Fabric characteristics of in situ sand with/without liquefaction verified by anisotropy of magnetic susceptibility

It is well known that fabric of sand may significantly affect mechanical behaviors and liquefaction resistance of sand.Various optical techniques are currently utilized to visualize the fabric,especially the distribution of the long axis of soil particles.However,none of these methods provides an ideal solution in laboratory tests and in situ observation.In this study,anisotropy of magnetic susceptibility(AMS)was first proposed as a convenient and efficient way to evaluate the liquefaction of clean sand.At first,investigations with scanning electron microscopy(SEM)and AMS were simultaneously conducted on two groups of soil specimens with different initial fabrics to verify the feasibility of the AMS technique.Then,80 in situ samples were collected to analyze the feature of liquefied and non-liquefied sand layers through AMS tests.It is clearly known from the test results that the natural sedimentary fabric was destroyed during liquefaction and the fabric anisotropy was greatly changed after liquefaction.The feasibility of evaluating soil fabric using the AMS survey was verified by the laboratory tests.Furthermore,the applicability of AMS in detecting liquefied layer in situ was confirmed for the first time.

Where are the Volcanic Calderas in the Xiangshan Volcanic Basin of Jiangxi？ Implications from Anisotropy of Magnetic Susceptibility

Objective As the world＇s third largest volcanic type uranium ore field,the Xiangshan volcanic basin has attracted much attention for its large industrial value.The ore hosting rocks are mainly the early Cretaceous rhyodacite and porphyroclastic lava,as well as small amounts of high level intrusive acidic rocks and metamorphic rocks.

Magnetic fabric and geomorphic characteristic of Neotectonic activity along strike direction of North Almora Thrust,Kumaun Lesser Himalaya,India

Structurally,the North Almora Thrust(NAT),is placed in the northern proximity of the Almora Nappe(AN)in the Kumaun Lesser Himalaya.The NAT is characterized by the presence of a zone of mylonitic rocks of basal Saryu Formation(1800±100 Ma)in the hanging wall and the metasedimentaries of Inner Lesser Himalayan rocks as the footwall block.The NAT has been dextrally offset by two estabilished maj or faults i.e.Saryu River Fault(SRF)in Saryu Valley and Dwarahat-Chaukhutia Fault(DCF)in Ramganga valley.In present study,we identified four new faults,based on field study,geomorphic landform and magnetic fabric analysis.These faults are N-S trending Pancheshwar Fault,NE-SW Rameshwer Fault,NNE-SSW trending Kosi Fault,and NNE-SSW trending Gagas Fault(GF).Our results show that the zone is bound by cross cutting relation with NAT and these fault zones are comparatively more active than other regions.Furthermore,we suggest that the steep and NW-SE orientation of magnetic foliation within the NAT zone is a result of NE-SW oriented progressive regional compression.The magnetic foliations represent the unseen internal foliations in the rocks developed due to preferred alignment of magnetic minerals and can be found through the Anisotropy of magnetic susceptibility(AMS)study.Variation in the alignment of the field and magnetic foliations are developed due to superimposed brittle deformation along the faults over the pre-existing field foliations.Magnetic foliations represent the impact of last stage deformation and significant to find brittle deformation and finite strain in the rocks of the study area.The lowering of anisotropy(Pj)away from the fault zone represents distribution of strain across the NAT zone.AMS fabric confirms the presence of faults developed across the NAT zone and also explains the deformation pattern along these faults.The geomorphic anomalies and steepness changes across the NAT zone are correlated with active deformation along the NAT and associated transverse faults.

Magnetic Fabric of Late Devonian Metasedimentary Rocks from Longyan,Southwestern Fujian,China

This study presents the results of magnetic fabric on Late Devonian metasedimentary rocks from Longyan , Southwestern Fujian , China .Measurements of low - field anisotropy of magnetic susceptibility (AMS)were made on 59 specimens (sites 48-53 ) using a Sapphire SI-2 induction coil instrument. Our data show that the magnetic fabric at sites 48- 51 and 53 is mainly carried by paramagnetic minerals (muscovite.chlorite and biotite), while at site 52 both paramagnetic minerals and hematite contribute to the total AMS . Separation of the two components of anisotropy indicates that they are approximately coaxial . The magnetic foliation is either parallel to an Indosinian (Late Triassic ) metamorphic schistosity (sites 48-52 ) or to a Yanshanian (Late Mesozoic ) microscopic mylonitic foliation (site 53). The magnetic lineation corresponds either to me regional structural trend (sites 48 - 52 ) or to the line of intersection between the schistosiry and the mylonitic surface (site 53 ).Our results suggest that Indosinian metamorphism and deformation took place simultaneously under the control of a compressional stress field which is mostly responsible for the generation of the observed magnetic fabric .However ,post-Indosinian deformation of the Yanshanian orogeny may exist at some locations and interfere with the Indosinian fabric .

Magnetic Characterization of Amphibolites from the Fomopéa Pluton(West Cameroon):Their Implication in the Pan-African Deformation of the Central African Fold Belt

The Fomopea granitic pluton is emplaced in gnessic and amphibolitic basement. These gneissic and amphibolitic basement rocks are represented in the pluton＇s body as sub-rounded, elongated or stretched xenoliths. Amphibolitic xenoliths display testimonies of two main tectonic events namely： （i） El flattening deformation event characterized by a NW-SE to E-W foliation with a best pole at 246/57 and a mineral stretched lineation with a best line at 293/47; and （ii） E2 compressive event typified by （1） N-S to NNE-SSW steeply dipping foliation; （2） S-type flexion-fold indicating a sinistral shear movement. These amphibolite rocks indicate a magnetic susceptibility magnitude （Km） range from 418 ~SI to 90092 IISI for 87% of the stations showing a ferromagnetic behavior. K-T curves reveal the presence of Ti-poor magnetite as susceptibility mineral carrier. Magnetic foliation and lineation suggest that the N-S strike direction observed in the Bamendou amphibolite is, as pointed out elsewhere in the Central African Fold Belt, of paramount importance in the tectonic evaluation of the Fomopea area, since it has most likely acted as major deformation phase in the second tectonic event in the belt.

Extensional tectonics and North China Craton destruction:Insights from the magnetic susceptibility anisotropy(AMS)of granite and metamorphic core complex

The craton is a long-lived stable geologic unit on the Earth’s surface.However,since the Mesozoic,the North China Craton(NCC)experienced large-scale lithospheric removal,the fundamental change of physical and chemical characteristics of the lithospheric mantle,widely distributed crustal deformation,and extensive magmatism.This complex evolution contrary to other cratons is called the NCC destruction.Widespread magmatism in the eastern NCC is an important response to the lithospheric removal at depth and crustal deformation on the surface.The plutons emplace under a tectonic context and therefore record the information of the tectonics;especially,the anisotropy magnetic susceptibility(AMS)pattern of the pluton was acquired with the influence of regional stress.In the past fifteen years,about 22 plutons intruding during the different periods from the Late Triassic to the late stage of the Early Cretaceous have been studied with AMS.The emplacement mechanisms of plutons and the contemporary tectonic setting were discussed to constrain their relationship with the NCC destruction in different stages of magmatism.As a result,the Late Triassic,Early Jurassic,and Late Jurassic plutons exhibit consistent N(E)-S(W)trending magnetic lineations.The early stage of Early Cretaceous plutons display NW-SE trending magnetic lineations,while the late stage of Early Cretaceous plutons show magnetic lineations with various orientations.Combined with previous studies,it is concluded that the emplacements of the plutons intruding in these three stages were controlled by weak N(E)-S(W)trending extension,regional NW-SE trending extension,and weak extension in the shallow crustal level,respectively.The transformation of regional extension from the N(E)-S(W)to the NW-SE direction was accompanied by a strain-increasing tendency.The extensional tectonics in the eastern NCC was interpreted to represent the interaction between Mongol-Okhotsk belt,PaleoPacific plate,and eastern Eurasian continent.

Magnetic Fabric and Petrofabric of Amphibolites from the Namcha Barwa Complex, Eastern Himalaya

The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility(AMS) and the crystallographic preferred orientation(CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.3×10^(-4) to 3.314×10^(-2) SI, with the Jelínek's anisotropy values(Pj) ranges from 1.094 to 1.487. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction(EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the [001] axes along the lineation and the [100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.

Paleomagnetic constraints on Paleogene-Neogene rotation and paleo-stress in the northern Qaidam Basin

Knowing the rotation and paleo-stress history in Qaidam Basin is a fundamental parameter to quantify the mechanism of intracontinental deformation in Tibetan Plateau.However,few studies have been conducted on tectonic rotation and stress evolution over long timescales in the Qaidam Basin.Here,we report new magnetic declination and the anisotropy of magnetic susceptibility(AMS)from a~52–7 Ma sequence of fluvial and lacustrine sediments in the Dahonggou(DHG)section in the northern Qaidam Basin.The magnetic declination revealed that the northern Qaidam Basin underwent a clockwise rotation 25.1°±8.6°during~33–17 Ma,followed by a counterclockwise rotation 16.9°±6.8°during~17–13.5 Ma.The AMS results showed that the“earliest deformation”fabrics were interrupted by the“pencil structure”fabrics in the intervals of~52–45 and~21–15 Ma.The interruption,synchronous with the marked deceleration of the India-Asia convergence rate,indicates pulse of strong tectonic compressive stress.In addition,the AMS results documented a transition in stress direction from S-N to SW-NE at~15 Ma,suggesting a kinematic shift in the northeastern TP.Our constraints on the rotation and stress from the northern Qaidam Basin support the two-stage evolution of the Altyn Tagh Fault(ATF).The fast-rate slip motion on the ATF during the early Oligocene caused the clockwise rotation in the northern Qaidam Basin;the second stage with enhanced thrusting since the middle Miocene caused extensive crustal shortening and dispersive NW-trending folds and faults in the Qaidam Basin and the northeastern TP.