Collision Site Research Articles

CRYSTALLIZATION OF AMORPHOUS ALLOYS BY HIGH ENERGY MECHANICAL DEFOFMATION

High energy ball milling has been used extensively in the last few years to produce amorphous metallic alloys by mechanically alloying pure crystalline elements or by high energy mechanical deformations of intermetallic compounds. The structural transformations occurring during the milling process is usually ascribed to a local effective temperature at the collision sites. At too elevated milling intensities, the effective temperature would be such that the precipitation of intermetallic compounds would occur/1. In this paper, the process of ball milling is used to crystallize already amorphous iron based alloys. The structural stability of the amorphous alloys against a thermal and mechanical deformation process is compared and the results indicate that the local effective temperature at the collision site is not the important parameter that controls the crystallization process during the deformations.

Northern Papua New Guinea: Structure and Sedimentation in a Modern Arc-Continent Collision: ABSTRACT

Northern Papua New Guinea and the Solomon Sea are the site of a modern oblique, arc-continent collision, which is progressing from northwest to southeast. By combining offshore seismic data from the Solomon Sea with geologic mapping in the Markham Valley area of northern Papua New Guinea the authors are predicting the outcome of this collision. The Huon Gulf is the present site of initial collision. Seismic profiles show this area is dominated by thin thrust sheets. Onshore, the bulk of the uplifted accretionary wedge is a melange with exotic blocks of a variety of lithologies. Structurally below the melange lies the Leron Formation composed of thick channelized sandstone and conglomerate. It dips north at approximately 40{degree} and is cut by several thrust fault with associated folds. Limestone blocks within the melange are reported to be 2 Ma, and Beryllium 10 anomalies from Bismarck arc volcanoes suggest that initial collision of the Finisterre block (375 km northwest of the present collision point) began no earlier than 3 Ma. This implies the collision is propagating laterally at about 125 km/m.y.. Large outcrops of basalt and gabbro within the melange suggest that segments of oceanic crust were incorporated into the accretionary wedge. Modernmore » sedimentation within the collision zone grades from fluvial sediments in the Markham Valley to deep-water turbidites ponded behind a structural ridge near the point of incipient collision. The Markham submarine canyon occupies the collision front here, and efficiently erodes the accretionary wedge. This setting may serve as a modem analog for deposition of much of the Leron Formation which exhibits tremendous sediment reworking.« less

Interdiffusion during the formation of amorphous alloys by mechanical alloying.

By probing the Ni content of Zr crystallites as a function of milling time using Auger spectroscopy, we report the first direct measurements of the interdiffusion which takes place during the amorphization of Ni-Zr by the mechanical alloying of pure-metal powders. Based on the concentration variation of Ni in Zr as a function of the milling time and using a simple interdiffusion model, we can estimate an effective local temperature at the collision site.

Ridge collision at convergent margins: implications for Archean and post-Archean crustal growth

Growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion (lateral and vertical) related to 1. (1) steady-state subduction, 2. (2) non-steady-state, episodic, processes. Ridge-forearc collision is one episodic process that can cause significant, highly diachronous, thermal and tectonic effects when a buoyant, bathymetrically-high and thermally-active oceanic spreading ridge approaches, collides with and potentially subducts below a continental margin. Near-field effects (within ~ 100 km of collision site) in the forearc may include anomalous heat, anomalous magmatism, ophiolite obduction, forearc deformation and tectonic erosion of the base of the forearc. Far-field effects generally postdate collision and may include changes in the thermal structure of the continent, changes in the composition of arc magmas, a reduction or hiatus in arc magmatism, uplift and/or subsidence of the continental margin, marginal basin formation, underplating of subducted slabs and intraplate deformation. Effects of ridge collision can vary depending on the triple junction geometry. The Chile margin triple junction (CMTJ), where the actively spreading Chile ridge is colliding with the South American forearc, is the best modern example of ridge collision. Near-field effects of Pliocene-Recent ridge collisions near the CMTJ include: 1. (1) emplacement of the Pliocene Taitao ophiolite less than 15 km from the trench 2. (2) intrusion of Pliocene granitic plutons and 3. (3) forearc deformation (formation of a pull-apart basin on the continental shelf, and late Tertiary (?)-Recent movement on a major fault along the arc-forearc boundary). Far field effects include reduction in subduction-related seismicity and volcanism, chemical changes in arc magmatism, and possibly uplift and/or subsidence. Ridge-collison features in southern Chile are similar to features of Archean greenstone belts, suggesting that some greenstone sequences may have formed in a ridge-collision setting and that ridge collision may have been an important process in Archean crustal growth. These features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin (mafic-ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc-alkaline and MORB character, intercalated with volcaniclastic sedimentary rocks). Differences between Archean and Phanerozoic tectonic processes, mainly related to higher heat production in the past, probably have affected the nature and importance of ridge collision in Archean crustal growth. During the Archean, greater ridge length and smaller plates probably resulted in more ridge collisions and related processes, contributing to a higher continental growth rate. In particular, underplating of subducted slabs, and direct slab melting may have been important processes in Archean continental growth, but are of relatively minor importance today.

Activation mapping of reentry around an anatomic barrier in the canine atrium. Observations during entrainment and termination.

We determined total right atrial activation sequences during entrainment and termination of flutter induced in dogs with a surgically induced atrial lesion. This type of atrial flutter is due to circus movement of an impulse around the tricuspid valve orifice. We recorded simultaneously from 96 bipolar intracavity electrodes in the right atrium of the isolated, perfused heart. By constructing isochronal maps, we demonstrated the pattern of atrial activation during atrial pacing protocols that either entrained or entrained and then terminated the reentrant rhythm. We show that during pacing the antidromic wavefront from the paced impulse (An) collides with the orthodromic wavefront from the previous paced impulse (On-1). During entrainment, the site of collision of the orthodromic and antidromic wavefronts was constant during pacing at a fixed rate but shifted in the antidromic direction as the pacing rate increased. Furthermore, the last paced beat was entrained only up to the site of collision of the previous paced beat. During one period of entrainment, termination of the reentrant arrhythmia occurred because On-1 blocked in the reentrant pathway due to refractory tissue left by On-2. However, subsequent An did not collide directly with On as was expected, but rather On blocked by an interaction with tissue left refractory by An. Because On was blocked, no reentry occurred when pacing ended.

Tectonic Evolution and Structural Styles of Cenozoic Basins around the Taiwan Area

The ocean margin island of Taiwan is a geodynamic body of young and complex build It is the site of an ongoing arc-continent collision between the Eruasian plate and the Phillippine Sea plate. The most manifest tectonic movements responsible for the deformation of this island arc are tephrogenic, collisional, and wrenching. These movements played an important role in the Cenozoic tectonic evolution of Taiwan and formed various types of hydrocarbon accumulated geologic struc­ tures. The continental margin crust around the Taiwan area was attenuated by rifting and developed north-northeast-trending Cenozoic basins. These basins have various graben-horst structures and are bounded by large faults. Extensional structure styles have prevailed in Cenozoic basins in offshore Taiwan, with listric faults, and tilted-biQ!:ks in the basement, and detached faults, growth faults, and roll-over anticlines or drape fold in the cover rocks. However, in onshore Taiwan, with the collision of Luzon island arc, those basins show a strong· response to the stress imposed upon the Cenozoic rocks by this Tertiary continent-arc collision. The collision remobilized the Mesozoic tectonized basement rocks and refolded them on various scales. Compressional and wrenching deformations are thus prevailed, with thrust faults, right-slip reverse faults and en echelon arranged folds in the cover rocks. The Cenozoic basins around Taiwan area have undergone different tectonics. In onshore areas the extensional structures have been changed into contractional structures. In offshore areas, those Cenozoic basins have extensional geologic structures until today. Since different tectonic evolution types and different structural styles in different tectonic levels, these Cenozoic basins around Iaiwan area have complicated hydrocarbon accumulations.

Isotope geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud

Geochronological data from the Golmud —Lhasa section across the Tibetan Plateau indicate progressively younger periods of magmatism from north to south associated with successively younger ocean closures. Pre -collision Eocene magmatism (50—4 0 Ma) exposed along the southern margin of the Lhasa Terrane in the Gangdise Belt resulted from anatexis of mid -Proterozoic crust (~ 1000 Ma) at depths greater than 10 km, but at higher crustal levels subduction-related intrusions were predominantly mantle-derived with ~ 30 % crustal assimilation . Intrusions from the northern Lhasa Terrane are early Cretaceous in age (130 —110 Ma). These form a bimodal suite comprised of two-mica granites derived from anatex is of Mid -Proterozoic crust and of biotite -hornblende granodiorites from about 60 % crustal assimilation by mantle magmas above a post-collision subduction zone. They place a minimum constraint on collision between the Lhasa and Qiangtang Terranes of 130 Ma . Granite magmatism from the Kunlun Mountains is late P ermian -early Jurassic in age (260—190 Ma). The Kunlun batholith represents reworked mid-Proterozoic crust (1400 —1000 Ma) at an active continental margin from 260 —2 4 0 M a . Post-tectonic granites were emplaced in a post-collision setting (200 -190M a). Collision between the Qiangtang and Kunlun Terranes is dated as end -Triassic. Nd model ages of sediments from across the plateau record up lift and erosion of young source regions throughout the Phanerozoic confirming that the Tibetan Plateau is the site of multiple continental collision through time. Phanerozoic magmagenesis throughout the plateau requires considerable crustal reworking and limited crustal growth which suggests thickened continental crust in the region may predate the most recent Eocene collision.

Early Paleozoic metamorphism at two crustal levels and a tectonic model for the Pennsylvania-Delaware Piedmont

Two high-grade metamorphic terranes of early Paleozoic age are exposed in southeastern Pennsylvania and northern Delaware. The Wilmington Complex in Delaware was metamorphosed at high temperatures to intermediate-pressure granulite facies during the early Paleozoic. In the West Chester prong, Pennsylvania, a granulite-facies metamorphism of Grenville age was overprinted by a high-pressure, moderate-temperature metamorphism during the Taconic orogeny. We propose a model to account for the different metamorphic conditions in these two terranes. In this model, the Piedmont of northern Delaware and southeastern Pennsylvania was the site of collision between a magmatic arc and the North American continent along an east-dipping subduction zone. The Wilmington Complex was the infrastructure of the arc, whereas the Grenville-age gneisses of the West Chester prong were basement of the North American continent. Between these two relatively rigid plates, there are nappes of remobilized basement and highly deformed forearc-basin and accretion-ary-prism sediments. The North American continent underthrust the hot infrastructure of the volcanic arc, resulting in the high-pressure (9-11 kb), moderate-temperature (650–700 °C), metamorphic overprint on Grenville assemblages of the West Chester prong. The Wilmington Complex was metamorphosed at moderate pressure (6–7 kb) and high temperature (∼800 °C) during the early Paleozoic, under the conditions of the high geothermal gradient that existed at the base of the volcanic arc. The intervening sediments and remobilized basement are all at amphibolite grade, but the grade is highest (above second sillimanite isograd) everywhere adjacent to the structurally overlying Wilmington Complex, suggesting that the isograds in the sediments are inverted beneath the Wilmington Complex. Uplift of this doubled thickness of crust was rapid enough that the gneisses of the West Chester prong were never heated above 700 °C.

A synthesis of the geologic evolution of Taiwan

The island arc of Taiwan is composed of Cenozoic geosynclinal sediments more than 10,000 m thick, lying on a pre-Tertiary metamorphic basement. Pleistocene to Miocene andesitic islands surround the main island and are related mostly to arc magmatism. The Penghu Island Group in the Taiwan Strait is covered with Pleistocene flood basalt. Neogene shallow marine clastic sediments are exposed mainly in the western foothills with Pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands. A thick sequence of Paleogene to Miocene argillitic to slaty metaclastic rocks underlies the western Central Range and forms the immediate sedimentary cover on the pre-Tertiary metamorphic complex to the east, which represents an older Mesozoic arc-trench system. The Coastal Range in eastern Taiwan is a Neogene andesitic magmatic arc, including also a large variety of volcaniclastic and turbiditic sediments. Cenozoic Taiwan is the site of arc-continent collision where the Luzon arc on the Philippine Sea plate overrides the Chinese continental margin on the Eurasian plate. East and northeast of Taiwan, the polarity of subduction changes whereby the oceanic Philippine Sea plate is subducting beneath the Ryukyu arc system on the Eurasian plate. Continent-arc collision in Taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well-defined plate boundary or subduction zone.

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

Abstract A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo‐silicate mineralogy, illite crystallinity and bo parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low‐pressure facies series type, with a geothermal gradient of ∼ 40°Ckm‐1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth‐related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn‐tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote‐actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite‐pumpellyite assemblages in metabasites and it is suggested that the latter represent non‐buffered, and therefore non‐diagnostic, assemblages.

圧密を伴う造粒のコンピューターシミュレーション

Computer simulation of agglomeration with compaction was carried out using a random addition model, which defined the probability of adhesion of particles at their collision sites as a function of the product of reciprocals of their radii. The agglomeration process simulated by this model was described by a non-random coalescence agglomeration. With as agglomeration proceeded, the outer surface of the agglomerate was compacted more closely than the inside one. It was found that the compaction process of the agglomerate was represented by a modified Kawakita's equation.

Action potential collision in heart tissue--computer simulations and tissue experiments.

The electrical effects of action potential collision were studied using a computer simulation of one-dimensional action potential propagation and tissue experiments from isolated cardiac Purkinje strands and papillary muscles. The effects of collision, when compared to normal one-way propagation, included quantitative changes in all of the measured indexes of action potential upstroke and repolarization. These changes can be attributed to spatiotemporal changes in the net membrane current. Parameter sensitivity and analytic techniques identified five factors which determine the collision-induced decrease in action potential area: conduction velocity, action potential height, cable radius, specific intracellular resistivity, and the specific membrane resistance during action potential repolarization. The simulations demonstrated that collision effects were independent of inhomogeneity in action potential duration, the spatial extent of the collision effects was greater than the passive space constant, and certain simulated abnormal conditions (e. g., discontinuous propagation, ischemic tissue) increased the magnitude of the collision effects. The tissue experiments supported the simulations regarding the changes in action potential configuration directly at and on each side of the collision site. Elevated [K+]0 increased the changes in action potential duration in both tissue preparations. In papillary muscles, collision effects in the transverse direction were confined to a narrower region than collision effects in the longitudinal direction with no difference in the peak magnitude of the changes. Action potential collision is a common occurrence in the heart.

Analysis of Deer-Vehicle Collision Sites in Pennsylvania

Nineteen habitat and highway characteristics thought to influence numbers of white-tailed deer (Odocoileus virginianus) vehicle collisions along Pennsylvania two-lane highways were tested for inclusion in a model used to predict probabilities of sections of highways being high deer kill sites. Two variables (in-line visibility and non-wooded) increased the probability of a section of highway being a high kill site. Seven variables (residences, commercial buildings, other buildings, shortest visibility, speed limit, distance to woodland, and fencing) decreased this probability. Removal of the variables speed limit and other buildings did not significantly change the model. The model showed strong discrimination between high kill and low kill (control) sections of highway.

Importance Function Biasing of the Deep Penetration Monte Carlo Calculation

The formal basis of the use of calculated importance information for biasing forward and adjoint Monte Carlo deep penetration sheilding problems is presented. The distinction between the ''point value'' and ''event value'' functions for adjoint problems is discussed. The analysis reveals that the emergent particle density, and not the particle flux density, is the proper choice of biasing function for the selection of the adjunctor's next collision site. This is analogous to the choice of the event value as the value function for the biased selection of the next collision site in the forward analysis. A numerical illustrative problem consisting of a concrete cylinder with an axial duct, a plane source on the bottom surface, and four joint detectors outside the emergent top surface is used to demonstrate this theory.

Atrioventricular sequential pacing: differential effect on retrograde conduction related to level of impulse collision.

Patients with DDD pacemakers who have intact retrograde conduction are known to be at risk of developing ventricular and "endless loop" tachycardia. To address this problem, a pacing protocol was designed in which V2A2 conduction was assessed in 16 patients during ventricular pacing alone (standard method) and during paced atrioventricular (AV) sequential drive (AV sequential method); the results were then compared. In eight of 16 patients who had intact retrograde conduction with both methods (group 1), the V2A2 intervals were significantly shorter (by 60 to 340 msec) with the AV sequential method. In the remaining eight patients, who demonstrated V2A2 block with the standard method, no V2A2 block occurred with the AV sequential method. In this study, two sets of AV intervals were programmed to produce collision of the two impulses (atrial and ventricular), either in the AV node or the His-Purkinje system (HPS). The site of V2A2 facilitation was related to the site of impulse collision. These results can be explained by earlier excitation by the atrial impulse (of AV node and/or HPS) during AV sequential pacing. However, in some cases it was evident that antegrade propagation of the atrial impulse was responsible for subsequent facilitation. The data suggest that assessment of retrograde conduction in candidates for DDD pacemakers can be made most accurately by the AV sequential method.

Studies of Achieving Buildup Against Charge-Exchange Losses in a Mirror-Confined Plasma

Mirror plasma buildup via neutral beam injection into a small low-density target is strongly affected by plasma losses resulting from charge-exchange (CX) with cold neutrals entering via chamber structures. The influence of CX events extends beyond the collision site due to the large ion orbits typical of small mirror plasmas. This study examines effects of key parameters that influence plasma buildup, using a 2½-dimensional, energy-dependent finite gyroradius model. Results presented for a 2X-like plasma show that buildup occurs if a “critical density” is achieved before CX losses erode the central plasma region. An efficient way to attain this density is to position the injected beams so that trapped ions have orbits circling inward, toward the plasma center.

Clues in pedestrian-vehicle collisions and their significance for reconstruction of accidents

Investigations of clues in 87 fatal pedestrian-car accidents which took place with erect posture of the body are reported. Traces of paint proved to be most important clues transferred from the vehicle to the pedestrian's clothing. They derived from the outer paint layer of the collision sites at the front of the vehicle and occurred in 49% of the cases investigated. As has been shown by light and scanning electron microscopic investigations, there is intensive contact between the textile fibers and the paint traces which can be explained by action of heat in the collision. The higher the collision speed, the more readily paint is rubbed off. Because traces of rubbed off paint mark the site of the vehicle impact and indicate the color of the vehicle, they have a high evidence value for reconstruction of the accident. This also applies to traces of molten plastic material as well as to rubbed off chrome and rubber. In contrast to the prevalent view, paint splitters are unsuitable for determining the impact direction. Textile microtraces, traces of blood, hair and tissue occur as transferred clues from the pedestrian to the vehicle. These clues do not permit any statements with regard to the impact direction. They must be evaluated in connection with the remaining clues and the injuries.

An efficient method of randomly sampling the coherent angular scatter distribution

Monte Carlo simulations of photon transport phenomena require random selection of an interaction process at each collision site along the photon track. The possible choices are usually limited to photoelectric absorption and incoherent scatter as approximated by the Klein-Nishina distribution. Coherent scattering events are usually ignored. One reason for this omission is the lack of documentation in the medical physics literature concerning an efficient and accurate procedure for sampling the coherent angular scatter distribution. The authors describe a technique to accomplish this task. Although similar methods have been described elsewhere (Cashwell et al. 1973, Chan 1981), the authors feel this technique should be made more easily available to the medical physics community.

The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest–trench interactions

New magnetic anomaly identifications W of the Shackleton Fracture Zone show 5 spreading sections, separated by fracture zones. In the 2 most southerly, the ridge crest collided with a trench at the margin of the Antarctic Peninsula only 6.5 and 4 Ma ago, the latest of a series of collisions starting at the base of the peninsula SO Ma ago. Following each collision, spreading and subduction both stopped. Opposite the South Shetland trench and actively extending Bransfield Strait, spreading also stopped 4 Ma ago, but before the spreading centre reached the trench. The tendency of the subducting plate to continue sinking probably initiated the opening of the Bransfield Strait. The parallelism between fracture zone orientation on the descending plate and convergence direction, previously thought responsible for the tectonic segmentation of the peninsula, was effective for only 10–30 Ma before collision. Before that, either the fracture zones did not extend to the trench, or the subducting ocean floor formed at the Farallon–Phoenix (Nazca–Aluk) boundary, with fracture zones oblique to the subduction direction. The Aleutian Arc–Kula Ridge model, in which arc magmatism virtually ceases when ocean floor younger than 25–30 Ma occurs beneath the arc, fits the distribution of Antarctic Peninsula radiometric dates, explaining a 50–60 Ma gap between collisions and youngest ages in the S, and possibly the migration of the youngest activity towards the trench. Thus, gaps in the geologic record of arc magmatism need not imply cessation of subduction. The progressive steepening of the peninsular margin towards the 4 Ma collision site suggests tectonic erosion of the fore-arc as the ridge crest approached.

Thermal models for the magmatic accretion and subsequent metamorphism of continental crust

I present one-dimensional conductive relaxation models for the thermal evolution of continental crust which has undergone magmatic thickening in a predominantly recumbent tectonic regime. The sustained addition of magma, and associated heat, profoundly influences the conductive relaxation of the affected sialic crust. The mechanism of accretion, successively beneath (under-accretion), or successively above (over-accretion), previously added material, strongly affects the type of P- T-time history sustained by the rocks in the middle and deep crust. The pressure and temperature conditions which might be recorded by mineral assemblages equilibrating in such tectonothermal regimes may bear very little relation to conductive heat supplies from the upper mantle. P- T values obtained by geobarometry and geothermometry in such complexes must be used with caution to constrain steady state heat flows and geothermal gradients. In a contemporary plate tectonic context I consider the tectonothermal models developed here to apply particularly to island arcs and sites of continental collision associated with extensive plutonic igneous activity. Precambrian orthogneiss complexes, such as the Archaean of southern West Greenland, probably represent exhumed deep crustal elements involved in the intense magmatic thickening processes considered here.