The Milos volcanic field includes a well-exposed volcaniclastic succession which records a long history of submarine explosive
volcanism. The Bombarda volcano, a rhyolitic monogenetic center, erupted ∼1.7 Ma at a depth <200 m below sea level. The aphyric
products are represented by a volcaniclastic apron (up to 50 m thick) and a lava dome. The apron is composed of pale gray
juvenile fragments and accessory lithic clasts ranging from ash to blocks. The juvenile clasts are highly vesicular to non-vesicular;
the vesicles are dominantly tube vesicles. The volcaniclastic apron is made up of three fades: massive to normally graded
pumice-lithic breccia, stratified pumice-lithic breccia, and laminated ash with pumice blocks. We interpret the apron beds
to be the result of water-supported, volcaniclastic mass-How emplacement, derived directly from the collapse of a small-volume,
subaqueous eruption column and from syn-eruptive, down-slope resedimentation of volcaniclastic debris. During this eruptive
phase, the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion.
The lava dome, emplaced later in the source area, is made up of flow-banded lava and separated from the apron by an obsidian
carapace a few meters thick. The near-vertical orientation of the carapace suggests that the dome was intruded within the
apron. Remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement.
Published online: 30 January 2003
Editorial responsibility: J. McPhie 相似文献
Comprehensive studies, based on isotope geochemistry of C, H, O, S and Sr, chronology, common element and trace element geochemistry of fluid inclusions for the epithermal Au, As, Sb and Hg deposits in the Youjiang Basin and its peripheral areas, suggested that the ore fluid was the basin fluid with abundant metallic elements and the large-scale fluid flow of the same source in the late Yenshan stage was responsible for huge epithermal mineralization and silicification. The ore fluid flowed from the basin to the platform between the basin and the platform and migrated from the inter-platform basin to the isolated platform in the Youjiang Basin. The synsedimentary faults and paleokast surface acted respectively as main conduits for vertical and lateral fluid flow. 相似文献
20301 Pn arrival time data are collected from the seismological bulletins of both national and regional seismic networks. Pn travel time residuals are tomographically inverted for the Pn velocity structure of uppermost mantle beneath North China. The result indicates that the average Pn velocity in North China is 7.92 km/s, and the velocity varies laterally from ?0.21 to +0.29 km/s around the average. The approximately NNE trending high and low velocity regions arrange alternatively west-eastward. From west to east we can see high velocity in the middle Ordos region, the Shanxi graben low, the Jizhong depression high, the west Shandong uplift and Bohai Sea low, and the high velocity region to the east of the Tanlu fault. In the southern boundary zone of the North China block, except for the high velocity in the Qingling Mountains region, the velocity is generally lower than the average. Obvious velocity anisotropy is seen in the Datong Cenozoic volcanic region, with the fast velocity direction in NNE-SSW. Notable velocity anisotropy is also seen around the Bay of Bohai Sea, and the fast velocity directions seem to show a rotation pattern, possibly indicating a flow-like deformation in the uppermost mantle there. The Pn velocity variations show a reversed correlation with the Earth's heat flow. The low Pn velocity regions generally show high heat flow, e.g., the Shanxi graben and Bohai Sea region. While the high Pn velocity regions usually manifest low heat flow, e.g., the region of Jizhong depression. This indicates that the Pn velocity variation in the study region is mainly aroused by the regional temperature difference in the uppermost mantle. Strong earthquakes in the crust tend to occur in the region with the abnormal low Pn velocity, or in the transition zone between high and low Pn velocity regions. The earthquakes in the low velocity region are shallower, while that in the transition zone are deeper. 相似文献
The design of a drainage system for a roofing slate quarry was implemented by the enhancement of discharge peak estimation, and the uncertainty inevitably associated with the engineering model was reduced.
The development of a topographical, geological, and vegetation cover database developed from a Geographical Information System (GIS) allowed for the definition of the drainage network for a hydraulic system, along with the calculation of the runoff coefficient. This is applied to the digital model of accumulated flow (DMF) as a weight correction coefficient, using a matrix-based model at 5×5 m resolution. The new digital model of corrected accumulated flow (DMCF) is the result of combining the thematic maps with the map of slope <3%, which was previously created from the slope model. It is demonstrated that this new model allows to apply the “Rational Method” on cartographic units defined by the GIS.
The DMCF is compared with other traditional applications of the Rational Method based on the calculation of the discharge peak considering: (1) the drainage basin as a single watershed or (2) defining an average runoff coefficient in each sub-watershed. Both approaches have bigger discharge peaks than those obtained by the DMCF since the slope, lithology, and vegetation cover have average values, and the runoff coefficient is poorly defined, increasing the uncertainty in the discharge peak. 相似文献