Showing papers on "Mohorovičić discontinuity published in 2019"

Deep electrical imaging of the ultraslow-spreading Mohns Ridge

Abstract: More than a third of mid-ocean ridges have a spreading rate of less than 20 millimetres a year1. The lack of deep imaging data means that factors controlling melting and mantle upwelling2,3, the depth to the lithosphere–asthenosphere boundary (LAB)4,5, crustal thickness6–9 and hydrothermal venting are not well understood for ultraslow-spreading ridges10,11. Modern electromagnetic data have greatly improved our understanding of fast-spreading ridges12,13, but have not been available for the ultraslow-spreading ridges. Here we present a detailed 120-kilometre-deep electromagnetic joint inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data. Inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Although the upwelling pattern shows several of the characteristics of a dynamic system3,12–14, it probably reflects passive upwelling controlled by slow and asymmetric plate movements instead. Upwelling asthenosphere and melt can be traced to the inferred depth of the Mohorovicic discontinuity and are enveloped by the resistivity (100 ohm metres) contour denoted the electrical LAB (eLAB). The eLAB may represent a rheological boundary defined by a minimum melt content. We also find that neither the melt-suppression model7 nor the inhibited-migration model15, which explain the correlation between spreading rate and crustal thickness6,16–19, can explain the thin crust below the ridge. A model in which crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates is more likely. Active melt emplacement into oceanic crust about three kilometres thick culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at the Loki’s Castle hydrothermal black smoker field. Fluid convection extends for long lateral distances, exploiting high porosity at mid-crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultraslow-spreading ridges. An inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data, reveals passive mantle upwelling controlled by slow and asymmetric plate movements.

Gravity and Magnetic Constraints on the Crustal Structure of the Ceara Plateau, Brazilian Equatorial Margin

Abstract: The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the largest one, the Ceara Plateau. This is a complex and poorly recognized area, with few available geophysical data. Gravity and magnetic data from the Equant I Project, seismic published lines and other previous studies are used to make several models and analyses. We recognized gravity and magnetic anomalies related to a denser, magnetized basement, internally comprised of two magmatic highs. Through gravity and magnetic forward modeling, we estimated the basement surface between 800 and 6000 m and the Mohorovicic discontinuity about 22-23 km below the Ceara Plateau. We also present and discuss the position of the continent-ocean boundary 40 km away from the continental shelf, placing the plateau at the oceanic crust domain, and the area of the transitional crust, with an extension of 40-50 km, formed during the rifting phase of the Atlantic Ocean opening.

Estimation of near-surface attenuation in the tectonically complex contact area of the northwestern External Dinarides and the Adriatic foreland

Abstract: . Seismically induced ground motion at a site is generally influenced by the seismic source, the propagation path and the local site conditions. Over the last several decades, researchers have consistently asserted that for near-site attenuation, the spectral parameter κ is subject primarily to the site conditions. In this research, we estimated the parameter κ based on the acceleration amplitude spectrum of shear waves from local earthquakes recorded by seismological stations situated in the western part of Croatia from the slope of the high-frequency part. The spatial distribution of κ values is comparable with seismological, geophysical and geological features, with the published coda- Q values for each station as well as with the isoseismal maps for selected stronger earthquakes in the study area. The complex pattern of longitudinal and transversal major late-orogenic fault zones dissecting early-orogenic thin-skinned tectonic cover in the Kvarner area and the shallow depth to the Moho (Mohorovicic discontinuity) in the Adriatic foreland (southern Istria) are probably responsible for a significant part of wave attenuation and for the anisotropy of attenuation. Regional near-surface attenuation distribution and modelled macroseismic fields point to the conclusion that attenuation properties of rocks in the northwestern External Dinarides are far from isotropic, and the most likely anisotropy sources are the preferential orientations of cracks and fractures under the local tectonic stress field, trapping of waves along major faults (waveguides), and/or attenuation within the fault zones. These results are important for gaining further insight into the attenuation of near-surface crust layers in the northwestern External Dinarides and the associated Adriatic foreland as well as in similar geotectonic settings.

Modeling of the earth’s crust and upper mantle beneath central part of Java Island using receiver function data

Abstract: The objectives of this research are to model the earth crust and Mohorovicic depth at part of Central java island. The subsurface modeling has been carried out using the inversion of receiver function techniques and stacking H-κ. The methodology of this research using the inversion technique and the H-κ stacking technique to obtain subsurface structures and determine the depth of Mohorovicic discontinuity. The results of this research are the average depth of Mohorovicic discontinuity is 33.54 km and the value of κ 1.65. The depth of Mohorovicic’s discontinuity in Yogyakarta and Semarang is 25-33km. Based on the inversion technique, there is a low speed anomaly between Mt. Merapi and Mt. Lawu at a depth of 11-19 km known as MLA (Merapi Lawu Anomaly) under the Kendeng Zone. The P wave velocity at a depth of 11 Km has a pattern which is interpreted as the long subduction zone of the dominant Maratus trend. The fact that proves the old subduction is the P wave velocity pattern near Karangsambung which has melange outcrops.

Studying the sensitivity of GOCE gravity gradients to the crustal structure: case study of Central Europe

Abstract: We discuss the determination of gravity gradients from the orbital ceiling to the depth of the Mohorovicic discontinuity (Moho) for Central Europe. Components of the Eotvos tensor were derived from “Heterogeneous gravity data combination for Earth interior and geophysical exploration research” project (“GOCE+”) by using the gridded data with a resolution of 0.2° per 0.2°. Gravity gradients to Moho boundary depth were modelled forward to the 255 km orbital height. We calculated gradient sensitivity using a 3D model divided into: sediments and consolidated crust including the precise location of the Moho boundary. To define tesseroids as mathematical model we need to set two parameters of the crust: density and thickness for each spherical layer separately. Altitudes for topography/bathymetry were derived from ETOPO1 model, sediments thickness from EuCRUST-07 model, and Moho boundary from Grad and Tiira (Geophys J Int 176(1):279–292, 2009. https://doi.org/10.1111/j.1365-246x.2008.03919.x ) seismic map. For high latitudes, we noted the largest changes for the gradients towards the poles, with particular values of 689.07 mE (milli-eotvos) and 1138.19 mE for VXX and VZZ gradients, respectively. We obtained extreme values for the location of the deep and shallow areas of the crust (Alps, North-Eastern Poland and areas of seas) equal to − 3 E and + 1.5 E, respectively. Most of the gradients showed strong correlation with anomalies in crustal density of − 2.5 E for VZZ and + 1.5 E for VYY in the extreme cases. We showed that changes in crust density and thickness by respectively 50 kg/m3 and 10 km entail changes in gradient values by 15% for density and 10% for depths. Numerical analysis considering Preliminary Reference Earth Model (PREM) showed importance of density modeling for determination of gravity gradients.