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1.
the single ignimbrite cooling unit E (average thickness, 28 m; volume, ca. 30 km3) forms the uppermost member of the Miocene Upper Mogán Formation on Gran Canaria. It is strongly chemically zoned from basal, first-erupted comendite (peralkaline rhyolite) to late-erupted trachyte, and, apart from an upper trachytic zone, it is densely welded. E was emplaced onto a surface inclined ca. 2–5° from the source caldera. Detailed mapping of key sections, up to 300 m long, exposed in barranco walls, ca. 10 km from the caldera margin, reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones. The shear zones formed within the lower-viscosity comenditic tuff. Extensional structures include mega-boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting. Mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs (compaction, lateral creep, folding, vertical density-driven diapirism) cannot account for these features, which reflect lateral (post-compactional) rheomorphic movement locally in excess of 800 m. We suggest the following sequence of events: emplacement of the several flow units; compaction, with little lateral movement; rheomorphic deformation. During and after compaction, layers of secondary porosity developed within the comenditic tuff, possibly where upward escape of gas was prevented by overlying, relatively impermeable layers of densely compacted ignimbrite. These structurally weak layers of high porosity subsequently acted as shear zones. 相似文献
2.
磁组构通常指磁化率各向异性,即AMS(Anisotropy of Magnetic Susceptibility),是一种重要的岩石组构,是弱变形沉积岩地区灵敏的应变指示计.近年来,AMS在造山带及前陆地区的广泛应用为构造变形研究提供了极大的帮助,同时提升了该方法的理论认识.本文在研读最新相关文献与著作的基础上,结合笔者及研究团队在龙门山地区获得的磁组构研究成果,综述了磁组构在沉积岩地区构造变形研究中的应用进展,并基于现有的研究认识对关键问题进行讨论,提出以下几点认识:(1)磁性矿物分析是AMS研究的关键,应结合多种岩石磁学实验及光学与电子显微构造研究手段展开详细的磁性矿物学分析;(2)磁化率椭球与应变椭球的对应主轴在绝大多数情况下相互平行,但在不同期次、不同种类复杂的磁性矿物组成,或者多期次构造变形的影响下,AMS与应变的关系相对复杂,应比对高场和低温AMS及非磁滞剩磁各向异性(AARM)测试结果,获得不同矿物的优选定向特征,并对获得的组构进行分期;(3)AMS可以揭示造山带及其前陆地区的构造演化历史,并且是分析断层相关褶皱的有限应变特征和变形机制的重要方法,同时也是厘定断裂带变形性状和期次及运动学分析的有效手段;(4)磁组构形成于成岩作用早期或构造变形的最早阶段,能很好地记录褶皱和逆冲作用之前的平行层缩短变形,因此可以揭示同沉积阶段的古构造应力方向.后期足够强烈的构造变形能局部改造或彻底掩盖先存AMS记录,构造流体有关的同构造期结晶矿物或先存矿物的重结晶导致的再定向被认为是其根本原因;(5)斜交磁线理是一种特殊的磁组构类型,反映了区域构造叠加或多期构造变形作用或隐伏斜向逆冲等可能的构造过程,有必要结合多方面的地质证据对其成因作出合理解释. 相似文献
3.
Rheomorphic ignimbrite D (13.4 Ma, Upper Mogán Formation on Gran Canaria), a multiple flow–single cooling unit, is divided
into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics
indicate contrasting deformation mechanisms during rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pure
uniaxial flattening perpendicular to the foliation; (b) an overlying shear zone characterized by asymmetric fabrics and a
significantly higher finite strain, with an ellipsoid geometry similar to stretched oblate bodies; (c) a central zone with
a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component;
and (d) a slightly deformed to non-deformed top zone where the almost random orientation of subspherical pyroclasts suggests
preservation of original, syn-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional
remobilization of a hot pyroclastic flow as shown by kinematic modeling based on: (a) the overall vertical structural zonation
suggested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of
the calculated vertical cooling progression at the base of the flow (formation of vitrophyre) and the related vertical changes
in strain geometry; (d) the complex lithification history; and (e) the consequent mechanisms of deformational flow. Rheomorphic
flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally
6–8°. We suggest that every level of newly deposited pyroclastic flow material of D first passed through a welding process
that was dominated by compaction (pure flattening) before rheomorphic deformation started.
Received: 25 June 1997 / Accepted: 28 October 1998 相似文献
4.
Perrine Paquereau-Lebti Michel Fornari Pierrick Roperch Jean-Claude Thouret Orlando Macedo 《Bulletin of Volcanology》2008,70(8):977-997
40Ar/39Ar ages and paleomagnetic correlations using characteristic remanent magnetizations (ChRM) show that two main ignimbrite sheets
were deposited at 4.86 ± 0.07 Ma (La Joya Ignimbrite: LJI) and at 1.63 ± 0.07 Ma (Arequipa Airport Ignimbrite: AAI) in the
Arequipa area, southern Peru. The AAI is a 20–100 m-thick ignimbrite that fills in the Arequipa depression to the west of
the city of Arequipa. The AAI is made up of two cooling units: an underlying white unit and an overlying weakly consolidated
pink unit. Radiometric data provide the same age for the two units. As both units record exactly the same well-defined paleomagnetic
direction (16 sites in the white unit of AAI: Dec = 173.7; Inc = 31.2; α95 = 0.7; k = 2749; and 10 sites in the pink unit of AAI; Dec = 173.6; Inc = 30.3; α95 = 1.2; k = 1634), showing no evidence of secular variation, the time gap between emplacement of the two units is unlikely to exceed
a few years. The >50 m thick well-consolidated white underlying unit of the Arequipa airport ignimbrite provides a very specific
magnetic zonation with low magnetic susceptibilities, high coercivities and unblocking temperatures of NRM above 580°C indicating
a Ti-poor titanohematite signature. The Anisotropy of Magnetic Susceptibility (AMS) is strongly enhanced in this layer with
anisotropy values up to 1.25. The fabric delineated by AMS was not recognized neither in the field nor in thin sections, because
most of the AAI consists in a massive and isotrope deposit with no visible textural fabric. Pumices deformation due to welding
is only observed at the base of the thickest sections. AMS within the AAI ignimbrite show a very well defined pattern of apparent
imbrications correlated to the paleotopography, with planes of foliation and lineation dipping often at more than 20° toward
the expected vent, buried beneath the Nevado Chachani volcanic complex. In contrast with the relatively small extent of the
thick AAI, the La Joya ignimbrite covers large areas from the Altipano down the Piedmont. Ti-poor titanomagnetites are the
dominant magnetic carriers and AMS values are generally lower than 1.05. Magnetic foliations are sub horizontal and lineations
directions are scattered in the LJI. The AMS fabrics are probably controlled by post-depositional compaction and welding of
the deposit rather than transport dynamics.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
5.
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
6.
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
7.
Michael H. Ort Giovanni Orsi Lucia Pappalardo Richard V. Fisher 《Bulletin of Volcanology》2003,65(1):55-72
The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions. 相似文献
8.
一般认为磁组构能有效地反映岩石所经历的应变特征.为了研究不同类型的磁组构和不同期次应变之间的关系,对来自华南地块两个地区的早三叠世灰岩样品进行了岩石磁学、磁组构以及应变特征的对比分析.来自湖北通山县的样品经历了三期构造变形,这为解析磁组构和多期次应变提供了理想的机会.岩石磁学结果显示携磁矿物主要为磁铁矿.磁化率各向异性（AMS）和非磁滞剩磁各向异性（AAR）结果显示其最小轴与层面垂直,最大轴和中间轴分布于层面内,反映了沉积和压实作用产生的应变,而后期构造应变在磁组构中没有体现.来自广东连县的样品发育有渗透性压溶缝面理和方解石脉,说明经历了构造应变.AMS结果没有显示占优势的组构方向.AAR结果显示三轴组构,其最大轴分布于最大应力方位,与构造应变特征吻合,最初的压实组构被构造应变组构所代替.上述结果表明：（1）AAR可以很好地反映渗透性应变的特征,而AMS有时会失效;（2）应变的尺度要小于样品的尺度,磁组构才能有效地反映应变. 相似文献
9.
B. Henry D. Jordanova N. Jordanova M. E. M. Derder B. Bayou M. Amenna D. Dimov 《Studia Geophysica et Geodaetica》2007,51(2):293-314
In a number of AMS studies, the presence and deciphering of composite magnetic fabrics is of major importance for a correct
interpretation of the data. On the basis of several examples from intrusive rocks (diorites and dolerites) we show that the
use of laboratory heatings can help to extract at least one component of the composite magnetic fabrics usually present. The
procedure includes comparison of the fabrics measured after stepwise laboratory heating with the fabrics determined by tensor
difference and by linear regression analysis.
In the diorite samples, the measured AMS results from the superimposition of different component fabrics and does not correspond
exactly to any of these fabrics. In these dykes, isolated magnetic fabric during thermal treatment corresponds to that of
the main magnetic mineral (Ti-poor titanomaghemite) and reveals an unknown structure. In volcanic flow or doleritic dykes,
a “parasitic” fabric related to late or post-magmatic evolution superimposed to the flow fabric can produce important scattering
of the AMS principal directions. Decomposition of magnetic fabric during thermal treatment allows isolation of the flow fabric. 相似文献
10.
近年来,针对秦岭造山带晚三叠世花岗岩体侵位机制的巨大争议,一些研究采用磁组构方法分析了岩体的内部组构特征及其与区域构造的关系,提出了具有重要意义的新认识.然而,目前这些研究均缺乏对岩体磁组构本质意义的分析,利用该方法约束岩体内部组构的可靠性并不十分清晰.针对这一问题,本文以秦岭造山带内具典型代表性意义的晚三叠世糜署岭花岗岩体为例,开展了该岩体的磁组构、岩石磁学、矿物形态组构和显微构造的综合研究.结果表明,糜署岭岩体的磁化率总体较低,属钛铁矿系列花岗岩.绝大部分样品的磁化率受控于顺磁性的黑云母等铁镁硅酸盐矿物,部分高磁化率样品包含了少量多畴磁铁矿等铁磁性组分的贡献,且随磁化率增大,铁磁性组分的贡献更为明显.样品的磁组构也主要是黑云母组构或由黑云母与磁铁矿的亚组构复合而成.由于样品中磁铁矿含量较低且与黑云母密切共生,磁组构与黑云母形态组构基本一致,因此,黑云母与磁铁矿的亚组构基本共轴.糜署岭岩体的磁组构本质上等同于黑云母组构,反映了黑云母等页硅酸盐矿物在岩体中的分布,可以有效的指示岩体的内部构造特征.宏观和显微构造观察还显示,糜署岭岩体的内部组构形成于岩浆侵位的晚期阶段,叠加了同岩浆期区域构造的关键信息,是从岩体构造角度开展区域构造演化的良好载体. 相似文献
11.
Till fabric associated with deformable beds 总被引：1，自引：0，他引：1
Jane K. Hart 《地球表面变化过程与地形》1994,19(1):15-32
This paper investigates the development of till fabric associated with glaciers overlying deformable beds. First, the till fabrics from both modern and ancient, soft bed and hard bed subglacial environments are investigated, and then soft bedded sites with specific strain histories are examined. The results indicate that lodgement tills had a strong fabric, whilst there is a wide range of much weaker fabric strengths associated with soft bed tills. It is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development. Field sites were chosen to isolate these two factors, and fabrics investigated in areas of known relative shear strain, and known relative thickness of the deforming layer. The results indicate that: (1) for a fixed deforming layer, fabric strength does increase with increasing deformation (except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain); and (2) lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength, whilst soft bed tills associated with a thick deforming layer have a low fabric strength. Where the deforming layer is thin, clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed. However, where the deforming layer is thicker and shear strains are high, melt-out processes encourage a parallel fabric, but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric. These processes combine to reduce the overall fabric strength. Till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till. 相似文献
12.
Neli Jordanova Diana Jordanova Vassil Karloukovski 《Studia Geophysica et Geodaetica》1996,40(1):36-49
Summary The anistropy of low field magnetic susceptibility has been studied for seven outcrops of loess sediments in North-Eastern
Bulgaria. Different sampling methods were applied in order to choose the best technique for obtaining the primary magnetic
fabric of such unconsolidated sediments. AMS results show significant changes in the petrofabric of samples collected by the
first technique which disturbs the original sedimentary fabric. The second applied technique does not cause such a strong
deformation but some disturbance of the magnetic fabric is probable. Typical sedimentary fabric is obtained from hand samples
and it is therefore concluded that this represents the best method for obtaining reliable AMS results from soft sediments. 相似文献
13.
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
14.
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
15.
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
16.
The magnetic anisotropy of two Miocene volcanic horizons south of the Bükk Mountains, northeast Hungary, was investigated (28 geographically distributed sites, over 400 samples). The horizons contain rocks from ignimbrites to porous tuffs. Microscopic and rock magnetic investigations showed that the main carrier of anisotropy is multidomain maghemite with varying composition. The complex magnetic fabric was separated into deformational and flow fabric in the upper horizon. This was carried out by monitoring the distribution of minimum susceptibility directions supposed to be elongated towards intermediates as a response to compressional strain. Jelínek's tensor statistics were used in the computations. Filtering out the effects of volcanic flow and compaction, north-south striking horizontal compressional strain was determined for the upper horizon in the geographic system. Owing to the lack of elongated distribution of minima, the characteristic intermediate susceptibility directions were taken as markers of strain directions in the lower horizon (WNW-ESE and NNE-SSW in the geographic system). The strain directions showed correlation with compressional stress orientations observed by microtectonics, thus a homogeneous stress-strain field was supposed within the area investigated. The tectonic component of the strain can be determined by taking the respective paleomagnetic declination rotations as the rotational components of the deformation. 相似文献
17.
To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal.In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence may have been affected by strain and could have originated as a DRM or a CRM.Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock. 相似文献
18.
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
19.
Original geological and structural data, which derive from the analysis of the rheomorphic Green Tuff ignimbrite unit of Pantelleria, have offered the opportunity to define its modes of emplacement and the location of the eruptive sources in terms of distribution and geometry. The Green Tuff displays a wide range of rheomorphic structures consisting of preserved penetrative foliations, lineations and folds which, developed at distinct times, have been assigned to three major (D1–D3) deformation events accompanying and following the ignimbrite unit emplacement. The first D1 event produced distinct sets of structures developed along ductile shear zones generated during the emplacement of pyroclastic density currents along current-deposit boundaries. Palaeoflow directions of this event are completely independent from topography and are directly related to high-energy currents generated from the eruption. D2 event is characterized by folding due to down-slope post-emplacement flows related to gravity sliding processes whereas the D3 event was dominated by semi-brittle to brittle structures developed after the complete emplacement of the flow units and their subsequent cooling and compaction. The statistical analysis of these structural data has led to the hypothesis that the Green Tuff eruption developed from fissural sources that are largely superimposed on the NNE-trending dip-slip normal fault zones of the island (the Zinedi and the Montagna Grande faults). Our model also implies that the Green Tuff ignimbrite deposit can be the result of several events within a single eruptive cycle. The orientation of the fissural eruptive systems is evidence that the feeding structures for this large-size explosive event were strongly controlled by the E-W to ESE-WNW directed extension structures that affect the island of Pantelleria and, as a whole, the entire region of the Sicily Channel. 相似文献
20.
The center-to-center method of strain analysis can be used to estimate flow lineation in high-silica ash-flow tuffs. It can be used as an alternative or supplement to other techniques for flow lineation identification, such as the examination of flow textures in thin sections and the measurement of anisotropy of magnetic susceptibility. The center-to-center method is a modification of a technique described by Fry (1979) and by Ramsay and Huber (1983) for determination of finite strain based on the spacing of particles within a deformed rock. In the present study, application of the method to an anticlustered array of phenocrysts in the flattening plane of an ash-flow tuff produces an ellipse with center to edge distances representative of the minimum distance between centers of phenocrysts in all directions within the flattening plane. The long axis of the ellipse corresponds to the maximum axis of finite strain; this direction is suggested to correspond to the flow lineation. The orientation of the stretching lineation was chosen both by eye and by least-squares analysis from center-to-center plots. The calculated orientation of the long ellipse axis can be varied by choice of a maximum distance between digitized objects which are included in the calculation. Comparison is made between late-stage flow lineations identified using the center-to-center method, and the AMS (anisotropy of magnetic susceptibility) method on samples of the high-silica Oligocene Bloodgood Canyon Tuff from the Mogollon-Datil volcanic field of southwestern New Mexico. Flow lineations based on center-to-center analyses of flattening plane-parallel rock slabs and thin sections agree well with AMS-derived flow lineations on most samples from which high-quality AMS lineations were obtained. Center-to-center analuses from flattening plane-perpendicular, lineation-parallel planes of ash-flow tuff produce ellipses inclined from 15° to 85° to the flattening plane, despite compaction of the ash, which should cause angles of inclination to be very low. The inclined ellipses may result from heterogeneities in grain size and distribution of phenocrysts in vertical sections of tuff, or from fragmentation of phenocrysts which occurred during the final stages of emplacement and compression. Center-to-center analyses on rock slabs rather than thin sections helps to avoid the effects of either textural heterogeneity and fragmentation of phenocrysts. With flow lineation identified by center-to-center analysis, petrographic examination of thin sections cut perpendicular to the flattening plane and parallel to the flow lineation allow for the identification of flow direction. 相似文献