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451.
452.
Relative sea-level change at the time of, and since, the most recent great earthquake at the Cascadia subduction zone is estimated from intertidal sediments at three marshes on western Vancouver Island, British Columbia. We compare the elevation of the pre-earthquake surface, which is marked by a tsunami sand sheet, with the modern depositional elevation range of the sediment type upon which the sand was deposited. At a site south of the Nootka fault zone, which is the northern boundary of the subducting Juan de Fuca plate, tidal mud overlies the pre-earthquake marsh surface. The stratigraphy at this site indicates 0.2–1.6 m of coseismic submergence and 1.1 m of subsequent emergence. In contrast, two sites to the north lack obvious stratigraphic evidence for coseismic land-level change and record between 0.1 and 1.7 m of post-earthquake submergence. These results indicate a difference in tectonic environment across the Nootka fault zone and suggest that plate-boundary rupture during the last great Cascadia earthquake probably did not extend north of central Vancouver Island. 相似文献
453.
454.
Marotta Anna Maria Seitz Kurt Barzaghi Riccardo Grombein Thomas Heck Bernhard 《Studia Geophysica et Geodaetica》2019,63(3):321-344
Studia Geophysica et Geodaetica - Forward modelling in the space domain is a very important task in geodesy and other geosciences. From topographical or isostatic information in the form of digital... 相似文献
455.
The occurrence of the invasive nonindigenous copepod Oithona davisae Ferrari and Orsi, 1984, is reported for the first time in the Aegean Sea. The data we collected in August 2017 from 14 stations along the Turkish coast of the Aegean Sea reveal the spatial distribution of O. davisae between the openning of the Dardanelles Strait in the north and the Izmir Bay in the south. The O. davisae individuals, in seven mesozooplankton samples collected from a single station, were consistently found in the inner part of the Izmir Bay from April 2015‐October 2016. The abundance of female O. davisae ranged from 4 ind./m3 in April 2015 to 31,524 ind./m3 in July 2016 and contributed to the total oithonid female population by 10.8% in April 2015 and 92.8% in September 2016. Our results show that this species is well established in the inner part of Izmir Bay and that it has become a permanent component of the copepod community in the area. 相似文献
456.
Fine- and microstructure observations indicate bottom-intensified turbulent dissipation above rough bathymetry associated with internal wave breaking. Simple analytic representations for the depth profile of turbulent dissipation are proposed here under the assumption that the near bottom wavefield is dominated by a baroclinic tide. This scheme is intended for use in numerical models and thus captures only the gross features of detailed solutions to the energy balance of the internal wavefield. The possible sensitivity of the magnitude and vertical variability of the dissipation rate profile to various environmental parameters is discussed. An expression for the diapycnal buoyancy flux is presented that explicitly treats the difference between the height of an isopycnal above the mean bottom and the actual bottom. This returns a diapycnal velocity estimate that is consistent with both tracer observations of downwelling and a basin scale mass budget that requires upwelling. 相似文献
457.
Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal‐scale strike‐slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal‐scale vertical strike‐slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike‐slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike‐slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength. 相似文献
458.
Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct
cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the
interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or
reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite
facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of
the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and
a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo
veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of
zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential
to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from
geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during
late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at
temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase
by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation
of biotite is the second important reaction in the alteration process. The reactions release components for the formation
of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall
rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall
rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction
front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase
alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor aqueous liquid. 相似文献
459.
A set of sheeted quartz veins cutting 380 Ma monzogranite at Sandwich Point, Nova Scotia, Canada, provide an opportunity to
address issues regarding fluid reservoirs and genesis of intrusion-related gold deposits. The quartz veins, locally with arsenopyrite
(≤5%) and elevated Au–(Bi–Sb–Cu–Zn), occur within the reduced South Mountain Batholith, which also has other zones of anomalous
gold enrichment. The host granite intruded (P = 3.5 kbars) Lower Paleozoic metaturbiditic rocks of the Meguma Supergroup, well known for orogenic vein gold mineralization.
Relevant field observations include the following: (1) the granite contains pegmatite segregations and is cut by aplitic dykes
and zones (≤1–2 m) of spaced fracture cleavage; (2) sheeted veins containing coarse, comb-textured quartz extend into a pegmatite
zone; (3) arsenopyrite-bearing greisens dominated by F-rich muscovite occur adjacent the quartz veins; and (4) vein and greisen
formation is consistent with Riedel shear geometry. Although these features suggest a magmatic origin for the vein-forming
fluids, geochemical studies indicate a more complex origin. Vein quartz contains two types of aqueous fluid inclusion assemblages
(FIA). Type 1 is a low-salinity (≤3 wt.% equivalent NaCl) with minor CO2 (≤2 mol%) and has T
h = 280–340°C. In contrast, type 2 is a high-salinity (20–25 wt.% equivalent NaCl), Ca-rich fluid with T
h = 160–200°C. Pressure-corrected fluid inclusion data reflect expulsion of a magmatic fluid near the granite solidus (650°C)
that cooled and mixed with a lower temperature (400°C), wall rock equilibrated, Ca-rich fluid. Evidence for fluid unmixing,
an important process in some intrusion-related gold deposit settings, is lacking. Stable isotopic (O, D, S) analyses for quartz,
muscovite and arsenopyrite samples from vein and greisens indicate the following: (1) δ18Oqtz = +11.7‰ to 17.8‰ and δ18Omusc = +10.7‰ to +11.2‰; (2) δDmusc = −44‰ to−54‰; and (3) δ34Saspy = +7.8‰ to +10.3‰. These data are interpreted, in conjunction with fluid inclusion data, to reflect contamination of a magmatic-derived
fluid (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}} ≤ +10‰) by an external fluid (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}} ≥ +15‰), the latter having equilibrated with the surrounding metasedimentary rocks. The δ34S data are inconsistent with a direct igneous source based on other studies for the host intrusion (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}} = +5‰) and are, instead, consistent with an external reservoir for sulphur based on δ34SH2S data for the surrounding metasedimentary rocks. Divergent fluid reservoirs are also supported by analyses of Pb isotopes
for pegmatitic K-feldspar and vein arsenopyrite. Collectively the data indicate that the vein- and greisen-forming fluids
had a complex origin and reflect both magmatic and non-magmatic reservoirs. Thus, although the geological setting suggests
a magmatic origin, the geochemical data indicate involvement of multiple reservoirs. These results suggest multiple reservoirs
for this intrusion-related gold deposit setting and caution against interpreting the genesis of intrusion-related gold deposit
mineralization in somewhat analogous settings based on a limited geochemical data set. 相似文献
460.
Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408?Ma; Dufferin, 380?Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their ??18Oquartz signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average ??18O values of +15.2?±?0.9?? (n?=?16) for The Ovens and +15.7?±?0.6?? (n?=?12) for Dufferin. Using an average ??18O value of +15.4?? for the two deposits, the corresponding ??18OH2O values, calculated for 400°C and 350°C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4?±?0.2?? and +10.2?±?0.2??, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30?Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in ??18OH2O values for deposits when arranged in their relative stratigraphic position such that ??18OH2O values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy. 相似文献