DEEP ELECTRIC STRUCTURE BENEATH NORTHEASTERN BOUNDARY AREAS OF THE NORTH CHINA CRATON

Magnetotelluric data are collected along a NW-SE trending and about 900km long profile within northeastern boundary areas of the North China craton(NCC). This profile extends from the Hegenshan belt within the Central Asian orogenic belt(CAOB), across the Baolidao arc, Solonker-Linxi suture zone, Ondor Sum accretion complex, Bainaimiao arc, Inner Mongolia paleo-uplift, Yanshan belt, and ends on the Liaohe depression of the NCC. Impedance tensor decomposition methods are used to study the dimensionality and geo-electric strike of MT data of the region. Two-dimension (2D) analysis is appropriate for this profile. The 2-D subsurface electrical resistivity structure along profile is obtained using the non-linear conjugate gradient (NLCG) algorithm. The electrical resistivity structure is characterized by lateral segmentation, and divided into high resistive, low resistive, and high resistive areas; The lateral variation of electrical resistivity is significant within the CAOB, but it is smooth in the NCC; The extensive high conductive body(HRB)is observed in the mid-low crust beneath the Solonker-Linxi suture zone and Inner Mongolia paleo-uplift, respectively; The low resistivity could be due to the partial melts and crustal flows. Based on our electrical resistivity structure and other geological, geophysical observations, we speculate that (1)the final suturing of the Siberian craton to the NCC could be along the areas between Xilinhot Fault and Xar Moron Fault; (2)the relatively thick high resistive body beneath the Yanshan belt may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection system, and lower the effect of tectonic evolution of CAOB on the destruction to NCC.     

岫岩陨石坑三维Q值层析成像

到2009年为止,世界上已发现176个陨石撞击坑.从20世纪80年代起,中国学者从地质、地球化学和地质钻探角度推断并证实辽宁省岫岩县的一个环形构造为陨石撞击而成,但该坑的深部构造及整体特征仍有待地球物理方法来确认.本文从地震勘探角度利用三维Q值层析成像方法首次对岫岩坑的地震波衰减结构进行了研究,以便对该撞击构造有个整体认识.Q值结构对了解地壳的非弹性性质、地壳介质非均匀性有着重要意义.利用基于吸收特征时间t^*的三维Q值层析成像方法,计算了岫岩坑地震折射方阵的观测数据的振幅谱,并通过拟合振幅谱得到了反映地震波衰减的t^*.利用t^*通过三维Q值层析成像方法获得了该坑的三维Q值结构.结果表明:该陨石坑内从地表到深度100 m的Q值层对应坑内上部107 m厚的湖泊相沉积物,深度100~300 m的Q值层对应坑内下部厚度为188 m的角砾岩堆积透镜体.深度300~700 m的Q值层对应角砾岩堆积透镜体下面的下元古界变质岩系程度较低的撞击破碎层.大约250 m深处的波阻抗界面与地质钻孔的260 m深处的陨石冲击分界面相对应.     

Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.     

On deep electric conductivity of the lithosphere in the Tobol-Ishim interfluve (West Siberia)

On the basis of exploration magnetotelluric soundings (MTS) carried out by the Tyumen Geological Survey in 1980–1981 within the Tobol-Ishim interfluve, the top of the conductive horizon with the electric resistivity of dozens of Ω m is identified in the geoelectrical section at a depth of 75–80 km. Two segments of the locally elevated top of this horizon are revealed. One segment, where the depth of the top of the conductor is 60–65 km, corresponds to the region of the Kiselevskii fault-the main fault in this territory; another segment is associated with the edge zone of the Ishim branch of the Triassic rift system within the West Siberian Plate. Within this segment, the top of the conductive layer rises to a depth of 55-60 km. It is assumed that the Ishim geoelectrical anomaly, which projects into the geothermal anomaly and is coherent with the features of the deep geological structure of its display region, is conditioned by the element of the mantle-crustal magmatogene fluid paleosystem.     

Results of electromagnetic sounding with JASC submarine cable

We present the results of long-term deep geoelectric studies using the JASC (Japan Sea Cable) submarine communication cable in the region of the Sea of Japan. In the 2D inversion of the amplitude and phase’s apparent resistivity curves and the frequency dependences of the tipper, we invoked the geological and geophysical information about the region and on-shore electromagnetic observations to fit the model to the observations. The resulting geoelectrical cross section of the region of the Sea of Japan along the JASC cable obtained in this way agrees well with the experimental data. The upper part of the section contains a conductive block beneath the bottom of the Central basin of the Sea of Japan at a depth of 10–40 km, a fault submerging below the continent in the marginal part and a deep fault in the continental region. In the lower portions of the cross section, the high-resistivity block interrupts the continuity of the horizontal conductive layers beneath the Yamato Uplift, and the conductive bottom part of the geoelectric cross section submerges under the continent. In the continental segment of the cross section, there is a large block with reduced electric resistivity, which is located between the conductive layers at a depth interval of 200–560 km. We analyze the characteristic features of the geoelectric cross section and the deep section imaged by seismic tomography in the region of the Sea of Japan.     

3D modelling of near-surface, environmental effects on AEM data

This study considers the three-dimensional (3D) modelling of compact, at-surface conductive bodies on frequency domain airborne electromagnetic (AEM) survey data. The context is the use of AEM data for environmental and land quality applications. The 3D structures encountered are typically conductive, of limited thickness (<20 m) and form ‘point’ source locations carrying potential environmental risk. The scale of such bodies may generate single-profile, ‘bulls-eye’ anomalies. In attempts to recover geological information, such anomalies may be considered to represent noise. In environmental AEM, the correct interpretation of such features is important. The study uses a combination of theoretical models and trial-fixed-wing survey data obtained in populated areas of the UK. Scale issues are discussed in terms of the volumetric footprints of the induced electric field generated by systems flown at both low and high elevation. One of the primary uses of AEM survey data lies in the assessment of conductivity maps. These are typically obtained using one-dimensional (1D) conductivity models at individual measurement points. In order to investigate the limitations of this approach, 3D modelling of conductive structures with dimensions less than 350×350 m and thicknesses extending to 20 m has been carried out. A 1D half space inversion of the data obtained at each frequency is then used to assess the behaviour of the spatial information. The results demonstrate that half space conductivity values obtained over compact 3D targets generally provide only apparent conductivity results. For thin, at-surface bodies, conductivity values are biased to lower values than the true conductivity except at high frequency. The spatial perturbation to both coupling ratios and 1D conductivity models can be laterally extensive. The results from 3D modelling indicate that the use of horizontal derivatives applied to the conductivity models offers enhanced edge detection. The practical application of such derivatives to both regional- and local-scale survey data is presented.. The special case of a near-surface, metallic pipeline has been modelled. The problem constitutes an inductive limit (current gathering) response in which the perturbation is largely confined to the in-phase coupling ratios. The main perturbations, in data and conductivity models, are within about 40 m of each side of the pipeline. The maximum perturbation to the conductivity model is only a factor of 1.5 above background. Detailed survey data across a former compact landfill (about 100×100 m) are used to compare the model behaviour predicted by the 3D modelling with survey results. The survey, conducted at two separate altitudes, provides a demonstration of 3D effects on 1D survey models as a function of frequency and elevation. Although the nature of the landfill materials and their location are not known precisely, the mapping information appears realistic.     

岫岩陨石撞击坑的速度结构和抗震性能分析

岫岩陨石撞击坑位于辽东半岛北部低山丘陵地区,直径1.8km,保存完好。由于陨石的撞击和此后的沉积作用在坑内形成了特殊的地貌、地质和地球物理环境,使坑内与坑外的介质在速度方面存在差异,尤其是湖相沉积的软弱淤泥层使得坑底上建造的房屋就像躺在一张巨大的海绵垫上一样,使人们常常感到有微震发生,表明坑底的软弱地层对地面的震动产生了放大作用,严重地影响了地基的稳定性,从而影响到地面建筑物的抗震性能。本文采用高分辨折射探测方法,获得了陨石坑内的速度结构和软弱地层的分布,分析了陨石坑地基的抗震性能,为坑内地基和地面建筑物的地震危险性评价提供了可靠的基础资料。     

Resistivity Structure of the Central Indian Tectonic Zone (CITZ) from Multiple Magnetotelluric (MT) Profiles and Tectonic Implications

The Central Indian Tectonic Zone (CITZ) is a major tectonic feature extending across the Indian subcontinent. It was formed in the Paleoproterozoic when the Bastar Craton and the Bundelkhand Craton were sutured together. This region is recognized in the geological record as a persistent zone of weakness with many tectonothermal events occurring over geologic time. The weakness of this region may have caused the late Cretaceous/early Tertiary Deccan volcanism to have been localized in the CITZ. The zone is still tectonically active, as evidenced by sustained levels of seismic activity. This paper presents the first systematic investigation of the resistivity structure of the CITZ using multiple magnetotelluric (MT) transects. Two-dimensional (2D) resistivity models were generated for five north–south profiles that cross the CITZ and encompass an area of ~60,000 km². The models were based on the joint inversion of transverse electric (TE), transverse magnetic (TM) and tipper (Hz) data. All the profiles showed a low resistive (10–80 Ωm) middle to lower crust beneath the CITZ with a crustal conductance of 300–800 S. The presence of an interconnected fluid phase and/or hydrous/metallic minerals appears to be the most likely explanation for the elevated conductivity that is observed beneath the CITZ. The presence of fluids is significant because it may indicate the cause of persistent weakness at crustal depths. A northward dip of both the crustal conductive layer and coincident seismic reflections favor a northward polarity of the subduction process associated with the formation of the CITZ.     

内蒙古东部古生代块体碰撞区的大地电磁测深研究——Ⅱ.二维解释

本文对内蒙古东部地区赤峰-东乌旗剖面上12个测点的MT资料进行了二维解释.针对测区具体情况,在计算中主要强调拟合有效视电阻率ρeff,ρaz和有效相位Φeff,Φaz.二维模型给出,地壳上地幔在西乌旗以北和翁牛特旗以南地区有5层结构,壳内高导层埋深20-25km,上地幔软流层顶面埋深约100km.大兴安岭火山岩区有6层或7层结构,上地壳10km左右和下地壳30-40km深度处分别有一高导层,软流层顶面埋深约75km.结合地质等资料得出:1.翁牛特旗以南地区至少在深部仍然属于中朝块体北缘;2.翁牛特旗-巴林右旗之间是中朝块体北缘古生代以前的一个增生体,西拉木伦河断裂是由于后期的构造活动在增生体内产生的一个逆冲断层;3.上地幔隆起和下地壳高导层（岩浆房?）的存在是产生大兴安岭火山岩的深部原因;4.大兴安岭地区可能存在推覆构造,推覆面即为上地壳高导低速层;5.二维模型没有反映出贺根山地区地下有岩浆通道或深断裂;6.中朝和西伯利亚块体早古生代和晚古生代的缝合带分别位于林西一带和西乌旗一带.     

A large hydrothermal reservoir beneath Taal Volcano (Philippines) revealed by magnetotelluric resistivity survey: 2D resistivity modeling

Taal Volcano, located in the southwestern part of Luzon Island, Philippines, has frequently experienced catastrophic eruptions from both the Main Crater on Volcano Island and flank eruptions. These eruptions have been magmatic, phreatomagmatic, and hydrothermal, with the latter implying the existence of a large-scale hydrothermal system beneath the volcano. We conducted an electrical resistivity survey using the magnetotelluric method in order to identify the location and geometry of the hydrothermal reservoir and sealing cap rock. Two-dimensional inversion using the observed data indicates four similar resistivity sections. The structure at shallow depths corresponds to volcanic deposits and an aquifer. Below 1 km, the structure features a relatively resistive zone beneath the main crater surrounded by a conductive shell. We interpreted these to be a large hydrothermal reservoir with an impermeable cap rock sealing it. Recent ground deformation detected by GPS measurements suggests that the hydrothermal reservoir is active. The interpreted cap rock thins just beneath the main crater and could easily be destroyed by an imbalance in the hydrothermal system. We conclude that this hydrothermal reservoir plays a significant role in driving catastrophic eruptions that begin with a hydrothermal explosion at the main crater.     

海拉尔盆地中-上地壳电性结构特征研究

本文通过对横穿海拉尔盆地的一条长约222km的北西—南东向大地电磁测深剖面数据的定性分析及二维定量反演解释,首次获得了海拉尔盆地高精度大范围的电性结构图.海拉尔盆地中-上地壳电性结构纵向上具有典型的分层特性,总体可分为四层,即低阻层-高阻层-低阻层-高阻层,而横向上又具有分块特点.海拉尔盆地边缘及内部分布的众多断裂将盆地划分为隆起与坳陷相间的格局,并发现盆地内部坳陷区也存在有小规模凸起,每一构造单元内部电性结构各具特点.海拉尔盆地中-上地壳低阻层底面最深达28km,通常在6~16km之间,但厚度变化不大,在4~10km之间,且隆起区与坳陷区底面埋深差别较大.据电性结构模型推测出两条新断裂F8和F9,且断裂F9规模较大,为基底断裂.中-上地壳的低阻层可能在一定程度上控制着海拉尔盆地内油气田的分布格局.     

A preparation zone for volcanic explosions beneath Naka-dake crater,Aso volcano,as inferred from magnetotelluric surveys

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.     

A methodology for 3D modeling and visualization of geological objects

Geological body structure is the product of the geological evolution in the time dimension, which is presented in 3D configuration in the natural world. However, many geologists still record and process their geological data using the 2D or 1D pattern, which results in the loss of a large quantity of spatial data. One of the reasons is that the current methods have limitations on how to express underground geological objects. To analyze and interpret geological models, we present a layer data model to organize different kinds of geological datasets. The data model implemented the unification expression and storage of geological data and geometric models. In addition, it is a method for visualizing large-scaled geological datasets through building multi-resolution geological models rapidly, which can meet the demand of the operation, analysis, and interpretation of 3D geological objects. It proves that our methodology is competent for 3D modeling and self-adaptive visualization of large geological objects and it is a good way to solve the problem of integration and share of geological spatial data. Supported by National High Technology Research and Development Program of China (Grant Nos. 2006AA12Z220, 2006AA12Z114, 2007AA12Z226), and Open Fund of State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing (Grant No. WKL(06)0304)     

喜马拉雅西北部逆冲带的地壳电性结构

印度板块北部地形起伏较大的喜马拉雅山地区由几个构造互异的地质单元组成,依地形高、低把喜马拉雅碰撞带分成低喜马拉雅和高喜马拉雅.为了研究与主要逆冲带（含主缝合带MCT和主边界带MBT）有关的地壳电性结构,沿Rohtangpass (海拔4000 m) 到Mandi (海拔400 m)剖面进行了MT探测.通过对16个测点观测资料的分析和考虑地形的二维反演,获得了沿剖面的二维电性结构.电性结构显示,在Chail和主逆冲边界带下方,东西走向的缝合带突然转向北.在下喜马拉雅的Rampur 区段的元古代基底为范围较大的高阻体,而浅部地壳被逆冲带分成向北倾的电导性块体和电阻性块体.Chail 逆冲带东侧低喜马拉雅Rampur 区段的推挤和它西侧的基底脊柱体导致主边界带及相关的逆冲带（Kangra 拐角）向北转弯,Kangra拐角处的应力可能是由于西侧基底脊柱体进入到Kangra 区引起的.     

New geological insights and structural control on fluid circulation in La Fossa cone (Vulcano,Aeolian Islands,Italy)

Electric resistivity tomography (ERT), self-potential (SP), soil CO₂ flux, and temperature are used to study the inner structure of La Fossa cone (Vulcano, Aeolian Islands). Nine profiles were performed across the cone with a measurement spacing of 20 m. The crater rims of La Fossa cone are underlined by sharp horizontal resistivity contrasts. SP, CO₂ flux, and temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of fluids. The Pietre Cotte crater and Gran Cratere crater enclose the main hydrothermal system, identified at the centre of the edifice on the base of low electrical resistivity values (<20 Ω m) and strong CO₂ degassing, SP, and temperature anomalies. In the periphery, the hydrothermal activity is also visible along structural boundaries such as the Punte Nere, Forgia Vecchia, and Palizzi crater rims and at the base of the cone, on the southern side of the edifice, along a fault attributed to the NW main tectonic trend of the island. Inside the Punte Nere crater, the ERT sections show an electrical resistive body that we interpret as an intrusion or a dome. This magmatic body is reconstructed in 3D using the available ERT profiles. Its shape and position, with respect to the Pietre Cotte crater fault, allows replacing this structure in the chronology of the development of the volcano. It corresponds to a late phase of activity of the Punte Nere edifice. Considering the position of the SP, soil CO₂ flux, and temperature maxima and the repartition of conductive zones related to hydrothermal circulation with respect to the main structural features, La Fossa cone could be considered as a relevant example of the strong influence of pre-existing structures on hydrothermal fluid circulation at the scale of a volcanic edifice.     

内蒙古东部古生代块体碰撞区的大地电磁测深研究——Ⅰ.观测与资料分析

内蒙古东部地区是古生代古亚洲洋闭合、中朝和西伯利亚块体碰撞区的中东段.本文分析了赤峰—东乌旗长约400km的剖面上12个点的大地电磁(MT)资料,介绍了数据处理方法,讨论了研究区的电性特征.研究区是一个二维性占主要成分的三维结构,受此影响,大多数点的视电阻率曲线含有明显的静态移动和畸变效应,对此采取了静态校正和畸变校正.校正后的视电阻率和相位及感应函数显示出:1.巴林右旗以南和西乌旗以北是两个相对的高阻值区,剖面中部的大兴安岭地区是相对的低阻值区;2.视电阻率曲线在周期大于1000s时普遍呈下降趋势,相应的相位则呈上升趋势,反映了该区上地幔高导层的存在;3.西拉木伦河北侧的早古生代蛇绿混杂岩带和锡林浩特杂岩带表现出横向上强的电性梯级变化.结合地质资料,初步认为上述两个电性梯级带是两个可能的地质分界线,分别对应中朝块体和西伯利亚块体早古生代和晚古生代的缝合带;4.东乌旗—西乌旗之间中地壳有一发育较好的高导层,观测中没有发现贺根山地区具有较深的“根”;5.赤峰—翁牛特旗之间电性存在较大差异,有可能表征了一个较大的断裂或地质界限.     

滇西地区地壳上地幔电性结构与地壳构造活动的关系

本文根据滇西地区18个大地电磁测深点资料的数据处理和分析结果,对测区深部导电率在纵、横向上的变化特征进行了研究。结果表明:滇西地区深部电性为多层结构,大致可分四至五个电性结构层;深部电性结构横向变化大,明显受区域构造控制;该区上部地壳内普遍存在低阻层;上地幔高导层明显存在两个隆起区,一个以剑川—鹤庆为中心呈北北西向展布的隆起区,另一个以腾冲—潞西为轴呈南北向展布的隆起区。 本文还讨论了地壳上地幔电性结构与大地构造的关系,滇西北裂陷区盆地的形成,以及该区地震活动与深部构造的关系     

Three-dimensional interpretation considering the static and the sea-effects of magnetotelluric data obtained in Jeju,Korea

Three-dimensional (3D) magnetotelluric (MT) surveys have been performed in Jeju, the largest volcanic island in Korea to figure out any possible structures or potential anomaly for remnant deep geothermal resources. Various approaches have been applied to interpret MT data observed in Jeju. MT dataset shows generally simple stratigraphy of four layers, though contains the severe static and the sea-effects. In our previous works, the induction vectors and 3D inversion results have commonly indicated the existence of a conductive anomaly in central parts of the island, beneath Mt. Halla. The 3D inversion dealt the static shifts as inversion parameters. The Jeju MT dataset, however, still contains the effect of conductive sea water surrounding the island.The sea-effect on MT impedance can be represented as a distortion tensor and excluded from the Jeju MT dataset by an iterative sea-effect correction. In this study, 3D inversion incorporating static shift parameterization was conducted using MT dataset corrected using 1D resistivity model obtained from the iterative scheme. Reasonably reconstructed images are obtained through the 3D inversion and using the MT dataset with sea-effect correction. The inversion result still shows the conductive anomaly in a similar depth. RMS misfits converged to a lower value than that of inversion using MT data before the sea-effect correction. From the fact, it is highly possible that the conductive anomaly is not an artifact but a real underground structure. Further investigation about the anomaly including exploration drilling is needed to see if it is from a fracture containing conductive sea water or related to the old volcanic activities.     

Controls on the geometry of a Holocene crater row: a field study from southwest Iceland

The Reykjanes Peninsula rift zone in southwest Iceland is a highly oblique segment of the Mid-Atlantic ridge system which accommodates NW–SE extension during rifting episodes that consist of eruptions and normal faulting, and E–W left-lateral shear strain along strike-slip faults during longer amagmatic periods. Dominant tectonic features on the peninsula are a series of generally NE-striking, sub-parallel eruptive fissures and normal faults, and a cross-cutting zone of N–S striking, right-lateral strike-slip faults. The last series of rifting episodes ended in 1227, and a proposed 1,000 year cyclicity predicts the start of a new series of eruptions within the next 200 years. In order to more accurately characterize the nature of eruptions on the Reykjanes Peninsula, we present a new, spatially accurate map of the ∼2,350 year old Sundhnúkur crater row in the western part of the peninsula, which was examined in detail in order to determine the structural controls on crater row geometry and to understand the interactions that take place between eruptive fissures and pre-existing geological structures. Volcanism is sometimes influenced by small perturbations in the surroundings such as gravitational loading, topography, changes in crustal properties or the presence of fault zones, but there are few field examples showing how fissures are influenced by these pre-existing structures. We identify 27 fissure segments, ranging in strike from 006° to 053°, with varying spacing and overlap between them. Significant local variability in strike and stepping sense of segments occurs in proximity to topographic highs as well as within zones of faulting that pre-date the crater row. Strike also varies at the northern end of the crater row as it approaches a region of older crust at the rift margin. Our data support numerical and laboratory modeling results which show that local topography, pre-existing fractures and crustal properties influence the path taken by magma on its way through the shallow crust.     

3D magnetotelluric characterization of the geothermal anomaly in the Llucmajor aquifer system (Majorca, Spain)

In the Llucmajor aquifer system (Majorca Island, Spain) some geothermal evidences have appeared. This phenomenon is not isolated to Majorca and it is present in other areas, where it can be associated with structural conditions, especially to the extensional event suffered by the island after the Alpine Orogeny. However, the origin of this anomaly in Llucmajor is not well known, and there is no surface geological evidence of these structural conditions. With the aim of delineating the geoelectrical structure of the zone and identifying the geological structure that allows the presence of this anomaly, an audiomagnetotelluric (AMT) survey was carried out. The AMT data was processed using a Wavelet Transform-based scheme. Dimensionality analysis indicates that the geoelectrical structure is mainly 3D. The 3D model was obtained by trial and error forward modeling, taking accounting of the responses from the determinant of the impedance tensor. The model shows a vertical resistivity distribution with three horizons associated with different units: on the top, a shallow high resistive media related to an unconfined shallow aquifer; in the middle, a conductive layer related to the aquitard, and below it, another resistive media related to the confined deeper aquifer. The intermediate horizon shows a sudden thinning beneath the thermal anomalous zone that can be identified as a weakness zone (fault or fracture) connecting both aquifers. An exploratory well was drilled after the AMT survey and reached almost 700 m in depth. This allowed correlating the resistivity distribution of the 3D model with data logging and lithology obtained from the well, showing a proper agreement between them.