Experimental investigation of crater growth dynamics

This work is a continuation of an ongoing program whose objective is to perform experiments and to develop scaling relationships for large-body impacts onto planetary surfaces. The centrifuge technique is used to provide experimental data for actual target materials of interest. With both power and gas guns mounted on the rotor arm, it is possible to match various dimensionless similarity parameters, which have been shown to govern the behavior of large-scale impacts. The development of the centrifuge technique has been poineered by the present investigators and is documented by numerous publications, the most recent of which are listed below. Understanding the dependence of crater size upon gravity has been shown to be key to the complete determination of the dynamic and kinematic behavior of crater formation as well as ejecta phenomena. Three unique time regimes in the formation of an impact crater have been identified

Pre-Drift Extension of the Atlantic Margins of North America and Europe Based on Paths of Permo-Triassic Apparent Polar Wander

We reconstruct the relative configuration of North America and Europe prior to separation using paths of apparent polar wander (APW) for the interval 300 to 200 Ma. The Bullard et al. (1965) reconstruction closely superimposes the 300 Ma points on the two APW paths but leaves the 200 Ma points far apart. Conversely, anomaly-based reconstructions for later times approximately superimpose the 200 Ma ends of the paths but leave the older ends far apart. This indicates that separation of the interiors of the two continents began during the interval 300 to 200 Ma, long before surficial rifting commenced in the late Mesozoic. This in turn requires pre-rift extension in the two continental margins. Extension appears to have occurred in two phases of approximately equal magnitude but significantly different direction; the change in direction occurred at about 200 Ma. The earlier (300 to 200 Ma) episode of extension appears to have involved a strong element of sinistral shear. Based on our preferred reconstruction, the total amount of pre-rift extension of the two continental margins may have been as much as 1400 km

Directional properties of p-wave velocities and acoustic anisotropies in different structural domains of the Northern Barbados Ridge accretionary complex

Ocean Drilling Program (ODP) Leg 156 revisited the northern Barbados Ridge, where the previous Deep Sea Drilling Program Leg 78A and ODP Leg 110 studied the frontal part of this accretionary prism. Drilling and logging-while-drilling at Sites 947, 948, and 949 successfully identified major thrust faults and the décollement, which was the target of several downhole experiments. Two of the eight holes drilled were equipped with borehole observatories that will monitor temperature, pressure, and fluid flow over the next years. Coring at Hole 948C recovered 180 m of sediment, centered around the décollement, which was positively identified based on structural information. The aim of this study is the evaluation of the possible correlation of preferred orientation of acoustic properties and the direction of maximum compressive strain in the frontal part of the accretionary prism. For this purpose, shipboard P-wave velocities from Holes 948C and 949B were reoriented. This information was then used to compare the directional properties of accreted and subducted sediments. In Hole 948C, lowest transverse velocities (Tmin ) were observed to be consistently oriented perpendicular to the maximum horizontal compressive stress, believed to be parallel to the convergence vector. In the underthrust domain of Hole 948C, several preferred orientations for Tmin were detected, but no correlation with the geotectonic reference frame could be identified. Acoustic anisotropy does not show a comparable pattern in Hole 948C. It is concluded that the observed directional dependence of P-wave velocity in the accreted sediment domain in Hole 948B is the result of moderate to steeply inclined bedding, although this conclusion can not adequately be tested due to the lack of corrected structural data

Magnetic Anisotropy of Barbados Prism Sediments

Anisotropy of magnetic susceptibility (AMS) results from sediments spanning the basal décollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is strongly overpressured. In the accretionary prism above the décollement, the minimum AMS axes are subhorizontal and nearly east–west trending, whereas the maximum AMS axes are nearly north–south trending, and shallowly inclined. At the top of the décollement, the AMS minimum axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the décollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the décollement are consistent with lateral shortening caused by regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American Plate, and the maximum axes are trench-parallel. This abrupt change in AMS orientations at the top of the décollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments from the underthrust sediments. The decoupling horizon occurs at the top of the décollement zone, coinciding with the location of flowing, high-pressure fluids. Comparison with magnetic fabrics and susceptibilities of the seaward reference site (Site 672) indicates that the AMS fabrics at Sites 948 and 949 record the orientations of neocrystallized (Ti)magnetite and or (Ti)maghemite, and so reflect decoupling of differential stresses (and perhaps also strains) at the top of the décollement. Further comparisons of susceptibility stratigraphy between sediments just above the lithostratigraphic Unit III/Unit II boundary at Sites 672 and 948 suggest that the lower portion of the structurally defined décollement at Site 948 may in fact be largely intact. This suggests that (1) there may be little displacement accommodated by sediments below about 498 mbsf; (2) the deformation structures observed in most of the décollement may have formed via low total strains (but perhaps under high strain rates?); and (3) the basal décollement of the Barbados prism is a narrow plane (490-492 mbsf), along which stresses are very effectively decoupled, rather than a thick zone of distributed deformation

Paleomagnetism and Rotation History of the Blue Mountains, Oregon, USA

An important element in reconstructions of the Cordilleran margin of North America includes longstanding debate regarding the timing and amount of rotation of the Blue Mountains in eastern Oregon, and the origin of geometric features such as the Columbia Embayment, which was a subject of some of Bill Dickinson’s early research. Suppositions of significant clockwise rotation of the Blue Mountains derived from Dickinson’s work were confirmed in the 1980s by paleomagnetic results from Late Jurassic–Early Cretaceous plutonic rocks, and secondary directions from Permian–Triassic units of the Wallowa–Seven Devils arc that indicate ~60° clockwise rotation of the Blue Mountains. This study reports new paleomagnetic data from additional locations of these Late Jurassic–Early Cretaceous plutonic rocks, as well as Jurassic sedimentary rocks of the Suplee-Izee area. Samples from three sites from the Bald Mountain Batholith, two sites from small intrusive bodies near Ritter, Oregon, and six sites from the Wallowa Batholith have well-defined magnetization components essentially identical to those found by previous workers. The combined mean direction of both sets of data from these Late Jurassic to Early Cretaceous intrusive rocks is D = 30, I = 63, α95 = 6°. Samples from Jurassic sedimentary rocks in the Suplee-Izee area include four sites of the Lonesome Formation, three sites of andesitic volcanics in the Snowshoe Formation, and three sites from the Trowbridge Formation. The Lonesome and Trowbridge samples all had very well-defined, two component magnetizations. The in-situ mean of the combined Lonesome and Trowbridge Formations is D = 28, I = 63, α95 = 15°. Upon tilt-correction, the site means of these units scatter and fail the paleomagnetic fold test in spectacular fashion. The similarity between the directions obtained from the remagnetized Jurassic rocks, and from the Late Jurassic to Early Cretaceous plutonic rocks suggests that a widespread remagnetization accompanied emplacement of the intrusives. Similar overprints are found in Permian and Triassic rocks of the Blue Mountains. Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k= 26, α95 = 3.7°. Comparing the directions with North America reference poles, a clockwise rotation of 60° ± 9° with translation of 1000 ± 500 km is found. Together with data from Cretaceous and Eocene rocks, clockwise rotation of the Blue Mountains has occurred throughout the past ca. 130 Ma, with long-term rotation rates of 0.4 to 1 °/Ma. Approximately 1000 km of northward translation also occurred during some of this time

ODP Leg 146 Examines Fluid Flow in Cascadia Margin

For the better part of 2 decades, it has been known that dewatering of sediments accreted to or subducted beneath accretionary wedges is a fundamental aspect of the subduction-accretion process. Yet, evidence for fluid flow in modern accretionary wedges is largely secondary and based on the presence of geochemical and/or thermal anomalies [e.g., Vrolijk et al., 1991]; the analysis of seismic velocity as an indicator of porosity, which suggests a progressive loss of pore volume in a landward direction [e.g., Bray and Karig, 1985]; and the occurrence of secondary sediment microstructures characteristic of fluid movement [e.g., Maltman et al., 1992]. The only quantitative measurements of fluid expulsion at surface vents are based on submersible-deployed, seepage-meter data [e.g.,Carson et al., 1990], and these results—coupled with the surface area of the vents—indicate flow rates significantly greater than can be supported by steadystate dewatering [Le Pichon et al., 1992]. The fluid budgets and mass fluxes associated with accretion are poorly constrained. Results of previous drilling suggest two distinct modes of fluid flow: channelized flow along fault zones (primarily the décollement [e.g., Mascle and Moore, 1990]) or diffuse flow, which is apparently accommodated by a pervasive fracture permeability [e.g.,Taira et al., 1992]

Searching for Saturn's Dust Swarm: Limits on the size distribution of Irregular Satellites from km to micron sizes

We describe a search for dust created in collisions between the Saturnian irregular satellites using archival \emph{Spitzer} MIPS observations. Although we detected a degree scale Saturn-centric excess that might be attributed to an irregular satellite dust cloud, we attribute it to the far-field wings of the PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such radial distances, and we expect PSF characterisation to be the main issue for future observations that aim to detect such dust. The observations place an upper limit on the level of dust in the outer reaches of the Saturnian system, and constrain how the size distribution extrapolates from the smallest known (few km) size irregulars down to micron-size dust. Because the size distribution is indicative of the strength properties of irregulars, we show how our derived upper limit implies irregular satellite strengths more akin to comets than asteroids. This conclusion is consistent with their presumed capture from the outer regions of the Solar System.Comment: accepted to MNRA

Prostatic diseases: is it toxic or genetic? Study of TNF alpha gene polymorphism and cigarette smoking in cases of prostate cancer and benign prostatic hyperplasia

Inflammation has been implicated as an etiological factor, in several human cancers. Allelic variants of the genes involved in inflammatory pathways are logical candidates as genetic determinants of prostate cancer risk. Besides genetic factors, environmental factors such as smoking are an important risk factor for prostate cancer. This study aimed to investigate whether 308 G/A single nucleotide polymorphism of tumor necrosis factor-α (TNF-α) gene promoter region was associated with outcomes of prostate cancer and to analyze the gene environment interaction between 308 G/A TNF polymorphism and cigarette smoking. A total of 282 patients with prostate cancer (143 smokers, 139 non smoker) and 212 patients with benign prostatic hyperplasia (105 smokers, 107 non smokers) along with 115 healthy control were enrolled in the study. Urinary Cotinine and serum TNF and PSA levels were measured using ELISA technique. TNF genotyping was performed using PCR-RFLP technique. Prostate cancer was significantly associated with TNF G/G genotype and this is accompanied by elevated plasma TNF, PSA and urinary Cotinine. Cancer smokers showed a high frequency of TNF-α 308 G allele compared with other patient groups associated with increased TNF levels. Results of this study support the hypothesis that polymorphism in proinflammatory genes may be important in prostate cancer development and the sequence variants in these inflammatory genes may interact with environmental modifiers such as cigarette smoking to increase prostate cancer risk

Collisional dust avalanches in debris discs

We quantitatively investigate how collisional avalanches may developin debris discs as the result of the initial break-up of a planetesimal or comet-like object, triggering a collisional chain reaction due to outward escaping small dust grains. We use a specifically developed numerical code that follows both the spatial distribution of the dust grains and the evolution of their size-frequency distribution due to collisions. We investigate how strongly avalanche propagation depends on different parameters (e.g., amount of dust released in the initial break-up, collisional properties of dust grains and their distribution in the disc). Our simulations show that avalanches evolve on timescales of ~1000 years, propagating outwards following a spiral-like pattern, and that their amplitude exponentially depends on the number density of dust grains in the system. We estimate a probability for witnessing an avalanche event as a function of disc densities, for a gas-free case around an A-type star, and find that features created by avalanche propagation can lead to observable asymmetries for dusty systems with a beta Pictoris-like dust content or higher. Characteristic observable features include: (i) a brightness asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open spiral or a lumpy structure in the case of face-on orientation. A possible system in which avalanche-induced structures might have been observed is the edge-on seen debris disc around HD32297, which displays a strong luminosity difference between its two sides.Comment: 18 pages, 19 figures; has been accepted for publication in Astronomy and Astrophysics, section 6. Interstellar and circumstellar matter. The official date of acceptance is 29/08/200