Wide-angle seismic reflections reveal a lithosphere-asthenosphere boundary zone in the subducting Pacific Plate, New Zealand.

New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)-km-thick layer at the bottom of the lithosphere with a P-wave (VP) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)-km-thick, low VP channel with a P-wave-to-S-wave velocity ratio (VP/VS) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high VP/VS ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.

High mantle seismic P-wave speeds as a signature for gravitational spreading of superplumes.

New passive- and active-source seismic experiments reveal unusually high mantle P-wave speeds that extend beneath the remnants of the world's largest known large igneous province, making up the 120-million-year-old Ontong-Java-Manihiki-Hikurangi Plateau. Sub-Moho Pn phases of ~8.8 ± 0.2 km/s are resolved with negligible azimuthal seismic anisotropy, but with strong radial anisotropy (~10%), characteristic of aggregates of olivine with an AG crystallographic fabric. These seismic results are the first in situ evidence for this fabric in the upper mantle. We show that its presence can be explained by isotropic horizontal dilation and vertical flattening due to late-stage gravitational collapse and spreading in the top 10 to 20 km of a depleted, mushroom-shaped, superplume head on a horizontal length scale of 1000 km or more. This way, it provides a seismic tool to track plumes long after the thermal effects have ceased.

Episodic kinematics in continental rifts modulated by changes in mantle melt fraction.

Oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor 'spreading centres' found between diverging tectonic plates. Modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres, but direct observation of this process is difficult beneath the oceans. Continental rifts, however-which are also associated with mantle melt production-are amenable to detailed measurements of their short-term kinematics using geodetic techniques. Here we show that such data can provide evidence for an upwelling mantle flow, as well as information on the dimensions and timescale of mantle melting. For North Island, New Zealand, around ten years of campaign and continuous GPS measurements in the continental rift system known as the Taupo volcanic zone reveal that it is extending at a rate of 6-15 millimetres per year. However, a roughly 70-kilometre-long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence, and is flanked by regions of extension and uplift. These features fit a simple model that involves flexure of an elastic upper crust, which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres. We propose that flexure is caused by melt-induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis, triggering a transient lower-crustal flow. A drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence, whereas melt accumulation reduces viscosity and allows uplift-processes that are also likely to occur in oceanic spreading centres.

Staff perceptions of syringe dispensing machines in Australia: a pilot study.

BACKGROUND/AIMS: Syringe dispensing machines were introduced into needle syringe programs (NSPs) two decades ago. The few published studies on dispensing machines have focused on feedback of machine users and service providers' feedback has rarely been reported. This study obtained the feedback of health staff of NSPs, other sectors of Drug Misuse Treatment Services and of other health services adjoining dispensing machines on the role and effectiveness of dispensing machines. METHODS: Between August and November 2006, questionnaires were anonymously completed by NSP and drug misuser treatment staff in an Area Health Service in Sydney, as well as by the staff of two nondrug-related health services located adjacent to needle syringe dispensing machines. The questionnaire was available in either paper or Internet based forms. RESULTS: Almost 80% of 94 participants rated dispensing machines as either moderately successful or successful in reducing sharing of needles and syringes. Staff considered that introduction of these machines to NSPs had improved services for injecting drug users without increasing unsafe disposal of used equipment, community drug use, or vandalism. However 78% of respondents felt that dispensing machines either reduce or may reduce IDUs' opportunity for staff contact and hence opportunity for engagement or education. The study limitations were noted. CONCLUSION: Syringe dispensing machines are perceived to be a successful and appropriate outlet of NSPs that complement other outlets. Lack of staff-user contact was seen as their main disadvantage.

Client satisfaction and risk behaviours of the users of syringe dispensing machines: a pilot study.

INTRODUCTION AND AIMS: The study examines risk behaviours of the users of syringe dispensing machines (SDMs) and evaluates the usefulness of these machines in providing injecting drug users (IDUs) with sterile injecting equipment. DESIGN AND METHODS: Self-administered questionnaires were used among users of SDMs in an Area Health Service of Sydney. RESULTS: The majority of the 167 participants reported being happy with the quality of the SDM services. Problems identified with machines were that they were often broken or jammed (32.8% respondents), not in the right place (21.9%) or require money (16.7%). Just over half (50.9%) of the IDUs use SDMs only from 5 p.m. to 9 a.m., the time when almost all other outlets for accessing sterile injecting equipment remain closed. Relatively young IDUs (age < or = 30 years) were more likely to prefer SDMs over staffed needle syringe programmes (NSPs) compared with older users (age > 30) and to identify stigma (a desire to hide their identity or not liking the way people treat them at staffed NSPs or chemists) as a main reason for using these machines. Primary users of SDMs do not differ from primary users of NSP/chemists in terms of sharing of needles. Those users who had shared in the last month were nearly four times as likely to have never used condoms in sexual encounters over that period (95% confidence interval: 1.2 - 14.5). DISCUSSION AND CONCLUSION: SDMs appear to complement other outlets of NSPs. Providing free-of-cost equipment from SDMs should be considered carefully, as needing money to buy equipment was a reason given for sharing of needles by 35% of those who shared.