Kilometers Of Crust Research Articles

Observations From a Global Database of Impact Craters on Mercury With Diameters Greater than 5 km

AbstractWe created a global database of Mercurian impact craters with diameters over 5 km, excluding obvious secondary craters. For craters over 10 km in diameter, we assessed basic morphological parameters, including an evaluation of whether significant postimpact filling occurred, the nature of any central structure, a three‐point assessment of degradation state, and a recording of the presence of rays and other atypical features. We used the database to evaluate the spatial patterns of different subsets of the crater population. The distribution of craters with diameters of 5 to 10 km compared to the distribution of larger craters shows many background secondaries associated with primary craters with diameters over 150 km, and their abundance would distort crater counts for surface dating. Counting only unfilled craters gives an estimate of the last possible date of endogenic geologic activity on a surface. Only a third of craters with diameters over 20 km are classified as unfilled, and the highest spatial density of unfilled craters is about half the total population at the same location. Our results support a globally rapid cessation in volcanism, with variations in the volume of late stage volcanism accounting for variations in the number of filled craters. We see regional variations of interior morphology for craters ~100 km in diameter that do not obviously correlate with other patterns derived from surface observations; our observations imply that there are variations in the mechanical properties of the upper several kilometers of crust.

Estimate of depths of source fluids related to mound fields on Mars

The investigation of the Martian surface through remote sensing allowed the identification of mound-like topographically positive features that, based on geomorphological observations, have been ascribed to different phenomena. New observations will be performed in the forthcoming future to look for possible methane sources, hence discriminating morphologically similar features is a key objective to efficiently investigate and target the Martian surface. Here, we performed fractal analysis on five Martian fields of mound-like features that have been interpreted respectively as mud volcanoes, pingos, tumuli, rootless cones and monogenetic volcanic vents successfully validating the major interpretations of these features and in turn, the applicability of the fractal clustering method for discriminating among such features. Indeed, this technique is able to assess if the analyzed features are directly related to underlying systems of connected percolating fracture networks and estimates their extension in the subsurface. Accordingly, volcanic vents and mud volcanoes appeared to be connected to percolating systems involving several kilometers of crust, pingos and tumuli resulted to be unequivocally unrelated to active percolating fractures while rootless cones outputted weak relationship with shallow active fracture systems.

Mantle compression affects seismic wave propagation

To try to understand the direction of motion of the Earth's mantle, which lies hidden beneath tens of kilometers of crust, researchers have relied on the property of seismic anisotropy. When seismic shear waves pass through some types of materials, known as anisotropic materials, the speed of the wave can vary depending on the direction in which it is moving. Traditionally, scientists have assumed that the direction in which waves move more quickly aligns with the direction of mantle motion. For subduction zones, however, this general rule seemed to break down—a discrepancy exists between numerical model simulations and observed seismic data.

Density and porosity of the lunar crust from gravity and topography

Newly obtained gravity and topography data of the Moon, combined with a lithospheric flexure model that considers both surface and subsurface loading, are used to place constraints on the density of the upper crust from a localized spectral admittance analysis. Subsurface loads are found to be relatively unimportant in the highlands, and when subsurface loads are neglected, the best fitting bulk densities for a number of highland regions are found to vary from 2590 to 2870 kg m−3, with a mean value of 2691 kg m−3. Crustal rock densities estimated from geochemical considerations and global iron and titanium abundances imply somewhat greater densities, which we interpret as porosity affecting the gravity‐derived bulk density estimates. The average porosity in the upper few kilometers of crust is calculated to be about 7.7%, which is consistent with porosity estimates of impact‐fractured meteorites and terrestrial impact craters.

A microearthquake survey of the high‐temperature vent fields on the volcanically active East Pacific Rise (9°50′N)

A 3 month deployment of nine ocean bottom seismometers (OBS) on the axis of the East Pacific Rise at 9°50′N detected 283 local microearthquakes in the spring of 1995. The earthquakes exhibit small, uniform seismic moments of 1014–1016 dyne‐cm (). Accurate locations were determined for 147 of the earthquakes, with hypocenters clustered beneath two high‐temperature hydrothermal fields (Bio9/P and Tube Worm Pillar/Y) at depths <1.4 km beneath the seafloor. Waveform cross‐correlation techniques relocated 76 events, 65 of which had lateral standard errors <100 m. Relocated hypocenters lie along two vertical columns from 0.7 to 1.1 km depth. A continuous level of seismic activity was observed beneath the Tube Worm Pillar and Y vent fields, while activity beneath the Bio9 and P vent fields was dominated by a vigorous swarm of 162 events during 3 hours that triggered a 7°C temperature increase in Bio9 exit fluids. The close correlation between the hydrothermal systems and the observed seismicity suggests the microearthquakes were generated by thermal strain associated with cooling of the shallow crust and that the water‐rock reaction zone is also a seismogenic zone. The earthquake depths suggest that brittle deformation on the rise axis is restricted to the upper kilometer of crust.

Delayed magnetization of the deeper kilometer of oceanic crust at Ocean Drilling Project Site 504

The Fe‐Ti magnetic carriers of the dike‐rich second kilometer of oceanic crust at Ocean Drilling Program Site 504 are shown to be secondary in origin, that is, their formation is likely to considerably postdate initial igneous crustal formation. An important consequence of the delay in formation of carriers is that the polarity of their magnetization has a significant probability of opposing that of the overlying, little‐altered extrusives. If this is the case, the linear magnetic anomalies in the area have sources crust, which, at least locally, is magnetized with successive layers of opposite polarities. The magnitude of the contribution of deeper layers to anomalies will depend both on their thickness and average magnetization, which are not as yet well known. Two types of secondary magnetite, SMI and SMII, separated spatially and by the steps leading to their formation, are recognized in the dikes of the second kilometer of the crust at Site 504. Both were formed in conditions of hydrothermal greenschist metamorphism. Since all newly formed oceanic crust is thought to experience this type of alteration at an early stage, polarity reversal with depth as a result of the delayed formation of magnetic carriers is likely to be widespread.

Geologic Hazards Assessment at the Idaho National Engineering Laboratory, Southeastern Idaho: ABSTRACT

The Idaho National Engineering Laboratory (INEL), an 890 mi{sup 2} facility on the Eastern Snake River Plain (ESRP) in southeastern Idaho, is operated by the US Department of Energy for energy research and development. The ESRP, a Neogene and Quaternary volcanic province produced by passage of the Yellowstone mantle plume, has intimate spatial and genetic relationships to the northern part of the Basin-and-Range province. Work is in progress in two major aspects of seismic hazards assessment: identification and characterization of seismic sources, and prediction of potential strong ground motions at specific sites. Investigations of mechanics of ESRP volcanic rift zone formation have furnished information important to both seismic and volcanic hazards assessment. Results to date include the following. Although some ESRP volcanic rift zones are colinear with basin-and-range faults, their surface deformation features are related to dike emplacement rather than to projection of basin-and-range tectonism onto the Plain. Trenching of a NE-trending scarplet along the NW edge of the ESRP shows that these features are not associated with faulting. The most serious seismic threat is associated with the southern segment of the Lemhi fault, a basin-and-range fault north of the ESRP. Based on the Band Limited White Noise methodology, strongmore » ground motions resulting from possible earthquakes on this fault depend upon the configuration of low velocity materials interbedded in the sequence of basaltic lava flows comprising the upper kilometer of crust in the ESRP. These materials consist mostly of alluvial and lacustrine sediments and a thick layer of unwelded rhyolitic tuff. Their seismic velocities (V{sub p}) range from 1 to 2 km/sec, whereas that for the basaltic lava flows varies from 3 to 5 km/sec.« less

Basin and Range style tectonics in East Africa

Geophysical and geological evidence suggest that the asthenospheric structure beneath the presently active east African rifts resembles that of the Basin and Range tectonic province of the western United States. Both regions are underlain by anomalously thin lithosphere riding on hot asthenospheric pillows that extend laterally for as much as 1500 km normal to structural trend. The continental rifts associated with these terranes do not appear to be leading to plate separation, and young oceanic rifts such as the Red Sea probably evolved from narrower, more restricted mantle thermal anomalies. Low-angle normal faults are interpreted to control the physiography and structural style in the east African rifts in a fashion analogous to that proposed for the Basin and Range. The limited horizontal extension that has occurred in east Africa, however, requires that mechanical thinning of the lithosphere must play a subsidiary role to thermal crosion from below. The upward migration of mantle isotherms has converted the base of the lithosphere to material with asthenospheric properties beneath thousands of square kilometers of crust. The presence of a broad zone of thin, warm lithosphere in both these tectonic provinces may help to distribute extensional strains over correspondingly broad regions. This in turn is expected to result in the development of numerous detachment surfaces (and corresponding surficial rifts), none of which ever completely thin the lithosphere or evolve to oceanic-style rifts.

Interpretation of some magnetic anomalies over Iceland

Simulated dike models with rectangular and normal probabilities are used to examine the distribution of volcanism implied by the magnetic anomalies over the Reykjanes ridge. The linearity and symmetry of the anomalies are lost once the area of Iceland is approached. This is attributed to a greater spread of the volcanism in space about the neovolcanic zone, lateral variations in thickness of the lava polarity groups, the introduction of weakly magnetic acid rocks at intervals, and variable amounts of remagnetisation and metamorphism within the lava pile. The source bodies of the magnetic anomalies over Iceland are complex and include shallowly dipping lava flows, and near vertical dikes and other intrusions. The earlier-formed lavas within the pile are probably completely remagnetised by subsequent dike injection towards the layer 2 – layer 3 seismic discontinuity and below this the crust is believed to be entirely intrusive. Most Icelandic volcanics have comparable magnetic intensities to other basaltic rock collections but the susceptibilities are an order or so higher than submarine lavas. The anomalies are not significantly affected by plausible magnetic decay rates or tilting of the lava pile. Rock magnetic properties show no regular change with distance from the neovolcanic zone. Magnetic polarity maps are presented for the two flanks of the neovolcanic zone in northern Iceland, the lava pile in eastern Iceland, and southwestern Iceland. The lava stratigraphy can be correlated with the aeromagnetic anomalies in the latter two areas, and models involving the top few kilometers of crust can be fitted to the observed data and set controls on its probable structure. The combined geologic and magnetic anomaly data allow a spreading rate to be deduced for southwest Iceland which is closely comparable to that inferred for the Reykjanes ridge. High amplitude and short wavelength anomalies correlate closely with the distribution of young volcanic rocks and they indicate that differential spreading is taking place between north and south Iceland.