Cenozoic Tectonic Evolution of the Central Wassuk Range, Western Nevada, USA

The central Wassuk Range of western Nevada is ideally located to investigate the interplay of Basin and Range extension and Walker Lane dextral deformation on the margin of the Basin and Range province. To elucidate the Cenozoic evolution of the range, the author conducted geologic mapping, structural data collection and analysis, geochemical analysis of igneous lithologies, and geochronology. This research delineates a three-stage deformational history for the range. A ~15 Ma pulse of ENE-WSW directed extension at high strain rates (~8.7 Ma) was immediately preceded by the eruption of andesites and was accommodated by high-angle, closely spaced (1-2 km), east-dipping normal faults which rotated and remained active to low angles as extension continued. A post-12 Ma period of extension at low strain rates was accommodated by a second generation of normal faults and two prominent dextral strike slip faults which strike NW, subparallel to the dextral faults of the Walker Lane at this latitude. A new pulse of extension began at ~4 Ma and continues today. The increase in the rate of range-bounding fault displacement has resulted in impressive topographic relief on the east flank of the Wassuk Range and supports a shift in extension direction from ENE-WSW during the highest rates of Miocene extension to WNW-ESE today. The total extension accommodated across the central Wassuk Range since the Middle Miocene is \u3e 200%, with only a brief period of dextral fault activity during the Late Miocene. Data presented here suggest a local geologic evolution intimately connected to regional tectonics, from intra-arc extension in the Middle Miocene, to Late Miocene dextral deformation associated with the northward growth of the San Andreas fault, to a Pliocene pulse of extension and magmatism likely influenced by both the northward passage of the Mendocino triple junction and possible delamination of the crustal root of the southern Sierra Nevada

Modern Strain Localization in the Central Walker Lane, Western United States: Implications for the Evolution of Intraplate Deformation in Transtensional Settings

Approximately 25% of the differential motion between the Pacific and North American plates occurs in the Walker Lane, a zone of dextral motion within the western margin of the Basin and Range province. At the latitude of Lake Tahoe, the central Walker Lane has been considered a zone of transtension, with strain accommodated by dip-slip, strike-slip, and oblique-slip faults. Geologic data indicate that extension and strike-slip motion are partitioned across the central Walker Lane, with dip-slip motion resulting in E–W to ESE–WNW extension along the present-day western margin of the central Walker Lane since approximately 15 Ma, and dextral strike-slip motion across a zone further east since as early as 24 Ma. GPS velocity data suggest that present-day strain continues to be strongly partitioned and localized across the same regions established by geologic data. Velocity data across the central Walker Lane suggest a minimum of 2 mm/yr extensional strain focused along the western margin of the belt, with very little extension across either the central or eastern portions of the Walker Lane. These data indicate very little dextral motion across the central and western portions of the domain, with dextral motion of 3–5 mm/yr presently focused along a discrete zone of the eastern part of the central Walker Lane, coincident with existing, mapped strike-slip faults. Historic seismic data reveal little seismic activity in areas of Late Holocene dip-slip motion in the west or dextral motion in the east, suggesting a period of quiescence in the earthquake cycle and the likelihood of future activity in both areas. Based on this and previous studies, it is likely that a combination of pre-Cenozoic crustal structure, a relatively weak lithosphere beneath the Walker Lane, and long-term low stress ratios in the crust have permitted the long-term partitioning of dextral and extensional strain exhibited across the central Walker Lane. The present-day location of dextral strain in the central Walker Lane is subparallel with dextral deformation documented in the northern Walker Lane, suggesting that as strain continues to accumulate, these two discrete zones could become a continuous strike-slip system which will play a more important role in the future accommodation of relative Pacific–North American plate motion

Segmentation of the Wassuk Range Normal Fault System, Nevada (USA): Implications for Earthquake Rupture and Walker Lane Dynamics

Normal faults are commonly segmented along strike, with segments that localize strain and influence propagation of slip during earthquakes. Although geometry of segments can be constrained by fault mapping, it is challenging to determine seismically relevant segments along a fault zone. Because slip histories, geometries, and strength of linkages between normal fault segments fundamentally control the propagation of rupture during earthquakes, and differences in segment slip rates result in differential uplift of adjacent footwalls, we use along‐ strike changes in footwall morphology to detect fault segments and the relative strength of the mechanical links between them. We apply a new geomorphic analysis protocol to the Wassuk Range fault, Nevada, within the actively deforming Walker Lane. The protocol examines characteristics of footwall morphology, including range‐crest continuity, bedrock‐channel long profiles, catchment area variability, and footwall relief, to detect changes in strike‐parallel footwall characteristics. Results reveal six domains with significant differences in morphology that we use to identify seismically‐relevant fault segments and segment boundaries. We integrate our results with previous studies to determine relative strength of links between the six segments, informing seismic hazard assessment. When combined with recent geodetic studies, our results have implications for the future evolution of the Walker Lane, suggesting changes in the accommodation of strain across the region. Our analysis demonstrates the power of this method to efficiently detect along‐strike changes in footwall morphology related to fault behavior, permitting future researchers to perform reconnaissance assessment of normal fault segmentation worldwide

The Impact of Inter-Bed Cohesion on Fold-Related Fracture Development, Stillwell Anticline, West Texas (USA)

The interpretation of fracture networks in contractional folds is challenging due to the range of factors that control fracture formation. We use outcrop-based analysis of fractures in plan-view pavements and in a 9-bed cross-sectional exposure to evaluate the fracture system within Cretaceous limestone layers of a Laramide fold in west Texas, the Stillwell anticline. Opening-mode extension fractures (veins) at high angles to bedding dominate the fracture population, although shear fractures and faults cut bedding at low angles within the forelimb. Analysis of extension fractures reveals NW-striking axial parallel and NE-striking axial-perpendicular fracture sets interpreted to have formed during contractional folding, a third N-striking fracture set formed during subsequent Basin and Range extension, and a fourth ESE-striking fracture formed due to unloading during exhumation. Fracture fill textures suggest that many fracture apertures increased during exhumation. The relative abundances of the four fracture sets and the intensity of each set vary from bed to bed in cross section. Because beds display no significant differences in mechanical strength and there is no correlation between bed thickness and fracture intensity, we attribute this bed-to-bed variability to differences in cohesion between beds. Bed decoupling, when combined with low extensional

The Unusual Temporal and Spatial Slip History of the Wassuk Range Normal Fault, Western Nevada (USA): Implications for Seismic Hazard and Walker Lane Deformation

We document temporal and spatial variations in vertical displacement rate across 6 temporal orders of magnitude to better under stand how the 100-km-long, east-dipping Wassuk Range normal fault system has accommodated strain in the context of the Walker Lane, a tectonically active, NNWtrending zone of dextral and extensional deformation that affects significant portions of western Nevada and eastern California. We combine 10Be and 26Al cosmonuclide exposure ages with shallow seismic and gravity data from the buried hanging wall of the Wassuk fault to derive a post-113 ka (105 yr time scale) vertical displacement rate of 0.82 ± 0.16 mm/yr. We also perform largescale fault scarp analysis to constrain the long-term (\u3e1 Ma; 106 yr time scale) displacement rate. Our fault-scarp analysis results imply similar vertical displacement rates, with higher long-term vertical displacement rates along the southern fault (-1.1 mm/yr) relative to the northern fault (6, 105, 103, and 101 yr time scales (this study and others) support a constant vertical displacement rate between 0.75 and 1.0 mm/yr for the Wassuk Range fault since ca. 4 Ma. An anomalously high vertical displacement rate at the 104 yr time scale is best explained by an earthquake cluster between ca. 15.5 ka and ca. 10.5 ka, potentially linked to rapid filling of the Walker Lake basin immediately prior to the ca. 13 ka Sehoo highstand of ancestral Lake Lahontan. We hypothesize that this flood event induced seismicity by placing an additional load on the hanging wall of the Wassuk Range fault and by increasing the pore-fluid pressure within and adjacent to the fault. Although an earthquake cluster like this is consistent with Wallace-type fault behavior, we suggest that a nontectonic stressor induced the cluster, resulting in the apparent discrepancy in vertical displacement rate at the 104 yr time scale. Thus, we posit that the long-term slip along Wassuk fault is better explained by slip-predictable Reid-type behavior, which deviates from the behavior of other well-documented fault systems. Based on these results, we suggest that similar, unrecognized nontectonic stressors may influence rates of strain release along other major fault systems worldwide. Finally, we present a revised model of central Walker Lane kinematics, based on data from this and other recent studies

The Unusual 3D Interplay of Basement Fault Reactivation and Fault-Propagation-Fold Development: A Case Study of the Laramide-age Stillwell Anticline, West Texas (USA)

Subsurface fault geometries have a systematic influence on folds formed above those faults. We use the extraordinarily well-exposed fold geometries of the Laramide-age Stillwell anticline in west Texas (USA) to develop a strain-predictive model of fault-propagation fold formation. The anticline is a 10-km long, NW-trending, NE-vergent, asymmetric fold system with an axis that displays a map-view left-stepping, en echelon pattern. We integrated field observations, geologic and structural data, cross-sections, and 2D kinematic modeling to establish an unusual 3D two-stage model of contractional fold formation, including: 1) reverse reactivation of a pre-existing, NW-striking, SW-dipping, left-stepping, en echelon normal fault system in Paleozoic basement rocks to generate monoclinal flexures in overlying layered Cretaceous carbonate rocks; and 2) the formation of a subsequent flat-ramp fault system that propagated horizontally along a mechanically-weak, clay-rich Cretaceous unit before ramping up at the hinge of the pre-existing monocline system. Strain is focused within the forelimb of the system, in front of the propagating fault tip, and is accommodated by a combination of interlayer slip, flat-ramp faulting, and fracturing proximal to planes of slip. This strain predictive model can be applied to similar, less-well-exposed contractional systems worldwide and provides a new, unusual example of Laramide-age contractional deformation

Developing Scientific Literacy in Introductory Laboratory Courses: A Model for Course Design and Assessment

Although science educators at all levels have focused on teaching students scientific literacy for nearly five decades, studies indicate that the average student remains far from scientifically literate. To address this issue at the local level, faculty at Trinity University, in San Antonio, Texas, significantly revised the curriculum of an existing introductory physical geology laboratory course. The course, which satisfies general education requirements at Trinity, was revised to provide students learning opportunities in a scientific process context as part of a new science literacy initiative. This effort was spurred by general dissatisfaction with the existing curricular structure of the course as well as a new interdisciplinary, National Science Foundation (NSF)–funded initiative to support the integration of research-grade instrumentation in curricula and undergraduate research across campus. The physical geology laboratory course revision was based on research that demonstrated the efficacy of learning through active participation, interpretation, iteration, and reflection, especially when knowledge and skills are gained within an explicit scientific process context. In addition to significantly revising laboratory activities, we added new activities within the course framework that involved the use of two new, NSF-funded instruments, including a handheld X-ray fluorescence spectrometer (XRF) and an inductively coupled plasma–optical emissions spectrometer (ICP-OES), which we used to improve student understanding of qualitative and quantitative elemental analyses. Finally, we introduced the use of a new course reader that provided both background materials for each activity as well as a new focus on providing a scientific process context for students. To assess student learning, we used in-class observations, student–instructor discussions, pre- and postlearning questionnaires, prelaboratory quizzes, course activities completed during class time, modified postactivity reflection questions, practical examinations, and a final examination. We also included faculty, staff, and administrator perspectives to qualitatively assess the impact of course changes upon student learning. Our results imply that students achieved the primary learning goals we developed for this scientific literacy initiative, including: (1) improved understanding of the scientific process and the nature of science; (2) improved understanding of qualitative and quantitative elemental research methods; and (3) improved understanding of the applicability of scientific research to real-world problems. Importantly, our findings suggest that the integration of research-grade instrumentation into introductory coursework, in a scientific process context, is an effective way to promote scientific literacy as well as to provide opportunities for students to understand and apply the knowledge and skills necessary to perform scientific research. We believe that this example of a significant course redesign provides a model that can be transferred to other geosciences departments as well as to other scientific disciplines

Thermochronological Constraints on the Timing and Magnitude of Miocene and Pliocene Extension in the Central Wassuk Range, Western Nevada

Apatite fission track and (U-Th)/He thermochronological data provide new constraints on the timing of faulting and exhumation of the Wassuk Range, western Nevada, where east dipping normal faults have accommodated large-magnitude ENE-WSW oriented extension. Extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the Grey Hills and central Wassuk Range. These fault blocks display westward tilts of ∼60° and expose preextensional paleodepths of up to ∼8.5 km, based on the structural reconstruction of tilted preextensional Tertiary andesite flows that unconformably overlie Mesozoic basement rocks. Apatite fission track and (U-Th)/He thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at ∼15 Ma. Fission track modeling results indicate rapid fault block exhumation occurred between ∼15 and 12 Ma, which is in agreement with Miocene volcanic rocks that bracket the tilting history. In addition, fission track and (U-Th)/He data suggest reduced rates of cooling following major extension, as well as renewed cooling related to active, high-angle faulting along the present-day range front starting at ∼4 Ma. Thermochronological data from structurally restored fault blocks indicate a preextensional Miocene geothermal gradient of 27° ± 5°C/km. The thermochronological constraints on the timing of extensional faulting and the eruptive history in the Wassuk Range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension, and where synextensional magmatism is suppressed during the highest rates of extension

Two-Phase Westward Encroachment of Basin and Range Extension into the Northern Sierra Nevada

Structural, geophysical, and thermochronological data from the transition zone between the Sierra Nevada and the Basin and Range province at latitude ~39°N suggest ~100 km westward encroachment of Basin and Range extensional deformation since the middle Miocene. Extension, accommodated primarily by cast dipping normal faults that bound west tilted, range-forming fault blocks, varies in magnitude from150% in the Wassuk and Singatse Ranges to the east. Geological and apatite fission track data from exhumed upper crustal sections in the Wassuk and Singatse Ranges point to rapid footwall cooling related to large magnitude extension starting at ~14-15 Ma. Farther to the west, geological and thermochronological data indicate a younger period of extension in the previously unextended Pine Nut Mountains, the Carson Range, and the Tahoe-Truckee depression initiated between 10 Ma and 3 Ma, and incipient post-0.5 Ma faulting to the west of the Tahoe-Truckee area. These data imply the presence of an extensional breakaway zone between the Singatse Range and the Pine Nut Mountains at ~14-15 Ma, forming the boundary between the Sierra Nevada and Basin and Range at that time. In addition, fission track data imply a Miocene preextensional geothermal gradient of 27 ± 5°C km -1 in the central Wassuk Range and 20 ± 5°C km -1 in the Singatse Range, much higher than the estimated early Tertiary gradient of 10 ± 5°C km -1 for the Sierra Nevada batholith. This might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting. Apatite fission track, geophysical, and geological constraints across the Sierra Nevada-Basin and Range transition zone indicate a two-stage, coupled structural and thermal westward encroachment of the Basin and Range province into the Sierra Nevada since the middle Miocene

Holocene Earthquakes and Late Pleistocene Slip-Rate Estimates on the Wassuk Range Fault Zone, Nevada

The Wassuk Range fault zone is an 80‐km‐long, east‐dipping, high‐angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0.7±0.1  mm/yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1  m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3‐m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range‐front fault has truncated and uplifted a remnant of the Penrod Canyon fan by \u3e40  m since the surface was deposited ∼113  ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at \u3e0.3–0.4  mm/yr for the Wassuk Range fault zone