马尼拉俯冲带中段增生楔精细构造特征及微型圈闭盆地发育模式探讨

对马尼拉俯冲带中段高精度地形数据编辑后绘制成图,联合之前所获取的地震剖面数据,对俯冲带增生楔的精细构造地貌特征及发育模式进行了深入探讨.在增生楔下构造区发现了大量狭长的微型圈闭盆地,经过对其平面断裂特点与深部应力状态的分析,发现增生楔从俯冲前缘到脊顶区其构造特点的转变对应着微型圈闭盆地4个完整的发育阶段,即初期的加积断裂阶段、中期的圈闭成盆阶段、后期的挤压消亡阶段乃至最终的隆升推覆阶段,揭示出一种新型的俯冲带增生楔发育模式,并推断其本质是深部板片俯冲活动所产生的压应力在海底面的表现.     

Global Plate Motions and Their Impact on the Earth's Surface Area Change

Based on their Euler poles,we calculated the relative velocities between every two plates in the typical global plate motion models,respectively,and estimated the area change along these boundaries. In our calculations,plates on both sides accommodated area changes depending on the boundary types: extensional, convergent or transform, so we can estimate area change of each plate and then globally. Our preliminary results show that the area of the southern hemisphere increased while that of the northern hemisphere decreased over the past 1 million years,and global area has increased by 26,000km2 to 36,000km2,which corresponds to the 160m～250m increment on the Earth's radius if all these area increments are attributed to Earth's expansion. Taking the NUVEL-1 model as an example,of the 14 plates in this model,11 are decreasing,but the global area has increased because of the larger increment amount from Africa,North America and Antarctica. Finally,we also discussed factors affecting the global area change such as subduction zone retreating and back-arc spreading.     

Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar–Eskisehir), NW Anatolia, Turkey

Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar–Eskisehir, NW Anatolia) within the Izmir–Ankara–Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to ⁴⁰Ar/³⁹Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene–Early Miocene).The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na_0.82–0.96Fe⁺³_0.68–0.83) rims and aegirine–augite cores (containing calcium, magnesium, and Fe⁺²). Some aegirine–augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites.Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high ⁸⁷Sr/⁸⁶Sr (0.706358–0.708052) and low ¹⁴³Nd/¹⁴⁴Nd (0.512546–0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent through the thickened crust in an extensional setting.     

双地震带的影响因素探讨

讨论了全球39个俯冲带内的双地震带层间距、应力类型与俯冲参数的相互关系,这些俯冲参数包括动力学参数(板块年龄、热参数、板片拉力)、运动学参数(俯冲板块速度、上覆板块运动速度、海沟迁移速度、弧后形变特征)、几何形态参数(浅俯冲角、深俯冲角、俯冲深度、长度)及上覆板块性质等.结果表明:(1)I型双地震带易形成于年龄较古老(...     

Analysis and interpretation of the October 21, 1766 earthquake in the Southeastern Caribbean

The October 21, 1766 earthquake is the most widely felt event in the seismic history of Trinidad and Venezuela. Previous works diverged on the interpretation of the historical data available for this event. They associated the earthquake either with the Lesser Antilles subduction zone, with strike-slip motion along El Pilar fault, or with intraplate deformation at the edge of Guyana shield. Isoseismal areas are proposed after a new search and analysis of primary and secondary sources of historical information. Two of the largest earthquakes of the twentieth century which occurred in the region, the 1968 (M _S 6.4, h = 103 km), and the 1997 (M _W 6.9, h = 25 km) events, for which both intensity data and instrumentally determined source parameters are available, are used to calibrate the isoseismal areas and to interpret them in terms of source depth and magnitude. It is concluded that the large extent of intensity values higher than V is diagnostic of the depth (85 ± 20 km) of the 1766 source, and of local amplifications of ground motion due to soft soil conditions and to strong contrasts of impedance at the edge of Guyana shield. It is proposed that the event occurred either in slab, or close to the bottom lithospheric interface between the Caribbean and South American plates (∼11°N; ∼62.5°W). The value of the magnitude is estimated at 6.5 < M _S < 7.5 depending on the source depth and on the decay of ground motion as a function of distance. Deep and intermediate depth earthquakes can induce important casualties in Trinidad, Venezuela, and Guyana, possibly more damaging than those induced by shallower earthquakes along the strike of El Pilar Fault.     

Thermal equation of state of magnesite to 32 GPa and 2073 K

Pressure–volume–temperature relations have been measured to 32 GPa and 2073 K for natural magnesite (Mg_0.975Fe_0.015Mn_0.006Ca_0.004CO₃) using synchrotron X-ray diffraction with a multianvil apparatus at the SPring-8 facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K₀ = 97.1 ± 0.5 GPa and K′ = 5.44 ± 0.07, with fixed V₀ = 279.55 ± 0.02 Å³. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂K_T/∂T)_P = −0.013 ± 0.001 GPa/K and zero-pressure thermal expansion α = a₀ + a₁T with a₀ = 4.03 (7) × 10⁻⁵ K⁻¹ and a₁ = 0.49 (10) × 10⁻⁸ K⁻². The Anderson–Grüneisen parameter is estimated to be δ_T = 3.3. The analysis of axial compressibility and thermal expansivity indicates that the c-axis is over three times more compressible (K_Tc = 47 ± 1 GPa) than the a-axis (K_Tc = 157 ± 1 GPa), whereas the thermal expansion of the c-axis (a₀ = 6.8 (2) × 10⁻⁵ K⁻¹ and a₁ = 2.2 (4) × 10⁻⁸ K⁻²) is greater than that of the a-axis (a₀ = 2.7 (4) × 10⁻⁵ K⁻¹ and a₁ = −0.2 (2) × 10⁻⁸ K⁻²). The present thermal EOS enables us to accurately calculate the density of magnesite to the deep mantle conditions. Decarbonation of a subducting oceanic crust containing 2 wt.% magnesite would result in a 0.6% density reduction at 30 GPa and 1273 K. Using the new EOS parameters we performed thermodynamic calculations for magnesite decarbonation reactions at pressures to 20 GPa. We also estimated stability of magnesite-bearing assemblages in the lower mantle.     

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low fH₂O and low fO₂) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ground motions observed during the 15 August 2007 Pisco, Peru, earthquake

A M_w 7.9 earthquake event occurred on 15 August 2007 off the coast of central Peru, 60 km west of the city of Pisco. This event is associated with subduction processes at the interface of the Nazca and South American plates, and was characterised by a complex source mechanism involving rupture on two main asperities, with unilateral rupture propagation to the southeast. The rupture process is clearly reflected in the ground motions recorded during this event, which include two separate episodes of strong shaking. The event triggered 18 accelerographic stations; the recordings are examined in terms of their characteristics and compared to the predictions of ground-motion prediction equations for subduction environments, using the maximum-likelihood-based method of Scherbaum et al. (Bull Seismol Soc Am 94(6):2164–2185, 2004). Additionally, macroseismic observations and damage patterns are examined and discussed in the light of local construction practices, drawing on field observations gathered during the post-earthquake reconnaissance missions.