RESURGENT MEXICAN PHOENIX*

ABSTRACT. Popular impressions of Phoenix, Arizona perpetuate the notion that this metropolitan area is an overwhelmingly Anglo place. We challenge this assertion and demonstrate that the city has substantial Mexican roots and is presently being shaped by a vibrant, resurgent Mexican population. Employing historical records, surveys, and landscape data, we articulate the Mexican character of early Phoenix and highlight how the revival of Mexican Phoenix has transformed the urban landscape. We then relate how Phoenix's Mexican population is a more nuanced regional subculture formed through both historical and contemporary connections with specific Mexican states. We conclude with a call for greater understanding of the internal heterogeneity of Mexicans in the United States and how this can inform our geographical interpretations of the growing Latinization of American cities.     

The southern cape conductive belt (South Africa): Its composition, origin and tectonic significance

Magnetometer array studies have led to the discovery and mapping of the Southern Cape Conductive Belt (SCCB) crossing the southern tip of Africa from west to southeast coasts. The SCCB lies just south of the Namaqua-Natal Belt of cratonic rocks remobilized about 1000 m.y. B.P. It is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m.y. Relationships between the SCCB and the basement geochronology, geology and tectonics are considered in detail. These relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a Proterozoic subduction which stopped about 1000 to 800 m.y. B.P. Associated with this subduction we propose a Proterozoic range of Andean mountains, whose roots are now exposed in the Namaqua-Natal Belt. Later subduction further south, near the present south coast, is proposed to account for the intrusion, between the south coast and the SCCB, of the Cape Granites in the time interval 600-500 m.y. B.P. There is some evidence for a third, yet more distant, subduction episode off Permian Gondwanaland. After outlining this tectonic history, the paper turns to a closer examination of the hypothesis that the Southern Cape Conductive Belt consists of partly serpentinized basalt accumulated at the top of a Proterozoic subduction. A large static magnetic anomaly, which correlates with the SCCB over most of its length, is well fitted by a model which strongly supports this hypothesis. Bouguer gravity anomalies along western and central profiles likewise support the hypothesis. A discussion follows of the process of formation of the proposed block of serpentinized marine rocks, beginning with serpentinization of the crust near oceanic ridges by reaction of warm, porous, newly-extruded basalt with seawater convecting through it. The serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction, where it accumulates because of its low density. Examples of such accumulations are cited. Finally, it is shown that any iron in the olivine and pyroxene in the original basalt precipitates, when these silicates are hydrated to serpentine, as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites. In particular the Beattie static magnetic anomaly requires, in our model calculation, a very high susceptibility readily attainable in basalt at 15–20% serpentinization. The authors know of no other rock able to provide this high susceptibility. A similar percentage of serpentinization provides the density required to model the gravity anomalies.     

Sensitivity of GPS and GLONASS orbits with respect to resonant geopotential parameters

The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combined GPS/GLONASS data during the International GLONASS Experiment (IGEX). The resulting precise orbits were analyzed using the program SORBDT. Introducing one satellites positions as pseudo-observations, the program is capable of fitting orbital arcs through these positions using an orbit improvement procedure based on the numerical integration of the satellites orbit and its partial derivative with respect to the orbit parameters. For this study, the program was enhanced to estimate selected parameters of the Earths gravity field. The orbital periods of the GPS satellites are —in contrast to those of the GLONASS satellites – 2:1 commensurable (P _Sid:P _GPS) with the rotation period of the Earth. Therefore, resonance effects of the satellite motion with terms of the geopotential occur and they influence the estimation of these parameters. A sensitivity study of the GPS and GLONASS orbits with respect to the geopotential coefficients reveals that the correlations between different geopotential coefficients and the correlations of geopotential coefficients with other orbit parameters, in particular with solar radiation pressure parameters, are the crucial issues in this context. The estimation of the resonant geopotential terms is, in the case of GPS, hindered by correlations with the simultaneously estimated radiation pressure parameters. In the GLONASS case, arc lengths of several days allow the decorrelation of the two parameter types. The formal errors of the estimates based on the GLONASS orbits are a factor of 5 to 10 smaller for all resonant terms. AcknowledgmentsThe authors would like to thank all the organizations involved in the IGS and the IGEX campaign, in particular those operating an IGS or IGEX observation site and providing the indispensable data for precise orbit determination.     

Late Cainozoic stratigraphy,palaeomagnetic chronology and vegetational history from Lake George,N.S.W.

The sedimentary record from Lake George provides the longest relatively continuous Quaternary continental sequence yet available from Australia, and may record one of the longest Upper Cainozoic lacustrine records in the world.

Palaeomagnetic analysis of a 36 m core from the lake floor identifies a sequence of deposition extending through the Brunhes and Matuyama, to the Gauss magnetic Chron. A longer core from the same site, but with incomplete recovery, extends to 72 m in lacustrine sediment; the age of the base of this core estimated by extrapolation is between 4.2 and 7 Ma. As there are still older and deeper sediments in the basin, extending to an estimated depth of 134 m, the age of the tectonic formation of the Lake George basin must be reckoned as Middle Miocene or older.

The pattern of facies organisation through time demonstrates a phase of deep water deposition extending from the base of the cored sequence (72 m) up to 51.5 m, at which time a major change took place. A disconformity developed at this level, associated with a period of deep weathering and a prolonged phase of slope mantle deposition (from 51.5 to 30.8 m). A gradual return to lacustrine environments, with diminishing proportion of slope wash detritus, resulted in increased rates of deposition coincident with the Jaramillo Subchron at 21.5 m. Thereafter, throughout the Brunhes magnetic Chron, lacustrine conditions dominated, varying from deep to lake dry conditions in a rhythmic fashion, and reflecting the major climatic oscillations of the past 700 000 years, becoming more regular in the past 400 000 years.

The pollen analytical record of the upper 8.6 m, covering the last 350 000 years, provides the main framework for the reconstruction of climatic history. The pollen and algal records indicate a sequence of vegetation and lake level changes, in which four major glacial/interglacial cycles are correlated with stages 1 to 10 of the ¹⁸0 marine record. This provides by far the longest continuous biostratigraphic framework for the Quaternary period in Australia.

Comparison between the palaeoclimatic record and the lake level evidence shows that there is no simple correlation between the lake level fluctuations and the glacial/ interglacial oscillations. In fact, major falls in the lake level occured both at the peak of cold glacials and during the warm interglacials. Though the falls in the lake levels during a warm period (interglacial) can be explained by high rates of evaporation, drying during maximum cold can be explained best in terms of a fall in precipitation. Permanent to deep‐lake conditions generally occurred during intermediate cool periods following warm intervals, when perhaps the seas were still warm and low rates of evaporation on land prevailed. On the other hand, short periods of shallow to deep lake levels also occurred during warm (interglacial) periods, showing that these were associated with reasonably high rates of precipitation.     

Period–luminosity relations for red supergiant variables – I.The calibration

We present CCD photometry of red supergiant long-period variables (LPVs) in the Per OB1 association, the Large Magellanic Cloud (LMC) and M33. The photometry was obtained in the Kron–Cousins R and I bandpasses and in a narrow bandpass ( λ ₀=8250 Å, FWHM=300 Å) chosen to avoid TiO bands in the spectral energy distribution of the LPVs. Because the strength of the TiO bands varies greatly with temperature, which varies with the phase of an LPV, avoiding TiO reduces the amplitude of the photometric variations seen in LPVs. The result is a lower dispersion and a well defined period–luminosity (PL) relation.
For the LMC sample we find an rms dispersion of 0.27 mag in the narrow-band PL relation and slightly larger dispersions for the LPVs in Per OB1 and M33. This dispersion is comparable to that of the Cepheid PL relation at similar wavelengths. Adopting a distance modulus of 18.5±0.1 mag for the LMC, we obtain distance moduli of 11.68±0.15 mag for Per OB1 and 24.85±0.13 mag for M33. These distances agree well with those based on main sequence fitting for Per OB1 and the Cepheid distance for M33. Since LPVs are ∼ 5 times more common than Cepheids and have a well defined PL relation, LPVs provide a promising method for estimating Galactic and extra galactic distances.     

A progress report on the design of the 8 m aperture Optical/Infrared Telescope

Progress of the design study carried out y the Instituto de Astrophysica de Canarias and Royal Greenwich Observatory joint project office for an 8 m aperture Optical/Infrared Telescope. This includes the development of an active pneumatic support system for both the radial and axial supports of an 8 m diameter thin meniscus mirror and the investigations of new technology encoding systems.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

Climate,environment, and humans in North America’s Great Basin during the Younger Dryas, 12,900–11,600 calendar years ago

Global climate change associated with the onset of the Younger Dryas chronozone affected different regions of the northern hemisphere in different ways. In the Great Basin of western North America, the effect was positive for human populations. Relatively cool temperatures causing effectively wetter conditions filled some pluvial basins with shallow but permanent lakes and other basins with well-watered marshes or meadows. Vegetation communities dominated by sagebrush and grasses promoted healthy and diverse animal populations. Ten archaeological sites from the region have been dated to the Younger Dryas chronozone. Evidence from these sites indicates that Paleoindians with skull shapes and mitochondrial DNA similar to modern western North American Indians occupied the region. These early humans produced a material culture characterized predominantly by large stemmed bifacial points, although one site contained a small fluted point. Curated tool forms and technological activities represented in analyzed lithic assemblages suggest a highly mobile settlement strategy, and redundant short-term occupations of sites indicate frequent and long-distance residential moves across territories spanning distances of up to 400 km. Paleoindian subsistence pursuits focused on artiodactyls (primarily mule deer, bighorn sheep, and pronghorn antelope), leporids (chiefly jackrabbits), birds (sage grouse and waterfowl), insects (grasshoppers), and possibly fish. Easy-to-process plants like cactus pads were also eaten, but small seeds do not seem to have been an important part of Great Basin human diets until long after the Younger Dryas, closer to 9500 cal BP. The Great Basin record contains no evidence for natural catastrophe at the onset of the chronozone. Instead, the Younger Dryas appears to have been among the best of times for human foragers in this region of North America.     

Deconvolution of teleseismic recordings for mantle structure

Delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances. This deconvolution problem can be posed in a log-spectral domain where the operation of time-domain convolution is reduced to an additive form. The introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers. The system is inherently singular, and stabilization is accomplished through the supply of an initial estimate of the source time function. Although alternative choices exist, an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular-value decomposition of the suite of aligned seismograms corresponding to a given event. The relation of the deconvolution scheme to simultaneous least-squares deconvolution is examined. Application of the methodology to broadband teleseismic P waveforms recorded on the Canadian National Seismograph Network demonstrates the retrieval of effective Green's functions including secondary phases associated with upper-mantle structure.