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118 records · Page 7Linked to original sources
USGS1978

The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415

A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg 1.14 Fe 0.80 Mn 0.02 Ca 0.04 )(Si 1.97 Al 0.03 )O 6 , has a unit cell in space group Pbca with a = 18.310(15)Å, b = 8.904(10)Å, c = 5.214(7)Å, containing 2 formula units. A set of 742 counter-measured intensity data made with MoK α radiation has been used to refine the crystal structure in isotropic thermal mode to R = 0.116. Anisotropic refinement led to R = 0.092, but thermal parameters are distorted by non-random errors resulting from poor crystal texture. The resulting structure is in close agreement with that obtained by Ghose [9] for a hypersthene from Greenland. A parameter q , which gives ( Mg q Fe 1−q ) for cation site M (1) and (Mg 1.14− q Fe q −0.18 Ca 0.04 ) for site M (2), was included in the least-squares analysis, yielding q = 0.90(1). This orthopyroxene has the high degree of cation order expected of pyroxenes subjected to Apollonian metamorphism at lower than 500–600°C. No evidence exists for a subsequent thermal event of sufficient intensity to disorder the pyroxene. On the basis of previous laboratory studies of argon-release patterns of lunar plagioclase and order-disorder kinetics of terrestrial pyroxenes, we attribute the reported isotopic age (3.9–4.1 AE) to cessation of metamorphism, perhaps caused by impact excavation.

Earth and Planetary Science Letters↗
USGS1982

Post-depositional alteration of titanomagnetite in a Miocene sandstone, south Texas (U.S.A.)

Petrographic and geochemical studies have yielded information on the time-space relationships of the post-depositional alteration of detrital titanomagnetite (Ti-mt) in fine- to medium-grained sandstone from unoriented core samples (taken below the water table at depths of 30–45 m) of the Miocene Catahoula Sandstone, south Texas. Aqueous sulfide introduced from sour gas reservoirs along a growth fault into part of the Catahoula shortly after deposition resulted in the replacement at the periphery of Ti-mt grains by iron disulfide (FeS 2 ) minerals. Remnants of Ti-mt in cores of the partly sulfidized grains show no evidence of earlier hematitic oxidation. After sulfidization, part of the sandstone body was invaded by oxygenated groundwaters flowing down a shallowly inclined (1°) hydrologic gradient. The boundary between oxidized and reduced facies is clearly defined by the distribution of ferric and ferrous iron minerals, and the concentrations of Mo, U, and Se. In oxidized (light-red) strata that had not been previously subjected to sulfidic-reducing conditions but that are correlative with strata containing FeS 2 minerals, Ti-mt has been partly to entirely replaced pseudomorphously by hematite to form martite. The absence of hematitic alteration of Ti-mt in the reduced facies is strong evidence that martite in the oxidized facies formed after deposition.

Texas↗
USGS2022

Comparison of Digital Terrain Models from two photoclinometry methods

We evaluate the horizontal resolution and vertical precision for digital topographic models (DTMs) of the Moon derived from image radiance information, a process known as photoclinometry (PC) or shape-from-shading (SfS). We use the implementations in two available planetary image processing software systems, single image PC in the U.S. Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) system, and multi-image SfS in the Ames Stereo Pipeline (ASP), and test results obtained with and without use of a starting solution from stereo, with single and multiple images, and for varying illumination conditions. To obtain the higher quality reference DTMs against which the products can be evaluated, we derived DTMs by stereoanalysis of Lunar Reconnaissance Orbiter Narrow-Angle Camera (LROC NAC) images at their native pixel spacing of ∼0.5 m, then produced a 16-m/post stereo DTM from images downsampled to 4 m/pixel and refined it with images at 16 m/pixel. When used with a single image, both algorithms improved resolution (by a factor of 1.4 for PC and 2.4 for SfS compared to stereo). An albedo map produced in ISIS by ratioing the image to a simulation based on the stereo DTM was well correlated with one output by SfS. The albedo correction was crucial for PC with ∼60° incidence but not at ∼80°. DTMs produced by PC and SfS without a starting stereo DTM had larger errors but good detail, and could be useful for many applications. In SfS, it was necessary to increase smoothing to get a usable DTM when the weighting on an a priori DTM was reduced. Multi-image SfS including modeling of spatially varying albedo reduced vertical errors by factors of 1.5 or more compared to single-image SfS.

International Archives of the Photogrammetry, Remo↗
USGS2023

Tracing magmatic genesis and evolution through single zircon crystals from successive supereruptions from the Socorro Caldera Complex, USA

Large volume rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, differentiation within the middle crust, and partial melting in the lower crust contributes to geochemical and isotopic evolution of the crust. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine rates of magmatic processes as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry for four ignimbrites from the nested caldera complex near Socorro, New Mexico (USA), within the Mogollon-Datil volcanic field. In agreement with past 40 Ar- 39 Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hells Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra became more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hells Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase in the proportion of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are likely the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections based on the incorporation of zircon material at the melt source. In contrast, the Hf isotope record is volumetrically dominated by autocrystic zircon domains and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision datasets to advance volcanic models.

Arizona, New Mexico↗
USGS2003

Geochronology and eruptive history of the Katmai volcanic cluster, Alaska Peninsula

In the Katmai district of the Alaska Peninsula, K–Ar and 40 Ar/ 39 Ar ages have been determined for a dozen andesite–dacite stratocones on the arc front and for 11 rear-arc volcanoes, 10 of which are monogenetic. Tied to mapping and stratigraphic studies, our dating emphasized proximal basal lavas that rest on basement rocks, in order to estimate ages of inception of each polygenetic cone. Oldest among arc-front cones is Alagogshak Volcano (690–43 ka), succeeded in the Holocene by the active Mount Martin cone. Mount Mageik consists of four overlapping subedifices, basal lavas of which give ages of 93, 71, and 59 ka, and Holocene. The three small prehistoric cones of Trident Volcano yield ages of 143, 101–58, and 44 ka. Falling Mountain and Mount Cerberus, dacite domes near the 1912 Novarupta vent, are related compositionally to the Trident group and give ages of 70 ka and 114 ka. Mount Katmai, which underwent caldera collapse in 1912, consists of two subedifices that overlapped in space and time, and is the only arc-front center here to include basalt and rhyolite; one cone began by 90 ka, the other by 47 ka. Snowy Mountain also consists of two contiguous cones, which started around 200 and 171 ka, respectively, the younger remaining active into the Holocene. Devils Desk, the only mafic cone on the arc front, was short-lived at about 245 ka. In the rear-arc, (1) Mount Griggs produced mafic-to-silicic andesite in several episodes between 292 ka and the Holocene; (2) the Savonoski River cluster includes a Pliocene dacite dome and five small mafic cones (390–88 ka); (3) Gertrude Creek cone (49.8% SiO 2 ) yields an age of 500 ka; and (4) the Saddlehorn Creek cluster includes five Pliocene basalt-to-andesite remnants. Eruptive volumes were reconstructed, permitting estimates of average eruption rates for edifice lifetimes. Since the mid Pleistocene, total volume erupted along the arc front here is 210±47 km 3 and in the rear-arc 39±6 km 3 , of which Mount Griggs alone accounts for 35±5 km 3 . Most productive has been Mount Katmai at 70±18 km 3 , yielding a rate of ∼1 km 3 /kyr, followed by Mount Mageik (0.33 km 3 /kyr) and Mount Griggs (0.3 km 3 /kyr since 50 ka).

Alaska↗
USGS2013

A new model for the growth of basaltic shields based on deformation of Fernandina volcano, Galápagos Islands

Space-geodetic measurements of surface deformation produced by the most recent eruptions at Fernandina – the most frequently erupting volcano in the Galápagos Archipelago – reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir. This includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera. A Synthetic Aperture Radar (SAR) image acquired 1–2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface. Galápagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes, and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes. Our findings allow us to reinterpret the internal structure and evolution of Galápagos volcanoes and of similar basaltic shields. Furthermore, we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures. Specifically, circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions. This mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western Galápagos volcanoes, as well as the alternation between radial and circumferential fissure eruptions at Fernandina. The same model suggests that the next eruption of Fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion, just southwest of the caldera rim.

Galápagos Islands↗
USGS2023

Geochemical evidence for evolving Proterozoic crustal thickness and orogenic styles in southwestern Laurentia

It has long been challenging for researchers to track the crustal thickness and mode(s) of crustal modification in ancient convergent margins, limiting evaluation of the tectonic styles and processes that modify continental crust during orogenesis. We present trace element igneous geochemical crustal thickness proxies that quantitatively track the crustal thickness evolution of the long-lived Proterozoic active margin in the southwestern U.S.A. We integrate these results with geobarometric data to constrain the mode of crustal modification. The data indicate a complex record of crustal thickness change in space and time and evolving orogenic styles. Geochemical proxies at 1.84–1.72 Ga are consistent with 20–40 km thick magmatic arcs that were locally thickened to ∼50 km during ∼1.75 Ga tectonism. During the Yavapai orogeny, 1.72–1.69 Ga, a ∼200-km-wide belt of 50-60 km thick crust extended from southern California to northern Colorado and was rapidly thinned and exhumed by ∼1.68 Ga. Crustal thickening and thinning during the Yavapai orogeny largely occurred by shortening and exhumation, respectively, in the upper 25 km of the crust. Subsequent 1.68–1.60 Ga tectonism involved crustal growth, local crustal thickening, and low-P, high-T metamorphism, consistent with extensional accretionary orogenesis. The 1.47–1.37 Ga Picuris orogeny was associated with 50–60 km thick crust across much of the Southwest and involved crustal shortening with ∼10 km of magmatic underplating. Advective heat from the emplacement of ferroan granites in the mid-crust likely contributed to elevated geothermal gradients and rheologically weakened the crust. Our results suggest evolving orogenic styles in the Southwest from 1.75–1.69 Ga short-lived crustal thickening associated with terrane accretion to 1.69–1.60 Ga largely extensional accretionary orogenesis, and regional, long-lived crustal thickening at 1.47–1.37 Ga involving extensive basaltic underplating. Contrasting with some recent hypotheses, our data document a complex middle Proterozoic record for the Southwest that was not orogenically quiescent or tectonically stagnant but involved complex mountain building styles.

Earth and Planetary Science Letters↗
USGS2024

Global variability of the composition and temperature at the 410-km discontinuity from receiver function analysis of dense arrays

Seismic boundaries caused by phase transitions between olivine polymorphs in Earth's mantle provide thermal and compositional markers that inform mantle dynamics. Seismic studies of the mantle transition zone often use either global averaging with sparse arrays or regional sampling from a single dense array. The intermediate approach of this study utilizes many densely spaced seismic arrays distributed around the globe. We systematically compute teleseismic P-to-S receiver functions for each seismic array and invert for the 1-D seismic velocity structure of the mantle transition zone beneath each array to facilitate a comparison between densely sampled regions. We stack 3,600 receiver functions on average at 67 arrays in total. The stack is used in a probabilistic inversion to estimate the mantle transition zone interface depths and velocities beneath each array. We focus on the 410-km discontinuity (410) because it is a prominent seismic interface that is clearly linked to a single mineral phase transition between olivine and wadsleyite. The depths and velocity contrasts of the 410 are mapped to temperatures and compositions using mineral physics constraints. The depth of the 410 ranges from ∼405–440 km, which is consistent with a ∼360 K temperature range in a dry mantle and a ∼260 K temperature range in a wet mantle (2 wt. % water). The Vs contrast across the 410 ranges from ∼2.5–8 %, which is consistent with ∼20–70 vol. % olivine composition in a dry mantle and ∼25–80 vol. % in a wet mantle. The bulk composition of the upper mantle near the 410-km discontinuity is typically considered to be well-mixed because there is no thermodynamic impediment to convection at the olivine to wadsleyite phase transition. However, the wide range of inferred olivine content from our study suggests that there are large lateral variations in the bulk composition of the upper mantle near the 410-km discontinuity.

Earth and Planetary Science Letters↗
USGS2016

Large along-strike variations in the onset of Subandean exhumation: Implications for Central Andean orogenic growth

Plate tectonics drives mountain building in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Growth of the Subandean thrust belt, which lies at the boundary between the arid, high-elevation Central Andean Plateau and its humid, low-elevation eastern foreland, figures prominently into debates of orogenic wedge evolution. We integrate new apatite and zircon (U-Th)/He thermochronometer data with previously published apatite fission-track data from samples collected along four Subandean structural cross-sections in Bolivia between 15° and 20°S. We interpret cooling ages vs. structural depth to indicate the onset of Subandean exhumation and signify the forward propagation of deformation. We find that Subandean growth is diachronous south (11 ± 3 Ma) vs. north (6 ± 2 Ma) of the Bolivian orocline and that Subandean exhumation magnitudes vary by more than a factor of two. Similar north-south contrasts are present in foreland deposition, hinterland erosion, and paleoclimate; these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms. Of particular interest is an abrupt shift to cooler, more arid conditions in the Altiplano hinterland that is diachronous in southern Bolivia (16-13 Ma) vs. northern Bolivia (10-7 Ma) and precedes the timing of Subandean propagation in each region. Others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded. These mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. In summary, we suggest that diachronous Subandean exhumation was driven by piecemeal hinterland uplift, orography, and the outward propagation of deformation.

Andes↗
USGS2015

Coral 13C/12C records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios ( ) in the skeletons of massive Porites corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral response to uplift (and subsidence) produced by the March 2005 M w 8.6 Nias–Simeulue earthquake, and uplift further south around Sipora Island during a M∼8.4 megathrust earthquake in February 1797. At Nias, the average step-change in coral was 0.6±0.1‰/m for coseismic displacements of +1.8 m and −0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll is 2.3‰/m , nearly four times greater than for the Nias Porites . Considering the natural variability in coral skeletal , we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding M w ∼7.2 in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral , light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia.

Earth and Planetary Science Letters↗