Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80-km-long Cheraw fault may be one of the most active faults in North America east of the Southern Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to 1) document the rupture history of the ~45-km-long southwestern section of the Cheraw fault over the past ~19 ka, and 2) evaluate slip-rate changes for the entire fault over the past ~200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface-rupture events since ~19 ka, each with an average vertical displacement of 0.75 m. An additional event is likely only slightly older than ~19 ka. Evidence for relatively small displacements at and near the Old Ranch site suggests that most of these earthquakes were M 7 or less and likely did not rupture the full length of the Cheraw fault. Since ~19 ka, the average slip rate is ~0.16 mm/yr near the Old Ranch site with an average interevent time of 3 - 5 kyr. New geochronologic data for mid- to late Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from ~145 ka to ~100 ka. Maximum vertical offsets of 7 to 9 m for these surfaces indicate that from ~19 to >200 ka the average slip rate was no greater than ~0.03 mm/yr. The accelerated slip rate since ~19 ka suggests a possible response to rapid erosional unloading and/or a limited late Cenozoic, <40 kyr, paleoseismic history for the Cheraw fault.

The following report summarizes the dating results from sedimentary deposits exposed by soil pits in Panamint Valley, CA. Within this report, we detail the methodology used by the USGS Luminescence Geochronology Laboratory to obtain ages including sample preparation methods, luminescence measurement, equivalent dose determination, and dating-related calculations.

Deep Springs Valley (DSV) is a hydrologically isolated valley between the White (north and west) and Inyo (south and east) Mountains that is commonly excluded from regional paleohydrologic and paleoclimate studies. Previous studies showed that uplift of Deep Springs ridge (informal name) by the Deep Springs fault defeated streams crossing DSV and hydrologically isolating the valley sometime after eruption of the Bishop Tuff. Here we present tephrochronology, clast counts, paleontology, and infrared stimulated luminescence (IRSL) data that reaffirms interruption of the Plio-Pleistocene hydrology and formation of DSV during the Pleistocene. Fossil gastropod, ostracodes, and charophytes along with IRSL dating document the 83.3-61.5 ka freshwater Deep Springs Lake, which roughly coincides with 71-57 ka Marine Isotope State 4 (MIS 4) glacial climate period. Documentation of the MIS-4 glacial climate in southwestern North America is sparse and pluvial Deep Springs Lake is indirect evidence of the MIS 4 glaciation that is corroborated by pluvial lakes in nearby Owens and Searles Valleys. We hypothesize that the MIS-4 Deep Springs Lake overflowed into Eureka Valley via the Soldier Pass wind gap. Hydrologic evolution of DSV has potential implications for understanding Pliocene and Pleistocene biotic dispersal pathways and endemism.

New bedrock and surficial geologic mapping in the Blue Ridge of NW NC and SW VA covers the Sparta East, Sparta West, and parts of the Glade Valley and Whitehead 7.5-minute quadrangles and includes the epicentral area of the 9 August 2020 Mw 5.1 earthquake near Sparta, NC. Mapping documented (1) the co-seismic surface rupture from the 2020 earthquake and brittle structures in the bedrock; (2) the fault contact between the western Blue Ridge (wBR) and eastern Blue Ridge (eBR); (3) lithostratigraphy in the Lynchburg Group, Wills Ridge Formation, Ashe Metamorphic Suite (AMS) and Alligator Back Metamorphic Suite (ABMS); (5) the nature of the contact between Lynchburg Group, AMS and ABMS; and (4) surficial deposits. The wBR consists of Mesoproterozoic granitoid gneiss (1.3–1.0 Ga) intruded by the Neoproterozoic Striped Rock pluton (~740 Ma). These rocks are overprinted by a late Paleozoic greenschist facies foliation that intensifies into several anastomosing high-strain shear zones of the Fries fault zone; rocks within the shear zones are white-mica phyllonites to ultramylonites. Kinematic indicators consistently document top-to-NW thrust motion. To the southeast, the polydeformed eBR is juxtaposed over the wBR along the Gossan Lead fault, a ~1 km-wide shear zone. Lithostratigraphy in the eBR separates interlayered graphitic mica schists (Zlp), metagraywacke and graphitic schist (Zl), metaconglomerate and metagraywacke (Zlc), and metagabbro (Zlg) into the Lynchburg Group, Wills Ridge Formation. A SHRIMP U-Pb zircon age of ~455 Ma was obtained on a ~1 m thick metagabbro dike that intrudes metagraywacke in the Lynchburg Group. The Lynchburg Group rocks are separated from metaultramafic-bearing rocks of the AMS by an unnamed fault. The AMS consists of metagraywacke and schist (_Za), muscovite schist and metagraywacke (_Zas), metaconglomerate and metagraywacke (_Zac), amphibolite (_Zaa) and metaultramafic schists and rocks (_Zau and _Zaud). NE-SW trending structures dominate the eBR: relict S0 bedding, S1 foliations and intrafolial F1 folds are transposed into a regional S2 foliation (mean 063/52). Map-scale isoclinal F2 folds are overprinted by an S3/F3 crenulation. The structurally higher ABMS consists of pinstriped mica gneiss and schist (_Zab). The contact between the AMS and ABMS is a dextral shear zone. In the epicentral area, the Little River fault (~110/45) is mapped for ~4 km and similarly oriented brittle faults occur in the Bledsoe Creek valley up to 4 km to the NW. Manganese-coated, striated brittle faults and surfaces are common; manganese cemented breccias occur locally. Terrace deposits are mapped above the New River, Little River, Bledsoe Creek, and major tributaries. Terrace deposits above Bledsoe Creek in the epicentral area overlie a brittle fault and yield cosmogenic burial ages of ~500 Ka. Potential paleoliquefaction structures were identified at 4 locations. This report consists of a surficial and bedrock geologic map with cross sections, correlation of map units, and description of map units, and a GeMS level 3 geodatabase. Mapping was supported by the National Cooperative Geologic Mapping Program and Earthquake Hazards Program; a EDMAP Grant supported mapping by A. Lynn and K. G. Stewart.

The following report summarizes the dating results from the. Within this report, we detail the methodology used to determine the storage time distribution for a 17 km length of Powder River in Montana, U.S.A. by the age distribution of eroded sediment. This data is used by the USGS Luminescence Geochronology Laboratory to obtain ages including sample preparation methods, luminescence measurement, equivalent dose determination, and dating-related calculations. We recommend that this report be included as the supplementary material for any publication(s) that use the ages within this report. This version supersedes all previous age estimates and reports.

The following report summarizes the dating results from Bradley Lake, Oregon. Within this report, we detail the methodology used by the USGS Luminescence Geochronology Laboratory to obtain ages including sample preparation methods, luminescence measurement, equivalent dose determination, and datingrelated calculations. We recommend that this report be included as the supplementary material for any publication(s) that use the ages within this report. This version supersedes all previous age estimates and reports.

The data release includes 35 new luminescence ages and 3 new radiocarbon ages. The new age data further brackets the ages of the Las Vegas basin Quaternary stratigraphy and provide new constraints on the timing of Quaternary fault activity.

Significant uncertainty remains in how and where crustal shortening occurs throughout the eastern Cascade Range in Washington State. Using lidar imagery, we identified a ~5 km long lineament in Swakane canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles across the lineament reveal a southwest-side-up break in slope with an average of ~3 m of vertical separation of the hillslope surface. We consider a range of possible origins for this feature, including differential erosion across a fault-line scarp, slope failure (sackung or landslide), and surface deformation across an active fault strand. Based on trenching, radiocarbon and luminescence dating, and ground penetrating radar (GPR) across the lineament, we conclude that warped saprolite observed in the shallow subsurface is most consistent with southwest-side-up folding caused by blind reverse faulting at depth. Following this reasoning, dating of overlying colluvial deposits suggests at least one Holocene earthquake occurred on this strand of the southern Entiat fault with an approximate vertical separation of ≥1 m. GPR reveals up to 4 m of cumulative vertical separation of the saprolite, suggesting a history of multiple earthquakes on the structure. Taken in context with other potential fault-related lineaments along the Entiat fault, our interpretation of Holocene earthquakes in Swakane canyon could suggest reactivation of longer sections of the Entiat fault, as well as other bedrock faults in the eastern Cascades. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, we conclude that the reactivated Entiat fault represents a seismogenic structure that should be considered in regional seismic hazard analyses. The difficulty of recognizing low slip-rate structures in forested and mountainous terrain underscores the importance of additional lidar surveys and geological and geophysical studies for fully understanding seismic hazard in regions with infrequent but potentially large earthquakes.

Significant uncertainty remains in how and where crustal shortening occurs throughout the eastern Cascade Range in Washington State. Using lidar imagery, we identified a ~5 km long lineament in Swakane canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles across the lineament reveal a southwest-side-up break in slope with an average of ~3 m of vertical separation of the hillslope surface. We consider a range of possible origins for this feature, including differential erosion across a fault-line scarp, slope failure (sackung or landslide), and surface deformation across an active fault strand. Based on trenching, radiocarbon and luminescence dating, and ground penetrating radar (GPR) across the lineament, we conclude that warped saprolite observed in the shallow subsurface is most consistent with southwest-side-up folding caused by blind reverse faulting at depth. Following this reasoning, dating of overlying colluvial deposits suggests at least one Holocene earthquake occurred on this strand of the southern Entiat fault with an approximate vertical separation of ≥1 m. GPR reveals up to 4 m of cumulative vertical separation of the saprolite, suggesting a history of multiple earthquakes on the structure. Taken in context with other potential fault-related lineaments along the Entiat fault, our interpretation of Holocene earthquakes in Swakane canyon could suggest reactivation of longer sections of the Entiat fault, as well as other bedrock faults in the eastern Cascades. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, we conclude that the reactivated Entiat fault represents a seismogenic structure that should be considered in regional seismic hazard analyses. The difficulty of recognizing low slip-rate structures in forested and mountainous terrain underscores the importance of additional lidar surveys and geological and geophysical studies for fully understanding seismic hazard in regions with infrequent but potentially large earthquakes.

The Bureau of Reclamation's Scoggins Dam lies within the Gales Creek fault zone southwest of Hillsboro, Oregon. Recent geologic mapping shows the dam to overlie a potentially active strand of the fault. This report describes the geology of the dam in detail and confirms that the dam overlies a strand of the Gales Creek fault. The report documents small faults in the reservoir and off the north end of the dam along the Parsons Creek strand of the fault. The report further documents the geology of an alternative dam site downstream of the existing dam.