Sea-surface topography around Australia

The sea-surface topographys, as represented by the separation between the ocean surface and a level surface, is viewed as a problem involving and concerning both geodesy and physical oceanography. The determination of this topography bygeodetic levelling processes, in conjunction with tide-gauge observation, is examined. Sources of error, difficulties, estimates of accuracies, and actual results are mainly related to the third-order Australian levelling net, which has indicated a sea-surface topography variation, with position, of 2 m, with a standard deviation estimated to be about 30 cm. The expectedoceanographic influences on the sea-level are described, the individual contributing factors being discussed separately. Around Australia, differences in water density can account for an estimated 60 cm of the above mentioned 200 cm sea-level variation, while the airpressure effect appears to account for another 10 cm only. The wind influence undoubtedly also contributes to the sea-surface topography but it is presently virtually impossible to provide a suitable figure. Some discussion is given to the apparent differences between the results from these separate sources, for this continent.     

KAr data for the age and evolution of Gettysburg Bank,North Atlantic Ocean

Gettysburg Bank forms the western end of the Gorringe Seamount which is situated in the North Atlantic 110 km west of the tip of the Iberian Peninsula, on the eastern end of the Azores/Gibraltar fracture zone.Gabbros dredged from the Gettysburg Bank record a complex history of events. K-Ar ages of separated mineral phases fall into three concordant groups (plus some discordant ages). The oldest ages are from three brown kaersutitic hornblendes and their mean age of135 ± 3Ma is taken to be that of their formation. Six plagioclase feldspars yielded concordant ages of105 ± 3Ma which is possibly a consequence of a thermal event occurring at that time. Ages from three deformed plagioclases are concordant with a mean of82 ± 3Ma which is believed to relate to a phase of shearing, perhaps occurring during transform motion at the plate boundary.     

The carbon isotope biogeochemistry of the individual hydrocarbons in bat guano and the ecology of the insectivorous bats in the region of Carlsbad,New Mexico

The structures and ¹³C contents of individual alkanes extracted from bat guano found in the Carlsbad region of New Mexico can be related to both the photosynthetic pathways of the local plants and the feeding habits of the insects that support the bats.Carbon isotopic analyses show that equivalent numbers of C₃ and C₄ native plant species occupy the Pecos River Valley, a very significant feeding area for the Carlsbad bats. During the seasons when bats frequent the area, the agricultural crops consist principally of alfalfa and cotton, both C₃ plants.The molecular composition of the bat guano hydrocarbons is fully consistent with an insect origin. Two isotopically distinct groups of insect branched alkanes were discerned. These two groups of alkanes derived from two chemotaxonomically distinct populations of insects possessing distinctly different feeding habits. It is possible that one population grazes predominantly on crops whereas the other population prefers native vegetation. This and other isotopic evidence suggests that crop pests constitute a major percentage of the bats' diet.Because the guano sample was less than 40 years old, this material reflects the present day plant community in the Pecos River Valley. Future studies of more ancient guano deposits should reveal a measurable influence of both natural and man-induced vegetative changes with time upon the ¹³C content of the bat guano hydrocarbons.     

Quaternary volcanism and faulting at O’A caldera,central ethiopian rift

O’a is the largest of the Quaternary caldera volcanoes that punctuate the axis of the Ethiopian rift valley. The known volcanic history of O’a is brief: eruptions of restricted ash-flow tuffs and «tufolavas» were followed by extensive pumice deposition with intervening paleosols, lacustrine sediments, and flows of occasional welded tuffs and rare basalts. Ensuing caldera collapse at c. 0.24 m.y. ago was accompanied by emplacement of two massive ignimbrite flow units comprising a single cooling unit: the first was much more severely welded than the second which shows lahar characteristics. Post-caldera volcanism at O’a has been sparse compared with most other Ethiopian rift centres. O’a volcano exemplifies the common rift association of a caldera set tightly between two offset segments of the Wonji fault belt. The Wonji fault belt marks the youngest tectonism of the rift floor, and in the vicinity of O’a has been active in a major way since caldera subsidence. This faulting is clearly younger than the massive rift margin faulting, which to the northeast of O’a occurred during a tectonic climax dated at c. 1.0 m.y. ago. Radiometric analysis suggests a rather regular level of initial⁴⁰Ar in O’a basalt lavas sampled near to their original vents. If this level also applies to near-vent basalts dated from other parts of the Ethiopian rift, a regional rift paroxysm of crustal extension and related silicic and basaltic volcanism is evident at c. 0.30–0.20 m.y. ago. Episodic dilatation and associated volcano-tectonism separated by long periods of quiescence appears to be a general feature of continental rift valleys.     

Lg Coda Q and its Relation to the Geology and Tectonics of the Middle East

—Regional seismograms were collected to image the lateral variations of Lg coda Q at 1 Hz (Q ₀?) and its frequency dependence <(eta)> in the Middle East using a back-projection method. The data include 124 vertical-component traces recorded at 10 stations during the period 1986–1996. The resulting images reveal lateral variations in both Q ₀ and <eta>. In the Turkish and Iranian Plateaus, a highly deformed and tectonically active region, Q ₀ ranges between about 150 and 300, with the lowest values occurring in western Anatolia where extremely high heat flow has been measured. The low Q ₀ values found in this region agree with those found in other tectonically active regions of the world. Throughout most of the Arabian Peninsula, a relatively stable region, Q ₀ varies between 350 and 450, being highest in the shield area and lowest in the eastern basins. All values are considerably lower than those found in most other stable regions. Low Q values throughout the Middle East may be caused by interstitial fluids that have migrated to the crust from the upper mantle, where they were probably generated by hydrothermal reactions at elevated temperatures known to occur there. Low Q ₀ values (about 250) are also found in the Oman folded zone, a region with thick sedimentary deposits. <eta> varies inversely with Q ₀ throughout most of the Middle East, with lower values (0.4–0.5) in the Arabian Peninsula and higher values (0.6–0.8) in Iran and Turkey. Q ₀ and <eta> are both low in the Oman folded zone and western Anatolia.     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

Lg Coda Q and its Relation to the Structure and Evolution of Continents: A Global Perspective

—Tomographic maps of Lg coda Q (Q ^c _Lg) variation are now available for nearly the entire African, Eurasian, South American, and Australian continents, as well as for the United States. Q ^c _Lg at 1 Hz (Q ₀) varies from less than 200 to more than 1000 and Q ^c _Lg frequency dependence (<eta>) varies between 0.0 and nearly 1.0. Q ₀ appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region. A plot of Q ₀ versus time since that activity indicates that a single Q ₀-time relation approximates most mean Q ₀ values. Those that deviate most from the trend lay in Australia, the Arabian Peninsula, and the East African rift. The increase in Q ₀ with time may be due to a continual increase in crustal shear wave Q (Q ) caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity. Extrapolated values of Q ^c _Lg at 5 Hz (using Q ₀ and <eta> values measured at 1 Hz and assuming that <eta> is constant in all regions between 1 and 5 Hz) show a similar percentage-wise increase with times that has elapsed since the most recent activity. Other factors that can reduce Q ₀ in continental regions include thick accumulations of sediment (especially sandstone and shale of Mesozoic age and younger), severe velocity gradients at the crust-mantle transition and, possibly, lateral variations in the depth, thickness, and severity of those gradients. Severe and large increases of Q in the mid-crust of some regions can cause relatively large values of <eta>, even if the frequency dependence of Q is small.     

An overview of climatic variability and its causal mechanisms

A variance spectrum of climatic variability is presented that spans all time scales of variability from about one hour (10⁻⁴ years) to the age of the Earth (4 × 10⁹ years). An interpretive overview of the spectrum is offered in which a distinction is made between sources of variability that arise through stochastic mechanisms internal to the climatic system (atmosphere-ocean-cryosphere) and those that arise through forcing of the system from the outside. All identifiable mechanisms, both internal and external, are briefly defined and clarified as to their essential nature. It is concluded that most features of the spectrum of climatic variability can be given tentatively reasonable interpretations, whereas some features (in particular the quasi-biennial oscillation and the neoglacial cycle of the Holocene) remain fundamentally unexplained. The overall spectrum suggests the existence of a modest degree of deterministic forms of climatic change, but sufficient nonsystematic variability to place significant constraints both on the extent to which climate can be predicted, and on the extent to which significant events in the paleoclimatic record can ever manage to be assigned specific causes.     

Late Precambrian subduction and collision in the Al Amar—Idsas region, Arabian Shield, Kingdom of Saudi Arabia

Recent work on outer arcs and collision belts provides for the first time a possible model for evolution of part of the Arabian Shield. The thick volcanic, volcaniclastic and sedimentary succession of the Proterozoic Halaban Group in the east of the Shield is intruded by synto late tectonic plutons and resembles Cenozoic subduction-related magmatic areas. West of the Halaban Group, and separated from it by a major east-dipping thrust with associated ultrabasic rocks and carbonates, are folded chlorite—sericite metasediments of the Abt Schists, comparable to Cenozoic outer arc successions. West of and beneath the Abt Schists calcareous and arenaceous metasediments of the Ar-Ridaniyah Formation are analogous to Mesozoic—Cenozoic continental margin shelf facies of the subducting plate. Eastward subduction with magmatism (Halaban Group) and tectonic emplacement of ocean-floor sediments (Abt Schists) was followed by continental collision and eastward underthrusting by the Ar-Ridaniyah Group and cratonized central part of the Shield. Collision-related post-tectonic granites were emplaced during and following the collision.     

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda,Africa

Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.