Detailed mapping and reevaluation of biostratigraphic data provide new insights into the regional stratigraphic significance of the Ordovician Comus Formation at its type locality at Iron Point, Edna Mountain, Humboldt County, Nevada. Mapping of the internal stratigraphy of the Comus Formation yielded six new subunits and a previously unrecognized formation that is potentially correlative to the Middle Ordovician Eureka Quartzite. The age designation of the Comus Formation was reexamined, using the most current understanding of Ordovician graptolite biostratigraphy. The species of graptolites found in the Comus strata at Iron Point are Late Ordovician, in contrast to the Middle Ordovician age assignment in previous studies. Structural analyses using the new detailed mapping revealed six deformational events at Iron Point. The first fold set, F 1 , is west-vergent and likely correlative to mid-Pennsylvanian folds observed nearby at Edna Mountain. The second fold set, F 2 , records north–south contraction and is likely correlative to Early Permian folds observed at Edna Mountain. The King fault is a normal fault that strikes north and dips east. It truncates the F 1 and F 2 fold sets and has not been active since the Early Permian. The Silver Coin thrust strikes east, places the Ordovician Vinini Formation over the Comus Formation, truncates the King fault, and is not affected by the F 1 and F 2 fold sets. Timing of the Silver Coin thrust is unknown, but it is likely post-Early Permian based on crosscutting relationships. The West fault strikes southeast and dips southwest. It truncates the Silver Coin thrust on the west, and the fault surface records several phases of motion. Finally, Iron Point is bounded on the east side by the Pumpernickel fault, a normal fault that strikes north and dips east. The movement on this structure is likely related to Miocene to Recent Basin and Range faulting. Several key findings resulted from this detailed study of the Ordovician rocks at Iron Point. (1) Based on detailed mapping of the internal stratigraphy of the Comus Formation at Iron Point, it is here interpreted to be correlative with the autochthonous Late Ordovician Hanson Creek Formation rather than the well-known “Comus Formation” that hosts Carlin-style gold mineralization in the Osgood Mountains to the north. (2) The Comus Formation at Iron Point is autochthonous, and the Roberts Mountains thrust is not present at Iron Point, either at the surface or in the subsurface. (3) The stratigraphic mismatch between Iron Point and Edna Mountain requires a fault with significant lateral offset between the two areas; its current expression could be the West fault. (4) West- and southwest-vergent structures at Iron Point and Edna Mountain are rotated counterclockwise relative to northwest-vergent structures at Carlin Canyon and elsewhere in northern Nevada. This relationship is consistent with large-scale sinistral slip along the continental margin to the west.

This paper reports the structural and stratigraphic history of Buck Mountain, Nevada, and its regional significance in the development of southwestern Laurentia during the late Paleozoic. The two distinct generations of folding have similar style and/or timing to other fold sets in late Paleozoic strata of northern Nevada. Unconformities in the upper Paleozoic strata at Buck Mountain are consistent with unconformities documented in northern and east-central Nevada. Northwest-vergent folds (F 1 ) in the Morrowan–Atokan Ely Limestone are erosionally truncated and unconformably overlain by the middle Desmoinesian Hogan Formation and middle Wolfcampian (Sakmarian) Upper Strathearn Formation. This upper Paleozoic stratigraphic package was subsequently refolded by the Buck Mountain Syncline and associated mesoscale folds (F 2 ). F 2 folds lack tight age control but are interpreted to be associated with the Cretaceous central Nevada thrust belt. Critically, none of these structures are localized above or below low-angle faults. The unconformity between the Ely and Hogan formations is consistent with the C5 regional unconformity. Importantly, it constrains the age of northwest-vergent deformation on Buck Mountain. West-vergent folds and west-directed thrusts are documented at several locations in northern and east-central Nevada, but because of the dominance of the C6 unconformity and/or lack of robust age control, the age of these structures has not been tightly constrained. The evidence at Buck Mountain indicates that west-vergent structures predate the C5 unconformity. Buck Mountain is important because it: (1) precisely brackets the age of west-vergent deformation in Nevada to pre–mid-Desmoinesian (sub C5-unconformity) and (2) defines a southeastern edge to the late Paleozoic west-vergent deformation in northern and east-central Nevada.

Recent tectonic reinterpretations of the Late Paleozoic Southwest Laurentian margins recognize widespread Late Paleozoic deformation as a critical component in the boundary region development. Overprinted late Paleozoic structures record repeated shortening events in both northern and southern Nevada, but spatial and temporal data are currently lacking to resolve the evolution of this margin. The Timpahute Range, south-central Nevada, bridges part of the spatial gap between previous detailed studies of Late Paleozoic deformation. The purpose here is to (1) evaluate structures in the area that do not appear to fit with recognized Sevier hinterland structures (the Central Nevada thrust belt [CNTB]) and (2) consider whether these contractional structures may be Late Paleozoic and possibly link, or not, structures to the north and south. New mapping in the Timpahute Range documents four geometrically or kinematically distinct sets of structures: Tempiute Ridge folds, Schofield Pass fault zone (SPFZ), structures of the CNTB, and Cenozoic extensional faults. The first three are interpreted to represent separate shortening events based on cross-cutting relations and differences in orientations of the Tempiute Ridge folds and SPFZ (north [N]), and structures of the CNTB (northwest [NW]). The Tempiute Ridge folds represent the oldest event, D 1 . These folds are large, trend N and verge east (E). The SPFZ is west (W)-vergent, cuts across the limb of a D 1 fold and represents D 2 . The SPFZ is interpreted to be older than the CNTB structures, D 3 , based on positions of fault cut offs, and differences in footwall and hangingwall facies. All of the shortening events predate the newly dated 102.9 ± 3.2 Ma Lincoln stock and its contact metamorphic aureole. New and previous correlations suggest that a belt of Permian deformation extends from southeast (SE) California northward at least to the Timpahute Range. The Tempiute Ridge folds and SPFZ have the same distinctive geometries, styles, and kinematics as structures in the Nevada National Security Site. The mountain-size, E-vergent Tempiute Ridge folds and the W-vergent SPFZ correlate to structures associated with the Belted Range thrust and the W-vergent CP thrust, respectively. The Belted Range thrust previously has been correlated southward into the Death Valley region. Thus, convergence created large-amplitude folds and thrusts for ~200 km along strike. Structures of this age are exposed in northern Nevada but are smaller. These new relations fill a data gap and suggest differences in the size and structural style of Permian structures along strike and corresponding variations in the plate boundary configuration.

Analysis and correlation of strata in ancient basins are commonly difficult due to a lack of high-resolution age control. This study tackled this problem for the latest Mississippian to middle Pennsylvanian Ely–Bird Spring basin. Here, 1095 new carbon isotope analyses combined with existing biostratigraphy at six sections throughout the basin constrain changes in relative sediment accumulation rates in time and space. The Ely–Bird Spring basin contains dominantly shallow-water carbonates exposed in eastern and southern Nevada, western Utah, and southeastern California. It formed as part of the complex late Paleozoic southwestern Laurentian plate margin. However, the detailed evolution of the basin, and hence the tectonic driver(s) of deformation, is poorly understood. The combined isotopic and biostratigraphic data were correlated using the Match-2.3 dynamic programming algorithm. The correlations show a complex picture of sediment accumulation throughout the life of the Ely–Bird Spring basin. Initially, the most rapid sediment accumulation was in the eastern part of the basin. Throughout Morrowan time, the most rapid sediment accumulation migrated to the northwestern part of the basin, culminating in a peak of sediment accumulation in Atokan time. This peak records tectonic loading at the north or northwest margin of the basin. Basin sedimentation was interrupted by early Desmoinesian time in the north by formation of northwest-directed thrust faults, folds, uplift, and an associated unconformity. Deposition continued in the south with a correlative conformity and increased clastic input. The combination of isotopic and biostratigraphic data for correlation is therefore a valuable tool for elucidating temporal basin evolution and can be readily applied to tectonically complex carbonate basins worldwide.

Abstract The Paleogene sedimentary deposits of the Colorado Headwaters Basin provide a detailed proxy record of regional deformation and basin subsidence during the Laramide orogeny in north-central Colorado and southern Wyoming. This field trip presents extensive evidence from sedimentology, stratigraphy, structure, palynology, and isotope geochronology that shows a complex history that is markedly different from other Laramide synorogenic basins in the vicinity. We show that the basin area was deformed by faulting and folding before, during, and after deposition of the Paleogene rocks. Internal unconformities have been identified that further reflect the interaction of deformation, subsidence, and sedimentation. Uplift of Proterozoic basement blocks that make up the surrounding mountain ranges today occurred late in basin history. Evidence is given to reinterpret the Independence Mountain uplift as the result of significant normal faulting (not thrusting), probably in middle Tertiary time. While the Denver and Cheyenne Basins to the east were subsiding and accumulating sediment during Late Cretaceous time, the Colorado Headwaters Basin region was experiencing vertical uplift and erosion. At least 1200 m of the upper part of the marine Upper Cretaceous Pierre Shale was regionally removed, along with Fox Hills Sandstone shoreline deposits of the receding Interior Seaway as well as any Laramie Formation–type continental deposits. Subsidence did not begin in the Colorado Headwaters Basin until after 60.5 Ma, when coarse, chaotic, debris-flow deposits of the Paleocene Windy Gap Volcanic Member of the Middle Park Formation began to accumulate along the southern basin margin. These volcaniclastic conglomerate deposits were derived from local, mafic-alkalic volcanic sources (and transitory deposits in the drainage basin), and were rapidly transported into a deep lake system by sediment gravity currents. The southern part of the basin subsided rapidly (roughly 750–1000 m/m.y.) and the drainage system delivered increasing proportions of arkosic debris from uplifted Proterozoic basement and more intermediate-composition volcanic-porphyry materials from central Colorado sources. Other margins of the Colorado Headwaters Basin subsided at slightly different times. Subsidence was preceded by variable amounts of gentle tilting and localized block-fault uplifts. The north-central part of the basin that was least-eroded in early Paleocene time was structurally inverted and became the locus of greatest subsidence during later Paleocene-Eocene time. Middle Paleocene coal-mires formed in the topographically lowest eastern part of the basin, but the basin center migrated to the western side by Eocene time when coal was deposited in the Coalmont district. In between, persistent lakes of variable depths characterized the central basin area, as evidenced by well-preserved deltaic facies. Fault-fold deformation within the Colorado Headwaters Basin strongly affected the Paleocene fluvial-lacustrine deposits, as reflected in the steep limbs of anticline-syncline pairs within the McCallum fold belt and the steep margins of the Breccia Spoon syncline. Slivers of Proterozoic basement rock were also elevated on steep reverse faults in late Paleocene time along the Delaney Butte–Sheep Mountain–Boettcher Ridge structure. Eocene deposits, by and large, are only gently folded within the Colorado Headwaters Basin and thus reflect a change in deformation history. The Paleogene deposits of the Colorado Headwaters Basin today represent only a fragment of the original extent of the depositional basin. Basal, coarse conglomerate deposits that suggest proximity to an active basin margin are relatively rare and are limited to the southern and northwestern margins of the relict basin. The northeastern margin of the preserved Paleogene section is conspicuously fine-grained, which indicates that any contemporaneous marginal uplift was far removed from the current extent of preserved fluvial-lacustrine sediments. The conspicuous basement uplifts of Proterozoic rock that flank the current relict Paleogene basin deposits are largely post-middle Eocene in age and are not associated with any Laramide synuplift fluvial deposits. The east-west–trending Independence Mountain fault system that truncates the Colorado Headwaters Basin on the north with an uplifted Proterozoic basement block is reinterpreted in this report. Numerous prior analyses had concluded that the fault was a low-angle, south-directed Laramide thrust that overlapped the northern margin of the basin. We conclude instead that the fault is more likely a Neogene normal fault that truncates all prior structure and belongs to a family of sub-parallel west-northwest–trending normal faults that offset upper Oligocene-Miocene fluvial deposits of the Browns Park–North Park Formations.

Abstract This field trip highlights recent research into the Laramide uplift, erosion, and sedimentation on the western side of the northern Colorado Front Range. The Laramide history of the North Park-Middle Park basin (designated the Colorado Headwaters Basin in this paper) is distinctly different from that of the Denver basin on the eastern flank of the range. The Denver basin stratigraphy records the transition from Late Cretaceous marine shale to recessional shoreline sandstones to continental, fluvial, marsh, and coal mires environments, followed by orogenic sediments that span the K-T boundary. Upper Cretaceous and Paleogene strata in the Denver basin consist of two mega-fan complexes that are separated by a 9 million-year interval of erosion/non-deposition between about 63 and 54 Ma. In contrast, the marine shale unit on the western flank of the Front Range was deeply eroded over most of the area of the Colorado Headwaters Basin (approximately one km removed) prior to any orogenic sediment accumulation. New 40 Ar- 39 Ar ages indicate the oldest sediments on the western flank of the Front Range were as young as about 61 Ma. They comprise the Windy Gap Volcanic Member of the Middle Park Formation, which consists of coarse, immature volcanic conglomerates derived from nearby alkalic-mafic volcanic edifices that were forming at about 6561 Ma. Clasts of Proterozoic granite, pegmatite, and gneiss (eroded from the uplifted at Laramide basin subsidence, sedimentation, and deformation in north-central Colorado, in Morgan, L.A., and Quane, S.L., eds., Through the Generations: core of the Front Range) seem to arrive in the Colorado Headwaters Basin at different times in different places, but they become dominant in arkosic sandstones and conglomerates about one km above the base of the Colorado Headwaters Basin section. Paleocurrent trends suggest the southern end of the Colorado Headwaters Basin was structurally closed because all fluvial deposits show a northward component of transport. Lacustrine depositional environments are indicated by various sedimentological features in several sections within the >3 km of sediment preserved in the Colorado Headwaters Basin, suggesting this basin may have remained closed throughout the Paleocene and early Eocene. The field trip also addresses middle Eocene(?) folding of the late Laramide basin-fill strata, related to steep reverse faults that offset the Proterozoic crystalline basement. Late Oligocene magmatic activity is indicated by dikes, plugs, and eruptive volcanic rocks in the Rabbit Ears Range and the Never Summer Mountains that span and flank the Colorado Headwaters Basin. These intrusions and eruptions were accompanied by extensional faulting along predominantly northwesterly trends. Erosion accompanied the late Oligocene igneous activity and faulting, leading to deposition of boulder conglomerates and sandstones of the North Park Formation and high-level conglomerates across the landscape that preserve evidence of a paleo-drainage network that drained the volcanic landscape.

As of October 2005, the semipermanent Global Positioning System (GPS) network called MAGNET (Mobile Array of GPS for Nevada Transtension) included 60 stations and spanned 160 km (N-S) × 260 km (E-W) across the northern Walker Lane and central Nevada seismic belt. MAGNET was designed as a cheaper, higher-density alternative to permanent networks in order to deliver high-accuracy velocities more rapidly than campaigns. The mean nearest-neighbor spacing is 19 km (13–31 km range). At each site, the design facilitates equipment installation and pickup within minutes, with the antenna mounted precisely at the same location to mitigate eccentricity error and intersession multipath variation. Each site has been occupied ~50% of the time to sample seasonal signals. Using a custom regional filtering technique to process 1.5 yr of intermittent time series, the longest-running sites are assessed to have velocity accuracies of ~1 mm/yr. The mean weekly repeatability is 0.5 mm in longitude, 0.6 mm in latitude, and 2.1 mm in height. Within a few years, MAGNET will characterize strain partitioning in the northern Walker Lane to improve models of (1) geothermal activity, which is largely amagmatic in the Great Basin, (2) seismic hazard, (3) the ways in which northern Walker Lane accommodates strain between the Sierra Nevada block and the extending Basin and Range Province, and (4) Neogene development of the northern Walker Lane and its broader role in the ongoing evolution of the Pacific–North America plate-boundary system. MAGNET’s design is generally applicable to regions with an abundance of vehicle-accessible rock outcrops and could be replicated elsewhere.

We present a velocity and strain rate model for the northern Walker Lane derived from a compilation of geodetic velocities and corrected for transient effects owing to historic earthquakes on the Central Nevada seismic belt. We find that from 37°N to 40°N, the Walker Lane is characterized by an ~100-km-wide zone with near-constant strain rates associated with ~10 mm yr −1 total motion across the zone. The strain rates depict predominantly shear deformation, but south of 39°N, the extensional component of the strain rate tensor increases and thus reflects more of a transtensional domain there. We conclude that this transtension is a kinematic consequence of the motion of the Sierra Nevada–Great Valley block, which is not parallel to its eastern margin, i.e., the eastern Sierra front, south of 39°N. While the orientations of several normal and strike-slip faults in the Walker Lane region are consistent with the strain rate model results at several places, the mode and rate at which geologic structures accommodate the deformation are less clear. Left-lateral faulting and clockwise rotations there may contribute to the accommodation of the velocity gradient tensor field, and most normal faults are properly oriented to accommodate some component of the regional shear strain, but significant additional right-lateral strike-slip faulting is required to accommodate the majority of the 10 mm yr −1 relative motion. Overall, the along-strike variation in the active tectonics of Walker Lane suggests that (1) various mechanisms are at play to accommodate the shear, (2) parts of the surface tectonics may (still) be in an early stage of development, and (3) inherited structural grain can have a dominant control on the strain accommodation mechanism.

We combine horizontal Global Positioning System (GPS) velocities from a new compilation of published and new GPS velocities, results from an interferometric synthetic aperture radar (InSAR) study, and paleoseismic data to evaluate the postseismic response of historic earthquakes in the Central Nevada seismic belt. We assume that GPS velocity has contributions from time-invariant (i.e., steady permanent crustal deformation) and transient (i.e., time varying and associated with the seismic cycle) processes that are attributable to postseismic viscoelastic relaxation of the crust and upper mantle. In order to infer the viscosity structure of Basin and Range lower crust, η LC , and upper mantle, η UM , we apply three objective criteria to identify rheological models that fit both geodetic and geologic data. The model must (1) improve the apparent mismatch between geodetically and geologically inferred slip rates, (2) explain the InSAR-inferred vertical uplift rate, and (3) not imply time-invariant contractions anywhere in the extending province. It is not required for the postseismic deformation field to resemble the time-invariant velocity field in pattern, rate, or style. We find that the InSAR and horizontal GPS velocities form complementary constraints on the viscoelastic structure, excluding different parts of the model space. The best-fitting model has a lower crust that is stronger than the uppermost mantle, with η LC = 10 20.5 Pa·s and η UM = 10 19 Pa·s, a finding consistent with the majority of similar studies in the Basin and Range. The best-fitting viscosity model implies that the majority of Central Nevada seismic belt deformation is attributable to postseismic relaxation, and hence that western Basin and Range time-invariant deformation north of 39°N latitude is more tightly focused into the northern Walker Lane than would be inferred from uncorrected GPS velocities. However, significant deformation remains after correction for postseismic effects, consistent with Central Nevada seismic belt faults slipping at rates intermediate between the Walker Lane belt and the central Basin and Range.

Deviation between velocity trajectories from global positioning system (GPS) networks and strain trajectories from earthquake focal mechanisms and fault-slip inversion within the central Walker Lane are reconciled as the consequence of non–plane strain (constriction) within a transtensional zone separating the Sierra Nevada and central Great Basin. Dextral transtension within the central Walker Lane is produced by differential displacement of the Sierra Nevada with respect to the central Great Basin, and it is partitioned into domains exhibiting simple shear–dominated and pure shear–dominated strain. From east to west across the central Walker Lane, GPS velocities change orientation from west-northwest to northwest and increase from 2–3 to 12–14 mm/yr as the incremental-strain elongation axis changes from west-northwest to west-southwest. The deviation between strain and velocity trajectories increases to 50° as the Sierra Nevada Range is approached from the east. This deviation in strain and velocity trajectories is consistent with analytical models linking kinematic vorticity, particle velocity paths, and incremental non–plane strain during transtensional deformation. We link field observations to the analytical models using a polar Mohr construction in a system that conserves kinematic boundary conditions to graphically demonstrate that the relationship between velocity and strain fields is a consequence of constrictional deformation.

We present marine high-resolution seismic reflection data from Upper Klamath Lake, Oregon, to discern the underlying structure and estimate Quaternary slip rates in this actively extending Basin and Range system. The sediment patterns and structures imaged on our seismic profiles reveal a complex geologic system that reflects a changing climate record, shallow water conditions, growth faulting, contrasting sediment sources, and high slip rates. We observe that Upper Klamath Lake is a sediment-saturated environment, and sediment accumulation rates are therefore controlled by basin subsidence rather than sediment supply. Published slip rates for Holocene extension are greater than our determined late Quaternary slip rates, assuming reasonable rates of deposition. The apparent increased Holocene fault-slip rates may be in part an artifact of long recurrence intervals between major earthquakes, with recent seismicity accommodating long-term strain. The quantity of observed faults below the lake is at least an order of magnitude greater than those mapped outside the lake, suggesting that many hidden faults throughout the region may be unaccounted for when estimating Basin and Range extension rates.