Transient Cavity Research Articles

Origins of Outliers of the Huronian Supergroup within the Sudbury Structure

The spatial relations between outliers of Huronian metasediment and the current outline of the Sudbury Igneous Complex (SIC) have been used previously to constrain the original size of the meteorite impact crater, despite uncertainty regarding their structure and origin. The outliers are tight to isoclinal, gently plunging synclines with northeast‐southwest‐trending axes, slightly overturned to the northwest. Major deformation of the Huronian Supergroup preceded the 1849‐Ma Sudbury meteorite impact event by sufficient time to permit peneplanation of the region. Impact occurred in a foredeep related to the subduction of the Laurentia plate under the Wisconson Magmatic Terranes. Faults truncate the northwest margins of most of the outliers. Where examined, at several localities, the northwest margin–bounding faults are invariably occupied by pseudotachylite. Structural considerations provide evidence for thousands of meters of slip along these surfaces, each of which was apparently accomplished in a single event, providing strong support for the contention that motion along these faults coincided with the collapse of the transient cavity after bolide impact to form the Sudbury Structure. The outliers, thus, owe their overall forms to preimpact folding long before the Penokean Orogeny. They owe their continued existence to large‐scale normal fault motions related to collapse of the transient cavity during the Sudbury impact event. The current map‐scale structural relations between the outliers and the outcrop area of the Sudbury Igneous Complex are largely due to the deep exhumation of a series of regional open folds that were formed during postimpact deformation, possibly at the termination of the Penokean Orogeny ca. 1840 Ma or during a later event ca. 1450 Ma.

Geochemistry and shock petrography of the Crow Creek Member, South Dakota, USA: Ejecta from the 74‐Ma Manson impact structure

Abstract— The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale Formation of eastern South Dakota and northeastern Nebraska, but it is the only unit that contains shock‐metamorphosed minerals. The shocked minerals represent impact ejecta from the 74‐Ma Manson impact structure (MIS). This study was aimed at determining the bulk chemical compositions and analysis of planar deformation features (PDFs) of shocked quartz; for the basal and marly units of the Crow Creek Member. We studied samples from the Gregory 84‐21 core, Iroquois core and Wakonda lime quarry. Contents of siderophile elements are generally high, but due to uncertainties in the determination of Ir and uncertainties in compositional sources for Cr, Co, and Ni, we could not confirm an extraterrestrial component in the Crow Creek Member. We recovered several shocked quartz grains from basal‐unit samples, mainly from the Gregory 84‐21 core, and results of PDF measurements indicate shock pressures of at least 15 GPa. All the samples are composed chiefly of SiO2 (29–58 wt%), Al2O3 (6–14 wt%), and CaO (7–30 wt%). When compared to the composition of North American Shale Composite, the samples are significantly enriched in CaO, P2O5, Mn, Sr, Y, U, Cr, and Ni. The contents of rare earth elements (REE), high field strength elements (HFSE), Cr, Co, Sc, and their ratios and chemical weathering trends, reflect both felsic and basic sources for the Crow Creek Member, an inference, which is consistent with the lithological compositions in the environs of the MIS. The high chemical indices of alteration and weathering (CIA' and CIW': 75–99), coupled with the Al2O3‐(CaO*+Na2O)‐K2O (A‐CN'‐K) ratios, indicate that the Crow Creek Member and source rocks had undergone high degrees of chemical weathering. The expected ejecta thicknesses at the sampled locations (409 to 219 km from Manson) were calculated to range from about 1.9 to 12.2 cm (for the present‐day crater radius of Manson), or 0.4 to 2.4 cm (for the estimated transient cavity radius). The trend agrees with the observed thicknesses of the basal unit of the Crow Creek Member, but the actually observed thicknesses are larger than the calculated ones, indicating that not all of the basal unit comprises impact ejecta.

Transient and disruption cavity dimensions of complex terrestrial impact structures derived from magnetic data

Accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact. Transient and disruption cavity size can, in principle, be inferred from morphometric relationships based on crater diameter. However, locating the crater rim can be difficult for eroded terrestrial craters, and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin. Fortunately, magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character. A lower bound on this parameter can be defined by the outer limit of short‐wavelength, intense magnetic anomalies produced by impact melt and/or suevite deposits. An upper bound is given by the inner limit of magnetic anomaly trends associated with the pre‐impact target rock configuration. Using published values of crater diameters (D) and values of collapsed disruption cavity diameters (DCDC) derived from magnetic data for 19 complex terrestrial impact structures, we derive the relationship DCDC = 0.49D. These data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation.

Exploring ocean waves from asteroid impacts

An asteroid the size of the Roman Coliseum crashes into the ocean somewhere on Earth, producing a transient cavity miles across. The spectacular collapse launches tsunamis in all directions, casting the asteroid's impact energy far and wide. Everyone living on these shores may be in for a bad day.Based partly on the assumption that relatively minor asteroid impacts spawn dangerous tsunamis, the National Research Council has recommended that NASA and the National Science Foundation design and build a Large Synoptic Survey Telescope (LSST) to image the entire sky every 7 days and detect thousands of asteroids to the 24th magnitude, especially those down to 300‐m diameter on near‐Earth orbits. The LSST will have many astronomical uses apart from asteroid hunting; nevertheless, before $100 million to $200 million gets spent, one must examine the justifications. Are sub‐kilometer asteroids really a significant threat? If not, then existing survey telescopes are adequate to catalog −98% of the 1100±100 near‐Earth asteroids larger than 1 km diameter in the next few decades. Of these, half have been detected so far, and most of the rest have orbits that are harder to discover from Earth. Perhaps the LSST money should instead be dedicated to a space‐borne observatory for bagging “holdouts” larger than a kilometer, those capable of causing global catastrophe.

The Acraman asteroid impact, South Australia: magnitude and implications for the late Vendian environment

Acraman, located in the 1.59 Ga Gawler Range Volcanics on the Gawler Craton, South Australia, is a complex impact structure that is now eroded ≥2.5 km below the original crater floor. The geology, geomorphology, apatite fission-track geochronology, and geophysical signature of Acraman suggest that the original crater comprised highly disturbed rocks of a central uplift, a transient cavity up to c . 40 km in diameter, and a possible final structural rim at 85–90 km diameter. Radial unfolded distance from the centre of Acraman, versus decompacted thickness for the Acraman ejecta horizon identified in late Vendian ( c . 580 Ma) mudstone in the Adelaide fold belt, Torrens Hinge Zone and Officer Basin up to 540 km from the impact site, accords with a transient cavity diameter of c . 40 km. The estimated impact energy for Acraman exceeds the threshold of 10 6 Mt set by earlier workers for global catastrophe. The impact occurred at a low palaeolatitude ( c . 12.5°) and probably perturbed the atmosphere in both the northern and southern hemispheres. The Acraman impact probably caused a severe perturbation of the late Vendian environment, a finding consistent with independent data from the Vendian palynology of Australia that the Acraman impact induced a biotic crisis.

Asteroid impact tsunami of 2880 March 16

SUMMARY NASA scientists have given a 1.1-km diameter asteroid (1950 DA) a 0.0 to 0.3 per cent probability of colliding with the Earth in the year 2880. This article examines a scenario where 1950 DA strikes the sea 600 km east of the United States coast. Travelling at 17.8 km s−1, the asteroid would blow a cavity 19 km in diameter and as deep as the ocean (5 km) at the impact site. Tsunami waves hundreds of metres high would follow as the transient impact cavity collapses. The tsunami disperses quickly; but because the waves are so large initially, destructive energy carries basin-wide. Within two hours of the scenario impact, 100-m waves make landfall from Cape Cod to Cape Hatteras. Within 12 hours, 20-m waves arrive in Europe and Africa. Water velocity at the deep ocean floor exceeds 1 m s−1 to 800-km distance, strong enough to leave a widespread tsunami signature in the sedimentary record.

Crustal attributes of lunar basins from terrain‐correlated free‐air gravity anomalies

For the major lunar basins between ±64° latitude, uncompensated mass components as well as crustal ring and transient cavity attributes were evaluated from the free‐air anomaly components of the Lunar Prospector gravity model (LP75G) that are spectrally correlated at 100‐km altitude with the gravity effects of the Clementine terrain model (GLTM‐2). Inversion of the terrain‐correlated anomalies inferred incipient radial adjustments of the Moho that equilibrate the basin topography, assuming the lunar crust is mainly compensated by thickness variations. Nearly all nearside multiring basins are marked by mascon gravity anomalies that reflect mass concentrations from superisostatic mantle plugs plus mare fill. Farside basins, by contrast, are mostly characterized by maslite gravity anomalies from mass deficiencies due to subisostatic mantle plugs with marginal or no mare fill. The preponderance of maslites on the farside is consistent with crust that responded more rigidly to impacts than the nearside crust, which had a higher thermal gradient due to enhanced abundances of radioactive elements in the nearside mantle and crust. However, the extended thermal evolution of the lunar lithosphere also may have promoted the development of interfering mascon and maslite effects for tectonically complicated regions like the farside Freundlich‐Sharonov basin. The Moho adjustments also revealed well‐defined concentric zones of maxima and minima about the central basins that are strongly correlated with photogeologically inferred basin rings. In addition, the innermost zero‐contour of the radial adjustments provides an effective estimate for the diameter of the transient cavity. The inferred transient cavity diameters correlated negatively with crustal thickness for basins with superisostatic mantle plugs and positively with the relative ages of the nearside basins. On the farside, superisostatic mantle plugs apparently developed in thinner crust up to thicknesses of about 30 km, whereas in thicker crust mantle plugs developed only to subisostatic levels. For the nearside, mass balance calculations between the equilibrium mantle plug and excavated basin materials support the proportional scaling relation of roughly 0.1 for the excavation depth‐to‐diameter ratio of the transient cavity that photogeologic studies have obtained. However, for the Serenitatis and Imbrium basins with possibly atypical nearside crustal properties, this proportionality may overestimate excavation depths by 28% and 37%, respectively. For nearside and farside basins apparently lacking mare flooding, shallow excavation depths were also inferred.

Risk Assessment by Myocardial Perfusion Imaging for Coronary Revascularization, Medical Therapy, and Noncardiac Surgery

Stress myocardial perfusion imaging (MPI) has become an important tool in risk stratification of patients with known coronary artery disease. A normal myocardial perfusion scan has a high negative predictive value and is associated with low annual mortality rate (< 1%). Patients with extensive ischemia (> 20% of the left ventricle), defects in more than 1 coronary vascular territory, transient or persistent left ventricular cavity dilation, and ejection fraction less than 45% have a high annual mortality rate (> 3%). Those patients should undergo coronary revascularization whenever feasible, as the cardiac event rate increases in proportion to the magnitude of the jeopardized myocardium. Stress MPI can be used to demonstrate ischemia in patients with symptoms early after coronary artery bypass surgery (< 5 years) or in those without symptoms late (>/= 5 years) after coronary artery bypass surgery. With respect to patients who underwent percutaneous interventions, stress MPI can help detect in-stent restenosis early after the intervention (3-6 months) or assess the progression of native coronary disease afterward. Since preliminary data suggest that a reduction in the perfusion defect size may translate to a reduction of coronary events, stress MPI can help assess the efficacy of medical management of coronary disease. Finally, stress MPI is indicated for perioperative cardiac risk stratification for noncardiac surgery in patients with intermediate risk predictors (mild angina, prior myocardial infarction or heart failure symptoms, diabetes mellitus, renal insufficiency) and poor functional capacity or in those who undergo high-risk surgery with significant implications in further preoperative management.

Geology, Mineralization, and Emplacement of the Whistle-Parkin Offset Dike, Sudbury*

The Whistle-Parkin dike is a 12-km-long radial offset dike located in the northeast sector of the 1.85 Ga Sudbury impact structure. The dike is connected to the Sudbury Igneous Complex via a 0.5-km-long, 250-m-wide embayment. The Whistle segment of the dike narrows to a width of approximately 30 m and extends for about 1.5 km from the complex, where it is cut by the Post Creek fault zone. This fault displaces the dike 2 km to the northwest, beyond which it continues as the Parkin portion of the dike. The Parkin segment extends a further 10 km north-northeast. The dike and genetically associated embayment are comprised of numerous rock types. These include the sublayer, radial breccia, mafic sulfide-bearing breccia, inclusion-rich quartz diorite, and inclusion-poor quartz diorite. Economic Ni-Cu-PGE mineralization is found in association with the sublayer, mafic sulfide-bearing breccia, and inclusion-bearing quartz diorite phases. Rare earth element (REE) data reveal that the dike lithologies are representative of bulk Sudbury Igneous Complex melt. We propose a multistage emplacement mechanism for the dike. The breccia units, including the inclusion-bearing quartz diorite phase, were forcefully injected laterally into an impact-generated radial crack during the excavation stage of crater formation. Subsequent early modification processes facilitated the intrusion of inclusion-poor quartz diorite, particularly along the dike margins. Inclusion-poor quartz diorite emplacement may have been gravitationally driven from the overlying impact melt sheet. Later stage modification (i.e., final transient cavity collapse) caused decoupling of the Whistle and Parkin dike segments via faulting. This occurred concurrently with the settling of the sublayer unit and associated economic sulfides into the embayment and main dike segments.

Crustal properties of Mercury by morphometric analysis of multi‐ring basins on the Moon and Mars

Abstract— We use satellite altitude free‐air and terrain gravity correlations to differentiate regional variations in crustal viscosity and transient cavity diameters of impact basins on the Moon and Mars that we combine with surface roughness for a rheological assessment of the crust of Mercury. For the Moon and Mars, we separate the free‐air anomalies into terrain‐correlated and terrain‐decorrelated components using the spectral properties of the free‐air and computed terrain gravity effects. Adjusting the terrain effects for the terrain‐correlated anomalies yields compensated terrain effects that we evaluate for crustal thickness variations of the impact basins. We interpret the terrain‐correlated anomalies for uncompensated elements of the crustal thickness variations that we find are strongly correlated with the distribution of basin rings from photogeologic analyses. Hence, we estimate the transient cavity diameter from the innermost diameter of the gravity‐inferred rings. Comparing these diameters with the related crustal thickness estimates clearly differentiates regional variations in the crustal rheologies. For the Moon, the analysis points to a farside crust that was significantly more rigid than the nearside crust during bombardment time. For Mars, the growth in transient cavity diameters with apparent crustal age also reflects increased viscosity due to crustal cooling. These results are also consistent with local estimates of surface roughness developed from the root‐mean‐squared topography over 64 times 64° patches centered on the basins. Hence for Mercury where gravity observations are lacking, rheological inferences on its crust may result from comparing photometric estimates of transient cavity diameter and local surface roughness with the lunar and martian estimates. These results for the Beethoven Basin, a typical multi‐ring impact feature of Mercury, suggest that the viscosity of the mercurian crust was relatively great during bombardment time. This enhanced rigidity, despite crustal temperatures that were probably much hotter than those of the Moon and Mars, may reflect an extremely dry crust for Mercury in its early development.

Folding within seconds

Hypervelocity impacts of cosmic projectiles larger than ∼200 m diameter are capable of forming complex craters on Earth. At these craters, shock loading, shock damage, and excavation flow are followed by a gravity-driven collapse of the deep transient cavity. Such impact structures are characterized by a central uplift, a flat crater floor, and a terraced crater rim. Collapse-induced deformation features, like folds and brittle fault zones, have many similarities to tectonic structures. Typical deformation patterns of complex terrestrial impact craters of 5–15 km diameter are compiled and analyzed with respect to their kinematic development. Unlike their tectonic counterparts, deformation structures are always the result of non-plane-strain deformation and are formed in a single event that takes place in seconds to minutes. To understand the high-strain-rate processes, the microstructure of an impact-induced fold of the Crooked Creek impact crater (∼7 km diameter), Missouri, United States, is investigated in detail. A period of 20–30 s at the most is determined for the collapse phase of this crater. The gross plastic deformation behavior of the fold is achieved by localized brittle deformation along millimeter- to centimeter-spaced fault zones, forming a network of veins. Shock damage has fractured ∼40% of grain boundaries. The onset of collapse and associated deformation started in rocks with a reduced cohesion and is friction controlled.

Comparison of pulmonary uptake with transient cavity dilation after dipyridamole Tl-201 perfusion imaging

Background. Elevated lung-heart ratio (LHR) and transient ischemic dilation (TID) have been identified as markers of severe coronary artery disease after both exercise and pharmacologic stress testing. We have previously demonstrated a very weak correlation between elevated LHR and TID after exercise, which suggests that they reflect different pathophysiologic manifestations of coronary disease. Because the physiology of pharmacologic vasodilation with dipyridamole is significantly different than that of physical exercise, we undertook this study to evaluate the relationship between elevated LHR and TID after pharmacologic stress testing with dipyridamole. Methods and Results. We identified 1129 consecutive patients who underwent pharmacologic stress imaging with dipyridamole and thallium 201. LHR and a dilation index were calculated and compared with each other and with relevant clinical parameters. Echocardiographic parameters were also compared in a subset of 475 patients who had echocardiography within 2 weeks of pharmacologic stress testing. There was no significant correlation between elevated LHR and TID despite the fact that both were associated with more severe thallium stress and redistribution scores. Patients with elevated LHR were more likely to have a history of myocardial infarction and coronary artery bypass grafting and to have lower ejection fraction. Patients with TID were more likely to have a positive electrocardiographic response (15% vs 7%, P = .0003), which was not seen in patients with elevated LHR (11% vs 8%, P = .23). Conclusions. Although both elevated LHR and TID were associated with more severe coronary disease, they have no significant correlation. Patients with elevated LHR are more likely to have a history of myocardial infarction or coronary artery bypass grafting, a larger left ventricle, and lower ejection fraction. Our results support the hypothesis that TID is due to diffuse subendocardial hypoperfusion and represents a different pathophysiologic response to ischemia than elevated LHR. (J Nucl Cardiol 2002;9:47-51.)

Numerical modeling of impact-induced modifications of the deep-sea floor

Abstract The mechanics of oceanic impact events differ in several respects from the process that accompanies the strike of an asteroid on land. In order to explore the cratering process on a water-covered target, a series of 2D hydrocode simulations for impact velocities of 10, 15, 20 and 30 km/s and water depth/diameter ratios between 5 and 7 have been carried out. In particular, these simulations were aimed to examine the effects of the asteroid on the deep-sea floor. The interaction of the impacting body and the water column causes loss of the body's kinetic energy both through oceanic drag and through mass loss due to hydrodynamic heating, prior to hitting the ocean bottom. Regardless of whether the impactor actually hits the ocean floor, the shock wave generated in the water column by the impact of the body on the water surface will leave behind unique traces in the marine sediments. For example an asteroid of 1 km in diameter arriving at the ocean surface with a velocity of 20 km/s induces a shock pressure of 6 GPa in the oceanic crust at a water depth of 5 km . The passage of the shock wave along the water–ocean bottom interface causes significant disturbance in the sediment layer. Moreover, the collapse of the transient cavity that results from the passage of the projectile through the water column causes a strong surge of water that generates a displacement and redeposition of sediments on the ocean floor. In order to examine sediment transport processes, we introduce computationally a large amount of massless particles within the projectile and the target material, and follow the movement of each individual particle. Tracer particles represent only forced material movement and do not account for any frictional forces. Thus, comparisions with real sediment disturbances may be qualitative only. For an impact velocity of 20 km/s and a water depth/diameter ratio of 5, this procedure reveals significant particle movements within a distance of 15 km from the center of the impact. The only known deep-sea impact structure so far, the Eltanin impact in the Bellingshausen Sea, Antarctica, is characterized by a zone of chaotically mixed sediments that most probably originated from impact-induced turbulent water currents. Based on the present observations, which do not allow the identification of an impact structure at the ocean bottom, our model yields estimates of a projectile diameter of 1 km and an impact velocity of 20 km/s .

Damage and rock-volatile mixture effects on impact crater formation

We explored simple geologic strength and material response models to determine which have the capability to simulate impact-induced faulting, complicated ejecta patterns and complex crater shapes. This led us to develop models for material damage, dilatancy, and inhomogeneous materials (mixtures). We found that a strength degradation (damage) model was necessary to produce faulting in homogeneous materials. Both normal and thrust ring faults may occur and extend relatively deeply into the planet during the transient cavity radial expansion. The maximum depth of fault development is about the depth of maximum penetration by the projectile. Dilatancy in geologic materials may reduce the final bulk density compared to the pristine state because of irreversible fracturing. When we include the effects of dilatancy, the radial position of faulting is displaced because of greater upward motions. In addition, the late time crater profile is shallower and the expression of features such as central peaks and rings may be more pronounced. Both damage and rock-ice mixtures effect the distribution of ejecta. The excavation flow field within the heavily damaged region is similar to flow fields in Mohr-Coulomb materials with no zero-pressure strength. In the outer, less damaged zone within the excavation cavity, the material trajectories collapse back into the crater. This effect creates a zone of reduced ejecta emplacement near the edge of the final crater. In the case of rock-ice mixtures, energy is preferentially deposited in the more compressible volatile component and the ejecta pattern is dependent upon the location of shock-induced phase changes in the volatile material.

Deep crustal structure of the Chicxulub impact crater

We present the results of a wide‐angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx‐A/A1 is a chord across the offshore portion of the crater, and Chicx‐B/F is an onshore‐offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land‐based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low‐velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40–60 km is observed on Chicx‐B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx‐B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx‐A/A1 profile, with adjacent Moho deepening of ∼1.25–1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.

Very-low-density lipoprotein assembly and secretion.

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.

An adaptive boundary‐element approach for 3D transient free surface cavity flow, as applied to polymer processing

An adaptive (Lagrangian) boundary-element approach is proposed for the general three-dimensional simulation of confined free surface flow of viscous incompressible fluids. The method is stable as it includes remeshing capabilities of the deforming free surface, and thus can handle large deformations. A simple algorithm is developed for mesh refinement of the deforming free surface mesh. Smooth transition between large and small elements is achieved without significant degradation of the aspect ratio of the elements in the mesh. Several flow problems are presented to illustrate the utility of the approach, particularly as encountered in polymer processing. These problems illustrate the transient nature of the flow in the extrusion through circular and square dies, the filling of circular and square disks as in conventional injection molding, and the flow during gas-assisted injection molding inside a duct, with relevance to the important problem of viscous fingering. Copyright © 2001 John Wiley & Sons, Ltd.

Transient left ventricular cavity dilatation during stress echocardiography is a strong predictor of cardiovascular mortality Binoy K. Singh, J. Mahenthiran, Varinder P. Singh, Madhu Jyothinagaram, Manish Undavia, Siu-Sun Yao, Farooq A. Chaudry. Columbia University St. Luke's Roosevelt Hospital Center, New york, NY

Transient left ventricular cavity dilatation during stress echocardiography is a strong predictor of cardiovascular mortality Binoy K. Singh, J. Mahenthiran, Varinder P. Singh, Madhu Jyothinagaram, Manish Undavia, Siu-Sun Yao, Farooq A. Chaudry. Columbia University St. Luke's Roosevelt Hospital Center, New york, NY

An adaptive boundary-element approach for 3D transient free surface cavity flow, as applied to polymer processing

An adaptive (Lagrangian) boundary-element approach is proposed for the general three-dimensional simulation of confined free surface flow of viscous incompressible fluids. The method is stable as it includes remeshing capabilities of the deforming free surface, and thus can handle large deformations. A simple algorithm is developed for mesh refinement of the deforming free surface mesh. Smooth transition between large and small elements is achieved without significant degradation of the aspect ratio of the elements in the mesh. Several flow problems are presented to illustrate the utility of the approach, particularly as encountered in polymer processing. These problems illustrate the transient nature of the flow in the extrusion through circular and square dies, the filling of circular and square disks as in conventional injection molding, and the flow during gas-assisted injection molding inside a duct, with relevance to the important problem of viscous fingering. Copyright © 2001 John Wiley & Sons, Ltd.

Peak-ring formation in large impact craters: geophysical constraints from Chicxulub

A seismic reflection and three-dimensional wide-angle tomographic study of the buried, ∼200-km diameter, Chicxulub impact crater in Mexico reveals the kinematics of central structural uplift and peak-ring formation during large-crater collapse. The seismic data show downward and inward radial collapse of the transient cavity in the outer crater, and upward and outward collapse within the central structurally uplifted region. Peak rings are formed by the interference between these two flow regimes, and involve significant radial transport of material. Hydrocode modeling replicates the observed collapse features. Impact-generated melt rocks lie mostly inside the peak ring; the melt appears to be clast-rich and undifferentiated, with a maximum thickness of 3.5 km in the center.