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.

The Late Permian to earliest Triassic Sonoma orogeny has long been envisioned as the result of an arc-continent collision that closed the Havallah oceanic basin, creating the Golconda allochthon, which was emplaced eastward onto the western edge of the continental margin along the Golconda thrust. Critical reevaluation of available stratigraphic, biostratigraphic, and structural data raise some fundamental issues with this scenario, including: (1) The Golconda allochthon experienced multiple phases of deformation both older and younger than the Sonoma orogeny; (2) the tectonostratigraphic successions in the Golconda allochthon record a disrupted depositional history; (3) these punctuated events and unconformities are mirrored by simultaneous punctuated tectonic disruptions of the adjacent continental margin; (4) some of the lithotectonic units within the Golconda allochthon have clear ties to a magmatic arc. These observations indicated that the Havallah basin did not originate as a simple, post-Antler orogeny rift basin, nor is the Mediterranean model for opening of a basin a solution to the initiation of this basin. Instead they imply a more complex paleogeography for the Havallah basin. The Late Permian–earliest Triassic closure of the Havallah basin did result in the development of the Golconda allochthon sensu stricto , but final emplacement of the Golconda allochthon was likely an Early–Middle Jurassic event.

ABSTRACT This field trip traverses the Sahwave and Nightingale Ranges in central Nevada, USA, and northward to Gerlach, Nevada, to the Granite, northern Fox, and Selenite Ranges. Plutonic bodies in this area include the ca. 93–89 Ma Sahwave nested intrusive suite of the Sahwave and Nightingale Ranges, the ca. 106 Ma Power Line intrusive complex of the Nightingale Range, the ca. 96 Ma plutons in the Selenite Range, and the ca. 105–102 Ma plutons of the Granite and Fox Ranges. Collectively these plutons occupy nearly 1000 km 2 of bedrock exposure. Plutons of the Sahwave, Nightingale, and Selenite Ranges intrude autochthonous rocks east of the western Nevada shear zone, while plutons of the Granite and Fox Ranges intrude displaced terranes west of the western Nevada shear zone. Integrated structural, geochemical, and geochronological studies are used to better understand magmatic and deformation processes during the Early Cretaceous, correlations with Cretaceous plutons in adjacent areas of Idaho and California, and regional implications. Field-trip stops in the Sahwave and Nightingale Ranges will focus on: (1) microstructure and orientation of magmatic and solid-state fabrics of the incrementally emplaced granodiorites-granites of the Sahwave intrusive suite; and (2) newly identified dextral shear zones hosted within intrusions of both the Sahwave and Nightingale Ranges. The Sahwave intrusive suite exhibits moderate to weak magnetic fabrics determined using anisotropy of magnetic susceptibility, with magnetic foliations that strike NW-NE and magnetic lineations that plunge moderately to steeply. Microstructural analysis indicates that these fabrics formed during magmatic flow. The older Power Line intrusive complex in the Nightingale Range is cross-cut by the Sahwave suite and contains a N-S–trending solid-state foliation that reflects ductile dextral shearing. Field-trip stops in the plutons of the Gerlach region will focus on composition, texture, and emplacement ages, and key differences with the younger Sahwave suite, including lack of evidence for zoning and solid-state fabrics. The field trip will utilize StraboSpot, a digital data system for field-based geology that allows participants to investigate the relevant data projects in the study areas.

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.