Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along- strike heterogeneity of strain distribu- tion, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We doc- umented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben- filling sediments from sources outside of the Klamath Mountains, and initiation of mid- Miocene or younger right- lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 +/- 2.1 Ma to 15.7 +/- 2.1 Ma. Inverse thermal modeling of these data and published apatite fission- track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 +/- 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 +/- 5.1 Ma on the downthrown side and 22.7 +/- 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaver- ville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south- central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these gra- bens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 +/- 0.4 Ma and 16.7 +/- 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault- controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to- bedrock contacts reported in well logs record E- side-up and right- lateral faulting at the eastern end of the Hayfork graben. A NE- striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Offset of both the base of the Weaver- ville Formation and the cataclasite- capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right- lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation. Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or "subcretion" in the southern Cascadia subduction zone beneath the southern Klamath Mountains.

ABSTRACT Thirty-seven portable accelerometers were deployed in the eastern San Francisco Bay communities of Walnut Creek and Concord to study site response in a fault-bounded, urban, sedimentary basin. Local earthquakes were recorded for a period of two years from 2017 to 2019 resulting in 101 well-recorded events. Site response is estimated by two methods: the reference site spectral ratio method and a source-site spectral inversion method. The reference site spectral ratio method allows investigation of the variability of site amplification with source azimuth and frequency. The source-site spectral inversion method yields the best least-squares fit to site response for a database of ground-motion records. Both methods show substantial amplification in the Walnut Creek–Concord basin below 2 Hz indicating strong surface-wave development. Greater amplification is seen for sources aligned along the long axis of the basin. Inversion using close-in sources at short distances yields lower amplification at longer periods than the entire data set due to reduced surface-wave generation for steeper angles of incidence. Inversion of site response spectra for shallow shear-wave velocity using a global search algorithm yields VS30 values consistent with generalized mapping results based on geology and topography but with greater variability due to local site variations. 3D finite-element modeling shows greater amplification in the Walnut Creek–Concord basin with a basin-edge effect likely contributing to higher ground motions. Topography is also seen to lead to increased scattering and shadowing effects.

The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpentine mineral stable at depth, in fault gouge near Lake Pillsbury.

First posted August 22, 2023 For additional information, contact: Geology, Minerals, Energy, & Geophysics Science CenterU.S. Geological SurveyBuilding 19, 350 N. Akron Rd.P.O. Box 158Moffett Field, CA 94035 Aeromagnetic surveys were conducted to improve understanding of the geology and structure in northeastern California, a region predominantly covered by Quaternary and Tertiary, mainly Neogene, volcanic rocks including Medicine Lake volcano. New aeromagnetic data are a substantial improvement over existing data and reveal structural details not resolved by older surveys. Here we show how these data (1) do not support the presence of a northwest-striking structural feature across the Modoc Plateau, (2) reveal a northeast-striking fault-bounded block of predominantly reversely magnetized material that may influence tectonism at Medicine Lake volcano, and (3) constrain possible right-lateral offsets along the Likely Fault Zone and other faults that traverse the region. The data also highlight possible extensions of mapped faults, such as those in Fall River Valley and the Tule and Lower Klamath Lake areas.

The evolution of strain in nascent continental plate boundaries commonly involves distributed deformation and transitions between different styles of deformation as the plate boundary matures. Distributed NW-striking faults, many with km-scale right-lateral separation, are prevalent near Blythe, California, and have been variably interpreted to have accommodated either Middle Miocene NE-SW extension as normal faults or Late Miocene to Pliocene dextral shear as strike-slip faults. However, with poor timing and kinematic constraints, it is unclear how these faults relate to known domains of Neogene deformation and the evolution of the Pacific–NorthAmerica plate boundary. We present kinematic data (n = 642 fault planes, n = 512 slickenlines) that demonstrate that these faults dominantly dip steeply northeast; ~96% of measured faults record normal, dextral, or oblique dextral-normal kinematics that likely reflect a gradational transition between normal and dextral oblique kinematic regimes. We constrain fault timing with 11.7 Ma and 7.0 Ma 40Ar/39Ar dates of rocks cut by faults, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite mineralized during oblique dextral faulting that demonstrates fault slip at ca. 10–7 Ma and perhaps as late as ca. 4 Ma. This Late Miocene dextral oblique faulting is best compatible with a documented regional transition from Early to Middle Miocene NE-directed extension during detachment fault slip to subsequent NW-directed dextral shear. We estimate 11–38 km of cumulative dextral slip occurred across a 50-km-wide zone from the Palen to Riverside mountains, including up to 20 km of newly documented dextral shear that may partly alleviate the regional discrepancy of cumulative dextral shear along this part of the Late Miocene Pacific–North America plate boundary.

ABSTRACT This field trip traverses a cross section of northern Sierra Nevada geology and landscape along two major corridors, Highway 49 (Yuba Pass) and Highway 70. These highways, and adjacent roadways, offer roadcuts, outcrops, and overviews through diverse pre-Cenozoic metamorphic rocks along the Laurentian margin, Mesozoic batholithic rocks, and Miocene volcanic rocks. Observing this array of rocks on a single trip provides an opportunity to examine the progression of tectonic forces in this region since the Paleozoic Era. Inspiration for this trip is a 1:100,000-scale geologic map and geophysical maps of the Portola 30′ × 60′ quadrangle that integrate decades of published and unpublished mapping with new geophysical data. The quadrangle map will seamlessly depict a geologically complex region along the boundary between the Sierra Nevada and Basin and Range provinces, dominated by transtensional tectonics of the Walker Lane. This field trip highlights many of the major units of the geologic map and will also feature new geochronological data on plutonic rocks.

Includes original, preferred, and alternate geophysical models across the Bartlett Springs fault zone and a figure showing the distribution of gravity measurements in the region.

First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in the Kingdom of Saudi Arabia. The gravity low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.

Figures 1–2, 4, and 6–7 are interactive. Use the radio buttons to toggle between different maps.

Two new joint gravity-magnetic models in northern Coachella Valley provide additional evidence for a steep northeast dip of the Mission Creek strand of the southern San Andreas fault (southern California, USA). Gravity modeling indicates a steep northeast dip of the Banning fault in the upper 1-2 km in northern Coachella Valley. The Mission Creek strand and its continuation to the southeast (Coachella segment) coincide with the northeastern margin of a Cenozoic basin and are marked by prominent gravity and magnetic gradients that are consistent with these strands of the San Andreas fault having accommodated >160 km of right-lateral and 1-5 km of vertical displacement. These anomalies are best fit by a moderate to steep northeast dip. Such a geometry is further supported by seismicity, reflectivity, geodesy, and boundary-element modeling. We explore the possibility that these fault strands forming the margin of Coachella Valley were originally near vertical and have rotated into their present orientation by underplating of a localized high-velocity, lower-crustal prong within the Peninsular Ranges batholith. Reconstructions of San Andreas fault offset suggest that this crystalline body was translated into the San Gorgonio Pass area at the time of major fault reorganization at 1.1-1.3 Ma.

The Packard Well fault zone (PWFZ) is likely one of the larger-displacement NW-striking Neogene faults in SE California, with variable dextral offset estimates in the literature of up to ~24 km, and may have formed a critical component of the Late Miocene to Pliocene paleo-eastern California shear zone (ECSZ). However, attempts to understand fault linkage from better-studied ECSZ faults in the Mojave Desert to a wider paleo-ECSZ system that included faults farther E are hampered by uncertainty that PWFZ deformation was compatible with paleo-ECSZ timing and kinematics. We use geologic mapping, fault kinematic analysis, and 40Ar/39Ar dating of faulted rocks at Palen Pass, California to constrain the slip history of the PWFZ. N-dipping strands of the PWFZ bound a 400 m-wide belt of asymmetrically folded alluvial fan conglomerate at Palen Pass. The northern fault strand thrust Mesozoic plutonic rocks over the conglomerate and preserves oblique dextral-reverse slickenlines consistent with a WNW-ESE restraining bend in a NW-SE dextral system. Sanidine 40Ar/39Ar dating of a steeply-dipping, lightly reworked ash bed in the deformed conglomerate indicates a population of young grains with a weighted mean average of 11.7 ± 0.1 Ma (2σ). A package of thin basalt flows ~5 km SW of the folded conglomerate records minor NNE-SSW shortening via conjugate strike-slip faults, and a published paleomagnetic study indicates the basalt experienced ~31° of clockwise rotation, both of which likely stem from local block rotation adjacent to the dextral PWFZ. New groundmass 40Ar/39Ar dating of this basalt yields a 7.0 ± 0.2 Ma (2σ) recoil model age. These findings suggest that significant dextral shear occurred on the PWFZ in the Late Miocene or Pliocene, prior to deposition of undeformed late Quaternary alluvial fans. The PWFZ likely connects along strike to concealed faults to the NW (e.g., Cadiz Lake and Iron Mountains faults), and towards Blythe, California to the SE (e.g., Cibola Pass fault zone), where steep horizontal gravity gradients bound a NW-SE-elongate gravity low. Our data demonstrate that the PWFZ was kinematically and temporally compatible with a paleo-ECSZ dextral system active in the Late Miocene to Pliocene and may have played a major role in accommodating Pacific-North America dextral shear.

First posted February 11, 2021 For additional information, contact: Director,Geology, Minerals, Energy, & Geophysics Science CenterMenlo Park, CaliforniaU.S. Geological Survey345 Middlefield RoadMenlo Park, CA 94025-3591 An aeromagnetic survey was conducted to improve understanding of the geology and structure in the area around Burney, northeastern California. The new data are a substantial improvement over existing data and reveal a prominent north northwest-trending magnetic grain that allows extension of mapped faults, delineation of plutons within the Mesozoic basement in the northern Sierra Nevada, and linear anomalies that limit the amount of strike-slip offset along various faults in the area.

In much of the western Cordillera of North America, the geologic framework of crustal structure generated in the Mesozoic leaves an imprint on later plutonic emplacement, subsequent structural setting, and present landscape morphology. The Merrimac plutons in the northern Sierra Nevada (California, USA) are a good example of the influence of pre-existing structure at a larger scale. This paper updates and refines earlier studies of the Merrimac plutons, with the addition of analysis of gravity and magnetic data and new 206Pb/238U zircon dates. The gravity and magnetic data not only confirm the presence of two different neighboring plutons, but also (1) support the presence of a third pluton, (2) refine the nature of the contact between the Merrimac plutons as being structurally controlled, and (3) estimate the depth extent of the plutons to be ∼4–5 km. The zircon 206Pb/238U dates indicate that the two main plutons have statistically different crystallization ages nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief, roughness and drainage density and generation of chi plots, indicate that the two main plutons are characterized by different elevations with large longitudinal channel knickpoints that we speculatively attribute to possible reactivation of pre-existing structure in addition to lithologic variations influencing relative erosion susceptibility in response to prior accelerated surface uplift.

Figures 2–5, 7–9, and 13 are interactive. For Figures 2–5, 9, and 13, use the radio buttons to toggle between different maps. For Figure 7, use the radio buttons to toggle between age range variations and uncertainties for different samples in each pluton. For Figure 8, use the radio buttons to toggle between zircon trace element compositions of different plutons.