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1.
《Earth and Planetary Science Letters》2006,241(3-4):978-992
The effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood. Therefore, we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the Foum Zguid dyke in southern Morocco. The study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments.The present study shows that: (1) close to Foum Zguid dykes, the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re-crystallization and Fe-metasomatism intensity. (2) The thermal experiments on AMS of samples collected farther from the dyke and, thus, less affected by heating during dyke emplacement, indicate that 300–400 °C is the minimum experimental temperature necessary to trigger appreciable transformations of the pre-existing magnetic fabrics. For temperatures higher than ca. 580 °C, the magnetic fabric transformations are fully realized, with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke. Therefore, measured variations of the magnetic fabric can be used to evaluate re-crystallization temperatures experienced by the host sedimentary rock during dyke emplacement. The distinct magnetic behaviour observed along the cross-sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 °C after dyke emplacement. (3) AMS data shows a gradual variation of the magnetic fabric with distance from the dyke margin, from sub-horizontal K3 away from the dyke to vertical K3 close to the dyke. Experimental heating shows that heat alone can be responsible for this strong variation. Therefore, such orientation changes should not be unequivocally interpreted as the result of a stress field (resulting from the emplacement of the dyke, for instance). (4) Magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations, thermally and chemically induced by dyke intrusion in hosting sediments. 相似文献
2.
Brooks B. Ellwood 《Earth and Planetary Science Letters》1984,67(3):367-376
Magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments. The first of these analyses includes samples from laminated, relatively undisturbed and bioturbated, totally disturbed horizons found in an Athabasca Oil Sand core. The second includes samples from biologically undisturbed sediments from Mammoth Cave in Kentucky. The third includes evaluation of the evolution of a magnetite horizon which was established by the author in December of 1982 in a biologically active tidal flat on Sapelo Island, Georgia. Changes in this magnetite horizon were monitored by periodic subsampling.Results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments. Such primary indicators include the anisotropy of magnetic susceptibility (AMS) parametersV used by Graham, always greater than 45°,Q used by Rees and Hamilton, always less than 0.69, and a subhorizontal magnetic foliation plane. I infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical, not biological factors are responsible for the observed fabric. These factors include dewatering and simple, low magnitude compaction. The data indicate that it cannot be assumed, a priori, that poor fabric distributions are the result of bioturbation. On the contrary, for the Sapelo Island experiments, the magnetic fabric actually improved with bioturbation and the Athabasca bioturbated magnetic fabric is very similar to that from the Athabasca laminated sediment.Data from the Sapelo Island, Georgia, tidal flat experiment are interpreted to indicate that long-axis AMS alignments develop within the sediment. Such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing. It is also observed that RM precision improves during the experiment in spite of bioturbation. 相似文献
3.
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
4.
在地层剖面中,洪泛沉积物与正常的河道沉积物的沉积学特征非常相似,仅靠沉积学特征很难将两者区分. 为了识别古洪灾事件,因而从环境磁学角度建立两种沉积物的识别标志是非常必要和有意义的. 本文通过对1998年长江黣洲湾溃口扇及其附近的长江现代河漫滩沉积物进行磁组构参数测量和对比分析,并结合长江下游河漫滩沉积物的磁组构参数特征,揭示出长江中游正常河流沉积物与河流溃口沉积物的磁组构参数特征具有明显的不同,并且它们与沉积时的沉积环境有着密切的关系. 因此,对长江中游地区沉积物磁组构参数的测试及磁化率各向异性量值椭球体形态的研究,是目前分辨洪泛沉积物和正常的河道沉积物的一个较有效的方法. 相似文献
5.
临夏盆地在东亚新生代地层、古气候、古生物研究方面占有举足轻重的地位.本文对盆地东部的郭泥沟剖面进行了详细的岩石磁学和磁组构研究,以揭示从早中新世到早上新世临夏盆地的沉积演化过程.郭泥沟剖面沉积物中的磁性矿物有磁铁矿、磁赤铁矿、赤铁矿和针铁矿,但剩磁载体以磁铁矿和赤铁矿为主.从上庄组和东乡组的褐红色粉砂质粘土到柳树组和何王家组的褐黄色粘土,赤铁矿含量呈现降低的趋势,与沉积物颜色变化一致.郭泥沟剖面沉积物磁组构类型为正常沉积磁组构.结合岩石磁学结果和磁组构参数特征可揭示临夏盆地早中新世-早上新世沉积的演化过程：早中新世上庄组为稳定湖相沉积,古水流方向为NNW,与南北向的大夏河方向一致;中中新世气候发生较明显的干湿波动,形成了东乡组的褐红色湖相粉砂质粘土夹粉砂、砂和青灰色泥灰质粘土条带,古水流方向主要为NNW,沉积过程主要受大夏河控制;中中新世晚期,受青藏高原构造运动影响,沉积相由湖相细粒沉积物转变为虎家梁组河流相砂砾层;同时,盆地的水动力条件也发生改变,晚中新世柳树组湖相沉积过程同时受南北向大夏河和东西向洮河控制,两个方向近垂直的河流共同作用导致柳树组内沉积各向异性度较低,面理和线理均不发育,磁化率最大轴偏角分布比较分散,磁组构确定的古流向为东西向和南北向;早上新世期间,由于受青藏高原隆升影响,沉积了何王家组下部的河流相砂砾层;受构造抬升影响,大夏河重新主导何王家组上部洪泛平原相沉积过程,水动力条件较为单一稳定,古流向主要为N向,与大夏河流向一致. 相似文献
6.
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
7.
Emö Márton Balázs Bradák Marta Rauch-Włodarska Antek K. Tokarski 《Studia Geophysica et Geodaetica》2010,54(1):121-134
AMS studies earlier carried out on the flysch sediments from the Outer Western Carpathians focused on the sandstone members
deposited from turbidity currents. The main conclusion of these studies was that the dominant features of the AMS fabric developed
during sedimentation and the tectonic overprint was weak. In recent years a large number of claystone (deposited also from
turbidity currents, but in calm water) localities were sampled from the Krosno beds of the Silesian nappe, primarily for paleomagnetic
study. Nevertheless, the AMS of the samples was also measured. The magnetic fabrics of the claystones were dominated by foliation,
but lineations were also expressed and attributed to deformation. The question arose how different are the reactions of the
claystone and the siltstone beds to deformation. 相似文献
8.
We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (< 5 μm) grains. Our investigations show that hydrothermal alteration does change the bulk susceptibility and the degree of anisotropy but not the directions of principal axes of the AMS ellipsoid which are predominantly corresponding to normal magnetic fabric. The stability of AMS directions, regardless of the degree of alteration, points towards the model of distribution anisotropy as the controlling factor for the observed magnetic fabric. 相似文献
9.
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
10.
塔里木盆地库车坳陷克拉苏河沿岸发育良好的新生界地层.我们对克拉苏河沿岸新生界剖面分A、B两段进行系统采样,获得定向岩芯样品1700余块. 岩石磁学研究表明,新生代沉积岩中磁性矿物以赤铁矿为主,含有少量磁铁矿;磁组构测试结果显示,两段剖面磁面理均较磁线理发育,最小磁化率主轴近于直立,显示原生沉积组构. A剖面磁化率主轴к1的偏角指示古流向. 库姆格列木群沉积时的古水流方向为NEE-SWW向,至苏维依组沉积时,流向变为NNE-SSW向. 河流古流向在苏维依组形成时发生急剧变化,说明南天山此时可能发生了明显的隆升,且以北西部隆升为主;B剖面为吉迪克组、康村组和库车组粗粒碎屑岩,磁组构显示к1-к2构成的磁面理与层理面小角度相交,к3呈叠瓦状分布,可以用来指示古水流方向.从吉迪克组至库车组下段,古流向均为NNW-SSE,但逐渐偏东,暗示天山在此期间处于缓慢的隆升期或东西部处于同步隆升,且西部隆升速度快于东部. 相似文献
11.
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
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.
H. Porath 《Pure and Applied Geophysics》1968,69(1):168-178
Summary The magnetic fabric of the Yampi Sound hematite ore bodies of Cockatoo and Koolan Island in Western Australia has been determined with a low field torque meter in an attempt to investigate the origin of the ores. The ore bodies are conformable with a ferruginous sedimentary sequence, which has been folded into overturned synclines and anticlines. Each ore body shows a consistent distribution of the principal susceptibility axes due to a preferred alignment of the trigonal axes of hematite, but the magnetic fabric is not related to the sedimentary structure of the ores. For the Cockatoo Island ore body the magnetic fabric indicates the presence of an axial plane foliation within the ore, suggesting that the preferred crystalline alignment is caused by arecrystallization of hematite during the folding of the sediments. The magnetic fabric of the ore bodies on Koolan Island is not related to either bedding or axial planes. This can be explained by assuming that the hematite recrystallized during an earlier stage of the folding process and that then the limbs of the fold were rotated into their present position. High field torque measurements on several ore specimens indicate a preferred crystalline alignment of hematite of about 25%. The ferruginous sediments of Cockatoo Island show a composite fabric caused by the superposition of two foliations, one being the bedding plane and the other the axial plane foliation impressed during the folding of the sediments. 相似文献
14.
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
15.
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
16.
Belén Oliva-Urcia Inma Gil-Peña Ruth Soto José María Samsó Borja Antolín Emilio L. Pueyo 《Studia Geophysica et Geodaetica》2018,62(2):291-322
Magnetic fabric allows to unravel the petrofabrics of sedimentary rocks and to assess their deformational history. The use of this technique, in addition to classical structural field observations in the limbs of seven asymmetric folds in the Pyrenees, helps to determine the differences of internal deformation as well as the folding kinematics. Three folds developed during the Variscan Orogeny in Ordovician and Devonian rocks, and four folds developed during the Pyrenean Orogeny in Eocene rocks, are studied. Folds show a variety of structural locations, in different thrust sheets of the Southern Central Pyrenees, different cleavage development, age, geometry and lithology. Sampling follows an equivalent lithological layer in the two limbs, except for one case, of the selected folds. Results show a modified tectonic magnetic fabric in most sites with the magnetic lineation on the tectonic foliation plane. A larger scattering of the magnetic lineation (maximum magnetic anisotropy axis) and a higher intensity of the preferred orientation of minerals (eccentricity of the anisotropy of magnetic susceptibility - AMS ellipsoid) is better observed in the overturned (short) limb of the asymmetric Variscan folds than in the normal (long) limb. On the other hand, the shape parameter in Alpine folds is generally larger in the overturned (short) limb then in the normal (long) one. A good clustering of the minimum magnetic anisotropy axes is observed in all limbs. The combination of the AMS data with the structural data helps to understand and better constrain the deformation degree in these asymmetric folds and to unravel the deformational history. 相似文献
17.
磁化率各向异性(AMS)是岩石的普遍特性,它反映岩石磁性矿物的择优取向,即磁组构(Magnetic fabric).近三十年来,磁组构技术逐渐应用于地质和地球物理学,显示广阔的研究前景.测量磁组构的仪器有多种,原理不一,故有必要用同一样品在不同原理的仪器上进行比测,以便确认数据的一致性和可靠性.用黄土、变质岩、玄武岩及掺有铁粉的断层泥制成正方体或圆柱体样品,在卡帕桥(KLY-1和KLY-2)和旋转式磁化率仪(Minisep)上进行比测发现:(1)前者的精度(即重复性)在多数情况下优于后者;2)用旋转式磁化率仪测量之前,必须先测得样品z轴的体磁化率,其标定值取决于厂家在标准样品上标示的数值及被测样品与标准样品的相对体积.就旋转样品测量AMS而言,其标定值应为z轴体磁化率测量标定值的一半. 相似文献
18.
19.
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
20.
对四川盆地东北部巴中地区和西部雅安地区早白垩世红层分别采集9块手标本,进行了详细的古地磁学研究.系统退磁获得两地的特征剩磁,其中巴中地区平均方向为,倾斜校正之前:D_g/I_g=25.3°/19.0°,k=18.6,α_(95)=8.8°;倾斜校正之后:D_s/I_s=25.8°/18.9°,k=24.3,α_(95)=7.6°.雅安地区平均方向为,倾斜校正之前:D_g/I_g=24.5°/45.0°,k=15.7,α_(95)=9.0°;倾斜校正之后:D_s/I_s=356.7°/35.6°,k=28.5,α_(95)=6.6°.磁化率各向异性实验结果显示两地均未受到显著的构造应力影响.等温剩磁各向异性(AIR)实验结果,巴中地区IRMz/IRMLx平均值为0.8194,表明18%的压实率;雅安地区IRMz/IRMx平均值为0.8909,表明11%的压实率,计算得到巴中和雅安地区校正后的磁倾角分别为22.7°和38.8°.根据等温剩磁各向异性(AIR)实验和Tauxe and Kent(2004)提出的EI校正法得到的结果表明,四川盆地早白垩世陆相碎屑岩层中存在沉积压实作用造成的磁倾角偏低现象,而且川东北巴中地区的偏低程度强于川西雅安地区. 相似文献