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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The interpretation of crustal dynamics data in terms of plate interactions and active tectonics of the Anatolian plate and surrounding regions in the Middle East

A detailed study was made of the consequences of the Arabian plate convergence against Eurasia and its effects on the tectonics of Anatolia and surrounding regions of the eastern Mediterranean. A primary source of information is time rates of change of baseline lengths and relative heights determined by repeated SLR measurements. These SLR observations are augmented by a network of GPS stations in Anatolia, Aegea, and Greece, established and twice surveyed since 1988. The existing SLR and GPS networks provide the spatial resolution necessary to reveal the details of ongoing tectonic processes in this area of continental collision. The effort has involved examining the state of stress in the lithosphere and relative plate motions as revealed by these space based geodetic measurements, seismicity, and earthquake mechanisms as well as the aseismic deformations of the plates from conventional geodetic data and geological evidence. These observations are used to constrain theoretical calculations of the relative effects of: (1) the push of the Arabian plate; (2) high topography of Eastern Anatolia; (3) the geometry and properties of African-Eurasian plate boundary; (4) subduction under the Hellenic Arc and southwestern Turkey; and (5) internal deformation and rotation of the Anatolian plate.

Toksoz, M. Nafi↗
NASA NTRSDate not supplied

Twinned magnetite in granitic samples from the Siljan impact structure, Sweden

Impact cratering is a ubiquitous process throughout the solar system and has played a key role for the geologic evolution of Earth and other planets [1]. Magnetic anomalies are common characteristics of impact craters [2], so understanding how magnetite (Fe23+Fe2+O42-), an important magnetic carrier in both terrestrial and extraterrestrial rocks, is affected by shock waves, especially structurally and magnetically, is crucial. Magnetic minerals have been studied to gain insight into the geologic evolution of Mars, with specific focus including understanding the Martian crustal structure and the oxidization state of Martian rocks (e.g., [3,4,5]). Shock waves permanently alter the intrinsic magnetic properties of rocks (e.g., [6]), as do twinning in natural environments with less extreme P/T conditions [7]. In fact, pressure conditions induced by impacts have been suggested to explain demagnetization signatures observed around Martian impact basins [8]. It is still poorly understood what effects shock has on the crystallographic structure of magnetite and thus it’s intrinsic magnetic properties, and , how that affects the bulk magnetic properties of rocks (e.g., [9]). Limited experimental studies have investigated the effects of shock compression on magnetite, especially with the aim of describing crystallographic deformation (e.g., [9] and references therein). In this work we report preliminary results of an investigation of microstructures in magnetite, an “unconventional” mineral in shock studies, in granitic samples from the ca. 380 Ma ~52 km-in-diameter Siljan impact structure, located in south-central Sweden.

shock metamorphism↗
DOE OSTI2026

The 10 September 2025 M w 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event

The 10 September 2025 M w 4.1 earthquake in northeastern Utah, United States, had a focal depth 68 km beneath sea level, which is ∼20–25 km greater than estimates of local crustal thickness, making it a rare example of a continental mantle earthquake (CME). The focal depth is well resolved from arrival-time inversion (nearest station ∼13 km away) and moment tensor inversion of regional waveforms. Similar to other CMEs in the Intermountain West, there were no obvious aftershocks or foreshocks, and the waveforms were enriched in high-frequency energy. Spectral modeling gives a stress drop of ∼80 MPa and a radiation efficiency of ∼0.08, albeit with large uncertainties. The high stress drop and low radiation efficiency are consistent with a dissipative source process such as thermal runaway. Also similar to previous Intermountain West CMEs, the event occurred along the boundary of the Archean Wyoming craton, where pressure–temperature conditions favor ductile deformation. We hypothesize that edge-driven or regional-scale mantle convection produces increased strain rates near the craton boundary that make either conventional brittle failure or thermal runaway feasible at relatively high pressure–temperature conditions. High conductivity inferred around the edge of the craton may suggest that fluids also contribute to CME occurrence.

Koper, Keith D. [Univ. of Utah, Salt Lake City, UT↗
NASA NTRS1996

Transient Creep of a Composite Lower Crust: A Polymineralic Basis for Rapidly Evolving Postseismic Deformation Modes - 2

Postseismic horizontal strain and displacement following the June 28, 1992, Landers, California, earthquake (M(sub W) 7.3) is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace. Both the observed strain at Pifion Flat Observatory (PFO) and observed Global Positioning System receiver velocities evolve rapidly after the Landers-Big Bear earthquake sequence. The observed exponential decay of these motions, with timescales of 4-34 days, may reflect a soft creep rheology in the lower crust and brittle-ductile transition zone or even within the seismogenic crust itself. Here a simple model of a two-dimensional screw dislocation in a layered Maxwell viscoelastic Earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes (approx. = 4-34 days) are consistent with the behavior of a biviscous lower crust. The lowest viscosity of this system is derivable from laboratory experimental data on the long-term creep of natural quarztites, and the highest viscosity is consistent with isostasy-related lower crustal flow in a continental extensional tectonic environment. The model predicts significant stress relaxation at the base of the seismogenic crust. Near the base of the seismogenic zone, and about 4 km away from the mainshock, the rate of predicted relaxation is of the order of 0.01 MPa/ d during the first 20 days of postseismic flow. Oblate spheroidal inclusions at 5% concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3-4 x 10(exp 15) Pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at PFO after the Landers mainshock. It is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are, in part, the cause of such rapid postseismic evolution in southeastern California. Unmylonitized quartz-rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes.

Ivins, Erik R.↗
DOE OSTI2024

Elastic thermobarometry on metapelites across the crustal section of the Famatinian Arc, Argentina

We applied elastic thermobarometry on garnet-bearing migmatites along two transects through the crustal section at Sierra Valle Fértil-La Huerta, Argentina. We performed quartz-in-garnet barometry and zircon-in-garnet thermometry on metapelites from different paleo-depths across the crustal section. Our work recovers entrapment pressures ranging from 240 to 1330 MPa and entrapment temperatures between 691 and 1574 °C. The entrapment conditions are broadly consistent with anticipated pressures and temperatures along the crustal section derived previously using conventional, thermodynamic thermobarometers. The quartz-in-garnet barometer reproduces those conventionally established entrapment conditions when samples only experienced conditions within the alpha-quartz stability field. Raman-derived pressures for samples that experienced beta-quartz reference conditions are commonly much higher than those established by conventional barometry. Samples that preserve compressive (positive) residual pressures best reproduce reference entrapment pressures. Entrapment temperatures show high variability and overestimation of temperature conditions compared to conventional results. These results indicate elastic thermobarometry over- or under-estimates crystallization conditions in rocks crystallized at high temperatures, as is common in the Famatinian Arc deep-crust. We suggest that modeling quartz behavior across the alpha–beta transition may present challenges, as does shape maturation, viscous deformation, and radiation damage in zircon.

58 GEOSCIENCES↗
NASA NTRSDate not supplied

Organic-rich fluids in Pluto’s interior—A source of cryovolcanic activity

The highly variegated surface of Pluto revealed by the New Horizons spacecraft is composed of both volatile ices (N2, CH4, CO) and H2O ice, all of which are intrinsically colorless. The colors on Pluto reveal a non-ice component generally acknowledged to be a refractory complex organic material (tholins) produced by photolysis in the planet’s atmosphere [1], and by photolysis and radiolysis of the surface ices [2-4]. These processes have produced a range of colors from pale yellow to red to brown, and result in a variation of a factor of 10in albedo across the surface. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups containing carbon. Nitrogen, oxygen, and other elements can be substituted in both the aliphatic and aromatic subunits. The detection of the spectral absorption of an ammoniated compound associated with surface exposures of red-colored H2O ice (herein, RAW) in the LEISA spectral mapping data from New Horizons suggests that a third source of a colored material is ejected from Pluto’s interior as a water-rich fluid(cryomagma) that rapidly freezes at the low temperature (~40K) of the surface [5,6]. The ammoniated compound is thought to be an ammonia hydrate or an ammoniated salt; the spectral data cover a limited wavelength range, resulting in ambiguity in the identification of the form of the ammonia. The distribution of RAW in the vicinity of tectonically stressed and deformed geological structures, particularly graben (fossae), is seen as the result of both fluid effusion and fountaining ejection of cryoclastic materials in Pluto’s Virgil Fossae region [5,6]. RAW is seen elsewhere on Pluto, both in association with graben complexes and on broad expanses of old surface, but primarily in zones of tectonic stress. The nature of the putative subsurface fluid is unknown, but it must have existed at relatively shallow depths such that the graben faulting could reach it. Geochemical modeling of fluids in the interiors of small planetary bodies in the outer Solar System[7] shows that ammonia-bearing H2O in contact with rock and metal components produce a rich chemical soup, including gases that can, in principle, help propel the fluid from shallow reservoirs through crustal fractures to reach the surface. An organic component of the fluid cryomagma is predicted to originate from thermal processing of the components [ref] and may be an example of one-pot synthesis of such complexes as amino acid precursors [ref].

Dale P Cruikshank↗
NASA NTRS2005

Lunar and Planetary Science XXXVI, Part 19

The topics include: 1) The abundances of Iron-60 in Pyroxene Chondrules from Unequilibrated Ordinary Chondrites; 2) LL-Ordinary Chondrite Impact on the Moon: Results from the 3.9 Ga Impact Melt at the Landing Site of Appolo 17; 3) Evaluation of Chemical Methods for Projectile Identification in Terrestrial and Lunar Impactites; 4) Impact Cratering Experiments in Microgravity Environment; 5) New Achondrites with High-Calcium Pyroxene and Its implication for Igneous Differentiation of Asteroids; 6) Climate History of the Polar Regions of Mars Deduced form Geologic Mapping Results; 7) The crater Production Function for Mars: A-2 Cumulative Power-Law Slope for Pristine Craters Greater than 5 km in Diameter Based on Crater Distribution for Northern Plains Materials; 8) High Resolution Al-26 Chronology: Resolved Time Interval Between Rim and Interior of a Highly Fractionated Compact Type a CAI from Efremovka; 9) Assessing Aqueous Alteration on Mars Using Global Distributions of K and Th; 10) FeNi Metal Grains in LaPaz Mare Basalt Meteorites and Appolo 12 Basalts; 11) Unique Properties of Lunar Soil for In Situ Resource Utilization on the Moon; 12) U-Pb Systematics of Phosphates in Nakhlites; 13) Measurements of Sound Speed in Granular Materials Simulated Regolith; 14) The Effects of Oxygen, Sulphur and Silicon on the Dihedral Angles Between Fe-rich Liquid Metal and Olivine, Ringwoodite and Silicate Perovskite: Implications for Planetary Core Formation; 15) Seismic Shaking Removal of Craters 0.2-0.5 km in Diameter on Asteroid 433 Eros; 16) Focused Ion Beam Microscoopy of ALH84001 Carbonate Disks; 17) Simulating Micro-Gravity in the Laboratory; 18) Mars Atmospheric Sample Return Instrument Development; 19) Combined Remote LIBS and Raman Spectroscopy Measurements; 20) Unusual Radar Backscatter Properties Along the Northern Rim of Imbrium Basin; 21) The Mars Express/NASAS Project at JPL; 22) The Geology of the Viking 2 Lander Site Revisited; 23) An Impact Genesis for Loki Patera? 24) Mars Polar Cap Edges Tracked over 3 Full Mars Years; 25) Elemental Abundance in Presolar SiC: Comparing Grains Separated by Acid Residue and Gently Separation Procedures; 26) First Results from the Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan; 27) Minor Element Behavior of Pallasite Olivine: Understanding Pallasite Thermal History and Chronology; 28) Canonical Anorthite in a Grosnaja Forsterite-bearing CAI; 29) Experimental Evidence for Condensation of 'Astrophysical' Carbonate; 30) Distribution and Classification of Multiple Coronae on Venus; 31) Recognition of Rayed Craters on Mars in THEMIS Thermal Infrared Imagery: Implications for Martian Meteorite Source Regions; 32) Geochemical Modeling of Evaporites on Mars: Insight from Meridiani Planum; 33) Hadean Crustal Processes Revealed from Oxygen Isotopes and U-Th-Pb Depth Profiling of Pre-4.0 Ga Detrital Zircons from Western Australia; 34) On Modeling the Seepage of Water into the Martian Subsurface; 35) Martial Gullies and Groundwater: A Series of Unfortunate Exceptions; 36) Olivine and Carbonate Globules in ALH84001: A Terrestrial Analog, and Implications for Water on Mars; 37) A Reevaluation of Mass Movements Within the Valles Marineris Region of Mars Using MOLA and MOC Data; 38) Evidence of Hydrated 109P/Swift-Tuttle Meteoroids from Meteor Spectroscopy; 39) Cr-54 Anomalies in the Solar System: Their Extent and Origin; 40) Reevaluation of the Mn-53-Cr-53 Systematic in the Basaltic Achondrites; 41) Effective Liquid Metal-Silicate Mixing Upon Shock by Power-Law Droplet Size Scaling in Richtmyer-Meshkov Like Perturbations; 42) Post-Impact Deformation of Impact Craters: Towards a Better Understanding Through the Study of Mjolnir Crater; 43) Cutting Silica Aerogel for Particle Extraction; 44) Liquid Hydrocarbons on Titan's Surface? How Cassini ISS Observations Fit into the Story (So Far); and 45) Mesoscale Simulations of Polar Circulations: Late Spring to Late Summe

Source record↗
NASA NTRS1983

Focal depths and fault plane solutions of earthquakes under the Tibetan plateau

Seismic data are used to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts of the Tibetan plateau. Synthetic seismograms were computed for large events from 1962 to 1976, and compared with the observed waveforms. All of the events studied are found to have occurred at depths less than 15 km and probably between 5 and 10 km, suggesting that the crust below 10-15 km is essentially aseismic, as in most of the Western United States. Fault-plane solutions show combinations of normal and strike slip faulting with T axes consistently oriented approximately east-west, supporting the inference that the upper crust of Tibet is actively extending in an east-west direction. In addition, solutions for two intermediate-depth events at depths of 90 and 85 km show primarily normal faulting with east-west T axes. Results show that brittle deformation occurs at shallow depths in the crust and in the uppermost mantle, but the lower crust seems to be aseismic.

Molnar, P.↗
NASA NTRS1994

Horizontal crustal motion in the central and eastern Mediterranean inferred from Satellite Laser Ranging measurements

Four campaigns to acquire Satellite Laser Ranging (SLR) measurements at sites in the Mediterranean region have been completed. These measurements to the LAGEOS satellite, made largely by mobile systems, cover a time span beginning in November 1985 and ending in June 1993. The range data from 18 sites in the central and eastern Mediterranean have been simultaneously analyzed with data acquired by the remainder of the global laser tracking network. Estimates of horizontal motion were placed into a regional, northern Europe-fixed, kinematic reference frame. Uncertainties are on the order of 5 mm/yr for sites having at least four occupations by mobile systems and approach 1 mm/yr for permanently located sites with long histories of tracking. The resulting relative motion between sites in the Aegean exhibit characteristics of broadly distributed pattern of radial extension, but at rates that are about 50% larger than those implied from studies of seismic strain rates based on seismicity of magnitude 6 or greater or across the region. The motion estimated for sites in Turkey exhibit velocity components associated with the westward motion of the Anatolian Block relative to Eurasia. These results provide a present-day 'snapshot' of ongoing deformational processes as experienced by the locations occupied by SLR systems.

Smith, David E.↗
DOE OSTI2020

Crustal Structure of the Mesopotamian Plain, East of Iraq

The crustal structure of Iraq was investigated through analyzing teleseismic data from 12 new seismic stations. Three seismic stations are located within the Zagros Fold-Thrust Belt, and nine are within the Mesopotamian Plain. Joint inversion of P wave receiver function and Rayleigh wave dispersion data were employed to resolve S wave velocity structure models beneath each station. Combining these models with available Moho depths within and around the study area reveals that the Moho depth is smoothly increasing from the Arabian platform toward the Zagros Mountains. Further, an exceptional deeper root is observed near the eastern edge of the Mesopotamian Plain where sedimentary pile is the thickest. This root was interpreted as a structure inherited from the successive Mesozoic rifting of the NE Arabian platform enhanced by sedimentary loading and progressive subsidence. A low-velocity uppermost mantle was observed beneath the Arabian Foredeep attesting the existence of a low-strength lithospheric mantle beneath the region southwest from the Zagros deformation front. The weak rheology of the uppermost mantle may have allowed local sinking of the crust and deepening of the Moho boundary due to vertical loading and the Late Miocene lateral contraction.

58 GEOSCIENCES↗
NASA NTRS1986

The Archean geology of the Godthabsfjord Region, southern west Greenland (includes excursion guide)

The part of the West Greenland Archean gneiss complex centered around Godthabsfjord and extending from Isukasia in the north to south Faeringehavn is studied. Extensive outcrops of 3800 to 3400 Ma rocks can provide some direct evidence of conditions and processes that operated on the Earth in the early Archean. However, the ways in which primary characteristics have been modified by later deformation, metamorphism, and chemical changes are first taken into account. The rocks exposed are the products of two major phases of accretion of continental crust, at 3800 to 3700 Ma and 3100 to 29 Ma. The main features of these two accretion phases are similar, but careful study of the least modified rocks may reveal differences related to changes in the Earth in the intervening period. The combination of excellent exposure over an extensive area, relatively detailed geological mapping of much of the region, and a considerable volume of isotopic and other geochemical data gives special insights into processes that operated at moderately deep levels of the crust in the Archean. Of particular interest is the effect of late Archean granulite facies metamorphism on early Archean rocks, especially the extent to which isotope systems were disturbed. Similar processes may well have partly or wholly destroyed evidence of more ancient components of other high grade terrains. This account does not attempt to be an exhaustive review of all work carried out on the geology of the region. Rather, it attempts to summarize aspects of the geology and some interest in the context of early crustal genesis.

Mcgregor, V. R.↗
NASA NTRS1994

Implications of a global survey of venusian impact craters

We present a global survey of the areal distribution, size-frequency distribution, and morphometric properties of the venusian impact cratering record. We explore the resurfacing history of Venus, crater degradation, ejecta emplacement, and cratering mechanics. The number of volcanically embayed and tectonically deformed craters from 0.5 to 1.0 km above mean planetary radius is disproportionately high for an otherwise crater-deficient elevation range. More resurfacing occurred in this range, an elevation range dominated by volcanic rises, rifts, and coronae, than elsewhere on Venus. Although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets, some craters appear particularly `fresh' because thay have radar-bright floors, a radar-dark halo surrounding the ejecta blanket, and a west facing parabola of low radar return; 20, 35, and 8%, respectively, of craters with diameters greater than 22.6 km have these features. Characteristics of ejecta deposits for venusian craters change substantially with size, particularly at 20 km crater diameter, which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens. Secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters. Comparison of complex craters found on Venus with those of other planets gave results that were consistent with the idea that interplanetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength. The interplanetary comparison indicates that Venus, the Moon, and Mercury appear to have stronger crusts than do Mars and Ganymede/Callisto.

Herrick, Robert R.↗
NASA NTRS1992

Asteroids and Archaean crustal evolution: Tests of possible genetic links between major mantle/crust melting events and clustered extraterrestrial bombardments

Since the oldest intact terrestrial rocks of ca. 4.0 Ga and oldest zircon xenocrysts of ca. 4.3 Ga measured to date overlap with the lunar late heavy bombardment, the early Precambrian record requires close reexamination vis a vis the effects of megaimpacts. The identification of microtektite-bearing horizons containing spinals of chondritic chemistry and Ir anomalies in 3.5-3.4-Ga greenstone belts provides the first direct evidence for large-scale Archaean impacts. The Archaean crustal record contains evidence for several major greenstone-granite-forming episodes where deep upwelling and adiabatic fusion of the mantle was accompanied by contemporaneous crustal anatexis. Isotopic age studies suggest evidence for principal age clusters about 3.5, 3.0, and 2.7 (+/- 0.8) Ga, relics of a ca. 3.8-Ga event, and several less well defined episodes. These peak events were accompanied and followed by protracted thermal fluctuations in intracrustal high-grade metamorphic zones. Interpretations of these events in terms of internal dynamics of the Earth are difficult to reconcile with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by mantle rebound response to intermittent extraterrestrial asteroid impacts is supported by (1) identification of major Archaean impacts from microtektite and distal ejecta horizons marked by Ir anomalies; (2) geochemical and experimental evidence for mantle upwelling, possibly from levels as deep as the transition zone; and (3) catastrophic adiabatic melting required to generate peridotitic komatites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem that arises from comparisons between modern continental crust and the estimated sial produced by continuous two-stage mantle melting processes. The volume problem is exacerbated by projected high accretion rates under Archaean geotherms. It is suggested that impact shock effects have been largely obscured by (1) outpouring of voluminous basic/ultrabasic lavas, inundating shock-deformed crust and extending beyond the perimeters of impact excavated basins; (2) gravity subsidence and downfaulting of terrestrial maria, accounting for the burial and anatexis of subgreenstone basement; and (3) extensive shearing and recrystallization at elevated temperatures of impact structure, breccias, and mineral deformation features beneath impact-excavated basins, relics of which may be retained in structural windows in high-grade metamorphic terranes.

Glikson, A. Y.↗
NASA NTRSDate not supplied

Impact of Stress-Activated Positive Holes on the Redox Timing in Organisms Living at the Surface of Rocks: Unlocking Nature's Secrets

Stressing and deforming igneous and/or high-grade metamorphic rocks activates electronic charge carriers known as positive holes, h•, that are defect electrons in the O2–sublattice, e.g. O-states. Like h•in semiconductors, the h•in rocks affect electrical and thermal properties. They produce electrochemical reactions, localized electrical signals, and currents. In this study, we explore the effects of positive holes on the electron flow in the electron transport chain (ETC) of organisms living at the surface of rocks such as gabbro or granite. We found that positive holes, h•, disrupt the in vivotemporal coordination governed by oscillating reduction-oxidation reactions, known as the redox cycle. Positive holes affect the timing of the redox cycle by interacting with essentialmolecules in vivo, leading to the formation of hydroxyl radicals and superoxide anions. We observed that positive holes significantly impede the growth of yeast Saccharomyces cerevisiae(i.e., colony size) and delay the sprouting of broccoli seeds. Additionally, positive holes were found to exert discernible impacts on plant development such as stem length and leaf size. Our findings highlight the intricate interplay between positive holes, redox timing, and biological processes, shedding light on the potentially significant role of positive holes in influencing the growth and development of organisms in tectonically stressed crustal environments. Understanding these effects has implications for a broader understanding of redox biology and of how environmental factors can influence cellular development in natural settings.

hypermutation↗
DOE OSTI2020

Landscape responses to intraplate deformation in the Kalahari constrained by sediment provenance and chronology in the Okavango Basin

The structural depression that occupies the Okavango Basin in southern Africa comprises a depo-centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26 Al/ 10 Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4–3 Ma during which the currently downthrown (southern) block received a mixture of sediments mostly from the Choma-Kalomo, Ghanzi-Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the upthrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3–2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time.

58 GEOSCIENCES↗
NASA NTRS2001

The Mountains of Io: Global and Geological Perspectives from Voyager and Galileo

To search for local and global scale geologic associations that may be related to the internal dynamics of Io, we have completed a global catalog of all mountains and volcanic centers. We have identified 115 mountain structures (covering approx. 3% of the surface) and 541 volcanic centers, including paterae (calderas and dark spots) and shield volcanoes. The average length of an Ionian mountain is 157 km, with the longest being 570 km. The mean height of Ionian mountains is 6.3 km, and the highest known mountain is Boosaule Montes (17.5 +/- 3 km). Five basic morphologic types of mountains have been identified; mesa, plateau peak, ridge, and massif. Very few mountains bear any physical similarity. to classic volcanic landforms, but many resemble flatiron mountains on Earth and are interpreted as tilted crustal blocks. This would be consistent with the hypothesis that most mountains are thrust blocks formed as a result of compressive stresses built up in the lower crust due to the global subsidence of volcanic layers as they are buried over time. More than one mechanism may be responsible for all Ionian mountains, however. The proximity of some mountains to paterae may indicate a direct link between some mountains and volcanism, although it is not always clear which came first. In contrast to earlier studies, a pronounced bimodal pattern is observed in the global distribution of both mountains and volcanic centers. The regions of highest areal densities of volcanic centers are near the sub- and anti-Jovian regions, but are offset roughly 90deg in longitude from the two, regions of greatest concentration of mountains. This anticorrelation may indicate the overprinting of a second stress field on the global compressive stresses due to subsidence. The bimodal distribution of volcanic centers and mountains is consistent with models of asthenospheric tidal heating and internal convection developed by Tackley et al.Over regions of mantle upwelling, compressive stresses in the lower crust induced by global subsidence might be reduced, encouraging volcanism and discouraging mountain building. In regions of mantle downwelling, these compressive stresses in the lower crust might be increased, discouraging volcanism and encouraging mountain building. Alternatively, the global pattern may be related to possible (but undocumented) nonsynchronous rotation of lo, which would produce two regions each of compression and extension in the crust. Evidence of layering and of mass wasting, including landslides, block sliding, debris aprons and downslope creep, on Ionian mountains suggests that the crust of Io is essentially a layered stack of partially consolidated volcanic lavas and plume deposits, becoming more consolidated with depth. The lower crust especially may also be ductily deformed, punctuated by volcanic intrusions and faulting at paterae, and broken into blocks, some of which have been uplifted to form mountains.

Schenk, Paul↗
NASA NTRS2023

Microstructural Changes and Pb Mobility During the Zircon to Reidite Transformation: Implications for Planetary Impact Chronology

Impact events modify and leave behind a complex history of rock metamorphism on terrestrial planets. Evidence for an impact event may be recorded in physical changes to minerals, such as mineral deformation and formation of high P-T polymorphs, but also in the form of chemical fingerprints, such as enhanced elemental diffusion and isotopic mixing. Here we explore laboratory shock-induced physical and chemical changes to zircon and feldspar, the former of which is of interest because its trace elements abundances and isotope ratios are used extensively in geochemistry and geochronology. To this end, a granular mixture of Bishop Tuff sanidine and Kuehl Lake zircon, both with well characterized Pb isotope compositions, was prepared and then shocked via a flat plate accelerator. The peak pressure of the experiment, as calculated by the impedance matching method, was ~24 GPa although a broader range of P-T conditions is anticipated due to starting sample porosity. Unshocked and shocked materials were characterized via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Raman spectroscopy. These methods show that the starting zircon material had abundant metamict regions, and the conversion of the feldspar to glass in the post-shock material. Analyses of the shocked product also yielded multiple occurrences of the high-pressure ZrSiO 4 polymorph reidite, with some domains up to 300 μm across. The possibility of U-Pb system disturbance was evaluated via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The isotopic data reveal that disturbance of the U-Pb geochronometer in the reidite was minimal (<2% for the main U-Pb geochronometers). To better constrain the P-T conditions during the shock experiment, we complement impedance matching pressure calculations with iSALE2D impact simulations. The simulated results yield a range of P-T conditions experienced during the experiment and show that much of the sample may have reached >30 GPa, which is consistent with formation of reidite. In the recovered shocked material, we identified lamellae of reidite, some of which interlock with zircon lamellae. Reidite {112} twins were identified, which we interpret to have formed to reduce stress between the crystal structure of the host zircon and reidite. These two findings support the interpretation that shear transformation enabled the transition of zircon to reidite. The size and presence of reidite found here indicate that this phase is probably common in impact-shocked crustal rocks that experienced ~25 to ~35 GPa, especially when the target material has porosity. Additionally, shock loading of the zircon and transformation to reidite at these pressures in porous materials is unlikely to significantly disturb the U-Pb system in zircon and that the reidite inherits the primary U and Pb elemental and isotopic ratios from the zircon.

Bishop Tuff↗
NASA NTRS1999

Diagnostic Clast-Texture Criteria for Recognition of Impact Deposits

It is difficult to find definitive evidence for impact in the geological record because there are many endogenous geological processes that can produce diamictites similar to those generated by impact ejecta. The classic impact criteria of shock fabrics in certain minerals, and iridium layers, for example, may be either difficult to find, or long-since erased from the impact site (shock fabrics also anneal with time). It is important to be able to recognize impact-generated materials in order to understand earth's crustal development and biological evolution. In future exploration of Mars and other solar-system bodies, recognition of impact materials will be important for elucidating planetary evolution, planetary volatile inventories, and exobiological issues. The cobble depicted is typical of many that have been found in diamictite deposits in Belize generated by the Chicxulub K-T impact event. The pebbles are roughly-hewn in general shape with smoothed corners and edges. Surfaces are almost uniformly frosted (on both protuberances and hollows), but some asperities are glazed. Optical microscopy and thin-section petrographic microscopy reveal the frosting to be only a few microns thick, with a well-defined granular structure; grains are the same size as those composing the bulk of the limestone, but their clearer appearance may represent annealing. One or two adjacent pebble faces are often decorated with striated gouges and closely-spaced hemispherical depressions representing indentation hollows produced by well-rounded impacting clasts of up to 0.5 cm in diameter. Some of the impactors are still embedded in the cobble surface. Non-destructive x-ray diffraction techniques showed the impactors to be of the same mineralogy as the target cobble. We believe this unusual glazing and frosting to be related to the impact event, but this must be reconciled with its survival for over 60 my. since it is composed of one of the most alterable substances, CaCO3. We focus, however, on the non-fractured rounded pebbles that appear to have impacted the larger cobble as a swarm of clasts, probably encountered during ballistic flight. It cannot be defined whether the cobble swept through the pebble cloud, or the pebbles rained upon a slowermoving cobble. Two interesting questions arise: (1) Where did large numbers of such well rounded pebbles come from in the ejecta curtain?, and (2) How did they embed themselves in a nominally brittle rock without suffering damage? Are the well-rounded pebbles crystalline (devitrified) melt spherules? Further investigations are in progress. If the cobble was cold and brittle, impact of well-rounded pebbles would have produced Hertzian fracture patterns (a) in the virtually elastically isotropic cobble target. For penetration depths of about 0.5 of the impactor diameter, the pebbles would require a relative impact velocity sufficient to cause pebble fragmentation and crushing, and the development of a complex Hertzian-Boussinesq fracture field (b) involving deep fracturing and lateral surface spalling (impact velocities > 50 m/s). The existing relationship could only evolve by the impact of cold, hard pebbles into a soft, plastic cobble surface (c). For limestone to have been plastic, it must have been at elevated temperature, but to prevent calcification of the material, the ambient pressure must also have been elevated. This would be possible either in the impact's gas plume, or within the confines of a thin aerodynamically-produced shock bow generated by supersonic ballistic motion of the cobble. In the latter case, it is implied that the cobble swept through the pebbles rather than vice-versa. Although there are high T-P conditions associated with volcanism, such textures have not been reported on volcaniclastic materials, nor from other high T-P environments; e.g., metamorphic (as far as the authors are aware). We propose that this very easily recognizable embedding and indentation surface texture can be used as a diagnostic criterion for the recognition of impact ejecta. The term "peening texture" is suggested, because it is absolutely analogous to the plastic-deformation induced, metal-surface textures generated by ball-bearing bombardment used in engineering metallurgy to work-harden metal surfaces. Additional information is contained in the original (Figures).

Marshall, J. R.↗