Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb-Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation ⁴⁰Ar/³⁹Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29-32 Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30 km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74-67 Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62-58 Ma), widespread Late Cretaceous-Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22-16 Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basins.     

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted ⁴⁰Ar-³⁹Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a ⁴⁰Ar-³⁹Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma.     

Feasibility of AFC models for the petrogenesis of calc-alkaline magma series

 One of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation (AFC). The equations governing these processes are well known and widely applied. The power and attraction of the model are that, with the minimum of constraints, petrogenetic schemes can be erected for essentially any group of rocks. These models are usually based on the observed variations in a small number of trace elements and/or isotopic systems. In this paper we have generated well-constrained AFC models for the Variscan, calc-alkaline Quérigut complex in the French Pyrénées. Our models are based on geological constraints on the identities of the parent magma and candidate assimilants, and the Sm-Nd and Rb-Sr isotope systematics of the rocks. The modelling is successful in mimicking the isotope array for the actual rocks. However, the results are massively inconsistent with those produced using the major- and trace-element concentrations. Our major conclusion is that AFC modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and the proposed assimilant, with due respect for crystal-liquid partition coefficients, in so far as they are known. The results should then be accepted only if the isotope-based model parameters are consistent with those derived from major- and trace-element variations. Received: 19 August 1994/Accepted: 23 March 1995     

Paleomagnetism of Late Cretaceous Poços de Caldas alkaline complex,Southern Brazil

The paleomagnetism of the Late Cretaceous Poços de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S (dp = 8°,dm = 13°). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic.     

Seeing Conditions at Sutherland. Results of the 1992/93 seeing campaign

In order to decide whether the seeing conditions at SAAO/Sutherland justify the erection of a 3.5 m telescope and also to compare Sutherland with the Gamsberg/Namibia site, a seeing campaign covering 15 months has been carried out. For direct comparison with the results of the seeing campaign at Gamsberg twenty years before the same QUESTAR telescope was employed. The seeing is determined by the scattering of the star-trail exposed on a film in the focal plane of the telescope. The campaign commenced in February 1992. Up to May 1993, data for 204 nights, that is 47.3% of the total number of nights, were collected. Due to wind speeds above 30 km h^-1, 25 out of the 204 nights were not considered in the final reduction. The useful 179 nights are evenly distributed over the campaign period. The median seeing value for the whole period is = 0.52. There are differences during the year: the best season gives = 0.42, the worst = 0.67. Each night was divided into three intervals, although data for each of the three intervals were not always available. Generally, there is an improvement in the seeing during the course of a night. The results are compared to the seeing values of Gamsberg/Namibia and ESO/La Silla.     

A Saturnian gas ring and the recycling of Titan's atmosphere

Atoms which escape Titan's atmosphere are unlikely to possess escape velocity from Saturn, and can orbit the planet until lost by ionization or collision with Titan. It is predicted that a toroidal ring of between ～1 and ～10³ atoms or molecules cm^?3 exists around Saturn at a distance of about 10 times the radius of the visible rings. This torus may be detectable from Earth-orbit and detection of nondetection of it may provide some information about the presence or absence of a Saturnian magnetic field, and the exospheric temperature and atmospheric escape rate of Titan. It is estimated that, if Titan has a large exosphere, ～97% or more of the escaping atoms can be recaptured by Titan, thereby decreasing the effective net atmospheric loss rate by up to two orders of magnitude. With such a reduction in atmospheric loss rates, it becomes more plausible to suggest that satellites previously thought too small to retain an atmosphere may have one. It is suggested that Saturn be examined by Lyman-α and other observations to search for the gaseous torus of Titan. If successful, these could then be extended to other satellites.The effect of a hypothetical Saturnian magnetosphere on the atmosphere of Titan is investigated. It is shown that, if Saturn has a magnetic field comparable to Jupiter's (～10 G at the planetary surface), the magnetospheric plasma can supply Titan with hydrogen at a rate comparable to the loss rates in some of the models of Trafton (1972) and Sagan (1973). A major part of the Saturnian ionospheric escape flux (～ 10²⁷ photoelectrons sec^?1) could perhaps be captured by Titan. At the upper limit, this rate of hydrogen input to the satellite could total ～0.1 atm pressure over the lifetime of the solar system, an amount comparable to estimates of the present atmospheric pressure of Titan.     

Three million years of monsoon variability over the northern Sahara

We present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of 400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the eastern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (21°N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan watershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from green Sahara climate models. Our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years. As would be expected, this orbital precession-scale mechanism is modulated on both short (100-kyr) and long (400-kyr) eccentricity time scales. We also observe a strong expression of the 41-kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also observe a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (0.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.