The obliquity of Pluto

Pluto's obliquity (the angle between its spin axis and orbit normal) varies between ～102 and ～126° over a period of about 3 million years. These oscillations are nearly sinusoidal and quite stable, leading to only modest changes in the insolation regime. Thus, Pluto's rotation has been slightly retrograde ever since its current orbit and rotation rate were established.     

RADIOCARBON DATING OF A PEAT DEPOSIT IN THE CUCAMONGA FAULT ZONE,CALIFORNIA*

A sequence of peat beds was deposited in a small sag pond along the Cucamonga fault zone during late Holocene. The stratigraphic section of this deposit is described; several beds have been radiocarbon dated. Comparison of radiometric and stratigraphic chronologies allows reconstruction of the geomorphic history of the sag pond, which extends back 3000 years, and establishes evidence that the fault has been active within the very recent geologic past.     

The association boninite low‐ti andesite‐tholeiite in the heathcote greenstone belt,Victoria; ensimatic setting for the early lachlan fold belt

The Heathcote Greenstone Belt is composed mainly of Lower Cambrian metavolcanic rocks and is one of three outcropping belts of the apparent basement to the Lachlan Fold Belt in SE Australia. The greenstones may be assigned to two broad magma series. A younger tholeiitic series with mid‐ocean ridge basalt (MORB) affinities has intruded through, and been erupted upon low‐Ti, intermediate SiO² lavas. The latter were originally boninites (both clinoenstatite‐phyric and more fractionated orthopyroxene‐phyric varieties) and plagioclase‐phyric, low‐Ti andesites. They have partially re‐equilibrated to the lower greenschist facies and outcrop mainly in the central segment of the Heathcote Greenstone Belt, where deeper stratigraphic levels are exposed. Tholeiitic lavas and sills metamorphosed to the prehnite‐pumpellyite facies dominate the northern and southern segments. As the association boninite/low‐Ti lavas/MORB is known only from modern West Pacific‐type settings involving island arcs and backarc basins, the early history of the Lachlan Fold Belt is inferred to have taken place in a similar setting.     

The lunar seismogram

The unexpected and unusual characteristics of the lunar seismogram have given rise to various speculations regarding their origin: secondary ejecta, diffusive wave propagation and wave propagation effects in a self-compacted powder layer with a linearly increasing velocity with depth. Many of the characteristics can be explained, qualitatively, by the simple theory of a self-compacting, dry powder layer for which the velocity varies as the sixth root of the depth. This gives a very low seismic velocity at the lunar surface which, in turn, allows the signal to have a long duration, a lack of correlation between horizontal and vertical displacements, a signal envelope that changes with source to receiver separation and a varying spectrum over the duration of the signal. To explain the long duration of the seismic signal quantitatively, it is necessary to include scattering of the normally incident rays at the surface by shallow surface undulations. The sixth root velocity-depth dependence is consistent with the measured variation, with pressure, of the compressibility and velocity of lunar samples.     

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of 0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of 2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at 27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.     

Transient and steady-state deformation of synthetic rocksalt

Synthetic rocksalt with a porosity less than 2.5% and an average grain size of about 0.35 mm was made by warm-pressing at 100°C, 150 MPa and for 15 min. Cylinders of this material, 25 mm in diameter by 50 mm long, were deformed at strain rates of 0.1 ksec⁻¹ at confining pressures of 20, 50, 100 and 200 MPa and at temperatures of 100, 200 and 300°C. The resulting stress-time data show transient-stress behavior before steady-state stress occurs. Very little variation in the stress/time data occurs for the above confining pressures at a constant temperature. Many of the tests reach steady state at 10% strain, where all the experiments were terminated. The differential stress at 10% varies from about 22 MPa at 100°C to about 6 MPa at 300°C. These “strengths” are slightly less than those measured by Heard (1972) (also on synthetic, polycrystalline rocksalt) and are similar to those measured by Wawersik and Hannum (1980) and by Hansen and Mellegard (1980) in modified creep tests on coarse-grained, natural rocksalt under similar pressure and temperature conditions. Activation energies computed from these steady-state stresses vary between 7.5 and 25 kcal/mole and are consistent with those obtained by Heard (1972) and by Parrish and Gangi (1977, 1981). The stress/time data were fit using one- and two-mechanism, transient-stress functions which assume independent mechanisms or processes. The rms errors decrease from about 1 MPa for the one-mechanism fits to about 0.2 MPa for the two-mechanism fits, indicating at least two mechanisms are operative in these tests. Similar one- and two-mechanism fits were made to creep tests performed by W. Wawersik et al. on New Mexico bedded rocksalt. Similar improvements in the fits were obtained for those tests that lasted long enough so that the effect of a second mechanism could be noted. It was found that the “steady-state” strain rate found in creep tests could be interpreted as the beginning of another mechanism. This raises the question of whether the “steady-state” phenomena exist at all or whether it is just an approximation to a mechanism with a time constant that is long compared to the length of the test.     

Circular meander pools

Aerial photographs led to the detection of a previously undescribed type of river meander. These features are characterized by nearly circular pools located at the apex of the bend and are widely distributed on small to medium sized coastal plain streams in the southeastern United States. Bathymetric evidence shows that the structure of meanders containing circular pools is distinctive. Field and laboratory evidence suggests that these features are formed by a short-lived countercurrent that develops during the flood cycle.     

Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes

A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Teñidas Este, Concepción, San Miguel, San Telmo and Cueva de la Mora) located in the northern Iberian Pyrite Belt. The isotopic analyses were mainly performed on quartz, chlorite, carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits, and also on some fluid inclusion waters. Homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 °C. Salinity of most fluid inclusions ranges from 2 to 14 wt% NaCl equiv. A few cases with T _h=80–110 °C and salinity of 16–24 wt% NaCl equiv., have been also recognized. In addition, fluid inclusions from the Soloviejo Mn–Fe-jaspers (160–190 °C and ˜6 wt% NaCl equiv.) and some Late to Post-Hercynian quartz veins (130–270 °C and ˜4 wt% NaCl equiv.) were also studied. Isotopic results indicate that fluids in equilibrium with measured quartz (d ¹⁸O _fluid ˜–2 to 4‰), chlorites (d ¹⁸O _fluid ˜8–14‰, dD _fluid ˜–45 to –27‰), whole rocks (d ¹⁸O _fluid ˜4–7‰, dD _fluid ˜–15 to –10‰), and carbonates (d ¹⁸O _ankerite ˜14.5–16‰, d ¹³C _fluid =–11 to –5‰) evolved isotopically during the lifetime of the hydrothermal systems, following a waxing/waning cycle at different temperatures and water/rock ratios. The results (fluid inclusions, d ¹⁸O, dD and d ¹³C values) point to a highly evolved seawater, along with a variable (but significant) contribution of other fluid reservoirs such as magmatic and/or deep metamorphic waters, as the most probable sources for the ore-forming fluids. These fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust.