The latitudinal dependence of the westward drift in the main geomagnetic field is examined by using the correlation analysis of moving random pattern. The study reveals the characteristics in the differential rotation...The latitudinal dependence of the westward drift in the main geomagnetic field is examined by using the correlation analysis of moving random pattern. The study reveals the characteristics in the differential rotation of the main field. The results show that the global geomagnetic field drifts westward with an average speed of 0.18/a during 1900—2000. The westward drift rate is not symmetrical with re-spect to the equator. The maximum westward drift rate, 0.31/a, occurs at the latitude j = -15? forming a Rapid Westward Drift Belt (RDB) around this latitude. Going northward and southward from this belt, the drift rate de-creases and reaches the minimum (0.12/a) at j = 50?and the minimum (0.14/a) at j = -56? forming a Northern Hemi-sphere Slow Westward Drift Belt (N-SDB) and a Southern Hemisphere Slow Westward Drift Belt (S-SDB). Three phases can be detected in the evolution of the westward drift. In the first phase (1900—1940), the RDB dominates the global drift pattern. The westward drifts in this belt are much faster than those in other areas. In the second phase (1940—1960), the drift rates in the RDB are less than those in the first phase, while the drifts in the N-SDB and S-SDB are relatively large. In this phase, the differential rotation becomes less obvious. In the third phase (1960—2000), the westward drift in the RDB increases again and the differen-tial rotation gradually becomes apparent.展开更多
The main geomagnetic field models of IGRF1900-2000 are used to study the latitude-dependence of the westward drift in the main field. The results show that the latitude-dependence exists in the magnetic components wit...The main geomagnetic field models of IGRF1900-2000 are used to study the latitude-dependence of the westward drift in the main field. The results show that the latitude-dependence exists in the magnetic components with different wavelengths (m=1-10). The global-average westward drift rate of the component of m=1 is 0.189°/a with the maximum of 0.295°/a at latitudes 40°-45°. The compo-nent of m=2 has an average drift rate of 0.411°/a with the maximum of 1.305°/a at latitude -60°. As for the compo-nents with further shorter wavelengths, the drift is generally restricted in a limited latitude range, and has many smaller drift rates. This latitude-dependence of westward drift can not be explained by rigid rotation of the earth’s core. The results of this note also show that there is a negative disper-sion in the westward drift, namely the components of long wavelengths drift faster than those of short wavelengths. This dispersion feature is not in agreement with Hide’s MHD model. It is likely needed展开更多
We analyze the gross crustal structure of the Atlantic Ocean passive continental margins from north to the south,comparing eleven sections of the conjugate margins.As a general result,the western margins show a sharpe...We analyze the gross crustal structure of the Atlantic Ocean passive continental margins from north to the south,comparing eleven sections of the conjugate margins.As a general result,the western margins show a sharper continental-ocean transition with respect to the eastern margins that rather show a wider stretched and thinner margin.The Moho is in average about 5.7±1dipping toward the interior of the continent on the western side,whereas it is about 2.7±1in the eastern margins.Moreover,the stretched continental crust is on average 244 km wide on the western side,whereas it is up to about 439 km on the eastern side of the Atlantic.This systematic asymmetry reflects the early stages of the diachronous Mesozoic to Cenozoic continental rifting,which is inferred as the result of a polarized westward motion of both western and eastern plates,being Greenland,Northern and Southern Americas plates moving westward faster with respect to Scandinavia,Europe and Africa,relative to the underlying mantle.展开更多
The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophy...The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources （below or within the asthenosphere, which decouples the lithosphere from the deep mantle） predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation （NNR） reference frame, and low （〈0.2°-0.4°/Ma） net rotation rates （deep hotspots source） predict an average net rotation in which some plates move eastward relative to the mantle （e.g., Nazca）. With fast （〉1°/Ma） net rotation （shallow hotspots source）, all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries.展开更多
Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable process of Earth's geodynamics? The reason why the tidal drag has been questioned as the mechanism determinin...Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable process of Earth's geodynamics? The reason why the tidal drag has been questioned as the mechanism determining the lithospheric shift relative to the underlying mantle is the apparent too high viscosity of the asthenosphere. However, plate boundaries asymmetries are a robust indication of the 'westerly'decoupling of the entire Earth's outer lithospheric shell and new studies support lower viscosities in the low-velocity layer(LVZ) atop the asthenosphere. Since the solid Earth tide oscillation is longer in one side relative to the other due to the contemporaneous Moon's revolution, we demonstrate that a non-linear rheological behavior is expected in the lithosphere mantle interplay. This may provide a sort of ratchet favoring lowering of the LVZ viscosity under shear, allowing decoupling in the LVZ and triggering the westerly motion of the lithosphere relative to the mantle.展开更多
基金support of K.C.Wong Education Foundation,Hong Kongsupported by the National Natural Science Foundation of China(Grant Nos.40204003 and 40074015).
文摘The latitudinal dependence of the westward drift in the main geomagnetic field is examined by using the correlation analysis of moving random pattern. The study reveals the characteristics in the differential rotation of the main field. The results show that the global geomagnetic field drifts westward with an average speed of 0.18/a during 1900—2000. The westward drift rate is not symmetrical with re-spect to the equator. The maximum westward drift rate, 0.31/a, occurs at the latitude j = -15? forming a Rapid Westward Drift Belt (RDB) around this latitude. Going northward and southward from this belt, the drift rate de-creases and reaches the minimum (0.12/a) at j = 50?and the minimum (0.14/a) at j = -56? forming a Northern Hemi-sphere Slow Westward Drift Belt (N-SDB) and a Southern Hemisphere Slow Westward Drift Belt (S-SDB). Three phases can be detected in the evolution of the westward drift. In the first phase (1900—1940), the RDB dominates the global drift pattern. The westward drifts in this belt are much faster than those in other areas. In the second phase (1940—1960), the drift rates in the RDB are less than those in the first phase, while the drifts in the N-SDB and S-SDB are relatively large. In this phase, the differential rotation becomes less obvious. In the third phase (1960—2000), the westward drift in the RDB increases again and the differen-tial rotation gradually becomes apparent.
基金This work was supported by the National Natural Science Foundation of China (Grant Nos. 49734140 and 49974014).
文摘The main geomagnetic field models of IGRF1900-2000 are used to study the latitude-dependence of the westward drift in the main field. The results show that the latitude-dependence exists in the magnetic components with different wavelengths (m=1-10). The global-average westward drift rate of the component of m=1 is 0.189°/a with the maximum of 0.295°/a at latitudes 40°-45°. The compo-nent of m=2 has an average drift rate of 0.411°/a with the maximum of 1.305°/a at latitude -60°. As for the compo-nents with further shorter wavelengths, the drift is generally restricted in a limited latitude range, and has many smaller drift rates. This latitude-dependence of westward drift can not be explained by rigid rotation of the earth’s core. The results of this note also show that there is a negative disper-sion in the westward drift, namely the components of long wavelengths drift faster than those of short wavelengths. This dispersion feature is not in agreement with Hide’s MHD model. It is likely needed
基金The Sapienza University supported this research.
文摘We analyze the gross crustal structure of the Atlantic Ocean passive continental margins from north to the south,comparing eleven sections of the conjugate margins.As a general result,the western margins show a sharper continental-ocean transition with respect to the eastern margins that rather show a wider stretched and thinner margin.The Moho is in average about 5.7±1dipping toward the interior of the continent on the western side,whereas it is about 2.7±1in the eastern margins.Moreover,the stretched continental crust is on average 244 km wide on the western side,whereas it is up to about 439 km on the eastern side of the Atlantic.This systematic asymmetry reflects the early stages of the diachronous Mesozoic to Cenozoic continental rifting,which is inferred as the result of a polarized westward motion of both western and eastern plates,being Greenland,Northern and Southern Americas plates moving westward faster with respect to Scandinavia,Europe and Africa,relative to the underlying mantle.
基金Research supported by Sapienza University of Rome and Miur-Prin2011
文摘The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources （below or within the asthenosphere, which decouples the lithosphere from the deep mantle） predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation （NNR） reference frame, and low （〈0.2°-0.4°/Ma） net rotation rates （deep hotspots source） predict an average net rotation in which some plates move eastward relative to the mantle （e.g., Nazca）. With fast （〉1°/Ma） net rotation （shallow hotspots source）, all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries.
文摘Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable process of Earth's geodynamics? The reason why the tidal drag has been questioned as the mechanism determining the lithospheric shift relative to the underlying mantle is the apparent too high viscosity of the asthenosphere. However, plate boundaries asymmetries are a robust indication of the 'westerly'decoupling of the entire Earth's outer lithospheric shell and new studies support lower viscosities in the low-velocity layer(LVZ) atop the asthenosphere. Since the solid Earth tide oscillation is longer in one side relative to the other due to the contemporaneous Moon's revolution, we demonstrate that a non-linear rheological behavior is expected in the lithosphere mantle interplay. This may provide a sort of ratchet favoring lowering of the LVZ viscosity under shear, allowing decoupling in the LVZ and triggering the westerly motion of the lithosphere relative to the mantle.
