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1.
Sheila J Seaman William C McIntosh John W Geissman Michael L Williams Wolfgan E Elston 《Bulletin of Volcanology》1991,53(6):460-476
Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K
1 axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K
1 susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K
1 orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information. 相似文献
2.
Flow directions are estimated from the measurement of the magnetic fabric of 106 samples, collected at 18 sites in four welded tuff units in the central San Juan Mountains of southern Colorado. The estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites. Errors in the estimation may arise from topographic variation, rheomorphism (post-emplacement mass flow) within the tuff, and other factors. Magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth. A test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff. This test is positive for the four units examined. Paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest Colorado, during the time (28.2–26.5 Ma) of emplacement. 相似文献
3.
In west-central Nevada, the Oligocene Candelaria pyroclastic sequence reaches a local thickness of up to 1.3 km, in what has
been referred to as the Candelaria trough, but more generally the accumulation of ash-flow tuffs and related volcanic rocks
is less than 300 m thick. Complete to near complete outcrops are scattered over about 3200 km2 in the Candelaria Hills and surrounding ranges of the Southern Walker Lane structural zone. Three regionally extensive compound
cooling units within the overall sequence (25.8 Ma Metallic City, 24.1 Ma Belleville, and 23.7 Ma Candelaria Junction Tuffs)
have distinguishing characteristics and are the focus of study. At 106 sites, anisotropy of magnetic susceptibility (AMS)
data provide an estimate of transport direction of each tuff. Inferred transport directions based on the AMS data are corrected
for a modest clockwise, yet variable magnitude, vertical axis rotation that affected these rocks in late Miocene to Pliocene
time, as revealed by paleomagnetic studies. The AMS data show a somewhat orderly pattern of magnetic fabrics that we interpret
to define unique transport directions for the Metallic City and Candelaria Junction Tuffs. The low susceptibility and degree
of anisotropy of the Belleville Tuff limits our interpretation from this pyroclastic deposit. The Metallic City and Candelaria
Junction Tuffs typically show gentle, south–southeast and southeast dipping magnetic fabric imbrication, respectively, and
very gently plunging magnetic lineations. These AMS fabric elements indicate the tuffs were transported to the north–northwest
and northwest, respectively. The AMS fabric data from the Metallic City and Candelaria Junction Tuffs suggest relatively unrestricted
flow during emplacement. Evidence across the 3,200 km2 area to support more regionally controlled channelized flow into and/or flow along the east northeast–west southwest axis
of the Candelaria trough is lacking. The ignimbrites clearly filled a topographic depression inferred to have formed concurrent
with early, localized Basin and Range extension during pyroclastic emplacement, but based on the uniformity of AMS fabric
data, we infer that the depression quickly filled and did not hinder flow across the region. Unrecognized eruptive centers
for the three ignimbrites may lie buried beneath Neogene basin fill sediments south–southeast of the Candelaria Hills or concealed
below younger deposits farther southeast into the Palmetto Mountains.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Revised and prepared for publication in the Bulletin of Volcanology. 相似文献
4.
结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及Kmin与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸. 相似文献
5.
H. -U. Schmincke 《Bulletin of Volcanology》1974,38(2):594-636
Peralkaline silicic welded ash-flow tuffs differ characteristically in a number of properties from most calc-alkaline welded tuffs, due to their generally lower viscosity and higher temperatures. For example, individual cooling units are relatively small (less than 30 m thick, less than 5 km3 in volume); rocks can be thoroughly welded and crystallized to feldspar, quartz, and mafic minerals; several primary deformational structures (e.g. lineations, stretching of pumice, folds, ramp structures) indicate late stage laminar creep, resulting from the low yield strength of the nearly homogeneous glass of very low viscosity. Theoretical considerations also suggest that peralkaline melts are of low viscosity and high temperature, as inferred from,e.g., their chemical composition (high iron- and alkali-, and low alumina-concentrations). The low viscosity may also be due to trapping of volatiles. Absence or paucity of OH-bearing phenocryst phases, paucity of pyroclastic rocks, other than ash flow tuffs, formed from highly explosive eruptions, and apparently high crystallization temperatures, indicate that peralkaline silicic magmas are comparatively dry. The common occurrence of peralkaline ash-flow tuffs may be due to an increased water content of the magmas, resulting also in amphibole phenocrysts in some welded tuffs, or to specific volcanotectonic conditions. Ash flows of peralkaline composition move as particularly dense particulate flows. This type of flowage and the very rapid welding of the low viscosity glass lead to a high degree of homogenization of the fine glass shards. This in turn inhibits complete degassing of the collapsing ash flow. Semiclosed systems result where gas overpressures can develop and where volatiles play an important role by fluxing crystallization and transporting dissolved matter. Several types of vesicles can form under these conditions: (a) Spherical vesicles within collapsed ash and pumice particles formed after deposition of the ash flow. (b) Round or irregular vesicles transsecting pyroclastic particles, vesicle sheets, and large cavities, several m in diameter, may form in a largely homogenized ash-flow tuff beneath tightly welded layers. (c) Lensoid cavities formed during granophyric crystallization of large pumice particles. Small ash particles of peralkaline composition may assume spherical shapes due to their low viscosity and in some cases, expansion of bubbles. They form during transport and are preserved under low load pressure in the top part of cooling units. Globule lavas and most froth flows are interpreted as welded ash-flow tuffs, some of their unusual features being due to their peralkaline composition. 相似文献
6.
Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process. 相似文献
7.
Schmincke andSwanson (1967) explained laminar flowage structures as indicators for flow direction of pyroclastic flows that show a radial flow pattern away from the source. Several other authors have reported similar examples, but the influence of pre-flow topographic relief has not been analyzed. Flow lineations were measured for the Ata pyroclastic flow deposit, southwestern Japan. This deposit has covered an undulating basement topography. Preferred orientation of crystals and lithic fragments were measured on thin sections cut parallel to sedimentary layering. The following three factors which control the flow lineation have been recognized. 1) Flow lineations oriented radially away from the source, as described by previous authors, were obtained only for samples collected from the surface of the pyroclastic flow plateau where the basement valleys were nearly filled by earlier flow units. 2) Lineations near the floor of narrow valleys were parallel to the strike of the valley. 3) Flow lineations near the wall of valleys tend to be parallel to the dip of the valley walls. These data suggest that the initial radial movement of pyroclastic flows from the source gradually changes direction to parallel the strike of deep valleys due to confining effect of valley wall. Flows which are trapped within a valley, tends to move towards the bottom of the valley just prior to the final settlement. After the basement topographic relief has been filled up with earlier flow units, the later flows maintain their original radial movement until final settlement. 相似文献
8.
Porosity, permeability, and fluid flow in the Yellowstone geothermal system, Wyoming 总被引：3，自引：0，他引：3
Patrick F. Dobson Timothy J. Kneafsey Jeffrey Hulen Ardyth Simmons 《Journal of Volcanology and Geothermal Research》2003,123(3-4):313-324
Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred. 相似文献
9.
H. C. Palmer W. D. MacDonald C. S. Gromme B. B. Ellwood 《Bulletin of Volcanology》1996,58(2-3):101-116
Anisotropy of magnetic susceptibility (AMS) and characteristic remanence were measured for 45 sites in the 0.76 Ma Bishop
tuff, eastern California. Thirty-three sites were sampled in three stratigraphic sections, two in Owens gorge south of Long
Valley caldera, and the third in the Adobe lobe north of Long Valley. The remaining 12 sites are widely distributed, but of
limited stratigraphic extent. Weakly indurated, highly porous to dense, welded ash-flow tuffs were sampled. Saturation magnetization
vs temperature experiments indicate two principal iron oxide phases: low Ti magnetites with 525–570 °C Curie temperatures,
and maghemite with 610°–640 °C Curie temperatures. AF demagnetization spectra of isothermal remanent magnetizations are indicative
of magnetite/maghemite predominantly in the multidomain to pseudo-single domain size ranges. Remeasurement of AMS after application
of saturating direct fields indicates that randomly oriented single-domain grains are also present. The degree of anisotropy
is only a few percent, typical of tuffs. The AMS ellipsoids are oblate with Kmin axes normal to subhorizontal foliation and Kmax axes regionally aligned with published source vents. For 12 of 16 locality means, Kmax axes plunge sourceward, confirming previous observations regarding flow sense. Topographic control on flow emplacement is
indicated by the distribution of tuff deposits and by flow directions inferred from Kmax axes. Deposition east of the Benton range occurred by flow around the south end of the range and through two gaps (Benton
notch and Chidago gap). Flow down Mammoth pass of the Sierra Nevada is also evident. At least some of the Adobe lobe in the
northeast flowed around the west end of Glass mountain. Eastward flow directions in the upper Owens gorge and southeast directions
in the lower Owens gorge are parallel to the present canyon, suggesting that the present drainage has been established along
the pre-Bishop paleodrainage. Characteristic remanence directions from 45 sites (267 samples) yield an overall mean of D=348°,
I=53° for the Bishop tuff. A correlation is found in two of the three profiles between density and remanence inclination.
A mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash-fall
tuff at Lake Tecopa is: D=353°, I=54°, k=172, α95=2.9°, N=15.
Received: 11 July 1995 / Accepted: 29 February 1996 相似文献
10.
Simon A. Stewart 《Earth and Planetary Science Letters》1995,130(1-4):57-67
The conventional corrections for bedding dip in palaeomagnetic studies involve either untilting about strike or about some inclined axis—the choice is usually governed by the perceived fold hinge orientation. While it has been recognised that untilting bedding about strike can be erroneous if the beds lie within plunging fold structures, there are several types of fold which have plunging hinges, but whose limbs have rotated about horizontal axes. Examples are interference structures and forced folds; restoration about inclined axes may be incorrect in these cases. The angular errors imposed upon palaeomagnetic lineation data via the wrong choice of rotation axis during unfolding are calculated here and presented for lineations in any orientation which could be associated with an upright, symmetrical fold. This extends to palaeomagnetic data previous analyses which were relevant to bedding-parallel lineations. This numerical analysis highlights the influence of various parameters which describe fold geometry and relative lineation orientation upon the angular error imparted to lineation data by the wrong unfolding method. The effect of each parameter is described, and the interaction of the parameters in producing the final error is discussed. Structural and palaeomagnetic data are cited from two field examples of fold structures which illustrate the alternative kinematic histories. Both are from thin-skinned thrust belts, but the data show that one is a true plunging fold, formed by rotation about its inclined hinge, whereas the other is an interference structure produced by rotation of the limbs about non-parallel horizontal axes. Since the angle between the palaeomagnetic lineations and the inclined fold hinge is equal on both limbs in the former type of structure, but varies from limb to limb in the latter, a simple test can be defined which uses palaeomagnetic lineation data to identify rotation axes and hence fold type. This test can use pre- or syn-folding lineations and could be useful in areas of non-coaxial folding. 相似文献
11.
The 29.5 Ma Wah Wah Springs Formation which erupted from the Indian Peak Caldera has an estimated volume of > 3900 km3 making it one of the largest ignimbrites on earth. The magma was calc-alkaline, dacitic (68 wt. % SiO2) and phenocryst-rich (38 vol.%). Phenocrysts include plagioclase (An 47), magnesio-hornblende, Mg-biotite, quartz, Fe-Ti oxides, diopsidic-augite, and rare Ca-poor pyroxene, in order of decreasing abundance. Apatite, zircon and pyrrhotite occurs as inclusions within phenocrysts. Atmospheric glass losses (1040 km3) account for bulk-rock compositions that have SiO2 contents ranging from 63 to 67 wt.%. Glass compositions are high-silica rhyolite.Phenocrysts equilibrated at temperatures ranging from about 790 to 850°C and oxygen fugacities approximately 2.6 log units above the QFM buffer. Confining pressure estimates using the aluminum-in-hornblende geobarometer calibrated for calc-alkaline volcanic rocks suggest a mean pressure of 230±50 MPa corresponding to 7.5±1.5 km depth. These estimates are consistent with caldera formation accompanying emplacement.Crystal compositions for phenocrysts and mineral inclusions within phenocrysts are remarkably homogeneous throughout the outflow tuff, although minor zoning does occur. Given the dacitic composition of the magma, the weakly zoned phenocryst population cannot be modeled to produce the observed high-silica glass (melt) indicating open-system behavior for the magma. The high-silica rhyolite glass is interpreted to be an artifact of efficient magma mixing accompanying addition of highly evolved magma, or melt to intermediate composition magma. Mixing was followed by magma hybridization. Additional support for this hybridization model includes: (1) physically and chemically distinct populations of augite; (2) minor but unbiquitous resorbed plagioclase, biotite and hornblende phenocrysts; and (3) reverse zoning in some of the plagioclase euhedra within pumice lapilli. 相似文献
12.
Anisotropy of magnetic susceptibility (AMS) has been used to interpret flow directions in ignimbrites, but no study has demonstrated
that the AMS fabric corresponds to the flow fabric. In this paper, we show that the AMS and strain fabric coincide in a high-grade
ignimbrite, the Nuraxi Tuff, a Miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well-defined
magnetic fabric. Natural remanent magnetization (NRM) data indicate that the magnetization of the tuff is homogeneous and
was acquired at high temperatures by Ti-magnetite crystals. Comparison between the magnetic fabric and the deformation features
along a representative section shows that AMS and anisotropy of isothermal remanent magnetization (AIRM) fabric are coaxial
with and reproduce the shape of the strain ellipsoid. Magnetic tests and scanning electron microscopy observations indicate
that the fabric is due to trails of micrometer-size, pseudo-single domain, magnetically interacting magnetite crystals. Microlites
formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff. The fabric was thus
acquired after deposition, before late rheomorphic processes, and accurately mimics homogeneous deformation features of the
shards during welding processes and mass flow. 相似文献
13.
Jean-Luc Schneider Claude Fourquin Jean-Claude Paicheler 《Journal of Volcanology and Geothermal Research》1992,49(3-4)
Pyroclastic deposits interpreted as subaqueous ash-flow tuff have been recognized within Archean to Recent marine and lacustrine sequences. Several authors proposed a high-temperature emplacement for some of these tuffs. However, the subaqueous welding of pyroclastic deposits remains controversial.The Visean marine volcaniclastic formations of southern Vosges (France) contain several layers of rhyolitic and rhyodacitic ash-flow tuff. These deposits include, from proximal to distal settings, breccia, lapilli and fine-ash tuff. The breccia and lapilli tuff are partly welded, as indicated by the presence of fiamme, fluidal and axiolitic structures. The lapilli tuff form idealized sections with a lower, coarse and welded unit and an upper, bedded and unwelded fine-ash tuff. Sedimentary structures suggest that the fine-ash tuff units were deposited by turbidity currents. Welded breccias, interbedded in a thick submarine volcanic complex, indicate the close proximity of the volcanic source. The lapilli and fine-ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales. Presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm-wave base.In northern Anatolia, thick massive sequences of rhyodacitic crystal tuff are interbedded with the Upper Cretaceous marine turbidites of the Mudurnu basin. Some of these tuffs are welded. As in southern Vosges, partial welding is attested by the presence of fiamme and fluidal structures. The latter are frequent in the fresh vitric matrix. These tuff units contain a high proportion of vitroclasis, and were emplaced by ash flows. Welded tuff units are associated with non-welded crystal tuff, and contain abundant bioclasts which indicate mixing with water during flowage. At the base, basaltic breccia beds are associated with micritic beds containing a marine fauna. The welded and non-welded tuff sequences are interbedded in an alternation of limestones and marls. These limestones are rich in pelagic microfossils.The evidence above strongly suggest that in both examples, tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment. The sources of the pyroclastic material are unknown in both cases. We propose that the ash flows were produced during submarine fissure eruptions. Such eruptions could produce non-turbulent flows which were insulated by a steam carapace before deposition and welding. The welded ash-flow tuff deposits of southern Vosges and northern Anatolia give strong evidence for existence of subaqueous welding. 相似文献
14.
An idealized numerical study of the influence of a tidal flow around an island has been undertaken with ROMS. The study focusses on coastal island wakes which are mainly controlled by elliptical tidal current flows on shallow shelves. This model is typical of some isolated continental shelf islands. The model is forced by a semi-diurnal barotropic inertia gravity wave imposed on the four open boundaries of a rectangular domain and its propagation results in an elliptical tidal flow within the domain in which the circular island lies. The influence of the surrounding island bathymetry and of the ellipse shape has been studied both in two and three dimensions. In the island vicinity, the residual circulation patterns over a tidal period show alongshore flow divergence along the major axis and convergence along the minor axis. A thin tidal ellipse (i.e. with a large ratio between major and minor axes) leads to strong eddy activity periods in the lee of the island during the flood and ebb phases, with eddy dissipation phases in between. By contrast, an almost round ellipse (axis ratio nearly 1) leads to vorticity filaments which continuously progress around the island without eddy shedding. The presence of a topographic slope in the vicinity of the island strengthens the eddy activity. This study suggests that the tidal current rotation favors the development of the eddy rotating in the same direction and weakens the development of the second eddy. In three dimensions with a surrounding bathymetry, an intense upwelling occurs in a large area in the lee of the island and the vertical velocities are stronger with thinner ellipses. With a flat bottom the vertical motions are almost fully generated by convergence and divergence of the secondary flow. With a varying bottom topography, the vertical motions come from a combination of this mechanism with convergence and divergence of the depth averaged flow. 相似文献
15.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen Yannick Branquet Dominique Panis Pierre Camps Pierre Rochette Angels Canals 《Earth and Planetary Science Letters》2009,277(3-4):501-513
The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric. 相似文献
16.
Along the south coast of Arabia, between Aden and the southern entrance to the Red Sea, there are six central vent volcanoes of probable Pliocene age. All are characterised by the interstratification of basic and acidic extrusives, the formation of large central calderas at a late stage in the volcanic cycle and the subsequent infilling of these calderas with horizontal acidic ignimbrites and basic lavas. Lying 60 miles to the west of Aden and of particular interest is Jebel Khariz, the largest and best preserved of the six volcanic centres, covering a roughly circular area of about 100 square miles and rising to a height of 2,766 feet. The volcanic sequence of Jebel Khariz is broadly divisible into two suites: a) alkali-rich rhyolites and trachytes which occur as flows and pyroclastic horizons and form about 80 per cent of the volume of the cone, and b) effusives of basaltic composition that occur in the caldera, locally on the south-east and south-west flanks and in a small parasitic cone on the northern flank. The alkali-rich acidic suite includes lavas, ash-flow and ash-fall rocks as well as vent and flow breccias, Generally, all rocks of this suite have phenocrysts of anorthoclase, and may contain phenocrysts of fayalitic olivine, aegirine-augite, magnetite and/or quartz. The fine grained matrix is composed of the same minerals with skeletal riebeckite and, in some cases, cossyrite. The basaltic suite is characteristically porphyritic, the phenocrysts being of calcic plagioclase, clinopyroxene, olivine and magnetite in a fine-grained mesostasis of plagioclase, olivine, clinopyroxene and ore. The plagioclase, on initial investigation, appears to lie in the labradorite-bytownite range, the olivine is commonly replaced by iddingsite and the clinopyroxene is most commonly a pale mauve titanaugite. Near the centre of the volcanic pile, as exposed in the caldera wall, masses of rhyolitic composition can be seen to form over half of the volcanic sequence. These masses are markedly lenzoid in cross-section normal to the flow direction and display intricate flow folding; they are considered to have been extruded as viscous lava. Further from the volcanic centre, these acidic extrusives become less markedly lenzoid until in the distal areas of individual units, some 5 miles from the caldera, they have spread out to form sheet-like masses covering as much as 10 square miles to a uniform thickness rarely exceeding 25 feet. The presence of agglomeratic bases, hard compact central sections and less compact upper divisions, together with the ubiquitous presence of columnar jointing and occasional shard textures suggest that these distal parts of each extrusive unit have been formed by an ash-flow/ash-fall mechanism. It is postulated that the majority of the Jebel Khariz volcanic pile was formed by emission of acidic material, effusive in the central area, but deposited mainly by an ash-flow mechanism around the flanks of the cone. This could be due to either the synchronous eruption of viscous lava from the central vent with ash flow eruptions on the flanks; or, more probably, to the progression of an individual volcanic episode through an initial ash-flow phase followed by the effusion of viscous lava, all emanating from the central vent. 相似文献
17.
At Shiotani, SW Japan, rhyolitic welded tuff forms a steep-sided funnel-shaped body, confined by Paleogene granitic rocks
to an elliptical area 1–1.5 km across. The Shiotani welded tuff is pervasively welded and foliated concordantly with the contact
that dips inward at angles of 70–90°. In contrast, nearby contemporary volcaniclastic deposits are non-welded and gently inclined.
Near the contact with the granite, the tuff is plastically deformed and shows lineations that plunge inward at angles of 40–65°.
Lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the
lineations indicating downward flow of the welded tuff. The geometry and internal structures suggest that the Shiotani welded
tuff was emplaced and welded in a funnel-shaped eruption conduit. Upon collapse of a plinian or phreatoplinian eruption column,
the majority of the conduit-filling pyroclasts probably fell back en masse into the conduit. Heat and steam from underlying
magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated
welding in the conduit, especially at deep levels. The hot welded pyroclasts then flowed down the conduit wall during welding
compaction and retreat of the magma. These processes resulted in increased welding toward the contacts and welding foliations
concordant with the steep wall. Emplacement of nearby correlative volcaniclastic mass-flow deposits in a shelf to upper bathyal
environment suggests a possibility that, when active, the Shiotani conduit was under the sea. Welding compaction would occur
even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access.
Received: 28 February 1996 / Accepted: 5 May 1997 相似文献
18.
Sheathfolds in rheomorphic ignimbrites 总被引：1，自引：1，他引：0
Structural reappraisal of several classic rheomorphic ignimbrites in Colorado, Idaho, the Canary Islands and Italy has, for the first time, revealed abundant oblique folds, curvilinear folds and sheathfolds which formed during emplacement. Like their equivalents in tectonic shear-zones, the sheathfold axes lie sub-parallel to a pervasive elongation lineation, and appear as eye structures on rock surfaces normal to the transport direction. With the recognition of sheathfolds, ignimbrites previously inferred to have undergone complex rheomorphic deformation histories are re-interpreted as recording a single, progressive deformation event. In some examples, the trends of sheathfolds and related lineations change with height through a single ignimbrite suggesting that rheomorphism did not affect the entire thickness of ignimbrite synchronously. Instead, we infer that in these ignimbrites a thin ductile shear-zone rose gradually through the aggrading agglutinating mass whilst the flow direction varied with time. This suggests that, in some cases, both welding and rheomorphism can be extremely rapid, with ductile strain rates significantly exceeding rates of ignimbrite aggradation.Editorial responsibility: T. Druitt 相似文献
19.
J. Urrutia-Fucugauchi 《Bulletin of Volcanology》1982,45(1):1-8
Anisotrophy of magnetic susceptibility (AMS) results from 27 specimens drilled from the top and two sides of a single columnar basalt segment are presented. The magnetic foliation plane is nearly horizontal for all parts of the column, which is consistent with a primary magma flow pattern, without evidence of local convection or differentiative processes. The shape of AMS ellipsoids is however predominantly prolate, which may be indicative of increased magnetic grain elongation due to crystal growth or grain realignment normal to a vertical stress field (due to thermal contraction). Apparent systematic variations related to column shape are found in bulk susceptibility, anisotropy degree and degree of lineation and foliation; some of the variation may also be related to weathering effects. The results are consistent with a primary AMS pattern resulting from thermal contractive stresses during column formation. Comparison of results from previous studies of columnar basalts reveals that there is a relatively large variation in AMS properties. There appears to exist a number of factors which may locally control the magnetic anisotropy of columns and very likely some of their other characteristics. 相似文献
20.
Low-field anisotropy of magnetic susceptibility (AMS) analyses were performed on 532 samples collected in 36 (mostly lower Pliocene to lower Pleistocene) marine clay sites from the Crotone basin, a fore-arc basin located on top of the external Calabrian accretionary wedge. The Crotone basin formed since mid-late Miocene under a predominant extensional tectonic regime, but it was influenced thereafter by complex interactions with NW–SE left-lateral strike-faults bounding the basin, which also yielded post-1.2 Ma ∼30° counterclockwise block rotations. The basin is filled by continental to marine sediments yielding one of the thickest and best-exposed Neogene succession available worldwide. The deep-marine facies – represented by blue-grey marly clays gave the best results, as they both preserved a clear magnetic fabric, and provided accurate chronology based on previously published magnetostratigraphy and calcareous plankton (i.e. foraminifers and nannofossils) biostratigraphy. Magnetic susceptibility range and rock magnetic analyses both indicate that AMS reflects paramagnetic clay matrix crystal arrangement. The fabric is predominantly oblate to triaxial, the anisotropy degree low (<1.06), and the magnetic foliation mostly subparallel to bedding. Magnetic lineation is defined in 30 out of 36 sites (where the e12 angle is <35°). By also considering local structural analysis data, we find that magnetic fabric was generally acquired during the first tectonic phases occurring after sediment deposition, thus validating its use as temporally dependent strain proxy. Although most of the magnetic lineations trend NW–SE and are orthogonal to normal faults (as observed elsewhere in Calabria), few NE–SW compressive lineations show that the Neogene extensional regime of the Crotone basin was punctuated by compressive episodes. Finally, compressive lineations (prolate magnetic fabric) documented along the strike-slip fault bounding the basin to the south support the significance of Pleistocene strike-slip tectonics. Thus the Crotone basin shows a markedly different tectonics with respect to other internal and western basins of Calabria, as it yields a magnetic fabric still dominated by extensional tectonics but also revealing arc-normal shortening episodes and recent strike-slip fault activity. The tectonics documented in the Crotone basin is compatible with a continuous upper crustal structural reorganization occurring during the SE-migration of the Calabria terrane above the Ionian subduction system. 相似文献