innovative applications of the EarthVision system or application of new computing/visualization software. Specifically, we will need software that can handle 3-D bodies moving and changing shape through time. It seems likely that 3-D animation software will be more effective in handling this problem. Answering the question of what tools are required will part of the work of this Project. Methodological problems in how to effectively record and(or) display 4-D information are yet to be resolved.

For the 3-D geologic map, we will combine surface geologic map data with potential field geophysical data and other datasets, following the techniques developed by the 3-D Geologic Mapping and Visualization Project. Some petroleum exploration well data will be available for certain parts of the zone.

The San Andreas Fault can be subdivided into 3 major sections within the study region based on the intersection with other major faults. The northernmost section (Northern San Andreas) extends north of the intersection with the San Gregorio Fault to the northern limit of the study region; the southernmost section (Central San Andreas) extends south of the intersection with the Calaveras Fault (the root of the East Bay Fault system) to the southern limit of the study region. Between the intersections is the Peninsula San Andreas (PSA). We intend to develop the 3-D model in three stages, corresponding to the 3 fault sections. We will start with the PSA, where we have the best current data sets (recently published digital geologic maps, good aeromag and gravity data coverage) and we can build on the activities of the San Francisco Bay Area Fault Hazards Project (detailed mapping of the surface traces of the PSA, 3-D fault surfaces of active faults in the region).

To develop the 4-D reconstruction of San Andreas Fault zone deformation, we will break the deformational history into 3 orthogonal components: fault parallel, fault normal, and vertical. Fault parallel (strike-slip) offset will be constrained by matching offset geologic units and(or) geophysical anomalies. Offset units of age less than the total age of the fault can provide details about changes in rates of strike-slip deformation over fault history and about activation/abandonment of various parts of the fault system. Many offset units do not provide exact piercing points, so estimates of uncertainty related to the size and shape of the unit will be incorporated into the reconstruction. It is not anticipated that fault parallel offset will be limited to the presently active San Andreas Fault. Fault normal deformation will analyzed by "unfolding" strata of known age. "Unfolding", which includes deformation perpendicular to the main fault by reverse and(or) normal faults as well as folds, is complex because of the juxtaposition of unlike stratigraphies by the large amount of strike-slip offset taken up by the fault system. Therefore "unfolding" must be accomplished for each block of unique stratigraphy (Assemblage). If strata of equivalent age are present in each Assemblage, then fault normal deformation can be calculated for that time period based on adding the deformation of each Assemblage and any fault normal component of the Assemblage-bounding faults. In cases where strata of equivalent age are not present, extrapolation/interpolation may be required. Vertical deformation will be studied based on vertical components of reverse and(or) normal faulting, paleoshoreline data and paleontological depositional environmental data developed in Task 2, analysis of erosion at unconformities, available fission-track and other radiometric analyses of uplift, and present topographic relations. As with the previous components of deformation, strata of different ages throughout the history of the fault system will be analyzed to detect time-dependant variations in vertical deformation.

Building on the observations and techniques in Graymer and others (2005, Geology, v. 33) and recent unpublished observations by Jachens, we will use the 3-D geologic map to prepare fault-face geologic maps for the active San Andreas Fault and will examine the correlation of the geology revealed with several categories of fault behavior, including microseismic clusters, surface creep rate, seismically quiet patches, fault shape, discontinuities in the active fault surface, and extent of coseismic rupture.

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