Mean Azimuth Research Articles

Seismotectonics and present-day relative plate motions in the Tonga-Lau and Kermadec-Havre region

An updated compilation of shallow seismicity in the Tonga-Kermadec arc and back-arc region, including especially 304 focal mechanisms from centroid moment tensor solutions files, provides new insights on the present-day tectonics and allows the relative motions between the Pacific, Indo-Australian and Tonga-Kermadec plates to be quantified. Azimuths of slip vectors along the trench change at 19°S. In the southern domain (35-19°S) the mean azimuth ranges from N280°E to N285°E, while in the northern domain it trends N276–277°E. This 19°S boundary also exits in the back-arc domain. South of 19°S, from 35°S to 20°S the direction of back-arc extension is respectively N135°E, N122°E and N111°E within the southern Havre trough, northern Havre trough and southern Lau basin. In the southern Lau basin the N111°E opening direction is compatible with the trend of the newly mapped Valu Fa ridge. North of 19°S, in the northern Lau basin (16–18°S), a N93°E trending crustal extension occurs along the N135°E Peggy ridge and along an inferred N5–10°E spreading zone. Between these two extensional features a fourth plate, the northern Lau microplate, is present. At 18–19°S a ridge-ridge-ridge triple junction, accommodated by N95°-115°E right-lateral strike-slip fracture zones, is proposed at the complex transitional zone between the southern and the northern parts of the Lau basin. The Fiji-North Tonga fracture zone which bounds northward the Lau back-arc basin is composed of two segments: a N95°E eastern segment extending from the northern end of the Tonga trench to 178°30′W, and a N75°E western segment lying north of the Fiji platform. A ridge-fracture-fracture triple junction is inferred at 178°30′W–15°45′S between the northern end of the Peggy ridge and the two segments of the Fiji-North Tonga fracture zone. Using the model RM-2 of Minster and Jordan (1978) and the directions of back-arc spreading and convergence at the trench, relative plate motions are estimated. The extension rates in the back-arc domain increase northward, and are respectively 0.8, 2.1 and 8 cm/y at 33°S, 28°S and 24°S. Farther north parallelism between directions of different motions does not allow the calculation of any accurate result. Therefore north of 24°S we used a spreading rate of 8 cm/y which corresponds to the 24°S result and also to the value deduced from magnetic anomalies. The consumption rates along the Tonga-Kermadec trench are respectively 7.2, 9.6, 16.4, 17.1 and 17.8 cm/y at 33°S, 28°S, 24°S, 20°S and 17°S. Both the back-arc opening rate and the consumption rate of the Pacific plate sharply increase between the northern Havre trough and the southern Lau basin. This limit coincides with the Louisville ridge-trench junction. Along the N95°E eastern segment of the Fiji-North Tonga fracture zone, a pure left-lateral strike-slip motion, parallel to the N93°E opening tectonics within the northern Lau basin occurs. In contrast along the N75°E western segment of the Fiji-North Tonga fracture zone, a left-lateral strike-slip motion of 7 cm/y should be accompanied by a N135°E extensional motion of 3.5 cm/y.

Lineaments in enhanced Landsat images from a portion of west Wales

Remotely sensed lineaments are often used as indicators of fractures in near-surface rocks. Previous lineament interpretations of Wales have used aerial photographs and relatively unenhanced satellite images. New images of part of Wales have been produced from Landsat MSS data using several digital image processing techniques intended to enhance the visibility of linear features. A lineament interpretation of these images is presented and described. Five populations of lineaments are classified, primarily on the basis of their azimuths. The most numerically significant of these has a mean azimuth of 075° and is here termed the ‘Ystwyth Lineament Set’. Typically the lack of rock exposure on the line of lineaments makes direct geological investigation difficult. However, field evidence and literature have been used to shed light on the origin of the Ystwyth Lineament Set. It is concluded that this set of lineaments is the visual representation of a set of fractures which have had a long and tectonically varied history.

Mancos B"Interval of Upper Cretaceous Mancos Shale, Douglas Creek Arch, Northwest Colorado: A "Shelf-Sand"Complex: ABSTRACT"

The Mancos B interval of the Mancos Shale, a major gas producer on the Douglas Creek arch is a shelf-sand complex represented by up to 1345 ft (404 m) of thinly interstratified claystone, siltstone, and very fine to fine-grained sandstone deposited approximately 100 mi (161 km) offshore in the Western Interior seaway. In the subsurface, the Mancos B is subdivided into four major units, ranging in thickness from 175 to 480 ft 953 to 146 m). Each unit thins over the arch's crest and, in the process, becomes more sand rich. In outcrops along the southern flank of the arch, the lower Mancos B forms a series of coarsening-upward sequences, 25 to 90 ft (8 to 27 m) thick. A typical sequence begins with silty claystone or bioturbated mudstone at the base, followed upward by bioturbated muddy sandstone, and topped with sandy dolomite (as concretions or thin beds). Sedimentary structures include horizontal lamination, wavy lamination, lenticular bedding, flaser bedding, and ripple lamination. Paleocurrent measurements from ripple foresets have a mean azimuth of 111/sup 0/. Burrows characteristic of the Cruziana ichnofacies are common to abundant in all lithofacies.

Orientation of American Kestrel Nest Cavities and Nest Trees

Balgooyen (1976), in his Sierra Nevada study, showed that nest cavities of American Kestrels (Falco sparverius) face east significantly more often than expected, and that nest trees are most often located on east-facing slopes. He based his results on comparisons of observed frequency distributions to a uniform distribution. But American Kestrels, typical of most secondary cavity-nesting birds, use abandoned nest cavities excavated by woodpeckers. It is possible, therefore, that the nonrandom orientations of kestrel nest cavities and slope exposure reflect selection by the original nest excavators rather than by the kestrels. Some studies have reported nonrandom orientations of woodpecker nest cavities (Lawrence 1967, Conner 1975, Inouye 1976, Korol and Hutto 1984). If most available cavities or slopes faced east, the apparent preference by kestrels could be random selection of available cavities or trees. To test this possibility, I compared Balgooyen's (1976) data on nest orientation to an independent sample of 105 nests of the two species whose nests kestrels most often use in California-Northern Flicker (Colaptes auratus) and Lewis' Woodpecker (Melanerpes lewis; Raphael and White 1984). I collected data from 1975 to 1978 on the same study areas reported by Balgooyen (1976). My null hypotheses were that: (1) no difference existed in orientation of kestrel nest cavities compared to that of available cavities, and (2) slope exposures were equal for kestrel nest trees and available trees. I tested the hypotheses using Watson's U2 test (Batschelet 1965:35). Both hypotheses were rejected. Nest cavity orientation differed significantly from expected (U2105,58 = 0.307, P < 0.005), and slope exposures also differed from expected (U293,58 = 0.490, P < 0.005; Table 1). Available cavities were generally oriented in a northerly direction (mean azimuth = 14*; Table 1), whereas most kestrel cavities faced east-northeastward (mean angle = 59*). Angular dispersions, which measure the relative concentration of points around the mean, were similar among nests and available trees for both cavity orientation and slope exposure (Table 1). Examination of differences between percentages of kestrel and woodpecker nests for each of the eight directions showed the greatest deviation for east-facing cavities and slopes (Table 1), supporting Balgooyen's original observations. Balgooyen (1976) speculated that nest cavities facing eastward might offer thermoregulatory advantages because of warmth of the morning sun and protection from storms and hot afternoon temperatures. These considerations should also apply to the woodpeckers that excavated the nests. If so, most woodpecker nests should have been excavated with east exposures. It is possible that woodpe kers excavated cavities at the place around t e tru k where decay conditions were best for nest excavation. In the present study area, however, both Lewis' Woodpecke s and Northern Flickers excavated nests in trees that were decayed throughout (Raphael and White 1984). Therefore, it is unlikely that their nests were oriented in relation to variable decay characteristics of the trees. While these results do not rule out the possibility that kestrels choose cavities to maximize thermal advantages, further study would be necessary before accepting such an explanation. The apparently nonrandom selection of east-facing cavities by kestrels nevertheless cannot be attributed to cavity availability.

Comments on the paper “bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments”, by Brooks B. Ellwood

Anisotropy of magnetic susceptibility (AMS) and remanent magnetization in semi-consolidated sediments reflect some ordered, spatial distributions of magnetic mineral grains, caused by forces acting during or shortly after deposition. Any preferred grain orientation is likely to be modified by biological disturbances, and in this paper by E1lwood [1] three different sediment types were investigated in order to elucidate the effect of bioturbation on magnetic fabric characteristics. We find his interpretation and conclusions derived from the tidal flat experiment (Sapelo Island) rather unsatisfactory, and offer hereby an alternative interpretation. The magnetic fabric in the established magnetite horizon gradually change towards what may be inferred to represent a primary depositional magnetic fabric, characterized by Q-values below 2/3 (oblate ellipsoids) associated with closely grouped, almost vertical distributions of kmin directions (minimum axis of the susceptibility ellipsoid). Mean azimuths of the flat-lying kma x axis all strike N-S, parallel to the shore line. Ellwood has inferred that this evidently post-depositionai reorientation of magnetite grains developed by subsurface long-axis grain alignment caused by water flowing horizontally through the (biological) expanded sediment. The apparent depositional (primary) characteristics of the magnetic fabric, is concluded to be secondary, improved during initial expansion followed by grain settling and dewatering after passage of organisms. In addition to

Registration of thunderstorm centers by automatic atmospherics stations

A new generation of VLF‐atmospherics receivers and analyzers is presented that operates completely automatically and allows the realtime identification of thunderstorm activity centers. The mean azimuth and angular spread of each thunderstorm center within the detection range as well as the mean statistical properties of three measured spectral parameters of the atmospherics originating at that center are determined within each measuring period (20 min) by a desktop computer. Using a suitable statistical procedure for determining the mean angular and spectral parameters and applying a new extended model for VLF‐propagation as function of the solar zenith angle and of the magnetic field component along the propagation path, the operation program determines in real time the distances and strengths (atmospherics/min) of the activity centers so that all data can be immediately printed out and stored on the internal magnetic tape cartridge. Additionally, an external plotter is used for marking the thunderstorm centers on a map. No maintenance is required apart from changing the data tape cartridge once every month. At present, two systems of this new kind are installed at Pretoria (South Africa), Tel Aviv (Israel), and a somewhat older one at Berlin. Three more stations are to be established toward the end of this year. First results show the characteristics of the method, the short‐ and long‐range capability and resolution, and the reliability of the thunderstorm localizations.

Stress orientations from oil-well fractures in Alberta and Texas

Many oil wells in Alberta exhibit spalling of the walls (known as breakouts), which elongates the holes with the longer axis aligned northwest–southeast. This alignment is observed over an area in excess of 4 × 105 km2, in siltstones, sandstones, carbonate sediments, and one shale formation, through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the beds. We have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls, in a stress field having large, unequal horizontal principal stresses and with the larger compression oriented northeast–southwest. It is probable that this northeast–southwest principal stress is σ1. This paper adds new data from oil wells in Alberta and northern British Columbia and shows that the breakouts, and by inference the stress orientations, are consistent through much of the western Canadian sedimentary basin. It also uses evidence from hydraulic fracturing in west-central Alberta and from steam-injection fracturing in eastern Alberta to support the view that σ1 is aligned northeast–southwest. The breakouts are consistent with either a thrust stress field or a strike–slip stress field, but the fractures formed by excess pressures in wells favour the latter. Recently Schafer has reported oil wells systematically elongated with a mean azimuth of N39°E in the Austin Chalk of southern Texas. We regard these elongations as formed by breakouts in a stress field with greater horizontal stress near N51°W, and this view is supported by overcoring measurements by Hooker and Johnson in granite of the Llano Uplift, 225 km from Schafer's wells, that reveal large horizontal compressions at the surface with the larger oriented N33°W.

Palaeocurrent and palaeohydrologic analysis of some vertical profiles through a Cretaceous braided stream deposit, Banks Island, Arctic Canada

ABSTRACTA study of directional variability has been carried out on five vertical profiles containing a total of approximately 195 superimposed planar crossbed sets. The latter are interpreted as the deposits of braid bars. Detailed measurements on one set revealed within‐set variance of 93. Within river tract, between‐bar variance was obtained by calculating a ten‐point moving average for each profile, and subtracting the actual azimuth at each point from the moving average value. Variance at this level is 980, which compares closely with data obtained from modern braided streams. Variability in the moving average azimuth is interpreted as the result of meander migration in the entire channel system. Sinuosity can be estimated from this variability, using a geometrical approximation. It ranges from 1.03 to 1.13.Changes in stream competency with time are indicated by vertical changes in directional variance and set thickness. When accompanied by changes in mean azimuth, as in one of the present five profiles, the data suggest successive deposition by two distinct stream systems.A Markov chain analysis of the data sequence indicates a weak memory effect. Short sequences of bar deposits were formed by streams which varied little in orientation, and these are separated from one another by sequences showing random directional fluctuation.The hydrology of the Isachsen rivers was investigated using Schumm's (1968a, b, 1969, 1972) empirical relationships for modern rivers. Owing to inherent uncertainties in these relationships and a wide margin of error in the input data (principally associated with estimates of depth and width of the Isachsen streams), the results have a validity only at the order‐of‐magnitude level. Individual Isachsen rivers are estimated to have had drainage areas of between 5000 and 30,000 km2; this information is of use in reconstructions of palaeogeography and tectonic history.

Paleocurrent analysis of the athgarh sandstone (upper Gondwana), Cuttack District, Orissa (India)

The Athgarh Sandstone (Upper Gondwana) of Jurassic age exposed in Orissa on the southeastern margin of India comprises almost horizontal medium- to coarse-grained pebbly sandstone interbedded with thin layers of white clay and carbonaceous shale. The cross-bedded sandstone beds within the formation display tabular cross-bedding, having an average thickness of 30 cm and average inclination of 13°. The mean foreset azimuth is 128° (nearly southeast). A total of 537 measurements of cross-beddings from 28 localities, over an area of 500 sq.km, were taken. The study of cross-bed data indicates transportation of sediments by water currents flowing in a southeasterly direction during the deposition of the Athgarh Sandstone. The mean azimuth, thickness and inclination of the cross-beds and sample variance of the area indicate a fluviatile environment of deposition. The dispersal pattern of the currents in the southeastern part of the area suggests proximity to the shore line.

Graphical Method for Calculating Vector Mean Azimuth and Magnitude for Paleocurrent Analysis

A graphical method is suggested in order to calculate the vector mean azimuth and vector magnitude for paleocurrent analysis.

Quiet homogeneous arcs at Halley Bay 1956–1965

Visual observations made at Halley Bay, Antarctica, in the years 1956–1965 are used to study variations in the position of quiet homogeneous arcs. The diurnal variations in median latitude and mean azimuth of the highest point on the lower border of the arcs are similar from year to year. The annual median latitudes and mean azimuths show no evidence of systematic variation during the solar cycle. The median latitude of all arcs is 66.7°S in θ 4 coordinates. The corresponding value at the peak of the auroral incidence curve presented by Stringer and Belon is found to be 67°. The diurnal variation in position, found using all measurements made from 1956 to 1965, can be fitted to the pattern of arc orientations found by Gustaffson.

Petrology of Krebs Subgroup (Pennsylvanian, Desmoinesian) of Western Missouri

The Krebs subgroup of Pennsylvanian age is a rock association of varied lithologic character that crops out in several counties of southwestern Missouri. The Krebs was divided into 10 rock types on the basis of lithologic differences observed in the field. Stratigraphic measurements were made of each rock type. Sandstone Type A (medium- to coarse-grained sandstone: submature to mature subarkose) constitutes 63 per cent of the rock sections measured. Other rock types present in the Krebs are orthoquartzitic (chert) conglomerate, fine- to very fine-grained subgraywackes (sandstone Types C1 and C2), supermature calcitic subarkose (sandstone Type B), black shale, gray shale, coal, underclay, and limestone (silty brachiopod biomicrite). Textural and miner logical analyses of each rock type indicate that the field division into rock types was generally valid. The Krebs subgroup is made up of six formations and in some places as many as three or four of the designated rock types occur within a single formation. Cross-bedding studies indicate that the Type A sandstones were deposited by currents with a modal southeasterly flow direction (mean azimuth = S 34° E). Cross-bedded units average 2.5 feet in thickness and cross-beds have a modal inclination of 22°. The provenance of these sediments is thought to be a combination of distant and local sources. The distant source was located northwest of the study area and was part of the Canadian shield, and the local source was the Nemaha uplift. Seven depositional environments--beach, lagoon, tidal channel, low tidal flat, high tidal flat, swamp, and shallow offshore marine--are represented by Krebs subgroup sediments. The Warner formation, composed of channel and sheet deposits, is of particular interest in that its grain-size characteristics and structures closely resemble those of the Recent Wadden Sea.

The ventricular gradient in space

1. 1. A simple, geometric method of determining the spherical coordinates of the ventricular gradient is presented. 2. 2. Electrocardiograms of eight normal subjects are thus analyzed, using the cube method of electrode placement. Mean magnitude, azimuth and elevation are found to be 47 microvolt-seconds, 16 ° and 52 °, respectively. 3. 3. The theory of the gradient is discussed. 4. 4. The role of the RS-T segment is elucidated and a method of evaluating its significance is suggested. 5. 5. Gradients are imposed on hypothetic muscle strips by means of a suitable equation, and the effects of change in rate and direction of activation on the form and area of the ventricular complex are demonstrated. It is concluded that these do not influence net area. 6. 6. A block diagram of an instrument for instantaneous determination of the gradient is included.