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At least 343 records · Page 19Linked to original sources
USGS1998

Grooved Terrain on Ganymede: First Results from Galileo High-Resolution Imaging

High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are extreme and give the visual impression of topographic shading. However, photometric analysis shows that the scene must owe its character to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct geological units are apparent across the region, and ridges and grooves are ubiquitous within these units. The stratigraphically lowest and most heavily cratered units ("lineated grooved terrain") generally show morphologies indicative of horst-and-graben-style normal faulting. The stratigraphically highest groove lanes ("parallel ridged terrain") exhibit ridges of roughly triangular cross section, suggesting that tilt-block-style normal faulting has shaped them. These extensional-tectonic models are supported by crosscutting relationships at the margins of groove lanes. Thus, a change in tectonic style with time is suggested in the Uruk Sulcus region, varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units. The morphologies and geometries of some stratigraphically high units indicate that a strike-slip component of deformation has played an important role in shaping this region of grooved terrain. The most recent tectonic episode is interpreted as right-lateral transtension, with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain. There is little direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region; instead tectonism appears to be the dominant geological process that has shaped the terrain. A broad wavelength of deformation is indicated, corresponding to the Voyager-observed topography, and may be the result of ductile necking of the lithosphere, while a finer scale of deformation probably reflects faulting of the brittle near surface. The results here form a basis against which other Galileo grooved terrain observations can be compared.

Icarus↗
USGS1998

Europa: Initial Galileo Geological Observations

Images of Europa from the Galileo spacecraft show a surface with a complex history involving tectonic deformation, impact cratering, and possible emplacement of ice-rich materials and perhaps liquids on the surface. Differences in impact crater distributions suggest that some areas have been resurfaced more recently than others; Europa could experience current cryovolcanic and tectonic activity. Global-scale patterns of tectonic features suggest deformation resulting from non-synchronous rotation of Europa around Jupiter. Some regions of the lithosphere have been fractured, with icy plates separated and rotated into new positions. The dimensions of these plates suggest that the depth to liquid or mobile ice was only a few kilometers at the time of disruption. Some surfaces have also been upwarped, possibly by diapirs, cryomagmatic intrusions, or convective upwelling. In some places, this deformation has led to the development of chaotic terrain in which surface material has collapsed and/or been eroded. ?? 1998 Academic Press.

Icarus↗
USGS2015

A review of the recent geochemical evolution of Piton de la Fournaise volcano (1927-2010)

Between 1927 and 2010, more than one hundred eruptions of Piton de la Fournaise produced ~1 km 3 of lava, and the volcano’s summit collapsed twice (in 1931 and 2007). These lavas display, respectively, 20 and 65 % of the Sr–Nd and the Pb isotope ranges reported for La Réunion volcanoes over their known eruptive record (3.8 Ma). Variations in major and trace element concentrations and Sr–Pb isotopes do not define a temporal trend at the scale of the century, but display systematic short-term cyclic fluctuations. The positive correlation between 87 Sr/ 86 Sr and ratios of trace elements that are more versus less incompatible during partial melting of the mantle (e.g., Nd/Sm, La/Sm) probably results from the sampling of small-scale heterogeneities within the plume source. Changes in the degree of melting and/or crystallization are debated, but these appear ultimately linked to source properties. Lead isotopes do not co-vary with Sr isotopes, in part because of the partitioning of Pb into dense metallic phases that are preferentially sampled during high-flux eruptions. Taken together, Sr–Nd–Pb–Os–Th isotopes do not support contamination of magma with genetically unrelated components, such as the underlying Indian oceanic crust, mantle lithosphere, seawater, or seawater-altered lavas. Yet, in some rare cases (e.g. the 1998 Hudson eruption), the compositional patterns suggest that the parental magma assimilated older volcanic products within the edifice, such as crystal cumulates and/or interstitial differentiated melts. The geochemical fluctuations over the 1927–2010 time period constrain the residence time of magma in the shallow reservoir to 10–30 years and its size to 0.1–0.3 km 3 . The magma residence time during the course of the long-lived 1998 eruption is estimated to be an order of magnitude shorter, but the reservoir was probably of similar size. Instead, the shorter magma residence for the 1998 eruption was probably due to a higher magma flux.

Book chapter↗
USGS1992

The Northeast Kingdom batholith, Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<2 (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<2O/Na2O (<1), K/Rb (60-350), Zr/Hf (30-50), Nb/Ta (2-22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5-22), Th/Yb (0-27), Ba/La (0-80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Ybcn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Ybcn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80-164), Th/Ta (<9), Rb/Cs (7-40), K/Cs (0.1-0.5), Ce/Pb (0.5-4), high values of Rb/Sr (1-18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals. ?? 1992 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1992

Subsidence of Surtsey volcano, 1967-1991

The Surtsey marine volcano was built on the southern insular shelf of Iceland, along the seaward extension of the east volcanic zone, during episodic explosive and effusive activity from 1963 to 1967. A 1600-m-long, east-west line of 42 bench marks was established across the island shortly after volcanic activity stopped. From 1967 to 1991 a series of leveling surveys measured the relative elevation of the original bench marks, as well as additional bench marks installed in 1979, 1982 and 1985. Concurrent measurements were made of water levels in a pit dug on the north coast, in a drill hole, and along the coastline exposed to the open ocean. These surveys indicate that the dominant vertical movement of Surtsey is a general subsidence of about 1.1??0.3 m during the 24-year period of observations. The rate of subsidence decreased from 15-20 cm/year for 1967-1968 to 1-2 cm/year in 1991. Greatest subsidence is centered about the eastern vent area. Through 1970, subsidence was locally greatest where the lava plain is thinnest, adjacent to the flanks of the eastern tephra cone. From 1982 onward, the region closest to the hydrothermal zone, which is best developed in the vicinity of the eastern vent, began showing less subsidence relative to the rest of the surveyed bench marks. The general subsidence of the island probably results from compaction of the volcanic material comprising Surtsey, compaction of the sea-floor sediments underlying the island, and possibly downwarping of the lithosphere due to the laod of Surtsey. The more localized early downwarping near the eastern tephra cone is apparently due to greater compaction of tephra relative to lava. The later diminished local subsidence near the hydrothermal zone is probably due to a minor volume increase caused by hydrous alteration of glassy tephra. However, this volume increase is concentrated at depth beneath the bottom of the 176-m-deep cased drillhole. ?? 1992 Springer-Verlag.

Bulletin of Volcanology↗
USGS1989

High velocity anomaly beneath the Deccan volcanic province: Evidence from seismic tomography

Analysis of teleseismic P-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to-1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of the P-wave residual data delineate the presence of a 600 km long approximately N-S trending anomalous region of high velocity (1-4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion of P-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1-4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period. ?? 1989 Indian Academy of Sciences.

Proceedings of the Indian Academy of Sciences - Ea↗
USGS2002

Cripple Creek and other alkaline-related gold deposits in the Southern Rocky Mountains, USA: Influence of regional tectonics

Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera. Alkaline igneous rocks and associated hydrothermal deposits formed at two times. The first was during the Laramide orogeny (about 70–40 Ma), with deposits restricted spatially to the Colorado mineral belt (CMB). Other alkaline igneous rocks and associated gold deposits formed later, during the transition from a compressional to an extensional regime (about 35–27 Ma). These younger rocks and associated deposits are more widespread, following the Rocky Mountain front southward, from Cripple Creek in Colorado through New Mexico. All of these deposits are on the eastern margin of the Cordillera, with voluminous calc-alkaline rocks to the west. The largest deposits in the belt include Cripple Creek and those in the CMB. The most important factor in the formation of all of the gold deposits was the near-surface emplacement of relatively oxidized volatile-rich alkaline magmas. Strontium and lead isotope compositions suggest that the source of the magmas was subduction-modified subcontinental lithosphere. However, Cripple Creek alkaline rocks and older Laramide alkaline rocks in the CMB that were emplaced through hydrously altered LREE-enriched rocks of the Colorado (Yavapai) province have 208 Pb/ 204 Pb ratios that suggest these magmas assimilated and mixed with significant amounts of lower crust. The anomalously hot, thick, and light crust beneath Colorado may have been a catalyst for large-scale transfer of volatiles and crustal melting. Increased dissolved H 2 O (and CO 2 , F, Cl) of these magmas may have resulted in more productive gold deposits due to more efficient magmatic-hydrothermal systems. High volatile contents may also have promoted Te and V enrichment, explaining the presence of fluorite, roscoelite (vanadium-rich mica) and tellurides in the CMB deposits and Cripple Creek as opposed to deposits to the south. Deep-seated structures of regional extent that formed during the Proterozoic allowed the magmas to rise to shallow crustal levels. Proterozoic sites of intrusions at 1.65, 1.4, and 1.1 Ga were also important precursors to alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, and are localized at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting.

Colorado, New Mexico↗
USGS2003

Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands

Most of the world's Mississippi Valley-type (MVT) zinc-lead deposits occur in orogenic forelands. We examine tectonic aspects of foreland evolution as part of a broader study of why some forelands are rich in MVT deposits, whereas others are barren. The type of orogenic foreland (collisional versus Andean-type versus inversion-type) is not a first-order control, because each has MVT deposits (e.g., Northern Arkansas, Pine Point, and Cevennes, respectively). In some MVT districts (e.g., Tri-State and Central Tennessee), mineralization took place atop an orogenic forebulge, a low-amplitude (a few hundred meters), long-wavelength (100-200 km) swell formed by vertical loading of the foreland plate. In the foreland of the active Banda Arc collision zone, a discontinuous forebulge reveals some of the physiographic and geologic complexities of the forebulge environment, and the importance of sea level in determining whether or not a forebulge will emerge and thus be subject to erosion. In addition to those on extant forebulges, some MVT deposits occur immediately below unconformities that originated at a forebulge, only to be subsequently carried toward the orogen by the plate-tectonic conveyor (e.g., Daniel's Harbour and East Tennessee). Likewise, some deposits are located along syn-collisional, flexure-induced normal and strike-slip faults in collisional forelands (e.g., Northern Arkansas, Daniel's Harbour, and Tri-State districts). These findings reveal the importance of lithospheric flexure, and suggest a conceptual tectonic model that accounts for an important subset of MVT deposits-those in the forelands of collisional orogens. The MVT deposits occur both in flat-lying and in thrust-faulted strata; in the latter group, mineralization postdated thrusting in some instances (e.g., Picos de Europa) but may have predated thrusting in other cases (e.g., East Tennessee).

Mineralium Deposita↗
USGS2024

Polyphase stratabound scheelite-ferberite mineralization at Mallnock, Eastern Alps, Austria

A peculiar type of stratabound tungsten mineralization in metacarbonate rocks was discovered and explored at Mallnock (Austria) during the late 1980s. It is the only tungsten occurrence in the Eastern Alps in which scheelite is associated with wolframite (96 mol% ferberite). The tungsten prospect is located in the Austroalpine Drauzug-Gurktal Nappe System recording polyphase low-grade regional metamorphism. Raman spectroscopy of carbonaceous material yield maximum metamorphic temperatures of 296 ± 27 °C and 258 ± 27 °C, which are assigned to Variscan and Eoalpine metamorphism, respectively. Scheelite and ferberite occur as polyphase stockwork-like mineralization in Fe-rich magnesite in the northern ore zone (Mallnock North), whereas in the western ore zone (Mallnock West), scheelite-quartz veinlets are exclusively hosted in dolomitic marbles. LA-ICP-MS analyses of scheelite and ferberite yield low contents of Mo, Nb, Ta, and rare earth elements, but high contents of Na and Sr. Uranium is particularly high in scheelite (up to 200 µg/g) and makes this mineral a suitable target for U–Pb dating. In situ U–Pb dating of scheelite yielded an early Permian age (294 ± 8 Ma) for Mallnock West and a Middle Triassic age (239 ± 3 Ma) for Mallnock North. A monzodioritic dike close to Mallnock yielded a U–Pb apatite date of 282 ± 9 Ma and supports the polyphase formation of this mineralization. The U–Pb scheelite ages indicate that a model for tungsten metallogeny in the Eastern Alps must also consider remobilization of tungsten by metamorphic fluids. In the Alps, the Permian to Triassic period (ca. 290–225 Ma) is characterized by an overall extensional geodynamic setting related to the breakup of Pangea. Lithospheric thinning caused higher heat flow, low-P metamorphism, and anatexis in the lower crust, which led to enhanced crustal fluid flow in the upper crust. These processes were not only responsible for the formation of metasomatic hydrothermal magnesite and siderite deposits in the Eastern Alps but also for this unique magnesite-ferberite-scheelite mineralization at Mallnock.

Mount Mallock↗
USGS1998

The Holocene sea-level highstand in the equatorial Pacific: Analysis of the insular paleosea-level database

A review of the literature provides 92 estimates of the middle to late Holocene sea-level highstand on Pacific Islands. These data generally support geophysical model calculations that predict a +1 to 3 m relative sea-level highstand on oceanic islands due to the Earth’s rheological response to the melting of the last continental ice sheets and subsequent redistribution of meltwater. Both predictions and observations indicate sea level was higher than present in the equatorial Pacific between 5000 and 1500 y B.P. A non-linear relationship exists between the age and elevation of the highstand peak, suggesting that different rates of isostatic adjustment may occur in the Pacific, with the highest rates of sea-level fall following the highstand near the equator. It is important to resolve detailed sea-level histories from insular sites to test and refine models of climatic, oceanographic, and geophysical processes including hydroisostasy, equatorial ocean siphoning, and lithospheric flexure that are invoked as mechanisms affecting relative sea-level position. We use a select subset of the available database meeting specific criteria to examine model relationships of paleosea-surface topography. This new evaluated database of paleosea-level positions is also validated for testing and constraining geophysical model predictions of past and present sea-level variations.

Coral Reefs↗
USGS2002

Geochemistry of Mesozoic plutons, southern Death Valley region, California: Insights into the origin of Cordilleran interior magmatism

Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with ε Ndi of –5 to –16, 87 Sr/ 86 Sr i of 0.707–0.726, and 206 Pb/ 204 Pb i of 17.5–20.0. Cretaceous granitoids of the region are mainly monzogranites with ε Ndi of –6 to –19, 87 Sr/ 86 Sr i of 0.707–0.723, and 206 Pb/ 204 Pb i of 17.4–18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust – some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unusually juvenile composition (ε Ndi –3.2, 87 Sr/ 86 Sr i 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordilleran plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane.

California↗
USGS2005

Experimental and geochemical evidence for derivation of the El Capitan Granite, California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.

Contributions to Mineralogy and Petrology↗
USGS2020

Four-dimensional thermal evolution of the East African Orogen: Accessory phase petrochronology of crustal profiles through the Tanzanian Craton and Mozambique Belt, northeastern Tanzania

U–Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper–middle crust record thermal quiescence since the Archean (2.8–2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga, > 900 °C zircon), the EAO (600–500 Ma rutile), and fluid influx during rifting (< 5 Ma apatite). Rutile in garnet granulite xenoliths exhibits partial Pb loss related to slow cooling of the lower crust after the EAO and suggests residence at 500–600 °C prior to entrainment. In contrast to the craton, the entire crust of the Mozambique Belt underwent differential cooling following the EAO. Both the upper and middle crust record metamorphism from 640 to 560 Ma (zircon, monazite, and titanite) and rapid exhumation at 510–440 Ma (rutile and apatite). Lower crustal xenoliths contain Archean zircon, but near-zero age rutile and apatite, indicating residence > 650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1–1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting.

Contributions to Mineralogy and Petrology↗
USGS2010

Gravity modeling of the Muertos Trough and tectonic implications (north-eastern Caribbean)

The Muertos Trough in the northeast Caribbean has been interpreted as a subduction zone from seismicity, leading to infer a possible reversal subduction polarity. However, the distribution of the seismicity is very diffuse and makes definition of the plate geometry difficult. In addition, the compressive deformational features observed in the upper crust and sandbox kinematic modeling do not necessarily suggest a subduction process. We tested the hypothesized subduction of the Caribbean plate's interior beneath the eastern Greater Antilles island arc using gravity modeling. Gravity models simulating a subduction process yield a regional mass deficit beneath the island arc independently of the geometry and depth of the subducted slab used in the models. This mass deficit results from sinking of the less dense Caribbean slab beneath the lithospheric mantle replacing denser mantle materials and suggests that there is not a subducted Caribbean plateau beneath the island arc. The geologically more realistic gravity model which would explain the N-S shortening observed in the upper crust requires an overthrusted Caribbean slab extending at least 60 km northward from the deformation front, a progressive increase in the thrusting angle from 8?? to 30?? reaching a maximum depth of 22 km beneath the insular slope. This new tectonic model for the Muertos Margin, defined as a retroarc thrusting, will help to assess the seismic and tsunami hazard in the region. The use of gravity modeling has provided targets for future wide-angle seismic surveys in the Muertos Margin. ?? 2010 Springer Science+Business Media B.V.

Marine Geophysical Research↗
USGS2019

Preface—Evaluating the response of critical zone processes to human impacts with sediment source fingerprinting

1) Background: Critical Zone Processes in the Anthropocene The Earth’s Critical Zone encompasses a suite of interconnected processes in the near-surface lithosphere, pedosphere, biosphere, atmosphere, and hydrosphere (Brantley et al., 2007; Lin, 2010) (Fig. 1). Processes and interactions both within and between these various Critical Zone components supports life-sustaining ecosystem services and resources that establish the foundation for humanity (NRC, 2001). This includes the formation production of fertile soils, flourishing vegetation, productive rivers, lakes and oceans, and our life-sustaining atmosphere (Gaillardet, 2014; Guo and Lin, 2016). Rapid population growth, land use intensification, and global environmental change are disturbing many of these fundamental Critical Zone processes. More than half of the Earth’s terrestrial surface is now impacted by anthropogenic activities (e.g., clearing, grazing, plowing, mining, and logging) (Hooke et al., 2012; Richter and Mobley, 2009). These changes are so widespread and pervasive that the great acceleration of socioeconomic development that occurred around 1950 (Fig. 2) has been recommended to delineate the dawn of the Anthropocene (Waters et al., 2016). Although the utility of adopting and delineating the Anthropocene as the current epoch is subject to debate (Crutzen, 2002; Ruddiman et al., 2015; Smith and Zeder, 2013), the concept effectively highlights both the nature and the extent of our global impact on Earth’s Critical Zone. Soil forming processes and ecosystem services provided by the pedosphere are central to the Critical Zone (Banwart et al., 2011; Lin, 2010). Many of these processes have been disturbed by the agricultural intensification that coincided with the great acceleration resulting in unsustainable land use practices now outpacing soil formation processes (Brantley et al., 2007). As agricultural landscapes now cover an area equivalent to what was scoured during the last glacial maximum (Amundson et al., 2007), the broad-scale intensification of anthropogenic activities has resulted in significant on- and off-site impacts. On-site, soil loss has resulted in decreases in soil fertility and agricultural yields (Ladha et al., 2009) threatening the ability to feed the world’s growing population (Brantley et al., 2007). Off-site, the excess delivery of particulate matter downstream is degrading riverine, lacustrine, and estuarine ecosystems (Bilotta and Brazier, 2008; Clark, 1985; Owens et al., 2005). The challenge, as noted by Brantley et al., (2007), is that despite our society having over 10,000 years of experience working with soils, our conceptual and quantitative models remain inadequate at predicting Critical Zone dynamics under current conditions. Notwithstanding growing pressure for improved environmental management, we still have a limited capacity to predict changes in the Critical Zone in response to anthropogenic activities owing to the multiple spatial and temporal scales at which these complex processes and feedbacks are manifest. As river basin systems are impacted by many of these processes, a deep understanding of soil-sediment continuum dynamics may provide a valuable framework for evaluating the disturbance response of Critical Zone processes. Understanding these processes may also provide land and resource managers with the information necessary to manage both the on-site and off-site effects of accelerated soil erosion.

Journal of Soils and Sediments↗
USGS1972

Oceanic ridges and transform faults: Their intersection angles and resistance to plate motion

The persistent near-orthogonal pattern formed by oceanic ridges and transform faults defies explanation in terms of rigid plates because it probably depends on the energy associated with deformation. For passive spreading, it is likely that the ridges and transforms adjust to a configuration offering minimum resistance to plate separation. This leads to a simple geometric model which yields conditions for the occurrence of transform faults and an aid to interpretation of structural patterns in the sea floor. Under reasonable assumptions, it is much more difficult for diverging plates to spread a kilometer of ridge than to slip a kilometer of transform fault, and the patterns observed at spreading centers might extend to lithospheric depths. Under these conditions, the resisting force at spreading centers could play a significant role in the dynamics of plate-tectonic systems.

Earth and Planetary Science Letters↗
USGS1975

Calculated geochronology and stress field orientations along the Hawaiian chain

A new method has been discovered for calculating ages of the main shield building stages of volcanoes along the Hawaiian chain from Kilauea to the Hawaiian-Emperor bend. The method is based on a graphical technique for hypothetical subtraction of distance intervals that theoretically represent regions of simultaneous volcanism along adjacent or nearly en-echelon loci of volcanism. Distances along the chain, measured from Kilauea, when progressively foreshortened by the distances of hypothetical “collapse” and plotted versus existing age data are found to give linear age-distance relationships. A calibration graph is presented that agrees closely with the measured ages in 17 of the 20 existing dated volcanoes. The criterion for simultaneous activity on different loci is based on the concept of equal azimuths of synchronous volcanic propagation within coeval segments of the chain. This is the predicted relationship when magmatic fluids inject the lithosphere along directions normal to a nearly horizontal least principal stress. It appears that the Pacific plate has been subjected to oscillatory, but principally clockwise, rotations of horizontal stress components during the last 40 m.y.

Hawaii↗
USGS1978

Isotopic composition of lead in oceanic basalt and its implication to mantle evolution

New data are given in this report for (1) Pb isotopic compositions and U, Th, and Pb concentrations of basalts from the island of Hawaii; (2) redetermined Pb isotopic compositions of some abyssal tholeiites; and (3) U, Th, and Pb concentrations of altered and fresh abyssal basalts, and basalt genesis and mantle evolution are discussed. The Th/U ratios of abyssal and Japanese tholeiites are distinctly lower than those of tholeiites and alkali basalts from other areas. It is thought that these low values reflect a part of the mantle depleted in large ionic lithophile elements. Thus a mantle evolution model is presented, in which Th/U ratios of the depleted zone in the mantle have decreased to ∼2, and U/Pb ratios have increased, showing an apparent ∼1.5-b.y. isochron trend in the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plot. The Pb isotopic compositions of basalts from the island of Hawaii are distinct for each of the five volcanoes, and within each volcano, Pb's of tholeiites and alkali basalts are similar. An interaction between partially melted material (hot plume?) of the asthenosphere and the lithosphere is suggested to explain the trend in the Pb isotopic compositions of Hawaiian basalts.

Earth and Planetary Science Letters↗