Travel time curves for complex inhomogeneous slightly anisotropic media

Summary The linearization approach is used to compute the travel times in inhomogeneous slightly anisotropic media. The basic formulae are outlined and their accuracy demonstrated in comparison with the exact solution based on the zero-order ray theory and the Backus formula (1965). The linearization is extended also to complex media with curved interfaces. The computer program for calculating travel times in 2D, inhomogeneous, slightly anisotropic, complex media is briefly described. The numerical results obtained for a realistic situation and various types of waves are presented to enable the effects of anisotropy and the effects of inhomogeneity on the resulting travel times to be compared.

---

na uauua n¶rt;¶rt; ¶rt; ama¶rt;aa , anmau aaumn ¶rt;a. ¶rt; u n¶rt; au m u n muu nuuuu u m¶rt; aa (1965). a uauua n¶rt;¶rt; ¶rt; a ¶rt; uuuauau a¶rt;a. am nuaa uuma naa ¶rt; ama¶rt;a ¶rt; ¶rt;. u mam ¶rt; a mun ¶rt;am m um m aumnuu u m ¶rt;¶rt;mu a a anmau .

     

Paleomagnetism of the Cambro-Ordovician McClure Mountain alkalic complex,Colorado

The Middle to Late Cambrian loop in the North American apparent polar wander path (APWP) has been variously attributed to tectonic rotations, remagnetizations and primary magnetizations. Although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated, the temporally self-consistent nature of the loop has been used as an argument for primary magnetizations. We have studied535 ± 5Ma nepheline syenites and syenites of the McClure Mountain alkalic complex, as well as495 ± 10Ma red trachyte dikes which intruded the complex, in an effort to find a primary TRM. Because Zijderveld analysis yielded consistent results for only one trachyte dike, remagnetization great-circle analysis was employed, giving a pole for the trachyte dikes at the tip of the loop (43°N, 114°E), while the syenites and nepheline synenites gave a pole at the base of the loop (18°N, 142°E). The magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration. KAr whole rock dating of the trachytes suggests a Pennsylvanian age for the alteration, and thus a late Paleozoic remagnetization of the trachytes. Thus, the low-latitude Cambrian pole is confirmed, but we find no evidence in this study to support the primary nature of the Cambrian APWP.     

Flow and grainsize pattern in a sharply curved river bend

The main characteristics of river flow and grainsize in a bend of the sand bedded meandering river Dommel, The Netherlands, are presented. Measurements were carried out at a relatively low discharge in a sharply curved bend following a long straight reach. In the studied bend, secondary circulation is restricted to the thalweg area; only in the downstream part of the bend it exists over the entire cross-section. Therefore, on the entire pointbar platform, which comprises the larger part of the bend, the median sedimentation diameter of the bedload material is governed by the distribution of the longitudinal components of the bed shear stress only.     

Structures caused by repeated freezing and thawing in various loamy sediments: A comparison of active,fossil and experimental data

In this paper, the authors present the results of both macroscopic and microscopic investigations on structure development created by repeated ice lensing in various loamy experiments. Experimental data are compared with observations performed on active forms in High Arctic and Alpine Mountain environments. Those observations are also compared with phenomena observed in fossil periglacial formations of Western Europe. Platy and short prismatic structure formation is bonded to the hydraulic and thermal conditions during ice segregation. When a long series of alternating freezing and thawing affects platy structures, the fabric evolves, also being influenced by slope and drainage conditions: cryoturbations, frostcreep, and gelifluction can appear. They are characterized by specific microfabrics which are better developed with an increasing number of cycles: this is clear in experiments where hydraulic and thermal parameters are better controlled. Vesicles are also a prominent characteristic of the surface horizon in experiments and arctic soils. The genesis of vesicles is discussed on the basis of new observations and is related to the mechanical collapse of frost-created aggregates under the mechanical work of soil air escape during soil saturation by water at thaw.     

Biogeographic implications of a packrat midden sequence from the Sacramento Mountains, south-central New Mexico

Thirteen packrat (Neotoma spp.) and two porcupine (Erethizon dorsatum) middens from 1555 to 1690 m elevation from the Sacramento Mountains, New Mexico, provide an 18,000-yr vegetation record in the northern Chiuahuan Desert. The vegetation sequence is a mesic, Wisconsin fullglacial (18,000–16,000 yr B.P.) pinyon-juniper-oak woodland; a xeric, early Holocene (ca. 11,000–8000 yr B.P.) juniper-oak woodland; a middle Holocene (ca. 8000-4000 yr B.P.) desert-grassland; and a late Holocene (ca. 4000 yr B.P. to present) Chihuahuan desertscrub. The frequency of spring freezes and summer droughts in the late Wisconsin probably set the northern limits of Pinus edulis and Juniperus monosperma at about 34°N, or 6° south of today's limit. Rising summer tempratures in the early Holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope. At this time pinyon-juniper woodland and pine forest dominated by Pinus ponderosa probably began their spectacular Holocene expansions to the north. Continued warming in the middle Holocene led to very warm summers with strong monsoons, relatively dry, cold winters, and widespread desert-grasslands. Desertscrub communities in the northern Chihuahuan Desert did not develop until the late Holocene when the biseasonal rainfall shifted slightly back toward the winter, catastrophic winter freezes decreased, and droughts in all seasons increased. The creosote bush desertscrub corridor across the Continental Divide between the Chihuahuan and Sonoran deserts was probably connected for the first time since the last interglaciation.     

Water chemistry and sedimentological observations in littlefield lake,michigan: Implications for lacustrine marl deposition

A combination of both water chemistry and sedimentological information was used to investigate the carbonate-producing mechanism in Littlefield Lake, a small lake located in Isabella County, central Michigan. Data on temperature, dissolved oxygen, pH, calcium carbonate (CaCO₃) saturation, alkalinity, calcium, and magnesium were obtained on a monthly basis over a 13-month period, with each parameter determined at 1m intervals over a depth range of 20m. The loss of dissolved carbon dioxide (CO₂) from warm surface waters during direct degassing, and to a lesser extent during photosynthetic uptake by lacustrine macrophytes and phytoplankton during the summer, results in massive precipitation of the low-magnesium calcite which predominates in all Littlefield Lake sedimentary facies However, despite the fact that carbonate precipitation in this rather typical temperate-region marl lake is directly related to, and may be driven by, seasonal variation in these physiochemical parameters, most calcite forms as encrustations around cyanophytic and chlorophytic macrophytes. Such relationships demonstrate that carbonate precipitation in marl lakes may result from complex interactions between both biochemical and physiochemical processes. As such, marl formation in this, and probably many other calcareous lake systems, can not be simply ascribed to one or the other of these two general mechanisms.     

Geomorphological evolution of a Lowland valley system during the Weichselian

Weichselian geomorphological and fluvial evolution has been recorded in the valley fill of the Reusel, a small tributary of the lower Maas river. It has been studied by means of borehole sections. A first deep incision into the substratum occurred during the Early Weichselian-Lower Pleniglacial. This was followed by aggradation, at first by a braided river, at a later date by a meandering river, dating respectively from the Lower and Middle Pleniglacial. The Upper Pleniglacial aggradation again points to a braided river system. The end of the Pleniglacial is characterized by aeolian infilling of the valley and decreasing carrying capacity of the river.     

Biomarker distributions in asphaltenes and kerogens analysed by flash pyrolysis-gas chromatography-mass spectrometry

Biomarker distributions in a suite of asphaltenes and kerogens have been analysed by flash pyrolysis directly coupled to a GCMS system. Attention has been focussed on biomarkers of the sterane and triterpane types. The sample suite under investigation consists of sediments with different kerogen types and some crude oils. Biomarker distributions in the pyrolysates have been compared with the “free” biomarkers in the corresponding saturated hydrocarbon fractions.The analyses show significant differences between the distributions of the free biomarkers and those in the pyrolysates. The latter have lower amounts of steranes while diasteranes are absent or present at low concentrations only. In the triterpane traces a shift of maximum intensity from C₃₀ (free compounds) to C₂₇/C₂₉ is observed. Furthermore, the pyrolysates contain a set of triterpenes (not present among the free compounds), and there is a selective loss of “non-regular” triterpanes that are present in the saturated hydrocarbon fractions. The observed differences between pyrolysates and free hydrocarbons can be explained partly by the processes occurring during pyrolysis such as bond rupture and subsequent stabilisation of primary pyrolysis products. To a certain extent these differences also show that maturation processes occurring in sediments have effects on free biomarker molecules different from those on molecules that are enclosed in a macromolecular matrix (kerogen or asphaltenes).Differences between biomarker distributions of asphaltene and kerogen pyrolysates are relatively small. A comparison with the pyrolysates from extracted whole sediments suggests that these differences are mainly caused by interactions between the organic material and the mineral matrix during pyrolysis.Oil asphaltenes behave differently from sediment asphaltenes as their pyrolysates are more similar to the corresponding saturated hydrocarbon fractions, i.e. the differences described above are observed to a much smaller extent. This different behaviour appears to be the result of coprecipitation of a part of the maltene fraction with the oil asphaltenes.     

High latitude river runoff in a doubled CO2 climate

A global atmospheric model is used to calculate the monthly river flow for nine of the world's major high latitude rivers for the present climate and for a doubled CO₂ climate. The model has a horizontal resolution of 4° × 5°, but the model's runoff from each grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. A routing scheme is used to move runoff from a grid box to its neighboring downstream grid box and ultimately to the mouth of the river. In a model simulation in which atmospheric carbon dioxide is doubled, mean annual precipitation and river flow increase for all of these rivers, increased outflow at the river mouths begins earlier in the spring, and the maximum outflow occurs approximately one month sooner due to an earlier snow melt season. In the doubled CO₂ climate, snow mass decreases for the Yukon and Mackenzie rivers in North America and for rivers in northwestern Asia, but snow mass increases for rivers in northeastern Asia.     

Seismically active fracture zones and distribution of thermal springs in Ecuador

Summary The analysis of the geometry of distribution of earthquake foci occurring in the continental South American plate above the subducted Nazca plate reveals that in the region of Ecuador these earthquakes accumulate in 8 well-separated fracture zones. These zones are interpreted as a system of deep seismically active fractures induced in the continental plate by the process of subduction. It is shown that the position of known thermal springs and of observed hydrothermal alterations in Ecuador correlates with the outcrops of the fracture zones.

---

¶rt;aumuu an¶rt;u a mu muma num uu a¶rt; ¶rt;uu a¶rt; naa m mu mu anm 8 m¶rt; a a, m umnmua au uu amu a a muma um n ¶rt;uu. aam, m nmu um ma umuu uu¶rt;ma uu a¶rt; a¶rt;am ¶rt;a mu a .