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Physical and compositional measurements are made at the ~ 7 km-long (~ 2200 years B.P.) Kings Bowl basaltic fissure system and surrounding lava field in order to further understand the interaction of fissure-fed lavas with phreatic explosive events. These assessments are intended to elucidate the cause and potential for hazards associated with phreatic phases that occur during basaltic fissure eruptions. In the present paper we focus on a general understanding of the geological history of the site. We utilize geospatial analysis of lava surfaces, lithologic and geochemical signatures of lava flows and explosively ejected blocks, and surveys via ground observation and remote sensing. Lithologic and geochemical signatures readily distinguish between Kings Bowl and underlying pre-Kings Bowl lava flows, both of which comprise phreatic ejecta from the Kings Bowl fissure. These basalt types, as well as neighboring lava flows from the contemporaneous Wapi lava field and the older Inferno Chasm vent and outflow channel, fall compositionally within the framework of eastern Snake River Plain olivine tholeiites. Total volume of lava in the Kings Bowl field is estimated to be ~ 0.0125 km3, compared to a previous estimate of 0.005 km3. The main (central) lava lake lost a total of ~ 0.0018 km3 of magma by either drain-back into the fissure system or breakout flows from breached levees. Phreatic explosions along the Kings Bowl fissure system occurred after magma supply was cut off, leading to fissure evacuation, and were triggered by magma withdrawal. The fissure system produced multiple phreatic explosions and the main pit is accompanied by others that occur as subordinate pits and linear blast corridors along the fissure. The drop in magma supply and the concomitant influx of groundwater were necessary processes that led to the formation of Kings Bowl and other pits along the fissure. A conceptual model is presented that has relevance to the broader range of low-volume, monogenetic basaltic fissure eruptions on Earth, the Moon and other planetary bodies.

Tertiary granitoid rocks including diorite, quartz diorite, quartz monzodiorite, monzonite, and quartz monzonite occur in southeast of Qom and to the east of Bidhand fault in the central Urumieh-Dokhtar Magmatic Arc (UDMA). Zircon U–Pb dating reveals an age of ca. 17 Ma for the crystallization of Kerogan quartz diorite. The study samples show sub-alkaline to transitional, metaluminous, and I-type affinity with low to middle SiO2 (53.9–63.9 wt%). Micro granitoid enclaves (MGEs) are widespread in these rocks, exhibiting diorite to monzodiorite composition (SiO2 = 56.75–58.4 wt%). MGEs and host rocks are slightly enriched in LREEs and LILEs, and depleted in HFSE including Nb, Ta and Ti as expected in granitoids formed in an active continental margin setting. The granitoid rocks display initial Sr isotopic compositions of 0.7047–0.7062, positive eNd(t) of 3.2–6.6, and 206Pb/204Pb (18.6431–18.8499), 207Pb/204Pb (15.6322–15. 6902), and 208Pb/204Pb ratios (38.5585–38.9325), and positive zircon eHf(t) values of 6.7–8.7. MGEs also display identical Sr-Nd isotopic compositions (87Sr/86Sri = 0.70537–0.70576; eNd(t) = 2.3–5.3) to the host rocks. The isotopic characteristics in combination with the geochemical signatures indicate that the granitoid rocks and MGEs most likely originated by interaction between mantle-derived mafic and basaltic lower crust-derived melts. Major and trace element modeling reveals that the mafic lower crust is composed of less ~40% partial melting of garnet-free amphibolite. Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves derived from the same parent by interaction between basaltic lower crust (~10%) and lithospheric mantle (~90%) during their generation that subduction related compositions have also been involved in their petrogenesis. This magma has experienced high fractional crystallization but minor crustal contamination during magma ascent to the upper crustal level. Petrographical, geochemical, and Sr-Nd-Pb isotopic similarities between granitoid and MGEs imply that MGEs are cognate late cumulates, formed by pressure quenching mechanism during the late stage of magma evolution in crustal magma chamber. The Tertiary magmatism in the study area originated in an arc-related setting induced by slab roll-back of northward Neo-Tethyan oceanic lithosphere subduction beneath central Iranian micro continent and resulted in lithospheric extensional regime and subsequently asthenosphere upwelling. The heat induced by the upwelling of the asthenosphere likely led to melting of upper mantle and the mafic crustal material during Early Miocene as the final magmatism phase in the central part of the UDMA before collision.

The mountains of Jupiter's volcanic moon Io are tall, steep, and tectonic in origin, yet their precise modes of formation and their associations with volcanic paterae are not fully understood. Global spatial statistics of paterae and mountains and their associated lineations reveal that both types of features are more common at low latitudes and tectonic lineations have preferred orientations, whereas straight patera margins are randomly oriented. Additionally, structurally controlled lineations tend to cluster with each other, and in areas of high concentrations these tectonic lineations are shorter in length than their global average. These results indicate that global-scale (rather than local or regional) processes are involved in forming Io's tectonic structures, but that the diversity of mountain characteristics and the collapse of paterae adjacent to mountain complexes are more locally controlled. Regional structural mapping of the Hi'iaka, Shamshu, Tohil, and Zal regions reveals Io's mountains reside in large, fault-bounded crustal blocks that have undergone modification through local responses of subsurface structures to variable stresses. Strike-slip motion along reactivated faults led to the formation of transpressional and transtensional features, creating tall peaks and low basins, some of which are now occupied by paterae. We propose Io's mountains result from a combination of crustal stresses involving global and local-scale processes, dominantly volcanic loading and tidal flexing. These stresses sometimes are oriented at oblique angles to pre-existing faults, reactivating them as reverse, normal, or strike-slip faults, modifying the large, cohesive crustal blocks that many of Io's mountains reside in. Further degradation of mountains and burial of faults has occurred from extensive volcanism, mass wasting, gravitational collapse, and erosion by sublimation and sapping of sulfur-rich layers. This model of fault-bounded blocks being modified by global stresses and local structural response accounts for the variation and patterns of mountain sizes, shapes, and orientations, along with their isolation and interactions with other features. It also provides a context for the operation and extent of global and regional stresses in shaping Io's surface.

Detailed analyses of slopes and arcuate planform morphologies of Titan’s equatorial mountain ridge belts are consistent with formation by contractional tectonism. However, contractional structures in ice require large stresses (4–10 MPa), the sources of which are not likely to exist on Titan. Cassini spacecraft imagery reveals a methane-based hydrological cycle on Titan that likely includes movement of fluids through the subsurface. These crustal liquids may enable contractional tectonic features to form as groundwater has for thrust belts on Earth. In this study, we show that liquid hydrocarbons in Titan’s near subsurface can lead to fluid overpressures that facilitate contractional deformation at smaller stresses (

The morphologies of craters on planetary surfaces reveal clues about the geologic mechanisms by which they originate and subsequently evolve, as well as the materials and physical variables inherent to the environment in which they formed. We carried out a quantitative multivariate analysis of shape descriptors derived from the outlines of craters formed by volcanic processes on Mars, Io, and Earth and by impact cratering on the Moon using elliptic Fourier analysis (EFA) and the Zahn-Roskies (Z-R) shape function. Canonical variate analysis (CVA) was used to construct a statistical model of differences between the crater groups to classify craters produced by various volcanic and impact processes. The classification model from canonical variate analysis of EFA shape descriptors yielded a 90% rate of success for the assignment of group membership among 406 examined craters. It correctly classified 138 of 154 (90%) ionian paterae,154 of 155 (99%) lunar impact craters, 31 of 35 (89%) terrestrial basaltic shield calderas, 32 of 38 (84%) terrestrial ash-flow calderas, and 12 of 24 (50%) martian basaltic shield calderas. The classification model from canonical variate analysis of Z-R shape function descriptors classified 84% of the total population of the examined craters correctly. The analysis correctly classified 96% of ionian paterae, 100% lunar impact craters, 51% terrestrial basaltic shield calderas, and 63% martian calderas, but only 16% of the terrestrial ash-flow calderas were correctly classified. Canonical variate analysis of EFA and Z-R results shows that the shapes of ash-flow calderas and paterae on Io differ the least of all groups included in this study, and basaltic shield calderas and martian calderas analyzed together also have few differences. The Z-R model successfully classifies more ionian patera and impact craters than the EFA classification model but performs poorly at classifying the other crater groups. This result shows that the descriptors convey different shape information. The Z-R model is robust in its ability to classify end-member differences in complexity while the EFA model is robust in its ability to reliably classify among more groups. These differences and similarities in shape confirm previously understood commonalities related to the origin and evolution of various types of craters. In general, basalt shield calderas on Earth and Mars are morphologically similar and are thought to have similar origins; this study confirms that the 2-D shapes of their craters are quantitatively correlated. Similarities have been noted between terrestrial ash-flow calderas and paterae on Io, principally in their large sizes, shallow magma chambers and complex evolution; this study confirms their shapes are also similar. Impact craters and ionian paterae are most dissimilar, as are their evolutions. This study demonstrates rigorous landform shape analysis can greatly increase our understanding of the diversity in craters and the processes involved in their formation.

Basaltic eruptive fissures of the Great Rift and surroundings on the eastern Snake River Plain of Idaho, USA, and selected volcanic features in Hawai’i, Iceland and northern Africa were surveyed for their relevancy as planetary analogs. Evaluated during field investigations and in satellite imagery for structures, physiography, and geologic setting, fissures were categorized into four broad types: (1) simple, monogenetic fissures with obvious volcanic constructs or deposits, (2) monogenetic fissures now obscured by low shields or relatively large cones, (3) polygenetic volcanic rift zones with multiple vents and deposits, and (4) compound regional fissure systems or dike swarms that comprise major rift zones or large volcanic terrains. Using this classification as an initial base, we surveyed imagery of volcanic features for likely fissure vents in two major geologic settings on the Moon: floor-fractured craters (FFCs) and mare and cryptomare provinces. Two major regions on Mars, the volcanic plains around Alba Mons and the greater Tharsis region, were also surveyed for fissure types and volcanic associations of fissure-like features. The planetary surveys suggest that the proposed classification provides a suitable analog starting point to interpret structures associated with fissure systems on the Moon and Mars. With few exceptions, our survey indicates that each of the studied terrains exhibits a dominant fissure type. Type 1 fissures, most with pyroclastic deposits, prevail in lunar FFCs and mare-like regions; whereas type 2 fissures are ubiquitous in the Tharsis region of Mars and a few exist on the Moon as low shields. Type 3 volcanic rift zones are not common on either the Moon or Mars, although they might become evident in future work on chemically evolved terrains. Type 4 fissures are inferred in mare terrains, often represented as the extensions of major linear rille networks or rimae, with possibly complex dike swarms that were buried beneath voluminous mare basalt lava flows. Likewise, numerous flood lavas on Mars are possibly associated with now-obscured or difficult to define type 4 fissure systems.

Analysis of large-scale morphological parameters can reveal the reaction of dunes to changes in atmospheric and sedimentary conditions. Over 7000 dune width and 7000 dune spacing measurements were obtained for linear dunes in regions across Saturn’s moon Titan from images T21, T23, T28, T44 and T48 collected by the Synthetic Aperture RADAR (SAR) aboard the Cassini spacecraft in order to reconstruct the aeolian surface history of Titan. Dunes in the five study areas are all linear in form, with a mean width of 1.3 km and mean crest spacing of 2.7 km, similar to dunes in the African Saharan and Namib deserts on Earth. At the resolution of Cassini SAR, the dunes have the morphology of large linear dunes, and they lack evidence for features of compound or complex dunes. The large size, spacing and uniform morphology are all indicators that Titan’s dunes are mature features, in that they have grown toward a steady state for a long period of time. Dune width decreases to the north, perhaps from increased sediment stabilization caused by a net transport of moisture from south to north, or from increased maturity in dunes to the south. Cumulative probability plots of dune parameters measured at different locations across Titan indicate there is a single population of intermediate-to-large-sized dunes on Titan. This suggests that, unlike analogous dunes in the Namib and Agneitir Sand Seas, dune-forming conditions that generated the current set of dunes were stable and active long enough to erase any evidence of past conditions.

Eruptions of topaz rhyolites are a distinctive part of the late Cenozoic magmatic history of western North America. As many as 30 different eruptive centers have been identified in the western United States that range in age from 50 to 0.06 Ma. These rhyolite lavas are characteristically enriched in fluorine (0.2 to 2 wt.% in glass) and lithophile trace elements, such as Be, Li, Rb, Cs, Ga, Y, Nb, and Ta. REE patterns are typically flat with large negative Eu anomalies; negative Nb–Ta anomalies are small or nonexistent; and F/Cl ratios in glasses are high (> 3). These features, together with high Fe/Mg ratios and usually low fO2, set them apart from subduction-related (I-type) silicic rocks. The rhyolites are metaluminous to only slightly peraluminous, lack indicator minerals of strongly peraluminous magmas, and have low P and B contents; these features set them apart from S-type silicic magmas. Instead, topaz rhyolites have the major and trace element, mineralogic, and isotopic characteristics of aluminous A-type or within-plate granites. Topaz rhyolites were formed during regional extension, lithospheric thinning, and high heat flow. Topaz rhyolites of the western United States crystallized under subsolvus conditions, and have quartz, sanidine, and Na-plagioclase as the principal phenocrysts. Fluorite is a common magmatic accessory, but magmatic topaz occurs only in a few complexes; both are mineralogical indicators of F-enrichment. Many also crystallized at relatively low fO2 (near QFM) and contain mafic silicate minerals with high Fe/(Fe + Mg) ratios. Some crystallized at higher oxygen fugacities and are dominated by magnetite and have titanite as an accessory mineral. Post-eruption vapor-phase minerals include topaz, garnet, red Fe–Mn-rich beryl, bixbyite, pseudobrookite, and hematite. They are genetically related to deposits of Be, Mo, F, U, and Sn. Topaz rhyolites erupted contemporaneously with a variety of other igneous rocks, but most typically they form bimodal associations with basalt or basaltic andesite and are unrelated to large collapse calderas. In their composition and mineralogy, topaz rhyolites are similar to the evolved members of rapakivi granite complexes, especially those of Proterozoic age in southern Finland. This suggests similarity in origin and lessons learned from these rocks may help us better understand the origins of their more ancient counterparts. For example, all topaz rhyolites in western North America seem to be intrinsically related to extension following a regional period of subduction-related volcanism. Cratonized Precambrian crust is found beneath almost all of them as well. Trace element models, Sr–Nd isotopic data, and geologic associations indicate that topaz rhyolites probably form by fractional crystallization of silicic magma which originated by small degrees of melting of hybridized continental crust containing a significant juvenile mantle component not derived from a subduction zone (i.e., intrusions of within-plate mafic magma). The Sr and Nd isotopic compositions of the topaz rhyolites lie between the fields of contemporaneous mafic magmas and older calc-alkaline dacites and rhyolites. Intraplate mafic magmas and their derivatives appear to have lodged in the crust and were then re-melted by subsequent injections of mafic magma. In turn, the mafic mantle-derived magma probably formed as a result of decompression related to lithospheric extension or to convective-flow driven by the foundering of a subducting lithospheric plate. Although significant uncertainty remains, we suggest that topaz rhyolites (and by extension rapakivi granites) are probably not simply melts of mid-crustal granodiorites, nor are they derived solely from felsic crust that was previously dehydrated or from which melt had been extracted as proposed in earlier papers.

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3 ) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3 . It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2 ) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

High precision 40 Ar/ 39 Ar laser-microprobe ages of individual sanidines, 40 Ar/ 39 Ar plateau age spectra on bulk sanidine concentrates, U-Pb zircon ages, and zircon and apatite fission-track ages from three bentonites bracketing the Cenomanian-Turonian boundary in the Western Interior of the United States suggest an age for the boundary of 93.1 ± 0.3 (2σ. The lowermost bentonite comes from the Upper Cenomanian Sciponoceras gracile biozone, and gives a weighted mean laser-fusion single-crystal 40 Ar/ 39 Ar age of 93.50 ± 0.52 Ma (2σ, standard error of the mean, n = 14) for sanidine. The middle bentonite comes from the Upper Cenomanian Neocardioceras juddii biozone, accepted in both North America and Europe as the uppermost Cenomanian ammonite zone; it gives an average single-crystal 40 / 39 Ar age of 93.33 ± 0.50 Ma ( n = 29), a bulk-sample 40 Ar/ 39 Ar plateau age of 93.09 ± 0.34 Ma (2σ) for sanidine, and concordant 206 Pb/ 238 U and 207 Pb/ 235 U ages of 93.48 ± 0.32 Ma on zircon. The upper bentonite comes from near the base of the Turonian, immediately above the first occurrence of the basal Turonian bivalve Mytiloides and sanidines from it give an average single-crystal 40 Ar/ 39 Ar age of 93.46 ± 0.60 Ma ( n = 12) and a bulk-sample 40 Ar/ 39 Ar plateau age of 92.87 ± 0.34 Ma. The composition of these Cenomanian-Turonian bentonites from Colorado and Utah, the types of phenocrysts present, and the morphology of included zircons all indicate that the pre-alteration ash was rhyolitic and probably generated in a subduction setting involving a significant crustal component.

Secondary age standards are valuable in intra- and interlaboratory calibration. At present very few such standards are available for fission track dating that is older than Tertiary. Several altered volcanic ash beds occur in the Middle Jurassic Carmel Formation in southwestern Utah. The formation was deposited in a shallow marine/sabhka environment. Near Gunlock, Utah, eight ash beds have been identified. Sanidines from one of the ash beds (GUN-F) give a single-crystal laser-probe 40 Ar/ 39 Ar age of 166.3±0.8 Ma (2 σ ). Apatite and zircon fission track ages range from 152–185 Ma with typically 15–20 Ma errors (2σ). Track densities in zircons are high and most grains are not countable. Apatites are fairly common in most of the ash beds and have reasonable track densities ranging between 1.2–1.5 × 10 6 tracks/cm 2 . Track length distributions in apatites are unimodal, have standard deviations μ m, and mean track lengths of about 14–14.5 μm. High Cl apatites (F:Cl:OH ratio of 39:33:28) are particularly abundant and large in ash GUN-F, and are fairly easy to concentrate, but the concentrates contain some siderite, most of which can be removed by sieving. GUN-F shows evidence of some reworking and detriaal contamination based on older single grain 40 Ar/ 39 Ar analyses and some rounding of grains, but the apatite population appears to be largely uncontaminated. At present BJK has approximately 12 of apatite separate from GUN-F.

New single-crystal, laser-fusion 40 Ar 39 Ar ages on plagioclase and alkali feldspar from the Brushy Basin Member of the Morrison Formation range from 153 to 145 ( ± about 1–2) Ma or Late Jurassic in age (mostly Tithonian based upon Palmer, 1983 ). All but one of these new ages come from a section of Brushy Basin Member near Montezuma Creek, Utah, USA. The other, which is the oldest sample at 153 Ma, comes from Dinosaur National Monument, from a bentonite previously dated at 135.2 ± 5.5 Ma (K-Ar biotite age) by Bowman et al. (1986) . The new feldspar ages are essentially concordant with previously reported fission track ages from Notom, Utah, with ages ranging from 144 to 135 ( ± about 15) Ma in the lower two-thirds of the Brushy Basin Member, but contrast with fission track ages from the upper one-third (about 15 m) of the Notom section, which ranged from 123 to 99 ( ± about 13) Ma ( Kowallis & Heaton, 1987 ). However, none of the new ages comes from the upper part of the Brushy Basin Member; the uppermost sample dated from the Montezuma Creek section is more than 20 m below the top of the Morrison Formation. Precise ages from the uppermost part of the Brushy Basin Member are still needed to determine if the Jurassic-Cretaceous boundary lies within the uppermost Brushy Basin Member.