scholarly journals Seismic and Gravity Investigations of the Central Volcanic Region, North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Timothy Andrew Stern
Keyword(s):  
Gravity Anomaly ◽  
Gravity Model ◽  
Upper Mantle ◽  
Seismic Data ◽  
Seismic Refraction ◽  
Volcanic Rocks ◽  
Low Density ◽  
Anomaly Field ◽  
Volcanic Region ◽  
Back Arc

<p>Gravity and seismic refraction studies were undertaken in order to investigate the geological structure of the Central Volcanic Region. A detailed analysis of density determinations from bore-hole rock samples, three seismic refraction surveys and a spectral analysis of the magnetic anomaly field are described. Interpretation of the observed gravity anomaly fie ld for the Central Volcanic Region is initially undertaken by analytically separating the observed anomaly field into its regional and residual components; the almost entirely negative residual anomaly field is then interpreted in terms of varying thicknesses of near-surface, low-density volcanic rocks. At Mangakino and just west of Taupo, however, it is found that the calculated gravity anomaly effect of the seismically determined thickness of low-velocity, and hence low-density, volcanic rocks is less negative than the observed residuals; at both locations "secondary residuals" of about -200 μN/kg remain unexplained. Models are presented that account for these secondary residuals as being due to discrete volumes of low-density molten rhyolite emplaced within the seismic basement. The second method of gravity interpretation used in this study involves modelling all components of the observed gravity anomaly field . This necessitated giving consideration to both the gravity effect of the subducted Pacific plate and to seismic data bearing upon the variation of crustal thickness and mantle density throughout the central North Island. A gravity model for the central North Island is developed for which the important features are:  i) The crust of the Central Volcanic Region is deduced to be only about half the normal continental thickness, and underlying the crust is an "anomalous", low-density upper mantle. This finding from the gravity model is supported by the results of a previous study of upper mantle seismic velocities and from the interpretation of a longrange seismic refraction survey carried out within the Region. These seismic data indicate the depth to, and the velocity of the upper mantle beneath the Region to be 15 km and 7.4 km/s respectively. ii) The positive gravity anomalies that predominate over the western and northwestern North Island can largely be explained by gravity edge-effects associated with variations in the crustal thickness and mantle density within the back-arc areas of the North Island. The gravity model is interpreted as lending support for a previously made proposal that the Region is the site of asymmetric back-arc spreading, and that the crustal rocks now being created are transitional in character between typical oceanic and typical continental.</p>

scholarly journals Seismic and Gravity Investigations of the Central Volcanic Region, North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Timothy Andrew Stern
Keyword(s):  
Gravity Anomaly ◽  
Gravity Model ◽  
Upper Mantle ◽  
Seismic Data ◽  
Seismic Refraction ◽  
Volcanic Rocks ◽  
Low Density ◽  
Anomaly Field ◽  
Volcanic Region ◽  
Back Arc

<p>Gravity and seismic refraction studies were undertaken in order to investigate the geological structure of the Central Volcanic Region. A detailed analysis of density determinations from bore-hole rock samples, three seismic refraction surveys and a spectral analysis of the magnetic anomaly field are described. Interpretation of the observed gravity anomaly fie ld for the Central Volcanic Region is initially undertaken by analytically separating the observed anomaly field into its regional and residual components; the almost entirely negative residual anomaly field is then interpreted in terms of varying thicknesses of near-surface, low-density volcanic rocks. At Mangakino and just west of Taupo, however, it is found that the calculated gravity anomaly effect of the seismically determined thickness of low-velocity, and hence low-density, volcanic rocks is less negative than the observed residuals; at both locations "secondary residuals" of about -200 μN/kg remain unexplained. Models are presented that account for these secondary residuals as being due to discrete volumes of low-density molten rhyolite emplaced within the seismic basement. The second method of gravity interpretation used in this study involves modelling all components of the observed gravity anomaly field . This necessitated giving consideration to both the gravity effect of the subducted Pacific plate and to seismic data bearing upon the variation of crustal thickness and mantle density throughout the central North Island. A gravity model for the central North Island is developed for which the important features are:  i) The crust of the Central Volcanic Region is deduced to be only about half the normal continental thickness, and underlying the crust is an "anomalous", low-density upper mantle. This finding from the gravity model is supported by the results of a previous study of upper mantle seismic velocities and from the interpretation of a longrange seismic refraction survey carried out within the Region. These seismic data indicate the depth to, and the velocity of the upper mantle beneath the Region to be 15 km and 7.4 km/s respectively. ii) The positive gravity anomalies that predominate over the western and northwestern North Island can largely be explained by gravity edge-effects associated with variations in the crustal thickness and mantle density within the back-arc areas of the North Island. The gravity model is interpreted as lending support for a previously made proposal that the Region is the site of asymmetric back-arc spreading, and that the crustal rocks now being created are transitional in character between typical oceanic and typical continental.</p>

Chronology of crustal growth and recycling in the Paleoproterozoic Amisk collage (Flin Flon Belt), Trans-Hudson Orogen, Canada

1999 ◽  
Vol 36 (11) ◽  
pp. 1807-1827 ◽  
Author(s):  
R A Stern ◽  
N Machado ◽  
E C Syme ◽  
S B Lucas ◽  
J David
Keyword(s):  
Upper Mantle ◽  
Tectonic Setting ◽  
Bulk Composition ◽  
Volcanic Rocks ◽  
Crust And Upper Mantle ◽  
Zircon Age ◽  
Magmatic Rocks ◽  
Main Period ◽  
Flin Flon ◽  
Back Arc

U-Pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the Flin Flon greenstone belt, now recognized as a Paleoproterozoic tectonic collage. The "Amisk collage" formed in two major magmatic periods that were separated by an interval of intraoceanic accretionary tectonics. Pre-accretionary volcanic and plutonic rocks of arc and ocean-floor tectonic affinities have crystallization ages of 1.906-1.901 and 1.888-1.881 Ga; the earlier period was dominated by juvenile tholeiitic arc basalts and related back-arc-basin basalts, and the younger period by juvenile calc-alkaline volcanic rocks and turbidites. Intraoceanic accretion of the diverse tectono-stratigraphic assemblages may have commenced between 1.90 and 1.89 Ga, but the main period was 1.88-1.87 Ga. The post-accretionary period (1.876-1.838 Ga) was characterized by intrusion of juvenile calk-alkaline plutons generated by a successor arc that stitched the diverse pre-accretionary assemblages. Marine to subaerial volcaniclastic and epiclastic units were deposited in successor basins <=1.87 Ga (Schist-Wekusko suite), succeeded by alluvial-fluvial (Missi Group) to marine (Burntwood Group) sediments after 1.85 Ga. Despite the dominance of juvenile magmatic rocks within the collage, U-Pb zircon age and Nd-isotopic data show that older (>2.2-3.0 Ga) basement fragments were present throughout the development of the Amisk collage. An arc-back-arc system close to an Archean craton is proposed as the most likely tectonic setting during formation and accretion of the Amisk collage from 1.90 to 1.84 Ga. Subsequent continental collision during peak orogeny (1.84-1.81 Ga) is interpreted to have delaminated the lower crust and upper mantle of the collage, preferentially preserving crust of intermediate bulk composition.

Mr

2020 ◽  
Author(s):  
Fayez Harash
Keyword(s):  
Density Distribution ◽  
Gravity Anomaly ◽  
Mediterranean Sea ◽  
Upper Mantle ◽  
Aegean Sea ◽  
Velocity Model ◽  
Low Density ◽  
Density Structure ◽  
Adjacent Region ◽  
The Mediterranean

&lt;p&gt;&lt;strong&gt;3-D density structure of the upper-mantle from gravity inversion constrained by seismic velocity model: A case study of the Mediterranean Sea and surrounding region &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Harash Fayez&lt;sup&gt;1&lt;/sup&gt;, Chao Chen&lt;sup&gt;1,&lt;/sup&gt;&lt;sup&gt;2&lt;/sup&gt;, Qing Liang&lt;sup&gt;1&lt;/sup&gt;&lt;sup&gt;,2&lt;/sup&gt;, Chenming Tu&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Institute of Geophysics &amp; Geomatics, China University of Geosciences, Wuhan 430074, P.R. China (Corresponding &amp;#160;author: &amp;#160;Harash Fayez).&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt;Subsurface Multi-Scale Imaging Lab, Institute of Geophysics &amp; Geomatics, China University of Geosciences,&lt;/p&gt;&lt;p&gt;&amp;#160;Wuhan 430074, P.R. China.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Summary &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;A 3-D density structure of the lithosphere and upper-mantle beneath the Mediterranean Sea and adjacent region was constructed based on inversion of gravity anomaly constrained by seismic tomography model. In this study, we have removed the terrain and crustal effects from the observed gravity field (EIGEN-6C4), in order to obtain the mantle gravity anomaly which was used to investigate the lithospheric and the upper-mantle density distribution. The 3-D inversion process is constrained by a reference density model estimated from shear-wave velocity model (SL2013sv). Our result shows some characteristics of density distribution in the lithosphere and upper-mantle that might be related to the tectonic signification beneath the Mediterranean Sea and adjacent region. A low-density zone dominates the lithosphere beneath the Mediterranean Sea except the area around Arabia shield and North Anatolian fault belt. A thinner high-density layer appears beneath the southwest of Mediterranean Sea, and it may be related to the older oceanic lithosphere fragments. The high-density anomalies appear below depth of 280 km beneath the Mediterranean Sea and the Turkish Aegean Sea Plate. However, the low-density anomalies appears at the top of the upper-mantle beneath trenches of the southwestern of Mediterranean Sea, the eastern of Aegean Sea, the Red Sea, the Black Sea and the middle of Arabia shield. It may indicate the intensity and origination of tectonic movement referring the deep structure below the Eratosthenes seamount in the Mediterranean Sea. Furthermore, the convergence region of two low-density anomaly zones may be interpreted as a significant tectonic unit.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

CRUSTAL STRUCTURE ON THE EASTERN SEABOARD OF CANADA: STUDIES ON THE CONTINENTAL MARGIN

1966 ◽  
Vol 3 (1) ◽  
pp. 65-76 ◽  
Author(s):  
Charlotte Keen ◽  
B. D. Loncarevic
Keyword(s):  
Continental Shelf ◽  
Upper Mantle ◽  
Seismic Data ◽  
Seismic Refraction ◽  
Ocean Basin ◽  
Mantle Structure ◽  
Upper Mantle Structure ◽  
Gravity Measurements ◽  
Vertical Density ◽  
Eastern Seaboard

The results of several seismic refraction profiles on the continental shelf and slope of the eastern seaboard of Canada are now available. Gravity measurements which begin near the coast of Nova Scotia and end over the abyssal plain have also been made along two tracks perpendicular to the shelf edge. Various models for the crustal and upper mantle structure are presented. A density distribution assumed for each model resulted in a computed gravity field satisfying the observed gravity measurements. The models in agreement with all seismic data suggest that horizontal and vertical density variations occur in the upper mantle down to 100 km. The results indicate a mantle density of 3.42 g/cm3 under the continental shelf and 3.32 g/cm3 under the ocean basin.

THE CRUST AND UPPER MANTLE STRUCTURES IN CENTRAL ANATOLIA, TURKEY, CONSTRAINED BY GRAVITY AND SEISMIC DATA

2020 ◽  
Author(s):  
Yagmur Yilmaz ◽  
◽  
Alain Plattner ◽  
Rezene Mahatsente ◽  
Ibrahim Çemen ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Data ◽  
Central Anatolia ◽  
Crust And Upper Mantle

scholarly journals Comparing global tomography-derived and gravity-based upper mantle density models

2020 ◽  
Vol 221 (3) ◽  
pp. 1542-1554 ◽  
Author(s):  
B C Root
Keyword(s):  
Seismic Tomography ◽  
Upper Mantle ◽  
Seismic Data ◽  
Gravity Data ◽  
Heterogeneous Data ◽  
Clear Correlation ◽  
Data Sets ◽  
Satellite Gravity ◽  
Similar Density ◽  
Similar Peak

SUMMARY Current seismic tomography models show a complex environment underneath the crust, corroborated by high-precision satellite gravity observations. Both data sets are used to independently explore the density structure of the upper mantle. However, combining these two data sets proves to be challenging. The gravity-data has an inherent insensitivity in the radial direction and seismic tomography has a heterogeneous data acquisition, resulting in smoothed tomography models with de-correlation between different models for the mid-to-small wavelength features. Therefore, this study aims to assess and quantify the effect of regularization on a seismic tomography model by exploiting the high lateral sensitivity of gravity data. Seismic tomography models, SL2013sv, SAVANI, SMEAN2 and S40RTS are compared to a gravity-based density model of the upper mantle. In order to obtain similar density solutions compared to the seismic-derived models, the gravity-based model needs to be smoothed with a Gaussian filter. Different smoothening characteristics are observed for the variety of seismic tomography models, relating to the regularization approach in the inversions. Various S40RTS models with similar seismic data but different regularization settings show that the smoothening effect is stronger with increasing regularization. The type of regularization has a dominant effect on the final tomography solution. To reduce the effect of regularization on the tomography models, an enhancement procedure is proposed. This enhancement should be performed within the spectral domain of the actual resolution of the seismic tomography model. The enhanced seismic tomography models show improved spatial correlation with each other and with the gravity-based model. The variation of the density anomalies have similar peak-to-peak magnitudes and clear correlation to geological structures. The resolvement of the spectral misalignment between tomographic models and gravity-based solutions is the first step in the improvement of multidata inversion studies of the upper mantle and benefit from the advantages in both data sets.

Le terrane de Slide Mountain (Cordillères canadiennes) : une lithosphère océanique marquée par des points chauds

2003 ◽  
Vol 40 (6) ◽  
pp. 833-852 ◽  
Author(s):  
M Tardy ◽  
H Lapierre ◽  
D Bosch ◽  
A Cadoux ◽  
A Narros ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Oceanic Island ◽  
Light Rare Earth ◽  
Isotopic Compositions ◽  
Incompatible Elements ◽  
Depleted Mantle ◽  
Mantle Sources ◽  
British Colombia ◽  
Ultramafic Cumulates ◽  
Back Arc

The Slide Mountain Terrane consists of Devonian to Permian siliceous and detrital sediments in which are interbedded basalts and dolerites. Locally, ultramafic cumulates intrude these sediments. The Slide Mountain Terrane is considered to represent a back-arc basin related to the Quesnellia Paleozoic arc-terrane. However, the Slide Mountain mafic volcanic rocks exposed in central British Colombia do not exhibit features of back-arc basin basalts (BABB) but those of mid-oceanic ridge (MORB) and oceanic island (OIB) basalts. The N-MORB-type volcanic rocks are characterized by light rare-earth element (LREE)-depleted patterns, La/Nb ratios ranging between 1 and 2. Moreover, their Nd and Pb isotopic compositions suggest that they derived from a depleted mantle source. The within-plate basalts differ from those of MORB affinity by LREE-enriched patterns; higher TiO2, Nb, Ta, and Th abundances; lower εNd values; and correlatively higher isotopic Pb ratios. The Nd and Pb isotopic compositions of the ultramafic cumulates are similar to those of MORB-type volcanic rocks. The correlations between εNd and incompatible elements suggest that part of the Slide Mountain volcanic rocks derive from the mixing of two mantle sources: a depleted N-MORB type and an enriched OIB type. This indicates that some volcanic rocks of the Slide Mountain basin likely developed from a ridge-centered or near-ridge hotspot. The activity of this hotspot is probably related to the worldwide important mantle plume activity that occurred at the end of Permian times, notably in Siberia.

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