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USGS2012

Rift flank uplifts and Hinterland Basins: Comparison of the Transantarctic Mountains with the Great Escarpment of southern Africa

Uplifted rift margins are a common feature of continents and oceans. Two variants of rift flank morphologies have been recognized: One in which the topography warps down from an inland high toward the continental margin, and one where the tropographic peak lies close to the continental margin. The Great Escarpment of southern Africa and the Transantarctic Mountains are examples of the first and the second variants of rift flanks, respectively. Both rift flanks are bordered on their landward side by broad continental basins: the Kalahari and the Wilkes hinterland basins. If these basins are interpreted as flexural “outer lows” that deepen in unison with the uplift of the rift flanks, the lithosphere on the uplifted side is very rigid in both cases (elastic thickness T e of 100 ± 20 km for southern Africa and 110 ± 20 km for East Antarctica). We suggest that the variation in rift flank morphology is caused by the isostatic response to uplift forces of elastic plates sharing different boundary conditions. We model the uplift of the Transantarctic Mountains as an upward deflection of an elastic plate which is broken at the front of the Transantarctic Mountains, and we model the uplift of the Great Escarpment as an upward deflection of a continuous elastic plate that is modified by the downward load of sediments on the continental margin. Although the Transantarctic Mountain uplift is young (60–0 Ma) and the southern African uplift is old (<100 Ma), the different isostatic responses of the two margins are not a function of age, because most loading (sedimentation) and unloading (erosion) took place shortly after rifting. Detailed modeling of topography, gravity, geological markers, and the locations of depocenters suggests that lithospheric rigidity decreases under the Transantarctic Mountains, whereas in southern Africa the decrease occurs not under the Great Escarpment but far seaward under the continental shelf and slope. If the distribution of lithospheric rigidity is indicative of the thermal regime of the lithosphere, then uplifted rift flanks are not always underlain by a thermal anomaly. This and other geological evidence indicate that a single mechanism cannot explain the uplift of both the Antarctic and the African margins.

Journal of Geophysical Research B: Solid Earth↗

USGS2013

Reliability of fish size estimates obtained from multibeam imaging sonar

Multibeam imaging sonars have considerable potential for use in fisheries surveys because the video-like images are easy to interpret, and they contain information about fish size, shape, and swimming behavior, as well as characteristics of occupied habitats. We examined images obtained using a dual-frequency identification sonar (DIDSON) multibeam sonar for Atlantic sturgeon Acipenser oxyrinchus oxyrinchus , striped bass Morone saxatilis , white perch M. americana , and channel catfish Ictalurus punctatus of known size (20–141 cm) to determine the reliability of length estimates. For ranges up to 11 m, percent measurement error (sonar estimate – total length)/total length × 100 varied by species but was not related to the fish's range or aspect angle (orientation relative to the sonar beam). Least-square mean percent error was significantly different from 0.0 for Atlantic sturgeon ( x̄ = −8.34, SE = 2.39) and white perch ( x̄ = 14.48, SE = 3.99) but not striped bass ( x̄ = 3.71, SE = 2.58) or channel catfish ( x̄ = 3.97, SE = 5.16). Underestimating lengths of Atlantic sturgeon may be due to difficulty in detecting the snout or the longer dorsal lobe of the heterocercal tail. White perch was the smallest species tested, and it had the largest percent measurement errors (both positive and negative) and the lowest percentage of images classified as good or acceptable. Automated length estimates for the four species using Echoview software varied with position in the view-field. Estimates tended to be low at more extreme azimuthal angles (fish's angle off-axis within the view-field), but mean and maximum estimates were highly correlated with total length. Software estimates also were biased by fish images partially outside the view-field and when acoustic crosstalk occurred (when a fish perpendicular to the sonar and at relatively close range is detected in the side lobes of adjacent beams). These sources of bias are apparent when files are processed manually and can be filtered out when producing automated software estimates. Multibeam sonar estimates of fish size should be useful for research and management if these potential sources of bias and imprecision are addressed.

Journal of Fish and Wildlife Management↗

USGS2012

Seismic reflection images of the crust of the northern part of the Chugach terrane, Alaska: Results of a survey for the Trans-Alaska Crustal Transect (TACT)

Deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the Aleutian trench. These data were collected across the Border Ranges fault system, a major suture zone that separates the Peninsular and Chugach tectonostratigraphic terranes. The Chugach terrane consists of metamorphosed, strongly deformed flysch, subduction melange as well as blueschist- and greenschist-facies rocks. Even so, highly reflective rocks lie at great depth within or beneath this terrane. The shallow part of the seismic section (0–5 s) reveals only a few reflections, which undulate and interweave, indicating that some of them originate outside of the plane of the seismic section. Other undulating events could reveal early Cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults. In contrast to this shallow reflection pattern, three reflection bands contain most of the events on the middle part of the seismic section (5–12 s or 12–34 km). We interpret divergent reflections within the uppermost band (5–5.5 s) as gently north dipping thrust faults, and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults. If the truncated reflections represent thrust faults, then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks, forming duplex structures. The upper and middle bands are separated by an area on the seismic section that reveals few reflections. This area correlates approximately in depth with a high velocity (7.5 km/s) layer evident in refraction models. The poorly reflective, high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the Chugach terrane. If so, then the bottom of the poorly reflective rocks indicates the base of the Chugach terrane. Rocks that cause the upper and middle reflection bands and the intervening, poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick. The deepest band of subparallel reflections (30–35 km) correlates closely in depth with the top of the Wadati-Benioff zone associated with the underthrusting Pacific plate. The reflections probably stem from the decollement that separates lithospheric plates within the subduction zone. This reflection band increases abruptly in thickness, possibly indicating local underplating of subducted sediment. No events from the oceanic Moho have been recognized. Neither the Border Ranges fault system, the suture between the Chugach and Peninsular terranes, nor deep crustal layers of the Peninsular terrane are evident in these seismic reflection data.

Journal of Geophysical Research Solid Earth↗

USGS2004

Granular avalanches across irregular three-dimensional terrain: 1. Theory and computation

To establish a theoretical basis for predicting and interpreting the behavior of rapid mass movements on Earth's surface, we develop and test a new computational model for gravity-driven motion of granular avalanches across irregular, three-dimensional (3-D) terrain. The principles embodied in the model are simple and few: continuum mass and momentum conservation and intergranular stress generation governed by Coulomb friction. However, significant challenges result from the necessity of satisfying these principles when deforming avalanches interact with steep and highly variable 3-D terrain. We address these challenges in four ways. (1) We formulate depth-averaged governing equations that are referenced to a rectangular Cartesian coordinate system (with z vertical) and that account explicitly for the effect of nonzero vertical accelerations on depth-averaged mass and momentum fluxes and stress states. (2) We compute fluxes of mass and momentum across vertical cell boundaries using a high-resolution finite volume method and Roe-type Riemann solver. Our algorithm incorporates flux difference splitting, an entropy correction for the flux, and eigenvector decomposition to embed the effects of driving and resisting forces in Riemann solutions. (3) We use a finite element method and avalanche displacements predicted by Riemann solutions to compute Coulomb stresses conjugate to the displacements in 3-D stress space. (4) We test the model output against analytical solutions, a sand cone conceptual experiment, and (in a companion paper) data from detailed laboratory experiments. Model results illustrate a complex interplay of basal traction and internal stress, and they successfully predict not only the gross behavior but also many details of avalanche motion from initiation to deposition.

Journal of Geophysical Research Earth Surface↗

USGS2022

Results of the collaborative Lake Ontario bloater restoration stocking and assessment, 2012–2020

Bloater, Coregonus hoyi , are deepwater planktivores native to the Laurentian Great Lakes and Lake Nipigon. Interpretations of commercial fishery time series suggest they were common in Lake Ontario through the early 1900s but by the 1950s were no longer captured by commercial fishers. Annual bottom trawl surveys that began in 1978 and sampled extensively across putative bloater habitat only yielded one individual (1983), suggesting that the species had been locally extirpated. In 2012, a multiagency restoration program stocked bloater into Lake Ontario from gametes collected in Lake Michigan. From 2012 to 2020, 1,028,191 bloater were stocked into Lake Ontario. Bottom trawl surveys first detected stocked fish in 2015, and through 2020 ten bloater have been caught (total length mean = 129 mm, s.d. = 44 mm, range: 96–240 mm). Hatchery applied marks and genetic analyses confirmed the species identification and identified stocking location for some individuals. Trawl capture locations and acoustic telemetry suggested that stocked fish dispersed throughout the main lake within months or sooner, and the depth distribution of recaptured bloater was similar to historic distributions in Lake Ontario and other Great Lakes. Predicted bloater trawl catches, based on modeled population abundance and trawl survey efficiency, were similar to observed catches, suggesting that post-stocking survival is less than 20% and contemporary bottom trawl surveys can quantify bloater abundance at low densities and track restoration.

Lake Ontario↗

USGS2015

Fluvial-aeolian interactions in sediment routing and sedimentary signal buffering: an example from the Indus Basin and Thar Desert

Sediment production and its subsequent preservation in the marine stratigraphic record offshore of large rivers are linked by complex sediment-transfer systems. To interpret the stratigraphic record it is critical to understand how environmental signals transfer from sedimentary source regions to depositional sinks, and in particular to understand the role of buffering in obscuring climatic or tectonic signals. In dryland regions, signal buffering can include sediment cycling through linked fluvial and eolian systems. We investigate sediment-routing connectivity between the Indus River and the Thar Desert, where fluvial and eolian systems exchanged sediment over large spatial scales (hundreds of kilometers). Summer monsoon winds recycle sediment from the lower Indus River and delta northeastward, i.e., downwind and upstream, into the desert. Far-field eolian recycling of Indus sediment is important enough to control sediment provenance at the downwind end of the desert substantially, although the proportion of Indus sediment of various ages varies regionally within the desert; dune sands in the northwestern Thar Desert resemble the Late Holocene–Recent Indus delta, requiring short transport and reworking times. On smaller spatial scales (1–10 m) along fluvial channels in the northern Thar Desert, there is also stratigraphic evidence of fluvial and eolian sediment reworking from local rivers. In terms of sediment volume, we estimate that the Thar Desert could be a more substantial sedimentary store than all other known buffer regions in the Indus basin combined. Thus, since the mid-Holocene, when the desert expanded as the summer monsoon rainfall decreased, fluvial-eolian recycling has been an important but little recognized process buffering sediment flux to the ocean. Similar fluvial-eolian connectivity likely also affects sediment routing and signal transfer in other dryland regions globally.

Indus Basin, Thar Desert↗

USGS2008

Spirit Mars Rover Mission to the Columbia Hills, Gusev Crater: Mission overview and selected results from the Cumberland Ridge to Home Plate

This paper summarizes the Spirit rover operations in the Columbia Hills of Gusev Crater from sols 513 to 1476 and provides an overview of selected findings that focus on synergistic use of the Athena Payload and comparisons to orbital data. Results include discovery of outcrops (Voltaire) on Husband Hill that are interpreted to be altered impact melt deposits that incorporated local materials during emplacement. Evidence for extensive volcanic activity and aqueous alteration in the Inner Basin is also detailed, including discovery and characterization of accretionary lapilli and formation of sulfate, silica, and hematite‐rich deposits. Use of Spirit's data to understand the range of spectral signatures observed over the Columbia Hills by the Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer (CRISM) hyperspectral imager (0.4–4 μ m) is summarized. We show that CRISM spectra are controlled by the proportion of ferric‐rich dust to ferrous‐bearing igneous minerals exposed in ripples and other wind‐blown deposits. The evidence for aqueous alteration derived from Spirit's data is associated with outcrops that are too small to be detected from orbital observations or with materials exposed from the shallow subsurface during rover activities. Although orbital observations show many other locations on Mars with evidence for minerals formed or altered in an aqueous environment, Spirit's data imply that the older crust of Mars has been altered even more extensively than evident from orbital data. This result greatly increases the potential that the surface or shallow subsurface was once a habitable regime.

Journal of Geophysical Research E: Planets↗

USGS2006

Characterization and petrologic interpretation of olivine-rich basalts at Gusev Crater, Mars

Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine-phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20-30%. Associated minerals in both include low-calcium and high-calcium pyroxenes, plagioclase of intermediate composition, iron-titanium-chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel-magnesium and chromium-magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine-rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.

Journal of Geophysical Research E: Planets↗

USGS2004

Holocene reef accretion: southwest Molokai, Hawaii, U.S.A.

Two reef systems off south Molokai, Hale O Lono and Hikauhi (separated by only 10 km), show strong and fundamental differences in modern ecosystem structure and Holocene accretion history that reflect the influence of wave-induced near-bed shear stresses on reef development in Hawaii. Both sites are exposed to similar impacts from south, Kona, and trade-wind swell. However, the Hale O Lono site is exposed to north swell and the Hikuahi site is not. As a result, the reef at Hale O Lono records no late Holocene net accretion while the reef at Hikauhi records consistent and robust accretion over late Holocene time. Analysis and dating of 24 cores from Hale O Lono and Hikauhi reveal the presence of five major lithofacies that reflect paleo-environmental conditions. In order of decreasing depositional energy they are: (1) coral-algal bindstone; (2) mixed skeletal rudstone; (3) massive coral framestone; (4) unconsolidated floatstone; and (5) branching coral framestone-bafflestone. At Hale O Lono, 10 cores document a backstepping reef ranging from ∼ 8,100 cal yr BP (offshore) to ∼ 4,800 cal yr BP (nearshore). A depauperate community of modern coral diminishes shoreward and seaward of ∼ 15 m depth due to wave energy, disrupted recruitment activities, and physical abrasion. Evidence suggests a change from conditions conducive to accretion during the early Holocene to conditions detrimental to accretion in the late Holocene. Reef structure at Hikauhi, reconstructed from 14 cores, reveals a thick, rapidly accreting and young reef (maximum age ∼ 900 cal yr BP). Living coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of ∼ 20 m where the reef ends in a large sand field. The primary limitation on vertical reef growth is accommodation space under wave base, not recruitment activities or energy conditions. Interpretations of cored lithofacies suggest that modern reef growth on the southwest corner of Molokai, and by extension across Hawaii in general, is controlled by wave-induced near-bed shear stress related to refracted North Pacific swell. Holocene accretion patterns here also reflect the long-term influence of wave-induced near-bed shear stress from north swell during late Holocene time. This finding is consistent with other studies (e.g., Grigg 1998; Cabioch et al. 1999) that reflect the dominance of swell energy and sea level in controlling modern and late Holocene accretion elsewhere in Hawaii and across the Pacific and Indian oceans. Notably, however, this result is refined and clarified for Hawaii in the hypothesis of Rooney et al. (2003) stating that enhancement of the El Niño Southern Oscillation beginning approximately 5000 years ago led to increased north swell energy and signaled the end to net accretion along exposed coastlines in Hawaii. The exposure of Hale O Lono to north swell and the age of sea floor there (ca. 4,800 cal yr BP), coupled with the lack of north swell incidence at Hikauhi and the continuous accretion that has occurred there over the last millennium, strongly supports the ENSO reef hypothesis as outlined by Rooney et al. (2003). Other factors controlling Holocene reef accretion at the study site are relative sea-level position and rate of rise, and wave sheltering by Laau Point. Habitat suitable for reef accretion on the southwest shore of Molokai has shrunk throughout the Holocene.

Hawai'i↗

USGS2012

Keanakākoʻi Tephra produced by 300 years of explosive eruptions following collapse of Kīlauea's caldera in about 1500 CE

The Keanakākoʻi Tephra at Kīlauea Volcano has previously been interpreted by some as the product of a caldera-forming eruption in 1790 CE. Our study, however, finds stratigraphic and 14 C evidence that the tephra instead results from numerous eruptions throughout a 300-year period between about 1500 and 1800. The stratigraphic evidence includes: (1) as many as six pure lithic ash beds interleaved in sand dunes made of earlier Keanakākoʻi vitric ash, (2) three lava flows from Kīlauea and Mauna Loa interbedded with the tephra, (3) buried syneruptive cultural structures, (4) numerous intraformational water-cut gullies, and (5) abundant organic layers rich in charcoal within the tephra section. Interpretation of 97 new accelerator mass spectrometry (AMS) 14 C ages and 4 previous conventional ages suggests that explosive eruptions began in 1470–1510 CE, and that explosive activity continued episodically until the early 1800s, probably with two periods of quiescence lasting several decades. Kīlauea's caldera, rather than forming in 1790, predates the first eruption of the Keanakākoʻi and collapsed in 1470–1510, immediately following, and perhaps causing, the end of the 60-year-long, 4–6 km 3 ʻAilāʻau eruption from the east side of Kīlauea's summit area. The caldera was several hundred meters deep when the Keanakākoʻi began erupting, consistent with oral tradition, and probably had a volume of 4–6 km3. The caldera formed by collapse, but no eruption of lava coincided with its formation. A large volume of magma may have quickly drained from the summit reservoir and intruded into the east rift zone, perhaps in response to a major south-flank slip event, leading to summit collapse. Alternatively, magma may have slowly drained from the reservoir during the prolonged ʻAilāʻau eruption, causing episodic collapses before the final, largest downdrop took place. Two prolonged periods of episodic explosive eruptions are known at Kīlauea, the Keanakākoʻi and the Uwēkahuna Tephra (Fiske et al., 2009), and both occurred when a deep caldera existed, probably with a floor at or below the water table, and external water could readily interact with the magmatic system. The next period of intense explosive activity will probably have to await the drastic deepening of the present caldera (or Halemaʻumaʻu Crater) or the formation of a new caldera.

Hawai'i↗

USGS1996

Hydrogeology of the Hawaii Scientific Drilling Project borehole KP-1 2. Groundwater geochemistry and regional flow patterns

A series of downhole and surface water samples were taken from the 1‐km‐deep KP‐1 borehole located on the eastern flank of the island of Hawaii. Early samples from depths of more than 700 m showed salinities nearly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and residual drilling mud clays. Later deep samples found only minor variations from seawater cation chemistry that are consistent with low‐temperature weathering of basalts; δ 18 O values are equivalent to seawater values and are consistent with this interpretation. Carbon 14 activities of dissolved inorganic carbonate indicate a water age ranging from 5890 to 7170 years B.P. and fluid transport rates of 1.8 to 2.2 m/yr. Fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the Mauna Kea/Mauna Loa interface; borehole resistivity logs indicate that it is ∼200 m thick. Although it has not yet been possible to obtain samples of the freshwater zone without contamination from the deep saline fluids, the chloride concentrations of the low‐salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg/L. Light stable isotope data indicate that the fresh water at 320 m is derived from recharge entering the island at an average elevation of 2000 m. Inferred 14 C activities of the dissolved bicarbonate in the freshwater zone indicate an average calibrated age of 2200 years B.P. and an average fluid velocity of at least 14 m/yr. A regional water flow model is proposed that suggests that the fresh water found at the 320‐m depth is derived from rainfall recharge from the middle elevations of Mauna Kea volcano. This rainfall is channeled beneath the Mauna Loa lavas by the thick soil layer separating the two volcanoes. A second shallow fresh‐to‐brackish water zone, derived from Mauna Loa recharge, is also inferred to exist below the carbonate formation that underlies the shallow basal lens. The results of our preliminary study of the groundwater system below the KP‐1 drill site demonstrate that intervolcano and interflow aquicludes can have a substantial impact on water circulation and discharge from young island volcanoes.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: evidence for near-surface water

The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor (MGS) mission has discovered a remarkable accumulation of crystalline hematite (α-Fe 2 O 3 ) that covers an area with very sharp boundaries approximately 350 by 350–750 km in size centered near 2°S latitude between 0° and 5°W longitude (Sinus Meridiani). Crystalline hematite is uniquely identified by the presence of fundamental vibrational absorption features centered near 300, 450, and >525 cm −1 and by the absence of silicate fundamentals in the 1000 cm −1 region. Spectral features resulting from atmospheric CO 2 , dust, and water ice were removed using a radiative transfer model. The spectral properties unique to Sinus Meridiani were emphasized by removing the average spectrum of the surrounding region. The depth and shape of the hematite fundamental bands show that the hematite is crystalline and relatively coarse grained (>5–10 μm). Diameters up to and greater than hundreds of micrometers are permitted within the instrumental noise and natural variability of hematite spectra. Hematite particles <5–10 μm in diameter (as either unpacked or hard-packed powders) fail to match the TES spectra. The spectrally derived areal abundance of hematite varies with particle size from ∼10% (>30 μm diameter) to 40–60% (10 μm diameter). The hematite in Sinus Meridiani is thus distinct from the fine-grained (diameter <5–10 μm), red, crystalline hematite considered, on the basis of visible, near-IR data, to be a minor spectral component in Martian bright regions like Olympus-Amazonis. Sinus Meridiani hematite is closely associated with a smooth, layered, friable surface that is interpreted to be sedimentary in origin. This material may be the uppermost surface in the region, indicating that it might be a late stage sedimentary unit or a layered portion of the heavily cratered plains units. We consider five possible mechanisms for the formation of coarse-grained, crystalline hematite. These processes fall into two classes depending on whether they require a significant amount of near-surface water: the first is chemical precipitation that includes origin by (1) precipitation from standing, oxygenated, Fe-rich water (oxide iron formations), (2) precipitation from Fe-rich hydrothermal fluids, (3) low-temperature dissolution and precipitation through mobile ground water leaching, and (4) formation of surface coatings, and the second is thermal oxidation of magnetite-rich lavas. Weathering and alteration processes, which produce nanophase and red hematite, are not consistent with the coarse, crystalline hematite observed in Sinus Meridiani. We prefer chemical precipitation models and favor precipitation from Fe-rich water on the basis of the probable association with sedimentary materials, large geographic size, distance from a regional heat source, and lack of evidence for extensive groundwater processes elsewhere on Mars. The TES results thus provide mineralogic evidence for probable large-scale water interactions. The Sinus Meridiani region may be an ideal candidate for future landed missions searching for biotic and prebiotic environments, and the physical characteristics of this site satisfy all of the engineering requirements for the missions currently planned.

Journal of Geophysical Research E: Planets↗

USGS2012

Structural evidence for northeastward movement on the Chocolate Mountains Thrust, southeasternmost California

The Late Cretaceous Chocolate Mountains thrust of southeastern California and southwestern Arizona places a block of Proterozoic and Mesozoic continental crust over the late Mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive Orocopia Schist. The Chocolate Mountains thrust is interpreted as a thrust (burial, subduction) fault rather than a low-angle normal (exhumation, unroofing, uplift) fault. An important parameter required to understand the tectonic significance of the Chocolate Mountains and related thrusts is their sense of movement. The Chocolate Mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds. Fabric relations, supported by regional geologic evidence, indicate that these folds are an integral part of and coeval with the thrust zone. On a lower hemisphere equal-area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities, spread over an area 60 by 10 km, Z folds plot northwest of and S folds plot southeast of a northeast-southwest striking vertical plane of overall monoclinic symmetry. The only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast. Asymmetric microstructures studied at several localities also indicate top to the northeast movement. Paleomagnetic data suggest that the original sense of thrusting, prior to Neogene vertical axis tectonic rotation related to the San Andreas fault system, was northward. The essential point is that movement of the upper plate of the Chocolate Mountains thrust evidently was continentward. Continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern California. The suture predicted by this model is elusive; but the Chocolate Mountains thrust and underlying Orocopia Schist themselves may represent the suture, at the present level of exposure. Alternative tectonic models involving subduction of the Orocopia Schist eastward beneath continental southern California circumvent the suture problem but are presently not supported by any direct structural evidence.

Journal of Geophysical Research↗

USGS2019

Application of an updated atmospheric model to explore volcano infrasound propagation and detection in Alaska

Winds and temperature gradients greatly affect the long-range propagation of infrasound . The spatio-temporal variability of these parameters must therefore be accurately characterized to correctly interpret recorded infrasound at long distances, specifically to differentiate between source and propagation effects. Here we present the first results of an open source reanalysis model, termed Alaska Volcano Observatory Ground-to-Space (AVO-G2S), constructed to accurately characterize the atmosphere and model long-range infrasound propagation from volcanic eruptions in Alaska. We select a number of case studies to examine recent eruptions of Alaskan volcanoes whose ash emissions posed a threat to air traffic, including the two most recent eruptions of Pavlof Volcano and two typical explosions from Cleveland Volcano. Strong tropospheric ducting and low noise at the station during the 21 July 2015 explosion of Cleveland Volcano led to an automated detection of the explosion at an infrasound array 992 km away, whereas low signal-to-noise ratio for the 6 November 2014 Cleveland Volcano explosion helps explain the non-detection in real-time of a predicted strong stratospheric arrival. For the November 2014 Pavlof eruption, discrepancies between local seismic data and a distal infrasound array 460 km away cannot be solely explained by changes in atmospheric conditions, though some features of the complex propagation predictions follow the trends in long-range infrasound signals. The most recent eruption of Pavlof Volcano in March 2016 shows minimal changes in propagation conditions throughout the eruption and therefore indicates that the signals detected at long-range primarily reflect source processes. These results show how detailed examination of the acoustic propagation conditions provides insight into detection capability and eruption dynamics. Future work will implement AVO-G2S and high-resolution long-range infrasound propagation modeling in real-time for Alaskan volcanoes of interest.

Alaska↗

USGS2006

Overview of the Spirit Mars Exploration Rover mission to Gusev Crater: Landing site to Backstay Rock in the Columbia Hills

Spirit landed on the floor of Gusev Crater and conducted initial operations on soil‐covered, rock‐strewn cratered plains underlain by olivine‐bearing basalts. Plains surface rocks are covered by wind‐blown dust and show evidence for surface enrichment of soluble species as vein and void‐filling materials and coatings. The surface enrichment is the result of a minor amount of transport and deposition by aqueous processes. Layered granular deposits were discovered in the Columbia Hills, with outcrops that tend to dip conformably with the topography. The granular rocks are interpreted to be volcanic ash and/or impact ejecta deposits that have been modified by aqueous fluids during and/or after emplacement. Soils consist of basaltic deposits that are weakly cohesive, relatively poorly sorted, and covered by a veneer of wind‐blown dust. The soils have been homogenized by wind transport over at least the several kilometer length scale traversed by the rover. Mobilization of soluble species has occurred within at least two soil deposits examined. The presence of monolayers of coarse sand on wind‐blown bedforms, together with even spacing of granule‐sized surface clasts, suggests that some of the soil surfaces encountered by Spirit have not been modified by wind for some time. On the other hand, dust deposits on the surface and rover deck have changed during the course of the mission. Detection of dust devils, monitoring of the dust opacity and lower boundary layer, and coordinated experiments with orbiters provided new insights into atmosphere‐surface dynamics.

Journal of Geophysical Research E: Planets↗

USGS2012

The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of 40 Ar/ 39 Ar thermochronology to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The 40 Ar/ 39 Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The 40 Ar/ 39 Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1°–5°C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman‐Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500°C which reset the K–Ar systems of minerals from Proterozoic rocks. High‐grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman‐Piute batholith at 73±1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were ∼100°C/m.y. between 73 and 70 Ma and 5°–10°C/m.y. from 70 to ∼30 Ma. Between 65 and 25 Ma the entire eastern Mojave Desert underwent a period of cooling at a rate of 2°–10°C/m.y. By 30 Ma, rocks exposed in the Old Woman‐Piute, Marble, Ship, Clipper, and Turtle Mountains were below ∼100°C. The 40 Ar/ 39 Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23±1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at ∼180°C and ∼350°C, respectively. This suggests that the exposed part of the Chemehuevi detachment fault initiated at a dip of 5°–30° or as a series of higher‐angle faults that cut to a depth of 10–12 km and were later rotated to their present dip. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10°–50°C/m.y. between 22 and 15 Ma.

California↗

USGS2024

Insights into glendonite formation from the upper Oligocene Sagavanirktok Formation, North Slope, Alaska

The type locality for the upper Oligocene Nuwok Member of the Sagavanirktok Formation (Carter Creek, North Slope, Alaska, USA) contains abundant occurrence of glendonite, a pseudomorph after the calcium carbonate mineral ikaite, which typically forms in the shallow subsurface of cold marine sediments. The region during the time of Nuwok Member deposition was located at a high latitude, similar to today, and the study site is characterized by sands and silty muds interpreted here to have been deposited in coastal and shelfal marine environments. Isotopic (Sr) and biostratigraphic (foraminifera) evidence presented here refine the depositional age of the outcrop to approximately 24 Ma. Glendonites occur in two basic forms: radial clusters, commonly centered around a single larger primary crystal ( approx. 10 cm; Type A) and larger single blades generally without accessory crystals (approx. 15–25 cm; Type B). Microscopic examination revealed a sequence of multiple types of replacive calcite that formed as a direct result of ikaite transformation: Type 1 rhombohedral crystals characterized by microporous and inclusion-rich cores and concentric zones, Type 2A, composed of clear calcite that overgrew and augmented Type 1 crystals, and inclusion-rich, microcrystalline Type 2B, which formed a matrix surrounding the rhombs and commonly dominates the outer rims of glendonite specimens. Type 3 calcite precipitated as fibrous, botryoidal epitaxial cement atop previous phases and is not ikaite-derived. These phases are distributed in similar ways in all examined specimens and are consistent with several previously described glendonite occurrences around the world, despite differing diagenetic and geologic histories. Stable isotope evidence (δ13C and δ18O) suggests sourcing of glendonite carbon from both organic and methanogenic sources. Glendonites of the Nuwok Member can therefore assist in the determination of a more comprehensive ikaite transformation model, improving our understanding of glendonite formation and the sedimentological and environmental context of their occurrence. Oligocene glendonites are uncommon globally; the well-preserved occurrence described here can allow future studies to better reconstruct Arctic environmental conditions and paleoclimates during this time.

Alaska↗

USGS2012

An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California

A 135‐km‐long, NW‐SE trending, seismic refraction/wide‐angle reflection profile provides a unique along‐strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two‐dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (< 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s −1 ) overlying a fairly homogeneous basement with velocities of 6.0 km s −1 ; (2) a localized, high‐velocity (6.4 km s −1 ) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s −1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s −1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a P n velocity of 8.0±0.10 km s −1 . The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s −1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle‐derived magmatism and lateral ductile inflow in the crust are proposed.

Arizona, California↗