ALBERT

Notes: On 1988 January 22 three earthquakes of Ms 6.3-6.7 occurred in a 12-hr period near Tennant Creek (Northern Territory) in the Proterozoic North Australian Craton and produced 32 km of surface rupture on two main scarps. the Lake Surprise scarp is shaped like a boomerang with the east arm trending ESE and the west arm, WSW. the Kunayungku scarp lies 7 km to the northwest and trends ESE. Locations of 211 aftershocks determined using data from portable seismograph arrays define a 10-km by 40-km aftershock zone elongated parallel to the trend of the surface ruptures. In the western and eastern part of the fault zone, aftershocks occur only south of the Kunayungku and eastern Lake Surprise scarps, respectively. In the central section, in contrast, aftershocks lie primarily to the north of the western arm of the Lake Surprise scarp. Focal depths range from the surface to 8km. In all sections the shallowest earthquakes lie closest to the scarps, whereas deeper events lie further away. In cross-section, the aftershocks lie in three distinct inclined zones, which delineate fault surfaces ruptured by the main shocks. Aftershocks are concentrated near the edges of the inferred fault surfaces suggesting that stress was relieved in the centre of the faults by rupture during the main shocks. the inferred fault-plane dips are: 45° to the SSW for the Kunayungku fault, 55° to the NNW for the western Lake Surprise fault and 35° to the SSW for the eastern Lake Surprise fault. These dip directions are consistent with the sense of thrust movement inferred from surface deformation. the reversal of fault dip and sense of movement between the central and end segments requires a complicated fault geometry. the aftershock data would allow a second, south-dipping plane in the central section that does not intersect the surface. This blind fault may be responsible for much of the moment release interpreted from the modelling of broad-band teleseismic waveforms (Choy & Bowman 1989). Rupture of the north-dipping plane may have been controlled by a pre-existing fault marked at the surface by a quartz ridge 5-km long and may be of secondary importance. Although the quartz ridge suggests previous faulting, the absence of significant topographic relief together with estimates of erosion rates suggest an average repeat time in excess of 104yr. Preliminary interpretation of water withdrawal histories and water-level data from a borefield adjacent to the eastern Lake Surprise scarp does not support a causal relationship between the borefield and the occurrence of the earthquakes.The nearby Warramunga (WRA) seismic array provides a record of local earthquake activity from 1965 to the present. the fault area was essentially aseismic until two small earthquakes in 1986 February and a series of six magnitude 4-5 earthquakes in 1987 January, which were followed by more than 1000 aftershocks. the frequency of occurrence of aftershocks declined very slowly in 1987, with p= 0.2 in the modified Omori formula, whereas for the 1988 aftershock sequence, p= 1. the low p during 1987 may have been diagnostic of high stress in the source and Water Authority provided partial financial support for J.R.B. and G.G.

Notes: Temporary array deployments of short-period seismometers in northern Australia have been used to build up composite record sections for waves interacting with the upper mantle. Stable measures of the seismic wavefield are provided by stacking the complex envelopes of all the seismic waveforms falling in a 10km distance interval away from the source.Two groups of sources (a) along the Flores Arc, Indonesia with propagation under northwestern Australia, and (b) in New Guinea with paths to the NNE of the array, have been used to construct composite record sections for both P and SV waves over the distance range 1300–2800 km. the timing and amplitude distributions for P waves from the two regions show noticeable differences. Detailed modelling of the record sections yields velocity models with significant variation in velocity for the two sets of propagation paths for which the midpoints are separated by about 1000km.The short-period SV-wave sections indicate efficient propagation of highfrequency S waves in a lithosphere extending down to 210km. Arrivals from the deeper mantle cannot be correlated with confidence because of a loss in high-frequency content revealed by broad-band observations. This requires a significant attenuation zone for S beneath 210 km.

Notes: A large array of short-period, portable seismographs was operated in the Northern Territory of Australia for 3 months in 1986 in order to record earthquakes in the island arcs to the north and east of Australia. The array consisted of 18 digital recorders, 28 analogue tape recorders and the 20 stations of the permanent Warramunga (WRA) seismic array. An unusual aspect of this experiment was the variable station spacing and apertures of the different elements of the hybrid array. The WRA and portable digital arrays had station spacing of 2.2 km and 5–13 km, respectively, and allowed confident identification of phases returned from the upper mantle, whilst the analogue array had station spacing of 40 km and allowed us to track mantle phases across its 500 km aperture.Seismograms from 17 shallow earthquakes (mb 4.3–5.1) in the Indonesian arc are used to investigate the P-wave velocity structure of the upper mantle beneath NW Australia. We combine seismograms from these events into a composite record section covering ranges from 1000 to 2600 km. Strong phases following the first P-wave arrivals in this composite record section clearly indicate the presence of significant structure in the upper mantle. Lateral heterogeneity in the upper mantle causes the timing and amplitudes of mantle phases to vary across the array and among earthquakes. In order to minimize the effects of lateral heterogeneity, we filter and stack the data and concentrate on features in the data that are seen for many individual seismograms and for several earthquakes. We calculate WKBJ and reflectivity seismograms in order to construct a vertical velocity profile that fits the observed traveltimes and waveforms in an average sense. Our preferred model NWB-1 includes second-order velocity discontinuities of 3.7 per cent near a depth of 200 km, 6.1 per cent near 400 km and 3.7 per cent near 620 km in order to reproduce the amplitudes of the later phases. The low amplitudes of the first arrivals in the range from 1600 to 2200 km require either a low-velocity zone below 230 km depth or a low velocity gradient between 230 km and the 400 km discontinuity. Model NWB-1 is smoother than some models that have been previously proposed for this region which may have mapped lateral heterogeneity into vertical velocity profiles.

Notes: Abstract Oxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite-kyanite to K-feldspar-muscovite-sillimanite zones, from Mica Creek, British Columbia. The δ18O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high-grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid.Temperature estimates based upon 18O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet-biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet-biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet-biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high-grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they significantly exceed 600°C.

Notes: Abstract Chemical analysis (including H2, F2, FeO, Fe2O3) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca18.92Mg1.88Fe3+0.40Al10.97Si17.81- O69.0.1(OH)8.84F0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca19Mg(MgAl7)Al4Si18O69(OH)9, which is related to Fe3+-vesuvianite by the substitutions Mg + OH = Fe3++ O in the B and O(10) sites and Fe3+= Al in the AlFe site.Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al2O3-SiO2-H2O-CO2 shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH2O 〉 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH2O 〉 0.97).