Dominant particle support mechanisms in debris flows at Mt Thomas, New Zealand, and implications for flow mobility

Within zones of little or no deformation by internal shearing in debris flows at Mt Thomas, about two-thirds of the weight of large particles is supported by buoyancy and about one-third by static grain to-grain contact. In boundary shear zones of low velocity flows and in high velocity, turbulent debris flow, grain-to grain contact is replaced by turbulence and dispersive pressure. Cohesive strength of the clay + silt + water interstitial fluid provides less than 2 % of the force keeping particles larger than 1 cm gravel in suspension. Excess pore pressure is generated in the interstitial fluid by the weight of coarse particles suspended in the slurry. According to Coulomb strength theory, pore pressures measured in these debris-flow slurries reduce the shear strength of the material to less than 10 % of what it is in the unsaturated state. The excess pore pressures are slow to dissipate because of the small connections between pore spaces that result from the extremely poor sorting of the debris and the presence of silt and clay in the pore fluid. Maintenance of sufficient pore space to trap fluid and facilitate flow on low-gradient slopes may be accomplished by dilatancy and subsequent partial liquefaction of the debris during shear.     

The Tugtutoq Younger Giant Dyke Complex, South Greenland: Fractional Crystallization of Transitional Olivine Basalt Magma

The Younger Giant Dyke Complex consists of a set of massivecoalescing dykes of Proterozoic age (c. 1170 Ma), resultingfrom intrusion of a suite of transitional olivine basaltic/hawaiiticmagmas in a continental rift setting. The suite, compositionallyrelated by low pressure (< 10 kb) olivine-plagioclase fractionation,is believed to have had a deeper level evolution dominated bypyroxene and possibly garnet fractionation. Slow cooling insitu of the interior parts of the dyke complex produced cumuliticsuites. Those exposed range from gabbroic to syenitic; residualbodies of riebeckite granite and, very subordinate, feldspathoidalsyenite were also generated. The basic magmas had notably lowfO₂ values, leading to delayed magnetite and clinopyroxene precipitation,relatively iron-rich differentiates and some residual liquidsof pantelleritic composition. The basic magmas had high F/Clvalues and are inferred to have had low H₂O contents. They werealso characterized by relatively high K/Rb and low ⁸⁷Sr/⁸⁶Srvalues; these characteristics imply a mantle source with highF/Cl but depleted in Rb relative to K and Sr. Basaltic magmasresponsible for (a) the preceding Older Giant Dyke Complex and(b) a suite of anorthositic xenoliths within the Younger GiantDyke Complex, are inferred to have been derived from separateprimary magma batches independent of those that yielded theYounger Giant Dyke Complex. The giant dykes are the highest-levelrepresentatives of a larger basic complex responsible for theextensive linear gravity ‘high’ in the Tugtutôq-Narssaqarea.     

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well‐studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time‐parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re‐deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near‐sea floor pore system is a first‐order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.     

Provenance and Magmatic-Metamorphic Evolution of a Variscan Island-Arc Complex: Constraints from U-Pb Dating, Petrology, and Geospeedometry of the Kyffhauser Crystalline Complex, Central Germany

The Kyffhäuser Crystalline Complex, Central Germany, formspart of the Mid-German Crystalline Rise, which is assumed torepresent the Variscan collision zone between the East Avalonianterrane and the Armorican terrane assemblage. High-precisionU–Pb zircon and monazite dating indicates that sedimentaryrocks of the Kyffhäuser Crystalline Complex are youngerthan c. 470 Ma and were intruded by gabbros and diorites between345 ± 4 and 340 ± 1 Ma. These intrusions had magmatictemperatures between 850 and 900°C, and caused a contactmetamorphic overprint of the sediments at P–T conditionsof 690–750°C and 5–7 kbar, corresponding toan intrusion depth of 19–25 km. At 337 ± 1 Ma themagmatic–metamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 7–11 km.This is inferred from a diorite, and conforms to P–T pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 5–7 to 2–4 kbar at 690–750°C.Subsequently, the metamorphic–magmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published K–Ar muscoviteage of 333 ± 7 Ma. With respect to P–T–D–tdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; U–Pb dating; hornblende; garnet; Mid-German Crystalline Rise; P–T pseudosection     

Constraints on the depth and variability of the lunar regolith

Abstract— Knowledge of regolith depth structure is important for a variety of studies of the Moon and other bodies such as Mercury and asteroids. Lunar regolith depths have been estimated using morphological techniques (i.e., Quaide and Oberbeck 1968; Shoemaker and Morris 1969), crater counting techniques (Shoemaker et al. 1969), and seismic studies (i.e., Watkins and Kovach 1973; Cooper et al. 1974). These diverse methods provide good first order estimates of regolith depths across large distances (tens to hundreds of kilometers), but may not clearly elucidate the variability of regolith depth locally (100 m to km scale). In order to better constrain the regional average depth and local variability of the regolith, we investigate several techniques. First, we find that the apparent equilibrium diameter of a crater population increases with an increasing solar incidence angle, and this affects the inferred regolith depth by increasing the range of predicted depths (from ～7–15 m depth at 100 m equilibrium diameter to ～8–40 m at 300 m equilibrium diameter). Second, we examine the frequency and distribution of blocky craters in selected lunar mare areas and find a range of regolith depths (8–31 m) that compares favorably with results from the equilibrium diameter method (8–33 m) for areas of similar age (～2.5 billion years). Finally, we examine the utility of using Clementine optical maturity parameter images (Lucey et al. 2000) to determine regolith depth. The resolution of Clementine images (100 m/pixel) prohibits determination of absolute depths, but this method has the potential to give relative depths, and if higher resolution spectral data were available could yield absolute depths.