The San Andreas fault system comprises an interactive network of right- and left-lateral strike-slip faults and related reverse and normal faults. In southern and central California, right- and left-lateral faults of the San Andreas system transect a crystalline terrane of Proterozoic through Mesozoic igneous and metamorphic rocks overlapped by Upper Cretaceous through Eocene marine sedimentary strata and by Oligocene and lower Miocene terrestrial volcanic and sedimentary strata. Paleogeologic patterns in these rocks define regional terranes and local reference domains, the reassembly of which permits determination of overall displacement on the strike-slip faults that disrupt them. Timing of fault movement is recorded by incremental displacements of upper Cenozoic sedimentary deposits and by the sequence of fault movements required to effect reassembly of the reference domains. Reassembling the pre-late Cenozoic regional paleogeologic framework of southern and central California leads to a balanced palinspastic reconstruction of the San Andreas system that differs from previously published reconstructions, both conceptually and in terms of magnitude of displacement restored on many of the principal strike-slip faults. Four reference domains that constrain the balanced reconstruction consist of paleogeologic patterns reassembled from crystalline and sedimentary rocks now found: (1) in the Transverse Ranges in the Frazier Mountain-Mount Pinos area, the eastern Orocopia Mountains and vicinity, and the Sierra Pelona-northern San Gabriel Mountains area; (2) in the Salinian block in the La Panza Range and in the Transverse Ranges in the Liebre Mountain block and western San Bernardino Mountains; (3) in the Salinian block in the Gabilan Range, in the southern tail of the Sierra Nevada in the San Emigdio and Techachapi Mountains, and in the Portal Ridge area of the northwestern-most Mojave Desert; and (4) in the southern part of the Transverse Ranges west of the San Andreas fault, in the northern Peninsular Ranges, and in the southern Chocolate Mountains. Simultaneous reconstruction of the four paleogeologic reference domains specifies the magnitude and sequence of displacement on the major right- and left-lateral faults of the San Andreas system. The Clemens Well-Fenner-San Francisquito fault is the earliest (and now abandoned) strand of the San Andreas fault system in southern California. It formed a continuous structure with the early San Andreas fault zone of central California, is today cut by the San Andreas fault in the Transverse Ranges, and diverges southeastward from the San Andreas fault east of the Salton trough at least as far as the Little Chuckwalla Mountains. The Clemens Well-Fenner-San Francisquito-early San Andreas fault accumulated a displacement of about 100 or 110 km during the interval between 22 and 13 Ma and probably during the more restricted interval between 18 to 17 and 13 Ma. The disposition of displacement southeastward from the Transverse Ranges is problematic because the Clemens Well-Fenner-San Francisquito fault neither rejoins the modern San Andreas fault in the Salton trough nor extends indefinitely into the continent. Hypothetically, the southeastward extension of the fault is absorbed by coeval sinistral kinking along the southern margins of the Transverse Ranges and Chocolate Mountains and/or by synchronous detachment faulting in southeasternmost California and southwesternmost Arizona. A zone of sinistral deformation trends roughly east-west along the southern boundary of the Transverse Ranges province. The earliest expression of this deformation is sinistral kinking that began to develop after 22 to 20 Ma and prior to 17 Ma. During the interval from 17 to 13 Ma, the zone of sinistral deformation was characterized by widespread volcanism and perhaps by faulting associated with left-oblique extension. Left-lateral displacement of about 40 to 45 km across this zone during this interval is attributed here to sinistral kinking. The zone of sinistral kinking and faulting apparently initiated in a right step between the southeastern terminus of the Clemens Well-Fenner-San Francisquito fault and the northwestern terminus of dextral faults such as the East Santa Cruz basin fault that were active coevally in the continental borderland. Subsequently, the zone served as the southern terminus of right-lateral strands of the San Gabriel fault system. Still later, it served as the northern terminus of the Elsinore and San Jacinto fault zones. In the Transverse Ranges, the San Gabriel fault system, including, from oldest to youngest, the Canton, San Gabriel, and Vasquez Creek faults, began to splay southward from the older Clemens Well-Fenner-San Francisquito-early San Andreas fault as early as 12 to 13 Ma. The San Gabriel fault system has since accumulated a displacement of 42 km, deforming the older Clemens Well-Fenner-San Francisquito-early San Andreas fault in the process. The Canton fault, active between 13 and 10 Ma, accumulated a displacement of 15 to 17 km; the San Gabriel fault, active between 10 and 5 Ma, accumulated a displacement of 22 to 23 km; and the Vasquez Creek fault, active between 6 Ma and the present, accumulated a displacement of no more than about 5 km. Displacement on these faults merged northwestward with that of the early San Andreas fault north of the Transverse Ranges, whereas movement ceased on the Clemens Well-Fenner-San Francisquito fault. In the Salinian block, the San Gregorio-Hosgri, Rinconada-Reliz, and Red Hills-Ozena faults developed coevally with the San Gabriel fault and also merged northwestward with the early San Andreas fault. Displacements of 45 km on the Rinconada-Reliz fault and 105 km on the San Gregorio-Hosgri fault south of Monterey Bay merge on the San Gregorio fault north of the bay for total of 150 km. Hypothetically, displacement on the San Gregorio fault is split between 70 km (after 6 to 6.5 Ma) west of the Montara Mountain block and 80 km (prior to 6 to 6.5 Ma) east of that block, so as not to leave the Montara Mountain block dangling north of the restored Salinian block. Displacement on the San Gregorio fault is transferred northward onto the San Andreas fault, thereby increasing its overall displacement to about 440 km north of the junction of the two faults. None of the displacements on the Salinian block faults and no more than 22 or 23 km of slip on the San Gabriel fault have been shown to remerge southeastward with the Clemens-Well-Fenner-San Francisquito-early San Andreas fault. Hypothetically, these displacements stepped west to the continental margin across the western Transverse Ranges, where left-oblique extensional faulting continued through the late Miocene into the early Pliocene. Along the southern boundary of the Transverse Ranges west of the San Gabriel Mountains, the ancestral Malibu Coast-Santa Monica fault system accumulated a sinistral component of displacement of as much as 35 km in addition to the earlier sinistral kinking, whereas to the east, the ancestral Raymond-Cucamonga-Banning fault system accumulated no more than about 10 to 20 km. The modern San Andreas fault emerged about 5 Ma. In central California, it coincides with the pre-5-Ma San Andreas fault, whereas in southern California it diverged from the older Clemens Well-Fenner-San Francisquito fault and actually crosscuts the older fault to merge southeastward with the Salton trough at the north end of the Gulf of California. Displacement on the post-5-Ma San Andreas fault varies along the fault because the crustal blocks adjoining the fault are deformed by coeval strike-slip faults, including right-lateral faults such as the San Jacinto, Calaveras-Hayward-Rodgers Creek-Maacama-Garberville, and San Gregorio-Hosgri faults, and left-lateral faults such as the Garlock fault and the east- to northeast-trending faults of the Transverse Ranges. Displacement restored on the modern San Andreas fault as measured along the present trace ranges from about 160 to 185 km. Simultaneous palinspastic reconstruction of the four reference domains is possible only in conjunction with restoration of slip along a zig-zag system of secondary strike-slip faults of the San Andreas system that distort the crustal blocks adjoining the modern San Andreas fault in southern California. This system includes right-lateral fault s in the Peninsular Ranges, Mojave Desert, and Death Valley area, and left-lateral faults in the Transverse Ranges and between the Sierra Nevada and Mojave Desert. Overall displacements restored on these secondary faults are generally well constrained by offsets of crystalline rocks and overlying Cenozoic strata. In the Peninsular Ranges, dextral displacement restored on the San Jacinto fault is 28 km, and that restored on the Elsinore fault is 5 km. About 10 km of Pliocene and Quaternary left slip is restored on faults along the southern boundary of the Transverse Ranges, where earlier left-oblique extensional faulting was overprinted by reverse and left-oblique reverse faulting by late Pliocene and Quaternary time. In the eastern Transverse Ranges, sinistral displacement restored on the major east-trending faults includes 16 km on the Pinto Mountain fault, 5 km on the Blue Cut fault, 11 km on the Chiriaco fault, and 8 km on the Salton Creek fault. In the Mojave Desert, dextral displacements restored on northwest-trending faults include 3 km on the Helendale fault, 3 km on the Lockhart-Lenwood fault, 4 km on the Harper-Harper Lake-Camp Rock-Emerson fault, 9 km on the Blackwater-Calico-Mesquite Lake fault, and 16 km on the Pisgah-Bullion fault. Although displacement on the central and eastern parts of the Garlock fault is well documented to be about 60 km, displacement at the western end, as limited by reassembly of the Gabilan Range-San Emigdio Mountains-Portal Ridge reference domain, can be no greater than 12 km.

The unsaturated zone at Yucca Mountain was investigated as a possible site for the nation's first high-level nuclear waste repository. Scientific investigations included infiltration studies, matrix properties testing, borehole testing and monitoring, underground excavation and testing, and the development of conceptual and numerical models of the hydrologic processes at Yucca Mountain. Infiltration estimates by empirical and geochemical methods range from 0.2 to 1.4 mm/yr and 0.2–6.0 mm/yr, respectively. Infiltration estimates from numerical models range from 4.5 mm/yr to 17.6 mm/yr. Rock matrix properties vary vertically and laterally as the result of depositional processes and subsequent postdepositional alteration. Laboratory tests indicate that the average matrix porosity and hydraulic conductivity values for the main level of the proposed repository (Topopah Spring Tuff middle nonlithophysal zone) are 0.08 and 4.7 × 10 −12 m/s, respectively. In situ fracture hydraulic conductivity values are 3–6 orders of magnitude greater. The permeability of fault zones is approximately an order of magnitude greater than that of the surrounding rock unit. Water samples from the fault zones have tritium concentrations that indicate some component of postnuclear testing. Gas and water vapor movement through the unsaturated zone is driven by changes in barometric pressure, temperature-induced density differences, and wind effects. The subsurface pressure response to surface barometric changes is controlled by the distribution and interconnectedness of fractures, the presence of faults and their ability to conduct gas and vapor, and the moisture content and matrix permeability of the rock units. In situ water potential values are generally less than −0.2 MPa (−2 bar), and the water potential gradients in the Topopah Spring Tuff units are very small. Perched-water zones at Yucca Mountain are associated with the basal vitrophyre of the Topopah Spring Tuff or the Calico Hills bedded tuff. Thermal gradients in the unsaturated zone vary with location, and range from ~2.0 °C to 6.0 °C per 100 m; the variability appears to be associated with topography. Large-scale heater testing identified a heat-pipe signature at ~97 °C, and identified thermally induced and excavation-induced changes in the stress field. Elevated gas-phase CO 2 concentrations and a decrease in the pH of water from the condensation zone also were identified. Conceptual and numerical flow and transport models of Yucca Mountain indicate that infiltration is highly variable, both spatially and temporally. Flow in the unsaturated zone is predominately through fractures in the welded units of the Tiva Canyon and Topopah Spring Tuffs and predominately through the matrix in the Paintbrush Tuff nonwelded units and Calico Hills Formation. Isolated, transient, fast-flow paths, such as faults, do exist but probably carry only a small portion of the total liquid-water flux at Yucca Mountain. The Paintbrush Tuff nonwelded units act as a storage buffer for transient infiltration pulses. Faults may act as flow boundaries and/or fast pathways. Below the proposed repository horizon, low-permeability lithostratigraphic units of the Topopah Spring Tuff and/or the Calico Hills Formation may divert flow laterally to faults that act as conduits to the water table. Advective transport pathways are consistent with flow pathways. Matrix diffusion is the major mechanism for mass transfer between fractures and the matrix and may contribute to retardation of radionuclide transport when fracture flow is dominant. Sorption may retard the movement of radionuclides in the unsaturated zone; however, sorption on mobile colloids may enhance radionuclide transport. Dispersion is not expected to be a major transport mechanism in the unsaturated zone at Yucca Mountain. Natural analogue studies support the concepts that percolating water may be diverted around underground openings and that the percentage of infiltration that becomes seepage decreases as infiltration decreases.