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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A compilation of information and data on the Manson impact structure

A problem for the impact hypothesis for the Cretaceous-Tertiary (K-T) mass extinction is the apparent absence of an identifiable impact site. The Manson Impact Structure is a candidate because it is the largest recognized in the U.S.; it is relatively close to the largest and most abundant shocked quartz grains found at the K-T boundary; and its age is indistinguishable from that of the K-T boundary based on paleontological evidence, fission track dates, and preliminary Ar-40/Ar-39 measurements. The region of northwest central Iowa containing the Manson Impact Structure is covered by Quaternary glacial deposits underlain by Phanerozoic sedimentary rocks (mostly flat-lying carbonates) and Proterozoic red clastic, metamorphic, volcanic, and plutonic rocks. In a circular area about 22 miles (35 km) in diameter around Manson, Iowa, this normal sequence is absent or disturbed and near the center of the disturbed area granitic basement rocks have been uplifted some 20,000 ft (6000m). Attention was drawn to Manson initially by the unusual quality of the groundwater there. Within the structure three roughly concentric zones of rock associations have been identified: (1) displaced strata; (2) completely disrupted strata, and igneous and metamorphic rocks. Manson was established as an impact structure based on its circular shape, its central uplift, and the presence of shocked quartz within the granitic central uplift. A gravity survey identified locations of low-density brecciated rocks and high-density uplifted crystalline rocks, but the outer boundary of the structure could not be established. Aeromagnetic and ground magnetic surveys showed locations and depths of shallowly buried crystalline rock and the locations of faults. A refraction seismic survey identified the crystalline central uplift, determined that the average elevation of bedrock is 70 ft (20 m) higher outside the structure than within, and was used to map the bedrock topography within the structure. A connection between the Manson impact and the K-T boundary may be established or refuted through study of the impact energy, the impact time, and composition of host rock, possible impactors, and impact melts.

Hartung, Jack B.↗
NASA NTRS1987

Source parameters for 11 earthquakes in the Tien Shan, central Asia, determined by P and SH waveform inversion

The style and the distribution of faulting occurring today in the Tien Shan region were studied, by digitizing long-period World-Wide Standard Seismograph Network P and SH waveforms of 11 of the largest Tien Shan earthquakes between 1965 and 1982 and then using a least squares inversion routine to constrain their fault plane solutions and depths. The results of the examination indicate that north-south shortening is presently occurring in the Tien Shan, with the formation of basement uplifts flanked by moderately dipping thrust faults. The present-day tectonics of the Tien Shan seem to be analogous to those of the Rocky Mountains in Colorado, Wyoming, and Utah during the Laramide orogeny in Late Cretaceous and Early Tertiary time.

Nelson, Michael R.↗
NASA NTRS1988

The Manson impact structure, a possible site for a Cretaceous-Tertiary (K-T) boundary impact

The Manson impact structure, about 35 km in diameter, is the largest impact crater recognized in the United States. Its center is located near the town of Manson, 29 km west of Fort Dodge, Iowa. The structure is not well known geologically because it is covered by tens of meters of glacial deposits. What is known about the structure was learned mostly from the study of water well cuttings. At Manson the normal Phanerozoic and Proterozoic sedimentary rocks were replaced by centrally uplifted Proterozoic crystalline rocks that are representative of the normal basement: This central uplift is surrounded by completely disrupted rocks which are roughly encircled by peripherally faulted and slumped sequences of normal sedimentary strata. Radially outward normal sedimentary strata are uplifted slightly. Manson, once interpreted as a cryptovolcanic structure, is now considered an impact structure based on its circular shape, its central uplift and the presence of multiple intersecting sets of shock lamellae in quartz grains from the central uplift. The Ar-40/Ar-39 age spectrum dating results for a microcline separate from the Manson 2-A core in the central uplift is shown. This spectrum is interpreted to indicate a nearly complete degassing of the microcline at the time of the Manson impact. The remainder of the gas released climbs in age with increasing temperature of release. This pattern of the age spectrum is interpreted to represent diffusional loss due to reheating at the time of the impact and during subsequent cooling. Shocked quartz grains, present in the iridium-bearing layer at the K-T boundary throughout the world, have a significantly larger size and are more abundant in the western interior of North America than elsewhere in the world. Furthermore, shocked feldspar and granitic fragments are found at the K-T boundary in North America. These observations indicate the K-T boundary impact must have penetrated continental crust in North America.

Hartung, J. B.↗
NASA NTRS1988

Mass mortality and extraterrestrial impacts

The discovery of iridium enrichment at the Cretaceous/Tertiary boundary resulted in formulation of hypothesis of a cometary or asteroid impact as the cause of the biological extinctions at this boundary. Subsequent discoveries of geochemical anomalies at major stratigraphic boundaries like the Precambrian/Cambrian, Permian/Triassic, Middle/Late Jurassic, resulted in the application of similar extraterrestrial impact theories to explain biological changes at these boundaries. Until recently the major physical evidence, as is the location of the impact crater site, to test the impact induced biological extinction was lacking. The diameter of such a crater would be in the range of 60 to 100 km. The recent discovery of the first impact crater in the ocean provide the first opportunity to test the above theory. The crater, named Montagnais and located on the outer shelf off Nova Scotia, Canada, has a minimum diameter of 42 km, with some evidence to a diameter of more than 60 km. At the Montagnais impact site, micropaleontological analysis of the uppermost 80 m of the fall-back breccia represented by a mixture of pre-impact sediments and basement rocks which fills the crater and of the basal 50 m of post-impact marine sediments which overly the impact deposits, revealed presence of diversified foraminiferal and nannoplankton assemblages. The sediments which are intercalated within the uppermost part of the fall-back breccia, had to be deposited before the meteorite impact. The post-impact deposits were laid down almost immediately after the impact as also supported by the micropaleontological data. In conclusion, micropaleontological studies of sediments from the first submarine impact crater site identified in the ocean did not reveal any mass extinction or significant biological changes at the impact site or in the proximal deep ocean basin.

Jansa, L. F.↗
NASA NTRS1988

Catastrophic volcanism as a cause of shocked features found at the K/T boundary and in cryptoexplosion structures

The presence of quartz grains containing shock lamellae at the Cretaceous/Tertiary (K/T) boundary is viewed by many as the single most compelling evidence of meteoritic or cometary impact because there is no known endogenous mechanism for producing these features. Similarly the presence of shocked quartz, shatter cones, coesite and stishovite at cryptoexplosion structures is comonly taken as conclusive evidence of impact. However, several recent studies have cast doubt on this interpretation. It is argued that basaltic volcanism, although not normally explosive, can under exceptional circumstances produce overpressures sufficiently high to produce shock features. The exceptional circumstances include a high content of volatiles, usually CO2, and no preestablished pathway to the surface. Rapid cooling of the saturated basaltic magma can occur if it underlies a cooler more evolved magma in a chamber. Initial slow cooling and partial exsolution of the volatiles will cause the density of the basaltic magma to become less than that of the overlying magma, leading to overturning and mixing. Gas will escape the magma chamber along planar cracks once the pressure becomes sufficiently high. In the vicinity of the crack tip there is a smallscale deviatoric stress pattern which is thought to be sufficiently high to produce transient cracks along secondary axes in the quartz crystals, causing the planar features. The CO2-rich fluid inclusions which have been found along planar elements of quartz in basement rocks of the Vredefort Dome were likely to have been emplaced by such a process. If the mechanism described is capable of producing shocked features as above, it would require a reassessment of the origin of many cryptoexplosion structures as well as seriously weakening the case for an impact origin of the K/T event.

Loper, D. E.↗
NASA NTRS1989

Martian tension fractures and the formation of grabens and collapse features at Valles Marineris

Simple models of the Martian crust are summarized that predict extensional deformation style on the basis of depth, material friction and strength, and hydraulic conditions appropriate to the planet. These models indicate that tension fractures may be common features on Mars, given adequate differential stress conditions. Examples of tension fractures on Mars inferred from morphological criteria are examined based on the probable geologic conditions in which they formed and on model constraints. It is proposed that the grabens and collapse features of Valles Marineris are controlled by tension fractures in intact basement rocks that lie below impact ejecta.

Tanaka, K. L.↗
NASA NTRS1989

Tectonic wedging in the forearc basin - Accretionary prism transition, Lesser Antilles forearc

This paper describes regional structure of the inner forearc of the southern Lesser Antilles, which contains an extensive 50-70 km wide inner forearc deformation belt (IFDB) developed above crystalline basement of the undeformed forearc basin (FAB), close to and perhaps above its probable subduction trace with Atlantic lithosphere. The IFDB is analyzed, with emphasis placed on five transects across the belt, using mainly migrated seismic sections and balanced model cross sections. The IFDB features and its evolution are discussed, with special attention given to the major structures divided by early and late stages of development, paleobathymetric history, event timing, displacement and strain, and alternative tectonic explanations.

Torrini, Rudolph, Jr.↗
NASA NTRS1989

Summaries of the thematic conferences on remote sensing for exploration geology

The Thematic Conference series was initiated to address the need for concentrated discussion of particular remote sensing applications. The program is primarily concerned with the application of remote sensing to mineral and hydrocarbon exploration, with special emphasis on data integration, methodologies, and practical solutions for geologists. Some fifty invited papers are scheduled for eleven plenary sessions, formulated to address such important topics as basement tectonics and their surface expressions, spectral geology, applications for hydrocarbon exploration, and radar applications and future systems. Other invited presentations will discuss geobotanical remote sensing, mineral exploration, engineering and environmental applications, advanced image processing, and integration and mapping.

Source record↗
NASA NTRS1990

Development of multiple source data processing for structural analysis at a regional scale

Various image processing techniques developed for enhancement and extraction of linear features, of interest to the structural geologist, from digital remote sensing, geologic, and gravity data, are presented. These techniques include: (1) automatic detection of linear features and construction of rose diagrams from Landsat MSS data; (2) enhancement of principal structural directions using selective filters on Landsat MSS, Spacelab panchromatic, and HCMM NIR data; (3) directional filtering of Spacelab panchromatic data using Fast Fourier Transform; (4) detection of linear/elongated zones of high thermal gradient from thermal infrared data; and (5) extraction of strong gravimetric gradients from digitized Bouguer anomaly maps. Processing results can be compared to each other through the use of a geocoded database to evaluate the structural importance of each lineament according to its depth: superficial structures in the sedimentary cover, or deeper ones affecting the basement. These image processing techniques were successfully applied to achieve a better understanding of the transition between Provence and the Pyrenees structural blocks, in southeastern France, for an improved structural interpretation of the Mediterranean region.

Carrere, Veronique↗
NASA NTRS1990

The shallow structure of the Martian lithosphere in the vicinity of the ridged plains

On Mars, the most prominent assemblage of regularly spaced plain ridges occurs in the plains of the Coprates and Lunae Planum regions. In this paper, a number of tectonic models is developed and quantitatively evaluated, that relate the regular spacing of these ridges to the shallow internal structure of Mars. It is shown that models with both rigid and deformable megaregolith-basement interface conditions can provide solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials. Implications of the models for the structure and state of stress in the Tharsis region at the time of ridge formation are discussed.

Zuber, M. T.↗
NASA NTRS1991

K/T age for the popigai impact event

The multi-ringed POPIGAI structure, with an outer ring diameter of over 100 km, is the largest impact feature currently recognized on Earth with an Phanerozoic age. The target rocks in this relatively unglaciated region consist of upper Proterozoic through Mesozoic platform sediments and igneous rocks overlying Precambrian crystalline basement. The reported absolute age of the Popigai impact event ranges from 30.5 to 39 Ma. With the intent of refining this age estimate, a melt-breccia (suevite) sample from the inner regions of the Popigai structure was prepared for total fusion and step-wise heating Ar-40/Ar-39 analysis. Although the total fusion and step-heating experiments suggest some degree of age heterogeneity, the recurring theme is an age of around 64 to 66 Ma.

Deino, A. L.↗
NASA NTRS1990

A discussion of 'Anomalous quartz from the Roter Kamm impact crater, Namibia - Evidence for post-impact hydrothermal activity?'

This paper presents arguments against the statement made by Koeberl et al. (1989) to the effect that various differences between the quartz of the three quartz pebbles from the Roter Kamm impact crater (Namibia) and the quartz of the pegmatites present in the basement rocks of this crater can be best interpreted as evidence that the pebbles were formed (or 'recrystallized') by a post-impact hydrothermal system. Arguments are presented that suggest that the three quartz pebbles are, most likely, fragments of a preimpact vein quartz of hydrothermal origin.

Roedder, Edwin↗
NASA NTRS1990

Thematic Conference on Remote Sensing for Exploration Geology - Methods, Integration, Solutions, 7th, Calgary, Canada, Oct. 2-6, 1989, Proceedings. Volumes 1 & 2

Recent advances in airborne and satellite remote-sensing technology for application to geological exploration are discussed in reviews and reports. Sections are devoted to basement tectonics and their surface expressions, spectral geology, hydrocarbon exploration applications, radar applications and future systems, engineering and environment issues, geobotanical remote sensing, advanced image processing, data integration and mapping, and mineral exploration. Extensive diagrams, graphs, and sample images are provided.

Source record↗
NASA NTRS1991

Graben and extension in northern Tharsis, Mars

A morphometric analysis of north-south trending graben at 35 deg N in northern Tharsis is used to provide the first quantitative measurements of crustal extension in and strain for areas of the Martian crust that have undergone both extreme and mild deformation. These estimates also demonstrate the viability of the Viking data for making such quantitative assessment. Graben in the region accommodate about 8 km of post Early Amazonian east-west extension. This extension corresponds to a net regional strain of 0.45-0.61 percent; local strains vary from less than 1 percent to 5 percent. Extension is nonuniform and localized near 110 deg (south of Alba Patera and north of Ceraunius Fossae) because of reactivation of preexisting fractures in underlying basement and possibly due to superposition of stress fields from Tharsis and Alba Patera. A major mechanical discontinuity in the shallow crust, probably representing the base of or a strength contrast within the megaregolith, is observed. It occurs at a depth of 2 km on the profile flanks and is depressed to about 7.5 km near 110 deg due to loading of the crust by the volcano.

Plescia, J. B.↗
NASA NTRS1992

The Manson impact structure - Its contribution to impact materials observed at the Cretaceous/Tertiary boundary

The Manson impact structure (MIS) in Iowa is an excellently preserved complex crater that formed 65.7 Ma ago at the K/T boundary. Drill and seismic data have been used to identify three primary terranes within the 35-km diameter crater: (1) an outermost ring graben composed of listric normal fault blocks that structurally preserve Paleozoic and Cretaceous strata, impact ejecta, and possibly earliest Tertiary lake sediments; (2) a crater moat region of slumped and fallback materials overlain by Tertiary lake sediments in most areas; and (3) a central peak of uplifted basement rock capped in many areas by impact breccia. It is argued that concentrations of Ir at a K/T boundary exposure near Gubbio, Italy and clasts of glass reported from the K/T boundary in Haiti are consistent with possible production in the MIS.

Anderson, Raymond R.↗
NASA NTRS1992

Reflection spectra of shocked ordinary chondrites and their relationship to asteroids

Spectral reflectance measurements are conducted of a number of mineralogically well-characterized, shock-blackened ordinary chondrites exhibiting four types of shock-generated black features: (1) opaque melt shock veins, (2) melt pockets and irregular interconnected melt veins, (3) melt dikes, and (4) black chondrites. While their spectra resemble those of C asteroids, these materials are found in impact crater basements and floors rather than surfaces and are of low abundances; they therefore cannot be responsible for large-scale spectral alterations of the parent asteroids of ordinary chondrites, and offer no support for the supposition that ordinary chondrite asteroids are hidden among C asteroids.

Keil, Klaus↗
NASA NTRS1992

Sudbury project (University of Muenster-Ontario Geological Survey): Origin of the polymict, allochthonous breccias of the Onaping Formation

The Sudbury structure has been interpreted as a deeply eroded remnant of a peak-ring basin. The polymict, allochthonous breccias of the Onaping Formation (OF) occur in the central part of the Sudbury structure, which is surrounded by the 1.85-Ga-old 'Sudbury Igneous Complex' (SIC). From bottom to top the OF can be divided into Basal, Gray, Green, and lower and upper Black members. The breccias were mapped in detail in the east range of the structure. The SIC and the lower part of the OF (Basal Member) are interpreted as the impact melt system. The overlying Gray Member is a breccia unit with a clastic matrix and has a sharp contact to the Basal Member. The Green Member is considered as a continuous uniform breccia layer on top of the Gray Member and comprises the former 'chlorite shard horizon'. The uppermost unit of the OF (Black Member) can be subdivided into a lower and an upper Black Member unit. The lower part (100-150 m thick) still shows petrographic features of suevitic breccias, small fragments of basement rocks, melt particles, chloritized particles, and breccia fragments in a dark, clastic matrix.

Avermann, M. E.↗
NASA NTRS1992

Structural review of the Vredefort dome

The structure of the older-than-3.2-Ga Archean basement and Archean-to-Precambrian sedimentary/volcanic rocks (3.07 to ca. 2.2 Ga) in the center of the Witwatersrand Basin to the southwest of Johannesburg (South Africa) is dominated by the ca. 2.0-Ga megascopic Vredefort 'Dome' structure. The effect of the 'Vredefort event' is demonstrably large and is evident within a northerly arc of about 100 km radius around the granitic core of the structure. Northerly asymmetric overturning of the strata is observed within the first 17 km (strata is horizontal in the south), followed by a 40-km-wide rim synclinorium. Fold and fault structures (normal, reverse, and strike-slip) are locally as well as regionally concentrically arranged with respect to the northern and western sides of the structure. The unusual category of brittle deformation, the so-called 'shock deformation', observed in the collar strata has attracted worldwide attention over the past two decades. These deformation phenomena include the presence of coesite and stishovite, mylonites, and pseudotachylites, cataclasis at a microscopic scale, and the ubiquitous development of multiply striated joint surfaces (which include shatter cones, orthogonal, curviplanar, and conjugate fractures). The macroscopic to microscopic deformation features have led to the formulation of various hypotheses to account for the origin of the Vredefort structure: (1) tectonic hypotheses--deep crustal shear model, doming and N-directed thrust fault model, fold interference model, and diapir model; (2) the exogenous bolide impact hypothesis; and (3) the endogenous cryptoexplosion model.

Colliston, W. P.↗