Evidence For Secular Variation Research Articles

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

Jupiter's Interior as Revealed by Juno

Jupiter is in the class of planets that we call gas giants, not because they consist of gas but because they were primarily made from hydrogen-helium gas, which upon gravitational compression becomes a metallic fluid. Juno, in orbit about Jupiter since 2016, has changed our view: The gravity data are much improved, and the simplest interpretation of the higher order even harmonics implies that the planet may have a diluted central concentration of heavy elements. Jupiter has strong winds extending to perhaps ∼3,000-km depth that are evident in the odd zonal harmonics of the gravity field. Jupiter's distinctive magnetic field displays some limited local structure, most notably the Great Blue Spot (a region of downward flux near the equator), and some evidence for secular variation, possibly arising from the winds. However, Juno is ongoing; it has not answered all questions and has posed new ones. ▪ Juno's mission reveals Jupiter's interior. ▪ A core exists but is diluted by hydrogen. ▪ The mission revealed wind depth and magnetic field.

Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

SUMMARY New estimates of crustal thickness, Poisson’s ratio and crustal shear wave velocity have been obtained for 39 stations in Angola, Botswana, the Democratic Republic of Congo, Malawi, Mozambique, Namibia, Rwanda, Tanzania and Zambia by modellingP-wave receiver functions using the H–κ stacking method and jointly inverting the receiver functions with Rayleigh-wave phase and group velocities. These estimates, combined with similar results from previous studies, have been examined for secular trends in Precambrian crustal structure within the southern African subcontinent. In both Archean and Proterozoic terranes we find similar Moho depths [38–39 ± 3k mSD (standard deviation)], crustal Poisson’s ratio (0.26 ± 0.01 SD), mean crustal shear wave velocity (3.7 ± 0.1 km s −1 SD), and amounts of heterogeneity in the thickness of the mafic lower crust, as defined by shear wave velocities ≥4.0 km s −1 . In addition, the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings. Thus, the results provide little evidence for secular variation in Precambrian crustal structure, including between Meso- and Neoarchean crust. This finding suggests that (1) continental crustal has been generated by similar processes since the Mesoarchean or (2) plate tectonic processes have reworked and modified the crust through time, erasing variations in structure resulting from crustal genesis.

Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation

We review the thickness and shear wave velocity structure of Precambrian crust in Africa and Arabia, where over 90% of the landmass is comprised of Archean and Proterozoic terranes, and examine the data for evidence of secular variation. The data come from many published 1D shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements, 35 new 1D shear wave velocity profiles for locations in eastern Africa, and a new map of crustal thickness for Africa and Arabia derived from modeling satellite gravity data. We find for both Archean and Proterozoic terranes a similar range of crustal thicknesses (~ 33–45 km), similar mean crustal shear wave velocities (~ 3.6–3.7 km/s), and similar amounts of heterogeneity in lower crustal structure, as reflected in the thickness of lowermost crust with shear wave velocities ≥ 4.0 km/s. There is little evidence for secular variation in crustal structure, indicating that there may have been few changes over much of Earth's history in the processes that form the continental crust. Post-formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize.

Model of Saturn's internal planetary magnetic field based on Cassini observations

We have derived a model of Saturn's internal planetary magnetic field from data obtained during the first three years of the Cassini Mission. This model is based on the most complete set of observations yet obtained in Saturn's magnetosphere and includes data from forty-five periapsis passes at a wide variety of geometries. Due to uncertainties in the rotation rate of the planet the model is constrained to be axisymmetric. To derive the model, the external currents are modeled explicitly as an equatorial ring current centered on Saturn's equator and the internal planetary magnetic field is derived using standard inversion techniques. The field is adequately described by a model of degree 3 and the spherical harmonic coefficients are g 10 = 21 , 162 , g 20 = 1514 , g 30 = 2283 . Units are nanoTeslas (nT) and are based on a planetary radius of 60,268 km. The model is consistent with a northward offset of the magnetic equator from the rotational equator of 0.036 Saturn radii. Reanalysis and comparison with data obtained by Pioneer-11 and Voyager-1 and -2 shows little evidence for secular variation in the field in the almost thirty years since those data were obtained.

Epidemiological studies of neural tube defects in Newfoundland.

A 9-year survey of neural tube defects in Newfoundland showed (1) evidence for secular variation, with a peak in 1980, but no general downward trend as seen in some other populations; (2) significant geographic variation which did not correlate with hardness or nitrate content of the lake water; (3) a tendency for the proportion of females among anencephalic births to increase with increasing frequency of anencephaly among geographic regions. This supports previous evidence for a female-specific contribution to the causes of increased liability to neural tube defect.

Paleomagnetic results from the Skaergaard intrusion, East Greenland

The natural remanent magnetization of 22 out of a total of 31 oriented cores from the layered series of the Skaergaard gabbroic intrusion (age: 55 m.y.) in East Greenland shows good stability in thermal and AF testing. The average direction of 22 AF and 9 thermally treated specimens is D = 170°, I = −59°, α 95 = 4.2 before correction for tilt. The mean directions after rotation around strike to horizontal and after rotation to original attitudes suggested by others yields poorer population statistics. It is therefore concluded that flexuring took place between solidification and acquisition of remanent magnetization, a range in temperature of about 500°C which may represent an interval of somewhat less than 250,000 years. No evidence for secular variation is observed which may also suggest slow cooling through the blocking temperature range. The polarity is reversed and the pole position without “tilt correction” is 165°E, 61°N, d m = 6.2 , d p = 4.6 , which is similar to pole positions reported by others for the overlying slightly older basalt.