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USGS2017

The interior structure of Ceres as revealed by surface topography

Ceres, the largest body in the asteroid belt (940 km diameter), provides a unique opportunity to study the interior structure of a volatile-rich dwarf planet. Variations in a planetary body's subsurface rheology and density affect the rate of topographic relaxation. Preferential attenuation of long wavelength topography (≥150 km) on Ceres suggests that the viscosity of its crust decreases with increasing depth. We present finite element (FE) geodynamical simulations of Ceres to identify the internal structures and compositions that best reproduce its topography as observed by the NASA Dawn mission. We infer that Ceres has a mechanically strong crust with maximum effective viscosity ∼10 25 Pa s. Combined with density constraints, this rheology suggests a crustal composition of carbonates or phyllosilicates, water ice, and at least 30 volume percent (vol.%) low-density, high-strength phases most consistent with salt and/or clathrate hydrates. The inference of these crustal materials supports the past existence of a global ocean, consistent with the observed surface composition. Meanwhile, we infer that the uppermost ≥60 km of the silicate-rich mantle is mechanically weak with viscosity <10 21 Pa s, suggesting the presence of liquid pore fluids in this region and a low temperature history that avoided igneous differentiation due to late accretion or efficient heat loss through hydrothermal processes.

Earth and Planetary Science Letters↗

USGS2017

Refining fault slip rates using multiple displaced terrace risers-An example from the Honey Lake fault, NE California, USA

Faulted terrace risers are semi-planar features commonly used to constrain Quaternary slip rates along strike-slip faults . These landforms are difficult to date directly and therefore their ages are commonly bracketed by age estimates of the adjacent upper and lower terrace surfaces. However, substantial differences in the ages of the upper and lower terrace surfaces (a factor of 2.4 difference observed globally) produce large uncertainties in the slip-rate estimate. In this investigation, we explore how the full range of displacements and bounding ages from multiple faulted terrace risers can be combined to yield a more accurate fault slip rate. We use 0.25-m cell size digital terrain models derived from airborne lidar data to analyze three sites where terrace risers are offset right-laterally by the Honey Lake fault in NE California, USA. We use ages for locally extensive subhorizontal surfaces to bracket the time of riser formation: an upper surface is the bed of abandoned Lake Lahontan having an age of 15.8 ± 0.6 ka and a lower surface is a fluvial terrace abandoned at 4.7 ± 0.1 ka. We estimate lateral offsets of the risers ranging between 6.6 and 28.3 m (median values), a greater than fourfold difference in values. The amount of offset corresponds to the riser's position relative to modern stream meanders: the smallest offset is in a meander cutbank position, whereas the larger offsets are in straight channel or meander point-bar positions. Taken in isolation, the individual terrace-riser offsets yield slip rates ranging from 0.3 to 7.1 mm/a. However, when the offset values are collectively assessed in a probabilistic framework, we find that a uniform (linear) slip rate of 1.6 mm/a (1.4–1.9 mm/a at 95% confidence) can satisfy the data, within their respective uncertainties. This investigation demonstrates that integrating observations of multiple offset elements (crest, midpoint, and base) from numerous faulted and dated terrace risers at closely spaced sites can refine slip-rate estimates on strike-slip faults.

California↗

USGS2017

Shallow microearthquakes near Chongqing, China triggered by the Rayleigh waves of the 2015 M7.8 Gorkha, Nepal earthquake

We present a case of remotely triggered seismicity in Southwest China by the 2015/04/25 M7.8 Gorkha, Nepal earthquake. A local magnitude ML3.8 event occurred near the Qijiang district south of Chongqing city approximately 12 min after the Gorkha mainshock. Within 30km of this ML3.8 event there are 62 earthquakes since 2009 and only 7 ML>3events, which corresponds to a likelihood of 0.3% for a ML>3on any given day by a random chance. This observation motivates us to investigate the relationship between the ML3.8 event and the Gorkha mainshock. The ML3.8 event is listed in the China Earthquake National Center (CENC) catalog and occurred at shallow depth (∼3km). By examining high-frequency waveforms, we identify a smaller local event (∼ML2.5) ∼15s before the ML3.8 event. Both events occurred during the first two cycles of the Rayleigh waves from the Gorkha mainshock. We perform seismic event detection based on envelope function and waveform matching by using the two events as templates. Both analyses found a statistically significant rate change during the mainshock, suggesting that they were indeed dynamically triggered by the Rayleigh waves. Both events occurred during the peak normal and dilatational stress changes (∼10–30 kPa), consistent with observations of dynamic triggering in other geothermal/volcanic regions. Although other recent events (i.e., the 2011 M9.1 Tohoku-Oki earthquake) produced similar peak ground velocities, the 2015 Gorkha mainshock was the only event that produced clear dynamic triggering in this region. The triggering site is close to hydraulic fracturing wells that began production in 2013–2014. Hence we suspect that fluid injections may increase the region’s susceptibility to remote dynamic triggering.

Chongqing↗

USGS2018

Strain partitioning in southeastern Alaska: Is the Chatham Strait Fault active?

A 1200 km-long transform plate boundary passes through southeastern Alaska and northwestern British Columbia and represents one of the most seismically active, but poorly understood continental margins of North America. Although most of the plate motion is accommodated by the right-lateral Queen Charlotte–Fairweather Fault (QCFF) System, which has produced at least six M > 7 earthquakes since 1920, seismic hazard ass essments also include the Chatham Strait Fault (CSF) as a potentially active, 400 km-long strike slip fault that cuts northward through southeastern Alaska, connecting with the Eastern Denali Fault. Nearly the entire length of the CSF is submerged beneath Chatham Strait and Lynn Canal and has never been systematically imaged using high-resolution marine geophysical approaches. In this study we present an integrated analysis of new marine seismic reflectiondata ac quired across Lynn Canal and tectonic block modeling constrained by data from continuous and campaign GPS sites. Seismic profiles cross the CSF at twelve locations spanning ∼50 km of fault length; they reveal thick (up to 300 m) packages of glaciomarine sedimentary facies emplaced on an unconformity su rface that formed during the Last Glacial Maximum (LGM). Localized warping of post-LGM stratigraphy (∼13.9 kyr B.P. to present) appears to correlate with sediment drape on basement topography and current-controlled deposition. There is no evidence for an active fault along the axis of Lynn Canal in the seismic reflection data. Crustal block models constrained by GPS data allow, but do not require, a maximum slip rate o f 2–3 mm/yr along the CSF; higher slip rates on the CSF result in significant misfit to GPS data in the surrounding region. Based on the combined marine geophysical and GPS observations, it is plausible that the CSF has not generated resolvable coseismic deformation in the last ∼13 ka and that the modern slip-rate is <1 mm/yr. We propose that models for strain transfer between the QCFF and the Denali Fault, and seismic hazard maps in general, may need to be reevaluated.

Alaska↗

USGS2018

Unraveling the dynamics of magmatic CO2 degassing at Mammoth Mountain, California

The accumulation of magmatic CO 2 beneath low-permeability barriers may lead to the formation of CO 2 -rich gas reservoirs within volcanic systems. Such accumulation is often evidenced by high surface CO 2 emissions that fluctuate over time. The temporal variability in surface degassing is believed in part to reflect a complex interplay between deep magmatic degassing and the permeability of degassing pathways. A better understanding of the dynamics of CO 2 degassing is required to improve monitoring and hazards mitigation in these systems. Owing to the availability of long-term records of CO 2 emissions rates and seismicity, Mammoth Mountain in California constitutes an ideal site towards such predictive understanding. Mammoth Mountain is characterized by intense soil CO 2 degassing (up to ∼1000 t d −1 ) and tree kill areas that resulted from leakage of CO 2 from a CO 2 -rich gas reservoir located in the upper ∼4 km. The release of CO 2 -rich fluids from deeper basaltic intrusions towards the reservoir induces seismicity and potentially reactivates faults connecting the reservoir to the surface. While this conceptual model is well-accepted, there is still a debate whether temporally variable surface CO 2 fluxes directly reflect degassing of intrusions or variations in fault permeability. Here, we report the first large-scale numerical model of fluid and heat transport for Mammoth Mountain. We discuss processes (i) leading to the initial formation of the CO 2 -rich gas reservoir prior to the occurrence of high surface CO 2 degassing rates and (ii) controlling current CO 2 degassing at the surface. Although the modeling settings are site-specific, the key mechanisms discussed in this study are likely at play at other volcanic systems hosting CO 2 -rich gas reservoirs. In particular, our model results illustrate the role of convection in stripping a CO 2 -rich gas phase from a rising hydrothermal fluid and leading to an accumulation of a large mass of CO 2 (∼10 7 –10 8 t) in a shallow gas reservoir. Moreover, we show that both, short-lived (months to years) and long-lived (hundreds of years) events of magmatic fluid injection can lead to critical pressures within the reservoir and potentially trigger fault reactivation. Our sensitivity analysis suggests that observed temporal fluctuations in surface degassing are only indirectly controlled by variations in magmatic degassing and are mainly the result of temporally variable fault permeability. Finally, we suggest that long-term CO 2 emission monitoring, seismic tomography and coupled thermal–hydraulic–mechanical modeling are important for CO 2 -related hazard mitigation.

California↗

USGS2018

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

California↗

USGS2018

Testing for the ‘predictability’ of dynamically triggered earthquakes in Geysers Geothermal Field

The Geysers geothermal field is well known for being susceptible to dynamic triggering of earthquakes by large distant earthquakes, owing to the introduction of fluids for energy production. Yet, it is unknown if dynamic triggering of earthquakes is ‘predictable’ or whether dynamic triggering could lead to a potential hazard for energy production. In this paper, our goal is to investigate the characteristics of triggering and the physical conditions that promote triggering to determine whether or not triggering is in anyway foreseeable. We find that, at present, triggering in The Geysers is not easily ‘predictable’ in terms of when and where based on observable physical conditions. However, triggered earthquake magnitude positively correlates with peak imparted dynamic stress, and larger dynamic stresses tend to trigger sequences similar to mainshock–aftershock sequences. Thus, we may be able to ‘predict’ what size earthquakes to expect at The Geysers following a large distant earthquake.

California↗

USGS2018

Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3–4 Ma pulse of basaltic magmatism.

Earth and Planetary Science Letters↗

USGS2018

Kinematics of the 2015 San Ramon, California earthquake swarm: Implications for fault zone structure and driving mechanisms

Earthquake swarms represent a sudden increase in seismicity that may indicate a heterogeneous fault-zone, the involvement of crustal fluids and/or slow fault slip. Swarms sometimes precede major earthquake ruptures. An earthquake swarm occurred in October 2015 near San Ramon, California in an extensional right step-over region between the northern Calaveras Fault and the Concord–Mt. Diablo fault zone, which has hosted ten major swarms since 1970. The 2015 San Ramon swarm is examined here from 11 October through 18 November using template matching analysis. The relocated seismicity catalog contains ∼4000 events with magnitudes between − 0.2 < M d < 3.6 "> −0.2< M d <3.6 . The swarm illuminated three sub-parallel, southwest striking and northwest dipping fault segments of km-scale dimension and thickness of up to 200 m. The segments contain coexisting populations of different focal-mechanisms, suggesting a complex fault zone structure with several sets of en échelon fault orientations. The migration of events along the three planar structures indicates a complex fluid and faulting interaction processes. We searched for correlations between seismic activity and tidal stresses and found some suggestive features, but nothing that we can be confident is statistically significant.

California↗

USGS2018

Carboniferous climate teleconnections archived in coupled bioapatite δ18OPO4 and 87Sr/86Sr records from the epicontinental Donets Basin, Ukraine

Reconstructions of paleo-seawater chemistry are largely inferred from biogenic records of epicontinental seas. Recent studies provide considerable evidence for large-scale spatial and temporal variability in the environmental dynamics of these semi-restricted seas that leads to the decoupling of epicontinental isotopic records from those of the open ocean. We present conodont apatite δ 18 O PO4 and 87 Sr/ 86 Sr records spanning 24 Myr of the late Mississippian through Pennsylvanian derived from the U–Pb calibrated cyclothemic succession of the Donets Basin, eastern Ukraine. On a 2 to 6 Myr-scale, systematic fluctuations in bioapatite δ 18 O PO4 and 87 Sr/ 86 Sr broadly follow major shifts in the Donets onlap–offlap history and inferred regional climate, but are distinct from contemporaneous more open-water δ 18 O PO4 and global seawater Sr isotope trends. A −1 to −6‰ offset in Donets δ 18 O PO4 values from those of more open-water conodonts and greater temporal variability in δ 18 O PO4 and 87 Sr/ 86 Sr records are interpreted to primarily record climatically driven changes in local environmental processes in the Donets sea. Systematic isotopic shifts associated with Myr-scale sea-level fluctuations, however, indicate an extrabasinal driver. We propose a mechanistic link to glacioeustasy through a teleconnection between high-latitude ice changes and atmospheric p CO 2 and regional monsoonal circulation in the Donets region. Inferred large-magnitude changes in Donets seawater salinity and temperature, not archived in the more open-water or global contemporaneous records, indicate a modification of the global climate signal in the epicontinental sea through amplification or dampening of the climate signal by local and regional environmental processes. This finding of global climate change filtered through local processes has implications for the use of conodont δ 18 O PO4 and 87 Sr/ 86 Sr values as proxies of paleo-seawater composition, mean temperature, and glacioeustasy.

Donets Basin↗

USGS2018

Tracing enhanced oil recovery signatures in casing gases from the Lost Hills oil field using noble gases

Enhanced oil recovery (EOR) and hydraulic fracturing practices are commonly used methods to improve hydrocarbon extraction efficiency; however the environmental impacts of such practices remain poorly understood. EOR is particularly prevalent in oil fields throughout California where water resources are in high demand and disposal of high volumes of produced water may affect groundwater quality. Consequently, it is essential to better understand the fate of injected (EOR) fluids in California and other subsurface petroleum systems, as well as any potential effect on nearby aquifer systems. Noble gases can be used as tracers to understand hydrocarbon generation, migration, and storage conditions, as well as the relative proportions of oil and water present in the subsurface. In addition, a noble gas signature diagnostic of injected (EOR) fluids can be readily identified. We report noble gas isotope and concentration data in casing gases from oil production wells in the Lost Hills oil field, northwest of Bakersfield, California, and injectate gas data from the Fruitvale oil field, located within the city of Bakersfield. Casing and injectate gas data are used to: 1) establish pristine hydrocarbon noble-gas signatures and the processes controlling noble gas distributions, 2) characterize the noble gas signature of injectate fluids, 3) trace injectate fluids in the subsurface, and 4) construct a model to estimate EOR efficiency. Noble gas results range from pristine to significantly modified by EOR, and can be best explained using a solubility exchange model between oil and connate/formation fluids, followed by gas exsolution upon production. This model is sensitive to oil-water interaction during hydrocarbon expulsion, migration, and storage at reservoir conditions, as well as any subsequent modification by EOR.

California↗

USGS2018

Rapid late Miocene surface uplift of the Central Anatolian Plateau margin

The Central Anatolian Plateau (CAP), Turkey, is bordered to its south by a steep mountain belt that emerged ∼8–7 Ma ago from the Mediterranean Sea. Knowledge of the onset, duration and rate of surface uplift and orographic barrier formation along the plateau margin is crucial for understanding the geodynamic drivers of plateau uplift. We present a new comprehensive data set that includes 12 40 Ar/ 39 Ar ages and lacustrine carbonate δ 18 "> δ18 O data ( n = 637 "> n=637 ) from 13 sections in upper Oligocene to Pliocene continental basins of the CAP interior. We aim at documenting the development of a rain shadow and therefore the surface uplift history of the CAP and its southern margin (Tauride Mts.). In the rain shadow of the Tauride Mts. we observe a gradual 3.9‰ decrease of δ 18 "> δ18 O values of lacustrine carbonate between ∼11 and 5 Ma that we interpret to originate from a similar change in δ 18 "> δ18 O values of precipitation owing to the late Miocene development of an orographic barrier. Our stable isotope paleoaltimetry data show that by 5 Ma the southern CAP margin had reached similar-to-present elevations of ∼2 km. Surface uplift was coeval with ignimbritic magmatism, forearc shortening and distributed compression. We suggest that the removal of lithospheric mantle below Anatolia led to surface uplift of the CAP interior, which was followed by surface uplift of the southern CAP margin due to crustal thickening as a result of northward subduction of the African plate below central Anatolia.

Earth and Planetary Science Letters↗

USGS2018

Pliocene erosional pulse and glacier-landscape feedbacks in the western Alaska Range

Pliocene–Pleistocene glaciation modified the topography and erosion of most middle- and high-latitude mountain belts, because the evolution of catchment topography controls long-term glacier mass balance and erosion. Hence, characterizing how erosion rates change during repeated glaciations can help test hypothesized glacier erosion-landscape feedbacks across a range of settings. To better understand how glaciations and landscapes coevolve on geologic timescales, I quantify erosion rates in the glaciated western Alaska Range with low-temperature thermochronometric data and modeling. Zircon (U–Th)/He and apatite fission track data suggest mountain-building was underway by the early Miocene. In contrast, lower-temperature apatite (U–Th)/He age-elevation and grain age-kinetic data indicate that erosion accelerated coincident with regional Pliocene glaciation ca. 4 Ma. Furthermore, erosion rates calculated within an eroding half-space indicate slow erosion at a rate ≤0.3 km/m.y. before 4.2 Ma, an initial pulse of rapid erosion at a rate of 1.0–1.6 km/m.y. during 4.2–2.9 Ma, and more moderate erosion at a rate of 0.4–0.7 km/m.y. since 2.9 Ma. The initial erosion pulse suggests a significant transient landscape adjustment to the introduction of efficient glacial erosion. The subsequent decrease in Pleistocene erosion rates is consistent with a negative feedback between continuing glaciation and glacier size/erosivity: If glacial erosion outpaces rock uplift, glacier erosion decreases over time as topography, mass balance, valley gradients, and ice flux are reduced. These findings imply that in areas of moderate rock uplift rates, the onset of local Plio–Pleistocene glaciation may have been punctuated by an initial pulse of rapid landscape change, after which change became more gradual.

Alaska↗

USGS2018

Sulfur cycle in the Valles Caldera volcanic complex, New Mexico – Letter 1: Sulfate sources in aqueous system, and implications for S isotope record in Gale Crater on Mars

Initial in situ sulfur (S) isotope measurements of the Martian bedrock in Gale Crater have revealed an unexpectedly wide range of δ 34 S values (−47 to +28%). Generally, it is unclear what processes could have contributed to these large isotope fractionations. Therefore, we studied S sources and aqueous SO 2− 4 cycling in the Valles Caldera volcanic complex, New Mexico to better understand S isotope fractionations related to S degassing, hydrothermal activity, and low-temperature processes in aqueous environment. Overall, our study demonstrates that volcanic systems show large spatial heterogeneity in δ 34 S. Magmatic S sources are obvious in steam-dominated H 2 S degassing and precipitation of secondary minerals from hydrothermal fluids with low δ 34 S values of +0.9 ±3%. Locally, however, hydrothermal processes have resulted in more negative δ 34 S values in sulfide minerals (−18 to −4%) and more positive δ 34 S values in sulfate minerals (−1 to +3%). Major aqueous SO 2− 4 sources are oxidation of H 2 S from modern hydrothermal gas emission, and oxidation and dissolution of sulfide and sulfate minerals present in the hydrothermally altered bedrock and crater-lake sediments. The δ 34 S of aqueous SO 2− 4 in surface water and groundwater varies widely (−8 to +5%) and is similar to major S endmembers that undergo oxidation and/or dissolution by active hydrological system. Minor SO 2− 4 contributions with more positive δ 34 S values (+9 to +14%) come from deeply circulating geothermal fluids and negligible amounts from atmospheric deposition (+5 to +7% in snow). Elevated SO 2− 4 contents are mainly associated with modern and past H 2 S emissions and oxidations near the surface. On regional scale, however, most of the intracaldera bedrock is S-depleted, thus the SO 2− 4 contents are usually low in the surface aquatic system and younger sedimentary lake deposits formed at times of negligible near surface hydrothermal activity. In general, magmatic-hydrothermal processes apparently cause the largest δ 34 S variation in S-bearing minerals on volcanic terrains. Therefore, we infer that the measured wide range of δ 34 S values in the Gale sediments by the Curiosity rover on Mars can be explained by S isotope composition of magmatic-hydrothermal sulfide and sulfate minerals that were present in the initial igneous/volcanic rocks prior to crater formation. Later aqueous processes involved oxidation and dissolution of S minerals initially present in these rocks and led to subsequent formation of diagenetic fluids and alteration products enriched in SO 2− 4 with relatively large δ 34 S variation. Additionally, physical erosion, transport and deposition of detrital hydrothermal S minerals from igneous/volcanic rocks might be in part responsible for the measured wide range of δ 34 S in Gale Crater. These unique S isotope results, measured in situ on another planet for the first time, imply the importance of magmatic-hydrothermal fluids in S transport on early Mars and their subsequent alteration in low-temperature aqueous environments.

New Mexico↗

USGS2019

Seismic velocity structure across the 2013 Craig, Alaska rupture from aftershock tomography: Implications for seismogenic conditions

The 2013 Craig, Alaska M W 7.5 earthquake ruptured along ∼150 km of the Queen Charlotte Fault (QCF), a right-lateral strike-slip plate boundary fault separating the Pacific and North American plates. Regional shear wave analyses suggest that the Craig earthquake rupturepropagated in the northward direction faster than the S-wave (supershear). Theoretical studies suggest that a bimaterial interface, such as that along the QCF, which separates oceanic and continental crust with differing elastic properties, can promote supershear rupture propagation. We deployed short-period ocean-bottom seismometers (OBS) as a part of a rapid-response effort less than four months after the Craig earthquake mainshock. During a 21-day period, 1,133 aftershocks were recorded by 8 OBS instruments. Aftershock spatial distribution indicates that the base of the seismogenic zone along the QCF approaches ∼25 km depth, consistent with a thermally-controlled fault rheology expected for igneous rocks at oceanic transform faults. The spatial distribution also provides supporting evidence for a previously hypothesized active strand of the QCF system within the Pacific Plate. Tomographic traveltime inversion for velocity structure indicates a low-velocity (V P and V S ) zone on the Pacific side of the plate boundary at 5–20 km depths, where NeogenePacific crust and upper mantle seismic velocities average ∼3–11% slower than the North American side, where the Paleozoic North American crust is seismically faster. Our results suggest that elastic properties along the studied portion of the QCF are different than those of a simple oceanic–continental plate boundary fault. In our study region, velocity structure across the QCF, while bimaterial, does not support faster material on the west side of the fault, which has been proposed as one possible explanation for northward supershear propagation during the Craig earthquake. Instead, we image low-velocity material on the west side of the fault. Explanations could include that part of the rupture was subshear, or that fault damage zone properties or fault smoothness are more important controls on supershear rupture than a bimaterial contrast.

Alaska↗

USGS2018

Eruptions in sync: Improved constraints on Kīlauea Volcano's hydraulic connection

Kīlauea Volcano is an archetype for the complex interactions that can occur between a volcano’s summit and flanks. Decades of monitoring at Kīlauea have demonstrated that magma rises beneath the summit and flows laterally at shallow depths to erupt along the rift zones. Kīlauea’s recent eruptions at Halema‘uma‘u and Pu‘u ‘Ō‘ō mark the first time in the historic record that long-term (>1 year) eruptions have been concurrent at the summit and a rift zone, offering a new opportunity to improve our understanding of the relationship between these two segments of the magmatic system. While magma supply rate beneath the summit has been shown in previous studies to be a primary control on magmatic system pressure and eruptive activity, the role of the eruptive vent has been less clear. Our study shows that a dynamic equilibrium is maintained between Kīlauea’s summit and East Rift Zone (ERZ) eruptive vent—and lava lake level fluctuations are closely coupled at the two eruption sites—providing new constraints on the hydraulic connection and ERZ conduit. We show that localized changes at the ERZ eruption site during 2010-2011 regulated summit behavior in an uprift direction over distances of ~20 km. Changes in the elevation and efficiency of the ERZ vent affect pressure in Kīlauea’s magmatic system and impact summit behavior. Thus, the hydraulic connection between the summit and rift zone is a “two-way street” that transmits both downrift- and uprift-directed changes. Our results support recent work at other volcanoes that shows a complex interplay between a volcano’s summit reservoir and flank conduit during flank eruptions, and suggest that explosive summit activity may in some cases be triggered by changes far away on a volcano’s rift.

Hawaii↗

USGS2019

Detrital K-feldspar Pb isotopic evaluation of extraregional sediment transported through an Eocene tectonic breach of southern California's Cretaceous batholith

Sedimentary provenance studies have come to be overwhelmingly based upon U–Pb geochronologic measurements performed with detrital zircon while alternative and potentially complementary approaches such as conglomerate clast studies and heavy mineral analysis have faded in importance. Measurement of Pb isotopic compositions in detrital K-feldspar is among the under-utilized approaches available to ascertain sedimentary source regions. While it has been long recognized that common Pb isotope compositions recorded by K-feldspar vary widely and reflect the crustal provinces from which the host basement rocks crystallized, use of the approach has suffered due to a lack of appropriate statistical models and ground truth compositional data from source regions. In this paper, we: (1) present high-throughput LA-ICPMS analysis protocols needed to generate statistically meaningful detrital K-feldspar Pb isotope data sets; (2) develop an interpretative approach based upon 208 Pb/ 206 Pb vs. 207 Pb/ 206 Pb that incorporate information from the U- and Th-decay systems into one two-dimensional plot that is amenable to analysis using two-dimensional Kolmogorov–Smirnoff statistical tests; (3) generate new Pb isotopic data from basement rocks from southwestern North America to improve knowledge of the Pb isotopic properties of potential source regions; and (4) generate new Pb isotopic data from Lower Eocene to Lower Miocene sedimentary rocks to evaluate changes in drainage patterns that occurred in response to deformation that affected the southern California margin. Through this case study, we demonstrate how our new analytical and interpretative methods could be profitably applied to future geochemical and provenance studies and tectonically driven re-organization of drainage patterns.

Earth and Planetary Science Letters↗

USGS2019

Hydrogen isotopes in high 3He/4He submarine basalts: Primordial vs. recycled water and the veil of mantle enrichment

The hydrogen isotope value ( δ D) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth's history. High 3 He/ 4 He ratios in some ocean island basalts, however, provide a clear geochemical signature of deep, primordial mantle that has been isolated within the Earth's interior from melting, degassing, and convective mixing with the upper mantle. Hydrogen isotopes were measured in high 3 He/ 4 He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau ( δ D = −51 to −90‰, 3 He/ 4 He = 7.6 to 14.1 R A ) and in submarine glasses from Loihi seamount south of the island of Hawaii ( δ D = −70 to −90‰, 3 He/ 4 He = 22.5 to 27.8 R A ). These results highlight two contrasting patterns of δ D for high 3 He/ 4 He lavas: one trend toward high δ D of approximately −50‰, and another converging at δ D = −75‰. These same patterns are evident in a global compilation of previously reported δ D and 3 He/ 4 He results. We suggest that the high δ D values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixed with primordial mantle, resulting in high δ D and moderately high 3 He/ 4 He. Conversely, lower δ D values of −75‰, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75‰ or lower. δ D values down to −100‰ also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with 87 Sr/ 86 Sr ratios near 0.703. These lower δ D values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δ D around −75‰ in the majority of mid-ocean ridge basalts and in high 3 He/ 4 He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material.

Earth and Planetary Science Letters↗