Detailed surface co-seismic displacement of the 1999 Chi-Chi earthquake in western Taiwan and implication of fault geometry in the shallow subsurface

The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations.     

Strain response and re-equilibration of CH4-rich synthetic aqueous fluid inclusions in calcite during pressure drops

Aqueous fluids in sedimentary basins often contain dissolved methane, particularly in petroleum environments. PVTX (Pressure-Volume-Temperature-Composition) reconstructions performed using fluid inclusion data are largely based on the assumption that inclusions do not change from the time of trapping until the present. Many authors, however, consider that fluid inclusions can re-equilibrate, particularly in fragile minerals like calcite. In order to understand this re-equilibration phenomenon in the metamorphic domain, previous experiments have been performed under high PT conditions, but few have been performed at low to medium PT conditions such as those associated with sedimentary burial diagenesis, and no previous studies have examined CH₄-bearing aqueous inclusions in calcite.An experimental study of the preservation/modification of CH₄-rich synthetic fluid inclusions in calcite during isothermal decompression was conducted. An autoclave was used for accurate PTX control allowing equilibrium between liquid and vapour in the CH₄-H₂O system. PTX conditions were maintained at four stages of decreasing pressure, with each stage held for 7 days to simulate an isothermal pressure drop. In order of decreasing pressure, the pressure-temperature conditions monitored were 276 ± 10 bar at 180 ± 7 °C, 176 ± 10 bar at 180 ± 7 °C, 76 ± 10 bar at 180 ± 7 °C and 10 ± 3 bar at 180 ± 15 °C. At the end of the experiment, the calcite was recovered and analyzed by microthermometry and Raman microspectroscopy for PTX reconstruction. A careful procedure was adopted to limit re-equilibration of inclusions during analytical procedures. Four types of inclusion shapes and four types of strain patterns were differentiated. Classification of the petrographic strain patterns was carried out. These strain patterns were associated with inclusion stretching and/or leakage regarding CH₄, T_h and P_h compared to experimental conditions. Factors controlling the preservation or acquisition of strain patterns included the initial shape and size of the inclusion, and the pressure differential (ΔP) between the confining pressure (P_cf) and the internal pressure (P_i) within the inclusion. Most fluid inclusions seemed to be trapped during the first 7 days of the experiment, although few (4%) of these preserved the initial PT conditions of 276 ± 10 bar, whereas 8% preserved the second and third run of PT conditions. Overall, the majority of inclusions (88%) did not reflect accurately the PTX trapping conditions. A petrographic guide to the inclusions is presented here that allows strain identification for PVT reconstructions. Re-equilibration patterns and evidence for preferential methane leakage from aqueous inclusions in calcite are important findings revealed by this study, and may be useful for the reconstruction of post-trapping events in investigations of natural samples, and in other experiments using synthetic inclusions in calcite.     

Modeling of the transport and deposition of tungsten in the scheelite-bearing calc-silicate gneisses of the Montagne Noire,France

Scheelite-bearing calc-silicate gneisses (CSG) oceur in the Montagne Noire within a series of dominant micaschists. Detailed petrographical and mineralogical studies reveal three successive stages of metamorphism and hydrothermal alteration: (1) stage 1, a regional metamorphism at 550°C and 4.5 kb where no mineralization is formed; (2) stage 2a, a hydrothermal alteration at 500 to 450°C and 4 to 3 kb which is characterized by an intense sericitization of feldpars and deposition of Sn in Sn-bearing cale-silicates; and (3) stage 2b, a hydrothermal alteration characterized by the crystallization of idocrasegrossular in CSG with concomittant precipitation of scheelite. Tungsten was transported through the micaschist environment and deposited as scheelite only in the CSG of stage 2b at relatively low pressures. To characterize the mechanism of tungsten transport, tungsten speciation at high P-T and scheelite solubility in aqueous solations buffered by the CSG and by the micaschists assemblages were calculated. It was found that H₂WO ₀ ⁴ , HWO _- ⁴ and WO _2- ⁴ are the dominant tungsten aqueous species in H₂O–NaCl (one molal) solutions at 500°C and 2–4 kb. Calculations also indicate that scheelite deposition is controlled by decreasing pressure and increasing activity of aqueous calcium in this system. This is consistent with the petrographical and mineralogical observations. The consequences of the presence of volatiles (N₂, CH₄, CO₂) in the regional fluids were examined by determining the effect of N₂ on tungsten speciation and scheelite solubility. The addition of N₂ (up to 10 mol%) to the mineralizing fluids results in a marked increase in H₂WO ₀ ⁴ and HWO _- ⁴ concentrations relative to WO _2- ⁴ and in a large decrease of scheelite solubility. This mechanism favours scheelite precipitation and accounts for the commonly observed association of W (and Sn) deposits with graphitic series generating mixed volatiles fluids.     

Pilot field experiment with canister-embedding clay under simulated repository conditions

Precompacted annular clay blocks surrounding a casing with a heater were emplaced in a borehole with 20 cm diameter and 2.8 m depth in granite. The heater was in operation for 8 months and raised the temperature of the clay to 176°C close to the casing. At the extraction the clay was completely water-saturated close to the rock and it had experienced an increased water content also close to the hot casing.

Mineralogical changes of the Ca-smectite clay were indicated by a slight increase in CEC and by an XRD-based conclusion that the content of low-charge smectite increased, while the initial content of kaolinite was reduced.     

The Effect of the Great Inequality on the Hecuba Gap

The effects of the "great inequality" (the quasi-resonance between Jupiter and Saturn) on the motion in the 2/1 mean motion resonance with Jupiter (the Hecuba gap) is investigated. We confirm the proposition made by Ferraz-Mello and collaborators that the great inequality generates secondary resonances which are likely to produce the slow diffusion observed in numerical investigations. We identify, in the restricted three body problem, the frequencies responsible for these secondary resonances. This revised version was published online in July 2006 with corrections to the Cover Date.