Propagation history and passive rotation of mesoscale normal faults: implications for synrift stratigraphic development

Field data from onshore exposures of the Oligo-Miocene Gulf of Suez Rift in the Sinai document the passive rotation of early formed mesoscale synthetic and antithetic faults and associated half-graben due to long-lived activity on large displacement (2–5 km) block-bounding faults. Early formed small-displacement (<350 m) mesoscale antithetic faults and half-graben within regional-scale fault blocks underwent progressive steepening due to footwall uplift, rotational faulting and footwall flexing on large-displacement, block-bounding faults. In contrast, mesoscale synthetic faults were progressively rotated to shallower angles. Analysis of palaeohorizontal surfaces within synrift sediments deposited in half-graben adjacent to the mesoscale faults indicate passive rotations of up to 25° about horizontal axes since deposition. Passive burial and in-filling of early formed mesoscale faults and half-graben by synrift sediments is consistent with extension being transferred from numerous mesoscale faults to few block-bounding macroscale faults as extension preceded. Furthermore, this transfer of extension appears to be associated with a marked change in basin configuration, synrift sediment dispersal patterns and facies development. Identification of early formed, passively rotated normal faults and half-graben is important for correctly reconstructing the early stages of basin palaeogeography and sediment dispersal, and for addressing models of rift basin evolution.     

Visco-Resistive Dissipation in Strongly Driven Transient Reconnection

Solar flare energy release mechanisms often neglect the role played by viscous effects. Here we perform incompressible planar reconnection simulations, driven by the Orszag–Tang vortex, for both classical and Braginskii forms of the viscosity. We show that strongly driven “saturated” flux pile-up current layers, which lead to weak reconnection rates at small resistivities, are accompanied by invariant global viscous losses. These results support the notion that viscous dissipation in flaring plasmas can account for a significant fraction of the flare energy release.     

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas (E 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos²⁵ α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of 10⁻⁴μAm⁻². The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.     

Reseeding of early earth by impacts of returning ejecta during the late heavy bombardment

Mounting attention has focused on interplanetary transfer of microorganisms (panspermia), particularly in reference to exchange between Mars and Earth. In most cases, however, such exchange requires millions of years, over which time the transported microorganisms must remain viable. During a large impact on Earth, however, previous work (J.C. Armstrong et al., 2002, Icarus 160, 183-196) has shown that substantial amounts of material return to the planet of origin over a much shorter period of time (< 5000 years), considerably mitigating the challenges to the survival of a living organism. Conservatively evaluating experiments performed [by others] on Bacillus subtilis and Deinococcus radiodurans to constrain biological survival under impact conditions, we estimate that if the Earth were hit by a sterilizing impactor ∼ 300 km in diameter, with a relative velocity of 30 km s⁻¹ (such as may have occurred during the Late Heavy Bombardment), an initial cell population in the ejecta of order 10³-10⁵ cells kg⁻¹ would in most cases be sufficient for a single modern organism to survive and return to an again-clement planet 3000-5000 years later. Although little can be said about the characteristics or distribution of ancient life, our calculations suggest that impact reseeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.     

A 1 Gyr climate model for Mars: new orbital statistics and the importance of seasonally resolved polar processes

New results from a 1 Gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long-term atmospheric pressure evolution. We present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the Tharsis uplift, and explore the long term effects on the martian climate. We find that seasonal polar cycles have a critical influence on the ability for the regolith to release CO₂ at high obliquities, and find that the atmospheric CO₂ actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form. At low obliquity, the formation of massive, permanent polar caps depends critically on the values of the frost albedo, A_frost, and frost emissivity, ?_frost. Using our model with values of A_frost=0.67 and ?_frost=0.55, matched to the NASA Ames General Circulation Model (GCM) results (Haberle et al., 1993, J. Geophys. Res. 98, 3093-3123, and Haberle et al., 2003, Icarus 161, 66-89), we find that permanent caps only form at low obliquities (<13°), suggesting that any permanent deposits on the surface of Mars today may be residuals left over from a period of very low obliquity, or are the result of mechanisms not represented by this model. Thus, contrary to expectations, the martian atmospheric pressure is remarkable static over time, and decreases both at high and low obliquity. Also, from our one billion year orbital model, we present new results on the fraction of time Mars is expected to experience periods of low obliquity and high obliquity.     

How large is Aroostook County? Exploring the historical mutability of US county area measurements

Area measurements play a prominent role in many types of geographical and historical analyses. The accuracy and precision of such measurements, however, are dependent on a host of input factors, including the type of instrumentation used as well as the scale and projection of the source map and its level of cartographic generalization. As a consequence of variability in these inputs, published area measures change over time. This paper reviews the record of land area measurements for counties published by the US Census Bureau between 1880 and 2000, documents the major changes in measurement instrumentation and procedures employed, and analyzes patterns of the greatest variability in reported values. Two states are used as case studies (Iowa and Maine). The results suggest that the greatest variability occurs in those areas with irregular borders, thus reflecting the interaction of the source maps with human measurement error and the encoding capabilities of the area measurement instruments. The observations also indicate that variability in area measurements for counties has decreased over time.     

SHRIMP U–Pb zircon geochronology and Nd signatures of supracrustal sequences and orthogneisses constrain the Neoproterozoic evolution of the Pernambuco–Alagoas domain,southern part of Borborema Province,NE Brazil

The Borborema Province is the western part of a major Brasiliano belt that extends from Brazil through NW Africa in pre-drift reconstructions. This province resulted from convergence and collision among the West African, Congo-São Francisco, and Amazonian cratons about 600 Ma. This study focuses on the Pernambuco–Alagoas (PEAL) domain, which is a complex of magmatic, migmatitic, and metamorphic rocks, located in the southern part of the Borborema Province. U–Pb geochronology and Sm–Nd data for metasedimentary sequences (Rio Una, Inhapi) of the PEAL domain and a sample from a sequence of the Transversal Zone domain suggest that their deposition occurred during a Cryogenian extensional event, within the interval 850–631 Ma (or slightly younger). This extensional event occurred in the PEAL, Transversal Zone, and Sergipano domains before the onset of the Brasiliano collision and was followed by syn- and post-collisional magmatism. The Rio Una sequence and the sequence from Transversal Zone domain were deposited over a Rhyacian (ca. 2.0–2.2) basement having a juvenile Palaeoproterozoic Nd signature, whereas the Inhapi sequence was deposited over an Early Neoproterozoic (Tonian) basement. The deposition of the studied sequences is coeval with metasedimentary sequences to the north and south in other domains of the Borborema Province. However, differences in Nd isotopic signatures between the sequences from PEAL, Transversal Zone, and Sergipano domains suggest that they were formed in distinct basins. Metasedimentary sequences from the PEAL domain have Meso- and Palaeoproterozoic T _DM model ages. These data suggest that the orogens where the metasedimentary sequences are located have a strong ensialic component. T _DM model ages of ca. 1.0 Ga and εNd (0.6 Ga) values around zero recorded in granites from the southern part of the PEAL, suggesting that juvenile material was accreted to the southern part of the PEAL domain crust during the Tonian. The migmatites from the PEAL domain have a wide range of T _DM ages along with a wide range of εNd (0.6 Ga) values. The transition between the northern and southern parts of the PEAL domain denotes the approximate northern margin of a Late Mesoproterozoic to Early Neoproterozoic rift.