Intrusive processes at ocean ridges: Evidence from the sheeted dyke complex of Masirah, Oman

Masirah Island largely consists of a late Mesozoic ophiolite which includes extensive areas of near-vertical, ENE—WSW striking, sheeted dykes. Previously the possibility has been suggested of a correlation between the similarly-aged ophiolites of Masirah and the Semail Complex of the Oman Mts. However, the Masirah ENE–WSW trend contrasts with N—S dyke trends from the Wadi Jizi area of the Semail, possibly suggesting two unrelated spreading centres. The dykes pass up into a pillow lava—minor sediment sequence, down into both layered and unlayered gabbros and are bounded to the west by a major N—S mélange zone which may have originated as a ridge transform fault. Age relations of the dykes and the gabbros are complex: the dykes contain a variable proportion of gabbro screens representing earlier crystallization, but they are also intruded by several small gabbro bodies which are themselves cut by still later dykes. The lava and dyke—gabbro screen sequence shows evidence of metamorphism from zeolite to low amphibolite grade. This metamorphism was caused by ridge hydrothermal activity which appears to have been effective approximately to the lower levels of the dykes. The rapid passage from low-amphibolite dykes to fresh gabbro suggests lithological control of the metamorphism. A combination of structural, geochemical and mineral phase studies may indicate generation in a slow spreading ridge environment and near-ridge metamorphism caused by a geothermal gradient of approximately 200°C/km.     

1) I. A. H. S. / A. I. H. S.

Abstract

A simple method is used to study the response of runoff in the Sahel to climate change. The statistical characteristics of rainfall are calculated over the western part of the Sahel for the period 1961–1990, using the BADOPLU network. Daily rainfall is simulated using a Markov process with Weibull distribution for rainfall depths. Runoff is modelled using a conceptual SCS model and the curve numbers are calculated for West Africa. Climate change is provided by simulations using the Arpège GCM (Scenario A1B), and a perturbation method is used on the parameters which describe the rainfall. Changes in rainfall are assumed to occur through increases in frequency, not intensity. Using Arpège, runoff is mainly found to increase, in depth and in number of events, by the end of the 21st century. Changes in evaporation and land use are not included in the analysis. The impact of this 21st century potential climate change (rainfall) on the runoff is found to be of the same magnitude as the impact of changes in land use.     

Inductive magnetic footpoint tracking by combining the minimum energy fit with the local correlation tracking and doppler velocity

Time-dependent magneto-hydrodynamic simulations of active region coronal magnetic field require the underlying photospheric magnetic footpoint velocities. The minimum energy fit (MEF) is a new velocity inversion technique to infer the photospheric magnetic footpoint velocities using a pair of vector magnetograms, introduced by Longcope (2004). The MEF selects the smallest overall flow from several consistent flows by minimizing an energy functional. The inferred horizontal and vertical flow fields by the MEF can be further constrained by incorporating the partial or imperfect velocity information obtained through independent means. This hybrid method is expected to give a velocity close to the true magnetic footpoint velocity. Here, we demonstrate that a combination of the MEF, the local correlation tracking (LCT) and Doppler velocity is capable of inferring the velocity close to the photospheric flow.     

Thermal and exhumation history of the central Rwenzori Mountains, Western Rift of the East African Rift System, Uganda

The Rwenzori Mountains (Mtns) in west Uganda are the highest rift mountains on Earth and rise to more than 5,000 m. We apply low-temperature thermochronology (apatite fission-track (AFT) and apatite (U–Th–Sm)/He (AHe) analysis) for tracking the cooling history of the Rwenzori Mtns. Samples from the central and northern Rwenzoris reveal AFT ages between 195.0 (±8.4) Ma and 85.3 (±5.3) Ma, and AHe ages between 210.0 (±6.0) Ma to 24.9 (±0.5) Ma. Modelled time–temperature paths reflect a protracted cooling history with accelerated cooling in Permo-Triassic and Jurassic times, followed by a long period of constant and slow cooling, than succeeded by a renewed accelerated cooling in the Neogene. During the last 10 Ma, differentiated erosion and surface uplift affected the Rwenzori Mtns, with more pronounced uplift along the western flank. The final rock uplift of the Rwenzori Mtns that partly led to the formation of the recent topography must have been fast and in the near past (Pliocene to Pleistocene). Erosion could not compensate for the latest rock uplift, resulting in Oligocene to Miocene AHe ages.     

On the instability of weakly radially anisotropic star clusters

We discuss contradictions existing in the literature in the problem on the stability of collisionless spherical stellar systems, which are the simplest anisotropic generalization of the well-known polytropic models. On the one hand, calculations of the growth rates within the framework of a linear stability theory and N-body simulations suggest that these systems should become stable when the parameter s characterizing the degree of anisotropy of the stellar velocity distribution becomes lower than some critical value s _crit > 0. On the other hand, according to Palmer and Papaloizou, the growth rate should be nonzero up to the isotropic limit s = 0. Using our method of determining the eigenmodes of stellar systems, we show that even though the mode growth rates in weakly radially anisotropic systems of this type are nonzero, they are exponentially small, i.e., decrease as γ ∝ exp(−a/s) when s → 0. For slightly radially anisotropic systems with a finite lifetime, this actually implies stability.     

青藏高原及邻近地区地壳与上地幔剪切波三维速度结构

本文利用30个基准台所记录的238条长周期面波资料,经过适配滤波和分格频散反演,得到中国大陆及邻区147个分格10—105s的纯路径频散,进而反演出青藏高原及邻近地区深至170km的剪切波三维速度结构.研究表明,青藏高原中西部地区和东部地区的地壳平均厚度分别为70±7km和65±7km,地壳平均剪切波速度分别为3.55和3.62km/s,上地幔顶盖平均速度分别为4.63和4.61km/s; 岩石层厚度均为120±10km;东部地区下地壳内30—40km深度处普遍存在低速层;青藏高原及其东侧的上地幔低速层内有横贯东西且明显向上隆起的低速腔.滇西缅北地区的地壳厚45±5km,上地壳及下地壳内都有低速层;上地幔顶盖的速度为4.42km/s,比青藏高原本体及恒河平原都低.恒河平原地壳厚34±2km,速度平均为3.45km/s;上地幔顶盖厚86±10km,速度平均为4.63km/s,顶盖内55—83km深处有一个低速夹层.     

Evidence for short term changes in the intensity and spectrum of hard X-rays from Cyg X-1

The paper presents experimental evidence for the existence of fast intensity fluctuations with time scales of the order of a minute in the X-ray emission from Cyg X-1 at energies greater than 29 keV. Spectral variations over time intervals of 20–25 min are also observed in the same energy range. Whereas, similar intensity and spectral fluctuations have been reported earlier at lower energies the observations presented here is the first evidence for the existence of similar fluctuations at high energies.     

Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions

In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled by interactions with the excess components albite, clinozoisite, chlorite, quartz and H₂O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K loop in the P-T plane with positive slopes at low and at high pressure. Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent heating and erosion. The record of evolving conditions may span some 30 Ma of geological history.