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At least 217 records · Page 12
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NASA NTRS2017

Evidence for a Low Bulk Crustal Density for Mars from Gravity and Topography

Knowledge of the average density of the crust of a planet is important in determining its interior structure. The combination of high-resolution gravity and topography data has yielded a low density for the Moon’s crust, yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates. By using well-chosen constraints derived from topography during gravity field model determination using satellite tracking data, we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data, using the admittance between topography and imperfect gravity. We find a low average bulk crustal density for Mars, 2582 ± 209 kgm−3. This bulk crustal density is lower than that assumed until now. Densities for volcanic complexes are higher, consistent with earlier estimates, implying large lateral variations in crustal density. In addition, we find indications that the crustal density increases with depth.

Goossens, Sander J.↗
NASA NTRS1985

Rifting on Venus: Implications for lithospheric structure

Lithospheric strength envelopes on Venus are reviewed and their implications for large scale rifting are discussed. Their relationship to crustal thicnesses and thermal gradients are explored. Also considered are the implications of a theory for rift formation.

Banerdt, W. B.↗
NASA NTRS1994

Formation of fold and thrust belts on Venus due to horizontal shortening of a laterally heterogeneous lithosphere

An outstanding question relevant to understanding the tectonics of Venus is the mechanism of formation of fold and thrust belts, such as the mountain belts that surround Lakshmi Planum in western Ishtar Terra. These structures are typically long (hundreds of km) and narrow (many tens of km), and are often located at the margins of relatively high (km-scale) topographic rises. Previous studies have attempted to explain fold and thrust belts in various areas of Venus in the context of viscous and brittle wedge theory. However, while wedge theory can explain the change in elevation from the rise to the adjacent lowland, it fails to account for a fundamental aspect of the deformation, i.e., the topographic high at the edge of the rise. In this study we quantitatively explore the hypothesis that fold and thrust belt morphology on Venus can alternatively be explained by horizontal shortening of a lithosphere that is laterally heterogeneous, due either to a change in thickness of the lithosphere or the crust. Lateral heterogeneities in lithosphere structure may arise in response to thermal thinning or extensive faulting, while variations in crustal thickness may arise due to either spatially variable melting of mantle material or by horizontal shortening of the crust. In a variable thickness lithosphere or crust that is horizontally shortened, deformation will tend to localize in the vicinity of thickness heterogeneity, resulting in a higher component of dynamic topography there as compared to elsewhere in the shortening lithosphere. This mechanism may thus provide a simple explanation for the topographic high at the edge of the rise.

Zuber, M. T.↗
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.↗
NASA NTRS2019

GRAIL-Identified Gravity Anomalies in Oceanus Procellarum: Insight into Subsurface Impact and Magmatic Structures on the Moon

Four, quasi-circular, positive Bouguer gravity anomalies (PBGAs) that are similar in diameter (~90-190 km) and gravitational amplitude (> 140 mGal contrast) are identified within the central Oceanus Procellarum region of the Moon. These spatially associated PBGAs are located south of Aristarchus Plateau, north of Flamsteed crater, and two are within the Marius Hills volcanic complex (north and south). Each is characterized by distinct surface geologic features suggestive of ancient impact craters and/or volcanic/plutonic activity. Here, we combine geologic analyses with forward modeling of high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission in order to constrain the subsurface structures that contribute to these four PBGAs. The GRAIL data presented here, at spherical harmonic degrees 6–660, permit higher resolution analyses of these anomalies than previously reported, and reveal new information about subsurface structures. Specifically, we find that the amplitudes of the four PBGAs cannot be explained solely by mare-flooded craters, as suggested in previous work; an additional density contrast is required to explain the high-amplitude of the PBGAs. For Northern Flamsteed (190 km diameter), the additional density contrast may be provided by impact-related mantle uplift. If the local crust has a density ~2800 kg/cu.m, then ~7 km of uplift is required for this anomaly, although less uplift is required if the local crust has a lower mean density of ~2500 kg/cu.m. For the Northern and Southern Marius Hills anomalies, the additional density contrast is consistent with the presence of a crustal complex of vertical dikes that occupies up to ~50% of the regionally thin crust. The structure of Southern Aristarchus Plateau (90 km diameter), an anomaly with crater-related topographic structures, remains ambiguous. Based on the relatively small size of the anomaly, we do not favor mantle uplift; however, understanding mantle response in a region of especially thin crust needs to be better resolved. It is more likely that this anomaly is due to subsurface magmatic material given the abundance of volcanic material in the surrounding region. Overall, the four PBGAs analyzed here are important in understanding the impact and volcanic/plutonic history of the Moon, specifically in a region of thin crust and elevated temperatures characteristic of the Procellarum KREEP Terrane.

Deutsch, Ariel N.↗
NASA NTRS1994

Lithospheric-scale buckling and thrust structures on Mars: The Coprates rise and south Tharsis ridge belt

Detailed photogeologic mapping documents for the first time the deformation sequence of the Coprates rise. Materials of Middle Noachian through perhaps Early Hesperian age were involved in the horizontal shortening of crustal and lithospheric rocks that produced uplift and asymmetric, east-vergent folding of these layers during the Late Noachian and perhaps Early Hesperian. Previous suggestions of normal faulting as the deforming mechanism are not supported by the new mapping or by dislocation models of rise topography. The most likely mechanism for producing the Coprates rise is folding due to buckling instability and perhaps thrust faulting. An extensive ridge belt concentric to south Tharsis is defined by a newly recognized system of approximately coeval long-wavelength topogtaphic ridges and swales that include the Coprates rise. Horizontal shortening of crust and lithosphere is manifested by periodically spaced structures at two different scales: wrinkle ridges spaced tens of kilometers apart and the south Tharsis system of ridges spaced hundreds of kilometers apart. Both types of structures may be associated with early volcanotectonic activity and crustal thickening in south Tharsis. These and other large ridges indicate that lithospheric buckling and thrust faulting may have been common on early Mars.

Schultz, Richard A.↗
NASA NTRS2005

Osmium-Isotope and Platinum-Group-Element Systematics of Impact-Melt Rocks, Chesapeake Bay Impact Structure, Virginia, USA

Osmium (Os) isotopes and platinum-group elements (PGEs) are useful for geochemically identifying a meteoritic component within impact structures, because meteorites are typically characterized by low (187)Os/(188)Os ratios and high PGE concentrations. In contrast, most types of crustal target rocks have high radiogenic Os and very low PGE concentrations. We have examined Os isotope and PGE systematics of impact-melt rocks and pre-impact target rocks from a 2004 test hole in the late Eocene Chesapeake Bay impact structure and from nearby coreholes. Our goal is to determine the proportion of the projectile component in the melt rock Additional information is included in the original extended abstract.

Lee, Seung Ryeol↗
NASA NTRS1992

Venus: Preliminary geologic mapping of northern Atla Regio

A preliminary geologic map of C1 sheet 15N197 was compiled according to Magellan data. Northern Atla Regio is dominantly a volcanic plain with numerous volcanic features: radar-bright and -dark flows and spots, shield volcanos, volcanic domes and hills with varied morphology, and coronalike constructions. Tesserae are the oldest terrains semiflooded by plain materials. There are many lineated terrains on this territory. They are interpreted as old, partly buried ridge belts. Lineated terrains have intermediate age between young plains and old tesserae. Ozza Mons and Sapas Mons are the high shield volcanos. The prominent structure of northern Atla Regio is Ganis Chasma rift. The rift dissected the volcanic plain and evolved nearly contemporaneously with Ozza Mons shield volcano. Ganis Chasma rift valley is highly fractured and bounded by fault scarps. There are a few relatively young volcanic features in the rift valley. The rift originated due to 5-10 percent crustal extension and crustal subsidence according to analysis of fracturing and rift valley geometry. Ganis Chasma is characterized by rift shoulder uplifts. Geological structures of Alta Regio and Beta Regio are very similar as assumed earlier.

Nikishin, A. M.↗
NASA NTRS1986

Three-dimensional P and S wave velocity structures of southern Peru and their tectonic implications

Arrival times of compressional and shear (S) waves from microearthquakes recorded in 1981 by an 18-station regional array are used to study the three-dimensional velocity structure of the crust and upper mantle of the central Andes. The data suggest a crustal thickness of about 40 km beneath the coast, increasing to about 70 km beneath the Cordillera Occidental. The inverse correlation between the dip in the Moho and the dip of the slab may indicate a broad-scale causal relation between the two. S wave velocities in the mantle between 70 and 130 km depth above the 30-degree dipping slab are low, possibly indicating the presence of a partially melted asthenosphere that may be responsible for the magmatic activity recorded in southern Peru.

Cunningham, Paul S.↗
NASA NTRS1987

NASA's geodynamics program

The NASA geodynamics program covers dynamics of the core; dynamics and structure of the mantle; dynamics and structure of the lithosphere; evolution and composition of the Earth; and comparative planetology. Projects include crustal dynamics/Earth observations; gravity field modelling; and magnetic field studies. Planned space flights include global gravity and magnetic field mapping; magnetic field secular changes; gravity gradiometer mission; LAGEOS-2; and Earth Observing System.

Mcadoo, D. C.↗
NASA NTRS1991

Tessera terrain, Venus - Characterization and models for origin and evolution

Tessera terrain is the dominant tectonic landform in the northern high latitudes of Venus mapped by the Venera 15 and 16 orbiters and is concentrated in the region between the mountain ranges of western Ishtar Terra and Aphrodite Terra. Tesserae are characterized by regionally high topography, a high-degree of small scale surface roughness, and sets of intersecting tectonic features. Available Pioneer Venus line of sight gravity data suggest that tessera terain is compensated at shallow depths relative to many topographic highs on Venus and may be supported by crustal thickness variations. Three types of tessera terrain can be defined on the basis of structural patterns: subparallel ridged terrain (Tsr), trough and ridge terrain (Ttr), and disrupted terrain (Tds). Observed characteristics of tessera terrain are compared to predictions of formational and modificational models in order to begin to address the question of its origin and evolution. It is found that horizontal convergence and late stage gravitational relaxation are basic observations for subparallel ridged terrain and disrupted terrain.

Bindschadler, Duane L.↗
NASA NTRS1997

Cryptoblemes: A New Discovery with Major Economic Implications and Profound Changes to the Geologic Paradigm

Cryptoblemes are subtle impact shock signatures imprinted by cosmic debris on the crustal surfaces of lunar planetary bodes. These signatures constitute a complex cumulative overprinting of topographic, structural geophysical, and tectonic patterns that have a conspicuous radial centric multiringed symmetry. The geometry and distribution of cryptoblemes on Earth is comparable to the size and density of impact features on lunar planetary surfaces. Analysis of satellite imagery, sea-floor sonar, side-looking radar and aerial photographs of specific sites reveals new criteria for the identification and confirmation of impact-shock signatures. These criteria include joint and foliation patterns with asbestiform minerals, ribbon-quartz, spheroidal weathering, domal exfoliation, pencil shale, and shock spheres, which may originate from hydrocavitation of water-saturated sedimentary rocks. Cryptoblemes may also be associated with breccia pipes, sinkholes, buttes, mesas, and bogs, high-Rn anomalies, nodular concentrations, and earthquake epicenters. Major implications of cryptobleme identification include exploratory targeting of hydrocarbon and mineral deposits and the explanation of their origins. Analysis of known mineral deposits, structural traps and sedimentary basins show a direct correlation with cryptobleme patterns. Significant geologic paradigm shifts related to cryptoblemes include mountain building processes, structural orogenies, induced volcanism, earthquake origins, hydrocarbon diagenesis, formation mineral deposits, continental rifting, and plate movements, magnetic overprinting and local regional, and global geologic extinction and speciation patterns. Two figures provide a comparison between a multiring impact overprint in water and multiring cryptobleme in the U.S. basin range. (Additional information is contained in the original document).

Windolph, J., Jr.↗
NASA NTRS1993

Diapirism on Triton - A record of crustal layering and instability

Cantaloupe terrain on Neptune's large, icy satellite Triton comprises an organized cellular pattern of noncircular dimples that structurally and geologically most closely resemble salt diapirs exposed on Earth. The mean separation of these cells is 47 km. Modeling of the cells as compositionally driven diapirs suggests that cantaloupe terrain forms by gravity-driven overturn within an ice crust about 20 km thick with a maximum viscosity of 10 exp 22 Pa s. These diapirs probably formed as a result of a density inversion in a layered crust composed partly of ice phases other than water ice.

Schenk, Paul↗
NASA NTRS1989

Workshop on the Archean Mantle

The Workshop on the Archaen mantle considers and discusses evidence for the nature of earth's Archaen mantle, including its composition, age and structure, influence on the origin and evolution of earth's crust, and relationship to mantle and crustal evolution of the other terrestrial planets. The summaries of presentations and discussions are based on recordings made during the workshop and on notes taken by those who agreed to serve as summarizers.

Ashwal, L. D.↗
NASA NTRS1992

Venus: Preliminary geologic mapping of southern Beta Regio-northern Phoebe Regio

New preliminary geologic maps of C1 sheets 15N283 and 00N283 were compiled according to Magellan data. The oldest terrains are tesserae that have fragmentlike shapes. Its margins are partly buried by younger plain materials. Volcanic plains are the dominant types of terrains. There are many different volcanic features on plains: radar-bright and -dark flows and spots, shield volcanos, volcanic domes and hills with varied morphology, and coronalike constructions. Devana Chasma rift crosses the surface between Beta Regio and Phoebe regio. The rift's normal faults dissect volcanic plains and shield volcanos. The rift valleys are relatively young structures. According to structural analysis of the rift valleys we conclude the rift originated due to 5-10 percent crustal extension and crustal subsidence. Devana Chasma rift is characterized by shoulder uplifts.

Nikishin, A. M.↗
NASA NTRS1988

Deformational models of rifting and folding on Venus

Features of presumed tectonic origin on Venus are reviewed, and lithospheric strength envelopes are derived based on laboratory measurements of the deformational properties of crustal and subcrustal rocks, extrapolated to conditions appropriate to Venus. Models for rifting and folding are developed that use this lithospheric structure and take into account both brittle and ductile yielding as well as finite elastic strength. For both rifting and folding, structures with characteristic widths and spacings are predicted whose size depends on the thickness of the lithosphere, density contrast, and elastic properties of the layer. Finally, the model predictions are compared with the widths and spacings of observed tectonic features, and it is concluded that they are consistent with a relatively strong mantle layer separated from a thin brittle surface layer by a ductile lower crust. These results allow constraints to be placed on the crustal thickness and thermal gradient on Venus.

Banerdt, W. Bruce↗
NASA NTRS1993

Porosity and the ecology of icy satellites

The case for a significant role for porosity in the structure and evolution of icy bodies in the Solar System has been difficult to establish. We present a relevant new data set and a series of structure models including a mechanical compression, not thermal creep, model for porosity that accounts satisfactorily for observed densities, moments of inertia, geologic activity, and sizes of tectonic features on icy satellites. Several types of observational data sets have been used to infer significant porosity, but until recently, alternative explanations have been preferred. Our first area of concern is the occurrence of cryovolcanism as a function of satellite radius; simple radiogenic heating models of icy satellites suggest minimum radii for melting and surface cryovolcanism to be 400 to 500 km, yet inferred melt deposits are seen on satellites half that size. One possible explanation is a deep, low conductivity regolith which lowers conductivity and raises internal temperatures, but other possibilities include tidal heating or crustal compositions of low conductivity. Our second area of concern is the occurrence and magnitude of tectonic strain; tectonic structures have been seen on icy satellites as small as Mimas and Proteus. The structures are almost exclusively extensional, with only a few possible compression Al features, and inferred global strains are on the order of 1 percent expansion. Expansions of this order in small bodies like Mimas and prevention of late compressional tectonics due to formation of ice mantles in larger bodies like Rhea are attained only in structure models including low-conductivity, and thus possibly high porosity, crusts. Thirdly, inferred moments of inertia less than 0.4 in Mimas and Tethys can be explained by high-porosity crusts, but also by differentiation of a high density core. Finally, the relatively low densities of smaller satellites like Mimas and Miranda relative to larger neighbors can be explained by deep porosity, but also by bulk compositional differences. Recent work has strengthened the case for significant porosity. Halley's nucleus was found to have a density near 0.6 g/cu cm, Janus and Epimethus were proposed to have densities near 0.7 g/cu cm, densities almost certainly due to high porosity. The irregular-spherical shape transition of icy satellites was quantitatively explained by low conductivity regoliths. A creative structure/thermal history model for Mimas simultaneously accounts quantitatively for Mimas' low density and moment of inertia by invoking initial high-porosity and subsequent compaction in the deep interior by thermal creep. The main problem with this promising model is that approximately 7 percent predicts a reduction in Mimas' radius, implying significant compressional failure and prevention of extensional tectonics, in contradiction to the observed extensional features and inferred 1 percent expansion in radius.

Croft, Steven K.↗
NASA NTRS2012

Solar Wind Electron Interaction with the Dayside Lunar Surface and Crustal Magnetic Fields: Evidence for Precursor Effects

Electron distributions measured by Lunar Prospector above the dayside lunar surface in the solar wind often have an energy dependent loss cone, inconsistent with adiabatic magnetic reflection. Energy dependent reflection suggests the presence of downward parallel electric fields below the spacecraft, possibly indicating the presence of a standing electrostatic structure. Many electron distributions contain apparent low energy (<100 eV) upwardgoing conics (58% of the time) and beams (12% of the time), primarily in regions with non-zero crustal magnetic fields, implying the presence of parallel electric fields and/or wave-particle interactions below the spacecraft. Some, but not all, of the observed energy dependence comes from the energy gained during reflection from a moving obstacle; correctly characterizing electron reflection requires the use of the proper reference frame. Nonadiabatic reflection may also play a role, but cannot fully explain observations. In cases with upward-going beams, we observe partial isotropization of incoming solar wind electrons, possibly indicating streaming and/or whistler instabilities. The Moon may therefore influence solar wind plasma well upstream from its surface. Magnetic anomaly interactions and/or non-monotonic near surface potentials provide the most likely candidates to produce the observed precursor effects, which may help ensure quasi-neutrality upstream from the Moon.

Solar↗