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1.
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2.
A tectonic mélange exposed on land is examined to reveal relationships between mélange formation, underplating, and deformation mechanisms, focusing on the deformation of basaltic rocks. The studied Mugi Mélange of the Shimanto Belt is composed of a shale matrix surrounding various blocks of sandstone, pelagic sediments, and basalts. The mélange was formed during Late Cretaceous to early Tertiary times in a subduction zone under P–T conditions of 150–200 °C and 6–7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions. The mélange represents a range of deformation mechanisms; pressure solution with micro-scale cataclasis in the shale matrix, brittle tension cracking in the blocks, and ubiquitous strong cataclasis in the basal portion of basaltic layers. The cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth.  相似文献   

3.
Seismic potential of Southern Italy   总被引：1，自引：2，他引：1  
To improve estimates of the long-term average seismic potential of the slowly straining South Central Mediterranean plate boundary zone, we integrate constraints on tectonic style and deformation rates from geodetic and geologic data with the traditional constraints from seismicity catalogs. We express seismic potential (long-term average earthquake recurrence rates as a function of magnitude) in the form of truncated Gutenberg–Richter distributions for seven seismotectonic source zones. Seismic coupling seems to be large or even complete in most zones. An exception is the southern Tyrrhenian thrust zone, where most of the African–European convergence is accommodated. Here aseismic deformation is estimated to range from at least 25% along the western part to almost 100% aseismic slip around the Aeolian Islands. Even so, seismic potential of this zone has previously been significantly underestimated, due to the low levels of recorded past seismicity. By contrast, the series of 19 M6–7 earthquakes that hit Calabria in the 18th and 19th century released tectonic strain rates accumulated over time spans up to several times the catalog duration, and seismic potential is revised downward. The southern Tyrrhenian thrust zone and the extensional Calabrian faults, as well as the northeastern Sicilian transtensional zone between them (which includes the Messina Straits, where a destructive M7 event occurred in 1908), all have a similar seismic potential with minimum recurrence times of M ≥ 6.5 of 150–220 years. This potential is lower than that of the Southern Apennines (M ≥ 6.5 recurring every 60 to 140 years), but higher than that of southeastern Sicily (minimum M ≥ 6.5 recurrence times of 400 years). The high seismicity levels recorded in southeastern Sicily indicate some clustering and are most compatible with a tectonic scenario where the Ionian deforms internally, and motions at the Calabrian Trench are small. The estimated seismic potential for the Calabrian Trench and Central and Western Sicily are the lowest (minimum M ≥ 6.5 recurrence times of 550–800 years). Most zones are probably capable of generating earthquakes up to magnitudes 7–7.5, with the exception of Central and Western Sicily where maximum events sizes most likely do not exceed 7.  相似文献   

4.
Biological marker maturity parameters were used to estimate the minimum HC generation temperatures of crude oils from Eastern Hungary. More than 50 oils and oil shows were analysed. Molecular- and homologous-ratios of biological marker compounds (triterpanes, steranes, mono- and triaromatic steroid hydrocarbons) were used as maturation parameters. The oils have at least five maturity stages, i.e. they have been generated under different thermal conditions. The highest reservoir temperature in each group was chosen as the best estimate of the groups' temperature just below the generation temperature, i.e. reservoirs of the group might be expected to be at shallower depths (lower temperatures) than those of the generation zone due to vertical migration into pools. For each maturation level, a threshold temperature range for genesis was inferred from reservoir temperatures; they are from 130–135°C for the least mature oils to 210–215°C for the most mature oils. In the least mature oils cracking was not observed, hence carbon–carbon cracking reactions had not taken place during their genesis. The most mature oils are intensively cracked oils; they are almost condensates. Two major genetic groups (families) of oils were found in the area. Both are present in each maturation level. The effects of migration were checked, and no influence on maturation was found. A number of the oils are in overpressured reservoirs within, or just above, the zone of the present-day active oil generation, hence the present-day temperatures of the pools must have been maximum temperatures. Contrary to the traditionally accepted temperature range for petroleum generation–maturation reactions (50–150°C), there is strong evidence from this study that the onset of oil generation requires temperatures higher than 130°C and is still proceeding above 215°C.  相似文献   

5.
In order to provide new data on the neotectonics and geodynamic properties of western Syria, studies of marine terraces have been carried out. The most attention was paid to the lower terraces in the range of 3–5 to 30–35 m above sea level, because they have more complete distributions along the shore. The lower terraces were examined along the coastal area from Tartus to Latakia, and along the carbonate cliff on Arwad Island. Seven 230Th/U dates for these terraces are in the range of 85–130 ka, suggesting the age interval of the last interglacial (MIS 5). New dates on the lower terraces provide a basis for stratigraphical and geomorphological interpretation as well as neotectonic reconstruction. According to the geomorphological data and lithological composition of those terraces, two main uplifted blocks can be established. One coincides with the Latakia block, and another corresponds to the western margin of the Banias volcanic plateau. These blocks are divided by a subsided structure corresponding to the Nahr el Kebir graben. The amplitude of neotectonic uplifting in the Latakia and Banias blocks reaches 15–20 m for the Late Pleistocene.  相似文献   

6.
Insights into oil cracking based on laboratory experiments   总被引：3，自引：0，他引：3  
The objectives of this pyrolysis investigation were to determine changes in (1) oil composition, (2) gas composition and (3) gas carbon isotope ratios and to compare these results with hydrocarbons in reservoirs. Laboratory cracking of a saturate-rich Devonian oil by confined, dry pyrolysis was performed at T=350–450 °C, P=650 bars and times ranging from 24 h to 33 days. Increasing thermal stress results in the C15+ hydrocarbon fraction cracking to form C6–14 and C1–5 hydrocarbons and pyrobitumen. The C6–14 fraction continues to crack to C1–5 gases plus pyrobitumen at higher temperatures and prolonged heating time and the δ 13Cethane–δ13Cpropane difference becomes greater as oil cracking progresses. There is considerable overlap in product generation and product cracking. Oil cracking products accumulate either because the rate of generation of any product is greater than the rate of removal by cracking of that product or because the product is a stable end member under the experimental conditions. Oil cracking products decrease when the amount of product generated from a reactant is less than the amount of product cracked. If pyrolysis gas compositions are representative of gases generated from oil cracking in nature, then understanding the processes that alter natural gas composition is critical.  相似文献   

7.
The Dabie–Sulu collision belt in China extends to the Hongseong–Odesan belt in Korea while the Okcheon metamorphic belt in Korea is considered as an extension of the Nanhua rift within the South China block. The Hongseong–Odesan belt divides Korea's Gyeonggi massif into northern and southern portions. The southern Gyeonggi massif and the Yeongnam massif are correlated with China's Yangtze and Cathaysia blocks, respectively, while the northern Gyeonggi massif is part of the southern margin of the North China block. The southern and northern Gyeonggi massifs rifted from the Rodinia supercontinent during the Neoproterozoic, to form the borders of the South China and North China blocks, respectively. Subduction commenced along the southern and eastern borders of the North China block in the Ordovician and continued until a Triassic collision between the North China and South China blocks. While subduction was occurring on the margin of the North China block, high-P/T metamorphic belts and accretionary complexes developed along the inner zone of southwest Japan from the Ordovician to the Permian. During the subduction, the Hida belt in Japan grew as a continental margin or continental arc. Collision between the North and South China blocks began in Korea during the Permian (290–260 Ma), and propagated westwards until the Late Triassic (230–210 Ma) creating the sinistral TanLu fault in China and the dextral fault in the Hida and Hida marginal belt in Japan. Phanerozoic subduction and collision along the southern and western borders of the North China block led to formation of the Qinling–Dabie–Sulu–Hongseong–Hida–Yanji belt.  相似文献   

8.
A detailed kinematic study in the Piedras–Girardot area reveals that approximately 32 km of ENE–WSW oblique convergence is accommodated within a northeast-trending transpressional shear zone with a shear strain of 0.8 and a convergence factor of 2. Early Campanian deformation is marked by the incipient propagation of northeast-trending faults that uplifted gentle domes where the accumulation of sandy units did not take place. Maastrichtian unroofing of a metamorphic terrane to the west is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodates less than 5% extension. This extensional fabric, distortion of fossil molds, and a moderate cleavage accommodating less than 5% contraction, developed concurrently, but before large-scale faulting and folding. Paleogene folding and southwestward thrust sheet propagation are recorded by syntectonic strata. Neogene deformation took place only in the western flank of this foldbelt. The amount, direction, and timing of deformation documented here contradict current tectonic models for the Cordillera Oriental and demand a new tectonic framework to approach the study of the structure of the northern Andes. Thus, an alternative model was constructed by defining three continental blocks: the Maracaibo, Cordillera Central, and Cordillera Oriental blocks. Oblique deformation imposed by the relative eastward and northeastward motion of the Caribbean Plate was modeled as rigid-body rotation and translation for rigid blocks (derived from published paleomagnetic and kinematic data), and as internal distortion and dilation for weak blocks (derived from the Piedras–Girardot area). This model explains not only coeval dextral and sinistral transpression and transtension, but also large clockwise rotation documented by paleomagnetic studies in the Caribbean–northern Andean region.  相似文献   

9.
The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO2 (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO2 (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.  相似文献   

10.
Rocks subjected to long-term loading have been known to suffer microcracking. The rate of cracking is sensitive to the type of the applied stress (tensile or compressive), and the magnitude of the stress relative to the instantaneous strength. In addition, crack growth is influenced by the environment (pressure and temperature) including the presence or absence of moisture.For tensile loading, the sensitivity of granite to time-dependent cracking is demonstrated through a fracture mechanics test known as double torsion. The crack velocity versus stress intensity function is established for two environments, room temperature and humidity and room temperature and 100 percent humidity.For compressive loading, time dependent cracking is evaluated from creep tests conducted in uniaxial compression in the same two environments. The rate of cracking is defined by finding the functional relationship between the rate of crack growth, expressed as the rate of crack volume strain, and uniaxial compressive stress.A variety of mathematical functions has been fitted to the obtained data. The traditionally-used power and exponential relationships give good correlation for both crack velocity and crack volume strain rate.The crack volume strain rate versus stress function can be integrated to obtain a lifetime estimate for Lac du Bonnet granite. After 1 000 years of loading in uniaxial compression at room temperature and 100 percent humidity, the strength of this granite could reduce from 225 MPa to 90–100 MPa.  相似文献   

11.
Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages.  相似文献   

12.
South Australia has the greatest utilisation of heritage or building stone in Australia because of its lack of timber resources. Consequently, natural stone was intensively used from the beginning of European colonisation. Building stones in South Australia, notable for their variety given the State’s diverse geology, can be challenging to designate as to their international importance. However, dimension stone in South Australia can also be designated as having national, regional, local or prospective importance. Commonly, stone in South Australia is restricted in use to a specific town, or even a single construction, and has only local significance except where use involves special stone characteristics, unusual stone masonry or use in a building with significant heritage. For instance, the town of Second Valley has a localised use of marble, likely the earliest use (1849) of this rock type in the State. Another example, ‘Adelaide Black Granite’, quarried since 1958, should probably be recognised internationally as a ‘Global Heritage Stone Resource’, as it has had intensive monumental use around Australia, utilisation as cladding in modern buildings, as well as paving and walling in Australia’s National Parliament in Canberra. It has also been exported, notably used for the Australian Embassy in Japan and for a major War Memorial at Le Hamel, France. South Australian slate quarries also provide significant heritage stone. For example, the Willunga Slate quarry south of Adelaide has been sourced for roofing slate and walling since 1840 and is arguably the longest continuous mining operation in Australia; this resource has obvious national significance given a period of quarrying extending to the present, coupled with national distribution of its products. Probably the most widespread stone sourced in South Australia is the surface limestone or calcrete, quarried in the Adelaide area until the 1850s and used for general construction. The stone continues to be used in rural areas. While arguably only of local significance, it could be considered to have national importance because of its extensive use in churches, public building, hotels, houses and simple walling across a vast area of South Australia. Additional research is needed to clarify the heritage status of many building stones used in South Australia.  相似文献   

13.
We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.  相似文献   

14.
In southern Patagonia, the occurrence of large seabird nesting colonies condition the features of the regional bone record, which is characterized by a large amount of bird remains, despite their small body size. Bird and mammal bones have remarkably different taphonomic histories in this region: mammal remains are less damaged by carnivores and resist weathering for long periods, whereas bird bones show a greater initial destruction by carnivore activities and faster deterioration by weathering. A comparison of these results with results from research conducted in several African parks (particularly Amboseli) shows that differences in both records are mainly due to differences in community composition and predator–prey dynamics in both ecosystems. Furthermore, while in both ecosystems bird bones weather more rapidly than mammal bones, destruction by this process is more rapid in Patagonia. Overall, the analysis reinforces the need to generate specific models to understand taphonomic histories of archaeofaunal assemblages from Patagonia.  相似文献   

15.
岩体化学风化的非连续性及其科学意义   总被引：5，自引：0，他引：5  
岩体化学风化在空间上具有高度的非连续性，这种非连续性广泛存在于从宏观、细观到微观的所有尺度。宏观结构面是化学风化最主要的发生场所；风化岩体内，新鲜岩块被沿结构面内法线方向发育的腐蚀带包围，呈斑点状分散于腐岩中。微缝等细观损伤普遍存在于各类岩石中；化学风化从岩块内不同空间位置的水力有效空隙向三维空间扩展，决定了细观尺度上化学风化的非连续性。矿物溶解是在晶体中具有过剩表面能的缺陷位置优先发生的，因而具有显著的微观非连续性。由于非连续特性，化学风化可增大水岩界面，提升矿物溶解反应的规模及速率。通过对既有损伤的扩展及在损伤空间堆积残余物，化学风化具有分离—裂化岩体、岩块及造岩矿物的重要作用，这种作用可使以新鲜岩石为主的岩体大规模脱离母岩，而堆积于坡脚的岩石块体在化学风化的继续作用下，可裂解为更小的岩屑或矿物碎屑，为向水体搬运创造条件，从而极大地促进斜坡夷平及地貌重塑进程。  相似文献   

16.
The mineral composition of sandstones from Cretaceous–Lower Paleocene terrigenous sequences of the western Kamchatka–Ukelayat zone (southern Koryak Upland, western Kamchatka) suggests that the Okhotsk–Chukot volcanogenic belt and fragments of the Uda–Murgal island arc served as the most probable provenance. Fission-track dating of zircon showed that sandstones from this zone contain detrital zircon of several different-age populations. Fission tracks in zircon grains were nor subjected to secondary ignition. The age of young zircon population coincides with the biostratigraphic age of host sequences. Thus, results of dating of detrital zircon grains from sandstones, which did not experience heating above 215–240°C, indicate that this method is appropriate for dating fossil-free terrigenous sequences. The young zircon population in the sandstones is related to erosion of plagiogranite and diorite intrusions of the Uda–Murgal arc and outer zone of the Okhotsk–Chukot volcanic belt exposed at the day surface owing to differential vertical movements and rapid exhumation of blocks.  相似文献   

17.
Three boreholes were drilled near the Nojima fault, which the 1995 Hyogoken–Nanbu earthquake occurred on. In order to research the properties and the healing process of the fault, water injection experiments were conducted every 3 years. In this report, we researched the permeability of the fault as a measurement of crack density or porosity of the fault zone. Pore water pressure changes in rock due to the water injections at one borehole were observed as discharge changes or groundwater level changes at the other borehole. Using numerical calculations, the permeability of the fault fracture zone was estimated for each experiment. The permeability has been decreasing as time passed, which is thought to show the fault healing process of the Nojima fault after the 1995 Hyogoken–Nanbu earthquake.  相似文献   

18.
This paper reviews the research on active and earthquake faults in Taiwan conducted prior and after the 1999 Chichi earthquake. The Chichi earthquake plays as a turning point of the relevant studies, since the 1999 coseismic surface rupture exactly follows preexisting fault scarps, created in turn by previous seismic events along the Chelungpu Fault. This fact indicates that the precise mapping on the other active faults is fundamental to predict the location of surface rupture caused by large future earthquakes. Since 1999, many trenching studies have been carried out along the Chichi earthquake fault. A few of them demonstrates that the penultimate event is as young as probably only 200–430 years old; however, some others show a rather old age of several hundreds years or even older for the last faulting event before 1999. More trenching studies are necessary for such a long fault in order to understand the possible segmentation features and the correlation of the paleoseismic events identified along the entire fault length. In addition, we further discuss the offshore faulting associated with seismic event along the eastern coast of Taiwan, where the multiple Holocene terraces are well known.  相似文献   

19.
A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.  相似文献   

20.
Electromagnetic experiments were conducted in 1995 as part of a multidisciplinary research project to investigate the deep structure of the Chyulu Hills volcanic chain on the eastern flank of the Kenya Rift in East Africa. Transient electromagnetic (TEM) and broadband (120–0.0001 Hz) magnetotelluric (MT) soundings were made at eight stations along a seismic survey line and the data were processed using standard techniques. The TEM data provided effective correction for static shifts in MT data. The MT data were inverted for the structure in the upper 20 km of the crust using a 2-D inversion scheme and a variety of starting models. The resulting 2-D models show interesting features but the wide spacing between the MT stations limited model resolution to a large extent. These models suggest that there are significant differences in the physical state of the crust between the northern and southern parts of the Chyulu Hills volcanic field. North of the Chyulu Hills, the resistivity structure consists of a 10–12-km-thick resistive (up to 4000 Ω m) upper crustal layer, ca. 10-km-thick mid-crustal layer of moderate resistivity (50 Ω m), and a conductive substratum. The resistive upper crustal unit is considerably thinner over the main ridge (where it is ca. 2 km thick) and further south (where it may be up to 5 km thick). Below this cover unit, steep zones of low resistivity (0.01–10 Ω m) occur underneath the main ridge and at its NW and SE margins (near survey positions 100 and 150–210 km on seismic line F of Novak et al. [Novak, O., Prodehl, C., Jacob, A.W.B., Okoth, W., 1997. Crustal structure of the southern flank of the Kenya Rift deduced from wide-angle P-wave data. In: Fuchs, K., Altherr, R., Muller, B., Prodehl, C. (Eds.), Structure and Dynamic Processes in the Lithosphere of the Afro-Arabian Rift System. Tectonophysics, vol. 278, 171–186]). These conductors appear to be best developed in upper crustal (1–8 km) and middle crustal (9–18 km) zones in the areas affected by volcanism. The low-resistivity anomalies are interpreted as possible magmatic features and may be related to the low-velocity zones recently detected at greater depth in the same geographic locations. The MT results, thus, provide a necessary upper crustal constraint on the anomalous zone in Chyulu Hills, and we suggest that MT is a logical compliment to seismics for the exploration of the deep crust in this volcanic-covered basement terrain. A detailed 3-D field study is recommended to gain a better understanding of the deep structure of the volcanic field.  相似文献   

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