Nicholas H. Hinz

Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary

In the western Great Basin of North America, a system of dextral faults accommodates 15%–25% of the Pacific–North American plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this system. This young evolving part of the plate boundary offers insight into how strike-slip fault systems develop and may reflect the birth of a transform fault. A belt of overlapping, left-stepping dextral faults dominates the northern Walker Lane. Offset segments of a W-trending Oligocene paleovalley suggest ∼20–30 km of cumulative dextral slip beginning ca. 9–3 Ma. The inferred long-term slip rate of ∼2–10 mm/yr is compatible with global positioning system observations of the current strain field. We interpret the left-stepping faults as macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. The strike-slip faults end in arrays of ∼N-striking normal faults, suggesting that dextral shear diffuses into extension in the Great Basin. Coeval extension and dextral shear have induced slight counterclockwise fault-block rotations, which may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating development of a throughgoing strike-slip fault.

Eocene–Early Miocene paleotopography of the Sierra Nevada–Great Basin–Nevadaplano based on widespread ash-flow tuffs and paleovalleys

The distribution of Cenozoic ash-flow tuffs in the Great Basin and the Sierra Nevada of eastern California (United States) demonstrates that the region, commonly referred to as the Nevadaplano, was an erosional highland that was drained by major west- and east-trending rivers, with a north-south paleodivide through eastern Nevada. The 28.9 Ma tuff of Campbell Creek is a voluminous (possibly as much as 3000 km 3 ), petrographically and compositionally distinctive ash-flow tuff that erupted from a caldera in north-central Nevada and spread widely through paleovalleys across northern Nevada and the Sierra Nevada. The tuff can be correlated over a modern area of at least 55,000 km 2 , from the western foothills of the Sierra Nevada to the Ruby Mountains in northeastern Nevada, present-day distances of ∼280 km west and 300 km northeast of its source caldera. Corrected for later extension, the tuff flowed ∼200 km to the west, downvalley and across what is now the Basin and Range–Sierra Nevada structural and topographic boundary, and ∼215 km to the northeast, partly upvalley, across the inferred paleodivide, and downvalley to the east. The tuff also flowed as much as 100 km to the north and 60 km to the south, crossing several east-west divides between major paleovalleys. The tuff of Campbell Creek flowed through, and was deposited in, at least five major paleovalleys in western Nevada and the eastern Sierra Nevada. These characteristics are unusual compared to most other ash-flow tuffs in Nevada that also flowed great distances downvalley, but far less east and north-south; most tuffs were restricted to one or two major paleovalleys. Important factors in this greater distribution may be the great volume of erupted tuff and its eruption after ∼3 Ma of nearly continuous, major pyroclastic eruptions near its caldera that probably filled in nearby topography. Distribution of the tuff of Campbell Creek and other ash-flow tuffs and continuity of paleovalleys demonstrates that (1) the Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma; (2) the Sierra Nevada was a lower, western ramp to the Nevadaplano; and (3) any faulting before 23 Ma in western Nevada, including in what is now the Walker Lane, and before 29 Ma in northern Nevada as far east as what is now the Ruby Mountains metamorphic core complex, was insufficient to disrupt the paleodrainages. These data are further evidence that major extension in Nevada occurred predominantly in the late Cenozoic. Characteristics of paleovalleys and tuff distributions suggest that the valleys resulted from prolonged erosion, probably aided by the warm, wet Eocene climate, but do not resolve the question of the absolute elevation of the Nevadaplano. Paleovalleys existed at least by ca. 50 Ma in the Sierra Nevada and by 46 Ma in northeastern Nevada, based on the age of the oldest paleovalley-filling sedimentary or tuff deposits. Paleovalleys were much wider (5–10 km) than they were deep (to 1.2 km; greatest in western Nevada and decreasing toward the paleo–Pacific Ocean) and typically had broad, flat bottoms and low-relief interfluves. Interfluves in Nevada had elevations of at least 1.2 km because paleovalleys were that deep. The gradient from the caldera eastward to the inferred paleodivide had to be sufficiently low so that the tuff could flow upstream more than 100 km. Two Quaternary ash-flow tuffs where topography is nearly unchanged since eruption flowed similar distances as the mid-Cenozoic tuffs at average gradients of ∼2.5–8 m/km. Extrapolated 200–300 km (pre-extension) from the Pacific Ocean to the central Nevada caldera belt, the lower gradient would require elevations of only 0.5 km for valley floors and 1.5 km for interfluves. The great eastward, upvalley flow is consistent with recent stable isotope data that indicate low Oligocene topographic gradients in the Nevadaplano east of the Sierra Nevada, but the minimum elevations required for central Nevada are significantly less than indicated by the same stable isotope data. Although best recognized in the northern and central Sierra Nevada, early to middle Cenozoic paleodrainages may have crossed the southern Sierra Nevada. Similar early to middle Cenozoic paleodrainages existed from central Idaho to northern Sonora, Mexico, and persisted over most of that region until disrupted by major Middle Miocene extension. Therefore, the Nevadaplano was the middle part of an erosional highland that extended along at least this length. The timing of origin and location of this more all-encompassing highland indicates that uplift was predominantly a result of Late Cretaceous (Sevier) contraction in the north and a combination of Late Cretaceous–early Cenozoic (Sevier and Laramide) contraction in the south.

3D geophysical inversion modeling of gravity data to test the 3D geologic model of the Bradys geothermal area, Nevada, USA

Three-dimensional geophysical inversion modeling of gravity data has been performed to test the validity of a 3D geologic model constructed for the Bradys geothermal area. Geophysical modeling was implemented in three different ways: (1) fully unconstrained (i.e., no geologic data included); (2) constrained by the 3D geologic model using homogeneous rock unit densities, and (3) constrained by the 3D geologic model using heterogeneous rock unit densities. We show that the existing 3D geologic model of the Bradys area is broadly consistent with the gravity data. At a more detailed level, however, our analysis suggests that some adjustments to the Bradys 3D geologic model would improve agreement between the observed gravity and the calculated gravity response. The results of the geophysical inversion modeling are important as they serve as a guide to show where and how the boundaries of the 3D geologic model may need to be adjusted to address density excesses and deficiencies. A 3D geologic model that has been independently tested prior to drilling (using a method such as that described in this paper) will be more robust and have less uncertainty than those which have not been tested. Such an approach will facilitate a reduction in drilling risk, lead to more successful drilling programs, and provide valuable geologic input to improve the accuracy of reservoir models.

Paleogeographic implications of late Miocene lacustrine and nonmarine evaporite deposits in the Lake Mead region: Immediate precursors to the Colorado River

Thick late Miocene nonmarine evaporite (mainly halite and gypsum) and related lacustrine limestone deposits compose the upper basin fill in half grabens within the Lake Mead region of the Basin and Range Province directly west of the Colorado Plateau in southern Nevada and northwestern Arizona. Regional relations and geochronologic data indicate that these deposits are late synextensional to postextensional (ca. 12–5 Ma), with major extension bracketed between ca. 16 and 9 Ma and the abrupt western margin of the Colorado Plateau established by ca. 9 Ma. Significant accommodation space in the half grabens allowed for deposition of late Miocene lacustrine and evaporite sediments. Concurrently, waning extension promoted integration of initially isolated basins, progressive enlargement of drainage nets, and development of broad, low gradient plains and shallow water bodies with extensive clastic, carbonate, and/or evaporite sedimentation. The continued subsidence of basins under restricted conditions also allowed for the preservation of particularly thick, localized evaporite sequences prior to development of the through-going Colorado River. The spatial and temporal patterns of deposition indicate increasing amounts of freshwater input during the late Miocene (ca. 12–6 Ma) immediately preceding arrival of the Colorado River between ca. 5.6 and 4.9 Ma. In axial basins along and proximal to the present course of the Colorado River, evaporite deposition (mainly gypsum) transitioned to lacustrine limestone progressively from east to west, beginning ca. 12–11 Ma in the Grand Wash Trough in the east and shortly after ca. 5.6 Ma in the western Lake Mead region. In several satellite basins to both the north and south of the axial basins, evaporite deposition was more extensive, with thick halite (>200 m to 2.5 km thick) accumulating in the Hualapai, Overton Arm, and northern Detrital basins. Gravity and magnetic lows suggest that thick halite may also lie within the northern Grand Wash, Mesquite, southern Detrital, and northeastern Las Vegas basins. New tephrochronologic data indicate that the upper part of the halite in the Hualapai basin is ca. 5.6 Ma, with rates of deposition of ∼190–450 m/m.y., assuming that deposition ceased approximately coincidental with the arrival of the Colorado River. A 2.5-km-thick halite sequence in the Hualapai basin may have accumulated in ∼5–7 m.y. or ca. 12–5 Ma, which coincides with lacustrine limestone deposition near the present course of the Colorado River in the region. The distribution and similar age of the limestone and evaporite deposits in the region suggest a system of late Miocene axial lakes and extensive continental playas and salt pans. The playas and salt pans were probably fed by both groundwater discharge and evaporation from shallow lakes, as evidenced by sedimentary textures. The elevated terrain of the Colorado Plateau was likely a major source of water that fed the lakes and playas. The physical relationships in the Lake Mead region suggest that thick nonmarine evaporites are more likely to be late synextensional and accumulate in basins with relatively large catchments proximal to developing river systems or broad elevated terranes. Other basins adjacent to the lower Colorado River downstream of Lake Mead, such as the Dutch Flat, Blythe-McCoy, and Yuma basins, may also contain thick halite deposits.