Cover. Example of LANDSAT 7 imagery draped on a digital elevation model and aligned with a corresponding three-dimensional hydrogeologic framework solid model, based on animation shown in appendix 1. For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS. Three-Dimensional HydrogeologicFor an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. AbstractAs part of a U.S. Geological Survey study in cooperation with the Bureau of Reclamation, a digital three-dimensional hydrogeologic framework model was constructed for the Rio Grande transboundary region of New Mexico and Texas, USA, and northern Chihuahua, Mexico. This model was constructed to define the aquifer system geometry and subsurface lithologic characteristics and distribution for use in a regional numerical hydrologic model. The model includes five hydrostratigraphic units: river channel alluvium, three informal subdivisions of Santa Fe Group basin fill, and an undivided pre-Santa Fe Group bedrock unit. Model input data were compiled from published cross sections, well data, structure contour maps, selected geophysical data, and contiguous compilations of surficial geology and structural features in the study area. These data were used to construct faulted surfaces that represent the upper and lower subsurface hydrostratigraphic unit boundaries. The digital three-dimensional hydrogeologic framework model is constructed through combining faults, the elevation of the tops of each hydrostratigraphic unit, and boundary lines depicting the subsurface extent of each hydrostratigraphic unit. The framework also compiles a digital representation of the distribution of sedimentary facies within each hydrostratigraphic unit. The digital three-dimensional hydrogeologic model reproduces with reasonable accuracy the previously published subsurface hydrogeologic conceptualization of the aquifer system and represents the large-scale geometry of the subsurface aquifers. The model is at a scale and resolution appropriate for use as the foundation for a numerical hydrologic model of the study area.

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We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ~175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which infl uenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude signifi cant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward-Rodgers Creek-Maacama fault system suggests that this system reactivated older basement structures, which appear to infl uence seismicity patterns in the region.

The Great Basin region in the western United States contains active geothermal systems, large epithermal Au-Ag deposits, and worldclass Carlin-type gold deposits. Temperature profi les, fl uid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems associated with gold mineralization share many common features, including the absence of a clear magmatic fl uid source, discharge areas restricted to fault zones, and remarkably high temperatures (>200 °C) at shallow depths (200-1500 m). While the plumbing of these systems varies, geochemical and isotopic data collected at the Dixie Valley and Beowawe geothermal systems suggest that fl uid circulation along fault zones was relatively deep (>5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small (<2.5‰) shifts from the meteoric water line (MWL). Many fossil ore-forming systems were also dominated by meteoric water, but usually exhibit δ 18 O fl uid-rock interactions with larger shifts of 5‰-20‰ from the MWL.Here we present a suite of two-dimensional regional (100 km) and local (40-50 km) scale hydrologic models that we have used to study the plumbing of modern and Tertiary hydrothermal systems of the Great Basin. Geologically and geophysically consistent cross sections were used to generate somewhat idealized hydrogeologic models for these systems that include the most important faults, aquifers, and confi ning units in their approximate confi gurations. Multiple constraints were used, including enthalpy, δ 18 O, silica compositions of fl uids and/or rocks, groundwater residence times, fl uid inclusion homogenization temperatures, and apatite fi ssion track anomalies.Our results suggest that these hydrothermal systems were driven by natural thermal convection along anisotropic, subvertical faults connected in many cases at depth by permeable aquifers within favorable lithostratigraphic horizons. Those with minimal fl uid δ 18 O shifts are restricted to high-permeability fault zones and relatively small-scale (~5 km), single-pass fl ow systems (e.g., Beowawe). Those with intermediate to large isotopic shifts (e.g., epithermal and Carlin-type Au) had larger-scale (~15 km) loop convection cells with a greater component of fl ow through marine sedimentary rocks at lower water/rock ratios and greater endowments of gold. Enthalpy calculations constrain the duration of Carlin-type gold systemsto probably <200 k.y. Shallow heat fl ow gradients and fl uid silica concentrations suggest that the duration of the modern Beowawe system is <5 k.y. However, fl uid fl ow at Beowawe during the Quaternary must have been episodic with a net duration of ~200 k.y. to account for the amount of silica in the sinter deposits.In the Carlin trend, fl uid circulation extended down into Paleozoic siliciclastic rocks, which afforded more mixing with isotopically enriched higher enthalpy fl uids. Computed fi ssion track ages along the Carlin trend included the convective effects, and ranged between 91.6 and 35.3 Ma. Older fi ssion tra...