Regional Transition Zone Research Articles

Application of Ps Scattering Kernels to Imaging the Mantle Transition Zone With Receiver Functions

AbstractSeismic imaging of the global boundaries of the mantle transition zone provides important constraints regarding mantle composition and convection. Additionally, modern hypotheses about mantle convection and increasing seismic data resources motivate attempts to image more complex regional transition zone structures such as dipping and discontinuous interfaces. Here we extend a 3‐D prestack migration method to make it more applicable for transition zone imaging with Ps receiver functions. After validation with 1‐D synthetic data, two types of hypothetical structures are adopted to demonstrate the strengths and weaknesses of different practical imaging parameters with 2‐D synthetic tests. The results show that the method can resolve dipping anomalies and laterally discontinuous low velocity layers. However, receiver spacing of 0.5° is inadequate to accurately migrate scattering from all possible angles in the transition zone at ~0.3 Hz. Application of a slowness cutoff window about the Ps raypath can mitigate artifacts due to low receiver density, but the tradeoff is a decrease in the maximum resolvable dip angle. Further synthetic tests evaluate source wavelet effects, addition of observed seismic noise to synthetic data, and imaging grid dimensions. Cumulatively, the tests indicate that generic application of migration algorithms to the limited source‐receiver distributions relevant for the transition zone should be treated cautiously. Imaging parameters, such as the slowness cutoff and wavelet, should be chosen based on synthetic tests for hypothetical targets and realistic source‐receiver geometries. Finally, observational Ps receiver functions from the USArray are migrated with the new method.

Diversity of wild yams (Dioscorea spp., Dioscoreaceae) collected in continental Africa

Plant genetic resources, especially crop wild relatives like wild yams, are under high risks of extinction due to habitat loss, climate change, unacceptable collection practices, shifting cultivation practice and over-exploitation. To contribute to their conservation, the diversity of wild yams (Dioscorea spp.) collected in continental Africa was analyzed. Specimens from Herbaria (B, BR, K, P, WAG and YA), databases, literature and actual collection from Cameroon were considered. The collection localities were georeferenced and projected to Lambert azimuthal equal-area projection. Spatial analyses of sampling and diversity were conducted at three different resolutions: 50 × 50-, 100 × 100- and 150 × 150-km sized grid cells. Four thousand and eighty-seven specimens, collected in 2895 localities, were inventoried and grouped into 55 taxa including 43 species, five subspecies, four varieties and seven morpho-species. The genus Dioscorea occurred in fifteen out of the eighteen phytochoria recognized on African mainland. The Guineo–Congolian regional centre of endemism was the most sampled and the richest phytochorion with 28 taxa. In terms of floristic composition, the Sudanian regional centre of endemism and the Guinea–Congolia/Sudania regional transition zone were the most similar (similarity coefficient SCi = 0.84). Globally, the most diversified grid cells were located in Sierra Leone, Liberia, Ivory Coast, Ghana, Togo, Benin, Nigeria, Cameroon, Gabon, Equatorial Guinea, CAR, Congo, DRC, Tanzania and Malawi. In terms of phytogeography, they were found in two regional centres of endemism (Guineo–Congolian and Zambezian), two regional transition zones (Guinea–Congolia/Zambezia and Guinea–Congolia/Sudania) and one regional mosaic (Zanzibar–Inhambane). These areas could therefore be considered as potential sites for in situ conservation of wild yams in continental Africa.

Mantle transition zone shear velocity gradients beneath USArray

Broadband P-to-s scattering isolated by teleseismic receiver function analysis is used to investigate shear velocity (VS) gradients in the mantle transition zone beneath USArray. Receiver functions from 2244 stations were filtered in multiple frequency bands and migrated to depth through P and S tomography models. The depth-migrated receiver functions were stacked along their local 410 and 660km discontinuity depths to reduce stack incoherence and more accurately recover the frequency-dependent amplitudes of P410s and P660s. The stacked waveforms were inverted for one-dimensional VS between 320 and 840km depth. First, a gradient-based inversion was used to find a least-squares solution and a subsequent Monte Carlo search about that solution constrained the range of VS profiles that provide an acceptable fit to the receiver function stacks. Relative to standard references models, all the acceptable models have diminished VS gradients surrounding the 410, a local VS gradient maximum at 490–500km depth, and an enhanced VS gradient above the 660. The total 410 VS increase of 6.3% is greater than in reference models, and it occurs over a thickness of 20km. However, 60% of this VS increase occurs over only 6km. The 20km total thickness of the 410 and diminished VS gradients surrounding the 410 are potential indications of high water content in the regional transition zone. An enhanced VS gradient overlying the 660 likely results from remnants of subduction lingering at the base of the transition zone. Cool temperatures from slabs subducted since the late Cretaceous and longer-term accumulation of former ocean crust both may contribute to the high gradient above the 660. The shallow depth of the 520km discontinuity, 490–500km, implies that the regional mean temperature in the transition zone is 110–160K cooler than the global mean. A concentrated Vs gradient maximum centered near 660km depth and a low VS gradient below 675km confirms that the ringwoodite to perovskite+magnesiowüstite reaction is the dominant cause of the 660 in the region.

A reconnaissance survey of the woody flora and vegetation of the Catapú logging concession, Cheringoma District, Mozambique

A checklist of the trees, shrubs and Hanes of Catapú, Cheringoma District, Mozambique, is presented. Floristically the study area falls within the Swahilian/Maputaland Regional Transition Zone. In total, 238 woody species and infraspecific taxa have been recorded, representing 59 families and 167 genera. Most species (64%) occur both to the north and south of the study area. 26% have their core distribution in the Swahilian Regional Centre of Endemism, 4% have a more southerly distribution. 14 are endemic to the Swahilian/Maputaland Regional Transition Zone and two near-endemic, extending into the Zambezian Regional Centre of Endemism only along the Zambezi River Valley as far west as Kariba. The checklist includes the Sena names for 191 species, 77 of which are recorded for the first time. Comparisons of the Catapú checklist are made with other checklists.

Regeneration Dynamics and Genetic Variability of Sugar Maple (Acer Saccharum [Marsh.]) Seedlings at the Species' Northern Growth Limit, Lake Superior Provincial Park, Canada

The transition from deciduous to boreal forest is abrupt regionally and topographically in Lake Superior Provincial Park, Ontario, Canada. The northern range limit of Acer saccharum is coincident with the forest transition to boreal forest. The goal of our study was to characterize the distribution of A. saccharum seedlings at the transition zone to determine variability in seedling demographics and genetics with topographic position and along a short north-south gradient. Seedling density, size, age, and growth were evaluated in permanent plots across the regional transition zone, and at the south-facing, ridge top and north-facing limits across the topographic transition. Growth over 5 yrs. was determined by measuring the distance between terminal bud scars, and compared with regional climate data. Genetic material was collected and analyzed from two of these sites. No significant differences were detected in density or growth of Acer saccharum across the regional transition but mean age increased and height decreased as the limit was approached. Across the topographic transition, ridge top seeding densities were greater than the south-facing or north-facing limits. Genetic variability is great with no cohort preference for topographic position detected.

Analyse phytogeographique de la region des Monts Kouffe au Benin

The Mounts Kouffe region is located in the central part of Benin between 1°40′ and 2°30′ N, and 8°25′ and 9°15′ E. This study dealt with the plant communities of the protected area of Wari Maro and Mounts Kouffe. The vegetation is composed of savannah, woodland and patches of dry forests. The aim of this study is to determine the phytogeographical position of the Mounts Kouffe region within Benin and western Africa. Between 1996 and 1998, floristical surveys were carried out. Some 159 phytosociological releves were done. They resulted in 954 herbarium specimens corresponding to 584 species. Species of the Sudanian base element represent 16.1% of the total flora spectrum of Mounts Kouffe region. Species of two of the three endemic genera to sudanian regional centre of endemism (RCE) were collected, i.e. Vitellaria paradoxa and Pseudocedrela kotschyi. The species present in both Sudanian and Zambesian RCE made up 13.5% of the phytogeographical spectrum and those of the Guineo-Congolian element accounted for 11.3% of the spectrum. They mostly belonged to gallery forests and dry forests because of the position of this region which is located in the most southern part of the Sudanian RCE close to the regional transition zone of Guinea-Congolia/Sudania. The other species widespread in several RCE's within continental african phytochoria represented 33% of the spectrum. Species of wide distribution represented 25.2% of the spectrum. This vegetation is entirely located into the Sudanian RCE as defined by White, with an important contribution of Guineo-Congolian species.

Coastal forests of eastern Africa: status, endemism patterns and their potential causes

Eastern African coastal forests are located within the Swahili regional centre of endemism and Swahili-Maputaland regional transition zone in eastern Africa, between 1° North and 25° South, and 34–41° East. Approximately 3167 km2coastal forest remains: 2 km2in Somalia, 660 km2in Kenya, 697 km2in Tanzania, 16 km2in Malawi, 3 km2in Zimbabwe and perhaps 1790 km2in Mozambique. Most forests are small (≤20 km2), and all but 19 are under 30 km2in area. Over 80% of coastal forest is located on government land, principally Forest Reserves; only 8.3 km2is found in National Parks (6.2 km2in Kenya (Arabuko-Sokoke), 2 km2in Tanzania (Mafia Island) and tiny patches in Zimbabwe). Coastal forests are an important and highly threatened centre of endemism for plants (c550 endemic species), mammals (6 species), birds (9 species), reptiles (26 species), frogs (2 species), butterflies (79 species), snails (>86 species) and millipedes (>>20 species). Endemic species are concentrated in the forests of the Tana River, between Malindi in Kenya to Tanga in northern Tanzania, and in southern Tanzania. Forests with highest numbers of endemics are: lower Tana River, Arabuko-Sokoke, Shimba Hills (Kenya); lowland East Usambara, Pugu Hills, Matumbi Hills, Rondo and Litipo and other plateaux near Lindi (Tanzania); the Tanzanian offshore island of Pemba; Bazaruto archipelago (Mozambique), and tiny forest remnants of southern Malawi, eastern Zimbabwe and Mozambique. Most coastal forest endemics have a narrow distributional range, often exhibiting single-site endemism or with scattered or disjunct distributional patterns. They are best interpreted as relicts and not the result of recent evolution. Relictualization probably started with the separation of the ancient Pan African rainforest into two parts during the Miocene. The coastal forests are interpreted as a «vanishing refuge» with the endemic species gradually becoming more and more relict (and presumably extinct) due historically to climatic desiccation and more recently to human destruction.

Coastal forests of eastern Africa: status, endemism patterns and their potential causes

Abstract Eastern African coastal forests are located within the Swahili regional centre of endemism and Swahili-Maputaland regional transition zone in eastern Africa, between 1° North and 25° South, and 34–41° East. Approximately 3167 km 2 coastal forest remains: 2 km 2 in Somalia, 660 km 2 in Kenya, 697 km 2 in Tanzania, 16 km 2 in Malawi, 3 km 2 in Zimbabwe and perhaps 1790 km 2 in Mozambique. Most forests are small (≤20 km 2 ), and all but 19 are under 30 km 2 in area. Over 80% of coastal forest is located on government land, principally Forest Reserves; only 8.3 km 2 is found in National Parks (6.2 km 2 in Kenya (Arabuko-Sokoke), 2 km 2 in Tanzania (Mafia Island) and tiny patches in Zimbabwe). Coastal forests are an important and highly threatened centre of endemism for plants ( c 550 endemic species), mammals (6 species), birds (9 species), reptiles (26 species), frogs (2 species), butterflies (79 species), snails (>86 species) and millipedes (>>20 species). Endemic species are concentrated in the forests of the Tana River, between Malindi in Kenya to Tanga in northern Tanzania, and in southern Tanzania. Forests with highest numbers of endemics are: lower Tana River, Arabuko-Sokoke, Shimba Hills (Kenya); lowland East Usambara, Pugu Hills, Matumbi Hills, Rondo and Litipo and other plateaux near Lindi (Tanzania); the Tanzanian offshore island of Pemba; Bazaruto archipelago (Mozambique), and tiny forest remnants of southern Malawi, eastern Zimbabwe and Mozambique. Most coastal forest endemics have a narrow distributional range, often exhibiting single-site endemism or with scattered or disjunct distributional patterns. They are best interpreted as relicts and not the result of recent evolution. Relictualization probably started with the separation of the ancient Pan African rainforest into two parts during the Miocene. The coastal forests are interpreted as a «vanishing refuge» with the endemic species gradually becoming more and more relict (and presumably extinct) due historically to climatic desiccation and more recently to human destruction.

Petrophysical properties of a prograde amphibolite-granulite facies transition zone at Sigerfjord, Vesterålen, northern Norway

The Vesterålen-Lofoten area in northern Norway is a province of deep-seated origin with associated regional magnetic and gravimetric anomalies. A regional transition zone between amphibolite and granulite-facies gneisses has been examined in detail with regard to interpretation of aeromagnetic and gravimetric data in northern Fennoscandia. The location of this petrographical boundary is defined by the first appearance of orthopyroxene in the intermediate gneisses. This orthopyroxene isograd is situated within an approximately 3 km wide magnetic transition zone where the average magnetite content increases fairly gradually from approximately 0.05% in the amphibolite facies ta 1.0% in the granulite facies. These estimates are based on in-situ susceptibility measurements which show an increase from 150 · 10 −5 SI to 3000 · 10 −5 SI. Magnetite is by far the most common Fe-oxide. The orthopyroxene isograd is situated approximately 500 m from the point where the susceptibility starts to increase. A few of the samples in the granulite fades have garnet occurring as coronas around Fe-Ti oxides and pyroxenes, indicating high-pressure retrograde metamorphism. Prograde textures, however, predominate. Density shows a moderate increase across the profile, varying from 2.75 · 10 3 kg/m 3 in the amphibolite-facies rocks, through 2.77 · 10 3 kg/m 3 in the transition zone, to 2.81 · 10 3 kg/m 3 in the granulite-facies rocks. The Q-values are 2.0 for the amphibolite facies, 0.45 for the transition zone and 0.27 for the granulite facies. The remanence is predominantly viscous, and probably influenced by coarse-grained multidomain magnetite. Based on susceptibility measurements and thin section studies it is concluded that the prograde metamorphism occurred at the same time as or before the development of numerous fractures in which fluids such as CO 2 had easy access. Sparse retrograde metamorphism in the granulite zone took place along more localized microshears that appear to have been carriers of H 2O.

Numerical modelling of SHLgwaves in and near continental margins

SUMMARY The effect of transition regions between continental and oceanic structures on the propagation of L, waves from continental sources is examined. In particular, the attenuation due to variations in layer thickness in such transition regions is calculated and explained for a suite of simple models. The measured attenuation, due tq the geometry of the transition regions between the oceanic and continental structures within a partially oceanic path with source and receiver in a continental structure, is at most a factor of four for frequencies from 0.01 to 1 Hz. This is inadequate to explain the observed extinction of L, along such paths. This extinction has previously been attributed to the effects of the transition region geometry. The method used to calculate the results presented in this study is developed and its validity and accuracy are demonstrated. Propagator matrix seismograms are coupled into a Finite Element calculation to produce hybrid teleseismic SH mode sum seismograms. These hybrid synthetics can be determined for paths including any regional transition zone or other heterogeneity that exists as part of a longer, mostly plane-layered, path. Numerical results presented for a suite of transition models show distinct trends in each of the regions through which the wavefield passes. The wavefield passes through a continent-ocean transition region, then a region of oceanic structure, and finally through an ocean-continent transition region. When an L, wavefront passes through a continent-ocean transition, the amplitude and coda duration of the L, wave at the surface both increase. At the same time, much of the modal L, energy previously trapped in the continental crust is able to escape from the lower crust into the subcrustal layers as body waves. The magnitude of both these effects increases as the length of the transition region increases. When the wavefront passes through the region of oceanic structure further energy escapes from the crustal layer, and produces a decrease in L, amplitude at the surface. The rate of amplitude decrease is maximum near the transition region and decreases with distance from it. When the wavefield passes through the oceancontinent transition region a rapid decrease in the L, amplitude at the surface of the crust results. The energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer. Some of the body wave phases produced when the wavefield passes through the continent-ocean transition region are incident on the continental crust in the ocean-continent transition region. These waves are predominantly transmitted back into the crust. The other body wave phases reach depths below the depth of the base of the continental crust before reaching the ocean-continent transition and, thus, escape from the system.

Extreme ultraviolet spectra of solar active regions and their analysis

Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 A) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm−2 (n e T = 6 × 1015 cm−3 K). The differential emission measure of the brightest 35″ × 35″ portion of an active region is obtained between 1.4 × 104 K and 5 × 106 K from HCO OSO-VI XUV (280–1370 A) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 A) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s−1. Downflows at 105 K are found throughout the plage with typical velocities of 10 km s−1. In the sunspot, downflows are typically 5 to 20 km s−1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s−1 downflows are found in the umbra in the same 1″ × 1″ resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1″ resolution element are derived. The velocities are greatest (∼ 10 km s−1) at 105 K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 104 K and 2 × 106 K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm−2 (n e T = 1.0 × 1016 cm−3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.