ALBERT

Notes: Laser action from barium vapor has been investigated using small-scale (20–40 cm−3) high-repetition-rate discharge-heated and low-repetition-rate externally-heated devices. Strongest output occurs at 1.13, 1.50, and 2.55 μm, with weaker emission at 2.92 and 4.72 μm. The range of discharge conditions for which laser action occurs at these wavelengths is described and the temporal occurrence of laser output is presented. The implications of this study to the future development of barium lasers are considered.

Notes: The palaeomagnetism of a Silurian succession (Lough Mask Formation lavas and sediments) and intrusive rocks (Kilbride dolerites and andesites and microgranodiorites, Killary Harbour area) in the Connemara–Mayo segment of the Irish Caledonides is described. Magnetizations are related to the regional tectonic history with the aid of fold, conglomerate and contact tests. They describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this Caledonian inlier. The shallow components appear to pre-date Siluro-Devonian folding while the intermediate ones post-date this episode. The change in palaeofield inclination is identical to the transition described by contemporaneous deep level (Silurian) to shallow and surface level (Lower Devonian) bodies in the British and Scandinavian Caledonides but declinations are rotated consistently to the west. The Lower–Middle Silurian vector is rotated clockwise by c. 100°. This difference is progressively reduced to c. 50° by the time of the Siluro-Devonian D2 folding, showing that this fold generation (now oriented E–W) developed in parallelism with contemporaneous folds on the NE strike continuation of the Caledonides. Post-folding remanences continue to deviate in a clockwise sense from the remainder of the Caledonides showing that rotation continued into Middle–Upper Devonian times and finally ceased during Carboniferous times. The integrated effect of this rotation is recognized in Ordovician magnetizations from the Dalradian metamorphic terrane of south Connemara. It can be explained by block rotations within a zone of distributed deformation according to the McKenzie–Jackson model and illustrated by many neovolcanic zones. Palaeomagnetism defines a tectonic regime commencing with rotations of c. 3° Myr−1 and lasting from mid-Silurian to Carboniferous times. Western Ireland is sited within a closure gap along the Iapetus suture extending from Ireland to Newfoundland, and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the Caledonian orogen.

Notes: In the southern sector of the Scandinavian Caledonides a metamorphic gneiss complex (the Basal Gneiss Region or BGR) underthrusts a nappe complex of Precambrian granulites (the Jotun Nappe) in an assemblage collectively recording orogenic events culminating in the Scandian (Upper Silurian and Lower Devonian) episode. Palaeomagnetic study of a large regional sample spanning the combined outcrop identifies properties strongly controlled by metamorphic retrogression to amphibolite and greenschist facies. Low blocking temperature (Ibt) components in the BGR have E+ directions with a mean of D = 69° 1 = 60° (N = 19 sites,α95= 7.0°); sporadic higher blocking temperature components define a dual polarity northeast negative/southwest positive (NE - /SW+) axis with a mean of D = 42°, 1 = -24° (20 sites, α95= 8.1°). Granulite facies rocks in the Jotun Nappe yield a high-quality palaeomagnetic record reflected in high $LQn values and dominated by medium and high blocking temperature (hbt) components. A dual polarity (‘A’) axis defines a palaeofield migration from NE+ to E+ during uplift and cooling of this terrane with a predominance of normal polarities. A smaller population of Ibt (‘B’) components show E+ directions distributed along the Mesozoic palaeofield migration path for Eurasia, and probably acquired during brittle tectonic events of this age. ‘A’ magnetizations from 86 sites define mean south pole positions between 301°E, 2°N and 325°E, 10°S linked to uplift-related cooling following the climactic Scandian orogenic episode and dated at 420–400 Ma from the collective geological and radiometric evidence. Both the outward and return segments of an APW loop may be represented. The polar swathe correlates with (i) a higher blocking temperature component from the BGR, (ii) the uplift magnetization record in the western orthotectonic Caledonides of Scotland, and (iii) primary magnetizations from the Siluro-Devonian molasse facies in Britain. This study shows that the westerly extension of the APW path identified by British Siluro-Devonian data is not peculiar to that crustal segment but applies to continental Europe, and presumably to Laurentia, as well. The pattern of remanence acquisition within the Caledonides suggests contrasting zones of thermal and thermochemical remanence, with the latter probably linked to fluid migration consequent upon orogenic loading.

Notes: The Upper Carboniferous–Lower Triassic Pyongan Supergroup is exposed in an E–W trending syncline comprising the Samcheok Coalfield in eastern South Korea; it borders the southern margin of a NE–SW trending (Okcheon) zone of Upper Jurassic–Lower Cretaceous deformation (Daebo Orogeny). Although a Recent field overprint widely recorded in Korean rocks is represented here, an ancient field component structure is recovered from the red bed facies in this Supergroup by detailed thermal demagnetization. A prominent Upper Jurassic–Cretaceous overprint is widespread (D/I= 56.1/54.8°, α,95= 9.6°, palaeopole at 200.4°E, 46.7°N) and records the magnetic effect of the Daebo Orogeny within the Okcheon Belt. It is post-deformational in origin and appears to have been confined mainly to the Cretaceous Normal Superchron. It has been rotated clockwise since these times in common with the main synclinal axis, as a consequence of dextral movements along the Tan Lu Fault System. Components predating this deformation are recovered from the Lower Triassic (D/I= 1.1/19.4°, α95= 18.2°, palaeopole at 306.1°E, 63.2°N), Permian (D/I= 358.3/11.5°, α95= 6.3°, palaeopole at 311.9°E, 58.7°N) and Upper Carboniferous (D/I= 341.1/-9.2°, α95= 7.2°, palaeopole at 335.7°E, 44.6°N) sediments.Second-order small circle dispersions of site palaeopoles record unaccommodated tectonic rotations and are largest in the oldest beds below an Upper Carboniferous-Lower Permian unconformity. However, the presence of reversals, probably acquired before and after the Carboniferous–Permian Reversed Superchron, defines ancient field axes and identifies an Upper Carboniferous-Lower Triassic APW path. This path correlates with the contemporaneous path from the North China Block and is removed from the path from the South China Block; agreement with North China is enhanced when these results are corrected for the rotation indicated by the Cretaceous overprint. We therefore identify a first-order correlation between the Korean Peninsula and North China at least since Upper Palaeozoic times. Establishment of, and movements along, the Tan Lu Fault System since Mesozoic times have not obscured this palaeomagnetic correlation.

Notes: Abstract The characteristics of high repetition rate CVL devices incorporating passive external heating have been investigated. Pre-heating of the CVL plasma tube using external heating has been shown to significantly reduce CVL warm-up time, while supplementary heating of the CVL plasma tube using external heating allows efficient CVL operation with reduced discharge power input without any degradation in output power.

Notes: Abstract Strong stimulated Raman emission has been observed on three Stokes' lines at 1.608, 1.744 and 1.896μm from 12.6 km of single-mode silica fibre pumped at 1.50μm with the output of a pulsed barium vapour laser. A study has been made of the spectral distribution of fibre output as a function of peak input power to the fibre (up to 47 MW cm−2), and the results are compared to theoretical predictions. Incomplete conversion of the pump to first Stokes', and first to second Stokes', lines is observed despite the length of the fibre.

Notes: Abstract The variations in the output power of a copper vapour laser resulting from H2 additive to neon buffer gas are investigated for a wide range of pulse repetition frequencies (3.2 to 19.7 kHz). Small improvements in laser output power (9%) were observed for ∼2% H2-Ne admixture at low repetition frequency, these improvements becoming progressively larger (up to 250%) as the repetition rate increased. These observations are consistent with added H2 modifying the interpulse kinetics, thereby altering the spatiotemporal evolution of the output pulse. In particular, the observed average output power increases at elevated repetition rates owing to both increased laser pulse energy and increased optimum pulse repetition frequency.