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USGS2012

Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range Province

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a P n of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low P n . Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous-textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO 2 -rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.

Arizona, California, Nevada↗

USGS2000

Geologic mapping of Europa

Galileo data enable the major geological units, structures, and surface features to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with various tectonic structures such as faults. Plains units are the most widespread. Ridged plains material spans a wide range of geological ages, including the oldest recognizable features on Europa, and appears to represent a style of tectonic resurfacing, rather than cryovolcanism. Smooth plains material typically embays other terrains and units, possibly as a type of fluid emplacement, and is among the youngest material units observed. At global scales, plains are typically mapped as undifferentiated plains material, although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size. Chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity; chaos is characterized by blocks of icy material set in a hummocky matrix. Band material is arrayed in linear, curvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain. Bilateral symmetry observed in some bands and the relationships with the surrounding units suggest that band material forms by the lithosphere fracturing, spreading apart, and infilling with material derived from the subsurface. Ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales. Ridge material includes single ridges, doublet ridges, and ridge complexes. Ridge materials are considered to represent tectonic processes, possibly accompanied by the extrusion or intrusion of subsurface materials, such as diapirs. The tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales. Crater materials include various interior (smooth central, rough inner, and annular massif) and exterior (continuous ejecta) subunits. Structural features and landforms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geological maps, including colors and letter abbreviations for material units. Implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial mapping results, a stratigraphic sequence is suggested in which ridged plains form the oldest unit on Europa, followed by development of band material and individual ridges. Band materials tend to be somewhat older than ridges, but in many areas the two units formed simultaneously. Similarly, the formation of most chaos follows the development of ridged plains; although chaos is among the youngest materials on Europa, some chaos units might have formed contemporaneously with ridged plains. Smooth plains generally embay all other units and are late-stage in the evolution of the surface. C1 craters are superposed on ridged plains but are crosscut by other materials, including bands and ridges. Most c2 craters postdate all other units, but a few c2 craters are cut by ridge material. C3 craters constitute the youngest recognizable material on Europa. Copyright 2000 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗

USGS2012

The mechanics and three-dimensional internal structure of active magmatic systems: Kilauea volcano, Hawaii

Interpretation of abundant seismic data suggests that Kilauea's primary conduit within the upper mantle is concentrically zoned to about 34-km depth. This zoned structure is inferred to contain a central core region of relatively higher permeability, surrounded by numerous dikes that are in intermittent hydraulic communication with each other and with the central core. During periods of relatively high magma transport, the entire cross section of the conduit is utilized. During periods of relatively low to moderate transport, however, only the central core is active. As the conduit penetrates the oceanic crust and enters the volcanic shield, it simultaneously supplies the deeper sections of the rift zones (6-to 10-km depth) and the roots of the summit reservoir with picritic magma. The rift zones at depth are inferred to be almost wholly molten and to possess a high degree of fluid continuity from Heiheiahulu in the East Rift Zone, 45 km westward through the roots of the summit magma reservoir, and well into the Southwest Rift Zone. Higher in the shield, the subcaldera magma reservoir and the shallow rift zones occupy the 2-to 4-km depth interval. Summit-differentiated olivine tholeiite (ρ ≈ 2.62 g cm −3 ) is periodically injected laterally along a horizon of neutral buoyancy within the rift zones, where the density of the magma is just balanced by the in situ density of the shield (Ryan, 1987 a , b ). Deep rift zone intrusions push seaward the deep tectonic blocks of the volcano's south flank. Shallow rift intrusions build a sheeted dike complex, inferred to be in isostatic equilibrium with the higher-density deep rift cores below. General finite element analyses are presented for the deformation and stress fields surrounding such dikes in the horizontal and vertical planes. The dike tip in two and three dimensions is surrounded by a tubular core of tensile (σ 1 , σ 2 ) and shear stress (τ max ). The displacement field is characterized by counterrotating cells on either side of the dike tip which, in vertical orientation, produce the characteristic subsidence above the dike complex, with uplift on either side, forming a ridge-trough-ridge structure. A finite element model of Kilauea's shield computes the displacement fields and principal stress (σ 1 ) distributions resulting from intrusive activity on each or both of the rift zones. Within the summit region, tensile stress lobes produced by the three-dimensional upward extension of the intrusions superpose constructively to produce calderawide regimes of tensile stress, conducive to caldera development. Parametric studies of (1) intrusion in the East Rift Zone only, (2) intrusion in the Southwest Rift Zone only, and (3) intrusion in both rift zones demonstrate their unique kinematic contributions. For case 1, the caldera undergoes a counterclockwise rotation (torque up state) conducive to the development of rightstepping en echelon eruptive fissures, as exemplified by the August 14, 1971, eruption. For case 2, the caldera undergoes a clockwise rotation (torque down state) conducive to the development of left-stepping eruptive fissures, as occurred during the December 31, 1974, eruption. For case 3, the caldera substructure is driven due southward, producing the southward migration of the upper portions of the summit magma reservoir.

Journal of Geophysical Research Solid Earth↗

USGS2012

Television observations from Surveyor 3

A total of 6315 pictures were taken by the television camera on Surveyor 3 after the lunar landing. These pictures have provided much new information about the location of the landing site on the moon, the detailed topographic and geologic characteristics of the lunar surface, and the appearance of the earth as seen from the moon, both during eclipse of the sun and during partial direct illumination by the sun. Surveyor 3 landed in a subdued crater slightly more than 200 meters in diameter, which has a low rounded rim and is about 15 meters deep. The spacecraft is situated on the east wall of the crater, about half way between the center of the crater and the rim crest. The spacecraft is inclined 14.7° ± 1.0° toward the west. The selenographic coordinates of the landing site are 2.94°S latitude, 23.34°W longitude, relative to selenodetic control adopted by the Aeronautical Chart and Information Center. Small morphologic elements of the landing site include small craters, linear ridges and troughs, and fragmental debris. The craters and fragmental debris resemble those observed at the Surveyor 1 landing site, both in distribution of shape and in distribution of size. Most of the craters in Surveyor 1 and 3 pictures are inferred to be of impact origin. Their size-frequency distribution corresponds to the distribution that would be produced by repetitive bombardment by meteoroids, a bombardment sufficiently prolonged that the crater population has reached a steady state or has come to equilibrium. Some of the craters observed at the Surveyor 3 landing site are inferred to be of secondary impact origin, and some probably have been formed either by subsidence or by drainage of fragmental debris into cracks or fissures in the subsurface. Fragmental debris at the landing site is inferred to have been derived primarily by the same process of repetitive bombardment that produced the majority of craters. The inferred volumetric size-frequency distribution of fragments, derived from the observed size distribution of fragments on the surface, is similar to the distribution that would be produced by repetitive bombardment of coherent rocks by meteoroids with a mass-frequency distribution like that found from observed meteors and recovered meteorites on earth. Two prominent strewn fields of blocky debris were observed around two craters, 13 and 15 meters across, at the Surveyor 3 site. The 13-meter crater has a sharp raised rim, and the 15-meter crater has a more subdued rounded rim. The blocks associated with the subdued crater have twice as high a mean roundness as the blocks associated with the raised-rim crater, and they are much more deeply buried. The size-frequency distribution function for the fragments in each of the strewn fields of blocks resembles the size-frequency distribution for fragments ejected by impacts in strong rock, such as Meteor Crater, Arizona. Most of the fragments at the surface of the Surveyor 3 landing site are evidently part of a layer of fragmental material of low cohesion that is at least 1 meter thick along the upper parts of the wall of the crater in which Surveyor 3 landed and may be much thicker near the center of the crater. The evidence from the Surveyor 3 pictures suggests that this layer of debris, or regolith, is subject to downslope creep or mass movement. Creep is probably caused by seismic shaking, due mainly to near and far impact events and perhaps due partly to internal lunar seismicity. Disturbances of the lunar surface produced by Surveyor 3, like those produced by Surveyor 1, exposed material at depths of a few centimeters or less that was darker than the material at the surface. The albedo of the fine-grained fragmental debris is probably 20 to 30% lower at depths of only a fraction of a millimeter than it is at the optically observed surface. All coarse fragments protruding above the general level of the surface have a higher albedo than the fine-grained matrix of the surface. These general photometric relationships can be explained if it is assumed that the surfaces of the particles in the shallow lunar subsurface tend to become coated with a dark substance; the term proposed for this hypothetical substance is ‘lunar varnish’ On the protruding surface of blocks and coarse fragments the lunar varnish is scrubbed off by the processes that cause rounding. The exposed surfaces of fine particles on the lunar surface are similarly affected, but, because they are mixed relatively rapidly with particles just beneath the surface, the process is incomplete and the fine-grained material exposed at the lunar surface, therefore, has a lower albedo than blocks and other large fragments. Coating of particles by lunar varnish evidently takes place just beneath the surface. The estimated normal luminance factor (normal albedo) of an undisturbed part of the lunar surface next to footpad 2 of the Surveyor 3 spacecraft is 8.5%. An area of the lunar surface disturbed by the surface sampler has an estimated normal luminance factor of 6.6%, and fine-grained material placed on footpad 2 by the surface sampler has an estimated normal luminance factor of 7.6%. The errors in all these estimates may be as high as 25% because of uncertainties of correction required for light scattered from the camera mirror. Preliminary search for color differences, by color reconstitution methods, revealed no determinable differences in color among various coarse blocks, the fine-grained matrix of the surface, or fine-grained material disturbed by the surface sampler. Surveyor 3 pictures of the eclipse of the sun by the earth revealed a bright region in the refraction halo surrounding the earth, which was correlated with the position of the sun, and a series of bright beads that occurred over regions of the earth largely clear of clouds. Clouds tend to occult the refracted rays of the sun, most of which pass through the lower part of the atmosphere at the limb; the beads occurred in the depressions in the optical silhouette of the earth. Preliminary reduction of the color of the refracted light showed that the brightest region, near the position of the sun, exhibited a correlated color temperature close to 4800°K. The color temperature tended to be lower for light that followed paths of greater atmospheric absorption. Preliminary analysis of Surveyor 3 pictures of the partly illuminated earth revealed colors similar to the colors recorded from orbit by the Mercury and Gemini astronauts.

Journal of Geophysical Research↗