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At least 415 records · Page 23Linked to original sources

USGS2022

Outgassing through magmatic fractures enables effusive eruption of silicic magma

Several mechanisms have been proposed to allow highly viscous silicic magma to outgas efficiently enough to erupt effusively. There is increasing evidence that challenges the classic foam-collapse model in which gas escapes through permeable bubble networks, and instead suggests that magmatic fracturing and/or accompanying localized fragmentation and welding within the conduit play an important role in outgassing. The 2011–2012 eruption at Cordón Caulle volcano, Chile, provides direct observations of the role of magmatic fractures. This eruption exhibited a months-long hybrid phase, in which rhyolitic lava extrusion was accompanied by vigorous gas-and-tephra venting through fractures in the lava dome surface. Some of these fractures were preserved as tuffisites (tephra-filled veins) in erupted lava and bombs. We integrate constraints from petrologic analyses of erupted products and video analyses of gas-and-tephra venting to construct a model for magma ascent in a conduit. The one-dimensional, two-phase, steady-state model considers outgassing through deforming permeable bubble networks, magmatic fractures, and adjacent wall rock. Simulations for a range of plausible magma ascent conditions indicate that the eruption of low-porosity lava observed at Cordón Caulle volcano occurs because of significant gas flux through fracture networks in the upper conduit. This modeling emphasizes the important role that outgassing through magmatic fractures plays in sustaining effusive or hybrid eruptions of silicic magma and in facilitating explosive-effusive transitions.

Puyehue Cordon Caulle volcanic complex↗

USGS2022

A reappraisal of explosive–effusive silicic eruption dynamics: Syn-eruptive assembly of lava from the products of cryptic fragmentation

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cordón Caulle 2011–2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive–effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011–2012 Cordón Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity – explosive, effusive, and hybrid – was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cordón Caulle, and from other similar eruptions, we further argue that hybrid explosive–effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cordón Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent non-fragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

Journal of Volcanology and Geothermal Research↗

USGS2023

Chemistry and petrography of early 19th century basaltic andesites and basalts from the Kamakaiʻa Hills in the Southwest Rift Zone of Kīlauea volcano, Hawaiʻi

Kīlauea is a frequently active, open-system volcano on the Island of Hawaiʻi known for erupting olivine-dominated tholeiitic basalt compositions. On rare occasions it erupts more differentiated magmas (<1% of erupted volume), such as basaltic andesites and andesites, from its rift zones. These differentiated magmas offer an opportunity to understand better the petrology, magma storage, magma mixing, and eruptive triggers that occur in Kīlauea's rift zone reservoirs. This study focuses on an eruption from the Southwest Rift Zone of Kīlauea, which is dominantly basaltic andesite with subordinate basalt. This eruption originated at the Kamakaiʻa Hills during the early 19th century and has two eruptive phases: 1) an early ‘a‘ā phase that is primarily exposed in the eastern part of the flow field, with minor western lobes, and 2) a late pāhoehoe phase that makes up most of the western part of the flow field. The early ‘a‘ā phase covers at least 5.8 km 2 with an erupted volume of ∼150 × 10 6 m 3 and consists of uniform composition basaltic andesites with 3.72–4.15 wt% MgO over its ∼7 km flow length. The late pāhoehoe phase reached >10 km from its vent, covers an area of ∼7.1 km 2 , has a volume of ∼100 × 10 6 m 3 , and initially erupted basaltic andesite near its vent (4.50–5.64 wt% MgO extending to 3.8 km from vent) with channel and tube-fed basalt (6.21–12.38 wt% MgO sampled at >3.8 km from vent) emplaced during its waning stages. Most Kamakaiʻa Hills lavas are crystal-poor, containing ≤1.5% glomerocrysts and individual phenocrysts of plagioclase + clinopyroxene + Fe Ti oxides ± orthopyroxene, as well as olivine in lavas with >6 wt% MgO. Major-oxide and trace-element concentrations throughout the Kamakaiʻa Hills lavas demonstrate the involvement of three distinct magmatic processes. First, the basaltic andesites of the early ‘a‘ā phase are the products of fractionation of plagioclase + clinopyroxene + Fe Ti oxides ± orthopyroxene, indicative of magmas that have been stored in rift zone reservoirs for decades or longer. Second, the near-vent (within ∼400 m of vent) basaltic andesites of the late pāhoehoe phase yield chemical concentrations that indicate magma mixing with a more differentiated magma (of a similar evolved composition to basaltic andesites at ∼55–56 wt% SiO 2 and ∼3.4–4.1 wt% MgO that erupted in the lower East Rift Zone in 2018). Third, the progressively more mafic magma (containing olivine + plagioclase + clinopyroxene) that continued to erupt throughout the waning stages of activity suggests an eruptive triggering process whereby an intruding summit or uprift reservoir basalt overpressurized and forced out the stored, differentiated magma of the Kamakaiʻa Hills rift zone reservoir.

Hawaiʻi↗

USGS2003

Thermal and chemical variations in subcrustal cratonic lithosphere: Evidence from crustal isostasy

The Earth's topography at short wavelengths results from active tectonic processes, whereas at long wavelengths it is largely determined by isostatic adjustment for the density and thickness of the crust. Using a global crustal model, we estimate the long-wavelength topography that is not due to crustal isostasy. Our most important finding is that cratons are generally depressed by 300 to 1500 m in comparison with predictions from pure crustal isostasy. We conclude that either: (1) cratonic roots may be 50 to 300 °C colder than previously suggested by thermal models, or (2) cratonic roots may be, on average, less depleted than suggested by studies of shallow mantle xenoliths. Alternatively, (3) some combination of these conditions may exist. The thermal explanation is consistent with recent geothermal studies that indicate low cratonic temperatures, as well as seismic studies that show very low seismic attenuation at long periods (150 s) beneath cratons. The petrologic explanation is consistent with recent studies of deep (>140 km) mantle xenoliths from the Kaapvaal and Slave cratons that show 1–2% higher densities compared with shallow (<140 km), highly depleted xenoliths.

LITHOS↗

USGS2018

Ore-forming adakitic porphyry produced by fractional crystallization of oxidized basaltic magmas in a subcrustal chamber (Jiamate, East Junggar, NW China)

Adakitic intrusions are supposed to have a close genetic and spatial relationship to porphyry Cu deposits. However, the genesis of adakitic intrusions is still under dispute. Here, we describe newly discovered intrusive complex rocks, which are composed of ore-bearing, layered magnetite-bearing gabbroic and adakitic rocks in Jiamate, East Junggar, NW China. These Jiamate Complex intrusions have diagnostic petrologic, geochronologic and geochemical signatures that indicate they were all generated from the same oxidized precursor magma source. Additionally, these layered rocks underwent the same fractional crystallization process as the ore-bearing adakitic rocks in the adjacent Kalaxiangar Porphyry Cu Belt (KPCB) in an oceanic island arc (OIA) setting. The rocks studied for this paper include layered magnetite-bearing gabbroic intrusive rocks that contain: (1) gradual contact changes between lithological units of mafic and intermediate rocks, (2) geochemical signatures that are the same as those found in oceanic island arc (OIA) rocks, (3) typical adakitic geochemistry, and (4) similar characteristics and apparent fractional crystallization relationships of ultra-basic to basic rocks to those in the nearby Beitashan Formation and to ore-bearing adakitic rocks in the KPCB. They also display similar zircon U-Pb and zircon Hf model ages. The Jiamate Complex intrusions contain intergrowths of magnetite and layered gabbro, and the intermediate-acidic intrusions of the Complex display typical adakitic affinities. Moreover, in conjunction with previously published geochronological and geochemistry data of the mafic rocks in the Beitashan Formation and in the KPCB area, additional data generated for the Jiamate Complex intrusions rocks indicate that they were formed from fractional crystallization processes. The Jiamate Complex intrusions most likely were derived from a metasomatized mantle wedge that was underplated at the root of the Saur oceanic island arc (Saur OIA). The ore-bearing adakitic intrusions in the KPCB and the adakitic Jiamate Complex intrusions were both probably generated from the same basaltic parental magmas through fractional crystallization. In addition, characteristics of the layered, magnetite-bearing, oxidized, basaltic Jiamate Complex intrusive rocks indicate that they are likely to be the parental arc magmas for the nearby porphyry Cu deposits. This conclusion is based on new interpretations of the regional and local geology, on interpretation of new geochemical analysis, new stable isotope analysis, new geothermobarometry, and new zircon age dating as well as other techniques and interpretations.

LITHOS↗

USGS2021

Hydrous pyrolysis of New Albany Shale: A study examining maturation changes and porosity development

The characterization of nanoscale organic structures has improved our understanding of porosity development within source-rock reservoirs, but research linking organic porosity evolution to thermal maturity has generated conflicting results. To better understand this connection, an immature (0.25% solid bitumen reflectance; BR o ) sample of the New Albany Shale was used in four isothermal hydrous pyrolysis (HP) experiment sequences at 300°, 320°, 340°, and 370°C, with residues collected periodically for a maximum of 103 days. The HP residues, along with the original immature sample and two naturally matured (1.49 and 1.56% BR o ) New Albany Shale samples were analyzed for organic petrology, total organic carbon (TOC) content, and organic porosity evaluation using correlative light and electron microscopy (CLEM). All of the HP series increased in thermal maturity with increasing duration of pyrolysis, though reflectance for each series plateaued within 25 days of maturation. Initially, TOC in the HP residues decreases (from 14.24 wt. %) with increasing thermal maturity until ∼1.0% BR o where TOC remains at ∼9–10 wt. % for all remaining residues. Qualitative CLEM observations within the 50–100 day 300° and 340°C HP sequences (0.95–1.70% BR o ), and the naturally matured samples, develop organic porosity in smaller (<5 μm in diameter), void-filling solid bitumen that occurs in spaces between clays and other fine-grained minerals. The 370°C HP residues developed significant organic porosity, relative to the other HP temperature series in all solid bitumen accumulations regardless of size. Overall, the study indicates that temperature and duration of artificial maturation play an important role in the abundance of pores in the HP residues. This work expands on our understanding of the conditions needed for the generation and development of organic porosity in the New Albany Shale and potentially in other marine source-rock petroleum systems.

Illinois, Indiana↗

USGS2022

Petrogenesis and rare earth element mineralization of the Elk Creek carbonatite, Nebraska, USA

Although carbonatites are the primary source of the world’s rare earth elements (REEs), the processes responsible for ore-grade REE enrichment in carbonatites are still poorly understood. In this study, we present a petrologic, geochemical, and isotopic evaluation of the Elk Creek carbonatite in southeast Nebraska to constrain the origin of REE mineralization. The Elk Creek carbonatite is a multilithologic carbonatite comprised of an early apatite-dolomite carbonatite, a middle/heavy REE-enriched magnetite-dolomite carbonatite, and a late-stage light REE-enriched, barite-dolomite carbonatite, as well as a suite of breccias. Neodymium, strontium, and carbon isotopic data from the early apatite-dolomite carbonatite, ε Nd (T) = 2.3 to 3.4, 87 Sr/ 86 Sr (i) = 0.702704 to 0.702857, and δ 13 C = −3.3 to −3.4, indicate that the parental magma and REEs were derived from the mantle, and textural and chemical data suggest that hydrothermal processes played an important role in reaching ore-grade enrichment. Higher initial 87 Sr/ 86 Sr values (∼0.7041) of REE-mineralized lithologies are evidence that these fluids were derived, in part, from meteoric water that interacted with the country rock. Modeling of the C-O isotopic data reveals that some of the isotopic variation results from closed-system Rayleigh fractionation of an evolving carbonatitic magma between 300 and 500 °C, but an excursion to heavier δ 18 O is likely the result of interaction with H 2 O-CO 2 -fluids at temperatures from 400 to 100 °C. Hydrothermal dolomite has higher 87 Sr/ 86 Sr values than early-formed magmatic dolomite, consistent with metasomatism by fluids derived, in part, from a more radiogenic source such as the Precambrian-age wall rock. Rare earth element mineralization occurs primarily in fine-grained, cavity filling minerals including monazite, bastnäsite, parisite, and synchysite along with barite, dolomite, quartz, and iron oxides. We interpret the LREE enrichment at Elk Creek to be the product of hydrothermal fluids derived from the evolving carbonatite magma and fluids from the wall rock. The REEs likely became enriched in late-stage fluids from the evolving magma as well as being remobilization by the dissolution of earlier formed minerals. Middle/heavy REE-enrichment in the magnetite-dolomite carbonatite is hosted in hydrothermal dolomite and is attributed to variations in the composition of hydrothermal fluids.

Nebraska↗

USGS2024

Layered intrusions in the Precambrian: Observations and perspectives

Layered intrusions are plutonic bodies of cumulates that form by the crystallization of mantle-derived melts. These intrusions are characterized by igneous layering distinguishable by shifts in mineralogy, texture, or composition. Layered intrusions have been fundamental to our understanding of igneous petrology; however, it is their status as important repositories of critical metals – such as platinum-group elements, chromium, and vanadium – that has predominantly driven associated research in recent decades. Many layered intrusions were emplaced during the Precambrian, predominantly at the margins of ancient cratons during intervals of supercontinent accretion and destruction. It appears that large, layered intrusions require rigid crust to ensure their preservation, and their geometry and layering is primarily controlled by the nature of melt emplacement. Layered intrusions are best investigated by integrating observations from various length-scales. At the macroscale, intrusion geometries can be discerned, and their presence understood in the context of the regional geology. At the mesoscale, the layering of an intrusion may be characterized, intrusion-host rock contact relationships studied, and the nature of stratiform mineral occurrences described. At the microscale, the mineralogy and texture of cumulate rocks and any mineralization are elucidated, particularly when novel microtextural and mineral chemical datasets are integrated. For example, here we demonstrate how mesoscale observations and microscale datasets can be combined to understand the petrogenesis of the perplexing snowball oiks outcrop located in the Upper Banded Series of the Stillwater Complex. Our data suggest that the orthopyroxene oikocrysts did not form in their present location, but rather formed in a dynamic magma chamber where crystals were transported either by convective currents or within crystal-rich slurries. Critical metals may be transported to the level of a nascent intrusion as dissolved components in the melt. Alternatively, ore minerals are entrained from elsewhere in a plumbing system, potentially facilitated by volatile-rich phases. There are many ore-forming processes propounded by researchers to occur at the level of emplacement; however, each must address the arrival of the ore mineral, its concentration of metals, and its accumulation into orebodies. In this contribution, several of these processes are described as well as our perspectives on the future of layered intrusion research.

Precambrian Research↗

USGS2007

The investigation of chemical structure of coal macerals via transmitted-light FT-IR microscopy by X. Sun

A recent paper by Sun [X. Sun, Spectrochim. Acta A 62 (1-3) (2005) 557] attempts to characterize a variety of liptinite, termed "barkinite", from Chinese Permian coals. The component identified does not appear to fundamentally differ from previously-described liptinite macerals included in the International Committee for Coal and Organic Petrology's system of maceral nomenclature. Further, chemical comparisons made with macerals from coals of different rank and age are flawed because the author did not account for changes in chemistry with rank or for the chemical changes associated with botanical changes through geologic time. The author has not satisfactorily proved his hypothesis that the component differs morphologically or chemically from known liptinite-group macerals. ?? 2006 Elsevier B.V. All rights reserved.

Spectrochimica Acta - Part A: Molecular and Biomol↗

USGS2021

Organic petrographic evaluation of carbonaceous material in sediments of the Kinnickinnic River, Milwaukee, WI, U.S.A.

This study examines the use of organic petrology techniques to quantify the amount of coal and carbonaceous combustion by-products (i.e., coke, coal tar/pitch, cenospheres) in sediments taken from the Kinnickinnic River adjacent to the former site of the Milwaukee Solvay Coke and Gas Company. These materials are of concern as contaminants like polycyclic aromatic hydrocarbons (PAHs) are known to readily adsorb to coal and combustion byproducts. Kinnickinnic River sediment samples (n = 36) ranging in depth (1–11 ft.) were collected from eight core locations to quantify and characterize carbonaceous material in the sediments. To determine the amount (vol%) of organic particulates, U.S. Geological Survey (USGS) modified the existing ASTM D2799 using the following categories: coal, coke, coal tar/pitch, inertinite organics, plant material, cenospheres, and mineral matter. Coal fragments were subdivided by rank using vitrinite reflectance (R o , %) and organic components were further subdivided into the size fractions of coarse (250–1000 μm), fine (63–250 μm), and very fine (<63 μm). Of the 36 samples analyzed, concentrations of coal, coke, and coal tar/pitch ranged from 0 to 18.2 vol%, 0 to 32.0 vol%, and 0 to 2.6 vol%, respectively, with the highest concentrations occurring near point sources (e.g. discharge pipe and coal unloading operations). Samples that were furthest upstream and downstream from the Solvay site exhibited a marked decrease in particulate organics, with exception of one upstream location which had 19.8 vol% coke. Overall, the modified ASTM method provided a means to quantify the abundance of carbonaceous material present in the sediments. Petrography and total PAH concentrations did not provide a clear correlation to organic matter type or size fraction but the samples with the highest vol% organic matter in each core generally corresponded to the sample with the highest bulk PAH content.

Wisconsin↗

USGS2023

Regional crustal structure of Indonesia from receiver functions

Characterizing the crustal structure of Indonesia is important to gain a better understanding of its geodynamic evolution and improve seismic hazard assessments in the area. However, a unified crustal model of the entire Indonesian region and its surroundings is lacking. We present new maps of crustal thickness and bulk V p /V s ratio in Indonesia and the surrounding area that are obtained using P-wave receiver functions at 36 seismic stations from several permanent regional networks. The measured crustal thickness varies from ∼24 km to ∼38 km. The thickest crust, ∼38 km, is beneath Flores Island, southern Maluku, and neighboring northernmost Australia, whereas the thinnest crust, ∼24 km, is found under eastern Malaysia. Thus, crustal thickness varies by ∼14 km (from ∼24 km to ∼38 km) despite the small changes in elevation at the measurement points. The V p /V s ratios are 1.79 ± "> ± 0.11, with high values (>1.85) found along the Banda-Sunda arc-trench system. We attribute these high values to: (1) the presence of mafic island arc and oceanic crust and (2) partial melting within this volcanic region, which causes a larger decrease in S-wave velocities compared with P-wave velocities. The comparison of the seismic properties of Indonesian island arc crust, particularly the V p /V s ratio, with laboratory measurements and the petrology of the exhumed Talkeetna island arc, Alaska, allows us to infer the crustal composition of Indonesian island arc crust.

Tectonophysics↗

USGS2004

Ultra-high chlorine in submarine Kı̄lauea glasses: Evidence for direct assimilation of brine by magma

Basaltic glass grains from the submarine south flank of Kı̄lauea, Hawai′i, have Cl concentrations of 0.01–1.68 wt%, the latter being the highest Cl content yet recorded for a Hawaiian glass. The high-Cl glass grains are products of brine assimilation by tholeiite magma. The glasses are grains in a sandstone clast from bedded breccias draping the southwestern margin of Kı̄lauea’s submarine midslope bench. The clast contains two distinct suites of glass grains: abundant degassed tholeiites, perhaps derived from subaerial lavas of Mauna Loa that shattered upon ocean entry, and a smaller population of Kea-type tholeiite ( n =17 analyzed) that erupted subaqueously, based on elevated S (780–1050 ppm), H 2 O (0.42–1.27 wt%), and CO 2 (<30–120 ppm), probably early in Kı̄lauea’s shield-building stage. Ten grains in this group have Cl>1000 ppm, six >5000 ppm, and two grains have >10 000 ppm dissolved Cl. Abundances of H 2 O, Na 2 O, K 2 O, and several trace elements increase regularly with Cl concentration, and we estimate that Cl enrichment was due to up to 13 wt% addition of a brine consisting of 78% H 2 O (wt), 13% Cl, 4.4% Na, 2.6% K, 2.6% Ca, 620 ppm Ba, 360 ppm Sr, 65 ppm Rb, and 7 ppm Pb. The large amounts of brine addition argue against bulk assimilation of low-porosity brine-bearing rock. The brine’s composition is appropriate for a seawater-derived hydrothermal fluid that reacted with basaltic wall rocks at T >100°C, losing Mg and S and gaining K, Ca, Rb, Ba, Sr, and Pb, followed by phase separation near 500°C and ∼50 MPa (5 km below sea level at hydrostatic pressure). Brine was assimilated at or near the depth it formed, as estimated on petrologic grounds, but under lithostatic conditions. The highest extents of assimilation either forced volatile saturation of the magma or enriched already coexisting magmatic vapor in H 2 O. Possible mechanisms for assimilation are: (1) forcible injection of brine into magma during bursting of overpressured pockets heated by new dikes, or (2) intrusion of magma into lenses or sills occupied by trapped brine.

Hawaii↗

USGS1960

Minor element abundance in a part of the Brazilian shield

Thirty-nine samples of granitic rocks from the Quadrilátero Ferrifero, Minas Gerais, Brazil, representing "mixed" ages and at least three distinct Pre-Cambrian ages were analysed for seventeen minor elements. The rocks are placed in five groups:, (1) 2400 million year old group;, (2) Bacao complex younger, a group of mixed rocks derived from, (1) and, (3);, (3) 1350 million year old group;, (4) a 500 million year old gneissic group;, (5) a 500 million year old silicic group, a later differentiate of a 500 million year old magma. Compared to granitic rocks elsewhere, these Brazilian rocks are about average in Ba, So, Zr, Pb, Nb, Co, Ni, Be and Mo; above average in La and Y; and below average in Cu, Sr, Ga, Sn, V and Cr. Compared to metasedimentary rocks elsewhere in the world., metasedimentary rocks of this region are about average in all seventeen minor elements. Sr has its greatest abundance in the 2400 million year old rocks; Ba, V, Co, Sc, Cr, Ni and Cu in the 1350 million year; and La, Nb, Sn, Zr, Y and Ga in the 500 million year silicic. These abundances are in accord with the petrology and chemistry of the rocks. The 1350 million year rocks are the most mafic of the group and should logically concentrate Ba, V, Co, So, Or, Ni and Cu which are most abundant in earlier crystallates. The 500 million year old silicic rocks are the most felsic and should concentrate La, Nb, Sn, Zr, Y and Ga, which are all most abundant in the latest fractions of granitic magna crystallization. The trace elements also lend support to the idea that the Bacao complex rocks developed by anatexis of 2400 million year rocks during the 1350 -pillion year orogeny. Cu, Sc, Ga, Zr, Sn and Nb have about the same magnitude in the Bagao and the 2400 million year rocks and Sr, V, Co, Or and Ni are lower in the BagAo than either the 2400 or the 1350 million yoar old rocks. This lower abundance is a probable effect of "debasif-cation" which is also brought out by the analysis of major constituents. The effects of a geochemical migration of the elements in this part of the earth's crust may be indicated by the increased abundance of Y in successivol y younger rock types. The increase of La, Nb, Pb, Sn, Y and Zr in the 1350 million year rocks compared to the 2,100 million year rocks might also be an effect of a secular migration.

southeast Brazil↗

USGS2002

Seismic-reflection profiles of the central part of the Clarendon-Linden fault system of western New York in relation to regional seismicity

Geological and geophysical research in upstate New York, with few exceptions, has not definitively associated seismicity with specific Proterozoic basement or Paleozoic bedrock structures. The central part of the Clarendon-Linden fault system (CLFS) between Batavia and Dale, NY is one of those exceptions where seismicity has been studied and has been spatially associated with structure. The CLFS is either a complex system of long faults with associated shorter branches and parallel segments, or a region of many short faults aligned north-south from the Lake Ontario shore southward to Allegany County, NY. Interpretation of 38 km of Vibroseis and approximately 56 km of conventional seismic-reflection data along 13 lines suggests that the CLFS is a broad zone of small faults with small displacements in the lower Paleozoic bedrock section that is at least 77 km long and 7-17 km wide and spatially coincident with a northtrending geophysical (combined aeromagnetic and gravity) lineament within the basement. The relative offset across the faults of the system is more than 91 m near Attica, NY. The CLFS is the expression of tectonic crustal adjustments within the Paleozoic rock above the boundary of two basement megablocks of differing petrologic provinces and differing earthquake characteristics that forms the eastern side of the Elzevir-Frontenac boundary zone. Deep seismic-reflection profiles display concave-eastward listric faults that probably merge at depth near the mid-crustal boundary layer. An interpretive vertical section provides the setting for refined definitions of the CLFS, its extensions at depth and its relation to seismicity. Most modern seismicity in western New York and the Niagara Peninsula of Ontario occurs in apparent patterns of randomly dispersed activity. The sole exception is a line of seven epicenters of small earthquakes that trend east from Attica, NY into the Rochester basement megablock. Earthquakes may be triggered at the intersections of north- and east-trending brittle faults within the Niagara basement megablock. Current interpretations of the mechanisms for earthquake generation in western New York and the Niagara Peninsula of Ontario require conservative estimates of seismic hazards that assume that an earthquake the size of the 1929 Attica, NY, event (Mb = 5.2) or larger could occur anywhere in the Eastern Great Lakes Basin (EGLB). The broad zone of small-displacement faults that marks the CLFS in the lower Paleozoic sedimentary section and the uppermost basement may not provide the structural environment for generation of earthquakes in western New York. If this interpretation is correct, most seismicity is generated within the Niagara basement megablock beneath or west of the CLFS. Consequently, we may have to look to the deeper tectonic regime of basement megablocks to understand the distribution of modern seismicity in the EGLB. ?? 2002 Elsevier Science B.V. All rights reserved.

Tectonophysics↗

USGS2002

On the relations between cratonic lithosphere thickness, plate motions, and basal drag

An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140-350 km), with a bimodal distribution for Archean cratons (200-220 km and 300-350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the "critical" surface area for cratons to have thick (>300 km) keels is >6-8 ?? 106 km2 . Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350-450 km decreasing to 280-400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small fast-moving cratons. Basal drag may have varied in magnitude over the past 4 Ga. Higher mantle temperatures in the Archean would have resulted in lower mantle viscosity. This in turn would have reduced basal drag and basal erosion, and promoted the preservation of thick (>300 km) Archean keels, even if plate velocities were high during the Archean.

Tectonophysics↗

USGS1999

Stratigraphic constraints on the timing and emplacement of the Alika 2 giant Hawaiian submarine landslide

Previous work has found evidence for giant tsunami waves that impacted the coasts of Lanai, Molokai and other southern Hawaiian Islands, tentatively dated at 100 + and 200 + ka by U-series methods on uplifted coral clasts. Seafloor imaging and related work off Hawaii Island has suggested the Alika phase 2 debris avalanche as the source of the ~ 100 ka "giant wave deposits", although its precise age has been elusive. More recently, a basaltic sand bed in ODP site 842 (~ 300 km west of Hawaii) estimated at 100 ?? 20 ka has been suggested to correlate with this or another large Hawaiian landslide. Our approach to the timing and linkage of giant submarine landslides and paleo-tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature, beginning with a suite of seven piston, gravity and box cores collected in the vicinity of the Alika 2 slide. We used U-series dating techniques, including excess 230 Th and 210 Pb profiling, high-resolution paleomagnetic stratigraphy, including continuous, U-channel analysis, δ 18 O stratigraphy, visual and X-ray sediment lithology, and the petrology and geochemistry of the included turbidites and ash layers. Minimum ages for the Alika phase 2a slide from detailed investigation of two of the cores are 112 ± 15 ka and 125 ± 24 ka (2σ) based on excess 230 Th dating. A less precise age for the Alika phase 1 and/or South Kona slide is 242 ± 80 ka (2σ), consistent with previous geological estimates. Oxygen isotope analyses of entrained planktonic foraminifera better constrain the Alika phase 2a maximum age at 127 ± 5 ka, which corresponds to the beginning of the stage 5e interglacial period. It is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers. Our study indicates the contemporaneity of the Alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on Lanai and Molokai through direct dating and compositional analysis of the landslide deposits.

Journal of Volcanology and Geothermal Research↗

USGS1986

Guidelines to classification and nomenclature of Arabian felsic plutonic rocks

Well-defined procedures for classifying the felsic plutonic rocks of the Arabian Shield on the basis of petrographic, chemical and lithostratigraphic criteria and mineral-resource potential have been adopted and developed in the Saudi Arabian Deputy Ministry for Mineral Resources over the past decade. A number of problems with conventional classification schemes have been identified and resolved; others, notably those arising from difficulties in identifying precise mineral compositions, continue to present difficulties. The petrographic nomenclature used is essentially that recommended by the International Union of Geological Sciences. Problems that have arisen include the definition of: (1) rocks with sodic, zoned or perthitic feldspar, (2) trondhjemites, and (3) alkali granites. Chemical classification has been largely based on relative molar amounts of alumina, lime and alkalis, and the use of conventional variation diagrams, but pilot studies utilizing univariate and multivariate statistical techniques have been made. The classification used in Saudi Arabia for stratigraphic purposes is a hierarchy of formation-rank units, suites and super-suites as defined in the Saudi Arabian stratigraphic code. For genetic and petrological studies, a grouping as ‘associations’ of similar and genetically related lithologies is commonly used. In order to indicate mineral-resource potential, the felsic plutons are classed as common, precursor, specialized or mineralized, in order of increasing exploration significance.

Journal of African Earth Sciences↗

USGS2009

III.-The Work of Prof. Henry Carvill Lewis in Glacial Geology

The recent notice of the life and work of Prof. Henry Carvill Lewis, whose lamented death occurred in Manchester, July 21st, 1888, in his thirty-fifth year, well indicates the wide range of his scientific labours. He published valuable results of investigations in astronomy, mineralogy and petrology, and especially in glacial geology, the last being based on his exploration of the drift and its terminal moraines in the United States, and later in Ireland, Wales and England. The present article reviews his contributions to our knowledge of these drift formations and of the history of the Ice Age, bringing into comparison and correlation the glacial records of America and Europe. Comprehensive as were Professor Lewis' observations and studies in this field, he was planning yet more thorough and extensive exploration of the drift in Britain, Germany and Scandinavia, when he was taken from us.

Geological Magazine↗