Electromagnetic fields in a halfspace scattered by a buried sphere by the method of transformation of local elements

Summary. A problem in modelling electromagnetic fields used in exploration geophysics is treated mathematically. Analytical expressions are obtained for the electric field due to a harmonic current in a horizontal loop on or above a conducting ground in which is buried a conductive and permeable sphere (ore body). The loop is coaxial with the sphere. For a general time-varying current in the loop, the analysis is carried to the stage where a Fourier inversion can be used to obtain readily the electric field in the time-domain. A new relationship between spherical and cylindrical wave functions is obtained as a transformation of local elements.
Solution of this problem has not been presented before in this form. Lee's solution of 1975 which uses an integral-equation formulation treats a similar problem without taking account of differences in magnetic permeability. The effects of magnetic permeability may have important and useful implications for geophysical explorations.     

Thermal maturity of the axial zone of the southern Apennines fold-and-thrust belt (Italy) from multiple organic and inorganic indicators

The reconstruction of the thermal history of folded and thrust units is crucial to define the pattern of tectonic loading and the time-space evolution of an orogen where tectonic exhumation processes occurred at shallow crustal levels. In the present study, a well-constrained reconstruction of the thermal maturity in the axial zone of the southern Apennines has been achieved by the combined use of different thermal indicators in diagenesis. The major results are: (i) documentation of a jump in thermal maturity from the Apenninic Platform derived tectonic unit (from immature to early mature stages of hydrocarbon maturation) to the Lagonegro Basin derived tectonic units (late diagenetic zone); (ii) documentation of along-strike slighter variations in the Lagonegro units, concerning thermal maturity (thus maximum burial temperatures). This can be related to changes in amounts of tectonic burial and erosion/exhumation because of the lack of cylindricity of contractional structures; (iii) recognition of an independent thermal evolution of the allochthonous chain compared with the Apulian Platform tectonic unit with Mt Alpi area (in the late mature stage of hydrocarbon generation) interpreted as a sector of localized, intense exhumation within the External Zone of the orogen.     

The impact of recent climate change on flooding and sediment supply within a Mediterranean mountain catchment,southwestern Crete,Greece

This paper presents work from a geomorphological investigation carried out in the Aradena Gorge, southwestern Crete, Greece. The gorge is typical of many steepland fluvial systems in the Mediterranean, with steep relief, coarse‐gravel sediments and high rates of sedimentation generated during intense winter storm events. Hillslope deposits and coarse‐gravel flood units within a 5 km section of the gorge have been mapped, dated (using lichenometry and dendrochronology), and their sedimentological characteristics recorded to establish a c. 200‐year record of flood frequency/magnitude and hillslope/channel sediment supply variability. This record has been compared with instrumented and previously published records of climate change from Crete and the Mediterranean region and used to establish the major controls on flooding and sediment dynamics within the Aradena Gorge. Rates of colluviation and sediment delivery to the channel appear to have been greater than the present sometime before c. AD 1800 and may be related to cooler climates with a more seasonal precipitation regime during the Little Ice Age (c. AD 1450 to 1850). In gorge sections where the present rate of sediment supply from hillslope colluvium is very low, the channel has incised into older alluvial and colluvial deposits. Conversely, in the few sections where sediment supply is currently very high, the channel is aggrading with a braided pattern. Major rock‐fall deposits at certain locations in the gorge have restricted any major downstream sediment transfer. Twelve periods of increased flooding during the last 150 years have been identified and these correlate quite well with negative or declining phases of the North Atlantic Oscillation (NAO). Analysis of daily precipitation data from Crete suggests negative phases of the winter NAO are characterized by an increase in the number of long‐duration, high‐intensity storms. These storms, particularly those with five‐day and greater duration, appear to be significant in triggering major floods in the Aradena Gorge. During the last 40 years the NAO index has been increasing and become locked into a positive phase. As a consequence of this, major flooding appears to have declined during the same period. Copyright © 2002 John Wiley & Sons, Ltd.     

On Afro-Arabian graben tectonics

This study of the Afro-Arabian gräben, which concentrates on the region between Kenya and the Levant reviews their evolution that begun not earlier than in the latest Oligocene or in the lower Miocene. The gräben continue to sink in several major downfaulting phases, especially during the transitional times of Miocene to Pliocene, of Pliocene to Pleistocene and in the Mid-Pleistocene. We have no proof of erstwhile major graben structures in the crystalline Precambrian basement or in the pre-Oligocene sedimentary cover revived during the young Cenozoics that could have redeemed the formation of the Afro-Arabian megagräben. Significant faults in the pre-Oligocene sedimentary cover are seldom observed. Where they occur, their tectonic direction scarcely runs parallel to the principal trend of the Cenozoic graben. At best these ancient faults may have had an impact on the development of cross — and diagonal faulting which played also a role in the disposition of the subgräben, grabensplays and other offshoots of the main gräben. Equally rarely has parallelism of trend between the basement metamorphics as well as their intrusive dykes and that of the main graben lineations been established. The Afro-Arabian gräben originated in a lower Tertiary peneplain of regional dimension which had expanded over large part of Gondwana. The graben dissection thus took place regardless of former tectonic highs and lows such as swells and basins — a phenomenon that negates the theory of key-stone dropping as a result of preceding upwarping. The occasional impression of vaulting with the graben as its axis is purely of morphological significance; it is due to “isostatic” uplifting and tilting of the main horsts attached to the main graben. Besides, most megahorsts are not “halfhorsts” but huge blocks mountains (Levant, Etbai, Ethiopia, Danakil, Yemen) bordered on both flanks by major tensional faults with throws of thousands of meters. These major uplifts, which are especially noticed in the Plio-Pleistocene, caused, moreover, the removal of the sedimentary cover by erosion and denudation in the elevated parts of the graben shoulders and its hinterland. The few known outcrops of Cretaceous-Eocene in the coastal area of the Red Sea, believed to be indications of ancient marine gulfs and thus forerunners of the Rift valley are in all likelihood the relics of such coverbeds that were saved from erosion in the down dropped blocks of the graben. The surveyed marginal faults are normal dip-slips of average 70° dip. These remained in the ensuing taphrogenic episodes the “localized” fixed graben frames. In the interior of the graben they are followed by a Vorberge zone consisting of tilted blocks that implicate also subsidiary horsts and gräben. In the superdimensional rifts of Afar and Red Sea, the subsidiary gräben and horsts of the Vorberge zone may approach a width comparable to that of the East Levant rift valleys. In the axial part of the Afar, Red Sea and Gregory Rift valley appears as youngest, that is, Pleistocene structural element a distinct graben-in-graben or rift-in-rift. This young structure is much less developed in the graben of the Gulf of Aden. The Aden graben differs also in other aspects from the pattern of the Red Sea, particularly in its submarine axial topography and in the peculiar transversal fault offsets, both morphotectonic features reminiscent of the Indian mid-oceanic ridges. A comparison of geophysical, especially of gravity measurements, carried out in the various main gräben reveals very conflicting results. While higher positive Bouguer values led to the conjecture of heavy magmatic material in the bottom of the rift-in-rift of the Red Sea, in most of the Afar fault-funnel and in the entire Gregory Rift valley there prevails an unusual large gravity deficiency that would point to an extremely thick crust. This is all the more remarkable as both rift valleys and horsts of Ethiopia and of Kenya are dominated by huge basaltic trap lavas and by other basic volcanic material. The hypothesis of anticlockwise rotation and drifting of the Arabian peninsula (including its supposed impact on the orogenic origin of the Zagros fold belt) has many pitfalls.

1. As the taphrogenic destruction is not restricted to the intercontinental Afro-Arabian gräben, but extended far into the Mediterranen and into the Indian Ocean, any anticlockwise rotation of Arabia must exceed the 7° postulated movement.
2. Yemen and Ethiopia, that is Arabia and Africa, were in the Miocene still a continuous continent. When they became separated in the Pliocene by the Bab el Mandeb splay, northern Sinai and northernmost Egypt had been transformed into a landlocked isthmus that has remained a continental bridge to this day. In fact at the end of the Miocene the Red Sea graben had already been turned — though for a short period — into an enclosed evaporitic basin.
3. As regards the axial graben-in-graben of the Red Sea that especially prompted the assumption of a drift between Africa and Arabia, its Pleistocene age would demand a drifting rate of half a meter per year. There are, however, at the coastal area of the Red Sea no prehistoric, historic or recent witnesses of lateral stretching at the Red Sea and certainly not of such excess. The deduced annual drift of half a meter there, exceed even the most optimistic calculations and images hitherto made by the adherents of such large drifting as has been postulated to have taken place between Africa and South America.
4. Finally, in the studies on drifts — and this also applies to the question of horizontal fault displacement — there is wanting a proper discussion on the mechanical deformations to which the rigid crustal parts must have been subjected in the course of such intensive tearing apart motions.

     

Divariant and trivariant reaction line slopes in FMAS and CFMAS: theory and applications

The theory of divariant reactions is investigated in some detail and approaches involving simultaneously operating reactions and X _Mg isopleths are combined. Using the Gibbs' method (Spear et al. 1982), a general equation is derived for X _Mg isopleth slopes in FMAS and CFMAS, which may be applied to any divariant assemblage in these systems. Isopleth slopes are calculated for a number of divariant FMAS and CFMAS reactions inferred to have operated in Sri Lankan granulites, giving more rigorous constraints on the local P-T path. Isopleth slopes are also calculated for the trivariant CFMAS reaction orthopyroxene + plagioclase = garnet + quartz. The resulting dP/dT range from +8 to +18 bar/K. Application to Sri Lankan granulites confirms an earlier hypothesis (Faulhaber and Raith 1991) that a retrograde path with a dP/dT of ～10–15 bar/K may cause garnet growth in charnockites and garnet breakdown in metabasites. It is shown here that dP/dT depends primarily on the Ca contents of garnet and plagioclase. Large variations in isopleth and reaction line slopes are possible, due to the thermodynamic effect of additional components in solid solutions, e.g. albite in plagioclase. This has a good potential for the assessment of the exact form of P-T paths. Equations to calculate isopleth slopes could be included in software packages aimed at defining P-T estimates, enabling geologists to derive realistic P-T paths.     

The Hohonu Batholith of North Westland, New Zealand: granitoid compositions controlled by source H2O contents and generated during tectonic transition

Geochemical studies on the Hohonu Batholith, of the West Coast, South Island, New Zealand, have recognised two distinct but chemically related suites of mid-Cretaceous granitoids. The suites are characterised by restricted radiogenic isotopic compositions (Sr_(i) = 0.7062 to 0.7085; ɛNd_(i) = −4.4 to −6.1), and represent melting of a mafic lithosphere source followed by interaction with Ordovician metasediments. The two suites (Te Kinga Suite and Deutgam Suite) are distinguished by contrasting contents of Al₂O₃, Na₂O, Sr, Ba, Eu and HREE, attributable to different residual asssemblages controlled by differing H₂O contents during melting of a metabasaltic source. The relatively mafic, metaluminous, I-type Deutgam Suite represents magmas derived by dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. In contrast, the peraluminous, high silica compositions of the Te Kinga Suite were produced by melting at higher H₂O contents, reducing the stability of plagioclase and resulting in a melt in equilibrium with a plagioclase-free eclogitic (garnet + amphibole) residue. Residual plagioclase during generation of the Deutgam Suite resulted in lower Al₂O₃, Na₂O, Sr, Ba and Eu contents, whereas residual garnet during generation of the Te Kinga suite resulted in depleted HREE contents. The mid-Cretaceous granitoids of the Hohonu Batholith were generated during a period of rapid tectonic transition from crustal thickening during collision to crustal thinning and core complex formation during extension. Received: 23 July 1996 / Accepted: 21 August 1997