Multiple Dikes Research Articles

Magnetic data interpretation for 2D dikes by the metaheuristic bat algorithm: sustainable development cases

Metaheuristic algorithms are increasingly being utilized as a global optimal method in the inversion and modeling of magnetic data. We proposed the Bat Algorithm Optimization (BAO) technique that is based on bat echolocation performance to find the global optimum solution. The best-estimated source parameters that correspond to the objective function minimum value are obtained after achieving the global optimum (best) solution. The suggested BAO technique does not require any prior knowledge; rather, it is a global search method that provides an effective tool for scanning the space of data to appraise sources parameters. The BAO technique is applied to magnetic data in the class of dipping and vertical dikes along 2D profiles to estimate the dimensional source parameters that include the depth to top, origin location, amplitude coefficient, index angle of magnetization, and width of the dipping dikes. The BAO technique has been used for single and multiple dikes structures. The accuracy and stability of the BAO technique are achieved on different synthetic examples of free and noisy data for single and multiple cases. Furthermore, the presented BAO technique was effectively utilized in three field examples from China and Egypt for iron ore deposits and metavolcanics basalt rock investigations. Overall, the BAO technique recovered inversion outcomes are in good agreement with borehole, geology, and published literature results.

Modeling of magnetic anomalies generated by simple geological structures through Genetic-Price inversion algorithm

A new approach to the interpretation of magnetic anomalies generated by geological structures resembling simple geometrical bodies is presented. The method is based on the Genetic-Price hybrid Algorithm (GPA) recently proposed by the authors for the inversion of potential-field data, specifically of self-potential signals. In this paper, the effectiveness of the proposed algorithm is tested on magnetic data for retrieving the parameters of the anomaly causative sources. First, the testing is performed on synthetic noise-free and noisy signals due to magnetized sphere-, dike- and fault-like models, then the analysis is extended to field magnetic anomalies. Concerning the synthetic data analyses, a very good agreement is obtained between assumed and retrieved source parameters. Specifically, the error between true and inverted parameter sets was found to be no higher than 9% even by adding 15% of Gaussian white random noise to the initial dataset. As for the study of field data, the values of depth, horizontal position, effective magnetization intensity and angle provided by the proposed GPA method compare well with those obtained by other interpretative approaches. Finally, the results of the GPA application to the inversion of magnetic data measured in the Mt. Somma-Vesuvius volcanic area are reported. In particular, the interpretation of the magnetic anomalies along a SW-NE profile in terms of multiple dikes provides information about depth and location of buried volcanic structures that match well those from other geophysical and geological analyses.

特集 火山活動と地殻応力場 中央海嶺のマグマ供給システム

Recent progress in studies on magma plubming system beneath mid-ocean ridges is reviewed. Mid-ocean ridges are not continuous, homogeneous series of crests, but are segmented by various topographical offsets of several orders of magnitude. Such topographical ridge segmentation is a surface manifestation of along-strike variations in the pattern of mantle convection and the supply of magma.Upwelling of the mantle material beneath the spreading center takes the form of a diapir, rather than sheet-like flow. Such diapiric upwelling has an along-axis dimension of several hundred to several tens of kilometers, which corresponds to first-and second-order segments, but some are as small as the scale of third-order segments. Where magma upwelling is intense, hot lithosphere and high magmatic productivity produces thick crusts, resulting in low gravity anomaly.When the spreading rate is low, mantle flow is cooled on route to the bottom of the lithosphere and ceases to melt at depths. Thus magmas produced beneath slow spreading ridges have low degrees of melting. On the contrary, fast spreading ridges are followed by intense mantle upwelling which melts significantly the mantle column from deeper part of the upper mantle up to just beneath the crust.Magmatic budget is so large at fast spreading ridges as to maintain large, steady-state magma chambers along the ridge axes with extensions comparable to second-and third-order segments. Such magma chambers have a very small melt pocket one to two kilometers wide and up to several hundred meters thick, underlain by a large mush of crystals and melt. Cumulate layers on the top of the mush sink and deform to S-shaped or downwarping layers as seen in large layered plutonic bodies in some ophiolites. However, low spreading ridges do not have enough supply of magma but only possess short-lived small magma chambers consisting of crystal mush.Volcanic landforms consist of major topographic features associated with small volcanic edifices : the former is a large shieled volcano several tens of kilometers long and is probably formed by episodic eruptions every several tens of thousand years ; the latter is a small conical lava cone comprising pillow flows which is constructed during a short-period eruption but some are active as long as 1.8 million years which are unequivocally central volcanoes.Rifting episodes at spreading axes depend on the spreading rate and the width of the volcanic zone. Slow spreading ridges such as Iceland have rifting every 100 years, but fast spreading ridges such as East Pacific Rise rift every year or two. Such rifting episodes continue for several years and are associated with multiple dikes with an injection interval of a month up to a year.Diapiric mantle upwelling, structure of magma chambers and volcanic landforms seen in mid-ocean ridges resemble those in some large ophiolites such as the Troodos, Cyprus and the Semail, Oman, and post-Teritary volcanism in Iceland.

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Comment and Reply on “Multiple dikes in the Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?”

Comment and Reply on “Multiple dikes in the Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?”

Comment and Reply on “Multiple dikes in the Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?”

Comment and Reply on “Multiple dikes in the Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?”

Multiple dikes in Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?

A mafic intrusive-extrusive complex at the base of the Kam Group in the Archean Yellowknife Supergroup at Yellowknife, Northwest Territories, Canada, grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments. Although rock types and primary structures resemble those in upper parts of Phanerozoic ophiolites, the Kam Group lacks an ultramafic base and is much thicker than typical ophiolites. Because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins, however, the Kam Group is viewed as a remnant of Archean oceanic crust rather than as a product of intracontinental rift volcanism. The dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea-floor spreading in an Archean marginal-basin setting.

Petrology of multiple dikes and related stratiform intrusions in lower Tunguska basin

Petrographic and geochemical evidence, geological-structural plan, and the absolute age of the stratiform and the multiple intrusions, as well as their positions with respect to other magmatic bodies in the basin, suggest a comagmatism of the intrusions and the dikes. Distinctions between the constituents of one and the same intrusive complex so visualized are explainable by the differences in their consolidation conditions, including the relative tectonic calm in the uplifts and the relatively active tectonism and volcanism in the downwaips -- V.P. Sokoloff.

Multiple Dikes of Cape Neddick

Cape Neddick lies on the southwest coast of Maine in York County. The Kittery quartzite crops out around nearly the entire periphery of the cape and contains a large number of structurally complicated multiple dikes. The quartzite in this area has been intruded by a soda granite and by the Cape Neddick gabbro. Each of these intrusives contains its own suite of dikes whose members are not, however, involved in multiple relationship with one another. Most of the multiple dikes strike nearly northeast, are nearly vertical, and are characterized by strong parallelism. Both structures and lithological associations were studied to determine features peculiar to these dikes. Certain types of behavior, for example, that called by the writer “braiding” and “intrasectate” and “extrasectate” combination, were found to be typical. No structural peculiarities in the host rock which might have localized the multiple dikes could be detected. The condition necessary for their formation is believed to be the repetition of periods of essentially parallel fracturing in association with periods of magma availability. The study suggests that recurrent fracturing and contemporaneous intrusion of magmatic differentiates are probably strictly interdependent phases of the dike-building activity. The cause of this activity must eventually be sought in the behavior and influence of subjacent igneous masses previously active in the area.