scholarly journals Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

2020 ◽  
Vol 224 (1) ◽  
pp. 290-305
Author(s):  
Fenitra Andriampenomanana ◽  
Andrew A Nyblade ◽  
Michael E Wysession ◽  
Raymond J Durrheim ◽  
Frederik Tilmann ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Plate Boundary ◽  
Active Regions ◽  
Shear Zones ◽  
Small Scale ◽  
Uppermost Mantle ◽  
Lithospheric Deformation ◽  
Pan African ◽  
Pn Velocity

SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (<8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (>8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

scholarly journals Lithospheric Deformation at the South Island Oblique Collision, New Zealand

2021 ◽  
Author(s):  
◽  
Sandra Bourguignon
Keyword(s):  
Wave Speed ◽  
Seismic Anisotropy ◽  
Plate Boundary ◽  
Southern Alps ◽  
Density Contrast ◽  
Mass Excess ◽  
Oblique Collision ◽  
Lithospheric Deformation ◽  
Shear Wave Speed ◽  
Central South

<p>Lithospheric deformation is investigated within the Southern Alps oblique collision zone of the Australian and Pacific plate boundary. Seismological methods and gravity modelling are used to estimate seismic anisotropy, wave-speed anomalies and mass anomalies in the uppermost mantle. While seismic anisotropy is generally interpreted to result from Cenozoic mantle shear, wave-speed and mass anomalies can be explained solely by thermal contraction of mantle rocks that results from the downward deflection of isotherms during mantle shortening. Along the eastern Southern Alps foothills and approximately 15' clockwise from their axis, earthquake Pn waves propagate at 8.54 +/- 0.20 km/s. This high wave speed is attributed to a high average Pn speed (8.3 +/- 0.3 km/s) and Pn anisotropy (7 - 13 %) in the mantle lid beneath central South Island. Two-dimensional ray-tracing suggests that the crustal thickness is 48 +/- 4 km beneath the Southern Alps' southern extent near Wanaka (western Otago). Such a thickness represents an 18 +/- 4 km thick crustal root that is thicker than necessary to isostatically sustain the approximately 1000 m topographic load of this region. A mass excess is proposed in the mantle below the region of over-thickened crust to compensate for the crustal root mass deficit. Assuming that the crustal root represents a -300 kg/m3 density contrast with the mantle lid, this mantle mass excess requires a minimum density contrast of 35 +/- 5 kg/m3, 110 +/-20 km width and 70 +/- 20 km thickness that will impart a downward pull on the overlying crust.</p>

scholarly journals Lithospheric Deformation at the South Island Oblique Collision, New Zealand

2021 ◽  
Author(s):  
◽  
Sandra Bourguignon
Keyword(s):  
Wave Speed ◽  
Seismic Anisotropy ◽  
Plate Boundary ◽  
Southern Alps ◽  
Density Contrast ◽  
Mass Excess ◽  
Oblique Collision ◽  
Lithospheric Deformation ◽  
Shear Wave Speed ◽  
Central South

<p>Lithospheric deformation is investigated within the Southern Alps oblique collision zone of the Australian and Pacific plate boundary. Seismological methods and gravity modelling are used to estimate seismic anisotropy, wave-speed anomalies and mass anomalies in the uppermost mantle. While seismic anisotropy is generally interpreted to result from Cenozoic mantle shear, wave-speed and mass anomalies can be explained solely by thermal contraction of mantle rocks that results from the downward deflection of isotherms during mantle shortening. Along the eastern Southern Alps foothills and approximately 15' clockwise from their axis, earthquake Pn waves propagate at 8.54 +/- 0.20 km/s. This high wave speed is attributed to a high average Pn speed (8.3 +/- 0.3 km/s) and Pn anisotropy (7 - 13 %) in the mantle lid beneath central South Island. Two-dimensional ray-tracing suggests that the crustal thickness is 48 +/- 4 km beneath the Southern Alps' southern extent near Wanaka (western Otago). Such a thickness represents an 18 +/- 4 km thick crustal root that is thicker than necessary to isostatically sustain the approximately 1000 m topographic load of this region. A mass excess is proposed in the mantle below the region of over-thickened crust to compensate for the crustal root mass deficit. Assuming that the crustal root represents a -300 kg/m3 density contrast with the mantle lid, this mantle mass excess requires a minimum density contrast of 35 +/- 5 kg/m3, 110 +/-20 km width and 70 +/- 20 km thickness that will impart a downward pull on the overlying crust.</p>

Joint inversion and collaborative interpretations in complex geodynamical context

2020 ◽  
Author(s):  
Christel Tiberi ◽  
Adeline Clutier ◽  
Matthieu Plasman ◽  
Stéphanie Gautier ◽  
Fleurice Parat ◽  
...  
Keyword(s):  
Seismic Velocity ◽  
Joint Inversion ◽  
Dynamical Evolution ◽  
Active Regions ◽  
East African ◽  
Lithospheric Deformation ◽  
Complex Interactions ◽  
New Information ◽  
Structure Heterogeneity ◽  
African Rift

&lt;p&gt;Active regions concentrate different geodynamical processes sometimes with complex interactions and retroactions. In order to understand the associated lithospheric deformation and evolution, scientists deduce crustal and mantle structures from sparse, inaccurate and indirect observations. In particular, geophysics aims at retrieving physical properties of crustal or lithospheric media from gravity, electric or seismic measurements. Those indirect tools have been used for decades now to image the Earth Interior at many different scales, from the surface down to the Core.&lt;/p&gt;&lt;p&gt;Besides, density, resistivity or seismic velocity retrieved from geophysical inversions are sensitive to many different factors (temperature, pressure, melt, composition&amp;#8230;), each of them impacting the parameters variously. Finally, each of these methods presents its own depth investigation and accuracy, which depends on time lap, network configuration, data wavelength, etc.&lt;/p&gt;&lt;p&gt;In order to distinguish the role of each factor in the lithospheric structure heterogeneity, and to counteract the different method limits, geophysicists have combined their observations in combined schemes for decades now. We will present here how jointly inverting seismic tomography and gravity may help to better understand complex zones implying melt, faults, crustal modification and plate interaction. When mathematical link between the parameters doesn&amp;#8217;t exist, we will present a combination of petrophysics and geophysics, that brings new information on past and present dynamical evolution in a magmatic area (East African Rift, Tanzania). Finally, we will address the question of the real benefit of a joint inversion, and whether we can combine all kind of data.&lt;/p&gt;

scholarly journals The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy

2003 ◽  
Vol 174 (1) ◽  
pp. 45-57 ◽  
Author(s):  
Sébastien Judenherc ◽  
Michel Granet ◽  
Jean-Pierre Brun ◽  
Georges Poupinet
Keyword(s):  
Shear Zone ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Velocity Model ◽  
P Wave ◽  
Lithospheric Deformation ◽  
Thermal Anomalies ◽  
The North ◽  
Chemical Anomalies ◽  
Armorican Massif

Abstract The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.

scholarly journals Lithospheric and sub-lithospheric deformation under the Borborema Province of NE Brazil from receiver function harmonic stripping

2019 ◽  
Author(s):  
Gaelle Lamarque ◽  
Jordi Julià
Keyword(s):  
Lithospheric Mantle ◽  
Seismic Anisotropy ◽  
Receiver Function ◽  
Seismic Station ◽  
Atlantic Coast ◽  
Shear Zones ◽  
Receiver Functions ◽  
Lithospheric Deformation ◽  
Borborema Province ◽  
Continental Interior

Abstract. The depth-dependent anisotropic structure of the lithosphere under the Borborema Province of northeast Brazil has been investigated through harmonic stripping of receiver functions developed at 39 stations in the region. This method retrieves the first (k = 1) and second (k = 2) degree harmonics of a receiver function dataset, which characterize seismic anisotropy beneath a seismic station. Anisotropic fabrics are in turn directly related to the deformation of the lithosphere from past and current tectonic processes. Our results reveal the presence of anisotropy within the crust and the lithospheric mantle throughout the entire Province, with the exception of a few stations in the continental interior that lack evidence for any anisotropic signatures. Most stations in the continental interior report consistent anisotropic orientations in the crust and lithospheric mantle, suggesting a dominant NE-SW pervasive deformation along lithospheric-scale shear zones developed during the Brasiliano-Pan African orogeny. The lack of anisotropy at a few stations along a NE-SW trend in the center on the Province is harder to explain, but might be related to heating of the lithosphere by an asthenospheric channel. Finally, several stations along the Atlantic coast reveal depth-dependent anisotropic orientations roughly (sub)perpendicular to the margin. These results suggest a more recent overprint, probably related to the presence of frozen anisotropy in the lithosphere due to stretching and rifting during the opening of the South Atlantic.

scholarly journals Decratonization by rifting enables orogenic reworking and transcurrent dispersal of old terranes in NE Brazil

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Carlos E. Ganade ◽  
Roberto F. Weinberg ◽  
Fabricio A. Caxito ◽  
Leonardo B. L. Lopes ◽  
Lucas R. Tesser ◽  
...  
Keyword(s):  
Shear Zones ◽  
Boundary Region ◽  
The North ◽  
Pan African ◽  
Juvenile Material ◽  
Oceanic Basin ◽  
Borborema Province ◽  
Isotopic Model ◽  
Late Neoproterozoic

AbstractDispersion and deformation of cratonic fragments within orogens require weakening of the craton margins in a process of decratonization. The orogenic Borborema Province, in NE Brazil, is one of several Brasiliano/Pan-African late Neoproterozoic orogens that led to the amalgamation of Gondwana. A common feature of these orogens is that a period of extension and opening of narrow oceans preceded inversion and collision. For the case of the Borborema Province, the São Francisco Craton was pulled away from its other half, the Benino-Nigerian Shield, during an intermittent extension event between 1.0–0.92 and 0.9–0.82 Ga. This was followed by inversion of an embryonic and confined oceanic basin at ca. 0.60 Ga and transpressional orogeny from ca. 0.59 Ga onwards. Here we investigate the boundary region between the north São Francisco Craton and the Borborema Province and demonstrate how cratonic blocks became physically involved in the orogeny. We combine these results with a wide compilation of U–Pb and Nd-isotopic model ages to show that the Borborema Province consists of up to 65% of strongly sheared ancient rocks affiliated with the São Francisco/Benino-Nigerian Craton, separated by major transcurrent shear zones, with only ≈ 15% addition of juvenile material during the Neoproterozoic orogeny. This evolution is repeated across a number of Brasiliano/Pan-African orogens, with significant local variations, and indicate that extension weakened cratonic regions in a process of decratonization that prepared them for involvement in the orogenies, that led to the amalgamation of Gondwana.

scholarly journals A pan-African medium-range ensemble flood forecast system

2015 ◽  
Vol 19 (8) ◽  
pp. 3365-3385 ◽  
Author(s):  
V. Thiemig ◽  
B. Bisselink ◽  
F. Pappenberger ◽  
J. Thielen
Keyword(s):  
Large Scale ◽  
Practical Implication ◽  
River Basins ◽  
Flood Forecasting ◽  
Small Scale ◽  
Flood Forecast ◽  
Flood Events ◽  
Forecast System ◽  
Pan African ◽  
Forecasting System

Abstract. The African Flood Forecasting System (AFFS) is a probabilistic flood forecast system for medium- to large-scale African river basins, with lead times of up to 15 days. The key components are the hydrological model LISFLOOD, the African GIS database, the meteorological ensemble predictions by the ECMWF (European Centre for Medium-Ranged Weather Forecasts) and critical hydrological thresholds. In this paper, the predictive capability is investigated in a hindcast mode, by reproducing hydrological predictions for the year 2003 when important floods were observed. Results were verified by ground measurements of 36 sub-catchments as well as by reports of various flood archives. Results showed that AFFS detected around 70 % of the reported flood events correctly. In particular, the system showed good performance in predicting riverine flood events of long duration (> 1 week) and large affected areas (> 10 000 km2) well in advance, whereas AFFS showed limitations for small-scale and short duration flood events. The case study for the flood event in March 2003 in the Sabi Basin (Zimbabwe) illustrated the good performance of AFFS in forecasting timing and severity of the floods, gave an example of the clear and concise output products, and showed that the system is capable of producing flood warnings even in ungauged river basins. Hence, from a technical perspective, AFFS shows a large potential as an operational pan-African flood forecasting system, although issues related to the practical implication will still need to be investigated.

scholarly journals The Evolution of the Solar Magnetic Field

2012 ◽  
Vol 10 (H16) ◽  
pp. 86-89 ◽  
Author(s):  
J. Todd Hoeksema
Keyword(s):  
Magnetic Field ◽  
Solar Cycle ◽  
Solar Magnetic Field ◽  
Coronal Holes ◽  
Active Regions ◽  
Statistical Characteristics ◽  
Field Pattern ◽  
Small Scale ◽  
Polar Caps ◽  
Magnetic Elements

AbstractThe almost stately evolution of the global heliospheric magnetic field pattern during most of the solar cycle belies the intense dynamic interplay of photospheric and coronal flux concentrations on scales both large and small. The statistical characteristics of emerging bipoles and active regions lead to development of systematic magnetic patterns. Diffusion and flows impel features to interact constructively and destructively, and on longer time scales they may help drive the creation of new flux. Peculiar properties of the components in each solar cycle determine the specific details and provide additional clues about their sources. The interactions of complex developing features with the existing global magnetic environment drive impulsive events on all scales. Predominantly new-polarity surges originating in active regions at low latitudes can reach the poles in a year or two. Coronal holes and polar caps composed of short-lived, small-scale magnetic elements can persist for months and years. Advanced models coupled with comprehensive measurements of the visible solar surface, as well as the interior, corona, and heliosphere promise to revolutionize our understanding of the hierarchy we call the solar magnetic field.

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