Pressure–temperature–time evolution of the Central East Greenland Caledonides: quantitative constraints on crustal thickening and synorogenic extension

Whereas geologists have known for three‐quarters of a century that there was significant crustal thickening in the central East Greenland Caledonides, the crucial role of extensional faulting during Caledonian orogenesis has only been recognized during the past decade. In this paper, new petrographic and thermobarometric observations are presented from migmatitic metasedimentary gneisses of the Forsblad Fjord region (c. 72.5°N). Samples of the Krummedal Sequence, collected from the footwall of the upper of two significant splays of the main extensional fault system in the region—the Fjord Region Detachment (FRD)—enable us to establish a relative sequence of metamorphism. Our pressure (P)–temperature (T) results imply a clockwise loop in P–T space. As recorded by mineral assemblages in the Krummedal gneisses, prograde metamorphism involved a net increase of c. 4 kbar and 250 °C, with peak conditions of c. 10.5 kbar at 785 °C. Early burial and heating was followed by near‐isothermal decompression of 4.5 kbar, a process which is attributed to roughly 18 km of tectonostratigraphic throw on the upper splay of the FRD. Combining data reported here with the published data, it is estimated that the approximate tectonostratigraphic throw along the lower splay of the FRD was c. 16 km. In situ U–Th–Pb‐monazite electron microprobe dating suggests that the earliest phase of metamorphism recorded in the Krummedal Sequence gneisses of Forsblad Fjord occurred during the Caledonian orogeny. Furthermore, the combination of our new data with existing conventional TIMS U‐Pb and ⁴⁰Ar/³⁹Ar data imply that: (1) movement along the uppermost splay of the FRD (c. 425–423 Ma) occurred at maximum time‐averaged slip‐rates equivalent to c. 9 mm of vertical displacement per year; and (2) that the final stages of metamorphism occurred prior to c. 411 Ma, although part of this denudation was likely accommodated on overlying extensional structures that may have been active more recently. There is close agreement between our data and results from the Krummedal Sequence north of the field area (72.5°?74°N), and rocks of the Smallefjord Sequence (75°?76°N) that are suggested to correlate with the Krummedal Sequence. This leads us to infer that the events recorded in the Forsblad Fjord region are of orogen‐scale significance.     

Giant cluster arcs as a constraint on the scattering cross-section of dark matter

We carry out ray tracing through five high-resolution simulations of a galaxy cluster, to study how its ability to produce giant gravitationally lensed arcs is influenced by the collision cross-section of its dark matter. In three cases typical dark matter particles in the cluster core undergo between 1 and 100 collisions per Hubble time; two more explore the long ('collisionless') and short ('fluid') mean free path limits. We study the size and shape distributions of arcs and compute the cross-section for producing 'extreme' arcs of various sizes. Even a few collisions per particle modifies the core structure enough to destroy the ability of the cluster to produce long, thin arcs. For larger collision frequencies the cluster must be scaled up to unrealistically large masses before it regains the ability to produce giant arcs. None of our models with self-interacting dark matter (except the 'fluid' limit) is able to produce radial arcs; even the case with the smallest scattering cross-section must be scaled to the upper limit of observed cluster masses before it produces radial arcs. Apparently the elastic collision cross-section of dark matter in clusters must be very small, below 0.1 cm² g⁻¹, to be compatible with the observed ability of clusters to produce both radial arcs and giant arcs.     

Stability of binary asteroids

The restricted problem of 2+2 bodies is applied to the study of the stability and dynamics of binary asteroids in the solar system. Numerical investigation of the behavior of the orbital elements and the maximal Lyapunov characteristic number of binary asteroids reveal extensive regions where bounded quasiperiodic motion is possible. These regions are compared to the bounded regions which are predicted by the classical restricted problem of three bodies. Regions of bounded chaotic solutions are also found.     

X-ray binaries

Summary The various types and classes of X-ray binary are reviewed high-lighting recent results. The high mass X-ray binaries (HMXRBs) can be used to probe the nature of the mass loss from the OB star in these systems. Absorption measurements through one orbital cycle of the supergiant system X1700-37 are well modelled by a radiation driven wind and also require a gas stream trailing behind the X-ray source. In Cen X-3 the gas stream is accreted by the X-ray source via an accretion disk. Changes in the gas stream can cause the disk to thicken and the disk to obscure the X-ray source. How close the supergiant is to corotation seems to be as much a critical factor in these systems as how close it is to filling its Roche lobe. In the Be star X-ray binaries a strong correlation between the neutron stars rotation period and its orbital period has been explained as due to the neutron star being immersed in a dense, slow moving equatorial wind from the Be star. For the X-ray pulsars in the transient Be X-ray binaries a centrifugal barrier to accretion is important in determining the X-ray lightcurve and the spin evolution. The X-ray orbital modulations from the low mass X-ray binaries, LMXRBs, include eclipses by the companion and/or periodic dipping behaviour from structure at the edge of the disk. The corresponding optical modulations show a smooth sinusoidal like component and in some cases a sharp eclipse by the companion. The orbital period of the LMXRB XB1916-05 is 1% longer in the optical compared to that given by the X-ray dip period. The optical period has been interpreted as the orbital period, but this seems inconsistent with the well established view of the origin of the X-ray modulations in LMXRB. A new model is presented that assumes the X-ray dip period is the true orbital period. The 5.2 h eclipsing LMXRB XB2129+47 recently entered a low state and optical observations unexpectedly reveal an F star which is too big to fit into the binary. This is probably the first direct evidence that an X-ray binary is part of a hierarchical triple. Finally the class of X-ray binaries containing black hole candidates is reviewed focusing on the value of using X-ray signatures to identify new candidates.     

Saltation threshold on Mars: The effect of interparticle force,surface roughness,and low atmospheric density

The effect of interparticle forces, as well as changes in surface roughness, particle diameter and density, and atmospheric density and viscosity are considered in new estimates of saltation thershold on Mars. These estimates result in somewhat lower minimum values of predicted threshold speed than by use of the “universal” A-B curve, with the minimum threshold speed occuring at smaller values of particle diameter. It is shown that the new predictions are much closer to the limited low atmospheric density data available than if the older method of estimation is used.